Handbook of radioactive contamination and decontamination

Studies in Environmental Science 47

HANDBOOK OF RADIOACTIVE C0 NTA MINAT10 N AND D EC0 NTA MINAT10 N JAN SEVERA

Purkyn6 Medical Research, Hradec Krdlov6, Czechoslovakia

JAROMJR

BAR

Military College of Ground Forces, VySkov, Czechoslovakia

E LSEVI ER Amsterdam-Oxford-New York-Tokyo,

1 991

Distribution of this book is handled b j the following publishers: jor the U.S.A. and Canada Elsevier Science Publishing Company. Inc. 655 Avenue of the Americas New York, NY 10017 for the East European Countries, Democratic Republic of Vietnam, Mongolian People's Republic, People's Republic of Korea. People's Republic of China, Republic of Cuba Alfa. Hurbanovo nam. 3, 815 89 Bratislava, Czechoslovakia for all remaining areas Elsevier Science Publishers Sara Burgerhartstraat 25 P.O. Box 21 1. lo00 AE Amsterdam. The Netherlands

ISBN 0-444-98757-6 (Vol. 47) ISBN 0-444-41696-X (Series) J. Severa and J. Bar, 1991 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, mechanical, photocopying, recording, or otherwise, without the prior written permission of the copyright owner

Printed in Czechoslovakia

Preface and Introduction

In common with all forms of matter, each atomic nucleus must have originated in the course of the history of the Universe and will eventually cease to exist sooner or later by transforming into another atomic nucleus or another form of mass, even without any external stimulus, solely from intrinsic causes. The capability of atomic nuclei to undergo spontaneous nuclear transformation is called r a d i o a c ti v i t y. Nuclear transformations are often accompanied by emission of nuclear radiation which is detectable and measurable by means of instruments and methods of nuclear technology. Nuclear radiations may be harmful to living organisms, in serious instances it can cause radiation sickness, even death. Atomic nuclei with a very long life emit an extremely weak nuclear radiation not intense enough to cause harm to human health, and are therefore for all practical purposes considered to be stable and thus nonradioactive. For some of these atomic nuclei the measuring techniques available at present are not capable of detecting any radioactivity at all. With a continuing refinement of the detection methods, however, the number of such nuclei steadily decreases. Absolutely stable forms of mass and consequently absolutely stable nonradioactive atomic nuclei may not exist. Chemical forms, such as molecules and ions, containing at least one radioactive atomic nucleus are designated as radioactive substances. Dispersed radioactive substances constituting a potential or real hazard to human health are defined as radioactive contamination. The term radioactive contaminant is also used to designate a contaminating radioactive substance or a mixture of such substances. It is impossible to rid a radioactive substance of its radioactivity. Neither any chemical reagents, nor any common factors of “classical” physics (e. g. temperatures up to 104K attainable in an electric furnace, pressures up to 103MPa, electric current, magnetic field etc.) can affect the rate of radioactive decay. There is no other way to render safe a radioactive substance than to’remove it from the place where it’couldimperil human health and safety, and then dispose of it in a safe locality where the risk of human exposure is excluded or substantially diminished. Apart from mechanical removal, various chemical agents, detergents, solvents, sorbents, ion exchangers and the relevant XX i

processes are applied to ensure the management of radioactive substances. The removal of contaminants from sites where they might endanger life, and their transfer to places where the risk is substantially less is called decontamination. Soon after H. Becquerel discovered radioactivity in 1896, it became obvious that ionizing radiation may have deleterious effects upon living organisms. The pioneers among the scientists working with radioactive substances gave little attention to protection against irradiation or radioactive contamination; consequently, many of them had to pay later on for their ignorance or sometimes their undue enthusiasm with poor health or with life shortening. Among others, this was the fate of Marie Curie Sklodowska. First impulses towards protection against ionizing radiation and radioactive contamination came from a variety of newly established institutions dealing with the application of radium in medicine and some other branches>of science involving a growing number of occupationally exposed workers. Yet specific safety standards and generally accepted regulations concerning radiation protection were not worked out until much later, and the codes of practice aiming at minimizing the health risk of radiation and radioactive contamination depended for long solely on the experience, sound judgment and initiative of those responsible for the operations in any given institution. The discovery of artificial radioactivity induced by nuclear particles, mainly neutrons, and later on the discovery of uranium fission resulted in a great upsurge of nuclear research. The Second World War and especially the time period shortly thereafter witnessed a rapid development of nuclear reactors, construction and testing of nuclear weapons and a wide expansion in the utilization of radionuclides as tracers in almost all kinds of scientific and technical fields. These advances in nuclear science and technology soon made it necessary to set up limits for maximum permissible doses of human radiation exposure and maximum permissible levels of radioactive contamination of both persons and the human environment. Hand in hand with these early developments came empirical knowledge about decontamination technology. The need to know more about the scientific principles and practical methods of decontamination gained a special urgency with the onset of nuclear energetics in 1954, and particularly since the early '603, marking the start of a steady increase in the number of nuclear power plants in all areas of the world. Decontamination as a scientific and technical discipline can conveniently be divided into general decontamination and several specific branches of applied decontamination. The subject matter of general decontamination is the set of basic underlying principles related to the removal of dispersed radioactive substances from the surface of contaminated solids and from contaminated fluids (i.e. liquids and gases). The special branches of decontamination deal with specific methods and means applicable to decontaminate objects of a particular

type, or adequately suited to a particular field of activity. For example, one can deal specifically with decontamination procedures applicable to operational installatims of nuclear power plants, equipment systems of nuclear laboratories, operational facilities of the uranium industry, and alternatively various items and areas contaminated with products of nuclear explosions etc. A realistic evaluation of the present state of knowledge in the field of decontamination necessarily requires that each particular branch be dealt with separately, since the individual special branches differ greatly one from another in the degree of their development. Decontamination technology for operational systems of nuclear power plants has essentially developed from previously known technologies of chemical cleaning and pickling of systems in conventional power plants. At the present time, this special branch of the art of decontamination belongs - next to waste water decontamination - to the most advanced and most progressive techniques. In part, it is a reflection of the fact that the nature of this particular branch of decontamination makes it possible for important advances in theoretical knowledge to be quickly implemented in practice. A milestone in this development is marked by the publication, in 1970, of the now classical monograph by J. A. Ayres et ale*)In a comprehensive way, the book presented all information available at that time on decontamination means and procedures which had been tried in the practice from the very beginning of nuclear energetics. It also included a concise description of related fields of chemical cleaning and pickling of equipment systems, corrosion and protection against the corrosive attack associated with decontamination procedures, and it evaluated the varying degree of success with any particular procedural scheme. The most important “classical” decontaminants and decontamination procedures listed in the monograph are now unequivocally classified as “hard” solutions and procedures for decontamination of metal surfaces. In the late 1970’s,new “soft” decontaminants and procedures began to replace the previously used ones in decontaminating nuclear power plant circuits. Prominent among the new procedures was initially the Can-Decon process applicable to decontamination of the circuits in heavy water as well as light water moderated reactors. The best procedure designed so far is the LOMI process (see Chapter 2.1.1.3.2). Whereas practically all the preceding decontamination procedures were based on knowledge gained empirically, the LOMI process is the first one derived essentially from scientific reasoning. An important source of invaluable experience with decontamination of reactor systems and other equipment nuclear accidents and the post-accident *) J. A. Ayres: Decontamination of Nuclear Reactors and Equipment. The Ronald Press Company, New York, 1970.

cleanup operations, as well as the dismemberment of energy reactors that for various reasons had been taken out of operation (i.e. the decommissioning). During the last decade, an entirely new system had to be designed for decontamination of nuclear power plants equipped with fast-breeding sodiumcooled reactors. The nature of contamination and the methods of dealing with it substantially differ from those encountered with reactors of any other type. The methods of decontamination of buildings, laboratories, environmental terrain and roads develop at a much slower pace relative to the other branches of decontamination. Except for laboratories, the chances to gather practical experience or to set up large-scale experiments are rather limited. Relatively simple and mostly mechanical procedures are usually adequate to deal with decontamination of buildings, surrounding grounds and roads. Laboratories are likely to require decontamination much more frequently, and also the demands concerning the efficiency of decontamination are more rigorous than are those deemed to be sufficient for environment and roads. In recent years, a problem of growing importance concerns contamination with tritium. Several technological systems have been designed which are capable of effectively reducing the concentration of airborne tritium in laboratories; on the other hand, no satisfactory procedure has hitherto been proposed for purifying tritium-contaminated water, especially a system that would be practicable in large-scale qperations. Thus, dilution with clean water down to the permissible tritium concentration still remains the only solution at present available. Protective clothing and footwear, as well as other personal protection gear often come into immediate contact with the human body; these items must therefore be decontaminated with a particularly high degree of efficiency before they may be reused. The decontamination of clothing, and textile products in general, originally drew amply upon the experience with technologies of laundering and chemical cleaning by means of organic solvents. However, the pioneering role of the cleaning technologies in providing empirical guidance to decontamination has virtually ended by now and - as in the case of metal surfaces - decontamination of clothing and means of personal protection will have to be put on its own footing, based increasingly on competent reasoning. The decontamination of the surface of human body, i.e. skin and the mucous membranes of the mouth cavity, nasopharynx and conjunctivae, is an important and exacting task, since a persisting contamination of the body surface constitutes a serious dual health risk: that due to a direct external irradiation and that due to an increased hazard of the intake of radioactive substances. Nevertheless, because of the intricacy of safety regulations and rigorous security requirements applied to any new health care procedure before it is considered acceptable, the decontamination of persons as a special branch xxii

of applied decontamination remains suprisingly conservative. Just slightly modified methods and means of the long-existing and well-tried techniques of personal hygiene continue to prevail here; non-traditional decontamination procedures have been developed only recently. The treatment of contaminated water must be regarded as one of the most important branches of decontamination, in view of both the extent of such operations and the overall significance for nuclear energetics as well as for healthy human environment. Water decontamination is a task of daily routine in a nuclear power plant. Reactor operation produces large volumes of radioactively contaminated water from literally dozens of different sources, requiring thus a continuous surveillance, monitoring and decontamination. Apart from the power plants, each working place using unsealed radiation sources - be it in science, research, health care, industry, agriculture or other - must be equipped with a relatively complicated and costly technological system enabling the control of radioactive waste water. The technology of water decontamination as a separate branch evolved from the methodology of purifying and processing energetic water in conventional power plants, which in turn had been based on public water supply technologies developed many years ago. The core of the modern water decontamination system is usually an ion exchange process, even though use is also made of other methods, such as coagulation, sedimentation, distillation, degassing, and recently also dialysis. All these methods are usually closely interrelated and integrated into a sophisticated system of water management at nuclear installations. The objective of this book is to present a comprehensive picture first of the fundamentals of general decontamination of solid surfaces and water and, in its second part, to review the main practical procedures and means of applied decontamination used in those fields of activity that appear at present to be of key significance. This concerns primarily decontamination of the operational facilities in nuclear power plants equipped with pressure water reactors, and decontamination of the equipment systems in radiochemical laboratories and nuclear medicine departments. Other special decontamination branches of current interest are also dealt with briefly. In common with other branches of applied science and technology, the art of decontamination is being continuously enriched by the progress achieved in a variety of relevant scientific disciplines, and it employs the advances in the practice. To keep pace with the rapidly developing technology, and to prevent a lagging art of decontamination from becoming a limiting factor in the further development of nuclear energetics and other utilization of radionuclides, it seems’that for some time to come the main trends in the development of decontamination will include : xxiii

- The development and use of new decontamination methods that would be highly efficient and, at the same time, nonaggressive to the decontaminated materials and economically attractive; - The utilization -of progressive elements of automation and robotics, including remote control systems, for decontamination of highly contaminated objects, inaccessible locations or any space where additional risk factors other than radiation can jeopardize human health and safety; - The development and use of such decontamination formulations as would minimize the volume of radioactive wastes and that would produce wastes in a form in which they could be either easily further treated or safely disposed of without any undue risk of endangering human health or polluting human environment; - The choice of suitable materials used both for the structural and the technological parts of nuclear installations with regard to their minimal contaminability and ease of decontamination.

XXiV

List of abbreviations, symbols and quantities

Abbreviations - nucleonic number of an atom, nuclide A - decontaminating solution containing ammonium citrate AC - solution containing AC plus EDTA ACE AISI-304 - U.S. Steel standard - “as low as resonably achievable” ALARA - decontaminating solution containing alkaline permanganate AP APAC, AP-AC - two-step decontamination process using AP and AC solutions APACE, AP-ACE - two-step decontamination process using AP and ACE solutions APOX, AP-OX - two-step decontamination process using AP and OX solutions - experimental fast reactor with 60 MWe power output (USSR) BOR-60 - boiling water reactor BWR - soft decontamination process developed for the CANDU reactor, using Can-Decon H202and low concentrations of decontamination reagents - heavy water moderated reactor (Canada) CANDU - decontamination process using citric and formic acids, Kraftwerksunion Co CAPA KWU - complete decontamination of persons CDP - decontaminating solution containing citric acid and EDTA CE - decontaminating solution containing citric and oxalic acids, and other comCITROX ponents - CITROX solution supplemented with EDTA CITROX E - carboxymethylcellulose CMC - decontamination centre DC - decontamination efficiency DE - Dounreay fast reactor (GB) DFR - diethylenetriaminepentaacetic acid DTPA - ethylenediaminetetraacetic acid EDTA - PWR primary circuit decontamination process ELPO - experimental reactor U.S.A. EPRI - facilities for CDP FCDP - decontaminating solution containing gluconic and citric acids GCA - International Organization for Standardization IS0 - ISO/Draft International Standard ISO/DI . - high activity wastes HAW - high density PE HDPE - heat exchanger HE - hydroxy-ethylene-diphosphoricacid HEDA

xv

H EPA HMP, HMPNa IAW LAW LDPE LET LOMI

M MOPAC NEP Nf NP NPP NW OPC OPF OPG OPP

ox PH PDP PE PEC PVA PVC PWR RAW RBMK RMU SFBR SG SI Sul Sulfox TMI TNT WWER

- high efficiency particle air filter - “hexametaphosphate”, i.e. polyphosphate; sodium HMP - intermediate activity wastes - low activity wastes - low density PE - linear energy transfer - “low oxidation-state metal ions” - decontamination method for metal

surfaces, particularly BWR primary circuit inner surfaces (applicable also to PWR) - general designation of a metallic chemical element - i. e. APAC process, Kraftwerksunion Co - nuclear explosion products - other modifying factors - decontaminating solution containing nitric acid and permanganate - nuclear power plant - nuclear weapon - decontaminating solution containing oxalate, peroxide and citrate a. o., see Table 2.2 - decontaminating solution containing oxalic and hydrofluoric acids and hydrogen peroxide - decontaminating solution containing oxalate, hydrogen peroxide and gluconic acid - decontaminating solution containing oxalate, hydrogen peroxide and peracetic acid - decontaminating solution containing oxalic acid - logarithm of the reciprocal of the hydrogen ion molar concentration (“hydrogen exponent”) - partial decontamination of persons - polyethylene - fast sodium reactor (Italy) - polyvinylalcohol - polyvinylchloride - pressure water reactor - radioactive wastes - i.e. BWR, Chernobyl, USSR - relative monetary unit - sodium fast breeder reactor - steam generator - International System of Units - decontaminating solution containing sulfaminic acid - decontaminating solution containing sulphuric and oxalic acids - Three Mile Island - trinitrotoluene (trinitrotoluol) - water-water power reactor, i.e. PWR - USSR

Symbols and quantities A

B

xvi

- absolute activity - benefit

Bq RMU

- becquerel - activity concentration (volume activity) - curie - absorbed dose, dose equivalent - Tompkin’s decontamination index - dose rate - decontamination factor - dose factor of ingestion - adnexa diffusion coefficient

voltage - gray - effective dose equivalent - particle flux (“relative activity”) - dose rate - adsorption coefficient - coefficient of contamination - desorption coefficient - dose constant - adhesion coefficient - coefficient of decontamination - radiactivity dose - lethal dose - solubility product of Me(OH), - number of atoms Np- costs - basic costs of decontamination - pascal - exposure rate - metal loss - environmental contamination - quantity of consumed reagents - radiation dose - surface - collective dose equivalent - specific surface - half-life of a radionuclide - biological half-life - effective half-life - degree of removal - gross value of decontamination - production of solid RAW - watt - work of adhesion - production of RAW disposed of - costs of radiation protection - total detriment from radiation exposure - residual contamination - relative adsorption - equilibrium adsorption -

Sln

T

U’ V

vp

W

’

S-1

Bq .m-’ 3.7.10’0 Bq

sv

I Bq. s - I I PGY. Bq-‘

m2.s-’ V

J .kg-I

sv

S-’

Gy .s - l

1 1

I

m‘. s-‘ 1

I Bq. a-I GY mol .dm-’ I RMU RMU N .m-’ cGy .h-‘ kg .m-’. s-’ dpm .cm-’ t, m’, a.0.

sv

m2

sv

m2. kg-I

S S S

1

RMU m3.a-I J .s-’ J .m-‘ m’. a-‘ RMU RMU I 1 1 XVii

adsorption kg . m-' area activity Bq .m-' - mass concentration 1 - count per minute min-l - equilibrium concentration 1 coefficient of removal 1 desintegrations per minute min-l - pitting factor 1 - factor of volume reduction for active liquid media YO - mean volume reduction factor pertaining to solid RAW treatment % - distance m - corrosion current A current density A . m-' - adsorption constant I - mass kg - number of positive electric elementary charges 1 - specific costs for the recovery of reagents and substances RMU .t-3 ; RMU . m-3 - specific costs of RAW disposal RMU . m-3 - specific costs of liquid RAW treatment media R M U . m-3 - specific costs of solid RAW treatment media RMU . m-3 - surface contamination Bq .m-2 - radius m - time S - velocity m . s-' - rate of investment % - distance m - permissivity A . m-I. s-' - corrosion inhibitor efficiency 1 - decay constant S-' - specific mass kg .m-' - surface (interface) tension between phases A and V J . m-2 - area density of electric charges c .m-' -

-

~

~

h i

i

k rn n 4,

xviii

~

1 Fundamentals of radioactive contamination and general principles of decontamination

1.1 Basic terms and concepts A set of atoms having the same proton number Z form a chemical element, ,X. A set of atoms having the same proton number Z and the same nucleonic number A form a nuclide, $X. Different nuclides of the same element are called isotopes: “$X, ‘$X etc. One can thus recognize for instance three isotopes of hydrogen: light iH,heavy :H,and superheavy :H. It is thus incorrect to speak about “isotopes” of a mixture of uranium fission products: ‘::Cs, !;Sr, ZSr, ’::Ce, ‘ZCe etc., but solely about nuclides of a mixture of uranium fission products. A set of atoms having the same proton number Z, the same nucleonic number A and the same energy content expressed for example as the same constant A of nuclear transformation (and consequently the same mean life span 5 and the same half-life T), form a nuclear individual $X(T). Two or more nuclear individuals of the same nuclide are isomeric with each other: $X(T), $X(T,). The nuclear isomer of a nuclide having the lowest energy content is taken for the basic isomer of the given nuclide $X(T,). All isomers of a given nuclide that have a higher energy content are metastable and are designated as Am ,X, or alternatively “iX. If there are more than one metastable isomers of a nuclide, they are denoted as AmlX, Ami X and so on. Each metastable isomer is characterized by its half-life. In common practice, the term nuclear individual is only very rarely used, since the terms nuclide $X and basic nuclear isomer of the given nuclide $X(T,) mostly have practically the same meaning. This slight conventional inconsistency is also sanctioned in this book. In the context of the perspectives of decontamination as a separate discipline dealing primarily with the development of technologies based now much more than previously on a rationale rather than on empirical experiences, it seems appropriate to outline shortly the underlying general notions. The radioactive substances may be accompanied by nonradioactive substances, the “ballast”, whose concentrations are higher often by several orders of magnitude. The ballast and the radioactive substances are mixed homogeneously. A carrier is a special kind of the ballast substance; the term denotes a nonradioactive admixture having a chemical form identical with or similar to 1

that of the accompanying radioactive substance and behaving in a similar way, at least in certain chemical and physicochemical processes. For example, when BaCl, and RaCl, are precipitated by means of H,SO,, a composite precipitate Ba(Ra)SO, is formed. The macro amount of the nonradioactive barium acts as a carrier for the micro amount of the radioactive radium. Micro amounts of uranium fission products '::Cs, GSr etc. in the primary circuit water of nuclear power plants (NPP) with pressure water reactors may be dispersed in a ballast macro amount of scales (Fe,O,) which form on the inside of the metal walls. In the majority of decontamination procedures, no attempt is deliberately made to separate the ballast from the radioactive substance; on the contrary, it is mostly preferable to remove the two components jointly. Similarly, a solid undissolved ballast makes decontamination of solid surfaces easier. A ballast dissolved in a contaminated liquid may sometimes facilitate its decontamination, such as when water is decontaminated by precipitation; on the other hand, other procedures, like water decontamination by means of ion exchangers, may be hindered by the presence of undissolved ballast substances. Contaminated solids are decontaminated by removing the contaminant from the surface, or alternatively by abrasion of a very thin surface layer of the material together with the contaminant. Fluids (i.e. liquids and gases) contaminated with coarse dispersions become decontaminated by spontaneous sedimentation, because of a marked effect of the gravitational force. Liquids contaminated with radioactive microdispersions can be decontaminated by precipitation, filtration through a layer of an ion exchanger or another sorbent, electroosmosis and other procedures. Gases contaminated with radioactive microdispersions (aerosols), as well as radioactive gases, may be decontaminated by adsorption and by methods utilizing the disparity in chemical reactions or differences in the solubility in liquids. The most common procedure is filtration through various aerosol and sorption filters. An important aspect is the question whether the contamination process took place in the presence or absence of liquids, chiefly water (moisture). Even very small amounts of liquids, such as the dew condensed on the surface of contaminated solids, are sufficient for the transfer of a fraction of the dry contaminant to the liquid phase. The ensuing secondary adsorption makes the subsequent decontamination more difficult. A similar situation occurs with dry contamination, if the first decontamination procedure involves the use of liquids, particularly water or water solutions not supplemented with suitable agents preventing the secondary adsoFption of the contaminant. For the same reason, when decontaminating liquids, the undissolved component of the contaminant is to be removed with a technique different from that used for removing the dissolved contaminants. In such a case, the undissolved component is left to

2

settle spontaneously or may be separated by mechanical filtration, whereas the dissolved component is precipitated, separated on ion exchangers etc. The dissolved contaminants become attached to solid surfaces mainly by adsorption (ionic, i.e. mediated by ion exchange, molecular, i.e. physical, and colloidal) and can be removed predominantly by desorption. The undissolved contaminants become attached to solid surfaces by the mechanism of adhesion, or simply by being entrapped on the surface microrelief, and are removed mainly by detergence in solutions of tensides, by precipitation with chemical precipitants in solutions, or electrostatically and mechanically in the dry state. Hence it is obvious that studies on contamination and decontamination of solid surfaces and liquids must distinguish the dissolved from the undissolved components of the radioactive contaminant and consider separately their respective behaviours. Some desorption agents are capable of removing from the contaminated surface primarily the originally dissolved component by desorption ; besides, they may also reduce adhesion and increase the detergence, improving thus the removal of the undissolved component as well. On the contrary, the surfactants which remove by their detergent action primarily the undissolved component, may at the same time increase desorption and reduce adsorption of the dissolved component. It follows from what has been said that questions involving the dissolved component of the contaminants belong principally to general (“classical”) radiochemistry in the sense conceived by I. E.Starik [ 11, who defined radiochemistry as a branch of chemistry dealing with chemical and physicochemical behaviour of radionuclides in trace amounts and trace concentrations. Questions involving undissolved components are pertinent to radiochemistry, chemistry of the interface, colloidal chemistry and to same extent physical chemistry in general. As for contamination and decontamination of gases, a certain analogy may perhaps be found in comparing the dissolved component to the contaminating radioactive gas and the undissolved component to the contaminating radioactive aerosols. Problems associated with decontamination of gases are the subject matter of physical chemistry and general chemistry. The general scope of problems connected with radioactive contamination of solid surfaces, liquids and gases, and their decontamination, are schematically outlined in Tables 2.2, 2.2 and 2.3. Decontamintion as a scientific and technical discipline has its own resources, operational elements and relations to other branches of science and technology (see Table 2.4). The possible main constituent elements of the underlying concepts of contamination and decontamination of solid phase surfaces are specified in Table 2.5.

3

TABLE 1.1 BASIC PATTERN OF THE GENERAL SCOPE OF PROBLEMS ASSOCIATED WITH RADIOACTIVE CONTAMINATION AND DECONTAMINATION OF SOLID SURFACES Radioactive contamination caused by radioactive solutions

radioactive solids I,

I

4

Conditions at contamination and decontamination

contact with liquids

1

Contaminant

dissolved

undissolved

1

Main processes causing contamination

1

adsorption

adhesion mechanical entrapping

4 _ _ _ -- -=----I-+ _ desorbents I surfactants 1

Main decontaminating agents Main processes for effecting decontamination

1

1

Jesorption, dissolution of surface films and surface layers

detergence, electrostatic and mechanical removal

TABLE 1.2 BASIC PATTERN OF THE GENERAL SCOPE OF PROBLEMS ASSOCIATED WITH RADIOACTIVE CONTAMINATION AND DECONTAMINATION OF LIQUIDS I

I

I

Radioactive contamination caused by radioactive liquids

I

radioactive solids

Conditions at contamination and decontamination I

Contaminant

I

4

dissolved

undissolved

Main processes causing contamination

diffusion

formation of suspension

Main decontaminating agents and facilities

adsorbents, chemical precipitants, mechanical filters

chemical precipitants mechanical filters

adsorption,

precipitation of coarse suspensions, mechanical filtration

Main processes for effecting decontamination

4

+c(

1

1

1.

co-precipitation

1

1 1

TABLE 1.3 THE BROAD OUTLINE OF PROBLEMS ASSOCIATED WITH RADIOACTIVE CONTAMINATION AND DECONTAMINATION OF GASES Radioactive contamination caused by radioactive gases Main processes causing contamination

radioactive aerosols

radioactive coarse dispersions mechanical motion, turbulence

diffusion liquid-containing absorbers,

Main decontaminating agents and facilities

Main processes for effecting decontamination

adsorbents

aerosol filters

chemical reaction and dissolution adsorption

mechanical separation

mechanical “sieve” filters

mehanical filtration, spontaneous gravitational sedimentation

TABLE 1.4 GENERAL SCOPE OF DECONTAMINATION AS A SEPARATE SCIENTIC AND TECHNOLOGICAL DISCIPLINE Resources

Operational elements

Theoretical basis of contamination and decontamination processes

planning, organization and management of decontaminating operations

Radiation safety analysis of the initial situation

agents, means, procedures and technical facilities of decontamination

Implementation of the principles of the ALARA and cost-benefit systems

economic aspects of decontamination

Related topic

waste management programme (storage, treatment and disposal of radioactive wastes arising from decontamination)

5

TABLE 1.5 THEORETICAL FOUNDATIONS OF CONTAMINATION AND DECONTAMINATION OF SOLIDS Studies on the underlying principles of processes resulting in contamination

Studies on the underlying principles of decontamination processes

Evaluation of the chemical forms of trace amounts of radionuclides in aqueos solutions

Processes resulting in weakening the bonds between the contaminant and the surface

Evaluation of the properties and behaviour of aerosols

Processes and methods for preventing the redeposition of the contaminant

Evaluation of the properties of solid surfaces

Evaluation of the corrosive attack caused by decontaminating agents

Interactions between the contaminant and the surface; particular types and their mechanisms

Standardization of experimental methods in the assessment of decontamination efficiency

Standardization of experimental methods in the assessment of contamination

Composition and properties of wastes arising from decontamination

1.2 Biological effects of io@zing radiation 1.2.1 Outline of radiation eflects The biological effects of ionizing radiation depend primarily upon the type of radiation, on the radiation dose and on how large a part of the body is exposed to radiation. Depending on the spatial relationship between the source of radiation and the irradiated subject, biological manifestations of the radiation damage may be either predominantly localized or predominantly general. The extent of the radiation damage resulting from a given radiation dose is affected by the distribution of the dose in time and by the repair capability of the exposed organism. The biological effects [2, 31 may be either somatic,if they become manifest in the irradiated subject himself, or genetic (hereditary), if they appear only in his descendants. For non-stochasdc effects, a causal dose-effect relationship is characteristic provided a certain radiation dose level is exceeded. These effects represent a well-reproducible type of radiation damage which from the medical viewpoint is considered as a pathogenetic entity, the “radiation syndrome”. 6

Stochastic effects, on the other hand, are those for which the probability of the occurrence, rather than their severity, is a function of the radiation dose and no threshold dose exists below which the effects are absent. Both somatic and genetic stochastic effects (leukaemia, malignant tumours and other conditions) appear also in unexposed populations and only their frequencies increase in the irradiated population; apart from radiation, age and sex also affect the degree of the risk. Riadiation effects are initiated by the absorption of the ionizing radiation energy in irradiated tissues; the ultimate manifestation of the damage is the result of a chain reaction in which three successive types of processes participate [41:

a) physical processes (dissipation of radiation energy, ionization and excitation of atoms and molecules in the exposed tissue); b) chemical processes (interaction of chemical radicals); c) functional and morphological changes (induced either by a direct or indirect action of radiation). Both direct and indirect effects of radiation on living matter may ultimately result in extensive biological alterations in the exposed organism. The types of radiation damage occurring at various levels of biological organization are summarized in Table 1.6 [5]. TABLE 1.6 TYPES OF RADIATION DAMAGE Level of biological organization

Radiation effect

Molecular

impairment of the integrity of macromolecules such as enzymes, ribonucleic and deoxyribonucleic acids; derangement of metabolic processes

Subcellular

damage to cellular membranes, cell nucleus, chromosomes, mitochondria and lysosomes

Cellular

blocking of cell division, cell death, malignant transformation of cells

Tissue, organ

systemic breakdown (central nervous system, blood, forming tissue, gastrointestinal tract) with pssible lethal outcome; induction of malignancy

Whole organism

death, shortening of life span

Population

alterations in genetic characteristics due to gene and chromosomal mutations in individuals

7

I .2.2 Dose-eflect relationship

The ultimate biological consequences of irradiation, irrespective of the level of biological organization, depend on a) the dose of ionizing radiation ( D ) , b) the dose rate, i.e. the time interval over which a given dose is delivered (JD),

c) the dose distribution in the exposed biological object. The quantities of the dose and dose rate are sufficiently dependable for basic considerations of the severity of the resulting detrimental effects of irradiation or the degree of risk. In the new system of units (SI), the unit of absorbed dose is the gray (Gy). It is defined as corresponding to the absorption of 1 J of radiation energy in 1 kg of matter, thus 1 Gy = 1 J .kg-I. A special temporary J. kg-', thus 1 Gy = 100 rad. The unit is the dosimetric unit D = 1 rad = basic unit of dose rate is 1 Gy .s-I, equal to 1 W . kg-I. The distribution of a given radiation dose in living matter can be considered from two aspects : i) Macrodistribution, i.e. the dose distribution in the entire exposed organism. Accordingly, one speaks of whole body homogeneous irradiation, whole body non-homogeneous irradiation and local (partial body) irradiation; ii) Microdistribution, i.e. the imparted energy distribution along the track of the ionizing particle, characterized by the linear energy transfer, LET. The magnitude of the LET is directly proportional to the charge and inversely related to the velocity of the ionizing particle. Relatively high LET radiations (alpha particles, protons, neutrons) are biologically more effective than p and y rays which are characterized by low LET. 1.2.2.1 Dose equivalent

The absorbed dose D is not in itself a sufficient parameter for determining either the severity or the probability of health detriment resulting from a radiation exposure under otherwise unspecified condition. A new value has therefore been introduced, the dose equivalent H. The dose equivalent at a given tissue site is defined by the relation

where D is the absorbed dose, Q - the quality factor and N,- the product of all other modifying factors (for most practical purposes a value of 1 is assigned to the factor N,). The unit of the dose equivalent is the sievert (Sv): 1 Sv = = 1 J . kg-', and is equal to 100 rem (if Q . N, = 1). 8

The quality factor Q depends upon the dose microdistribution. It is defined as a function of the collision stopping power, L , [keV . pm-’1, at a given point in water. If the dose distribution according to L , is unknown, the following approximate values for Q are at present applied: 1 - for y and X ray photons and for electrons - for neutrons and particles of a rest mass equal to, or greater than 10 1 atomic mass unit and particles of unknown energy - for a particles, multiply-charged particles (or particles of un20 known charge) of unknown energy - for thermal neutrons 2.5

1.2.3 Evaluation of risk and detriment after human exposure The risk is the probability that an exposed individual will suffer from a serious late stochastic deleterious effect as a result of a dose of ionizing radiation. The system of individual dose limitation assumes that the risk is proportional to the effective dose equivalent (HE) accumulated by the individual [2]. The evaluation of social acceptability is concerned with detriment, which is defined as the mathematical concept, “expectation”, of the harm incurred from a radiation dose, taking into account both the probability and the severity of each deleterious effect occurring in a group of exposed subjects. An objective health detriment, disregarding any adverse mental reactions, is proportional to the collective effective dose equivalent (SE) [2]. For the purpose of radiation protection, two types of irradiation are distinguished : a) human exposure is anticipated in the course of a particular operation or practice, and is controlled by protective countermeasures at the radiation source by a system of dose limitation and by a choice of appropriate operational procedures ; b) human exposure results from an uncontrollable source of a radiation field and requires the adoption of a system of remedial countermeasures. Most of the decontamination operations will fall under the first heading, i.e. human exposure is anticipated and is restricted by a system of pre-planned countermeasures which will be outlined in more detail below. As concerns external irradiation, human exposure can be either whole-body or partial (homogeneous or inhomogeneous), and a dose may be delivered either in a single short-time exposure or in a protracted exposure. The radiation damage is clearly related quantitatively to the radiation dose and the actual manifestation of the symptoms is preceded by a latent period: the higher the radiation dose, the shorter the latent period. In most cases, deleterious effects of radiation develop with a few weeks immediately following the

9

exposure. The damage resulting from a partial exposure is less severe than that occurring after an equivalent whole-body exposure. As to the consequences of a radiation exposure, several critical levels can be recognized. For example, acute whole-body irradiation with a dose of about 1 Gy gives rise to a mild acute radiation sickness. Doses in the range of 2 4 Gy cause a clear manifestation of acute radiation sickness, and the medium lethal dose (LD,,) for a single short-time exposure (causing death of 50 % of exposed subjects) lies in the region of 3.5 Gy, provided the absorbed dose is homogeneously (uniformly) distributed throughout the whole body. It is well known that an organism is capable of recovering from a sub-lethal radiation damage. The repair capability and the speed of recovery depend on a number of factors, primarily on the absorbed dose. If the dose is less than 1 Gy, non-stochastic effects are almost completely reversible. If the dose range is between 2.5-5 Gy, recovery is still possible. Doses exceeding 6 Gy cause a serious acute radiation sickness, and long-term survival is very unlikely. 1.2.4 Internal contamination

Radioactive compounds can enter the organism essentially in three ways [4]: - by inhalation (via the respiratory tract), - by ingestion (via the gastrointestinal tract), - percutaneously (through both damaged and intact skin). The significance of internal contamination is determined by the dose equivalent commitment which is realized in an organ or a tissue. The dose equivalent commitment is equal to the cumulative dose incurred until a total disappearance of the incorporated radionuclide; for practical purposes this means 50 years after the deposition. The dose commitment depends on: - the amount (radioactivity) of the nuclide deposited in the organ or tissue, - the type and energy of radiation emitted by the radionuclide, - the effective half-life T,, (i.e. the time necessary to reduce the amount of the deposited radionuclide to one half). T,, depends on the half-life of the radionuclide T and the biological half-life G ;since always &/ T > 0, K , is defined by the following relation: consequently, the effective half-life is always shorter than G ; the radiosensitivity of the organ or tissue in which the radionuclide is deposited. Radionuclides of biological importance can be classified by their behaviour in biological material either as transportable or nontransportable. Transport-

10

able nuclides are soluble in biological fluids and are able to penetrate through tissues. They usually appear in the organism in a physiological form and accompany either their stable isotopes (e.g. iodine) or their chemical analogues (e.g. caesium-potassium or strontium-calcium complexes). Examples of transportable isotopes are radionuclides of elements belonging to the 1 st and the 2nd group in the Periodic Table. Nontransportable radionuclides are those of the rare earth metals and transuranic elements. O n the basis of their distribution in the organism once they penetrate into the bloodstream, four groups of radionuclides are distinguished : a) radionuclides with homogeneous distribution: Na, K, Rb, Cs, H, C; b) osteotropic (bone-seeking) radionuclides: Ca, Sr, Ra, Pu, Np, heavy lanthanoides ; c) radionuclides retained in the reticulo-endothelial system : light lanthanoides; d) radionuclides with selective accumulation in particular organs : iodine in the thyroid, iron in the erythropoetic tissue. Internal contamination results in an exposure of the contaminated organism to radiation emitted by the incorporated internal source. As with external irradiation, the effects of internal irradiation can be classified as asymptomatic damage, acute radiation damage, chronic radiation damage and late effects. The present state of knowledge justifies the assumption that internal radiation exposure, particularly in connection with decontamination activities can only exceptionally lead to the development of an acute radiation syndrome. 1.2.5 Surface contamination of persons The handling of dissolved or powdered radioactive material is associated with a risk of contamination of garments and protective clothing, implements, tools, as well as any uncovered body parts (face, neck, wrists etc.). If the region around the nose and mouth is found to be contaminated, it must be taken as a serious warning of possible internal contamination. Different transfer mechanisms may contribute to the contamination of surfaces. For externally contaminated persons, the tissue most at risk is the basal layer of the epidermis lying underneath the outer covering of mostly keratinized cell layer about 0.5 - 1 mm thick. The radiation dose imparted to the basal layer is primarily due to /3 rays; variations in the thickness of the shielding layer greatly modify the ratio between the measured surface dose and the actual absorbed dose in the basal cell layer. Not only this ratio, but also the surface dose rate per unit of area radioactivity depend on the energy spectrum of beta radiation. Moreover, this spectrum may differ with time, since it can vary with changes in the radionuclide composition of the contaminant (eg. when the skin 11

is contaminated with a mixture of fission products or with corrosion products). The skin is believed to be much less susceptible (in comparison with other organs such as gonads, active bone marrow, thyroid gland and others) to the development of radiation-induced malignancy. In order to reduce the incidence of stochastic effects to an acceptable level and to prevent the appearance of non-stochastic damage, a limit for irradiation of restricted parts of the skin (hands, forearms, soles, ankles) has been set to 20 Gy over the whole occupational lifetime [2]. Surface contamination results in a certain - mostly localized - harm to skin and easily accessible mucous membranes. The post-irradiation skin damage usually manifests itself as radiation dermatitis. The symptomatology and the consequences of acute skin lesions depend upon the extent of surface contamination, its localization, the radiation dose absorbed in the skin, and the dose distribution in particular skin layers and in the subcutaneous tissue, as well as upon the quality of radiation (type of radiation and its energy). These factors affect the latency period and the kinetics of skin lesion development. The following stages can be distinguished : I. reddening (erythema), 11. blisters (oedema), 111. necrosis. Details concerning radioactive contamination of skin can be found in Section 1.6.2 and 2.9.1. 1.2.6 Gui& values of area radioactivity

A regular monitoring system for controlling surface contamination must be set up for all working conditions involving operations with unsealed radioactive substances; the level of surveillance must be appropriate to the category of the establishment (or individual work place) and the working methods. Measurements of surface radioactive Contamination of the work place and of all persons involved must also be carried out upon the discontinuation of each working operation, and all objects and persons must be monitored when leaving the controlled area (upon termination of the decontamination procedure). The monitoring of surface radioactive contamination is carried out either by direct measurements of surface radioactivity or indirectly by smear tests. The derived limits of area contamination are set for each working place in relation to the actual working conditions, and represent the upper limits of surface contamination values which, in compliance with all principles of safety, may accompany normal operations at a given working place and are at the same time reasonably measurable. Unless the operational conditions necessarily require otherwise, the derived limits shall not be higher than the guide values of area activity a, (kBq.m-*) listed in Table 2.7. These guide values have been designed in order to ensure that the basic limits of absorbed dose of external or

12

TABLE 1.7 GUIDE VALUES O F AREA RADIOACTIVITY FOR DETERMINATION OF DERIVED LIMITS AND EXAMINATION LEVELS O F SURFACE CONTAMINATION - a, (kBq .m-') 7 A ~Z28Th, , 'qh 1 2 n 231pa,2 3 2 u , 233u Z M u 2 3 6 ~ 7

9

,

natural Th, alpha emitters with Z > 92

I7Sm, '"Pb, 227Th 2 3 5 ~ .2 3 8 u , 241pu

natural, impoverished and enriched U

Other iuclides - alpha :mitters

14C, "S, "Mn Dther '7C0, "Zn, 67Ga 'H, "Cr, 'Fe, 63Ni, *adio"Se, 77Br,"Sr %Tc, IwCd, Iz3I 13nqn, 1311 luclide I2'I, '%Cs, 197Hg

-

~~

Inner surfaces and contents of air-tight containment hoods, laminar flow hoods, fume hoods and other types of enclosures*' Surfaces (of floors, walls, ceilings, benches etc.) in the controlled zone of a work place designed for unsealed sources and in nuclear establishments; outer surfaces of protective and operational devices, transport containers, shieldings, materials etc., as well as outer surfaces of personal protective clothing and gear Body surface and inner surfaces of protective personal clothing and gear Surfaces in work places outside the controlled area except for places handling radiation sources; surfaces of transport vehicles, transport casings and objects leaving the controlled zone (including personal effects) c

-

-

-

-

-

-

for areas greater than ImZ

30

300

300

3000

3000

300

for area: smaller than 1 mz

3

30

300

3000

3000

300

3

3

3

300

3000

30

3

3

30

300

300

30

*)Guide value not fixed; homogeneously sttainable minimum is required

internal irradiation are not exceeded as a result of even frequent or prolonged radioactive contamination of large areas.

1.3 Basic safety standards for radiation protection in the course of decontamination The concept of radiation protection involves the following principles which must be observed if the radiation dose is to be kept within safe limits [2]: - Any activity resulting in human radiation exposure is justified by the benefit accrued; - Any radiation dose resulting from such justifiable activities are kept As Low As is Readily Achievable (the principle’s acronym is ALARA), economic and social factors being taken into account; - The limits of the dose equivalent to individuals set for particular types of radiation are not exceeded; - No inadmissible human exposure to radiation is likely to be incurred any time in the future. Before any major decontamination operation is carried out, it is necessary to weigh all aspects of the situation with the aim at achieving the maximum decontamination effect not only with minimum costs but also at the highest level of radiation protection of the personnel involved. The individual dose equivalents for any given type of radiation must not exceed the relevant dose limits established by a competent authority. Any decontamination activity entails a risk of both external and internal human radiation exposure from, respectively, the external radiation sources and the ingested or inhaled radionuclides. The risk of internal contamination is higher when handling unsealed radiation sources, in particular solutions, powders and aerosols. Radiation exposure may also be due to radioactive substances contaminating the human body surface. Prior to starting a decontamination operation, it is necessary to work out a monitoring plan. The plan includes: - The programme of measurements (type, method and frequency of measurements, places to be monitored; - Instructions as to how to evaluate (interpret) the obtained data; - The values of derived limits and reference levels including the way they were derived and suggestions for the remedial measures to be taken if the limits are exceeded ; - Specifications concerning the measuring equipment and methods. The monitoring plan makes it possible to verify that the requirements of the dose limitation system have been met. It includes regular (routine), occasional and operational monitoring. 14

A model of a defined situation relates the derived limits to the basic limits. The reality of the model used for deriving the limits relies on how accurately and dependably the derived limits will reflect the basic standards. The reference level expressed by the value of the limited or directly measured magnitude indicates what step of the pre-planned programme of actions has to be implemented. The following reference levels are distinguished : - The registration (recording) level, above which the dose is sufficiently noteworthy to be recorded; - The examination level, above which the dose is sufficiently significant to warrant further examination (it is derived from three tenths of the annual permissible occupational dose); - The action (intervention) level, signifying an early warning and indicating the need for implementing the pre-planned protective measures, particularly in the case of an emergency or accident, or an inadvertant escape of radioactive material into the environment. These measures are usually of two types: precautionary and consequential. 1.3.1 Radiation safety analysis and monitoring

The actual decontamination operation of any major extent ought to be preceded, among other preparative measures, by an analysis of the situation from all main aspects of radiation safety. As a rule, the following are the essential points of the analysis: - An overall assessment of the nature of the work site; - An evaluation of the contaminant including its radiobiological and radiochemical properties and chemical toxicity; - Fundamental facts needed for a qualified assessment of the risk and the benefit of decontamination; - Specifications regarding the requirements for human monitoring and health protection. The monitoring of an area (work place) subjected to decontamination is intended to provide the following data: The dose rate determined on the work place (or the ground) to be decontaminated; Results of the dosimetric control of the personnel obtained by means of personal and operational (group-monitoring) dosimeters including some with dose-alarm devices (indicating any transgression of the derived operational limits); The level of surface contamination of persons; The level of surface contamination of equipment, implements and tools used for decontamination and protection of the involved personnel; 15

-

The radioactivity of wastes and effluents to be released into the atmosphere, public sewage or water streams.

1.3.2 Basic rules of radation hygiene and principles of radiation protection The following are the basic rules of radiation hygiene and the principles of health protection applicable to decontamination operations: - Separation of the clean from the potentially contaminated zones of the work site; - Demarcation of the work area to be decontaminated, and prevention of access of unauthorized persons; - Containment of liquid effluents and their treatment in special water processing units, disposal of solid wastes according to their characteristics and level of activity in segregated containers (or disposal sites); - Prohibition of food and drink intake, smoking and resting inside the zone contaminated with radioactive substances; - Obligatory use of available means of personal protection to prevent the contamination of skin and the intake of radionuclides via the respiratory tract; - A complete, or at least a partial, decontamination of personnel leaving the contaminated area for a shift change, rest, eating, drinking; this procedure is obligatory at the end of the duty period; - Careful planning of the action (organization, choice of methods, supply of materials) aiming at minimizing the duration of the operation; - Use of physical protection (shielding) and mechanical remote-controlled manipulators wherever the conditions so allow; - Prohibition to transfer items employed for decontamination and to use them to other purposes; - Availability in sufficient quantities of operational and protective equipment, detergent solutions and solutions of acids and alkalies needed for neutralization; - Clear instructions, provided to each member of the staff engaged in the action, as to the working methods and required safety precautions pertaining to the planned decontamination action and to the expected working conditions and operational problems, as well as instructions for potential unforeseen emergency situations; - Adequate supply of the equipment necessary for implementing the radiation protection programme, in particular the personal monitoring dose-alarm dosimeters; - Availability of an easily accessible first aid medical kit suplemented with 16

means for partial decontamination of uncovered body parts and for rinsing the mouth cavity, the nasopharynx and the conjunctival pouch; - Regular surveillance of the health fitness of the personnel by means of the prescribed system of medical examinations, i.e. prior to the employment, at regular time intervals during the occupational activity and following any suspected or actual over-exposure.

I .3.3 Hygienic (medical) surveillance The decontamination actions of major extent or those conducted under especially unfavourable or dangerous conditions (high temperature, risk of fire, possibility of toxic compound escape, lack of oxygen, high relative humidity in the environment etc.) require a permanent hygienic or medical surveillance in order to provide an immediate and competent emergency aid if needed. Members of the surveillance team are best qualified to accompany an accidentally affected or injured worker to a medical institution for professional or specialized examination or treatment. They are also qualified to cope with an actual or suspected case of internal contamination, in that they can administer adequate remedial compounds that are likely to prevent absorption of the ingested or inhaled radionuclides, or to accelerate the elimination of the contaminant from the organism. Among such remedial compounds are for instance potassium iodide (KI) administered 2 - 3 times daily in a dose of 0.1 g in case of an intake of radioactive iodine or a fresh mixture of fission products; activated barium sulphate administered in the form of a suspension in a dose of 40 g in 100 ml water repeatedly 1-3 times a day for the duration of one to five weeks to prevent deposition of bone-seeking radionuclides (Sr, Ba, Ra) ;ferric cyanoferrate (11) (Prussian blue) used to reduce the rate of caesium absorption from the gastrointestinal tract, as well as some other compounds.

1.4 Contamination with radioactive substances Apart from a negligible contamination caused by natural sources of radiation, environmental contamination mostly originates from sources generated by human activities, known as technogenous sources. Technogenous radiation sources may be of two kinds, controllable and uncontrollable.

I .4.1 Controllable technogenous sources This category includes primarily the nuclear fuel cycle, radioactive effluents and wastes arising in nuclear plants, and applied sources of radiation. 17

1.4.1.1 Nuclear fuel cycle

The nuclear fuel cycle-encompassesall sectors of nuclear energetics, such as uranium ore mining and milling, uranium hexafluoride production, uranium enrichment, production of fuel rods and assemblies, operation of nuclear reactors, storage and reprocessing of burn-out fuel assemblies, decommissioning of nuclear power plants and reclassification, and the treatment and storage of radioactive wastes. Possible alternative pathways in the nuclear fuel cycle of a light water reactor are schematically shown in Fig. 1.1.

/

UO,/PuO,

FUEL PINS

........................, L.. /‘

//’

.......

//

.....&O,

u

FUEL PRODUCTION

ii!I

,’ ,,’/ /’

il I

BURNOUT FUEL

LONG-TERM STORAGE

COOLING OFF

Fig. 1.I . Nuclear’fuelcycle for light-water reactors [6]. Without burnout fuel reprocessing (-),

with U recycling (- - -), with U and Pu recycling (. .... .)

I .4.1.I .I Sources of radioactive contamination arising from uranium mining and milling, and nuclear fuel manufacturing

The mining of uranium ore and its processing are activities which give rise to significant quantities of contaminants. The main representatives of naturally occurring radioactive elements are uranium, radium, polonium, radon and their daughter decay products. Their relative proportions in rocks, mine waters and air vary greatly. As an example, the average content of Ra and U in underground waters is several hundred times less than that found in rocks. The highest Rn concentration is detected in air, much lower in water. The radon release from a mine amounts to approximately 190 TBq per 1 GWe per year, 18

whereas the release from a quarry is only about 3.7 TBq per 1 GWe in a year 161* Assuming that the ore contains 1 YOuranium in equilibrium with all its daughter products, then 60 alpha disintegrations and 30 beta disintegrations occur in one minute in each milligram of the ore. The uranium ores exploited commercially usually contain about 0.3 YOU30, (approximately 2.5 kg uranium in lo3kg of ore). For a better understanding of the effect of uranium ores on environmental contamination, it is useful to know that the radioactivity of the uranium together with its daughter products contained in one ton of the ore amounts to approximately 122 GBq (3.3 Ci). The extent of contamination and the physicochemical forms of the contaminant depend on the way the ores are mined and further processed (whether mechanically or chemically). Mechanical processing is carried out as either a wet or a dry process. Chemical processing consists in leaching the ore in acid or carbonate solutions followed by concentration on ion exchangers or extraction with organic reagents. A rising content of 235Uin enriched natural uranium increases the alpha activity: at a 20 % enrichment with 235Uby a factor of 12, at 90 % enrichment by a factor of 80 relative to the activity pertaining to the naturally occurring mixture containing only about 0.7% 235U.With an increasing degree of the enrichment of natural uranium with the isotope 234Uthe proportion of the total alpha activity attributable to 234Urises sharply, attaining as much as 91.5% when the 234U content reaches 20 % ; this apparent disproportion is due to the substantially shorter half-life of 234U. Apart from uranium and the products of its decay chain, attention must also be given to another alpha emitter, 210Po,because of its high specific Q activity (about 155 TBq .g-I). Detectable amounts of polonium occur in uranium ores, concentrates of the rare earth elements and wastes arising from ore processing. The uranium ore processing plants produce in the first step the nonrefined concentrate of uranium oxides, the so-called “yellow cake”, which may contain 60-90 % of U30s.A qualified estimate of the activity, related to 1 GWe per year, remaining in gaseous wastes discharged from model processing plants specifies 0.85GBq 238U,234Uand 234Th,0.36 GBq 226Ra,230Th,210Pb,2’oBiand *‘OPo,and about 55 TBq 222Rn:radon is the most important constituent from the radiation protection aspect. Another source of potential human exposure are the sites selected for disposal of wastes arising in ore processing plants. An average uncovered dump site has an area of about 20 hectares and contains approximately 20 kBq 23”T’h and 20 kBq 226Raper 1 kg of mass. The mean yearly dose equivalent to lung tissue incurred at a distance of 50 m from such a dump has been estimated to 19

be 80 mSv; the same calculation arrives at 3 mSv and 1 mSv at the distances of 1 km and 2.2 km respectively. The U30s concentrate as it leaves the processing plant is subjected to reduction, hydrofluorination and fluorination to give volatile uranium hexafluoride UF,. The concentrate is first purified by chemical solvents, or alternatively the crude uranium hexafluoride is refined by fractional distillation. The amounts of radionuclides discharged as wastes arising in plants producing the hexafluoride are shown in Table 1.8. TABLE 1.8

RADIONUCLIDES RELEASED IN THE COURSE OF UF, PRODUCTION (RADIOACTIVITY PER 1 GWe. year) [7] Amount of released wastes Radionuclide

gaseous (MBq)

liquid (GBq)

23?h 23"Th 226Ra 222Ra

28.5 28.5 1.2 17.8 0.08 0.08 0.3

1.07 1.07 0.05 1.11 0.33 0.01 -

Total

77.7

3.66

238"

2 3 4 ~

2 3 5 u

The gaseous UF, is used to enrich the light-water reactor fuel; the initial proportion of 0.7 % 235Ufound.in natural uranium is incresed to 2 4 %. The effluents released from the enrichment plants contain 0.74MBq of 2MU, 0.74 MBq of 238Uand 0.03 MBq of 235U;the gaseous discharges contain 25 MBq of 234U,0.74 MBq of 23sUand 3.33 MBq of 238U,all related to 1 GWe per year [71. The enriched UF, is used to produce powdered uranium oxide, the actual constituent of fuel pellets, manufactured by compression and sintering of the oxide. The pellets are inserted into sheathed fuel pins arranged subsequently in clusters making up usually rod-like fuel assemblies. Plants engaged in nuclear fuel production release appreciable amounts of radioactive wastes, both gaseous and liquid. The following are the estimates, in MBq related to 1 GWe per year, of gaseous and liquid releases, respectively: 234Th - 1.1 1 and 260; 234U- 7.4 and 1850; 235U- 0.22 and 37; 238U- 1.1 1 and 260 [7].

20

1.4.1.2 Contaminants arising during operation of nuclear power plants

The operation of nuclear reactors generates radionuclides by several processes : fission of nuclear fuel; neutron capture in uranium and transuranic elements nuclei ; and neutron activation of construction materials, corrosion products and impurities in the coolant (heat-carrying medium). The type and amount of the contaminant arising, including its physical and chemical properties, depend not only on the reactor type, but also on the course and duration of the reactor operation. With regard to the off-site radiation risk, the following radionuclides deserve special attention: "Kr, a nuclide which cannot be easily controlled: it is nonreactive, relatively long-lived, highly mobile in the biosphere; 'H and 14C, biogenous long-lived elements which take part in the global turnover of water and organic matter; iodine radionuclides (in particular 13'1 and 1291); aerosol particles containing long-lived radionuclides of Sr, Cs, Ba, La; and corrosion products, particularly Co, Fe, Mn and Cr. On-site surface contamination with radioactive substances can occur both under normal operational conditions and under special circumstances of an accident. Even normal operation of a nuclear reactor leads to contamination of technological elements such as the inner surfaces of the core or the primary circuit. The replacement of fuel assemblies, which is to be considered a normal step of the operational technology in a nuclear facility, results in a transfer of the contaminant from the primary circuit to some other compartments of the main production block and to other parts of the plant. The main transfer medium of the contaminant is the coolant. The processes which contribute to contamination of the cooling medium are: a) Neutron activation in the reactor core of the coolant itself or the impurities therein; b) Surface contamination of fuel assemblies with nuclear fuel components; c) Escape from defective fuel assemblies of a mixture of fission products; d) In-core activation of corrosion products formed on the inside of the construction materials. The experience gained with the operation of NPP with pressure water reactors shows that a certain category of radionuclides in the mixture of uranium fission products are particularly important from the viewpoint of radiation protection. They are:I4'Ce, l4Ce, I3'Cs, Io3Ru,IMRu,"Ba, "La, I3'I, 95Zr,"Nb and, among the radionuclides appearing in corrosion products, 58C0,6oCo, "Mn, "Fe, "Zn, 95Zr, """'Ag, "Cr, '24Sb,9sNb. The total activity of fission products contained in 1 kg of natural uranium irradiated with neutrons for a duration of 300 days at a neutron density flux of lOI3cm-*.s-' has been estimated to be 74 TBq (2 kCi); at the same time about 1 g of 239Puis generated. 21

Radioactive corrosion products are represented by a mixture of radionuclides which are induced by nuclear particles in the products of corrosion that attacks the construction -materials of the reactor and the adjacent primary heat-absorbing circuits. The induction process takes place in the reactor core, but the target material need not have its origin in the core itself, but may be carried in by the circulating coolant from quite distant sectors of the entire primary circuit. The neutrons emitted when the fuel atoms nuclei undergo fission are first slowed down by a moderator and then captured by atomic nuclei of substances present in the core. In this way, unstable atoms are generated with a concurrent emission of y rays. Neutrons thus induce radioactivity in originally non-radioactive substances, and they do so with an incomparably higher efficiency than any other type of nuclear particles. For example, the radioactivity induced by p or a particles can be completely disregarded as far as the requirements for decontamination are concerned. The highest efficiency in inducing radioactivity pertains to thermal neutrons, i.e. those with a kinetic energy TABLE 1.9

IMPORTANT RADIOACTIVE CORROSION PRODUCTS OF CONSTRUCTION AND TECHNOLOGICAL MATERIALS IN NUCLEAR REACTORS AND COOLING CIRCUITS Half-life T Nuclide Z

X

A

h d a

hours - days - years -

Type of transformation

Energy of particulate radiation

fJ

MeV

-

Energy of y radiation

--

n

-

MeV

-

15 24 25

P Cr Mn

32 51 54

14.3 d 27.8 d 291 d

beta beta K

274 121 134

1.71 0.756 0.838

-

25 26 27

Mn Fe Co

56 59 58

2.58 h 45.1 d 71 h

beta beta K

458 73.7 77.7

2.86 0.46 0.485

134.5 176 130

0.84 1.10 0.81

27 28 29

Co Ni Cu

60 65

64

5.29 a 2.56 h 12.8 h

beta beta K, beta T

49.7 336 96

0.3 1 2.10 0.6

213 239 215

1.333 1.49 1.34

30 40 41

Zn Zr Nb

65 95 95

245 h 65 d 35 d

beta beta beta -

52.8

0.33 0.40 0.16

176

1.11

I15 123

0.72 0.77

42 47

Mo Ag

99 llOm

64 h 253 d

beta beta

118.5 105.6

0.74 0.66

51 74

Sb

124 187

60 d Id

beta beta -

96 110.5

0.60 0.69

22

w

'

+

-

64 25.6 197 13.8 86.4 99.2 101

1.23 0.086 (55 Yo) 0.54 (43 Yo) 0.62 0.63

51.3 -

0.32 -

comparable to that of thermal movements of molecules. The principal nuclear reaction inducing radioactivity is the radiation capture of the type (n, y) which can be written as or in an abbreviated form A + I x

'

$ x < n , Y)

z

(1 -4)

The reaction (n, y ) produces an isotope of the element X which has in its nucleus one neutron in excess of the number corresponding to the original element. As a consequence, the nucleus of the new isotope is usually beta-active; the emitted j3 particle is often accompanied with the emission of a y quantum. An example of induced radioactivity in corrosion products is provided by a nuclear interaction of neutrons with the stable nuclide of ::Mn: 55Mn+0n 1 25

+

;:Mn+:y

Some typical radionuclides induced in corrosion products of the reactor and the adjacent circuits construction materials are listed in Table Z.9. When comparing the values of the half-lives of typical radionuclides appearing in the corrosion products with those typical for a mixture of uranium fission products (Table 1.10) or the nuclear fuel (Table Z.11) it becomes obvious that the mean life-time of radioactivity in corrosion products is shorter than that of either of the two other components. Similarly, the total radioactivity of corrosion products corresponds just to a minor fraction of the activity of fission products arising in a nuclear reactor or released as a result of a nuclear explosion. However, the significance of activated corrosion products is due to the fact that they represent a permanent component of the contaminant in the primary and the adjacent circuits (as they are generated in the course of normal reactor operation), whereas a mixture of fission products can only appear in heat-transferring circuits of pressure water reactors as a consequence of fuel element failure. Activation products of the coolant and its admixtures Corrosion products occur predominantly in pipelines and tanks in the form of oxides or as sludge. The amount of the corrosion products of materials characterized by a relatively low corrosion rate may attain about 0.8 g .m-2. s-', which-for a major reactor with large inner surface area of the primary circuit would represent about 100 g per day. In gas-cooled reactors, the nature of the contaminant depends on what coolant is used. If the coolant is a highly purified air, the prevailing radionuclide 23

TABLE 1.10 IMPORTANT FISSION PRODUCTS OF URANIUM

Half-life

Nuclide Z

X

A

37 38 38

Rb Sr Sr

86 89 90

39 39 40

Y Y

9 0

h - hours d - days a - years 19.5 d 54 d 28 a

Type of

:ransformatior MeV

n

MeV

beta beta beta

290 240 96

1.8 1.5 0.6

180

1.1

2.2 1.5 0.4

-

-

-

beta

350 240 64

110

0.7

26 190 46

0.16 1.2 0.29

120 118

0.75 0.74

32 6 150

0.2 0.04 0.96

80

0.5

I60 320 260

2.0 1.6

54

0.34

150

0.96

220 380 96

I .4 2.4 0.6

300 96

1.9 0.6

138 110 290

0.86 0.7 1.8

120 67 130

0.72 0.042 0.8

98 83 160

0.61 0.52 1.02

58 110

86

0.36 0.66 0.54

210 93 48

1.3 0.58 0.3

256 23 small

0. I45 small

150

0.93 0.8 0.22

-

beta beta -

Zr

61 h 61 d 65 d

41 42 43

Nb Mo Tc

95 99 99

35 d 67 h 2.105 a

44 44 46

Ru Ru Pd

103 106 109

40d la 14 h

47 47 48

Ag Ag Cd

111 113 115m

7.6 d 5.3 d 43 d

50

Sn Sn Sb

123m 125 125

136 d 9.4 d 2.7 a

Sb Te Te

127 127m 129m

3.7 d 115 d 33.5 d

56

I Cs Ba

131 137 140

8.1 d 33 a 12.8 d

beta beta beta beta beta beta beta beta beta beta beta beta -

57 58 58

La Ce Ce

140

141 144

40h 33.1 d 282 d

beta beta beta

59 60 61

Pr Nd Pm

143 147 147

13.7 d 11.3 d 2.6 a

beta beta beta

51 51

52 52 53

55

,

Energy of y radiation

fJ

91 95

50

Energy of )articulate radiatioi

beta beta beta

-

beta beta beta

-

-

130

35

1

.o

-

-

-

-

14

-

-

-

-

1.60

0.09 -

is 41Ar(half-life 1 12 min) resulting from neutron activation of 40Ar;then follows the gaseous fission products -krypton and xenon radionuclides. If the coolant is carbon dioxide, the most abundant constituents of the contaminant are l6N, 24

TABLE 1.11 FISSILE NUCLEAR FUELS

Nuclide Z

92 92 92 94

X U

U U Pu

A

233 235 238 239

Half-life (Years)

1.6.10' 7.1. lo8 4.5. lo9 2.4. lo4

Energy of particulate transformation

alpha alpha alpha alpha

I

fJ

760 710 665

810

I

MeV 4.8 4.5 4.2 5.1

y radiation

I

fJ 8 30 8 2

I

MeV 0.05

0.185 0.05

0.01

I

of the muclide MBq .kg-' 350 OOO 70 I1 2200000

41Ar,14Cand some less important krypton and xenon radionuclides. Virtually no activation at all takes place if the system is cooled with helium. Corrosion product activity is also low in this case, except for helium-cooled high.-temperature reactors where the coolant is contaminated with fission products, particularly with radionuclides of I, Te, Ru, Sr, Ba. Liquid sodium-cooled reactors produce appreciable amounts of 22Na(half-life 2.6a) and "Na (half-life 15h); the specific activity of the latter in the primary circuit may reach the order of magnitude of 10" Bq .1-' (10'Ci. 1-I). In heavy-water reactors, interactions of neutrons with deuterium (*H) produce tritium (3H). Tritium, incidentally, is produced in a low concentration by any type of reactor. Gaseous efluents Gaseous wastes arising as a result of light water reactor operation contain, in the order of significance for radiation protection, the following radionuclides: noble gases, in particular '33Xe (half-life 5.27 d) and "Kr (half-life 10.6a), tritium, I4C and I3lI (apart from other iodine radionuclides of minor importance). The estimated approximate values related to 1 GWe per year for pressure water reactors (PWR) are 550 TBq of '33Xe,6.3 TBq of "Kr, 2-7.5 TBq of 'H, 0.2 TBq of I4Cand 2-20 GBq of 13'I. For boiling water reactors (BWR), the values are 7400 TBq of 13'Xe, 1 110 TBq of "Kr, 2 TBq of 'H, 0.2 TBq of 14C and 75-185 GBq of I3'I [8].

Liquid efluents

- The radioactivity of liquid wastes is due mainly to tritium (some 75 TBq per 1 GWe per year for PWR and about one fifth of that amount for BWR). In addition to tritium, a host of other radionuclides add up to the total liquid wastes radioactivity averaging 0.3 TBq (PWR) and 2.2 TBq (BWR); up to 40 % 25

and 15 YO of this additional radioactivity is attributable to iodine and cobalt radionuclides respectively. Aerosols of solids

The mean total radioactivity of solid aerosols per 1 GWe .a is estimated to be 20 GBq for PWR and 40 GBq for BWR. The emissions from PWR are 'To, and composed of "Rb (over 80 YOof the total activity), 134Cs,137Cs, 54Mn.Aerosols released from bWR include larger proportions of IMBa, '@La, "Sr plus smaller fractions of I3lI, ' T o , @Co,134Cs and 137Cs(contributing less than 10% each). Contaminant resulting from decommissioning of a nuclear facility taken jinally out of operation The total radioactivity of a 1 GWe pressurized water reactor decommissioned after 30 years of operation amounts to about 350-750 PBq (fuel and control rods excluded). Most of the radioactive substances are contained inside the reactor. The prevailing radionuclides are those of iron, cobalt, nickel, chromium and manganese. The 6oCoand "Fe are responsible for about 90 YOof the activity [9]. Contamination arising at the storage of spent fuel When storing the burn-out fuel, a release of fission products mostly has the same cause as that described for a reactor, i.e. a failure of fuel assembly cladding. Assuming that the probability of the damage is 0.2% and that 300 tons of spent fuel are handled in a year, the yearly amounts of radioactivity are close to 4 TBq of 3H, 80 TBq of 85Kr,0.8 MBq of '291, 0.4 GBq of I3'Cs and 4 kBq of 239Pureleased into the water in the plant's storage pool, except that iodine and tritium also become partly, and krypton predominantly, airborne. The contribution to the dose equivalent accrued by a member of the population at large in the vicinity of a plant has been calculated to reach Sv per year. According to a conservative estimate, a yearly release of 4.8 GBq during a 30-year operation period could add up to a collective dose equivalent to the population in the region of 3.5 man. Sv [lo, 1 1 , 121. Reprocessing of nuclear fuel

The reprocessing of nuclear fuel can be a source of serious environmental contamination. A plant with the capacity to handle 1500 t of U 0 2releases into the air around 40 PBq a year, assuming a retrieval efficiency of 99 % for 1291and 75 YOfor tritium. Advanced technology employed by large reprocessing plants allows working with even higher retrieval efficiencies for gaseous effluents : 99.5 % for 1291,80-90 % for 'H,90 % for I4C,95 % for "Kr: the decontamination factor for aerosols is likely to be of the order of lo9 [13].

26

1.4.1.3 Contamination resulting from the production and application of artificial radionuclides The economic benefit which the utilization of radionuclides in medicine, agriculture and industry can convincingly bring about leads to a continuous growth, on a global scale, of the variety and the amount of radionuclides produced. The circumstances accompanying the rising production of radioactive nuclides, their transport and use in various fields of human activity unavoidably enhance the risk of environmental contamination. The following table (Table 1.12) reviews the most frequently used artificial radionuclides and specifies also their main fields of application [14, 151. With only a few exceptions, the listed nuclides are beta or gamma emitters. Those emitting high-energy y rays are used primarily in the irradiation of malignant tumours, to radiation sterilization (of foodstuffs, drugs and medicines, dressing material, hospital bedding etc.) and in defectoscopy. They are used almost exclusively in the form of sealed sources. On the other hand, unsealed beta-gamma emitters very frequently find an application in medical diagnostics and therapy, biology and agriculture, and are invaluable tools in research as tracers. The physical and chemical forms of the used radiation sources vary with the aim and the particular way of application. Most of the radionuclides are not applied in their elemental form, but rather after incorporation into “labelled” compounds. The spectrum of the available labelled compounds is virtually boundless. The principal methods of production essentially consist in irradiation of the target atomic nuclei in nuclear reactors or cyclotrons. The latter facility, though less efficient as far as the yield is concerned, makes it possible to obtain specimens with high specific activity and nuclide purity.

1.4.2 Uncontrollable technogenous sources Very serious consequences with high-level contamination of the earth’s surface and extensive internal contamination of living organisms can be caused by processes based on uncontrolled fission reaction or thermonuclear fusion, as well as accidents in nuclear facilities resulting in massive releases of radiotoxic substances into the environment. 1.4.2.1 Contamination resulting from a nuclear explosion 1.4.2.1.1 Radioactive contamination as an aggravating factor of the destructive eflect of nuclear weapons

Nuclear explosions are unavoidably associated with the generation of extensive zones in which the earth’s ground and the structures erected on it

27

TABLE 1.12 THE MOST FREQUENTLY USED RADIONUCLIDES Nuclide

'H ENa 42

57.

K

"Cr 58.60c o

S9Fe "Br "Kr %"'TC

Form diagnostic radioactive drugs [I41 tritiated water chloride chloride serum albumin, chlorate cyanocobalamine

citrate sodium bromide

gas serum albumin, aggregate, pertechnetate ll3m1~ DTPA complex, chloride various, most often serum albumin, iodide, 1311 o-iodo-hippurate, Bengal red chloride '37cs 197:203Hg chlormeradrine colloidal solution I9'Au

32P

60co

% 9oy 1311

'8zTa i92~r '98A~

35s

"co 89Sr '37Cs IaBa 147Pm 226Ra

Half-life

12.3 a 2.58 a 12.5 h 27.8 d 270 d 71 d 5.29 a 45 d 36 h 10.8 a 5.9 h 1.7 h 8.05 d 9.7 d 65 h; 47 d 2.1 d

therapeutic radioactive drugs [I41 phosphate needles, metal wire applicators colloidal solution sodium iodide metal needles metal grains colloidal solution, metal grains or wire

14.2 d 5.29 a 28 a 64.8 h 8.05 d 115 d 14.5 d 2.1 d

radiation therapy, industry, agriculture [ 151 various compounds metal chloride metal chloride, sulphate chloride various

87.2 d 5.29 a 51 d 33 a 12.8 d 2.6 a 1620 a

Applied radioactivity (MBq) 5-10 0.14.8 4-10 0.4-12 0.01--0.04

0.2-1

0.8-2 4-15 20-500 4(%400

0.1-20 6 4-40 &-6

0.04-Q.08 as required as required 0.24.4 0.1-5

as required as required 1-6

I 03 of -103 102- 103 1-103

370-1 850 various 10-103

become heavily contaminated with nuclear explosion products. Radioactive contamination of the ground is not limited to the epicentre, but extends also along the trajectory of the radioactive cloud movement forming the S.C. radioactive trail where the nuclear explosion products gradually deposit onto the ground. The intensity of contamination is much higher in the epicentre than in the trail, the size of the trail greatly exceeding the dimensions of the explosion site. Apart from a mixture of deposited fission products, the high intensity of ionizing radiation at the explosion site is also due to radionuclides induced by neutron capture. The size of the trail as well as the dose rate measured on the earth’s surface and in the structures thereon depend on the size of the weapon, type of explosion, meteorological conditions and characteristics of the terrain. Radioactive contamination constitutes one of the destructive factors of the effect of nuclear weapons. The following components, arranged in the order of their relative significance for radiation safety, play a role: - A mixture of fission products; - Radionuclides arising from the interaction between neutrons and atomic nuclei (induced radioactivity) ; - Unreacted fragments of the nuclear charge. The maximum ground contamination occurs if the weapon is exploded very close to the earth’s surface, underground or on a water surface. Among the meteorological factors, it is primarily the wind direction and wind speed that affect the size and orientation of the radioactive trail, and the degree of contamination. The nuclear explosion products arising in a ground explosion are ejected as an aerosol consisting of particles of spherical shape with a diameter ranging from 0.1 pm to 1 mm. The fraction of the total activity released at the moment of explosion which will be eventually deposited within the radioactive trail depends primarily on the type of the soil (it is likely to be the highest for sandy, i.e. light soils). The bulk of the radioactivity is carried by particulate material having the diameter range from 100-1000 pm. The soil type also determines the chemical composition of the aerosol. It can be assumed that the fission process in an exploded nuclear weapon of 1 kt TNT equivalent gives rise to approximately 57 g of mixed fission products, and the radioactivity 1 h after the explosion corresponds to 2. lOI3MBq. In a ground explosion, about 3 . lo4kg dust particles are dragged into the cloud per each kt TNT equivalent. The mean activity d calculated for a single particle depends on its surface area and is independent of the weapon TNT equivalent: d = 9.25. d2.2. t-’.2 (Bq) (1 -6) where d is the particle diameter (pm) and t - the post-explosion time (h).

29

The number of particles deposited on a unit area of the earth’s surface depends on the weapon size, type of the soil bed in the epicentre, meteorological conditions and the distance from the epicentre. On average, lo4to lo6particles deposit on each square meter of the ground. The contamination of the terrain is a complex physical process consisting of - Gravitational sedimentation of particulates (the mean sedimentation rate depends on the particle diameter and height above the earth’s surface); - Initial vertical distribution of particles according to the effective source height above ground; - Particulate fallout caused by the down wind; - Dispersion of particles due to a turbulent diffusion in the atmosphere. 1.4.2.1.2 Mixture of fission products Nuclear explosives, i.e. ’;;U, *;;U and ’;zPu, can undergo a nuclear fission reaction, either spontaneously or triggered by neutron capture. A reaction of this type can be schematically written as

The compound atomic nucleus of 236Uis highly unstable and disintegrates “immediately” into two daughter elements, fragments X and Y,which are newly generated radioactive nuclei having the mass numbers between 72 and 166 and half-lives ranging from fractions of a second to tens of seconds. The diversity of the generated radionuclides is a consequence of the fact that the nucleons (i.e. protons and neutrons) of fissile elements (233U,235U,239Pu) are randomly distributed among the newly emerging fission products. The arising radioactive nuclei are mostly members of the “disintegration chains”. By emitting beta particles and photons, the newly formed fission products transmute to other radionuclides until finally the nucleus appearing after 3 to 5 transformation steps is stable. As a general rule, one can say that the half-life of any subsequent radionuclide is longer than that of its predecessor (precursor). Fission products represent a complex mixture of various radionuclides (about 250 in number) belonging to as many as 35 chemical elements whose proton numbers range from Z = 30 to Z = 64. Among the more important members are radionuclides of the following elements: Sr, Y, Zr, Nb, Ru, Te, I, Cs, BayCe, PryNd and Pm (see Table 1.10). The longest half-lives exhibit I3’Cs (33a) and wSr (28a); the latter is usually regarded as the nuclide among the fission products which is biologically most hazardous. Because individual radionuclides in the mixture decay with different rates, the composition of the mixture varies with time : Short-lived radionuclides predominate at an early stage, whereas later stages are characterized by a prevalence of long-lived nuclides. 30

*

At the stage of condensation and aerosol formation, the isotopic composition of a mixture of short-lived fission products changes considerably. This phenomenon, known as primary fractionation, implies that the short-lived precursors of longer-lived radionuclides, such as for instance !%r, '37Cs,95Zrand lace, do not take part in the condensation process. Thus, noble gases and volatile radionuclides with short half-lives (the precursors of ?3r and I3'Cs) are not condensed and/or incorporated in aerosols. Consequently, daughter elements do not become incorporated into the newly formed particles either. Instead, they take part in processes leading to their adsorption on the surface of the already existing particles, primarily the finely dispersed aerosols easily soluble in various biological fluids. The isotopic composition of fallout is further modified by secondary fractionation due to, firstly, a differential sorption capability of aerosol condensates which form during the cooling of the vapourized environmental materials at the site of the explosion and, secondly, a different rate with which particular radionuclides are washed out by radioactive rain resulting from water condensation on the aerosol particles as the temperature falls. This fractionation plays a relatively greater role in the explosions of small size weapons [4]. The relative proportions of particular radionuclides in the mixture depend on the type of fissionable material, the energy of neutrons that trigger the fission reaction and the time elapsed after the explosion. The decrease in the total activity of fission products with time can be expressed by an empirical relation deduced by Way and Wigner [16]: A(t) = A(l).t-'.2

(1 -8)

where t is the post-explosion time in hours and A(1) denotes the activity at the time t = 1 h. The energetic spectrum of gamma radiation emitted by a mixture of fission products is rather complex, and varies with time. It extends over energy regions ranging from 0.3205 fJ (98mlT~) to 1.07364 pJ (86Br, S7Br).Fission products exhibit an immense specific activity of up to 2.4. le2Bq .kg-l one minute after the explosion. 1.4.2.1.3 Induced radioactivity The induction of radioactivity, i.e. transformation of a stable nuclide into a radioactive one, is caused by neutron capture in the nucleus. Since both fission and fusion reactions generate high neutron fluxes, radionuclides are induced in the construction material of the explosive device and in environmental materials. In nuclear weapons of the three-phase type (fission-fusion-fission), the 238U in the charge is transformed to radionuclides 23%Ipand 237Uwith respective 31

half-lives 2.35 d and 6.6 d ; their initial activities are very high and for the first 2-8 days even exceed the activities of the generated fission product mixture. The following radionuclides are induced ,in the construction materials: 54Mn, 28A1,"Fe, 59C0,6oCo,6'Zn, 9 9 Mand ~ other actinides. In addition, I4Cis induced in the air, and 22Na,24Na,28A1,32P,42K,45Ca,"Mn, 56Mn,84Rb,134Cs and others in the soil. The activities of induced radionuclides depend on the size and type of the weapon and the altitude of the explosion. The highest activity levels are those of 28A1,24Na,42Kand 56Mn.The thermonuclear fusion reaction generates also 'H and 14C. An explosion of a thermonuclear weapon gives rise to about 1 kg 'H (4.4.10'' Bq) and about 13.8 kg 14C(2.5.10" Bq) per each Mt of TNT equivalent. Radionuclides induced in the charge and the construction materials become dispersed in radioactive fallout. The majority of those induced in the soil are retained in the surface layers of the earth to a depth of 20-30 cm in a region of 1.5 km radius around the epicentre, and represent a radiation source of relatively low activity. The peak of activities has been detected at a depth of about 5 cm. It should be noted that the radioactivity induced in materials can neither be removed from the matrix nor reduced to any practical extent without matrix removal. 2.4.2.1.4 Unreacted part of the charge

The radionuclides '"U, 235U,239Puused as nuclear explosives (see Table 2 . 2 2 ) are alpha emitters. Compared to the beta-gamma emitters among the fission products, their radioactivities in the fallout are negligible. From the radiobiological point of view, however, they are classified as extremely toxic radionuclides because of their high relative biological effectiveness attributable to the high ionization density, i.e. the rate of energy transformation by formation of ion pairs along the track of the ionizing particles. The sites of their preferred deposition in higher organisms are bone and lung tissues. Plutonium as an element belongs to the most toxic nuclides of all. 1.4.2.2 Accidents in nuclear facilities On the basis of past experience with the design and operation of NPP with reactors of the PWR type, conceivable accidents resulting in an escape of the primar circuit coolant can be divided into five classes [17]: Class I - Accidents caused by a rupture in the primary circuit piping system of the greatest diameter accompanied with a partial melting of the core. A core disruptive accident in which 10% of the fuel load has melted is considered to be the maximum credible accident. Estimates of the proportions of individual fission products expelled from the molten fuel are as follows: gases 32

100 YO,I, 80 YO,Cs 50 YO, Sr 5 YO,Ba 5 YO,Mo 2 YO,Te 10 YO, Cr 0.5 YO, Ru 10 YO, Zr 0.5 YO. Class I1 - Accidents caused by a rupture in the primary circuit piping system with a subsequent failure of fuel cladding but falling short of fuel melting. The escaped radioactivity accumulates in the primary circuit coolant. Any leak of fuel assemblies results in fission products escaping from the primary circuit. This type of accident is thought to be the maximum credible aEcident in NPP equipped with passive, fast-acting emergency systems of core cooling. Class I11 - Accidents caused by a rupture of the primary circuit piping manageable by means of the standard systems of coolant’s refilling, and not accompanied with any damage to fuel cladding. The accident results in a release of a steam-air mixture containing fission products. Because of excessive steam formation, a rising pressure at the leak site can expel the volatile gaseous as well as the more refractory fission products into the vault and ultimately into the environment. Class IV - Accidents caused by escapes from the primary circuit system occurring at increased temperatures exceeding 37 1 K. This category includes events resulting in an escape of liquid and gaseous substances under increased temperatures and pressures in amounts approximating 100 1. h-*. Class V - Accidents caused by escapes from the primary circuit system occurring at lower temperatures of the leaking medium. Accidents of this type include water leakage from volumes containing radioactive substances at temperatures below 371 K. Among such volumes are: primary circuit at cold state, water circuit of the biological shield, water of the secondary circuit (up to the steam generator) and water of the liquid waste storage pool. There is no risk of serious radiation hazard due to an escape of radioactive gases. Uncontrollable scattering of radioactive materials is conceivable. It holds true in principle that the kind and the amount of the contaminant, including its physical and chemical properties depend not only on the reactor type, but also on the course and duration of reactor operation. The time course and consequences of hypothetical nuclear accidents may be analyzed by applying a probability method, such as that derived by Rasmussen V81. Several accidents involving nuclear facilities have occurred in the past and have been extensively analyzed and reported. The three most widely publicized accidents are those which happened at Windscale (1957), Three Mile Island (1979) and Chernobyl (1986). The accident which involved a gas-cooled reactor at Windscale (Great Britain) resulted in a release of approximately 750 TBq of I3*I,22 TBq of I3’Cs, 3 TBq of 89Srand 0.33 TBq of %, Among the most important radionuclides released into the environment at 33

Three Mile Island (U.S.A.) after an accident at the TMI-2 reactor were '"Xe, '35Xeand I3'I. The collective dose equivalent to the population accrued from the release during the first post-accident days was estimated to have been about 20 man.Sv, which is below 1 YO of the dose accumulated from the natural background radiation in a year. Only three members of the plant's personnel received a dose equivalent of 30-40 mSv [19]. The most serious accident so far involving power excursion in a nuclear reactor occurred on April 26, 1986, at the 4th Unit of the nuclear power station at Chernobyl, USSR. With regard to the extent of the release of radionuclides. this was by far the gravest accident experienced up to now in the history of nuclear energetics. The accident had serious consequences for the surrounding environment, and the widely dispersed radioactive material could be detected in most of the European countries, even those far away. The accident destroyed one of the four RBMK-1000 type reactors operating at Chernobyl. The main technical parameters of the RMBK-1000 reactor are summarized in Table 2.23. TABLE 1.13

MAIN CHARACTERISTICS OF THE RBMK-1000 REACTOR Parameter Thermal power Fuel enrichment Mass of uranium in an assembly Number/diameter of fuel elements in sub-assemblies Burnup Coefficient of inhomogeneity of power output over the core radius Coefficient of inhomogeneity over the core height Limiting theoretical channel power Isotopic composition of fuel unloaded: Uranium 2s5U Uranium 238U Plutonium 239Pu Plutonium % 'I Plutonium 24'Pu

Value 3200 MW 2 Yo 114.7 kg 18 pc/13.6 mm 20 MWd. kg-l 1.48

1.4

3250 kW 4.5 2.4 2.6 1.8 0.5

kg . t - ' kg .t - l kg .t-' kg.t-' kg. t - l

The core of the ChernobyLUnit 4 contained at the time of the accident a radioactive inventory of about 4 . lOI9Bq ( lo9Ci). The isotopic composition of this inventory is shown in Tub& 2.24. On the basis of radioactivity measurements and analyses of samples taken from a 30 km radius around the plant and throughout the European part of the Soviet Union it has been estimated that about 1 .lo'* to 2 . 1OI8Bq (3. lo7 to 5 . lo7Ci) were releasedfrom the fuel during the accident, not counting contribu34

TABLE I .I4 INVENTORY OF FISSION PRODUCTS IN THE REACTOR CORE AND THEIR ESTIMATED ESCAPE Radioactivity (Bq)

Element

"Kr

'"Xe 1311

'"Te '"cs '"CS

*Mo 9 3 3

'"Ru

'"Ru ImBa I4'Ce W e *Sr 90Sr 239Np 238pu

2 3 v u 240pu

241pu 242Cm

3930 5.27 8.05 3.25 750 1.1.104 2.8 65.5 39.5 368 12.8 32.5 284 53 1.02.104 2.35 3.15.104 8.90. lo6 2.40.106 4800 164

3.3.1016 1.7. 10l8 1.3. 10l8 3.2. 1017 1.9. 1017 2.9. 1017 4.8. 10l8 4.4.10'8 4.1. lob8 2.0. 1Ol8 2.9. 10" 4.4. 10'8 3.2. 10" 2.0.10'8 2.0. 1017 I.4.1017 1.0.1015 8.5. 1014 1.2. loi5 1.7. loi7 2.6. lot6

Relative escape

(%I

-- 100 100

20 15

10 13 2.3 3.2 2.9 2.9 5.6 2.3 2.8 4.0 4.0 3 3 3 3 3 3

tions to the release by noble gases (xenon and krypton), as the noble gas radionuclides are thought to have completely escaped the plant. The estimates have an error of & 50 YO.The integrated releases of individual radionuclides estimated by Soviet experts are also shown in Table 1.14. About 10-20 YOof the volatile radionuclides of iodine, caesium and tellurium were expelled from the fuel. Releases of the more refractory radionuclides, such as barium, strontium, plutonium, cerium etc. amounted to 3-6 YO. The release of radionuclides did not occur in a single massive event. Rather, only about 25% of the total release took place during the first day of the accident, the rest occurring as a protracted process extended over a nine-day period. Throughout this time, samples of the air and ground deposits in the Soviet Union were obtained and measured. From these data, the experts constructed the time depended release rate curve. This curve can be categorized into four segments: 1. Initial intense release on the first day of the accident;

35

2. A period of five days over which the release rate declined to a minimum value six times lower than the initial release rate; 3. A period of four days over which the release rate increased to a value which was about 70 YOof the initial release rate; 4. A sudden drop in the release rate nine days after the accident to less than 1 YOof the initial rate and a continuing decline in the release rate thereafter. The distribution of fuel deposited around the destructed Unit was as follows: a) on-site 0.3-0.5 YOof the core; b) 0-20 km, 1.5-2 YOof the core; c) beyond 20 km 1-1.5 YOof the core. Samples of UO, were found to be oxidized to U,O, . Chemical forms of the aerosolized materials were quite variable. Particle sizes of aerosols evolved from the reactor core were in the broad size range of less than 1 micrometer to tens of micrometers. Initial calculations and, at a later stage, direct measurements (I3’I in the thyroid and whole body counting) made it possible to estimate that the collective dose to the population in the European part of the USSR over the next 50-70 years is likely to be of the order of 2 . 1O6 man . Sv, with most individuals receiving a dose over their lifetime which is less than that from natural background radiation. Iodine I3’I gave rise to comparatively high doses to the thyroid of some individuals in the short term, but it is not expected to be an important factor contributing to the total dose in the long term either for individuals or for the whole population [20].

I .4.3 Methods of contamination assessment Determination of the contamination levels is an integral part of radiological monitoring within the framework of the radiation protection programme (see Sect. 1.3.1). The surface contamination monitoring is the of the Health Physics Group and is performed for instance prior to, and in the course of, any decontamination action, whenever an item is transferred from the controlled zone, or when site personnel enter or leave the zone; protective clothing and other means of personal protection are monitored before they are either declared “clean” or segregated as contaminated items. The frequency, type and methods of measurements, and the appropriate measuring instruments used vary in relation to the purpose and extent of the work. The obtained data on surface contamination form the basis for decision making. The decision must comply with the requirements of the binding limits both in the sense of the doses of external human radiation exposure and the permissible levels of surface activity and activity concentration. The two most frequently measured parameters are the following: First, the dose (exposure) rate determined at appropriate locations of the work place with 36

respect to the anticipated occupancy on the part of the personnel engaged in a particular operation, as well as the dose (exposure) rates resulting from surface contamination of relevant objects. The second parameter is the level of radioactive contamination determined by measuring (either directly or by means of the wipe test) the area activity or the activity concentration (in liquids). Monitoring data are evaluated before a decision is made as to the organization of the work, choice of working methods, rotation of workers, temporal shielding etc. to minimize the external radiation exposure. Monitoring data also determine whether or not a given item (object) is radioactively contaminated and is to be subjected to decontamination. By comparing the values of dose rates or activities before and after decontamination it is possible to assess the decontamination efficiency. There are essentially two methods of assessing the level of radioactive contamination, viz. a) calculation (this has the character of a prediction with a certain degree of probability), and b) measurements by means of the appropriate measuring instruments. 1.4.3.1 Assessment of the radiation situation by calculation 1.4.3.1.1 Relationship between the radioactivity of a site and the radiation

dose rate A serious accident at a nuclear facility involving an uncontrolled escape of fission products can cause an extensive radioactive contamination of the environment. In order to estimate the radiation dose to which human subjects might be exposed within the contaminated zone, the following relation between radioactivity and dose rate can be invoked: If a sizeable area (several hundred square meters) is contaminated with 37 GBq (1 Ci) per each square meter, then the dose rate measured 1 m above the ground surface varies from 44-90mGy.h-’, depending on the relief of the land. A dose rate of 8.6 mGy .h-’ (i. e. 1 rad per hour) corresponds approximately to an area activity of 4.0 to 8.0 GBq .m-’ (approximately 100 to 200 mCi .m-2). This rough relationship is valid only if no significant changes occur during the evaluated time period in the level of the earth’s surface radioactivity. Neruda et al. [21] calculated the dose rates of y radiation for various radii in a locality contaminated with a mixture of fission products. The values listed in Table 1.15 have been derived by using the equation for the calculation of dose rates over a “zero weight circular source in vacuum” Ka r2

z2+r2

0, = 2 In -

2

37

TABLE 1.15 DEPENDENCE OF THE DOSE RATE OF GAMMA RADIATION 0, ON THE RADIUS OF IRRADIATION Radius r (m)

Height (m)

D, (pGy .h-'/Bq. cm-2)

1 .o

0.04 1.oo

1.3. 1.4.10-3

10.0

0.04 1 .oo

50.0

cc

0.04 1 .oo

I .oo

Gamma dose rate

2.4.

9.7.10-3 3.0. loW2 I .6. 2.2. 10-2

The value for r = x has not been calculated but taken over from commonly accepted data assuming that the contamination of '*a real terrain" attaining an average area activity of 37 GBq. m-' (1 Ci . m-2) would give rise to a dose rate measured I m above the ground surface equal to approx. 70 mGy , h-'.

where Kgis the dose constant, r -the radius of the source, ap-the area activity and z - the distance of the point of interest from the centre of the circle. The dose constant for a mixture of fission products used in these calculations is 0.67 (pGy .h-'/Bq .cm-2). 1.4.3.1.2 Relationship between the surface contamination and the content of airborne radioactive substances

A direct proportionality exists between the level of surface contamination and the concentration of aerosols, depending on the nature of the surface, the frequency of the total air volume exchange, the overall content of dust in the room, and the chemical and physical characteristics of the radioactive contaminant. It has been found that an amount of powdery radioactive material corresponding to an area activity of 37 MBq .m-* (1 mCi . m-2) would result, if handled in a closed space, in an activity concentration of 1.48 kBq. m-3 (4. mCi .mP3),whereas if an operation involving radioactive dust formation was carried out in an open space, the increase in the activity concentration unit per cubic meter. The movement of persons across a attained only 2 . floor contaminated with a-emitters releases into the air the unbound fraction of a-emitting radioactive substances in such a way that one unit activity per 1 m2 corresponds to an increase in the aerial concentration of a-emitters by 2 . to unit per 1 m3.As an example, a single person crossing a relatively small non-ventilated room with its floor contaminated with 1 unit of radioactivity per

38

square meter would cause an increase in the airborne activity by 4 . units. m-3. By generalizing the published data, it is possible to conclude that the level of surface contamination and the degree of aerial contamination are mutually related; the relationship may be epressed as

C = K,.q

(1.10)

where C stands for the concentration of airborne radioactive substances in the working space (MBq .cm-3 or mCi .cm-4, K, is the coefficient defining the fraction of radioactivity released from the contaminated surface into the air (cm-I) and q is the level of surface contamination, i.e. its area activity (MBq . cm-2 or mCi .cm-2). The value of K, can vary in a broad range ( lop2to lo-". cm-I) depending on the kind of material and characteristics of the contaminated surface, physicochemical properties of the contaminant, the type of work performed and additional factors, such as the degree of saturation of the working place with equipment and tools, frequency of the total air volume exchange etc.

1.4.3.2 Assessment of the level of surface Contamination by means of measuring instruments Methods for determining the level of surface contamination by means of suitable measuring instruments are, in principle, either direct or indirect. Direct methods and techniques are those where the detector of the measuring instruments either registers the signals in direct contact with the radioactively contaminated surface, or is placed in the contaminated space in such geometrical arrangement which allows the measurement of the desired quantity in the entire .space. Alternatively, the detector may be partially shielded to detect only radiation emitted within a defined limited solid angle. Indirect measurements of surface contamination are in principle performed outside the contaminated space in a more suitable environment, thus avoiding the interference of a high radiation background which would increase the counting error beyond acceptable limits, or which might occasionally make the measurement entirely impossible, because the working range of the measuring instrument might be exceeded. If it is feasible to move out the facility or to dimantle it, the whole system or its dismantled parts are subjected to radiation assessment preferably in a suitable place characterized by a low radiation background. However, in many cases it is impractical or utterly impossible to separate parts of the equipment or the surface to be measured (e.g. floor covers, wall paints etc.), and representative samples must therefore be taken from the contaminated surface by a suitable collection method. Among the most com39

mon methods are the wipe test using dry or wet swabs (of cellulose, cotton and other material) or sampling of the contaminant by means of an adhesive tape or strippable lacquers (paints). Practical experiences point to a low reproducibility of measured data obtained by such methods [22]. 1.4.3.3 Indirect methods of measurements

To apply these methods, it is necessary to know beforehand the value of the coefficient of radioactivity removal from the given surface by the chosen means and technique. This coefficient of removal, cs, expresses the ratio of the radioactivity of the swab (or tape, as it may be) A, to the actual radioactivity of a surface AS c ='

A

(1.11)

As

The value of the coefficient of removal can only be determined empirically and depends on a number of factors, above all on how the wipe test is made. Its value is fraught with a high degree of uncertainty due to many subjective factors, such as the pressure applied to the swab, the technique and duration of swabbing, moisture content of the swab etc. The following Table 1.16 lists the values of c , for ~ some of the commonly used swab techniques. It is evident at first glance that dry swabs are much less efficient. On the other hand, some of the wet techniques may cause partial dissolution of the contaminant and consequently facilitate its possible penetration into deeper layers of the decontaminated surface. Furthermore, contaminants become adsorbed also on fibres inside the wad, a fact which may unfavourably affect the accuracy of the measurements, particularly if low-energy @emitters are to be measured. TABLE 1.16 MEAN VALUES O F THE COEFFICIENT O F REMOVAL PERTAINING T O VARIOUS METHODS (231 Removal technique

40

Mean coefficient of removal k,

Filter paper Water-soaked gauze swab

0.2 0.6

Gauze swab dampened with 1.0 to 1.5 NHNO, Successive swabbing with two gauze swabs dampened with 1.0 to 1.5 N HNO, followed by wiping with a dry swab

0.9

o . e 1 .o

I .4.4 Decrease in radioactivity with time Dependence on time t of a number N of atoms that did not undergo radioactive transformation is expressed by the law of radioactive trasmutation (decay) (1.12) N = N ~e-A1 . where No is the number of atoms at zero time ( t = 0), and 1 - the constant of radioactive transformation (decay constant). Similarly, an activity A, defined as the number of atoms undergoing transformation within a unit time is proportional to the number of atoms N and is thus expressed by an analogous expression A = Ao.e-" (1.13) where A, = A at zero time. If one considers a particular radionuclide, or more precisely (see Section 1.1) a nuclear individual $ X ( T ) , one half of its initial number No of atoms (i.e. N = N0/2) will have undergone radioactive decay in a time interval equal to the half-life T of the given radionuclide (i.e. t = T), thus the law of radioactive transformation will read (1.14) where n = t / T , i.e. a quotient expressing the time interval in the number of half-lives. By comparing the two relations (1.12) and (1.14) one obtains 1.T = In2 (1.15) The graphic representation of the dependence N = f ( t ) in the form N / N o = f ( t / T) is shown in Fig.1.2.

1

2

3

4

5

tlT

Fig. 1.2. The law of radioactive decay: NIN, = e-' N

= 2-l"

number of atoms of a radioactive nuclide at time I; No number of atoms o f the nuclide at time I = 0; T - half-life of the nuclide; A - decay constant ~

41

The activity A has the dimension of inverse time (s-I). The unit in the SI system is 1 Bq (becquerel) and is equal to 1 radioactive disintegration per second. The previous unit-of radioactivity still transitionally in use is the curie (Ci) defined as 1 Ci = 3.7.10’’ Bq corresonding approximately to the activity of 1 g of radium *i;Ra. It is very difficult to directly determine the activity A experimentally. As a rule, one usually measures a quantity I = k .A which is directly proportional to the activity A. This quantity I may be the flux of particles passing through the detector, the ionization current etc., and is sometimes termed “relative activity”. The proportionality coefficient k depends on the conditions of the measurement such as the geometrics applied, efficiency and sensitivity of the detector etc. Under definite and unvarying conditions of measurements, the coefficient remains constant. The relation I =f(t) then assumes a form analogous to that of the law of radioactive decay. 1.4.5 Peculiarities in the behaviour of contaminants

Even those objects that show heavy radioactive contamination are likely to carry only very small and chemically hardly detectable quantities of radionuclides. One speaks in this connection of trace amounts, trace concentrations, or simply traces of radioactive substances. Very early in the development of radiochemistry it became obvious that traces of radioactive material behave in a somewhat different way compared to substances that are chemically identical but do not decay, and are present in easily measurable concentrations. For example, itohas been found that solutions of radioactive substances often showed attributes of colloidal solutions, even through some of the characteristic features of colloids, such as for example the Tyndall effect, were absent. Furthermore, radioactive substances exhibited an uncomparably higher adsorption tendency than their non-radioactive analogues. At first, it was the radioactivity itself which was thought to account for those discrepancies. However, it has soon been proved that the main reason is due to strikingly differing concentrations and quantities of the radioactive substances compared to the non-radioactive ones. Thanks to the development of sensitive radiometric methods, radioactive substances could be detected and studied in trace quantities and trace concentrations several orders of magnitude below the limits of quantitative detectability of stable substances by chemical methods available at that time. The radioactivity itself, or more precisely the ionizing radiation, does not affect the behaviour of solutions of -radioactive substances, unless the particle flux density and the energy of the particles are such that the exposure results in radiation-induced chemical changes in the solution, as for example radiolysis. This is however a different category of 42

changes, quite unlike the differences in behaviour described above. Within the range of relatively low flux densities of particulate radiation that are not likely to cause changes through their radiation effect, it holds true that radioactive and non-radioactive substances of the same chemical form behave identically, provided they are present in comparable quantities and comparable concentrations. Because of the negligible concentrations of radioactive substances it is often difficult to perform successfully even such elementary chemical operations as the separation from a solution by precipitation. Although the precipitate may be practically insoluble, one seldom attains the concentration of a saturated solution necessary for the precipitate to separate. Occasionally, such as in the case of solutions of radionuclides Zr and Nb, it is even difficult to transfer a solution from one glass vessel to another. The sorption on the glass wall may be so high that practically no radionuclide is left in the solution. For these and a number of other reasons the chemistry of radioactive substances (radiochemistry) must resort to the use of “carriers”. A carrier is a macro amount of the non-radioactive substance which is added to a trace amount of the radioactive substance and which behaves - at least in certain chemical processes -in a similar way as that particular radioactive substance. Carriers may be either isotopic or non-isotopic. An isotopic carrier is a macro amount of the non-radioactive nuclide of the same chemical element and in a chemical form identical with that of the radionuclide. For example, a macro amount of non-radioactive SrCl, is added to a trace amount of radionuclide ZSr (28 years) in the form of 90SrCI,; this mixture, dissolved in water, can be easily precipitated with sulphuric acid. The resulting precipitate of SrSO, contains practically all the radioactivity, only a negligible fraction remains dissolved. Without the isotopic carrier it would be entirely impossible to reach the limit of the solubility product and thus to precipitate the radioactive %3+.An obvious disadvantage of the use of an isotopic carrier is the fact that it is no longer possible to recover the pure carrier-free radionuclide from the mixture by any of the common chemical separation procedures. In the actual practice of decontamination, one example where radionuclides with isotopic carriers are encountered is the case of radioactive corrosion products present in the primary cooling circuit of a nuclear power plant. These radioactive corrosion products are composed of a bulk of non-radioactive ballast carrier completely pervaded with trace amounts of radionuclides belonging to the same elements and occurring in the same chemical form as those constituting the ballast corrosion scales. On the other hand, uranium fission products may also sometimes contain isotopic carriers and, as in the previous case, perhaps 1 to 3 orders of magnitude in excess of the concentrations of the relevant radionuclides, but these mixtures of radionuclides and their isotopic carriers as a whole 43

still remain in the realm of trace amounts and trace concentrations. Such a mixture is commonly considered as a “carrier-free” radionuclide, or more precisely a “radionuclide-wihout any carrier added”. As a matter of fact, a genuine carrier-free radionuclide is encountered only rarely (examples are Tc, Ra). If an element different from that of the radionuclide in question is used as a carrier, it is said to be non-isotopic. For instance, a macro amount of non-radioactive BaCl, is added to a solution containing ?Sr in the form of ‘?3rC12. Traces of ‘%r can in this case be separated from the solution by precipitation with sulphuric acid together with Ba. The resulting precipitate BaSO, contains almost all the %Sr radionuclide. This type of process is called co-precipitation. In this case, however, the recovery of %r from the precipitate is feasible by suitable chemical separation processes. Both theoretical and practical aspects of the question of how the efficacy of surface decontamination can be modified by an addition of carriers to the decontaminating solutions have been studied by J. Alexa [24]. His conclusions can be summarized as follows: 1. If the addition of a carrier to a decontaminating solution is intended to increase the value of the decontamination factor, the molar concentration of the dissociated anion of the complexing agent present in the same solution must exceed at least twice that of the carrier. 2. A carrier can conveniently be used in all cases where it is necessary to reduce the concentration of other constituents of the decontamination solution, such as organic acids or complexing agents; this may be required to improve the economy, to facilitate the processing of liquid radioactive wastes resulting from the decontamination process, to mitigate the corrosive aggressivity of the solution, or to prevent the harmful effects of the solution on human skin. 3. If the efficiency of a decontaminating solution, particularly its complexing component, is sufficiently high, addition of a carrier to the solution is worthless. The mechanism of co-precipitation can be based either on an isomorphism of the non-isotopic carrier and the relevant chemical form of the radionuclide, or on adsorption of the radionuclide upon the carrier. It is therefore appropriate to distinguish between an isomorphous and an adsorption co-precipitation. The latter is more important in the art of decontamination, because it is frequently utilized when liquid radioactive wastes are to be decontaminated by means of precipitating reagents.

1.4.6 Chemical forms of contaminants Dissolved contaminants (i.e. radionuclides) may assume different chemical forms in the solution: ionic, molecular and colloidal. An ionic form contains

44

only a single type of atoms, namely that of the relevant radionuclide: I3’Cs+, ”Zr4+, I3’I- etc. The term “compound ionic form” indicates that the ion consists of other atoms in addition to an atom of the relevant nuclide, such as (18sW02)2+, (131103)-,[9sZr(OH)i]4-i,where i = 1, 2, 3, 5, 6 (not 4). Similarly, molecular forms are either single molecules, for example I3’I2, or molecular complexes, such as [9SZr(OH),]0,[9’Zr(OH)i(N03)4- i ] O , where i = 1, 2, 3. Colloidal forms of radionuclides may be either true colloids or pseudocolloids. True colloids are formed by relatively insoluble compounds of radionuclides of colloidal character. For example, water solutions of salts of trivalent and multivalent metal radionuclides undergo hydrolysis resulting in the formation of hydroxides of the given metallic radionuclides. The solubility products (see next section) of these hydroxides have infinitesimal values, for example for Y(OHX it is about lo-’,, for Ce(OH), about lo-&, for Zr(OH), about and for Nb(OH), (provided that this formula for niobium hydroxide is correct) as low as lo-”. Consequently, even though the trivalent and multivalent metal radionuclides are present in trace concentrations only, the hydroxides can attain the solubility product limits, and separate from the solutions in forms of imperceptible ultramicroscopic colloidal particles. These particles do not show typical Tyndall effect, because they are too small and too diluted, but can for instance be spun down in an ultracentrifuge. False colloidal forms, the “pseudocolloids”, consist of more complex and more voluminous colloidal particles than are those of true colloids. The pseudocolloidal particle nuclei are not formed by a chemical form of the given radionuclide, but foreign non-radioactive insoluble colloidal impurities function as a kind of a non-isotopic carrier and adsorb on their surface other forms (ionic, molecular, even truly colloidal) of radionuclides. Silicon hydroxide colloid leached from the wall of glass vessels, iron hydroxide colloids leached from the wall of steel pipes and many others are examples of substances that can become pseudocolloidal particle nuclei. Traces of alkali metal radionuclides form exclusively simple univalent cations of the Me+ type. Traces of radionuclides of alkali earths can form not only simple bivalent cations of the MeZ+type, but also pseudocolloids. The tendency to pseudocolloid formation increases here with the proton number of the metal. Traces of radionuclide of trivalent and multivalent metals form a polydispersed water solution which can in principle contain all the described forms of radionuclides in mixture or singularly, where one particular form dominates over the other. It-must be pointed out that chemical forms of radionuclides, and the chemistry in general, are often neglected when dealing with decontamination. Various techniques of radioactivity removal have been studied without due consideration being paid to the chemical form in which the nuclides appeared, 45

or even with no respect at all to what chemical element has actually been dealt with.

I .4.7 Chemical aspects of contamination and decontamination Chemical problems confronted in studies on contamination and decontamination touch upon many facets of problems that are the domain of physical chemistry and nuclear chemistry (radiochemistry). For a better comprehension of the scope of chemical topics related to contamination and decontamination, it seems apropriate to explain the following terms familiar in the two abovementioned branches of chemistry : phase, dispersion, colloid, solubility product, specific surface of solid material, sorption, adhesion, and detergence. Phase A substance or a homogenous mixture of substances sharply delimited from another substance or another homogeneous mixture of different composition form a phase. All gases form a single gaseous phase. Liquids that can mix one with the other only to a limited extent, such as oil and water, form more phases of different composition, for example in the given example, a water phase and an oil phase. Since no two substances exist that would be absolutely unmixable, each phase contains a higher or lower concentration of individual components of all other adjoining phases. Thus, the oil layer above the water contains some water as well as traces of all substances of the vessel wall; similarly the water covered by an oil layer contains some oil and traces of all structural constituents of the vessel. The designation of the phase usually reflects the predominance of a certain component in that particular phase: aqueous phase, oil phase etc. Solid states also form various phases; for example steels and other alloys contain a number of different phases. Dispersion Two distinct substances, A and B, can become mutually dispersed, i.e. dissipated one in the other, for instance by mixing, dissolving, melting etc.; a dispersion is thus formed that is a mixture, solution, melt etc., where the substances A and B are components of the dispersion. If two dispersed components form a single phase, the dispersion is said to be homogeneous; if two or more phases result, the dispersion is heterogeneous. Even a multi-component dispersion can be obtained. In a heterogeneous dispersion A / B , the A component is called the dispersed substance and its concentration prevails in multiple small volumes consisting of one phase which are separated from each other by a single continuous volume of the other phase -the dispersing environment - in which the concentration of the B substance predominates (see Fig. 1.3). 46

a

b

Fig.I.3. Heterogeneous dispersions A/B a - emulsion of oil phase (A) in water (8); b - foam. i.e. dispersion of gaseous phase (A) in water (B)

Two components forming a dispersion may be represented by substances appearing in various physical states, gaseous (G), liquid (L), and solid (S). Accordingly, various types of dispersion may arise; they are listed in Table Z.17. If the dispersed substance A forms dispersions of various kinds in the same dispersing environment, one speaks of a polydispersion system of two components. Similarly, polydispersion systems of more than two components can be TABLE 1.17 CATEGORIES OF DISPERSIONS OF TWO COMPONENTS OCCURRING IN GASEOUS (G), LIQUID (L) OR SOLID (S) PHASES Dispersing environment

Combination of phases

Designation of dispersion homogeneous

GIG L/G

mixture of gases nonexisting

SIG

nonexisting

G/L LIL SIL

tru solution true solution true solution

G

L

'

heterogeneous nonexisting liquid aerosol - mist solid aerosol - fume foam emulsion suspension

homogeneous dispersion of gas in a solid

heterogeneous dispersion of gas in a solid

isomorphous composite crystal, homogeneous alloy and others

heterogeneous composite crystal, heterogeneous alloy and others

47

obtained. If the polydispersing environment is liquid, such a system is called a polydispersion solution. Here the same substance can concurrently be dispersed in the form of ions, molecules and colloids. Detergent solutions and solutions of radionuclides at trace concentrations are such polydispersion solutions. Colloih The constituent particles of a solid state substance (i.e. atoms, ions, molecules) may be arranged in a relatively simple ordered pattern, forming a “crystal lattice”. Such a solid phase is called a crystalline phase. The crystalline phase is usually bounded by plane faces meeting at a certain angle along lines of some symmetry, making up a crystal. In contrast to this, other homogeneous solid phases may have particles of the constituting substance arranged in a more complicated structural pattern (they may be “disordered”). Such phases are designated as colloidal phases or colloids. More detailed studies of colloids disclosed that colloidal phases contained also localities (sites) characterized by a relatively simple arrangement of particles, i.e. localities of crystalline nature; however, other sites exhibit a more complex (“disarranged”) structural pattern, and those are then typical colloidal localities. Thus, it will be seen that no sharp borderline exists between the crystalline and the colloidal phases of solid state matter. Both forms can sometimes be detected simultaneously for the same substance and it often depends solely on the external conditions (i.e. under what conditions the solid phase separates out of the solution) whether crystals or colloids will be formed. Typical examples of crystals known from everyday life are rock-salt crystals, soda crystals, sugar crystals, diamonds etc. Typical examples of commonly known colloidal phases are glue, gelatin, white and yolk of an egg etc. If a colloidal substance is dispersed in a solvent (e.g. fresh white of an egg in water), one can under certain conditions obtain a colloidal (inhomogeneous) solution of relatively low viscosity. Such a colloidal solution is called a sol. Other conditions may cause the viscosity of the solution to increase to such an extent that a jelly like substance is formed by the colloidal solution, i.e. a “gel”. For example, a 2 per cent solution of gelatin in hot water would form a semisolid gel. Upon further increasing the temperature, or adding water under vigorous stirring, the gel will change again to a sol. Addition of a certain amount of an electrolyte (inorganic salt) to the sol results in separation of the dissolved substance in clusters of precipitate. This phenomenon is called the “salting out” of colloids.

Solubility product Substances having a finite solubility in a given solvent will dissolve up to a point where the limiting value of the solubility product, L, is reached. The value 48

of L is characteristic for each particular substance and a given solvent under the given conditions (especially the temperature). The solubility product L of a hydroxide Me(OH), with limited solubility in water is given by the expression:

LMHOH), = [Me"+].[OH-]n

(1.16)

where [Me"+] stands for the number of mols of the element Me per litre of saturated aqueous solution of Me(OH),, in other words its molar concentration. If the ions Me"+ and OH- in the solution are in an equivalent ratio, i.e. the ratio of the numbers of their mols in aqueous solution is excatly 1 :n, the magnitude of the solubility product can be calculated from the concentration of Me(OH), in a saturated solution in water. The solubility of a substance characterizes the ability of that substance to form true solutions in a given solvent; it is expressed as the concentration of a saturated solution of the substance in a particular solvent. Expressing the solubility of Me(OH),, by its molar concentration as [Me(OH),], then it holds that I [Me(OH),J = [Me"+] = - - [OH-] (1.17) n It further holds true that

n. [Me"'] = [OH-]

(1.18)

With substitution of the equation (1.18) in (1.16), the equation reads (1.19)

An increase in the concentration of the hydroxide [Me(OH),,] as such is not the only way to reach the solubility product in a solution; the same effect can be achieved if the concentration of just one or the other ion is increased. Either an addition into the solution of a soluble form of MeX, would be sufficient to increase the cationic concentration, or any added soluble hydroxide, such as NaOH, KOH etc. could raise the concentration of anions. Specific surface of solids The area of a surface which can be determined directly by mechanical geometric measuring instruments is called the geometric surface S, . Some parts of the surface (microscopic cracks, fissures, pores etc.) cannot be measured directly by mechanical geometric tools, but can be assessed by appropriate physical or physicochemical techniques, such as optical (microscopical) methods or the adsorption method. If the application of several precise methods 49

yields results which do not substantially differ from each other, their mean value can be considered to represent the “real surface”, S. The specific surface S,,, of a solid X is the real surface S of a mass unit of the relevant material X,and can thus be expressed by the equation S s,,, = -

mX

(1.20)

where S denotes the real surface of the given quantity of X expressed in suitable units, such as m2, and mx is the weight of the same material X expressed in appropriate units, for instance kg. Sorption

Let us consider two phases in close contact, A and B, that do not mix one with the other (e.g. air and water, water and vessel wall, water with suspended charcoal particles, water and oil, air and charcoal etc.); the boundary where the two phases meet is called the interface S. Let us further assume that the phase A contains another dispersed substance X (e.g. soap dissolved in water or gaseous chlorine diffused in air) and thus a dispersion X’A results. The mobility of X in the dispersing medium A is presumed to be unrestricted (this condition can only be fulfilled if the phase A is represented by a liquid or gas). Near the interface S, phase B (i.e. the vessel wall or charcoal) will generally affect the distribution of both the dispersed matter X and the substance constituting the phase A environment (i.e. both soap and water, or for the second example both gaseous chlorine and molecules of oxygen and nitrogen in the air). As a consequence, the concentrations of the dispersed matter (soap, chlorine) on the one hand and the substance constituting the medium of the phase A (water, components of the air) on the other, will differ near the interface S from the concentrations of the corresponding substances freely extended in the bulk of phase A. This phenomenon is called sorption. Matter X is the sorbed substance or sorbate, phase B - the sorbent and phase A - the environment of the sorption or the sorption medium. If the concentration of the matter in question near the interface S becomes higher than that in the bulk, the phenomenon is called positive sorption, or adsorption, of the given substance on the interface in question. Conversely, if the concentration decreases, the term negative sorption, or desorption, is used. Adsorption of the sorbate X (soap, chlorine) at the interface S must necessarily be accompanied by desorption of the substance constituting the sorption medium A (water, air constituents) at the same interface; by analogy, desorption of the sorbate X must be necessarily associated with concurrent adsorption at the same site of the substance constituting the sorption medium A. 50

The sorption of the sorbate X on an interface S can often be accounted for by unsaturation of the intermolecular van der Waals forces or electric forces exerted across particles (molecules, ions, colloidal particles) of the sorbent B along the interface of phase B and by the resulting influence on particles of the sorbate X.The sorption on an interface S of sorbate X molecules is illustrated in the following figure (Fig.1.4). Molecules of the sorbent attract each other. The attractive forces acting on molecules inside the phase B are reciprocal and cancel each other. Within the boundary layer of the B phase, the forces directed towards the inner layers of the sorbent are counterbalanced and therefore offset by mutually opposed forces of other sorbent molecules, whereas those force components that act in the direction away from the interface are not counterbalanced (not saturated) and can consequently exert an attractive force on the sorbate X molecules dispersed in the phase A.

S

6

Fig. 1.4. Intermolecular forces acting along an interface S (air-charcoal) on particles of sorbate X (chlorine) in air (A) and inside the continuous phase of sorbent B (charcoal)

A similar phenomenon occurs also with ionic sorption (which is involved in “ion exchange”), the only difference being that electric (Coulomb) forces account for the interaction between two ions instead of van der Waals intermolecular attraction. Sorption can in some cases be also caused by chemical bonds (covalent and polar); this type of sorption is called the chemisorption. Somewhat more complex is the nature of the sorption of colloidal particles (micelles); here the detailed mechanism is still far from being satisfactorily explained. It appears from what has been already described that the following types of sorption may be distinguished: - ionic sorption, caused by electric forces; - molecular sorption, due to intermolecular forces; - chemisorption, caused by chemical bonds; and - sorption of colloidal particles which is of a more complicated nature and also is concerned with adhesion.

51

Since the boundary separating two phases is characterized by the exertion of a force, it is in theory possible to for measure any given surface the surface tension 0 at the given interface. The surface tension is defined as the amount of energy acting on a unit area of the interface and is expressed in energy units per area unit, i.e. joule per square meter (J .m-2), or alternatively (which is just another form of the expression) force units along unit length, i.e. newton per meter (N .m-I). More often than not, surface tension concerns the boundary between a given phase and air. If two phases are involved neither of which is the atmospheric phase (air), the term interfacial tension is preferred to surface tension. The extent of adsorption depends primarily on the chemical composition and the relative amounts of substances constituting the phase A, phase B and sorbate X.For constant chemical compositions and amounts of substances A, B and X in the given system, the extent of adsorption depends on the area of the interface S, i.e. on the size of the real surface of the sorbent S. The extent of adsorption a’ is expressed as the weight of the sorbate mx which becomes adsorbed on a unit area of the real surface S of sorbent B: a’ = m x (1.21) S and may be given for instance in kg.m-2. It is often presumed that the real surface of individual unit mass of the sorbent is constant. Under this assumption the real surface S of the sorbent B is proportional to its weight; consequently, the extent of adsorption is (1.22) mB One can see that the adsorption value a is a dimensionless quantity expressing simply the mass ratio of the adsorbed sorbate X and the adsorbing sorbent B. Let us now assume that a concentration c of the sorbate Xis freely extended in a continuous phase A (e.g. in an aqueous solution) and that a sorbent B is added to this system. A fraction of the sorbate becomes gradually adsorbed on the sorbent until an equilibrium is reached in the distribution of the sorbate between the sorbent and the solution. At that point, the extent of adsorption a of the sorbate Xis called the equilibrium adsorption a, and the concentration of the sorbate X in phase A under equilibrium conditions is called the equilibrium concentration c,. The relationship between the equilibrium adsorption a, and equilibrium concentration c, is given by Langmuir ’s equation KCr a, = 1 kc,

+

52

(1.23)

where K and k are constants depending on the type and composition of phases A and B. If the concentration c, is high (c + a), the equilibrium sorption a, then which under the approaches the constant value of maximum adsorption amax given conditions (volumes of the phase A and sorbent B, temperature, pressure etc.) is limited, because the sorbent is saturated. Conversely, if the concentration is very low (c, -P 0), the quantity kc, in the denominator of Langmuir’s equation can be neglected and the equation may be written as a simple direct proportionality between a, and c, a, = Kc, (1.24) In the region of low equilibrium concentrations (c, -P 0) it is even possible to express the adsorption by a quantity independent of the sorbate concentration, i.e. directly by the constant K denoting the slope of the Langmuir’s line for that region K,!! (1.25) Cr

The constant K is called the adsorption coefficient. There is no doubt that sorption can be regarded as the toughest obstacle interfering with successful decontamination of solid surfaces and, conversely, as a significant help facilitating decontamination of liquids and gases. All types of sorption (ionic, molecular, colloidal and chemisorption) play a role in contamination and decontamination. Adsorption studies

If a solid sorbent is immersed in a radionuclide solution, a sample of its unit area surface will emit an ionizing particle flux I depending on time t and characterized by the curve shown in Fig. 1.5. For any time t beyond t, (t > t,), I

0

tr

Fig. 1.5. Adsorption kinetics

t

Particle flux /emitted from a sample of solid sorbentadsorbing a radionuclidefrom its solution as a function of the time of adsorption 1. I, - particle flux corresponding to sorption quilibrium; I, -- time necessary to attain a virtual sorption equilibrium

53

the sorption will have already reached equilibrium and the concomitant particle flux I, remains practically unchanged. Likewise, the equilibrium concentration of the dissolved radionuclide characterized by the particle flux I, emitted from a unit volume of the solution remains virtually constant. Dependence of the equilibrium adsorption I, on the equilibrium concentration Z: is expressed by Langmuir's relation K, . I : I, = 1 + kI;

(1.26)

Since the preceding comments apply to trace concentrations, where 1:-+ 0,

I,

= K,I:

(1.27)

and the adsorption coefficient K,

=! ! l

I:

(1.28)

For rational solution of the problems of radioactive contamination it is important to have reliable information on the dependence of the coefficient K, on the solution's pH, concentration of the element of the dissolved radionuclide, concentration of foreign substances present in the solution, temperature and -if radionuclides of trivalent and multivalent elements are involved -also the age of the solution. Detailed studies dealing with those dependences have been reported by Starik [I] and recently also by Mikhailov [25]. Dependence of the adsorption coeficient of radionuclides on the p H value of the solution for sorption on cation exchanger surfaces A number of surfaces significant in decontamination (plastics, cellulose and

others) possess properties of weak cation exchangers. When simple ionic forms of radionuclides adsorb on cation exchanger surfaces, the coefficient of adsorption for cations increases with rising pH value, whereas the adsorption of anions decreases, a phenomenon which can be explained by their competition with H + and OH- ions. A more complicated picture of the dependence K, =f(pH) is obtained for the adsorption of radionuclides of trivalent and multivalent metals, because the relevant compounds tend to undergo hydrolysis in water solutions and consequently even true colloidal solutions can be involved. The minimum pH value at which a radionuclide Me"+ can form true colloidal solution of its hydroxide Me(OH), is defined by the equation 1 pH = - log LMc(OH), + 14 - -1 - log [Me"+] n n 54

(1.29)

where is the solubility product of the hydroxide of the element of the radionuclide Me(OH), and [Me”+]- the molar concentration of the same element, inclusive of its isotopic carrier. The appropriate curves expressing the dependence K,, =f(pH) show a characteristic maximum. An acceptable explanation has been published by . Mikhailov [25], who corrected the earlier data of Starik [l].

Dependence of the adsorption coeficient on the concentration of the element of the radionuclide In principle, this dependence is characterized by the equation derived by Langmuir (1.23, 1.26). Since the arbitrary choice of the radionuclide concentration is confined to very narrow limits, the limitation is by passed in practice by adding an isotopic carrier. The radionuclide concentration further affects the value of the minimum pH compatible with the existence of a true colloidal solution of the hydroxide as discussed above (equation 1.29). Dependence of the adsorption coeficient on the concentration of simple ions Simple non-complex ions present in a solution can compete with simple ionic forms of the radionuclides. In addition, they may affect the separation of colloids and perhaps even molecular forms of the radionuclide by the “salting out” effect [l]: a rising concentration of electrolytes increases the tendency of colloidal forms of radionuclides to separate out of the solution. Desorption studies Desorption is quantitatively expressed by the coefficient of desorption Kd, which is the weight ratio of the remaining fraction of the adsorbed sorbate after desorption to the amount of the same sorbate prior to desorption. As the particle fluxes are directly proportional to the radionuclide masses, then (1.30) where I,,is the particle flux emitted by a sorbent sample at time t , measured from the onset of desorption, and I. is the initial particle flux emitted by the same sorbent before desorption. The “per cent desorption” is simply % Kd = 100. Kd (yo)

(1.31)

Reversibility of sorption A sorption process is reversible if it proceeds simultaneously in both directions, i.e. if adsorption and desorption take place concurrently. Upon reaching the sorption equilibrium, the adsorption and desorption rates of the same 55

sorbate are equal. In principle, ionic and molecular sorption are reversible processes, whereas colloidal sorption is irreversible. In practice, however, the sorption processes are complicated by diffusion of the sorbate into the pores and other irregularities of a highly rugged sorbent surface, penetration through semipermeable cell membranes, and through a compact surface film etc. It is therefore preferable to speak only of a higher or lower grade of sorption reversibility. To test the sorption reversibility, it is convenient to measure the rate of desorption in a solution which lacks the radionuclide, but has otherwise the same composition as that in which the adsorption of the radionuclide took place. When the sorption equilibrium is reached, a coefficient &+ 1 characterizes reversible sorption, while & < 1 signifies that the sorption is irreversible. When dealing with irreversible sorption, it is illogical to talk about reaching a sorption or desorption equilibrium. It is therefore customary to select a certain conventional value of the time t known empirically to be sufficient for reaching a practically stationary particle flux, and then to use this time interval as an arbitrary duration of desorption tests. The sorption irreversibility does not in itself necessarily mean that the sorbate (the radionuclide) cannot be desorbed. Another desorption medium must be applied to bring about desorption.

Eflect of simple ions on desorption As already explained in the discussion on adsorption, simple (non-complex) ions can, on the one hand, compete with ionic forms of radionuclides in the course of reversible sorption and thus increase the rate of desorption from the surface of cation exchanger sorbents and, on the other, salt out colloidal hydrolytic forms and perhaps also molecular forms of nuclides of trivalent and multivalent metals, thus inhibiting their desorption. Eflect of the p H on desorption As a rule, hydrogen ions H + increase the desorption rates by several mechanisms: They compete in the course of reversible sorption with simple cationic forms for sites on cation exchanger surfaces. Further, at a certain pH value they can obstruct hydrolysis and dissolve the colloidal forms. Finally, they may reduce the sorption capacity of the sorbent. Eflect of complexing ions on desorption Complexing agents capable of forming stable soluble complex ions and molecules with radionuclides generally increase the desorption of trace amounts of radionuclides. It is essential for effective decontamination that the formation of these complexes take place in a sufficiently short time. The complexes increase 56

the extent of desorption for two reasons: First, they usually have large-size molecules and are therefore only slightly attracted to the sorbent; and secondly, many complexing agents are capable of dissolving hydrolytic and colloidal forms of radionuclides, and of forming stable soluble complexes. Adhesion Two phases (phase A and phase B ) which are in contact but which do not mix one with the other attract each other (stick to each other). This phenomenon is called adhesion. Forces of two kinds underlie the phenomenon of adhesion: intermolecular van der Waals forces and electrostatic Coulomb forces. Intermolecular forces at adhesion Van der Waals intermolecular attractive forces are exerted among all molecules inside each phase, and between molecules along the interface of the two phases that are in cotnact. Near the phase boundary, these forces account for the soption of molecules of some substances, such as polyvalent radionuclide molecules on activated charcoal. Intermolecular forces are the main cause of adhesion between two liquid phases and between a liquid and a solid phase. Adhesion between two liquid phases Let us assume‘that a continuous volume of one liquid phase A (e.g. water) is freely extended in air in weightless state. Unless other external forces affect this volume, the intermolecular forces would force this liquid to assume a spherical shape. Let us further assume that in addition to the volume of phase A another spherical volume of liquid phase B not mixing with the former one exists in air under the same conditions of weightlessness and that the two phases are brought into contact. A contact area will be formed and the phase will attract each other. If the contact area is of unit size (1 m’), the work needed to overcome the adhesive force exerted on this unit area is the work of adhesion W,. If one knows the values of the surface tension oAYbetween the phase A and the between the two liquid phases air V , aBYbetween the phase Band the air and aAB (“interface tension”), one can calculate the work of adhesion according to the equation

(1.32) W , = OAAY + OEF’ - O A E The interface tension aAB in the eqution has a negative sign, because the

contact area between the phases A and B disappears as soon as the work of adhesion W is consumed and is replaced by other surfaces formed anew: one between the phase A and the air V with the corresponding surface tension nAv, the other between the phase B and the air V with a surface tension oBY. 57

Adhesion between a liquid and a solid phase

Let us assume a single drop of a liquid phase A resting on a support, i.e on the surface of a solid phase B. If one knows the values of the surface tension oAv and the wetting angle Q (see Fig. 2.6), the work of adhesion between the liquid phase A and the solid phase B can be calculated by using the equation

w,= b A V . (1 - COSQ)

(1.33)

The wetting angle Q is the maximum angle between the tangent of the drop’s curvature and the plane of the support at the meeting point of the three phases; its construction is illustrated in Fig. 2.6. As long as the angle is acute, the surface of the phase B in relation to the liquid A is said to be lyophilic. On the other hand, an obtuse angle characterizes a lyophobic surface. The smaller the wetting angle, the greater is the wetting effect of the liquid A on the solid phase B surface. A good wetting effect is due to the fact that the attractivity among particles (e.g. molecules) of different phases (A, B) is greater that that exerted among particles of the same liquid-phase A.

B

B

Fig. 1.6. Wetting angle

Q,

Phases: A - liquid., B - solid, V - air

Electrostatic forces a t adhesion It is much more difficult to offer a simple explanation of adhesion between two solid phases, for instance between the layer of a dried adhesive (glue, paint etc.) and a solid support. It has been shown convincingly [26] that electrostatic forces play a role here in addition to intermolecular forces, and that the former may be decisive in a number of cases. Macromolecules of strongly adhesive substances - the “adhesives” - are strongly polar. Polar substances have asymmetrical molecules and consequently a positive and a negative pole of their electrostatic charges, whereas in non-polar substances the positive and negative poles fuse in one point because of the symmetry of their molecules. An illustrative example of an adhesive is the macromolecule of the polyester resin originating from a copolymerization of glycerol with phthalic acid anhydride. It is used as a paint, and therefore needs to be strongly adhesive. Parts of the macromolecule containing the -C6H,- residue have a marked tendency to yield some of the negative electrons to those parts of the macromolecule in which the oxygen 58

atoms are located. As a consequence, a chain macromolecule is formed composed of parts with alternating electric charges (see Fig. 2.7). Depending on their chemical composition, adjoining phases also have a tendency either to attract electrons from the surface of the neighbouring phase or to repel them. Due to this tendency, polar molecules present in one of the two phases along the phase boundary become adsorbed in an oriented way (Fig. 1.7)

+

+

+

+

+

+

b

+

+

+

+

+

+

Fig. 1.7. Oriented adsorption of polar molecules a - simple molecules, b

~

macromolecules consisting of polar parts with alternating electric charges

and thus produce an electric charge of a certain sign on the surface of their own phase. This charge then induces an equivalent electric charge of an opposite sign on the surface of the other phase in contact. In this way, two charged layers of mutually opposite signs are generated -the electric double luyer. The two layers attract each other. Strongly adhesive polymers (glues, paints etc.) harden in time if spread on a surface. Any attempt to pull the adhering layer off its bed must overcome not only the intermolecular adhesive forces, but mainly the electric forces. If the adhesive layer is stripped off at a very high speed, the electric charges of the two layers are not given enough time to “drift away” (to become discharged gradually in the air). On the other hand, a very slow separation of the two phases makes discharging possible and the electric forces do not then contribute to the resistance to separation. The values of the work of adhesion W determinend by a direct measurement of the resistance which must be overcome when slowly removing an adhering film from a solid bed, decreases to the same order of magnitude as that which is characteristic for adhesion between two liquid phases or a solid and a liquid phase, i.e. for situations in which intermolecular van der Waals forces are exclusively operative and electric forces play no role whatsoever. The dependence of the work of adhesion on the speed of the removal of an adhesive film from a solid phase bed is graphically expressed in Fig. 2.8. The figure shows that if the film is stripped off at a rate of tenths of cm.s-’ and greater, electric forces are the main cause of adhesion 59

which makes the nitrocellulose film stick to a glass surface; quantitatively, they surpass the van der Waals intermolecular forces by about three orders of magnitude. If the speed of film removal, falls below 0.01 cm .s-', the gradual discharging begins to be increasingly felt, until finally at a speed of 10-4cm. s-' it is complete and the electric forces add practically nothing at all to the resistance. Only the van der Waals intermolecular forces must be overcome when a film adhering to a glass surface is stripped offat such a very low speed.

L . . . . . . . -6

-5 -4

-3

-2

-1

0

log v i c m d

Fig. 1.8. Dependence of the work of adhesion Won the speed Y with which a sticking film of adhesive

(nitrocellulose) is stripped off a solid (glass) surface

All conditions that modify the air conductivity and thus the speed at which the electric charges are balanced out can change the work of adhesion: air pressure, humidity, ionization density etc. The magnitude of the work of adhesion reaches its peak in vacuum. With a rising air pressure the work of adhesion decreases. An increasing air humidity likewise tends to reduce the work of adhesion, although only up to a certain limit. Very close to the level of absolute saturation of the air with water vapours, capillary condensation of water can take place in tiny crevices between the supporting bed and the adhering solid phase, and the work of adhesion can again increase. Undoubtedly, ionization of the air due to irradiation is also likely to reduce the work of adhesion [26]. Taking into account the area density of electric charges in the double layer, a,,Deryagin et al. [26] introduced a method of calculation enabling then to quantify the work of adhesion W, for an adhesive film sticking to a solid supporting bed : E . AE a, =*(1.34) 4x.h where E is the permissivity of the medium separating the double layer (i.e. air) measured in farads per meter (F .m-I), h is the distance between the two layers 60

in the double layer (usually of the same order of magnitude as the size of molecules) and A E is the difference of electric potential between the two layers measured.in volts (V).The area density of the electric charge a, is expressed in Coulombs per square meter (C . m-2). The work of adhesion W , is equal to the amount of work necessary to separate the layers of a double layer to infinity. As known from electrostatics, such work is equal to a.AE (1.35) w,= 9 L thus E. A ~ E w,= (1.36) 8nh A qualitatively similar situation is likely to occur if radioactive fallout adheres to surface of hardened paints or to any other solid material. Yet, due to the fact that once the polymers solidify, the polar macromolecules of an adhesive paint can hardly become orientedly adsorbed on deposited particulate radioactive fallout, the resulting adhesion will be quantitatively much weaker. It cannot be excluded that even a simple contact of two solid bodies results in adhesion and that also here both intermolecular and electric forces are operative to a smaller or greater extent. However, apart from a restricted mobility of the molecules of solids, several obstacles may prevent a sufficiently intimate contact of two solid phases, such as the unevenness and other structural irregularities of the contact surfaces, adsorption of foreign substances (even air molecules) or adhesion of dust particles on the contact area between the two phases.

Detergence The detergence, or cleansing process, is a complex phenomenon of colloidal and chemical nature based on the sorption of surface active agents (surfactants) at the boundary of a liquid phase. The process consists of three consecutive steps : - Wetting the soiled surface and the impurities with the cleaning solution; - dispersing the impurities in the solution (formation of emulsions and suspensions); and - stabilizing the dispersed impurities in the solution. Surfactants (tensides) tend to become sorbed at the boundary between a solid and a liquid phase as a consequence of two specific features of their molecules. The molecules consist of two parts: The lyophilic part is characterized by bonds and often even, a composition similar that which that characterizes the solvent in which the surfactant is dispersed. The characteristics of 61

both the bonds and the composition of the lyophobic part differ substantially from those prevailing in the solvent. Whereas the lyophilic part tends to be retained within the solvent phase, a distinct tendency to leaving the solvent phase is a characteristic feature of the lyophobic part. These two antagonistic tendencies cause the molecules, ions and colloidal micelles of a surfactant to become adsorbed at the phase boundary in such a way that the lyophilic parts point inward and the lyophobic parts are directed towards the outside of the phase (oriented adsorption). At the interface with a gaseous phase (fluid level) the lyophobic parts of the molecules even protrude out of the liquid phase into the air, as schematically depicted in Fig.Z.9.

(0-) of a tenside at the boundary between a liquid phase (solution) and another phase (air, vessel wall and vessel bottom). The lyophilic part of a micelle, molecule or ion is represented by a circle (0). the lyophobic part by a attached to the circle segment (-) Fig. 1.9. Oriented adsorption of micelles, molecules or ions

As long as water is the dispersing medium for a surfactant (and this is so in a great majority of cases), one speaks of a hydrophilic and a hydrophobic part of the surfactant molecules (or ions). The hydrophilic part is generally polar, often even ionic, while the hydrophobic part of the molecule is rather less polar (with bonds of a prevalently covalent nature).

a

b

Fig. 1.10. Colloidal micelles composed of molecules or ions of a tenside in solution a - spherical micellc. b - lamellar micelle

62

At higher concentrations of surface active agents when the interfaces between the solution and the adjoining phases are fully occupied with adsorbed molecules or ions, the lyophilic groups of these molecules and ions tend to shield their lyophobic parts from the surrounding solution. As a consequence, two kinds of colloidal micelles are formed: spherical and lamellar (Fig. 1.10).Spherical micelles are composed of a number of molecules or ions that ran into the thousands. In the detergent process, colloidal micelles behave in a way similar to that described for surfactant molecules and ions, since they are similarly adsorbed on the boundary between the solution and the adjacent phases. The same symbol (0-) is therefore used to represent colloidal micelles in the schematic illustrations. The appearance of micelles in detergent solutions is desirable, as they usually enhance the cleansing effect. Wetting Surface active agents adsorbed on the boundary between a solution and an adjoining phase cause a substantial decrease in surface tension at the interface. This occurs even when the concentration of the surfactant is relatively low (as low-as 0.1 or even 0.01 %). With some oversimplification, the surface active agents are sometimes defined as substances capable of strongly reducing the surface tension even if present at very low concentrations. As an example, water under normal conditions has a surface tension relative to air amounting to 730 pJ .m-*. Surfactants reduce not only the surface tension at the water-air interface, but also the interphasic tension at the boundaries with all solids or with fat and other impurity particles which are in contact with the solution. Because the wetting angle Q, in the equation (1.33) is reduced, all surfaces and all attached impurities become more thoroughly wetted when contacted with a surfactant solution. The wetting of liquid and solid impurities is schematically outlined in Fig. 1.11.

Fig. f . f f . Wetting of solid (A) and liquid (B) impurities by a surfactant solution

63

Good wetting is a necessary, but not in itself a sufficient prerequisite of detergence (cleansing). Diethylether has a surface tension relative to air only 170 pJ .cm-') consequently, it is a much more efficient wetting agent than a solution of 1 g sodium oikate per 1 litre water (which has a surface tension relative to air of 250 pJ .m-*). Yet the latter solution is superior to diethylether in removing many diethylether-insoluble impurities (such as graphite or clay dirt) from fabrics. Ethanol with its surface tension relative to air of 220 pJ .m-' is capable of wetting greasy deposits on cloth somewhat better than the sodium oleate water solution, yet the oleate is superior to ethanol in removing grease from textile materials, the latter being in fact entirely ineffective. It can be inferred from what has just been said that the cleansing process depends not merely on wetting (which incidentally can be well defined quantitatively by the wetting angle or the reduction in interphasic tension), but also on other processes induced by the adsorption of surface active agents, particularly the formation and stabilization of impurity dispersions in the solution. The last mentioned processes cannot be simply quantified by any known physicochemical quantity. It is necessary to determine the total detergent effect achieved by a certain solution under the given conditions empirically, such as by washing standard samples of fabrics in the same washing machine using a standardized procedure. F o r m a t i o n of i m p u r i t y d i s p e r s i o n s in t h e c l e a n s i n g solution Ions, molecules and micelles become adsorbed anywhere on the interface between the solution and any adjoining phase. They penetrate through the narrowest interspace separating the impurity particles and the solid surfaces which are in contact with the solution, they force their way even into fissures and pores of rugged solid impurity particles. By doing so, they impart to the impurities a certain amount of energy sufficient to set free small particles from the soiled surface or to break down larger particles into smaller ones. Dispersions (suspensions) of solid impurities are produced in this way. The energy transmitted by ions, molecules and micelles of tensides may, however, be insufficient to release larger solid particles and to set loose liquid impurity particles from soiled surfaces, and some additional mechanical stimulus or movement of the solution may be necessary to accomplish this by using a scrubbing-board or a scrub-brush, jet-sprays, rotation of the washing drum, sonic or ultrasonic vibrations etc. Merely a momentary and relatively weak impulse (such as sound) may be sufficient to detach the impurity particle and disperse it. Foam formation is also a kind of dispersion process. Foam is a dispersion of a gas in a solution. The degree of foaming power and the stability of foam used to be considered a chief criterion of the cleaning effect. However, many

64

detergents with negligible foaming power are known, yet their cleansing efficiency may be substantial. The mechanism of foam production is illustrated in Fig. 1.12.

Fig. 1.12. Foam bubbles in a surfactant solution

S t a b i l i z a t i o n of i m p u r i t y d i s p e r s i o n s in t h e cleansing solution Once the impurities are dispersed in the cleansing solution (in the form of an emulsion or a suspension), ions, molecules and micelles of the surfactant become largely adsorbed on the dispersed particles as well as on the surfaces being cleaned, and in this way isolate them from each other as well as from the solution. This process is important, as it prevents an agglomeration (clustering) of tiny particles into larger bodies and greatly reduces their sedimentation rate and redeposition on the surfaces that have just been cleaned. The stabilization of impurity dispersions is further assisted by a continuing breakdown of dispersed impurities into still smaller particles due to the persisting action of the surfactants and of some additional factors - the pep t i z a t i o n. Classification of cleansing agents Cleansing agents may be characterized either by their main function in the detergent process (wetting agents, dispersing agents, foaming agents, emulsifiers, stabilizers) or by the mechanism of their effect. Some agents, like fat soaps, alkylsulphates and other substances are good detergents in themselves, since their wetting, dispersing and stabilizing properties are equally good. Other reactants, particularly some of the specialized detergents (see below) have only one particular pronounced effect, and must therefore be combined or supplemented with auxiliary reagents to achieve the desirable cleaning action. A detergent can thus be defined as a system of chemical agents having a good cleansing (detergent) efficiency. 65

As to the mechanism of their action, the following types of detergents can be distinguished : 1. Surface active agents, or tensides; 2. Solid emulsifiers; 3. Protective colloids; 4. Alkaline degreasers. Among the most commonly used tensides are fat soaps and saponifiers. Some surface active agents dissociate in water into ions and are therefore said to be ionogenic; those that do not, are said to be non-ionogenic. The ionogenic surfactants can be either anion-active or cation-active, depending on whether the anion or the cation, respectively, is the carrier of their surface activity. The following types of anion-actiue surfactants are among the more common agents: - Fat soaps with a general formula R . COO-Me+, where R is a hydrocarbon radical of acids resulting from hydrolyzing natural fats, mainly stearic, palmitic and oleic acids, and where Me+ is the sodium, potassium or ammonium cation (Na+, K+, NI-I:). The hydrocarbon radical R represents the hydrophobic part of the surface active anion, whereas the polar group 4 0 0 - is hydrophilic. The Me+ cation is not by itself surface active, but its alkalinity contributes to the cleaning effect of the anion. - Alkylsulphates of the general formula R . 0 . SO;Me+, where R is the alkyl radical with a chain of 10-17 carbon atoms. - Alkylarylsulphonates of the general formula R,. R, . SOyMe+, where R, is the alkyl radical, R, is the benzene, diphenyl, naphthalene or similar residue. - Alkylsulphonates of the general formula R . SO;Me+. Anion active surfactants are at present the most important chemical cleansers, distinguished by a strong cleaning effect in an alkaline environment. Among the cation-activesurfactants, the following representatives are noteworthy:

- alkylammonium halogenides of the general formula [R .NHJ'X-, X- is a chloride or bromide ion; - alkylmonoethylammonium halogenides [R .NH, ,C,H,]+X-; - alkyldiethylammonium halogenides [R .NH(C,H,)J+X- ; - alkyltriethylammonium halogenides [R.N(C,H,),]+X- ; - alkylpyridiniumhalogenides.

66

where

Under customary conditions (neutral and alkaline environment), the cation-active agents have no cleaning effect at all. However, they are efficient detergents in a strongly acidic environment; this may be of particular significance for decontamination. Representatives of the non-ionogenic surface active agents are substances of the type R, . R,.CH,. CH,. O(CH, .CH,. 0)”.CH,. CH,. OH where R, is usually an alkyl group, R, may stand for 6, -S-, -CH,NH-, 40-NH-, -C,H,--O-, -Cl0H6-Oetc., and n is the degree of polymerization (usually below 10). The alkyl radical R, is the hydrophobic part, whereas the chain containing the -CH, . CH, . 0- group is hydrophilic. Water molecules become attached by means of hydrogen bonds to the oxygen atom in the chain. Next to the anion-active agents, these substances are the second significant group of detergents exhibiting a very strong cleansing action. Solid emulsifiers This term designates substances which facilitate the formation of dispersions of solid substances in a cleaning solution. The dispersed particles are in the form of flat tiny platelets whose one side is somewhat more hydrophilic than the other due to its crystalline structure. As a consequence, the platelets have a tendency to accumulate at the interfaces between the solution and the adjoining phases, in particular at the boundary with oily impurities. They become oriented so that their hydrophilic side faces the solution and the more hydrophobic side is turned towards the oily or other impurity phase. Both the impurity particles and the surfaces being cleaned thus become “armored” against agglutination and redeposition. Solid emulsifiers do not wet, neither do they by themselves form emulsions or suspensions of impurities in the solution. This must be done by applying intense mechanical impulses (scrubbing). The solid emulsifiers just stabilize the once generated emulsions of oil or grease impurities in the washing solution. An example of a solid emulsifier is a particular component of fat soaps, sodium stearate; a major part of it remains undissolved at temperatures below 293 K. It does not therefore act as a surfactant in the form of ions, molecules or colloidal micelles, but rather as a less efficient solid emulsifier. This is one of the reasons why higher temperatures increase the cleaning efficiency of soaps. The most important representatives of solid emulsifiers are washing powders, clays and ground minerals often used to facilitate washing dishes soiled with grease. The most efficient washing minerals are rnontmoril[onites, appearing also in bentonites. Ordinary loams, clays and silts also exhibit a certain degree

67

of stabilizing effect, though much weaker than that of the above-mentioned minerals. It is clear that the cleansing efficiency of solid emulsifiers is far inferior to the detergent action of soaps and saponifiers. Protective colloids Some colloidal substances used either separately or preferably as additives to composite detergent agents also have a stabilizing effect on emulsions and suspensions formed in a cleaning solution. Starch, used otherwise for linen and cloth stiffening, and polysaccharides, such as carboxymethyfcelfufose(CMC), are frequently added as stabilizers to powdered detergents. Also other adhesives, such as glue or gum arabic, in some cases even mineral colloids such as silicic acid gel can be used as protective colloids. Like the solid emulsifiers, protective colloids as such have a low cleaning effect and their significance is merely as detergent additives. Alkaline d e g r e a s i n g a g e n t s Cleaning of greasy surfaces of objects made of metal, ceramics, enamel or other material can also be achieved by means of inorganic alkaline degreasers. With increasing alkalinity of the water solution, the interphasic tension between the grease and the water decreases, and consequently the wetting of greasy surfaces improves. The grease must be emulsified in water by applying an intense mechanical impulse (scrubbing, brushing etc.). Some alkaline substances, such as caustic soda or soda ash do not sufficiently stabilize the produced emulsion, while other compounds, such as silicates, become hydrolysed to colloidal substances (silicic acid gel) which - acting as protective colloids stabilize the emulsion. This is the main reason why silicates as degreases are superior to stronger alkalies like NaOH. The most potent inorganic alkaline degreasers are the alkali metasilicates, for example sodium metusilicate Na2Si0, . 9 H20. Comparable in efficiency are alkaline silicates, such as trisodium orthosilicate Na3HSi04.SH,O, water glasses Na20.xSi0, (x = 1 to 4), sodium orthosilicate Na4Si04and other silicon-base agents, further various phosphate-base compounds, e.g. alkaline phosphates, such as polyphosphates, in particular the for example sodium “hexametaphosphate ” and sodium tripofyphosphate;also other sodium metaphosphates, sodium pyrophosphate Na4&0,. 10H,O, and alkaline orthophosphates, such as Na3P04.12H,O. Soda ash and caustic soda are also used, though their degreasing effect is less. In some cases, particularly in the. absence of any stronger mechanical impulse (running water, scrubbing) the alkaline degreasers are preferably used in combination with detergents; the removal of greasy impurities is thus substantially improved. 68

1.5 Standardization of experimental methods in studies on contamination and decontamination of solid phase surfaces No internationally recognized binding system of standards applicable to decontamination has as yet been adopted; moreover, quite a number of countries still lack even a pertinent national norm. However, with an ever growing urgency the practical problems require a unification of techniques; methods and equipment used when testing and assessing solid surfaces as to their contaminability with radioactive substances and the ease of their decontamination. In 1986, the IS0 (International Organization for Standardization) submitted for consideration by its Member States a draft proposal of an international norm (TC85 -nuclear energy ISO/DIS8690) entitled : Decontamination of radioactively contaminated surfaces -Methods for testing and assessing the ease of decontamination [27]. The following aspects ought to be taken into account, if the norm is to be of a real value in practice: .a) First of all, it must be clear what is the basic intention of a normative regulation, i.e. for what field and to what kind of users the norm will serve, what are its ultimate objectives and what real contribution to standard practice is expected from its adoption. In our own view, the adoption of an international standard would make it possible: - to objectively assess materials (in particular construction and engineering material, materials used for manufacturing protective devices, protective clothing, cover paints etc.), to compare and classify various materials as to the ease of their contamination and decontamination with respect to a reference contaminant; - to objectively compare the properties of various decontaminants and the efficiency of decontamination methods and techniques using a (standardized) reference surface and a reference contaminant. b) It appears highly desirable that a unified terminology be agreed on in order to prevent any misunderstanding and subjective interpretation of the meaning of technical terms. c) The actual performance of the contaminability and decontaminability tests ought to be such that the data provide complete adequate information, including particulars, on - the tested specimen (characteristics, size, number, processing prior to testing etc.); - technical facilities employed in the test; - nuclear equipment used (radiation detection principle), nature of the measured quantity, sensitivity and the required accuracy and precision of the measurements; 69

properties of the reference contaminant: If the test contaminant is in the form of an aerosol, the following additional data would be relevant: characteristics of both the dispersing environment and the dispersed phase (per cent ratio of individual components), and the activity concentration. If the test contaminant is in the form of a solution, the data that would be useful include the identification of the radionuclide (or composition of the mixture of radionuclides), specific activity, the kind of solvent, the pH value, electrolyte concentration, conductivity, duration of the contact with the surface, environmental temperature, and conditions of the test contamination (whether static or dynamic). Additional data may also be of use, such as the surface properties of vessels used for storing the radionuclide solutions, surface properties of the experimental system, specification of the equipment and chemicals used etc. - characteristics of the decontamination agent (composition, concentration, temperature, pH), duration of contact, method of decontamination (under static or dynamic conditions) and other. d) Auxiliary data concerning the-dosimetry may occasionally be required, such as the background radiation, correction of the measured values for natural decay, the minimum number of measurements required (number of samples) etc. e) Basic instructions for evaluation of the results and the statistical methods preferred indication of the reproducibility of the results in view of the total error of the measuring method employed. -

1.5.1 Terminology

Surface - any outer face which is solid at ambient temperature. The basic dimension is the area, the unit is (m2). Radioactive substance - a substance containing atoms undergoing spontaneous disintegration accompanied by a change in the composition or energetic state of the atomic nucleus. Contaminant - a radioactive substance undesirably dispersed in the environment and constituting a direct or potential hazard to human health. Surface contamination - a process whereby a contaminant comes into contact with a surface resulting in a bond formation between the surface and the contaminant. Contaminability of a surface - characterized by the strength of the bond between the surface and the contaminant, and by the capacity of the surface to adsorb a given contaminant. It is expressed as the coefficient of contamination or the adhesion coefficient. Coeficient of contamination (K,) -the ratio between the specific activity of

70

a liquid contaminant at the onset of contamination and the surface area activity at the end of the contamination process. Adhesion coeficient (KO&)- the ratio between the specific activity of a solid contaminant at the onset of contamination and the surface activity at the end of the contamination process. Surface decontamination -a process resulting in a removal of the contaminant from the contaminated surface. It can be carried out as a dry process, i.e. under dry conditions, or a wet process, i.e. by means of solution or the solvent itself, usually water. Decontaminability of a surface - characterized by the degree to which a contaminant can be released from the bond with the solid phase surface, in other words by the degree of the removal of a given contaminant from the surface. It is quantitatively expressed as the decontamination factor. Decontamination factor (OF)- defined as the ratio of activities (or count frequencies or dose rates) measured prior to decontamination and after it. 1.5.2 Test methods for assessing the surface contamination with radioactive substances Tests for evaluating the contamination of a solid phase surface with radioactive substances are according to different whether they involve contamination with radioactive solids and aerosols (“dry state” contamination), or with radioactive liquids, i.e. solution of radionuclides in a water or liquid non-water environment (“wet state” contamination). Solid surface can be contaminated with radioactive solutions either under static or under dynamic conditions. Dynamic tests make use of two alternative arrangements: the radioactive solution is made to circulate around, or to flow past, an anchored sample of the tested surface; or a free or fixed sample is made to move or vibrate while submersed in a solution of the contaminant. 1.5.2.1 Test methods for the “dry state” contamination of solid surface Principle: The solid phase surface is exposed for a specified time interval to radioactive aerosol of a single radionuclide or a mixture of radionuclides with known specific activity and a defined degree of dispersion in a certain dispersing medium. The weight of the radioactive dust used to produce the aerosol must exceed by at least an order of magnitude the weight of the dust fraction retained by the tested surface. This requisite has to be verified empirically with inactive dust particles and by employing any of the conventional analytical methods, such as gravimetry. At the end of the selected contact time interval, the surface radioactivity (in cpm) of the sample is determined. The sedimentation rate of the 71

given aerosol must be taken into account and the time needed for complete sedimentation (calculated or empirically assessed) must be added. The ratio of the area activity of the contaminated sample A, to the specific activity of the contaminant (dust) A, gives the adhesion coefficient KO& (1.37)

or (1.38)

The drawback of this method is the difficulty in achieving a reproducible surface contamination level. To this end, several preconditions must be strictly observed : constant degree of dispersion of the contaminating solid; constant weight and specific activity of the contaminant; identical technique (and conditions) for the contamination, best accomplished by means of an automated device; homogeneity of the contaminated surface, including its identical pretreatment; standardized geometrics of all radioactivity measurements.

1.5.2.2 Test methods of contamination of solid surfaces by means of a solution under static conditions Principle: The solid surface is exposed for a specified time interval to a direct contact with the solution of a contaminant (a radionuclide or a mixture of radionuclides) having a specified volume with known activity concentration, pH value and temperature. The sample is then removed from the solution, rinsed repeatedly with a given volume of water or other fluid and left to dry. The area radioactivity of the sample (in cpm) is determined.

1.5.2.3 Test methods of contamination of surfaces by a solution under dynamic conditions Principle: This method does not differ basically from that described for static conditions, except that the solution driven by a pump moves in a circuit at a given flow rate around the usually anchored sample, or alternatively a specified volume of the solution flows past the sample driven by a hydrostatic pressure due to the difference in height of the fluid level in the stock container and the test vessel. Repro'ducible results can be obtained with this method provided that the following rules are carefully observed: 72

The solution volume is such that the samples are fully submersed; the activity concentration and the length of the contact time are sufficient for a sorption equilibrium to be attained between the radioactivity in the solution and that on the tested surface. - A constant pH value is maintained throughout the entire duration of the contact between the surface and the solution. - The surface of all samples is pretreated in a standard reproducible manner (e.g. removal of grease, rinsing with an acid or alkaline solution etc.). - Multiple samples are arranged carefully in the test vessel so as to avoid overlapping or touching each other. - After the test contamination the samples are washed in a standardized way to get rid of the unbound fraction of the contaminant. A common standard procedure is three rinses with approximately the same volume of 96% ethanol supplemented with 1 YOgasoline or ether. - All radioactivity measurements (both the activity concentration in the solution and the area activity of the solid samples) are performed under identical geometrics. The experimentally estimated degree of the contaminability of materials is quantitatively characterized by the magnitude of the coefficient of contamination K, (see p. 70) calculated as the ratio of the area activity (in cpm) of the tested sample A, measured at the end of the test interval of time to the initial activity concentration (in cpm) of the contaminating solution A, : -

A

K, = 2 or

(1.39)

A0

1.5.3 Decontamination of surfaces contaminated with radioactive substances By analogy with the contaminability test methods, procedures used for the assessment of decontamination are likewise classified as dry and wet. “Wet state” decontamination can be assessed by means of a decontaminant dissolved in polar or non-polar solvents. Either static or dynamic conditions may again be chosen for the test. Moving the samples in the decontaminating solution, for example by shaking, is to be regarded as one of the procedures carried out under dynamic conditions. Experimental methods assessing the decontaminability do not differ in principle from those used in contamination studies, at least as far as the wet methods are concerned. In this case, however, the tested (contaminated) surface 73

is brought into contact with the chosen decontaminating solution of a standard composition and with standard properties. Otherwise all procedural steps, devices and equipment are identical. The degree of the decontaminability of tested surfaces is quantitatively characterized by the magnitude of the coefficient of decontamination, &, calculated as the ratio of the area activity (dose rate, cpm) before and after decontamination ( A , and A d , respectively): (1.41)

or (1.42)

In contrast to the methods for assessing the contaminability, it may be difficult in some cases to classify a particular decontamination method unequivocally as a dry or a wet procedure, especially if various emulsions, foams or pastes are applied, because their water content can seldom be defined. The prerequisite of a good reproducibility of the obtained results is that the same rules are observed as those mentioned in subsection 1.5.2.3, except that the requirements specified for the contaminating solution (concerning the temperature, pH etc.) apply here to the decontaminating solution. 1.5.4 General rules for the standardization of testing the contaminability and decontaminability of surfaces

Samples: The selection of samples must be representative. The geometric shape of the sample ought to be such that it facilitates an easy control of the process and a dependable measurement. Its size should preferably be smaller than the size of the effective area of the detector. Those parts of the surface area that are not to be evaluated (such as side walls of porous materials) should be excluded from taking part in the processes, in order to avoid any undue interference with a precise evaluation. The simplest way is to cover the surfaces to be eliminated with parafin, epoxy resin or some other inert material. Careful removal of grease from the surface to be tested must precede the test; this can be done by means of aqueous solutions of detergents, alkalies or organic solvents. Only materials stable in the solutions used in the assay may be accepted for testing. The minimum number ofreplicate samples to be assessed in any single test is six. Detection of ionizing radiation: The choice of the detector depends on the properties of the contaminant, in particular the type and energy of its radiation, 74

on the radioactivity level and other parameters. It is not important what variable is actually determined: it may be the relative or absolute radioactivity, dose rate, exposure rate, count rate (cpm) or frequency of disintegrations (dpm). The objective is always to achieve the highest possible sensitivity of detection. The sensitivity threshold of the measurement depends on the characteristics of the radiation detector, the radiation source and the geometrics of the detection system. With high activities (count frequencies) it may be necessary - depending upon the nature of the detector and the method of evaluation - for the results to be corrected on the “dead time” of the counter. The background count must always be determined and deducted from the measured value. It is desirable to keep the background level as low as possible, because it is the limiting factor for the working range of the detector; it determines the lower limit of detectable radioactivity and affects the magnitude of the counting error. Contaminant: The contaminant may be a single radionuclide, an exactly defined mixture of radionuclides, a mixture of fission products, or any operational contaminant of known composition. The choice of particular radionuclides depends on the objective of the test. The simplest model acceptable for most cases seems to be a mixture of fission products containing the following set of radionuclides in trace concentrations (without an addition of isotopic carriers): ”Sr, I3’I, 134Cs (or I3’Cs) and 14’Ce.If the contaminant is a solution, it is necessary to define its composition, the pH value, the electrolyte specification and concentration, the conductivity, temperature and activity concentration. The radioactivity of the contaminant must be appropriate to the aim of the test, yet the basic rule is to employ only a minimum necessary amount of radioactivity limited by the threshold of detectability by the method chosen. The most widely used solvent is deionized water characterized by the values of its conductivity and pH. Since the pH value also affects the state of trace concentrations of radionuclides in aqueous solutions, the reaction of the environment must be adjusted to the exact desired level by means of solutions of acids or alkalies (analytical grade). Even though the effect of temperature on the degree of surface contamination and decontamination is usually small, it seems sensible to maintain the environmental temperature at a constant level. It is however essential that each particular test be preceded by a new exact determination of the activity concentration in the contaminating solution, because this often changes with time as a result of varying adsorption of radionuclides on the vessel walls. Duration of the contamination and decontamination processes: It can be safely assumed that it takes approximately 1 hour under static conditions and at a temperature of about 298 K before dynamic equilibrium is attained between a contaminating solution and a tested surface, or between a contaminated surface and a decontaminant solution. It thus appears adequate to restrict the 75

duration of the tests to 1 hour, except in those cases when the assay is intended to provide information of the dependence of adsorption or desorption on time. Under dynamic conditions, less than 1 hour is usually sufficient to reach an equilibrium state. Surfaces of vessels and technological systems: Vessels, implements and equipment used in the described tests are made of various materials differing in chemical composition. The composition is largely the determining factor for the formation of a certain type of bonds with radionuclides. In some other cases, however, the effect of chemical composition may be less important than the effect of the surface roughness. It is essential that the vessels and all devices used throughout the test be made of materials that do not interact with the radionuclide solution, that do not become highly contaminated and that can be safely decontaminated (good examples are polyethylene, polytetrafluoroethylene, polypropylene and similar materials). It is appropriate to “saturate” the surface with inactive solutions of the same composition prior to the experiment for about 1 - 6 hours. All fittings and joints connecting individual parts of the equipment system must be dry, without vaseline, fat or lubricant of any kind Decontaminating solution : By far the most commonly used decontaminating agent is water. It is therefore appropriate to perform in all cases a decontamination test with deionized (distilled) water as a comparative experiment. Reproducibility of the tests is improved if the pH and the conductivity of the water are standardized. A use of decontaminating solutions uniform in chemical composition is preferable and helps in comparing the decontaminability of a standard surface contaminated with different contaminants, except for special cases and special practical objectives. The following formula is recommended as a widely applicable solution : - 0.7 wt. YO disodium EDTA (analytical grade) - 0.1 wt. % anion-active tenside in water; the pH value is adjusted to 6.0 f 0.5 by means of 0.01 NHCl and the solution is used in the temperature range 295-298 K. Evaluation of results: Student’s t-test at the level of 5 YO significance is recommended as an adequate method of statistical treatment of data obtained in a minimum of 6 separate tests for each assay. The results are best presented by giving the arithmetical mean and the t-product of the standard error. Dixon’s test is used to eliminate extreme values. If more than one result is to be rejected from a series of 6, the assay ought to be repeated. 1.5.5 Evaluation of the eficiency of ‘decontaminationprocedures

In addition to the proposed decontamination coefficient, there are also other convenient quantitative indicators of the efficiency of a given deconta76

mination agent or decontamination method; these are residual contamination, degree of removal, decontamination factor, and Tompkin’s decontamination index. Residual contamination Z’ denotes the fraction of the initial radioactivity A, remaining on the decontaminated surface: Z’

-

= Ad

(1.43)

A0

or Z’=’d

(1.44)

10

A,, and A , being the activities measured on the surface at the end and the beginning, respectively, of the decontamination process and I, and I, being the corresponding count frequencies. The degree of removal U‘ expresses the proportion of the radioactivity taken off by the decontamination procedure

U‘= 1 -Z‘

(1.45)

U ’ = 1 --1 (1.46) DF The decontamination factor DF denotes the quotient of the reduction of the

initial radioactivity (of a surface, volume etc.)

(1.47) or (1.48)

and is in fact a reciprocal value of the residual contamination

DF=

1

(1.49)

Tompkin’s decontamination index Diis the base ten logarithm of the decontamination factor (1.50)

or (1.51)

77

1.6 Surfaces of solid decontaminated materials and sorbents prevention of contamination and facilitation of decontamination The best decontaminationis no decontamination. It is much better, rationally and beforehand, to prevent contamination than to have to resort to risky, laborious and costly processes of decontamination. High costs of the decontamination procedures can be substantially reduced by prudent, premeditated prophylactic measures. Several examples of can be briefly outlined. Solid objects likely to become contaminated should be made of materials that can be easily decontaminated. The outer surfaces are treated with impervious protective coatings, strippable paints or protective layers of surfactants to make decontamination easy and efficient to the highest possible extent. Water in the primary circuit of nuclear power plants is often supplemented with additives intended to reduce the contamination of inner surfaces, and conversely to make decontamination of the coolant easier. The decontaminability is also one of the main criteria which decides what sorbent to choose for filling the filters used in the coolant’s decontamination. It foll-ows from this outline that a good knowledge of the surface properties of solid materials is required to rationally prevent contamination and to facilitate efficient decontamination. This concerns primarily some physical efficient attributes, in particular the surface structure, and the chemistry of pertinent surfaces. Pressing problems in the field of contamination and decontamination are mainly connected with physical, physicochemical and chemical interactions between the contaminant and the surface. This applies not only to solid phase surfaces, but partially also to decontamination of water and other fluids. Smooth materials The surface of untreated metals, glass, paintings covering the surfaces of operational equipment and technological parts made of miscellaneous material, plastics, personal protection equipment, rubber etc. have seemingly a compact, smooth outer face. In reality, however, these surfaces show a definite degree of unevenness, though very fine, as can be observed under a microscope as the microrelief of the surface. Its specific surface depends on the surface roughness. The smaller the surface roughness, the finer the microrelief. A certain degree of porosity is a characteristic feature of rubber, plastics and protective paintings. All in all, however, the specific surface of the listed materials is small compared to other categories of materials; consequently, their decontamination is easier. Penetration of radioactive solutions into the depth of materials of this category can for all practical purposes be disregarded. Porous materials Natural fibres and some synthetic fibres (cotton, wool, nylon etc.), leather

78

and human skin and similr materials are characterized by a considerable degree of porosity. It has to be expected, therefore, that contamination and decontamination (particularly if wet procedures are involved) will result in a spreading of the radicactive substances to a certain depth, though not as deep as in the case of coarse-grained materials. Porous materials are too fine in texture to be decontaminated by removing the upper layer. Coarse-grained materials The surfaces of clay, sand, rock and the vegetation covering the land, as well as unpainted wood, are so rough that deep contamination with radioactive substances (dry or wet) must always be presumed. Decontamination is effected by removing the outer contaminated layer to a depth which depends on the contamination intensity, depth of penetration, and the characteristics of the surface. 1.6.1 Chemical composition of decontaminated materials

In the course of contamination and decontamination, mainly if the processes take place in the presence of water, radionuclides often react chemically with the solid surfaces. Depending on the the nature of the interaction between the radionuclide and the surface, materials can be divided in three distinct groups: materials with restricted reactivity, materials with ion exchange properties, and metals. 1.6.1.1 Materials with restricted reactivity These materials are characterized by a very low degree of chemical interaction with the given chemical form of radionuclides in water environment, in decontaminating solutions and in solvents. The properties of such materials are particularly amenable to decontamination procedures for a number of reasons : a) They usually have a very compact, non-porous and smooth surface; b) They are hydrophobic and may even be oleophobic (showing a low grade of wetting with water and oil, respectively), a quality which reduces the contact of the sorbent with the fluid and hence lowers the amount of the adsorbed radioactivity; c) They have few dissociated and chemically reactive groups, a fact which again diminishes the capacity for ionic sorption or chemisorption. Materials of this type are preferred whenever there is a risk of higher Contamination, particularly by means of radioactive solutions. They are used as laboratory bench covers, as construction and engineering materials in radiochemica1 and radiometric laboratories, surface coatings of nuclear measuring in79

struments etc. The most important representative is teflon and, to a somewhat lesser extent, also other fluorinated hydrocarbon polymers, namely polyethylene and polystyrene. Only molecular or colloidal forms of radionuclides are likely to become adsorbed on those plastic substances, but even so to a substantially limited extent, because of their small Specific surface. Ionic sorption is virtually excluded. Teflon Teflon is chemically a polytetrafluoroethylene with the formula - ( C F 2 b . Its macromolecule is linear without any branching. Coatings and linings made of teflon offer the best protection of solid phases against radioactive contamination. Teflon vessels are the containers of choice for all kinds of laboratory operations involving unsealed radionuclides in the form of solutions and powders. They are not suitable for exact volumetric measurement and for temperatures above 473 K because teflon undergoes thermal decomposition. Another favourable aspect important for the prevention of contamination and for easy decontamination is the fact that the surface properties make teflon not only hydrophobic, but oleophobic as well. As a consequence, fabrics made of teflon fibres and used to manufacture exposed parts of clothing such as cuffs or collars repel oily impurities, resist soiling and can be easily cleaned, often simply by a stream of water. However, their other hygienic qualities of teflon fabrics are far from being ideal. Po l y e t h y l en e Polyethylene (PE) is a macromolecular polymer of the type - ( C H 2 b where the degree of polymerization may vary quite considerably. The molecular weight of the chain molecule mostly lies between 12000 and 22000. When exposed to higher temperatures (> 353 K) in the presence of air, a slow gradual oxidation of PE results in its destruction and polarization of molecules. It seems likely that this may explain why ionic forms of some radionuclides (Cs’, S;Z+ and other) may become adsorbed on PE. Another possible explanation of this phenomenon is the fact that apart from the inert methyl groups, reactive -CH==CH, double bond-containing vinyl groups may form the end groups of PE macromolecules. Moreover, the low density PE macromolecules sometimes ramify and thus contain an asymmetrical (and hence to some extent polar) carbon atom in the chain: ...-CHflH--CH,-. .. The assymetrical carbon atom may also be the cause of adsorption of some forms of radionuclides in trace amounts. The low density PE cannot therefore be classified as an absolutely inert material. Still, this substance is otherwise considered as “virtually non-polar” and is accepted as a good electric insulator. Resistance to aggressive chemicals and great tear strength are properties which make PE the material of 80

choice for manufacturing squeeze bottles, many types of vessels, containers, tubes and hoses. Utensils and connecting tubing made of PE are now commonplace in chemical and radiochemical laboratories for all types of work with unsealed radionuclides in the form of solutions and powders. PE sheeting is often used in radiation protection of both personnel and equipment, aiming particularly at preventing direct radioactive contamination. Polypropylene Polypropylene is a macromolecular polymer of the type +CHdgfwith molecular weight of about 10OOO. It is very light (density 0.9 kg .l-’) and makes a good insulator. It is a material of great toughness and its tensile strength is high. Since the melting point is much above 373 K, it can be sterilized by heat. Polypropylene fibres are strong, take up very little water (only about 0.5 %) and are stable at higher temperatures. Additives as well as oxidation may give rise to strongly polar or dissociated groups. Because the marked hydrophobity makes the fibres hygienically less suitable (they do not absorb sweat), hydrophilic surfaces are often “grafted” on the polypropylene threads. This grafted hydrophilic phase may be acrylonitrile, methylmethacrylate and other compounds. Both the physical and chemical properties of the surfaces of grafted fibres change. Polystyrene Polystyrene, known also under its alternative name polyvinylbenzene, has the chemical formula - ( C H 2 T H ) , , - . Its molecules are linear. Temperature

0

above 358 K cause polystyrene product to become deformed; heat sterilization is therefore impossible. It is used for manufacturing various moulded containers, vessels, bottles and household utensils.

1.6.1.2 Materials with a low ion exchange capacity Substances of this category cannot be regarded as ion exchangers for macroscopic amounts of ions. Ion exchange processes taking place on their surfaces can only involve trace quantities of compounds; yet, this property may be of special significance for sorption of radionuclides in the absence of any 81

macroamounts of isotopic carriers. Cation exchange can take place on several functional groups ( - CO O H , --OH, -SH etc.) irrespective of whether the groups appear in the core or the end part of the macromolecule. The commonly-cited chemical formula of the appropriate macromolecule usually does not show any carboxyl group; this group frequently originates from subsequent oxidation or decomposition of the macromolecule. Anion exchange can take place on basic functional groups (=NH, EN) present in the macromolecules of many polymers. Some representatives of this category possess both acid and basic groups in the same molecule, hence both cationic and anionic sorption of traces of radionuclides can take place on their surfaces. Some of the materials characterized by low ion exchange capacity are particularly important with respect to decontaminability, and their relevant properties will be briefly described in this context. Further, the properties of some porous substances belonging to this group, such as cellulosic and proteinaceous materials, may explain why decontamination of textile and paper materials is so fraught with technical problems. Above all, however, it is glass which finds a wide application in chemical and radiochemical laboratories as the most common constituent material of vessels, appliances and equipment. Quartz glass Chemically the simplest type of glass, it is in fact solidified molten silica, i.e. silicon dioxide SO,. The crystalline lattice of quartz consists of regular tetrahedrons with a silicon atom placed in the centre and four oxygen atoms attached one to each comer. When molten, the structure of SiO, becomes disordered and individual tetrahedrons are irregular. On the surface and on broken faces there appear free bonds which can bind metal ions present in solutions. The active groups formed -Si--O-; -Si--OH; -Si-OM give to quartz glass the characteristic of an ion exchanger (M =metal). O t h e r t y p e s of g l a s s Apart from silicon dioxide, normal glass usually contains also other oxides of alkaline metals and lead (Na', K+, Ca2+,Pb2+). In addition to the above mentioned active groups, the following groups may also be present on glass surfaces : Si-OM'; -?i-,

where M' is for example Na, K, and M" is Ca, Pb or other divalent elements. Acids and water wash out alkaline substances from the glass surface and give rise to a thin film of hydrated silicon dioxide, silica gel, covering the surface. 82

Silica gel has a large specific surface and, since it contains active groups, possesses a relatively large capacity to adsorb radionuclides in trace amounts. Contrary to acids, alkalies are capable of cleaving the Si--O-Si bonds changing them to ==Si&M groups. Silicic acid gel is leached in small quantities from the glass surface and passes to water or water solutions. This is one of the reasons why pseudocolloids can form in solutions of trace concentrations of radionuclides kept in glass vessels. It explains also why used glass has a substantially greater sorption capacity than glass which has not yet been in use.

c el l u 1o s i c

m a t e r'i a 1s Cellulose is a polymer consisting of a cellobiose chain, the most frequent single element of which has a formula shown in Fig.1.13. The degree of polymerization, i.e. the number of repeating elements in a cellobiose chain, may be up to 2000. Cellulose has a formal molecular weight, i.e. the molecular weight of a cellulose monomer (a single member of the C,H,,O, cellobiose chain) equal to 162. Cellulosic materials also contain a certain number of carboxyl groups attached to the polymeric chain in a way illustrated in Fig.1.14. The carboxyl CH20H

j(FXH 0-.

OH

...o-

H

Fig. 1.13. A single element of the cellobiose chain

CH, OH

Fig. 1.14. Possible location of carboxyl groups in cellobiose chain elements

groups confer on cellulose the properties of a weak cation exchanger. In pure water the cellulose surface has a negative electrokinetic potential The value of this potential depends on the composition of the solution and especially on the pH value. The curve expressing the cellulose electrokinetic potential as a function of the pH value of the solution is shown in Fig.1.15. It indicates that

c.

a3

at a pH less than 2 cellulose in water solution is practically free of any electic charge. Within this region the cellulose thus lacks the properties of a cation exchanger. Actual values of the electrokinetic potential differ for various types of cellulosic material (cotton, paper etc.). The sorption capacity of cellulose is also a function of its chemical form (H', Na+, K+, Ca2+etc.).

- 30 - 20 - 10

0

2

4

6

8

1

0

1

2

~

~

Fig. 1.15. Electrokinetic potential 5 of cellulose aqueous solution as a function of the pH value

Towards strong alkalies, cellulose behaves as a weak cation exchanger. Reaction with the hydroxide group corresponds to equilibrium expressed by the equation CelL-OH MOH e C e l - O M + H,O (1.52)

+

Alkalies partially dissolve cellulose. The degree of its solubility in alkalies depends on the type of cellulose fibres. Acids, especially mineral acids, cause a profound decomposition of cellulose macromolecules. Oxidizers may also cause depolymerization, but they primarily induce cellulose oxidation which for the most part results in a decrease in the mechanical strength of cellulose fibres much more pronounced than that caused by the effect of strong acids and alkalies. Cellulose is strongly hygroscopic. The capacity of fibres to absorb moisture is due to the abundance of free hydroxyl groups, particularly those that are located in the amorphous regions, outside the crystalloidal structures. When cellulose fibres are placed in a detergent water solution, the emulsified and suspended detergent particles cannot at first permeate the intercellular spaces because of a discrepancy between the size of the particles and the diameter of the channels. However, as water readily passes through, it causes swelling and widening of the intercellular channels, making it thus possible for the detergent to penetrate. Still, most of the colloidal chemical detergent processes take place 84

rather on the surface of microfibrils as well as in submicroscopical channels of larger size. Cotton Cotton fibres rank among the most important textile raw materials. Fabrics made of cotton are frequently used for manufacturing outer clothing and underwear. Even though cotton fibres are far from ideal with regard to decontaminability, they cannot as yet be substituted by any other material including synthetic fibres, because all the substitutes are markedly inferior in their hygienic properties in that they infavourably affect the physiological functions of human skin. Paper Filter paper, particularly the “low in ash” grade used in analytical chemistry, is essentially pure cellulose with the chemically and mechanically slightly modified structure of natural cellulose. In most of the common type papers, cellulose appears as a cation exchanger partly in the form of Ca2+,due to contact with calcium solutions during the production process. Paper materials cannot be decontaminated by any wet procedure; the chemical composition of paper is therefore of no relevance for decontamination. C a r bo x y m e t h y 1 cell u 1o se (CMC) CMC is an acid ether derivative of cellulose known mainly in the form of its sodium salt with --O.CH,.COONa groups. In this form it is used as a thickening agent in textile additives and in cosmetics, and also as a protective colloid in various suspension and emulsion formulations. Wood This is a hard fibrous substance consisting to a large extent also of cellulose. Apart from the cellulose, wood contains lignin, minerals and other substances. Decontamination procedures for unpainted wooden surface by means of wet methods are largely ineffective, because radioactive solutions readily spread through the wood to a considerable depth and become strongly adsorbed on the coarse surface. Wooden surfaces are more effectively decontaminated by mechanical procedures, preferably dry, either with or without removing the surface layer of the wood itself (by means of grinding, planing etc.). The chemical composition of wood is therefore irrelevant to decontamination practice. Proteinaceous materials Proteins are macromolecular substances which under the effects of mineral acids or enzymes undergo hydrolysis. The breakdown proceeds ultimately to a 85

complex of amino acids of a general formula R. CH(NH,),. COOH where R is the carbohydrate residue. Amino acids are amphoteric, being capable of forming salts with both acids and bases. The protein molecules are composed of simple amino acid residues which are mutually interconnected by bonds between their carboxyl and amino groups

...- N H . CH(R). CO. N H . CH(R’). CO. N H . CH(R”). CO-

...

Particular types of proteins differ primarily in the carbohydrate residues R and their mutual position in the protein macromolecule. The (=NH) group is basic, and it can bind a water molecule, forming thus =NH$-OH-. The hydroxyl group can be exchanged for other anions. Proteins therefore have the properties of an anion exchanger. On the other hand, a certain number of carboxyl end groups -COOH may be present in a protein macromolecule, both in the main polypeptide chain and in the side chains (R, R’, R ) . Thus, a protein macromolecule may at the same time exhibit the function of a cation exchanger also. Both the type of the protein and the surrounding environment (solution), in particular its pH value, determine whether the a nion or cation exchange effect will prevail in a given protein. In a strongly acid environment protein normally exhibit a positive charge, whereas a negative charge predominates in an alkaline environment. The following proteinaceous materials are relevant to the topics dealt with in this book: sheep wool, leather and live skin. Sheep wool Sheep wool is made up of fibres consisting of linear aggregates of keratin characterized by a covering of minute projections of scales (subcutis) encircling the cortex composed of short spindle-like cells, “fibrils”. Coarser fibres have a distinct matrix, i.e. larger cells in the core of the fibre. In its natural state wool fleece is richly saturated with fatty impurities (suint, yolk etc.); those are almost completely removed by the tanning process. Keratin is a sulphur-containing protein of a chemical formula which is shown in Fig.ZJ6. The main polypeptide chains of keratin are interconnected as well as salt bonds joining by means of disulfide bonds of the type -S-Sthe adjacent side chains. Like the cellulose fibers, keratin also has the properties of a colloid. Beside the amorphous (non-crystalloidal) regions, it also contains crystalline regions. As to their structure, wool fibres consist of micelles and are pervaded with submicroscopical channels and intermieellar spaces. Wool fibres contain some water taken up from humid air. In the temperature range between 283-353 K wool adsorbs more moisture from the atmosphere than cotton and natural or artificial silk fibres, as can be seen from

86

CO CHR NH

co

disulphide bonds

CH 10.

2

4

6

8 9 1 0 l2

pH

Fig. 1.21. Dependence of steel corrosion rate 0,. on the pH value of still (1) and flowing (2) aqueous solution

1.6.1.4.3.2 Effect of oxygen in water on the rate of metal corrosion

This effect varies with changing concentrations of other components of the solution. If CI- ions are present, the corrosion rate is directly proportional to the oxygen concentration in water. In the absence of aggressive ions, notably C1-, for instance in high purity demineralized water, the relationship between the corrosion rate 0,. and the oxygen water concentration c,, is fairly complex (see Fig.2.22). ,As can be seen from the graph, there are two regions where the vc

80 60

40 20 =0

0

100

200

300

p g . i’

Fig. 1.22. Corrosion ra :u,. of iron in water of high purity as a function of oxygen water concer ration c,

corrosion rate is low: one lies at concentrations below 10 pg O2per litre of water, the other at concentrations exceeding 250 pg per litre. This is the reason why under standard conditions the oxygen concentration in feed water of primary cooling circuits is kept below 10 pg..I-’. On the contrary, if the operating conditions require the presence of oxygen, its concentration is kept close to 0.2-0.3 mg .1-’. A necessary prerequisite, however, is a minimum concentration of all other impurities, especially C1- ions. 96

1.6.1.4.3.3 Effect of salts on metal corrosion rate in water

Salts, such as NaCI. affect the oxygen solubility in water. The maximum amount of oxygen dissolves in a 3 wt. % NaCl aqueous solution. This is also the critical concentration of NaCl corresponding to the maximum rate of iron corrosion, as can be deduced from Fig.2.23.

-

I I

3 5

Fig. 1.23. Steel corrosion rate

P,

20 010

10

‘Naa

as a function of sodium chloride water concentration cNaC,

Salts of strong acids and weak alkalies hydrolyse in water to form acid products; the presence of such salts therefore augments the corrosion rate. It applies particularly to chlorides of Al, Ni, Mn, NH, and especially Fe3+.On the other hand, salts of strong alkalies and weak acids (Na,PO,, Na2B40,, Na2Si0, and other) hydrolyse to form alkaline solutions and inhibit the corrosion rate. 1.6.1.4.3.4 Effect of temperature on metal corrosion in water

In closed systems, such as the primary cooling circuits of nuclear power plants, the cooling water does not become degassed and the corrosion rate is directly proportional to its temperature. In open systems operating under normal pressure, the same proportionality of corrosion rate to temperature holds true for temperatures up to about 353 K. However, the closer the temperature approaches the boiling point, the greater the inhibition of the corrosion rate because of an increasing effect of degassing, until in boiling water corrosion stops completely (see Fig. 2.24). A qualitatively similar pattern of the dependence is obtained for other pressures.

80

loo

t l O C

Fig. 1.24. Steel corrosion rate in water u,. as a function of temperature rocat atmospheric pressure

97

I h.1.4.4 Types of corrosive attack Uniform corrosion, ile. an equal loss of material over the entire area, can only appear if the same physical and chemical conditions prevail over the boundary between the metal and the corrodent. The probability of anodic and cathodic reactions must be identical for any point on the metal surface. An example of uniform corrosion is the loss of metal caused by the presence of carbon dioxide in water. All other types of corrosion discussed below are non uniform. Pitting corrosion is a very dangerous type of corrosion caused by oxygen depolarization. In a relatively short time, it can result in total penetration of quite thick metal walls, even though the average metal loss may be very small. The depth of the pits is usually greater than their width. The rate ud of the pitting attack is expressed as the maximum depth of a pit for a certain time interval. Another approach is to compare the rate vdwith the rate u,. of uniform metal loss at the same site and for the same time interval by means of the “pitting factor” f, vd

(1.57) vc Pitting corrosion attacks stainless steel in the presence of C1-, Fe3+,Cu2+, as well as copper and aluminium. It occurs also in those sites in which the protective oxide film on the surface of materials is systematically removed or damaged by mechanical factors (erosion, friction, cavitation etc.). Erosion corrosion results from rapid streaming of water in the system. Flowing water removes the protective layer of oxides and the bare metal is exposed to the corrosive attack. Fretting is a type of enhanced corrosion occurring at the contact area of two solids (at least one of them being metal) slightly moving against each other and thus rubbing off the surface layer of oxides in a manner similar to that of the erosion corrosion. Cavitation corrosion is caused by cavitations, i.e. tiny implosions of bubbles originating in a layer of liquid which is directly in contact touch with a rapidly moving metal part, such as revolving parts of pumps and water turbines. The implosions exert immense local pressures at high temperatures, attaining the order of magnitude, respectively, of 102MPa and 104K. As a consequence, a number of pits are formed, often arranged in a lattice pattern consisting of narrow relatively long fissures. Crevice corrosion appears predominantly in crevices, as the crevice is insufficiently flushed with water. Chloride ions greatly increase this type of corrosion. Oxygen depolarization uses up all the oxygen in the crevices, the metal dissolves and its cations accumulate in the narrow space. To compensate for the increased

f,=-

positive charge, anions from the environment, particularly CI-, diffuse into the crevice. Since the metal compounds hydrolyse, acidity in the crevices increases. Higher concentrations of H+, C1- and other anions upset the surface oxide film and facilitate corrosion. Pits, channels, tunnels etc. are formed as a consequence. Wastage - local thinning of the outer side of metal tube walls. This type of damage attacks tubes made of nickel alloy Inconel 600, austenitic stainless steel 304 (U.S.A.) and Incoloy 800 alloy, and occurs in systems operating in phosphate regimens in localities with a sluggish water flow. It has been often observed localized above the horizontal tube plate of vertically situated steam generators, where the water flow rate is low and phosphate deposits accumulate, and also in crevices between the tubes and the bored distance plates. Denting attacks steam generators in nuclear power plants with PWR, and occurs between the tubes made of Inconel 600 or other alloys and the bored distance plates made of carbon steel. A relatively rapid corrosion process in the crevice gives rise to a magnetite layer deposited on the inner side of the distance plate hole. Since magnetite is about twice as voluminous as the steel from which it originated, the deposits completely fill up the crevices and compress the tubes, often causing visible deformations. It has been experimentally verified that acid chloride is the most likely corrosive agent for crevices [28]. As a result of denting, the outside diameter of tubes decreases, the distance plate holes are widened and deformed, resulting in a S.C. “hour glass deformation”, expansion of distance elements, leakage of tubes and deformation of the steam generator casing [29]. Selective corrosion occurs in multicomponent alloys with greatly differing phase activities. One particular structural or chemical component of the alloy is preferentially attacked. An example is a selective loss of zinc from brass condenser tubes. The corroded tubes retain their original shape and the damage is not readily apparent, but their ductility and strength may be seriously impaired, the material becomes brittle and permeable to the corrosive solution. Intercrystalline corrosion is due to an increased corrosive activity on the plane faces of neighbouring crystalline particles. The attack results in loosening the crystals and in subsequent crumbling. The process usually has a rapid progression and the lesions may be very deep, leading to grave deterioration of mechanical properties of the attacked metal. Faulty technology in heat processing of the metal often accounts for this type of corrosion. Corrosive cracks and breaks occur primarily with austenitic steels and alloys, and are caused by a concurrent action of corrosive environment and mechanical stress. It is also called stress corrosion cracking. The cracks are either intercrystalline or transcrystalline; in the latter case, the cracks split the crystal granules apart. The same material placed in the same corrosive environment though not subjected to any stress usually does not crack. However, even without cracking, stress often enhances the rate of the corrosion process, giving

99

rise to srress corrosion which does not stop even if the stress has ceased to act. Practically all construction metals are susceptible to stress corrosion cracking, as long as the environment is corrosive. to at least one (less noble) component of the alloy (usually the base metal). Since however the corrosion cracking is restricted just to certain specified environments and requires stress of considerable intensity, it is possible to practically eliminate the risk by careful advance testing of the construction materials. 1.6.1.4.5 Protection against corrosion

Rational effective measures of corrosion control must necessarily take into account the actual cause of the corrosive process. In principle, protection against corrosion can be accomplished in two ways: 1. By reducing the thermodynamic instability of the system, i.e. by selecting more resistant construction materials or their coatings, creating an environment which has a lower affinity towards the metal, or inducing in the metal a potential which falls within the passivity region. 2. By reducing the rate of the corrosion process, for instance by the use of materials and coatings characterized by a slower progression of the anodic reaction, by addition to the environment of substances inhibiting the corrosive attack, or by induction in the metal of a potential which lies in the passivity region. In practice, the following measures can be taken: - to apply a surface protection to the material; - to change the composition of the corrosive environment; and - to convert the metal potential to that which is more suitable. The possibility of finding materials appropriate for an operation and at the same time resistant to corrosion without any protection at all is restricted just to a certain specific environment. No material exists which would be suitable under all conditions. 1.6.1.4.5.1 Surface protection

This consists, for example in applying a surface layer of a nobler metal or alloy by means of plating, filling, air blasting, metal spraying, gas-phase reaction etc. Alternatively, the metal is coated with a plastic or rubber layer, cemented or painted, or provided with inorganic coatings: layers of oxides, phosphates or chromates (surface protection can also be achieved by means of a thin layer of oil or Vaseline, but this method of metal protection is inappropriate for decontamination.) The environment can be modified in a desired direction by choosing the parameters of temperature, pressure, flow rate and water oxygen content in such 100

a way that the oxidative potential is diminished and the reaction driving force decreases. Modifications of the electrode potential can be induced in two ways: by anodic or cathodic protection. 1.6.1.4.5.2 Protective layers of oxides on metal surfaces Iron and steel surfaces are always covered with oxide layers. On carbon steels, the oxide layer that forms spontaneously under common atmospheric conditions is usually porous, inhomogeneous and lacking strength. Salts often concentrate in porous layers and enhance the corrosion rate. Such porous layers do not provide sufficient anticorrosive protection. An ideal protective oxide layer must be compact over the entire surface area, nonporous, homogeneous and firm. A film which forms on carbon steel in steam is likely to be composed of several oxides in the following succession : Fe/FeO/Fe304/Fez03/vapourized H20. In liquid water, magnetite (Fe304)prevails, whereas the amount of Fe,O, formed is negligible. The Fez+ions diffuse across the film in the direction from the metal into the water, while oxygen from the water diffuses in the opposite direction and reaches the metal after it passes through the film. Low temperatures favour the generation of Fe(OH), . At temperatures higher than about 423 K, Schikor’s reaction gives rise to a continuous film of magnetite which provides a good protection to the metal. The reaction rate increases with rising temperature. Hydrogen is liberated by Schikor’s reaction :

3 Fe(OH),

-,

Fe,04

+ 2 H 2 0 + H,

(1.58)

After a long period of operation, hydrogen may be generated as a result of a reaction of water vapour with the metal according to the general reaction xM+yH,O

+

M,O,.+yH,

(1.59)

3Fe+4H20

--t

Fe304+4H2

( I .60)

or specifically The protective layer consists of two components: the outer one which is porous, and the inner one compact. To offer good protection, the covering film must not form on a clean metal surface. On the surface of clean metal, the protective layer forms only slowly and does not adhere firmly to the metal phase. Water in steam generators often contains Fe2+ and Mn2+ ions, mostly in the form of- hydrated oxides, i.e. polymeric Fe(OH), and MnOOH. These may generate porous deposits on those sites that are exposed to high thermal fluxes, substantially impairing the heat transfer and accelerating corrosion. Serious damage to steam generator tubing systems has been observed after only 101

1000-2000 hours of operation, if the heat flux density across the tube walls, g, amounted to 800 kW. m-’ and the iron concentration in water, cFe,exceeded 20 pg. 1-1. Stainless steels in contact with water become coated with an inner compact film sticking firmly to the metal, and covered at the outer side by a less adhering deposited layer. The following sequence of oxides can be found layered one on top of the other in the inside compact film: metal, Cr203,NiCr20,, FeCr,O,, Ni,Fe, - ‘i - ,Cr,O,. The last oxide in the row represents the transition layer between the inner and the outer deposits; the outer layer is made up of NiFeO, and Fe30,. The Fe30, is the common surface oxide in primary circuits of pressure water reactors. In primary circuits of boiling water reactors, the magnetite is overlain with an additional layer of Fe203, and occasionally also FeOOH [30, 311. A similar situation obviously occurs in secondary circuits of PWR in those locations where the metal is in direct contact with water vapour. Loosely bound deposits of iron and other metal oxides result from the elimination of dispersions suspended in water and their deposition on the heat exchanger walls. Salts concentrated in the pores accelerate the corrosive attack, and the wall surface under the deposit may be seriously damaged. Oxide films on the inner surfaces of the primary circuit tubing systems of PWR and BWR made of austenitic steel (AISI 304,321) were analyzed by means of scanning electron microscopy. X-ray microscopy and Auger’s electron microscopy. The results show that the surface films have a stratified structure. The outer layer is composed of large, prominent oxide crystals; the inner homogeneous layer is formed by small crystals uniform in size. The following cations can be detected in the oxide layer: Fe2+,Fe3+,Cr3+and Ni2+.In reactors of the PWR type, the surface films contain M30, spinels (where M may be represented by Fe, Cr, Ni), whereas in boiling water reactors, hematite (an alpha Fe203)is an additional component of the film apart from the M30, spinels. Surface layers of corrosion products and other deposited substances form also on the inner surfaces of circuits in nuclear power plants with fast sodiumcooled reactors, as well as in sodium experimental loops. Under normal operating conditions of these nuclear facilities the surface films are very resistant to most attempts to dissolve them. On stainless steels, they are composed of “zerovalent” metals, their carbides and only a low percentage of oxides. They are crystalline and at most only a few pm thick. Yet they adhere firmly to the steel. In loops and circuits with a low oxygen content it is possible to detect M,,C, crystals, where M = Cr, Fe and Mo. Even elemental metals (Mn, Co, Fe and Ni) can appear in the deposits.. Surfaces of aluminium are coated with a microscopic transparent film of A1,0,. When removed, it is immediately renewed in air, and perfectly protects the metal against deep corrosion. 102

Articles and equipment items made of copper and its alloys (brass, bronze) may become coated with verdigris, which is basic copper carbonate Cu(OH), . cuco, 1.6.1.4.5.3 Impairment of the protective oxide layer

The protective oxide layer may be damaged by several mechanisms: 1. A high local specific thermal pressure; 2. Sudden changes in thermal load - the “thermal shocks”, for instance during launching or run-down manoeuvers; 3. Oxidation of magnetite at higher aqueous concentrations of oxygen as a result of faults in the water degassing system or oxygen content control; 4. Dissolution of magnetite at higher concentrations of NaOH; 5. Anodic dissolution of magnetite at a pH value above 1 1 ; 6. Dissolution of magnetite by the action of alkalies concentrating at the sites of porous deposits; 7. Erosion resulting from the effect of high fluxes of water containing solid particles and colloids. The volume of the oxide layer is greater than the volume of the metal that has been transformed to the oxides. Due to this fact, the boundary between the oxide layer and the metal is exposed to a considerable tension which rises further with each thermal impact (shock). As long as the adhesion of the film to the metal is greater than the cohesion inside the layer itself, the layer splits; conversely, if the cohesive forces prevail, the film peels off and cavities appear which subsequently accumulate impurities and accelerate the local corrosion. 1.6.1.4.5.4 Corrosion inhibitors

Corrosion inhibitors are substances which are capable of modifying corrosive reactions in such a way that the corrosion rate decreases. By lowering the concentration of hydrogen ions (increasing the pH value), the hydrogen depolarization is reduced and the formation of insoluble products is fostered. Anodic inhibitors slow down the anodic reaction. They are mostly used in neutral environments. Alkalization and formation of insoluble products generate well-protecting layers, sometimes of a passivating nature. Cathodic inhibitors inhibit the cathodic reaction. Reduction in an acid environment results in an increase in the polarization of the cathode, hence curbing the corrosion progression. In neutral environments cathodic inhibitors may also give rise to hydroxide layers that inhibit the hydrogen depolarization. Two categories of inhibitors differing in their action mechanism are distinguished: physical and chemical. Physical inhibitors are substances that can reversibly adsorb on the metal surface without being themselves changed by the process, thus blocking the 103

electrode reaction, either slowing it down or stopping it completely. These inhibitors are substances possessing strongly polar groups which are preferentially adsorbed on the active sites of the electrode surface. Examples are cation-active surfactants of the alkylammonium halogenide type and alkylpyridinium halogenides, in particular alkylpyridinium bromides:

On the cathodic surface, these substances form a film with a high chemical resistance. Physical inhibitors induce a high cathodic polarization. They are used almost exclusively means for pickling and chemical cleaning of steam generators. Chemical inhibitors inhibit corrosion by chemically reacting with the metal or the corrosive environment components. Examples are passiuating inhibitors, such as strongly oxidizing chromates or nitrites forming passivation layers about 2 . 10-2pm thick; also substances reacting with metal ions and forming thicker layers of insoluble compounds (phosphates, chromates, arsenates, silicates etc.); and further, electrochemical inhibitors, i.e. electrochemical passivators. These substances exhibit a considerable hydrogen overpressure thus obstructing the hydrogen depolarization. They maintain the metal within the passivation region. They are reduced on cathodic areas and become deposited on anodic sites. The passivators based on Hg, As, Sb are representatives of this category. When applying these substances, it is essential that their concentrations in the solution be kept within certain optimum limits. A decrease in concentrations below the limit may have unfavourable consequences in that a substance which is an effective passivator under optimum conditions acts conversely as a stimulant accelerating the corrosion attack. Inhibitors control and restrict the corrosion of metals in aqueous solutions. This applies to corrosion caused by acid solutions during alkaline decontamination (e.g. by means of silicates), and during alkaline-oxidizing decontamination (by means of chromates and permanganates). An effective inhibitor must restrict corrosion, it must be soluble in the decontaminating solution to be used, it must not impede the removal of the deposits, it must be easily removable with the solutions or the subsequent water rinses, it must be stable during the decontamination process, and must not form undesirable reaction products when used. If one designates the corrosion rate associated with the application of 104

decontaminating solution lacking the inhibitor as u, and that containing the inhibitor as vc, then the inhibition efficiency q can be calculated as q = -vco - v c

(1.61)

CCO

and the relative efficiency as 17% = 100. q

(1.62)

The most effective inhibitors of acid solutions are complex organic compounds: the nitrogen compounds forming ions of the ammonium type (> C+-N=N-H) the oxygen compounds, such as aldehydes, ketones and acids, forming ions of the oxonium type (>C+-O-H), and the sulphur compounds, such as mercaptans, disulfides, forming ions of the sulfonium type (>C+-S-H). Though there is no common principle of inhibition applicable to all these compounds, the general understanding is that they are cathodic inhibitors, i.e. they adsorb on the cathode and prevent recombination of hydrogen atoms to hydrogen gas molecules. An inhibitor which is frequently used for decontamination purposes is either urea or the more efficient thiourea. Ayres [32] recommends phenylthiourea as the most suitable inhibitor for decontamination because of its high effectivity and economic advantages. More detailed information about inhibitors can be found in specialized publications [e.g. 331. When considering the suitability of a particular inhibitor, it must be also borne in mind that the inhibitor must not contain any chlorides or fluorides. The same requirement applies to any other chemical additive combined with decontaminating solutions.

I 61.4.6 Corrosion eflects on metal of decontaminating solutions Corrosion effects of the chief constituents of decontaminating solutions (citric acid, oxalic acid, EDTA) on sensitized AISI 304 stainless steel were analyzed by Speranzini et al. [34]. Evaluation of the electrochemical data indicates that it is most probably the oxalic acid which accounts for the intergranular corrosive attack. The oxide films on carbon steel are removed by a number of inhibited acid decontaminating solution. These solutions contain sulfaminic acid, sodium hydrosulphate, biammonium citrate and phosphoric acid. Oxylic acid solutions generate a precipitate of iron oxalates on the steel surface.together with co-precipitated radionuclides. Continuing operation at a high temperature leads to a decomposition of the precipitates; the process gives rise to a black adherent film which resists other decontamination procedures. Improved formulations of decontaminating solutions which contain oxalic, 105

citric and sulfaminic acids do not form such precipitates even after extended treatment at high temperatures. With rational schemes of decontamination operations, the rate of uniform corrosion of carbon steel usually does not exceed 5.6 mg. m-'. s-' (2.0 mg.cm-'. h-I). When two kinds of steel of differing quality are in contact galvanic corrosion is induced; it appears for instance at the weld regions between carbon and stainless steels. Galvanic corrosion may result in pitting, occasionally in the appearance of quite deep chasms in the metal. The pits are located at some distance from the weld. High oxygen concentration accelerates this type of corrosion attack; conversely, elimination of the oxygen during the decontamination procedure, for instance by using an inert atmosphere of N, or Ar, as well as subsequent rinsing, inhibit the attack. High-alloy steels and nickel-base alloys withstand the effect of alkalies and particularly of acids. The oxide films adhere strongly to the metal and are very resistant. To remove them, it is necessary to apply a two-step decontamination procedure consisting of the consecutive use of an oxidative solution and, after a rinse with demineralized water, an acid solution. The former solution oxidizes mainly Cr"' to Cr"', i.e. to chromates; by partly dissolving the chromium, it softens the surface film and makes it more susceptible to the subsequently applied acid solution (oxalic acid, nitric acid, citric acid and its salts, sulfaminic acid, phosphoric acid, sodium hydrosulphate etc.) which ultimately removes the deposits. Among the classical oxidants are solutions of alkaline potassium permanganate, i.e. mixtures of potassium permanganate and potassium or sodium hydroxide. An alternative method of oxidation has been developed recently, namely that of a direct feeding of oxygen (in the form of H,O,) into the primary circuit water at a reduced pressure of the coolant [35]. Even more effectiveis the use of ozone [36],usually in the form of ozone-saturated water. The oxide film deposited on high-alloy steels and alloys can be loosened not only by oxidation of Cr3+,but also by its reduction to Crz+,or reduction of Fe3+ to Fe2+.The latter method is particularly suitable for boiling water reactors, because their primary circuit contains much more Fe3+chiefly in form of Fe,O,, then does the primary in PWR; in fact, the magnetite is almost completely absent on the primary circuit inner surfaces of PWR. Nevertheless, attempts have also been made to apply this method to PWR circuits as well [37, 381. The reduction mentioned above is accomplished by means of transition metal ions in a lower oxidation state, hence the acronym LOMI designating the method (Low Oxidation-state Metal Ions) [30, 381. A particularly suitable reagent is vanadium picolinate solution at a concentration of 4 . mol .I-' and pH 5. Although the oxidative potential of the reaction V2+ + V3+ is low, the appropriate reaction proceeds quite fast. Strong acids also dissolve the oxide layer on stainless steels, but this reaction is very slow; consequently, the volume of liquid 106

wastes would be prohibitively high. The chief advantage of the LOMI method is the fact that the reactants remove merely the oxide layer, while the metal itself is not attacked. The methods described so far of oxidation by means of oxygen or ozone, and reduction by means of low oxidation-state metals are all based on the use of relatively low concentrations of the reagents. Acid solutions, when used for the purpose of oxide film removal, are applied in low mass concentrations of the order of i.e. values at least 10 times lower than those which are common for decontamination reagents in the usual concentrated decontamination solutions. Methods that use low concentrations of decontaminants are characterized by decontamination factors ranging from 1.5 to 7, which are low compared to values close to 20, common for classical methods using concentrated reagents. However, the relatively low efficiency is compensated for by a number of advantages: The removed radionuclides may be continually intercepted on ion exchange columns inserted into the cleaning loop of the primary circuit which is being decontaminated. It is possible to do without a complicated and expensive system of treating a large volume of liquid radioactive wastes. The reaction kinetics are also favourable for a continuous ion exchange regeneration of spent decontaminating reagents, i.e. acids, complexing agents and LOMI chemical reactants. Procedures of primary circuit decontamination which would have extended over several months with concentrated solutions may be shortened to mere 72 or perhaps only 36 hours. This brings about great savings in terms of the time period of the unavoidable interruption of power production. The decontamination procedures may also be repeated more frequently, and often make use of the shutdown of reactor operation imposed by other reasons. With heavy water moderated reactors, such as for isntance the Candu type reactor (Canada), the method using low concentrations brings additional savings in that it prevents radioactive contamination of the moderator. On this principle, the Canadians devloped a special decontamination method designated as Can-Decon [39]. Decontaminating solutions must not contain any chlorides, as the prolonged action of C1- may cause irregular intercrystalline corrosion of stainless steels and nickel-base alloys with ensuing stress cracking. Mere traces of chloride ions remaining in the tubing system in crevices, or resulting from inadequate rinsing etc., may suffice to initiate the attack. Chlorides can be tolerated only if perfect subsequent rinsing of all predisposed sites of the cleaned system is feasible. In practice, however, this condition can only be met when simple machinery parts are being decontaminated in dismantled state in large decontamination vats, but hardly for assembled complex equipment. A prolonged effect of bases on steels results in a “alkaline embrittlement” and cracking. The only alkaline solution that may be safely used for extensive 107

time periods is the alkaline permanganate (AP) solution. Steels exposed to AP at the commonly used concentration of 10 YONaOH and 3 YOKMnO, and at a temperature of 378 K are-not likely to crack even after extended periods of time, provided that the rinse terminating the treatment is sufficiently thorough. The suppressive effect of permanganate on corrosion cracking is clearly a function of its concentration. The optimum effect is observed with 2-3% solutions, whereas concentrations below 0.5 YOare ineffective [32]. Neutralization after the procedure is also suitable. Aluminium is used as a fuel assembly cladding material. In strongly alkaline solutions it is attacked by corrosion. Aluminium can be decontaminated with sulfaminic or oxalic acids, or with SULFOX, a mixture consisting of 0.3 MH,SO,, 0.1 MH2C204and an inhibitor. Very good results can be achieved 0.1 MH,SO, 1 g .I-’ phenylwith a solution containing 0.5 MCr,(SO,), thiourea (or another equivalent inhibitor). Citric acid, phosphoric acid and hydrosulphates supplemented with an inhibitor are also candidates as effective decontaminants of aluminium. It is absolutely essential that any solution used as aluminium decontaminant contain no C1- or F- ions. Buffered solutions of oxalic acid and hydroperoxide exhibit a low rate of corrosion attack on aluminium ranging from 7-14 mg . m-,. s-’ (2.5-5 mg . cm-’. h-I). Zirconium and titanium-based alloys undergo rapid corrosion in the presence of fluorides. All media which come into contact with Zr or Ti must be absolutely free of F- ions; the fluorine level must be permanently controlled. In the absence of fluorides, Zr and its alloys exhibit an outstanding resistance to AP, oxalic acid, sulfaminic acid, phosphoric acid, hydrosulphate or oxalic acid supplemented with H , 0 2 . No weight loss due to corrosion can be detected in such cases. Titanium corrodes in hydrofluoric acid, fluorides and oxalic acid. Other decontaminating solutions, provided they do not contain oxalic acid, practically do not attack Ti, the corrosion rate being below the detectable limit, i.e. less than 30 pg . m-2. s-’ (10 pg. ern-,. h-I).

+

+

1.6.1.4.7 Conditions of decontamination with respect to corrosion When complex facilities are being decontaminated, it is necessary to comply with the following basic rules: The time period specified for each particular decontamination procedure must be strictly observed. Some of the inhibitors decompose when exposed to increased temperatures for an extended period of time. Even if a relatively weak corrosion process takes place durig the decontamination procedure initially, the corrosion rate may suddenly increase rapidly if a certain critical “induction period” is exceeded. 108

The temperature of the decontaminating solution substantially influences the rate of the film removal. An icrease by about 10 K approximately doubles the reaction rate. Pitting of inhomogeneous welds (joining two steel types) induced by sulphuric acid does not usually occur below the temperature of 358 K. Some inhibitors lose their stability when exposed to temperatures only slightly higher than those that are specified as the operating temperatures of the decontaminating procedures (333-358 K). With a rising temperature the decomposition of some reactants increases in proportion with the increase in temperature. For instance, a 10% solution of sulfaminic acid hydrolyses to ammonium hydrosulphate

N H , . SO,H

+ H,O

-P

NH4HS04

(1.63)

at a rate of 0.1 YOper day when at room temperature, whereas at 353 K the rate increases to 8.3 % per hour. Aluminium may be decontaminated by means of an 8 % solution of inhibited sulfaminic acid, but the temperature must not exceed 338 K. The same solution at boiling point temperature attacks the aluminium with a pitting corrosion. It is desirable that new formulations of decontaminating solutions be designed for complex technological facilities, such that they would not be excessively corrosive even beyond the critical limits of temperature and other conditions prescribed by the operating instructions. It has already been said that air and oxygen increase the tendency to pitting corrosion of inhomogeneous welds. Scaled off corrosion products may accumulate in the decontaminating solution and the increased concentration of ions (ferric and other) may result in an increased corrosion. The concentration of individual components in the decontaminating solution is usually not an overly important factor, because the inhibitor is likely to be present in a certain excess. For this reason, diluted solutions are often more aggressive than the same solutions in a concentrated state due to the dilution of the inhibitor. When the decontaminating procedures are carried out in vats, it may be relatively easy to optimize the conditions. However, when complex technological equipment systems are decontaminated in situ, various difficulties may arise. It the time interval specified for the procedure is short, say 1 hour, the filling and the draining of the system may take up a considerable portion of the entire time interval, say half an hour. A diversiform inner relief and “deadlegs” can make it extremely difficult to adhere to the specified requirements regarding the temperature, concentration and composition of the solution. An ideally decontaminable equipment system would have smooth non-diversified inner surfaces, no deadlegs, could be quickly filled and drained, and quickly heated to the 109

desired temperature. Real equipment systems will, of course, at best only remotely approach this ideal. Nevertheless, serious consideration should already be given to the decontamination aspect when designing the system. 1.6.1.4.8 Radiation eflects in decontaminated primary circuits of nuclear power

plants Apart from the effect of ionizing radiation that are commonly observed in primary circuits (radiolysis of the coolant water and other effects), there are two radiation effects which have a direct bearing on decontamination: 1. Radiolysis of the inhibitor reported [40] to appear within six days of decontamination with ammonium citrate at 355 f 2 K. 2. Radiation dissolution of hematite which reportedly [41] takes place in the reactor core decontaminated by means of EDTA and oxalic acid solutions in a strongly acid environment (pH at weight concentrations of the order of 2.2). 1.6.1.4.9 Some conclusions related to metal surface decontamination

Radioactive substances not only pervade the surface oxide films, but diffuse to some extent also into the surface layer of the metal. Over 97% of a test sample of @Co was found absorbed within the magnetite film, about 3 % penetrated into the metal [42]. It is obvious that, unlike chemical cleaning of machinery parts, the “classical” decontamination by “hard” methods requires removal of a thin layer of the metal itself together with the overlying oxide film. Corrosion of the metal is therefore and unavoidable occurrence in a complete decontamination. On the other hand, typical decontamination by “soft” methods and means does not even remove the entire depth of the oxide film and the metal is left intact, if the procedure is carried out as planned. It does not seem difficult to invent a highly efficient decontamination process for metals “per se” ; the problem is rather to find an efficient decontaminant that would not at the same time excessively damage the metal. Metal decontamination, in this sense, is a controlled metal corrosion. In the past, the advice of corrosion chemists has not been taken much into consideration when forming the design concepts of nuclear energetic systems. The traditional attitude of the designers was to emphasize the significance of mechanical stress resistance, and to underestimate the corrosion stress exetted on mechanical parts. Within the framework of this philosophy, the wall thickness of pressure vessels, pipings, machinery parts and other energy facility components was being designed according to a simplified formula: If the calculated mechanical stress required a wall thicknes d,,, and the presumed 110

corrosion loss of the thickness for the entire life time of the facility was d,, the minimum total wall thickness was calculated as

dmin= d,,,

+ 2d,

(1.64)

The doubling of the corrosion loss in the equation functions as a safety factor. The presumed corrosion loss must necessarily include not only the operation corrosion, but also the corrosion due to all decontamination cycles, both regular and exceptional. If the corrosion loss resulting from a single decontamination cycle amounts to a thickness decrement designated as d,, and the entire life time of the facility encompasses the number of regular and irregular decontamination cycles corresponding to, respectively, m and n, then the entire envisaged loss can be calculated as

+ +

d, = do, (m n) dd

(1.65)

I .6.2 Surface firms The films which may occur on the surface of various materials significantly modify the efficiency of decontamination procedures. A survey of surface films on materials important for nuclear technology is presented in Fig.1.25.

1.6.2.1 Greasy films on metal surfaces Surfaces of metal items, such as machinery parts and means of transport, may be coated with greasy films formed by residues of lubricants and oils, heavier fractions of crude petroleum, tar combustion products originating in vehicle exhausts etc. These greasy films can be removed easily by less polar organic solvents, such as for instance liquid fuels, chlorinated hydrocarbons etc., or by water solutions of alkaline degreasers and detergents.

1.6.2.2 Other impurities Technical devices and means of conveyance are often covered with deposits of dust, mud and clay. The deposited impurities can be removed by jetting onto the surfaces water under pressure or by rubbing the surfaces wetted with water and cleansing solutions.

1.6.2.3 Colloidal protective surface layers To make decontamination easier, it has been proposed to cover the surface likely to become contaminated with a protective colloidal coating containing a 111

fiFl

c c

CORROSION

h)

SURFACE FILMS

ON METAL OBJECTS

-

-

GREASE

PROTECTIM COLLOID FILMS

PAINTS

i

STRIPABLE PAINTS COMMON PAINT COATS

IMPURITIES

IMPURITIES SOLUBLE

ON CLOTHING

EXAMPLES: GLYCIDS. UREA, PROTEINS. INORGANIC SALTS

AGENTS (IMPREG-

-

WATER INSOLUBLE

-

NORGANIC

MAMPLES : DIRT, CEMENT. SOOT

4

POLAR

ORGANC

NONPMAR

Fig. 1.25. Characterization of surface films with respect to their composition, function and properties

D ( A ~ E : MAT, FATTY ACIDS.

1

FATS

EXAMPLES : HYDROCARBONS. LUBR CANTS, ASPHALTS. TARS, PAINTS, LACWERS

-

detergent, i.e. a tenside, which would facilitate the decontamination of the surface later on rapidly and effectively, by a simple rinsing with water. The Eollowing aqueous solution is an example of the possible formula of a film-forming mixture : - 5-1 5 YOof sodium dibutylnaphthalene sulfonate, pretreated with ethanolic extraction in order to get rid of the acid sodium sulphate which commonly accompanies the commercial preparations. - 10 YOof glue purified by dissolution in water and sedimentation to eliminate the undesirable coarse particles of calcium carbonate. - 1-2 % of triethanolamine soap of oleic acid. - 0.5 YOof potassium chromate. - Water added to 100%. The sodium dibutylnaphthalene sulfonate is a detergent, the glue has a function of a film-forming component and improves also the film adhesion to the painted surface, the triethanolamine soap confers flexibility upon the film, and the chromate is a corrosion inhibitor. Decontamination can further be substantially facilitated by generating a protective layer of a tenside on the metal surface prior to the expected radioactive contamination. This can be done by submersing the metal part into a 1 YO aqueous solution of a detergent washing powder [43]. 1.6.2.4 Strippable covering paints

Work surfaces in laboratories designed for handling solutions of radionuclides may be protected with coatings of strippable paints. The strippable film is simply removed upon termination of the work and discarded. The most common base for strippable paints is a co-polymer of vinylacetate and vinylchloride. Strippable paints can be used not only as a means of preventive protection, but also as a means of subsequent decontamination. By spreading the paint over the contaminated area and allowing some time for complete drying, a considerable part of the contaminant is usually removed with the layer of the stripped film. Susumu et al. [44]recommended a paint based on a styrene-butadiene co-polymer. The polymer results from mixing 60-80.wt. YOof styrene (plus a varying proportion of other modifying ingredients) with 0.5-5 wt. YO of a higher alkylamine containing between 12 and 18 carbon atoms in the molecule. 1.6.2.5 Covering paints

Most cover paints are composed of several layers; usually there is a priming coat, a main coat and an upper finishing coat. The completed coating thickness 113

commonly extends over a range of 0 . 1 5 4 . 3 0 mm; if it is thinner, it does not protect sufficiently, if thicker, it is not resilient enough and it therefore develops cracks. The paints used for coating usually consist of film-forming, colouring and volatile components. The film-forming components may be the following: boiled oils (i.e. plant oils polymerized ue to the effect of atmospheric oxygen), natural resins (colophony, shellac), synthetic resins, asphalts and pitches (natural asphalt, synthetic tar etc.), cellulosic derivatives and chlorinated rubber. The colouring component is represented by pigments, and one of the common organic solvents (acetone, ethyl acetate, turpentine etc.) forms the volatile component. The film-forming components of coatings used for protection of industrial facilities and machinery parts are usually derived from synthetic resins. The two which find the widest use are polystyrene and epoxy paints. Polystyrene (glyptal, alkyd) resins, are polyesterified polyvalent alcohols and polybasic carboxylic acids. An example of such a resin is the polyester of glycerol (G) with phthalic acid anhydride (P) :

-P

1 d -P+P

e

-P-G-P-G-P4-

1

where G stands for - O . C H , . H.CH2.C- and P is -CO.O,H,.CO-

Glyptal resin coatings are durable, can be easily cleaned and decontaminated without-damage to the substrate, retain the resilience and colour, are grease resistant, dry quickly in air and can sometimes be subjected to stoving. Epoxy paints contain epoxide resins as their film-forming component. Their chemical formula is

114

Epoxy paint coatings are strongly adherent, and can therefore be applied directly without a priming coat. They are noted for their excellent resistance to wear and chemical reagents, i.e. acids and bases of medium concentrations, as well as their resistance to liquid fuels, moisture and low temperatures. They can easily be decontaminated. For all these reasons, epoxy paints are being widely applied as suitable protective coatings in nuclear energy facilites as pointed out in a review published by Schwarzenauer [45]. Among others, epoxy paints have contributed to an effective management of the TMI-2 accident. 1.6.2.6 Surface layers on clothing

The soiling layers that may cover the clothing surface are generally composed of dirt and substances which are deliberately applied as finishers. The dirt layers occurring commonly on clothes (see Fig. 2.25) can be categorized as: - Water soluble, such as carbohydrates (starch, fluor), urea, organic compounds (acids), proteins (blood, slime, dead cells) and inorganic salts (sodium chloride, lime etc.). - Water insoluble, inorganic, for example various earth components (clay, silicates, road dust), cement, plaster, soot etc. These substances are insoluble in organic solvents. - Water insoluble, organic, apolar, for instance hydrocarbons (gasoline, petroleum, lubricating and burning oil), lubricants, asphalts, tars, paints, lacquers etc. These compounds are soluble in organic solvents. - Water insoluble, organic, polar, such as fatty acids, sweat, plant and animal fats etc. These compounds are soluble in some organic solvents. Substances that are deliberately applied to the surface of clothes are various finishing and impregnating agents, such as for instance water repellents. The finishers generally facilitate decontamination, though at the same time they usually worsen the hygienic properties of the fabric. Wet decontamination procedures mostly remove the finisher, and renewal of the layer is necessary. The most widely used finisher for linen is starch. 1.6.3 Prevention of contamination, and conditionsfacilitating decontamination in nuclear energy facigties

Ever increasing attention is centered has been recent years on prevention of radioactive contamination in nuclear facilities; this concerns in particular the primary circuits of PWR. More consideration is also given to proper design and construction aspects which would reduce the future need for decontamination and/or make the decontaminating procedures easier. These new decontamina115

tion aspects in nuclear power plant design philosophy have been summarized in appropriate regulations [46]. The following are the basic desirable requisites which the designers should take into consideration in order to strengthen the preventive aspect of the contamination risk, and to make decontamination easier : 1. Construction materials of the reactor core and the adjacent primary cooling circuit should be made of steel or alloys which are free of cobalt. It this requirement is not met, after ten to twelve years of operation the long-lived radionuclide @'Co (half-life 5.3a) would make up about 80% of the total radioactivity contaminating the inner surfaces of the primary circuit, and the area activity of contaminated surfaces would amount to about 8.10'lo6Bq .mW2[47]. 2. The core and the cooling systems should have smooth, non-diversiform inner and outer surfaces, both macroscopically (no crevices) and microscopically (minimum roughness). The surface finishing of metals should best be carried out by electropolishing or by passivation. The outer surfaces of equipment systems that are exposed to contamination fallout or solutions should be coated with protective paintings, or possibly with strippable films. 3. All systems of fluid transportation should be designed so as to keep the flow rate throughout the entire system as constant at any point as feasible, and to avoid as completely as possible sharp changes in the flow orientation. Low spots and deadlegs are undesirable. On the other hand, sudden variations (shocks) in operating parameters, such as flow rate, pressure, temperature, or generation of gas in form of bubbles, may by themselves exert a decontaminating effect on the circuitry. 4. The draining of the inner system should be easy and quick; it is desirable that the entire volume of the liquid flow out spontaneously, driven only by gravitational force. 5. The dimensions of the fluid transportation system, and the performance of the main as well as the ancillary pumping systems should be such that all the inner spaces could be filled and drained an as short as possible a period of time, in order to increase the efficacy of decontamination by circulation methods. 6. A heating system should be provided allowing the warming up in a reasonably short time interval of the fluids to a temperature of at least 373 K, or possibly higher if the fluid is under pressure. 7. All parts that are fragile, likely to be easily damaged or excessively worn, or parts exposed to stress and corrosive attack (screws, nuts, bellows, bearings etc.) should be easily replaceable. . 8. The processing of the primary circuit water should ensure an adequate quality of the coolant by eliminating the solid particulates and dissolved substances whose activation in the core could give rise to harzardous long-lived 116

radionuclides, by adjusting the pH to the neutral point, and by keeping the redox potential at its minimum. 9. In order to prevent the deposition of radioactive scales (thick films) on inner surfaces of the PWR primary circuits, it is advisable to add continuously alpha diketones to the coolant, primarily acetyl acetone. With the metal ions and iron oxides, which are the chief constituents of the scales, alpha diketones form complexes that react with metals and give rise to neutral molecules which are somewhat water soluble. It has been estimated [48] that up to a total concentration of the metal in the coolant of 1 . lo-' (10 ppb), all the metal remains dissolved. 10. The radioactive fluid-containing system should be efficiently shielded from its surrounding. Careful detailed planning of normal operations as well as the optimization of technological procedures associated with the maintenance and repair can substantially reduce the risk of radioactive contamination. Systematic education and training together with an increasing competence and skill of the personnel operate in the same direction.

1.7 Behaviour of trace amounts of radionuclides in the course of decontamination 1.7.1 Behaviour of trace amounts of radionuclides in the course of decontamination of solids Even though the contamination of a solid is essentially a dry process, i.e. the radionuclides are not parts of macroscopic volumes of solutions, it can never be absolutely excluded that a smallei or larger proportion of the radionuclides become dissolved and then adsorb onto the contaminated surface. An almost imperceptible bedewing or moistening of the surface may be sufficient for a fraction of the contaminant to pass into a liquid state. Moreover, if the initial decontamination procedure involves the use of water, aqueous solutions or other liquids, part of the radionuclides may become adsorbed subsequently. Any decontamination scheme must therefore include - in addition to an effective elimination of adsorbed radionuclides - also precautionary measures which reduce readsorption.

1.7.1.1 Limitation of adsorption All items that are likely come into contact with radioactive contaminants should be designed and constructed with proper decontamination aspects in mind. Minimum macroscopical diversiformity of the surface and a smooth 117

microrelief are the first two logical requirements. Adsorptoin can be effectively reduced by an appropriate choice of structural materials (teflon, polyethylene, synthetic fibres), by an _appropriate pr,etreatment of the surface (polishing, honing), and by coating with common paints or thin films of water soluble substances. If a surface contamination is caused by a contaminant in dry state, it is appropriate to try first a dry decontamination procedure before applying a wet method. In some cases, it is prudent to begin with procedures involving organic solvents or organic solvent-based solutions, because such procedures create less favourable conditions for radionuclide readsorption. The degreasing by means of organic solvents and chemical dry cleaning*) are illustrative examples. Only after the dry and the organic solvent-base procedures have failed to achieve the desirable degree of decontamination is it proper to apply water-based decontaminating solutions. Dry decontamination methods would be operative particularly when large territories were contaminated with radioactive fallout as a result of nuclear explosions. The decontaminating solutions inhibit the radionuclide adsorption by two mechanisms. First, they readily convert radionuclides into stable complexes soluble in the given solution and, secondly, they reduce the value of the pH (as long as this is compatible with the decontaminated system). The following reagents used in decontaminating solutions form stable and soluble complexes with a majority of the contaminants : polycarboxylic acids, complexing agents, phosphates of all types, in particular polyphosphates (and “sodium hexametaphosphate”), sulfaminic acid, hydrofluoric acid, and fluorides etc. 1.7.1.2 Removal of adsorbed radionuclides by the decontamination process Radionuclides adsorbed on a contaminated surface may be removed by desorption, abrasion of the contaminated surface layer and by electrochemical separation. For reversibly adsorbed radionuclides, desorption is the reciprocal process to adsorption. Thus, all conditions that inhibit reversible adsorption foster at the same time the desorption. This concerns both the generation of stable soluble complexes and the lowering of the pH. It should be noted, however, that these two processes support the dissolution and thus the desorption of radionuclides that are adsorbed irreversibly, i.e. in their colloidal forms.

*’The term “dry cleaning” is misleading, as it is not in fact a dry process, but uses organic solvents instead of water

118

Removal of the contaminated surface layer together with the adsorbed radionuclides can be effected either mechanically or chemically. Chemical methods consist in dissolving, and thus removing, away, the surface film or paint coat, or etching away the surface of the contaminated item by means of suitable reagents.

1.7.2 Methods of decontamination of solids 1.7.2.1 Dry methods

Mechanical dry decontamination methods of solid surfaces include sweeping, wiping off with swabs or rags, and brushing etc. If a metal surface is to be decontaminated, additional procedure are available : wire brushing, grinding by means of a grinding wheel, abrasive paste, sand paper, emery paper or cloth, abrasive powder, planing, machining, sand blasting, vacuuming etc. The use of abrasives moistened with water or with a decontaminant solution, and also as an attractive use of ice as an abrasive can no longer be classified as strictly dry methods [49]. Fabrics can be decontaminated in the dry state by various dust removing procedures, such as mechanical beating, vacuuming, sound vibrations, ultrasound, electrostatic separation or various combinations of these. All these methods are usually used with simultaneous vacuum sweeping. Each mechanical dry decontamination technique must necessarily be coupled with safety precautions aimed at averting the spreading of the stirred radioactive dust. Two measures can substantially help in achieving this goal: isolation of the space where the decontamination processes take place, and exhaustion of the radioactive dust and its retention on efficient dust and aerosol filters. It is self evident that common filters with which ordinary vacuum cleaners are provided are entirely insufficient for this purpose. On the contrary, the use of a vacuum sweeper without an additional special filter fitted in cannot but disperse the contaminant more into the environment. Dry methods based on the use of adhesives include the application of adhesive tapes or foils (suitable just for small contaminated areas) and in a sense also strippable films. A summarizing classification of methods used to decontaminate solid surfaces is given in Fig. 2.26. The electrostatic methods are numerous. In a broad sense, even a simple wiping off with a dry dust cloth, or brushing with common-type brushes, are electrostatic separation processes. Much more effective, however, are special anti-static cloths or brushes provided with special anti-static rubber or plastic bristles. The most advanced electrostatic methods of dust removal are those that have been worked out for electrostatic separation of particles in smoke. 119

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j%G--pWST REMVAL

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OF aDTHwj

N WATER DMKW€NT IN oRGpEuc UXMNT DMIIMNT
ls2Ta> @Co> 12,Sb. The sorption of 65Znand ',Mn on nickel substrate was greater than on any other metal substrate. No sorption at all was detected for Ag: this radionuclide was found almost homogeneously dispersed in the hot primary coolant. After 20 000 hours of operation, the cold trap contained just 6'Zn and ',Mn, and only insignificant amounts of 6oCoand Is2Ta. The diffusion rate in liquid sodium of activated corrosion products (',Mn and @%ZO) and fission products (134Csand I3'Cs) in the primary circuit pipeway made of 12Kh18N9T stainless steel was regularly measured in the cold and hot legs of the BR-10 sodium reactor (USSR) in the course of 120000 hours of operation. The minimum depth of the examined metal was 3 4 pm. A twice repeated decontamination cycle using a succession of solutions (0.3 YOKMnO,, 1 YO oxalic acid, 5 YO HNO, at 363 K, 1 min for each solution) resulted in a D , 100. The diffusion rates were less for Mn and Co than for Cs [78]. Three categories of uranium and plutonium fission products can be distinguished with respect to how they behave in liquid sodium: Nonvolatile fission products become quickly deposited on the circuit's inner surfaces; volatile fission products, such as I, Cs and Te, dissolve in sodium and become reversibly sorbed in the cold leg of the circuit; and gases (Kr and Xe) diffuse into the blanketing argon atmosphere above the sodium level [79]. Measurements were carried out with the BOR 60 reactor [80] to determine how efficiently individual radionuclides are entrapped in the cold trap. Fission products can be adsorbed from liquid sodium on a stainless steel wire screen [81]. This process is practically independent of the temperature and the flow rate of the coolant. Cold traps are often combined with sorption filters filled with stainless steel chips. Their purpose is, among others, to adsorb the fission products. Caesium Forms a homogeneous solution in sodium so that only a small fraction of the element becomes sorbed on the pipe walls. In contrast, most of the amount of Zr, Ru, Te and I become sorbed on metal surfaces, chiefly in the 198

cold leg. A marked temperature dependence is observed for I and Te. This is the reason why the cold traps are considerably more effective for I and Te than for other elements. Sorption of Cs in cold traps is quite negligible [82] and special traps are being constructed for Cs radionuclides [83]. Caesium is readily adsorbed on carbon or carbon-rich materials [84]. Traces of carbon which then appear in the coolant are removed by adsorption on Ti, Fe, Cr or Nb. The volatile fission products (I, Cs, Te) become entrapped in cold traps [80], while the argon atmosphere is cleared of the gaseous fission products by their adsorption on charcoal [85], usually at low temperature. This permits generation of the sorbent simply by warming it up, thus desorbing the radioactive rare gases. The I3’Cs activity in the coolant of the BOR 60 sodium reactor (USSR) substantially decreased when the reactor was shut down; the same phenomenon was observed in the L- and U-shaped segments of the pipeway when the reactor was restarted. About 30 % of the total activity of caesium in the primary circuit of a shut-down reactor appeared to be adsorbed on the metal walls, with the exception of the cold trap. When the temperature was increased, a part of the radioactivity diffused into the coolant [86]. Non-isotopic cobalt carriers (Ni, Zr) decrease the level of 6oCocontamination in the primary sodium circuit. With the weight concentration of 6oCoand (1 ppb), the non-isotopic carrier added in a its isotopic carrier equal to 1 . concentration either 5 . (5 ppb) or 1 . lo-’ (10 ppb), reduced the diffusion of @Cointo the metal walls to, respectively, one fifth and one twelfth [87].

2.1.2.2 Decontamination of primary circuits after depletion of the coolant After a fast reactor has been shut down and cooled off, the sodium coolant may be displaced from the circuit by argon gas. The inert argon atmosphere is maintained in the circuit to facilitate further clearing of the system of traces of sodium. Neutrons in the reactor core generate ::Na (15 h), a beta-gamma emitter whose activity concentration in the coolant may rise to 10” Bq .1-’. Because of the short half-life, the activity rapidly decreases after shutdown of the reactor and practically disappears after one or two weeks. Machinery parts that have been in contact with liquid sodium for some time have, or may have, residues of sodium sticking to their surfaces. Traces of sodium may be found not only in deadlegs, depressions and fissures, but even a smooth flat surface may display a visible sodium mirror or an almost invisible sodium. coating. All remnants must be carefully removed because sodium is inflammable in contact with water, and caustic. Partial decontamination is performed as the macro amounts of sodium residues are removed. The subsequent decontamination procedure must rid the surface of all, often tenaciously 199

sticking, traces of radioactivity. Machinery parts, in particular piping and vessels, must not be cleaned by rinsing or filling with water, because the exothermic reaction of sodium with water would endanger the personnel and might also damage the facility. It must be also borne in mind that the reaction with water (even with steam or mist) generates hydrogen, and the simultaneous presence of air may therefore, in a closed space, give rise to an explosive gas mixture. For these reasons, it is recommended that pipings, circuits and closed vessels be cleaned with a mixture of steam and an inert gas (most often nitrogen) in a volume ratio 1 : 1, heated to about 423 K, or with atomized water at ambient temperature (water spray of tiny particles dispersed usually by means of pressurized nitrogen). After the temperature has fallen and the acoustic and visible signs of a reaction between sodium and water have disappeared, the treated object may be submersed in water or cautiously filled with water, left for 24 hours, and then drained and dried in hot air. The cleaning methods described are safe, effective, relatively rapid, and leave only small volumes of RAW. In some cases, the cleaning process can be visually controlled. An obvious disadvantage of all cleaning methods involving the use of water is the alkaline embrittlement of steels increasing the risk of intergranular corrosion and stress cracking. Those parts that are most at risk (nuts, bolts, belows etc.) must therefore be replaced by new parts after each decontamination cycle. Another category of cleaning methods involves the use of alcohols, chiefly ethanol and butanol. Because of fire danger, the cleaning is performed in an inert atmosphere. A technology described for of cleaning and decontaminating the plating steel [88] recommends a mixture of 2 YOmethanol, 88 YOethanol and 10 YOisopropanol in an argon atmosphere. After drying the cleaned object in hot air, it is necessary to remove the sediment of the alcoholate off the surface. This is first done in dry state and only after it is certain that no traces of sodium are left can water be used. Many radioactive corrosion products are removed along with the alcoholate; thus, the cleaned object is partially decontaminated at the same time. Smaller and more delicate machinery parts can be cleared of the sodium residues by means of liquid ammonia. Sodium becomes dissolved without an exothermic reaction. However, the reagent is ineffective for sodium oxide and sodium peroxide; these compounds must be subsequently removed by rinsing with water. The vapour pressure of ammonia is considerable and increases with the temperature. The cleaning must therefore be performed in a pressure vessel or in a refrigerated facility. The handling of ammonia is difficult in itself and the procedure is rather costly. Although the cleaning process removes some radioactivity along with the sodium residues, it must be followed by an actual decontamination procedure 200

until all the tenacious, maintly surface-bound contaminant, is effectively removed. The GCA process developed for the hot leg of the SFBR primary circuit [73,74] makes use of an aqueous solution of 2.5 wt. YOgluconic (hydroxyacetic) acid and 2.5 wt.% hydrated citric acid at 343-363 K. The duration of the action depends upon the depth to which the "Mn radionuclide has penetrated the metal surface. The GCA process is effective and produces RAW which can be easily managed. The reported decontamination factor achieved in 36 hours was 6.9. Corrosion rates for different types of stainless steel at 343-363 K ranged between 0.5 and 3.6 pm per day. Wrought products showed a higher decontamination efficiency than castings. The material which is characterized by the deepest penetration of radionuclides also exhibits the highest values of D, when treated with the GCA process. The D, increases in the following order: stainless steel 316, CM-8M, 304 wrought, CF8 (308 cast). The reagents, acting for a period of 14 days effectively decontaminated all the materials listed. An extended duration of the process gave no further improvement of the decontamination efficiency; on the contrary, the damage to the type 316 stainless steels was augmented [89]. Several illustrative examples of SFBR decontamination will be described in more detail: In 1960, the BR 5 reactor (USSR) was subjected to decontamination to get rid of the accumulated fission products. After the sodium was drained from the primary circuit, the residual radiation dose rate was estimated to be several mGy .h-I. The circuit was cleaned by steam jetting and decontaminated with solutions of 0.5wt.% KMnO,, 5wt.% HNO,, 1 wt.% oxalic acid and demineralized water, and then dried in vacuum at 423 K. The decontamination liquid wastes contained predominantly 137Cs,65Zrand 95Nb. After the short-lived 24Nahas decayed, some 90 % of radioactivity of the sodium coolant could be accounted for by I3'Cs and 5 % by 22Na,the rest being due to the activity of '"Ba--'"La and 9'Zr-95Nb. A treatment of contaminated parts with steam resulted in the following reduction of radioactivity of particular radionuclides : 137Csby a factor of 10; '"Ba--'"La by a factor of 3; and 9'Zr-g5Nb insignificantly. The argon atmosphere above the coolant contained mainly 133Xeand I3'Xe, and to a smaller extent I3'Xe, 13'Cs, "Kr-"Rb and 8'Rb [85]. Machinery parts of the DFR reactor (GB) stained with residues of the eutectic coolant (a mixture of sodium and potassium) were cleaned by immersion for 12 hours in vats containing butylalcohol. The decontamination which followed was performed with solutions of 12 N HNO, and 2 vol.% concentrated HF. Fuel assemblies were cleaned and decontaminated in hot cells with dibutylalcohol, followed by a methanol bath [90]. Contaminated parts of the Rhapsodie reactor (France) were cleared of the sodium remnants by heating to 453 K in a nitrogen atmosphere, and then 20 1

subjected to a sequence of treatments consisting of steam jetting at 423 K, spraying of water mist, submersing in water and drying in nitrogen at 421 K. This complex procedure removed 100 YO"Na and 90 % 137Csand I3,Cs. The actual decontamination process then consisted of two successive sprayings with 10 % H,P04, rinsing with water, drying in nitrogen at 423 K and one manual decontamination cycle with 10 YOHNO, plus alcohol. The treated parts could then be disassembled and subjected to a decontamination process which removed between 17 and 30 % of ',Mn; the ultimate DFranged from 10 to 100 [911. Tests were performed at the same reactor [92] with samples of stainless steel decontaminated by two alternative methods. The (A) process involved the use of 2 g . I - ' KMnO, 20 g . I - ' NaOH at 353 K for 4 h. The procedure was repeated three times, using fresh solutions each time, and resulted in a removal of 92.7-99.3 % of the contaminant and a 0. I pm thick layer of stainless steel (type 316 SS). The (B) process made use of a 10-25 vol.% solution of H2S04 at 313 K and led to a removal of 95.2 % of the contaminant and a 6.7 pm layer of the steel. The results clearly indicate that the (A) process is superior. The Phoenix reactor (France) operated for 2 years in the temperature range 671-821 K. The dominating contamination of 316 SS stainless steel was from ',Mn, whereas other radionuclides, ,C o@ ' "Co, 137Csand "Na, were of minor importance. After precleaning the parts with steam, decontamination was performed with a solution of 2 g . I-' KMnO, followed by a mixture of phosphoric and sulphuric acids. The radiation dose rate in the hot leg (8.6 mGy . h-') decreased by a factor of 100 [93]. The decontamination factor was even much higher in the cold leg. The circulation pump installed in the cold leg of the primary circuit of the EBR-11 reactor ( U S A . ) was decontaminated after 7 years of operation [92]. Radioactive contamination was due mainly to 54Mn,,C o@ ' 137Csand Ix2Ta.The coolant was removed by a mixture of ethanol and water. The pump was then submersed in a solution containing a mixture of ammonium oxalate and ammonium citrate (Turco 4521). This solution, warmed up to 338 K, was allowed to act under stirring for 4 h. Final rinsing was made with 95 'YOethanol. About one half of the 54Mnradioactivity was removed by this procedure. The following method was employed to decontaminate the PEC reactor (Italy) [94]. Sodium residues were removed by warming the equipment to be cleaned with a stream of hot nitrogen to which steam was gradually added in increasing amounts. When the rection between sodium and water came to an end (this was manifested by a temperature decrease), the equipment was sprayed with water, submerged in a water vat and then dried in hot nitrogen. Decontamination with a special solution then followed. Even parts of the reactor core were subjected to this procedure. Certain systems were taken apart and the

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components, such as parts of the test loops, fuel assemblies, control rods, shielding etc. were decontaminated in hot cells. This unique decontamination operation yielded a wealth of valuable information, in particular the following: Dismantling and decontamination must follow as early as possible after the removal of sodium traces. The decontaminated surfaces must be treated by pickling in the same way as a new equipment before returning to normal operation. All parts that are brittle or those that are under particular mechanical or thermal stress must be replaced. The decontamination aspects should be taken into account when designing the reactor systems. Among others, the access to those parts that must frequently be dismantled and replaced should be easy.

2.2 Decontamination of hot cells Hot cells are specially constructed and equipped facilities which allow the handling of materials with extremely high levels of radioactivity, such as are encountered in plants reprocessing the burnup nuclear fuel or producing plutonium, when handling fuel assemblies or manipulating with components of the reactor core, or when decontaminating such components. A hot cell perfectly isolates the radioactive material from the working personnel and provides an effective shielding and protection from harmful effects of high radiation fields which inside the cell may reach a level of the order of 103Gy.h-’ [2]. The design of hot cells must be conceived with due respect to the requirements of decontamination, in particular it should make it possible to decontaminate the cell effectively, safely and with relative ease despite the extreme radioactivity level, by means of remote control equipment. The decontamination is substantially facilitated if the entire inner surface of the cell has a stainless steel lining. In addition, it is advantageous to apply strippable paints to exposed surfaces. It is further required that the hot cell be provided with apropriate equipment enabling spraying and rinsing with various liquid reagents, particularly decontaminating solutions, outlets for high pressure water and steam, and appropriate connectors and fittings which would enable decontamination to be carried out by mechanical means, air blasting, ultrasound, electrochemical methods etc. Adequate provisions must also be made to manage the solid, liquid and gaseous radioactive wastes. The design concept should also take into account the demands for a safe and easy decommissioning at the end of the useful operational life. It is sometimes convenient, particularly in fuel reprocessing plants and plutonium production plants, to group the hot cells into large functional units lip to several hundred meters in length, called “hot canyons”. In general, decontamination of hot cells is a tough problem and represents one of the most exacting tasks in the art of decontamination and radiation safety.

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2.2.1 Decontamination of hot cells designed for handling solid radioactive substances Hot cells of this type are designed for assembling and disassembling operations, cutting and various shaping procedures involving radioactive items. Such working activities often produce voluminous radioactive wastes in the form of grinding dust, saw dust, shavings, cuttings, as well as gaseous wastes, such as when sealed fuel elements are mechanically opened. H. J. Blythe divided the process of decontamination of such hot cells in three successive stages [12]: In thefirst stage, all mobile and easily removable items are taken out of the cell by means of manipulators and remotely handled tools; the remote-control tooling is an integral part of the hot cell equipment. The cells are further equipped with exhaustion, spraying and rinsing systems, as well as various ancillary equipment facilitating decontamination. Adhesive tapes, wads, filtration paper, newsprint and various other sorbents, sponges, foam plastics etc. are often used at this stage to collect and contain the contaminant. Later on in this stage, all visible deposits and spots likely to have more resistance to the removal effort, such as rust spots, evaporated salt residues, soiled and greasy spots, are removed from the walls and equipment surfaces by separate treatment. Various tools and means can be used to accomplish this task : brushes, wire brushes, steel wool, floor cleaners, rags, mops, sponges, cleaning powders, bentonite, powderized tuff and other sorbents, either dry or soaked in decontaminating solutions. Flat areas (ceilings, floors, walls) are most conveniently scrubbed by means of a self-moisturing floor scrubber. Greasy surfaces are best decontaminated with organic solvents, such as trichloroethylene. Unless the contaminant firmly adheres to the surface, a mixture of steam and water from an ejector is often effective. Contaminants that are likely to become air-borne and to form an aerosol are immobilized on the surface by any of the available means, for example polyurethane foam, glycerin, strippable paint, adhesive foil etc. In planning the course of the action, one of the principal rules, which incidentally applies also to decontamination of buildings, rooms and any large object that cannot be turned upside down, is that decontamination always starts up with the top side and proceeds to the bottom, which means beginning with the ceiling and the upper side of the equipment. Particular caution is required when electric appliances and installations are to be decontaminated. It is also important to keep the screen of the vision slots transparent. The inner side is often provided with a mechanical squeegee for that purpose. From the beginning of the second stage, the contamination levels are constantly monitored. In particular, radioactivity is measured before and after each step or operation, and the data are entered in a special record chart.. (Measurements are not necessary during the first stage.) When the results of the 204

initial monitoring show the activity to be too high, some steps of the first stage may be repeated. If still unsuccessful, other procedures, more efficient and perhaps more drastic, are used. The same procedure is usually not repeated more than three times. The second stage is characterized by the use of chemical decontaminating solutions, electrochemical methods, ultrasound, steam jetting and other cleaning procedures. A consistent recording of all measuring data makes it possible at any moment to judge whether further decontamination is necessary, and in particular what objects can be moved out, what parts of the equipment or spots on the surface require repeated treatment, and ultimately when it is possible for members of the decontamination team to enter the hot cell. The second stage ends as soon as the radiation field allows work inside the hot cell. Either the personnel normally servicing the hot cell or a special decontamination technical team may carry out the decontamination. It appears that a combination is the most convenient arrangement; the servicing technicians are better acquainted with the operation of the manipulators, remotely handled tools and the overall utilization of the equipment of the cell, and are therefore better suited for the first and the second stages. It is understood that the work is done in the presence and with the assistance of the decontamination specialists. Conversely, operations in the third stage may better be assigned to the specialized decontamination team assisted by the technical personnel of the hot cell unit. The thirdstage begins when it is possible to step in the hot cell and manually complete those decontamination procedures which could not be satisfactorily accomplished by means of remote handling. Each person entering the hot cell must be provided with special protective clothing fully covering the entire body and with a full-face respirator. He must wear two pairs of overshoes and two pairs of rubber gloves. The radiation dose is permanently monitored by means of personal dosimeters including the alarm dosimeters signalling an excess dose or dose rate. The accumulated radiation dose and dose rate to which members of the staff have been exposed must be registered for the entire period of the work inside the cell and entries are systematically made in the record. When the decontamination is finished, the residual radiation field inside the hot cell must not exceed 860 pGy .h-’, the limit depending on the actual conditions. During the third stage, all necessary repair work is done and new equipment needed for resumption of the activity is installed. When the cell is newly equipped, the persons who work inside may be dressed in a simple protective clothing, must wear a half-face respirator ready to use and must be provided with the prescribed personal dosimeters. The entire process of decontamination of a hot cell usually takes 2 to 6 weeks. When special protective boxes (the “box in cell”) are used for certain 205

operations that are associated with greater contamination risks, the decontamination period may be cut down to a mere 1 to 3 days. If the hot cell is lined with stainless steel, the nuinber of possible complete decontamination cycles is practically unlimited. However, as time goes on, residues of the contaminants from previous operations do become accumulated, so that successive decontaminations are more difficult. 2.2.2 Decontamination of hot cells designed for work with radioactive solutions

The sources of contamination in this case are spills of radioactive solutions and dried-up evaporates of the solutions. The equipment prevailing in hot cells of this type is that needed for laboratory work in chemistry and radiation chemistry, i.e. various steel and glass vessels and utensils, pipeways, tubings, pumps etc. Though in principle the ways and means of decontamination are the same as those described above, (2.2.1) it is necessary to decontaminate also all the internal surfaces of vessels, tube systems etc. The decontamination is therefore more laborious and greater amounts of decontaminating reagents are needed. According to Blythe [12], decontamination of this type of hot cells can conventionally be divided in six stages: 1. The hot cell interior is surveyed by remotely controlled radiometric and dosimetric measuring devices, and the data obtained are entered in a special record. 2. Pipeways, reservoirs and vessels are repeatedly rinsed with chemical reagents and detergents until the dose rate measured inside the vessels and tubes no longer decreases. 3. Equipment is taken apart and the disassembled parts are placed in shielded containers to be either decontaminated or disposed of; with only a few exceptions, the contaminated equipment is discarded. 4. The cell with the remaining installations is decontaminated by means of pressurized steam jetting. 5 . The cell is opened and the rest of the equipment and installations are dismantled, cutting the large items to pieces as may be necessary. All this is done with tools provided with long handles. 6. When the cell is emptied, strongly contaminated spots are treated separately and finally the cell as a whole is systematically decontaminated, usually by steam jetting. By analogy with what has been explained in Section 2.2.1, it seems more appropriate to charge the hot cell servicing staff with the responsibility for decontamination work in the first four stages, and let then the special decontamination unit team take over and complete the task. An illustrative example of a reported operation [I21 will demonstrate the 206

gradual decrease in radiation dose rates in the course of the decontamination process: The starting dose rate determined before the action was about 4 Gy . h-,' (400 rad. h-I), in certain "hot spots" even more than 6 Gy . h-'. When the remotely controlled decontamination operations were completed (i.e. after the second stage in the case of a hot cell of the previous type), the mean dose rate fell to 10 mGy . h-', with spots exhibiting up to 100 mGy . h-'. Removal of the equipment reduced the dose rate to an average of 270 pGy . h - ' and reached a final level not exceeding 100 pGy .h-' at any spot after ultimately cleaning the cell of the residual contamination.

2.2.3 Peculiarities of radioactive contamination in hot cells Inner surfaces of vessels in hot cells may sometimes exhibit radioactivity levels which are higher than the activity of liquids contained therein. By a kind of natural selection, the most stable and most tenacious chemical compounds accumulate inside the vessels and leave gradually increasing amounts of firmly-sticking contaminant. Removal of such well-attached films poses a serious problem for the decontamination effort. The deposits contain salts with restricted solubility, hydroxides, silicates, chromates, plumbates and niobates. Important are the Si and Zr compounds, because they readily undergo hydrolysis and polymerization, and thus form colloids and pseudocolloids. The process gives rise to aluminosilicates, ferrosilicates and zirconates of radionuclides, as well as nearly insoluble oxides of Si, Mg, Ca, Mn and alkalies. These deposits in hot cells attain as much as 1.5 kg . m-' in terms of weight and 10" Bq . kg-' in terms of specific activity. A peculiar effect of aging is observed with such deposits due to dehydration, polymerization, transformation of amorphous colloids into crystalloids, changes in crystalline structure, diffusion interchange and similar processes. As a consequence, the deposit becomes thicker and forms thermodynamically stabler systems [2]. The resulting compact films show a high chemical and mechanical stability. The decontaminating solutions are then ineffective, unless the reagents dissolve the deposited layer or at least precondition it in that they partially dissolve certain components or loosen the layer by chemical transformations, making it in this way more accessible to the attack by the subsequently applied decontaminant.

2.2.4 Solutions for decontamination of hot cells As a rule, stainless steel surfaces are precleaned with dilute HNO, and live steam. In this way, U, Pu and Np are dissolved and washed away, and may be recovered from the effluents. The subsequent actual decontamination may be performed by means of the reduction-oxidation process (Redox) using the AP

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solution followed by a mixture of oxalic and nitric acids. The AP solution in this case hydrates and oxidizes the oxides, breaks down the silicates and the niobates, oxidizes the radionudides of Ru and Ce, and transforms them to more soluble anionic forms. Some bivalent elements, such as %r, form soluble permanganates. Another important method is the oxalate process using a mixture of oxalic and hydrofluoric acids followed by HNO,. The H F complexes Zr, Nb and Pu and dissolves silicates; at the same time, its corrosive attack on steel is milder and more homogeneous (no intercrystalline corrosion or pitting) than that of HCl [2]. Stainless steel can further be decontaminated by a sequence of solutions in the following order: A 10 vol.% solution of HNO, ; 10 wt.% citric acid; 10wt.% NaOH 2.5 wt.% tartaric acid; 10wt.% oxalic acid, 0.003 M HIO,, 3 wt.% NaF + 20 vol. YOHNO,. Another alternative is a sequence of three solutions in the following order: oxalic acid + H202+ H F (OPF see Table 2.2); 15 M HNO, + 0.02 M H202;1.5 M H2S04 0.1 M H202,at 358-363 K. Vessels and circuits may be decontaminated by rinsing with a succession of: detergent water solution, 5 vol.Y~HNO,, water, 5 wt.% NaOH, water, 0.1 M disodium EDTA, water, 5 wt.% NaOH, water. Instead of rinsing, the same solutions may be applied as sprays using steam as a carrier. The following succession of reagents in water solution are commonly used to remove fission products and Pu from hot cells: 0.15 wt.% household detergent; 2.2 wt.% degreasing' powder (containing 0.22 YO NaOH + 0.45 % Na,P04 + 0.6 YO Na2C03+ 0.1 YO detergent); 0.22wt.Yo oxalic acid; 1 wt.% NH,CO,H; 10 vol.% HNO, ;0.08 wt.% NaF; 36 wt.% AP solution, 2.7 wt.% sulfaminic acid; 10 wt.% oxalic acid; 0.1 wt.% NaOH; 0.6 YONaOH; oxalic acid solution and H202(OP see Table 2.2), water. Acids and alkalies are little effective, however, when it is necessary to remove the insoluble deposits containing fission products. Better results can be achieved with pressurized water, steam and detergents, and with mechanical decontamination by means of brushes, adhesive tapes, floor cleaning and polishing machines and abrasive dish washing powders. Carbon steels are best decontaminated with solutions of ammonium oxalate, ammonium citrate and H202(OPC), inhibited 1 M H3PO4 and common rust removing agents. Aluminium is effectively decontaminated with the OPC solution, dilute NaOH and a mixture of oxalic acid and detergent. Dilute HNO, is also used for decontamination of copper and brass; commercial household brass cleaning agents are also a possibility. Glass utensils are effectively cleaned with detergent solution in combination with ultrasound, dish washing agents etc. Painted surfaces are treated with commercial cleansers, with a mixture of water and steam From an ejector and a mixture of paint-removing solvents. Plants exist to elaborate the technique of molten salts for hot cell decontamina-

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tion an attractive prospect here is the easy management of resulting solid wastes [12]. 2.2.5 Experience with hot cell decontamination

Hot cells belonging to the Eurochemic company [95] were decontaminated in 1961 after four years of operation. The dominating contaminant was plutonium. The total space to be decontaminated was 60m3, surface area 3000 m2 and the internal surfaces of pipeways (70 km in length) amounted to 10000 m2.The total quantity of radioactivity removed was 4.8. lO',Bq (13 kCi) and the dose rates determined at representative locations within the cell decreased from the starting 9 Gy .h-' to 20-200 pGy . h-'. The following noncorrosive reagents were used: 2-10 M HN03 and 0.5-5 M NaOH (alternately), 1 wt.% sodium citrate + 5 wt.% naOH. Slightly corrosive reagents were 5 wt.% sodium tartarate + 5 M NaOH (preceded by a rinse with 6 M HNO,); and 0.1 M KMnO, + 0.1 M NaOH (AP), oxalic acid (the last two solutions alternately). Finally, corrosive reagents were also used consisting of 3 wt.% NaF + 5 M HNO,, 0.1 M Al(NO,), to form complexes with F- ions. The application of the corrosive agents was restricted to 5 hours. The volume of RAW was kept low by recycling the decontaminating solutions, starting first with less contaminated areas and proceeding with a repeated cycles to more and more heavily contaminated spots; the volume was further reduced by evaporation. The resulting volume of RAW was only 169 m3, total beta-gamma activity 4.8 . loi4Bq (13 000 Ci). The operation lasted for 4 months. The value of the decontamination factor for each single decontamination cycle ranged between 1 . 1 and 5. The best results were obtained with a combination of HNO, and NaOH, the D , varying from 1.3 to 5. For other decontaminants the range was D, = 1.1-1.7. The D,value for one working day could be evaluated as ranging between 1.05 and 1.5. It took 0.12 to 1.85 h to remove the activity of 37 GBq (1 Ci). In the final stage, the volume of liquid wastes containing 37 GBq ranged from 10-76 1. In another operation, glove boxes strongly contaminated with plutonium were decontaminated after they had been badly damaged by an explosion. The operation took 9 months and cost 1300000 Swiss francs [96]. The radiochemical laboratories at Hanford (U.S.A.) used the APOX process to decontaminate stainless steel surfaces in hot cells [97]. In addition, ultrasonic vibrations, water jetting at 70 MPa and sandblasting were also used. A report on the decontamination of a hot canyon at the same laboratory was also published [98]. The canyon, contaminated predominantly with plutonium, was a 300 meter-long facility used for plutonium production. Treatment was 209

performed by rinsing with 60 YOHNO,, water, AP solution and again HNO,. The plutonium production facility was decontaminated either by hexone, 75 % HNO, 1 YOH3B03and 75 YOHNO, 6 YOAlF, 1 70H,BO,, or by a two-stage method using solutions of AP and HNO, with a water rinse in between. The roof of the canyon was decontaminated in the same way, except that the last HNO, step was substituted with 2.5 wt.% oxalic acid and water. Deposits in glove boxes were removed mechanically, followed by cleaning with a solution of HNO, HF. The starting radiation field (dose rate of 10 Gy . h-I, i.e. 1000 rad . h-'), was reduced, to 2 2 4 pGy . h-I) with hot spots reaching 50.05 mGy . h-' (50CL-5 rad . h-'). The steam emulsion method at 400 kPa and 343-353 K was used in the USSR [99] to decontaminate a hot canyon, corridors, stainless and carbon steel large vessels and epoxy-sealed surfaces. The procedure required 15 1 decontaminating solution per square meter; the solution consisted of 0.5 wt.% oxalic acid 0.5 wt.% sodium hexametaphosphate and 0.1 wt.% anionactive detergent. For decontamination of a nuclear fuel reprocessing plant where ruthenium was the major contaminant [ 1001, the following reagents were found effective: HN03, NaOH, KMnO, NaOH, and EDTA. At the KFK plant, Karlsruhe (FRG), the double-lid transfer technique has been substantially improved [ 1011. The replacement of defective power manipulators and slave arms of the master/slave manipulators as well as disposal of wastes took place via the double-lid locks using special airtight containers. Techniques which would be applicable to decontamination of the remotely operated bridge cranes at the Savannah River Plant were evaluated in laboratory scale tests. The most attractive technique available appeared, in 1985, to have been high-pressure freon blasing ; the attitude to freon release, however, has changed substantially since that time. Strippable coatings are considered to be an acceptable substitute [102].

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2.3 Decontamination of disassembled components of nuclear power plants, machinery parts and tools These operations are usually performed in Decontamination Centres. When the level of contamination is too high, there must be an initial partial on-site decontamination to permit safe dismantling. The disassembled parts are wrapped in plastic foils, or the contaminant is immobilized (preferably by a polyurethane foam layer) to prevent a spread of radioactivity during the transfer to the Decontamination Centres. Most of the applicable decontamination methods have already been described in Sect. 1.7.2 and 2.1.1.3. Only those aspects 210

that have not been sufficiently dealt with in previous sections will be discussed here. A variety of decontamination methods are available for the treatment of machinery parts, tools and implements : chemical methods, electrochemical techniques, ultrasound, pressurized water, steam emulsions and steam-water jetting, foams, solvents, grinding, abrasive treatment, molten salts, thermal erosion, smelting, adhesive foils, strippable paints and gels, pastes, pneumatic methods and others. It may be sometimes of advantage to cover the contaminated external surface with a layer of particulate pigment with grain size about equivalent to the particle size of the deposits (usually between 1 and 10 pm). Since presumably the physical behaviour will be similr, the visible release of the pigment will be indicative of the effectivity of decontamination [ 1031. 2.3.1 Chemical methods

Chemical methods are the basic and most effective decontamination techniques, though their major disadvantage is a large volume of resulting radioactive effluents, which makes decontamination of machinery parts can be performed in two alternative arrangements: they are either inserted in a loop with circulating decontaminants, or submersed in a vat containing the decontaminating solution.

2.3.1.1 Circulation methods The methods are analogous to those used for decontamination of NPP circuits (see Sect. 2.1). Here, however, an ancillary “ad hoc” decontamination loop must be specially constructed. Even separable components of the primary circuit may be successfully decontaminated in this way, usually those that exhibit the heaviest contamination. As an example of this technique, a flow chart is shown (Fig. 2.6) of the decontamination loop which was built to clean the rotor of the main circulation pump of a WWER-type NPP [l]. The two tubs (3 and 5 in Fig.2.6) are schematically shown in more detail in Fig.2.7. The decontaminating solutions may be warmed up by steam; the steam condensate is utilized for rinsing. The solutions may be recycled. Other optional vats may be attached to the loop for decontamination of smaller parts and tools. The D , accomplished with the described system ranged between 10 and 100. The recent trend in decontaminating the primary circuits is to separate the most contaminated parts or sections and decontaminate them in specially constructed loops either on site or in Decontamination Centres after disassembling. The decontaminating solutions have essentially the same chemical com21 1

1Q

5

4"

4

6

4

LN

- 1

W

7

Fig. 2.6. Flow chart of a loop for decontaminationof dismantled parts of the main circulation pump (NPP with WWER) [I] I -- circulation pump driving the decontamination solution, 2 - lower fluid level gauge. 3 - tub for the bottom part of the pump, upper fluid level gauge, 5 - tub for the rotor, 6 - AP solution reservoir. 7 - acid solution reservoir. 8 -- water reservoir, 9 -- water pump. 10 - compressed air. I 1 - steam. I2 - condensate, -s- steam, --Econdensate

4

~

~

position as those described for the on-site decontamination of entire circuits. In one reported operation [ 1041, the main circulation pump was decontaminated with a two-step procedure consisting of (1) 4 wt.% NaOH + 0.2 wt.% KMnO,, and (2) 2.0 wt.% oxalic acid + 0.2 vol.% H N03at 353-363 K. By using for the decontamination loop a circulation pump with an output of 12.5 1. s-' (45 m3.h-I), a decontamination factor of D , = 10-100 was achievved. Radiochemical analyses disclosed, however, that the 51Crradionuclide was insufficiently removed. A modified LOMI process applied to one side and the Can-Decon process to the other side was the method used for decontamination of a SG removed from service 1551. The LOMI and Can-Decon processes have also been employed for fuel bundles and highly irradiated stainless steel samples taken from a BWR. The Can-Decon as a typical dilute chelating process, and the LOMI as a typical more strongly reducing process were preceded .by application of the AP oxidizing step [ 1061. A two-stage chemical decontamination method based on inhibited solutions of citric or oxalic acid was developed to decontaminate high-pressure 212

turbine rotors of BWR-type NPP. Acridine and hexamethylenetetramine were found to be the best inhibitors for that purpose. The oxide film along with 1 to 3 pin of the underlying metal matrix was removed by the process, resulting in a D', = 5.9 [107]. Aqueous solutions of fluoro-boric acid, silico-fluoric acid and potassium silim-fluoride were shown to possess the capability of dissolving the radionuc1idc:-rich oxide layers. They are therefore suitable decontaminating reagents for the construction materials of high temperature gas turbine blades : Ni-Cr alloy Nim ocast 713 LC, Mo-alloy TMZ. The corrosion damage was very moderate and no signs of pitting corrosion appeared. However, the same reagents caused sevexe pitting in type 21CrMoV511 mild steel, which is the structural material of tlhe turbine rotor. Although an addition of suitable solution-specific in-

&

2

3

I

F

4 i i

i

--+A

i i

i

4

I

-

K

i -

b7ig.2.7. Special tubs to contain the decontaminated parts of the main circulation pump [l]

- lo,ver fluid level gauge. 2 - upper fluid level gauge. 3 - decontaminating solution inlet. 4 -tub

for the bottom part. 5 - tub for the rotor. 6 - steam inlet, 7 - compressed air inlet. 8 - solution outlet

213

hibitors may reduce the corrosion rate of mild steels by up to two orders of magnitude, the formation of pits cannot be entirely avoided [IOS]. 2.3.1.2 Chemical decontamination performed in vats These methods are suitable for small and medium-size objects. Machinery parts and tools to be cleaned are submersed in decontaminating solutions maintained at a preset temperature and preferably agitated. Long-term experience shows that it is convenient to have three tubs available differing in :size: a big one with a volume of 20 m3 or more, one with 2-3 m3 and the third one of 1 m3or less [ 131. Agitation of the liquid is ensured by stirring, by letting st eam pass through the solution or by ultrasonic vibrations. The following are some selected examples of successfully employed methods : For stainless steel parts, a two-bath procedure was recommended, consisting of: (1) 3 wt.% NaOH + 0.2-0.5 wt.% KMnO,; (2) 1--3 wt.% omlic acid + 0.5-1 vol.% H,O, at 363-368 K for 10-15 min. Another oxidationwt.% NaOH 0.1 wt.% reduction two-stage process used: (1) 1-5 KMnO, + 0.4 wt.% sodium hexametaphosphate; (2) 5 vol.% HNO, -t 0.2 wt.% oxalic acid + 0.2 wt.% NaF + 0.5 wt.% Na,CO,. Rusty carbon steel parts can be treated with an inhibited solution of 1-3 vol.% H,P04 at 3,43 K for 30-60 min. Heavily contaminated stainless steel is effectively decon taminated with a three-stage procedure: (1) 3 wt.% NaOH 0.3wt. YOKMnO,; (2) 5 vol. YOHNO, + 0.3 wt. YO KMnO,; (3) 0.5 wt. YOoxalic acid, each stage a t 363-368 K for 10-15 min. A universally applicable reagent for various materials with not very resistant contamination is a solution of0.5-1 .O wt.% oxalic acid 0.5 wt.% sodium hexametaphospha te + 0.1 wt.% detergent containing a mixture of sodium alkylbenzenesulfonates at 323-353 K for 1(r-20 min. Bulky components are decontaminated in solutions containing HNO,, for example 1.5 vol.% H F + 10 vol.% HNO,. Other solutions, such as AP, 1 wt.% oxalic acid, AP-Citrox, and particularly Citrox (0.2 M citric acid + 0.5 M oxalic acid) may be used. A damaged evaporator which had been used for acid regeneration [21] was decontaminated by a two-step process: (1) HNO, NaOH; (2) KMnO, HNO, + EDTA. A reservoir of high-activity wastes contaminated with sludges was decontaminated by two repeated cycles of 50 h each in which about 95% of the sludge was dissolved in an approximately twenty fold volume of 8 wt.% citric acid at 358 K. Corrosion of the material was largely prevented by subsequent neutralization [39].

+

+

+

+

+

2.3.1.3 Manually performed chemical ‘decontamination Manual procedures are also used to decontaminate smaller machinery parts, although it is prudent to restrict the extent of manual operations as much 214

as possible, because it is technically difficult to safeguard the working personnel from radiation hazard. The most common utensils and tools are brushes (usually bound to long sticks), rags, mops and pails with water and decontaminating solutions. Complexing agents, acids, sodium hexametaphosphates in a concentration between 0.1 and 0.5 wt.%, and detergents in 0 . 1 4 . 5 wt.% solutions are commonly used. Vessels, electrodes and other utensils made of platinum can be decontaminated in boiling aqua regia (HCl + HN03).The effect is increased by adding an isotopic, or less conveniently nonisotopic, carrier of the contaminating element in a chemical farm which is not precipitable by ammonium chloride. When the contaminant is dissolved, the object is thoroughly rinsed and the platinum in the solution may be recovered by precipitation with NH,Cl. The precipitate is washed several times with a solution of the isotopic carrier and annealed on a platinum sponge. Up to 99 O h of the dissolved platinum can be recovered [49]. 2.3.1.4 Ultrasound and other vibration chemical methods The combination of chemical action and vibrations is particularly suitable for removal of firmly fixed contaminants which require a mechanical impulse in addition to the chemical effect of the solution, and where the high radiation field precludes a manual intervention. The application of ultrasound to decontamination purposes has however certain limitations do to the relatively short distance of the effective range of the vibration field: it is only about 30 cm for currently available ultrasound generators. On the other hand, the system is easily amenable to remote control. It has been utilized in decontamination of spent fuel storage pools, reactor fuelling channels and fuel assemblies themselves [52]. Another possible dual method is the use of polishing reagents in combination with mechanical vibrations [50]. The removal of particulate magnetic corrosion products from fuel element cladding is facilitated by magnetic field vibratios. Magnetite particles 1 to 5 mm in size are effectively removed by vibrations of the magnetic field ranging from 2&200 Hz [105].

2.3.2 Electrochemical methods Electrochemical decontamination of metallic surfaces is a cleaning process which takes place in a decontaminating electrolyte. The object to be decontaminated is connected to the positive pole of the electric power source. Electrolysis is one of the advanced, highly effective and rapid decontamination techniques characterized by a very little damage to the metal surface. Its applicability is somewhat restricted by the fact that it involves the use of electric current and 215

requires therefore appropriate precautionary measures, particularly safe insulation. An unequal electric resistance and thus also the current density over an intended metal surface results in a tendency to smoothen the roughness of the surface, hence the alternative term “electropolishing”. The induced very slight corrosion is entirely homogeneous and causes no mechanical stress in the material. A smooth surface reduces the retention of the contaminant and corrodes less than an unpolished surface. High current densities cause water to undergo electrolysis to oxygen and hydrogen. In a closed space, this creates a risk of explosion. Hydrogen generated on the cathode may be the cause of hydrogen embrittlement and subsequent stress corrosion cracking, particularly of noble steels and alloys. It is therefore necessary to bind the hydrogen chemically. The optimum conditions for electropolishing must be determined empirically on a case by case basis. The most effective, and also most common, electrolyte in electropolishing is phosphoric acid used in concentrations ranging between 20 and 80 wt.% and at a fairly temperature, such as 353 K. Since however the volume of resulting liquid RAW is high (see also Sect. 4), other electrolytes more convenient in this respect are often preferred, even though their effectivity is lower. Examples are solutions containing 10 YO formic acid 0.5 M potassium bromide, or 5 ‘YO oxalic acid + 0.5 M potassium bromide [109]. Other electrolytes have also been tested with some success, such as a mixture of phosphoric and chromic acids at 353 K under vigorous stirring, or various alkaline electrolytes [I 101. After the electrolytic process has been terminated, the cleaned object is rinsed successively with water, alcohol and an organic solvent, and then dried. The room or cell set aside for electrolytic polishing must be properly equipped. A collection system for radioactive effluents, a ventilation system with filters entrapping radioactive aerosols and gases, and exhaustion of explosive gases are obligatory. When the iron concentration in the electrolytic bath exceeds 3.5 wt.%, or if the electrolyte is excessively contaminated, it must be replaced at least partly, to keep the iron concentration within the optimum range of 2.5 and 3.0 wt.% [12]. The surfaces of PWR primary circuits were decontaminated by a specially developed ELPO process based on the use of phosphoric acid. Anodic oxidation takes place in the oxide layers:

+

Fe2+

+

Fe3+ + e -

+

+

(2.8)

2Cr3+ + 7 H 2 0 + Cr,O:14H+ 6e(2.9) Simultaneously with the dissolution of the oxide layer from the face side, the layer also- begins t o peel off from behind as a result of the base metal dissolution : Fe + Fe2++2e(2.10) 216

The deposits of hematite appearing in BWR circuits must first be transformed to magnetite, so that a cathodic reduction Fe3++ e-

+

Fe2+

(2.1 1)

must precede the anodic oxidation. The ELPO process is applicable to surfaces with a relatively simple geometries. Experience shows that the effectivity of the method is as follows: for PWR pipeways -DF = 150-400; BWR pipeways - DF 25; SG pipes - D F 50. Even when repeated several times, the ELPO process does not accelerate the corrosion rate of austenitic and chromium steels and the nickel-base alloy Incoloy 800. The Inconel 600 did show a tendency to selective corrosion, but it was possible to prevent it by using a modified electrolyte [30]. Apart from electropolishing, special electrochemical decontamination methods have been developed which find an application in nuclear power plants. The electrolyte composition resembles that of decontaminating solutions, for example (1) 2.0-3 YOoxalic acid; (2) 1.5-2.0 YOH2S04+ 1.5-2.0 % H,PO,; (3) H N 0 3 + H,CrO, ; (4) 20 YOH2S04(allowing a semi-dry procedure) [l 111. It is advantageous and timesaving to place the smaller metal parts in a stainless steel wire cage and submerse the entire cage in the electrolyte. The cage itself serves as the cathode. Straight segments of pipes can be decontaminated both outside and inside by means of movable electrodes. An interesting electrochemical method had been suggested for decontamination of tools [112]. It makes use of a mixture of dilute hydrochloric and sulphuric acids. The method enables the recovery of valuable contaminants, in particular plutonium and other transuranic elements. Contaminants removed from the treated tools, connected to the positive pole as the anode, become deposited on a stainless steel cathode. When the decontamination is completed, the tools are taken out and replaced by a carbon anode. A second electrolytic process eliminates the rest of the contaminants from the solution and deposits them upon the cathode. As soon as the electrolyte is free of radioactivity, the cathode is subjected to ultrasonication. The vibrations release the oxide layer of recovered contaminants. Large-size flat areas are conveniently decontaminated by a reported modification [l 111consisting of an external movable electrode equipped with a nozzle which continuously moistens a porous dielectric with an electrolyte. The current density is 0.5-1 A . cm-’ and the electrolyte consumption 0.1-0.2 1. m-,.Any source o f direct electric current with a sufficient output can be used, such as an arc welding generator. Electrochemical methods have been successfully applied to decontamination of a variety of machinery parts in NPP and in nuclear fuel processing plants

-

-

217

and to decontamination of those spots on metal surfaces which resist chemical decontamination, and also to decontamination of molybdenum casings used in the production of ceramic nuclear fuel. They are also the decontamination methods of choice in all cases where recovery of plutonium and other transuranics is the objective. The main circulation pump case, steam generator collectors and the main gate valve of a PWR primary circuit were decontaminated, with a D, ranging from 50-500, by an electrochemical process using a movable electrode and an electrolyte consisting of sulphuric, oxalic and phosphoric acids. Not more than between 100 and 200 1 of liquid RAW were produced in eight decontamination cycles when treating the main circulation pump case [I 131. Metal parts with radioactive deposits on the surface were placed in a container and connected to the negative pole of a DC power source. After the anode had been positioned in the same container, the whole system was submersed in an electrolyte. The required concentration range of the solution was 0.1 -10 wt.%. Below 0.1 YO, the electric resistance increased to a level which makes the process inefficient, while with concentrations beyond 10% the rate of natural corrosion of non-metal materials began to increase. The method made it possible to clear the metallic surface of the cruds by a synergistic effect of the induced reduction and the physical action, without damaging the matrix material [114]. Deposits of plutonium on stainless steel were removed by the Redox decontamination procedure which uses a solution of a suitable metallic salt to promote stainless steel corrosion. The Ce4+--Ce3+system was selected as the decontaminating agent, because it has sufficient stability in an acidic solution, dissolves stainless steel at a suitable rate and the ions are efficiently entrapped by electrolytic oxidation. The Redox method combines the favourable features of chemical decontamination and electrolysis [ 1151. The decontamination efficiencies of electropolishing and that of ultrasonic cleaning were compared in a test study. Significant effects were observed after ultrasonication during the first 5 min, but not in the following 20 min. The sample thus treated was then electropolished for 5 min. The contaminant was removed below the limit of detectability. The same effect was observed when the sample was immersed in the eletrolyte prior to the electrolytic process. The two ways of pretreatment were effective in decontaminating the surface, though they could not remove such substances as Pu existing within the bulk or at grain boundaries of metal crystals [116]. After a completed electrochemical decontamination, the metallic surface has a grey colour typical of a nonoxidized metal. The efficiency of electrochemical methods depends on electric current density, on how uniform is the contact of the felt-lined cathode with the decontaminated surface, on the nature of the 218

contaminant, the degree of soiling and also on the operational conditions of the equipment. 2.3.3 Methods based on the use of pressurized water Decontamination utilizing water under pressure is carried out in closed boxes or in cells and either manually directed or rotating jets are used. If the effluents are drained via the waste water system, no aggressive additives are permitted. The method suffers from one major problem, namely the large volume of effluents. To reduce this, the decontaminating liquid is collected and recycled as long as the radioactivity of the treated part continues to decrease and this requires a continuous monitoring. If a closed circuit modification is used, the method is not limited just to the use of water, but various other cleaning solutions may be applied in this way, even with solid abrasives as additives. It is convenient to arrange small items to be decontaminated in steel wire cages. A thorough rinsing must follow decontamination. The effluents are treated as liquid RAW, the air is exhausted and filtered as it passes through a ventilation system before it is discharged through a stack into the atmosphere. Water at about 8-10 MPa overpressure is used to decontaminate large-size vessels, SG chambers etc. The equipment designed originally for cleaning the milk tanks in the dairy industry has just the required characteristics. To improve the effectiveness of the decontaminating procedure, hot demineralized water may be supplemented with citric acid or sodium hydroxide. In this way, the D F value may be raised to 50-100. Fully automated, sophisticated systems with revolving jets have been devised. An advanced piece of equipment of this type is the hydromonitor described in Section 2.1.1.5. Transport tanks are decontaminated by a combination of high-pressure water jetting and a mechanical cleaning, usually by means of revolving brushes. The modern highly effective equipment is constructed for pressures as high as 400 MPa. 2.3.4 Steam emulsion and steam water methods Steam ejected at 0.8-1.2 MPa pressure carries along the decontaminating reagent, either a solution or solvent. A solution containing 2 wt.% oxalic acid, 0.5 wt.% non-ionogenic detergent and 0.5 wt.% sodium hexametaphosphate has been recommended. The method is particularly suitable for decontamination of internal surfaces of tanks, fuel storage pools, as well as ceilings, walls and floors of rooms and cells, external surfaces of technological systems, machinery, equipment and tools (see also Sect. 2.1.1.5). 219

2.3.5 Foam methods These methods deserve more attention than they have hitherto received for two reasons: they have good wetting capability, and they give rise to low volumes of resulting RAW. Foam is generated by letting air or steam pass through a solution of special foaming agents, with functional components, such as detergents, complexing reagents, sodium hexametaphosphate acids etc. Special foam generators have also been constructed. 2.3.6 Methods based on the use of solvents Solvents are the reagents of choice for decontamination of greasy surfaces. Because solvents are relatively expensive and can in most cases be easily evaporated, they are frequently recovered from the effluents by means of mechanical or sorption filtration and subsequent distillation. The most common solvents are tetrachlorethylene and trichlorethylene, although other chlorinated solvents are also available. They are incombustible, but their wider application in nuclear technology is severely limited by the fact that their decomposition is accompanied by a release of C1- ions. Chloride ions are highly undesirable because they strongly increase the tendency to intercrystalline corrosion of stainless steel under stress. The application must therefore be selective; they are very useful for instance in decontaminating metal scrap. Other solvents which are effective degreasers (kerosene, gasoline and other) are mostly combustible. 2.3.7 Grinding Portable grinders, polishers, honing machines and similar equipment using grindstones, emery disks, sandpaper and other abrasive means can be utilized to remove strongly adherent contaminant along with a thin surface layer of the base material. Unless wet grinding is used, containment of the resulting aerosols must be ensured. Very fine grinding (honing) finds a frequent application in decontaminating metal containers for high-level activity wastes. A similar effect is achieved with a vibration finishing process using ceramic and metal powders. This was found particularly convenient for decontamination of glove boxes, equipment and tools contamjnated with plutonium ahd'other transuranics, because the contaminants can be effectively recovered from the solid wastes. The low volume of exclusively solid wastes is the chief merit of grinding methods.

220

2.3.8 Abrasive methods

Sandblasting in the dry or wet state (possibly in combination with chemical methods) is an effective method of decontaminating machinery parts, means of transportation of all kinds including automobiles and ships, concrete, brickwork and paving etc. The abrasive material is usually recycled. A veriety of particulates have been used as abrasives : sand, carborundum, silica gel, boron carbide, B203,glass cullet, chopped wire, infusorial earth, pumice, corundum, saw dust, nut shells, and chaff etc. Fragile abrasives cannot be recycled; moreover, they cause excessive dust formation. metal abrasives prevent the spread of radioactivity. An attractive modification appears to be the use of ice, both frozen water and “dry ice” (solidified carbon dioxide); in the latter case, the particles are carried by a stream of nitrogen [117]. The ice is crushed and . particles of a desirable size are segregated by means of a separating sieve. Dry ice has been found superior to frozen water because CO, undergoes sublimation and the adsorbed contaminant is then retained in a small volume of solid wastes. A modification which completely removes the risk of spreading radioactivity is vacuum abrasive blasting performed under an evacuated bell-shaped transparent cover. Normally, the abrasive particles are blasted upon the surface, propelled by a stream of compressed air, and appropriate measures must be taken to limit the scattering of contaminated particles. The method is less hazardous in this respect if centrifugal force is used to propel the abrasive particles. A high-pressure water stream can also be used as the vehicle (see Sect. 2.3.3). The inlet and outlet chambers of NPP SG were successfullydecontaminated by air blasting with B 2 0 3 .The residues cause no problem here, because boron is a regular additive to the coolant. Citric and oxalic acids and phosphates in the solid state may be used in a similar sense. In the course of further treatment, the residues of these compounds used originally as abrasives dissolve and act secondarily as chemical decontaminants. The grain size ranges between and 1 mm in diameter; the air pressure is commonly 50-100 kPa. The wastes are usually continually aspirated and may be recycled after passing through efficient separators or aerosol filters.

2.3.9 Molten salts method This is another method aiming at minimizing the RAW volume. Flame-molten salts are applied to surfaces to be decontaminated as described in Sect. 1.7.2.1. The method can be used to decontaminate metallic machinery parts and tools provided that they tolerate high temperatures. 22 1

2.3.10 Thermal erosion method

This term is practically synonymous with the use of a laser beam. Under the effect of coherent infrared radiation, the upper contaminated layer of metal or concrete evaporates. The vapourized material must be entrapped on filters by means of an efficient ventilation. Since the technique can very readily be adapted to remote control handling, it is particularly valuable in extreme radiation fields [I 181. 2.3.11 Methods based on the use of adhesivefoils

Adhesive foils are appropriately used for decontamination of not very extensive areas with relatively simple geometries such as those of laboratory equipment, tools and plastic parts that would be easily damaged by other more aggressive methods. The chief advantage is the low volume of RAW in a solid form; the volume can be further reduced by burning the foil in an appropriately equipped incineration plant. 2.3.12 Methods based on the use of strippable paints and gels

Laboratory equipment, hot cell wall lining, internal surfaces of drums and containers used for transporting the spent nuclear fuel are preventively coated with a film of strippable paint to facilitate the subsequent decontamination. Suitable paints are those prepared on the basis of latex, polyvinylchloride and polyvinylacetate emulsions, or polyacryl, polyethylene and polypropylene resins. The resinous paints are water insoluble, but can be dissolved in special solvents (such as acetone, toluene, xylene etc.) or fused by high temperature. The paints are applied to the surface by means of a brush or by spraying. The use of gels is analogous. They are superior to paints in that the removal is easier: quite often, the gels together with adsorbed contaminants can be simply washed off with water. 2.3.13 Methods based on the use of pastes

Again, the chief merit is a low volume of RAW obtained in solid form. Apart from efficient decontaminants, such as complexing agents, acids, detergents etc., pastes contain a bulk of various inert fillings: Ti02, BaSO,, A1203, polyethylene and other materials (see also Sect. 1.7.2.1). 222

2.3.14 Pneumatic methods This category includes cleaning by suction, i.e. vacuuming, and less frequently air blowing. High-output suction fans are used, preferably supplemented with settling bags, cyclones and absolute filters. A suction nozzle is often inserted permanently in glove boxes; manually performed decontamination by vacuuming inside the glove box is a common procedure. The effectivity of the procedure is potentiated by dry ultrasonication. Such a combination is adequate for decontamination of the interior of dry vessels and pipeways, piping systems of gas cooled reactors, and also clothing.

2.4 Decontamination and decommissioning of nuclear facilities 2.4.1 Basic stages of decommissiong Decommissioning of nuclear facilities retired from operating service is implemented in three stages, though not necessarily in a complete stepwise succession [ 119-1 241. Stage I (safe storage, “mothballing”) consists in removing all disposable radioactive material, in particular the fuel assemblies, the primary coolant and all other contaminated liquids, and confinement of fixed residual radioactivity within the primary containment barrier in a way similar to that applicable to the operating state, except that all mechanical opening systems are permanently blocked and sealed in order to preclude any accidental escape or intentional release of the contaminants across the barriers into the environment. The barriers require continuous radiological surveillance with periodic inspections. Almost all structures and buildings are retained, the dismantling and demolishing are deferred. Stage 2 (entombment) is characterized by a removal of all easily handled parts to reduce the volume of the active zone, and a strengthening of the containment barriers. The contaminated area must be secured for a period of 100 to 120 years. An entombment of the reactor vessel in concrete is an effective way of hardened passive protection. Radiological surveillance and periodic inspections can be relaxed, but are still required. Partial dismantling of the inactive secondary structures of the plant is possible. Stage 3 (total dismantling) requires removal of all materials, equipment and components of the facility in which activity is still significant despite decontamination, and their transfer away from the plant site for safe disposal. Total demolition makes the site ultimately available to unrestricted use for other 223

purposes [125]. Stage 3 is the final goal for nuclear plant decommissioning. Its immediate implementation is, however, associated with often unbearably high costs. Total dismantling is iherefore mostly deferred to some future time and a combination of stage 1 and a part of stage 3 is recommended [119] as the basic approach to decommissioning. The holding period may be some 100 to 120 years, a time sufficient to outlast the confined radiological hazard, because most of the radionuclides, with the exception of an insignificant fraction, will have decayed by that time. If the radioactivity is then released into the environment, the resulting radiation exposure of the general public would be likely, in the next 10000 years, to cause a radiation damage that would be acceptable [ 1221. As of the end of 1986, the number of nuclear reactors which had finally terminated their operating lifetime was estimated at 110 (plus about 10 reactors operated for plutonium production). Out of this total, 41 were commercially utilized reactors with an output higher than 100 MWe, including two 1000 MWe blocks which had experienced serious accidents (the TMI-2 in the U.S.A. and the RBMK at Chernobyl, USSR). Of the small reactors with MWe less than 20, some 60 were completely decommissioned to the dismantling stage. Most of the remaining reactors (85 %) with an output between 20 and 100 MWe are maintained either in stage 1 or stage 2. It can be expected that the latter mode of decommissioning (i.e. safe storage and entombment) will prevail in the foreseeable'future [123]. 1

2.4.2 General consideration of the disassembly techniques and decontamination methods applicable to the decommissioning of nuclear facikties

The decommissioning operation begins with removal of all free, mobile and transportable equipment and facilities. This concerns also the asbestos insulation around the vessel and the piping systems, because asbestos in itself represents a health hazard [126]. Coolants and other radioactive liquors are drained from the vessels and pipelines. Dust is collected by means of highperformance vacuum cleaners. The contaminant remaining on the inner and outer surfaces is immobilized by applying a paint or by spraying with polyurethane foam. Vessels, pipes and equipment systems are cut into pieces of a size suitable for plugging the openings and wrapping the parts into plastic foils. The hermetically sealed packages are placed in containers and finally disposed as solid RAW. The emptied boxes, cells and rooms are subjected to decontamination. Dry, mechanical methods are used first, and only subsequently is use made of wet procedures, but methods employing low water volumes are preferable, to minimize the amount of resulting effluents. Metal objects are best decontaminated

224

by means of electrochemical methods, as they produce much lower volumes of RAW than chemical decontamination techniques. It must be borne in mind that the costs of RAW management, particularly the treatment of liquid wastes, are by far the most significant single item in the financial implications of a decommissioning operation [ 1271. Scrap metal can be decontaminated by smelting. Structures of concrete and brickwork are decontaminated and then cut, spalled or blasted apart with explosives. Any mechanical method of demolition and mechanical cutting generates radioactive dust and aerosols, and must therefore be accompanied by an efficient vacuuming and separation of particulates by filtration. Heavily contaminated objects are preferably handled by remotely controlled tooling techniques. For large-scale operations, automated devices, robots, wrecking machines and other special equipment with remote control are essential. These techniques have proved very helpful in the effective management of major nuclear accidents, such as those at Chernobyl and Three Mile Island [128]. In the course of the entire decommissioning procedure it is necessary to maintain a continuous radiological survey of the radiation situation in the locality where the decommissioning work is done. This requirement also includes continuous measurements of the radioactivity of all dismantled and decontaminated objects, as well as a regular monitoring of occupational exposures. The bulk of wastes resulting from decommissioning are low-activity solid wastes. These are better transported away from the site in large containers, in order to reduce the extent of the necessary cutting and other segmenting operations that are likely to enhance the risk of radiation exposure of the personnel involved, and which would increase the costs. It has been estimated that the dismantling of a 1100 MWe power reactor (PWR) yields about 5.0 to 5.5. 108kgof wastes. Some 98 % of the wastes are concrete debris of low or very low activity which can be reused as building material [129]. The highest-activity wastes are the contaminated metal parts of the disassembled structure and operational systems of the energy block. Even though wastes of this type account for only 2 % of the total bulk of the decommissioning wastes, they nevertheless represent a weight of lo7kg. High-activity liquid RAW are stored in large-volume tanks (10' to lo3m3),and are finally embedded at a temperature of 1373 K in borosilicate glass and disposed at the repositories. The costs associated with decommissioning of nuclear facilities are immense, but at no rate do they exceed 1 %' of the value of the energy produced during the operating lifetime of the plant. Current estimates indicate that for a NPP the costs of total demolition reach about 5 to 12 YOof the installation costs. As an example, decommissioning of the Biblis A power station (FRG) with a 1205 MWe PWR, is expected to cost 200 million DM [130]. The budget of the 225

planned decommissioning of the Shippingport NPP (U.S.A.) in 1990 allocates the sum of 98 million US $, a sum which makes up about 12 YOof the present equivalent of the installation costs [131]. It is now believed that the decommissioning costs for plants in the power range between 600 and 1300 MWe and possibly higher are independent of the size and type of the NPP. About one half of the total costs are expended on RAW management, the other half on demolition, surveillance, radiation protection and decontamination, as well as planning, provision of the information base and administration [I 231.

2.4.3 Immobilization of the contaminant on solid surfaces The dust particles which, after vacuuming, remain on the surfaces of solid objects must be immobilized. The objective of this operation is twofold: it prevents dust from swirling and thus stop an uncontrolled spreading of the contaminant which would represent a serious occupational radiation hazard, and it reduces the volume of the resulting liquid RAW. A fixation layer must be applied to the surfaces of all items leaving the decommissioned site. A temporary immobilization is conveniently performed with paints (e.g. an epoxy paint [132]), lacquers and adehesives applied by means of a brush or spray. The best agents for dust immobilization are polyurethan foams. They are fireproof, resistant to climatic factors (UV rays, thermal changes, humidity and water) as well as to mechanical aggression (impact, abrasion, erosion). A particularly relevant advantageous property of the UREFLEX foamy polyurethane is the low degree of initial migration of 6oCoand I3’Cs, increasing significantly only at later stages [133]. A layer of foam can be quickly and safely applied by means of a spray gun; it solidifies in a short time. Remotely controlled spray guns have been designed and used with success during the dismantling of the Marcoule reactor [133]. The device is equipped with a biological shield for the operator and an attachment enabling vacuum collection and filtration of loose foam particles. A similar immobilization tooling system has been developed for other situations: disassembling of glove-boxes used previously for handling the transuranic elements, in particular plutonium, and packaging large items to be shipped in transport containers to the RAW repositories.

2.4.4 Cutting and a5sassembling of contaminated metallic parts Segmenting operations on radioactive metallic parts are mostly performed by mechanical and thermal techniques. A new electrochemical segmenting method has also been developed.

226

2.4.4.1 Mechanical methods of segmenting and dismantling

These include sawing and grinding. Sawing is effective for segmenting large equipment systems. Circular saws are particularly adaptable to remote control and automation. Sawing generates less aerosols than grinding or controlled blasting with explosives. Grinding is suitable for slightly contaminated non-ferrous metals and thinwalled stainless steel tubes and poles. The amount of aerosols formed is considerable. The technique is unsuitable for segmenting thick metal sections. 2.4.4.2 Thermal methods

Thermal methods include flame cutting by means of oxyacetylene, oxygen or plasma arc torches. Laser cutting has also been tried and found useful primarily for fracturing concrete blocks. Flame cutting, including the electric arc, is suitable for poorly accessible sites, for curved kerfs, thick segments and large-diameter pipes. The plasma arc system shortens the duration of the operation relative to using a hack saw by a factor of about 180 [109]. If a mixture of argon and hydrogen is used is used as the fuel gas and carbon dioxide as the protective gas, the fume generation is negligible. The plasma arc can safely segment pipes with a measured exposure rate of 0.25 Gy .h-’. A remote control adaptor has also been developed [109]. Cutting by means of an oxygen burning torch or a plasma arc can be performed in air as well as under water. For example, a pressure vessel of wall thickness between 270 and 420 mm with a 4-12 mm stainless steel lining could be segmented under water [ 1291. The cutting was performed from within because of poor access to the outer side. A special and relatively cheap device has been constructed in Japan for this purpose weighing only 13 kg including the torch, the electric arc extension and the support plate. The unit can revolve over the range of 90°. The tool makes it possible to segment a 1100 MWe PWR pressure vessel with stainless steel lining up to a total wall thickness of 450 mm [129]. When cutting is done under water, contaminated particles become scattered not only in the water, but radioactive aerosols also appear above the water surface [1331. The amount of radioactive aerosols which are produced by thermal cutting, including the plasma arc, is lower when compared to grinding, but is somewhat greater than with sawing. Containment of dust particles and aerosols, and effective filtration of potentially contaminated particulate materials must be ensured with all types of metal cutting, be it mechanical or thermal. The HEPA (High Efficiency Particle Air) filters and electrostatic precipitators 227

are options of about equivalent efficiency. The use of protective respirators is compulsory for the working staff. It has been reported- [1341 that flame cutting of stainless steel glove-boxes which had been in use for a considerable time gave rise to more fume than what appeared when new and unused glove-boxes were segmented. The difference may be due to residues of chemical substances that had penetrated the walls in the course of many years’ operations. If plasma arc underwater cutting was applied, the fume formation was considerably less, regardless of whether or not the stainless steel walls were lead-lined. 2.4.4.3 Electrochemical method This technique of metal cutting is based on a similar principle of metal removal as that which applies to electropolishing. The method was used for segmenting the reactor pressure vessel. It is reportedly a relatively cheap and safe technique allowing moreover the decommissioning occupational doses to be kept very low [133]. The following figure (Fig.2.8) shows the electrochemical 1

Fig. 2.8. Schematic drawing of an equipment for electrolytic cutting of metals I

~

brass electrode (cathode), 2 - work-piece (anode), 3 - nonconducting cathode guide (made of glass fibres and epoxy resin), 4 - electrolyte (NaN03 water solution), 5 - kerf, 6 - cathodic insulator

metal cutting device. The space between a brass cathode (1) and the metallic work-piece, which forms the anode (2) is filled with a circulating electrolyte (4), for instance a sodium nitrate water solution. A high amperate direct current carries the metal along with the electrolyte and produces a kerf (5). The resulting hydroxides of the metal are continually separated from the electrolyte by means of sedimentation and/or centrifugation, and are concentrated in a filtration press. The whole process may be automated [133]. 228

2.4.5 Removal of surface contamination at decommissioning of nuclear facifities Surface contamination is generally removed only after it has been immobilized and the object disassembled, packaged and transferred to special Decontamination Centres. The exception is the on-site removal of radioactive dust by vacuuming. In some cases [135], decontamination is carried out both before and after disassembling.A survey of methods used for removal of surface contamination and their qualitative evaluation is given in Table 2.8.

2.4.5.1 Electrochemical methods There are two main procedures: the electropolishing method and the “Cerium-Redox” method. Electropolishing can either be performed manually or may be automated. Remotely controlled operation is usually preferred if the contamination exceeds the level of 100-200 Bq , cm-* area activity. The process can be carried out without prior disassembling, i.e. in situ. The initial stage of the procedure gives rise to a relatively small volume of a contaminated electrolyte. Phosphoric acid is the most effective electrolyte [135]. It is also used in a modification, designated as “swab electropolishing, employing a current density of 125 A . dm-2 and a flow rate of the electrolyte ranging from 5-20 1. h-I. Under these conditions, the steel corrosion rate ranged from 60-100 pm .h-’. The final surface roughness found with electrolyte flow rates of 5.1.h-’ and 75 1. h-’ was, respectively, 0.11 pm and 0.15 pm. Mild steels suffered a corrosive attack of 300600pm.h-’, irrespective of the flow rate [133]. Though the phosphoric acid electrolyte is highly efficient, its use brings considerable difficulties concerning the subsequent treatment of the resulting RAW.For instance, cementation of a 60% phosphoric acid solution requires its neutralization and dilution to a volume which is much greater than the starting volume of the contaminated acid. Other formulations for electrolyte composition are therefore preferred for decontamination of mild as well as stainless steels, such as water solutions containing 10 YOformic acid plus 0.5 M potassium bromide, or 5 YOoxalic acid plus 0.5 M potassium bromide. In these cases, the resulting liquid RAW can be easily solidified by cementation. The addition of KBr increases the conductivity. The formic acid exhibits high avidity for iron, while the oxalic acid produces almost insoluble salts which can be separated by precipitation and filtration. A remotely controlled in situ electropolishing modification was successfully applied to the decontamination of a stainless steel tank at the Fission Products Development Laboratory (USA) The tank, approximate volume 0.5 m3, was contaminated with deposited crystals of I3’Cs in an amount corresponding to 229

0

TABLE 2.8

METHODS USED FOR REMOVAL O F SURFACE CONTAMINATION IN THE COURSE O F DECOMMISSIONING OF NUCLEAR FAClLlTlES Methods$of decommissioning decontamination

lvaluation of suitability from the standpoint o Technical means

~

Wet

safety

costs

AW contro

+

++

++

+

+++

+

vacuuming

vacuum cleaners

grinding

abrasion

grindstones grinding and polishing disks abrasive tools

foams

foam generators

++

++

+

+

adhesive foils

adhesive foils

++

+ + t

-

+

strippable paints

brushes, spray guns

++

_ _

_

++

electropolishing

electropolishing equipment

+

++

++

etching

etching reagents ultrasound generators

paint removing

inorganic reagents organic solvents

Dry

Semi dry

efficiency

+ +

(+)

++

+

++

_ _ _

+ (-1

Mode of evaluation: + and - designate. mpcctively, favourable and unfavourable reported experience

Suitable mainly for decontamination of all parts of buildings and facilities

floors, painted surfaces of minor size small-size dismantled parts

floors, walls, ceilings, cable lines smooth surfaces of minor size metal, plastic and painted surfaces metal surfaces

all machinery parts painted surfaces painted surfaces

activity of the order of 10l6Bq ( lo6Ci). Chemical decontatmination procedures (various acids and alkaline decontamination solutions, high pressure jetting) resulted in a reduction of the exposure rate in the closed space to 0 . 0 8 4 . 1 Gy . h-’, or up to 1 Gy . h-’ on direct contact with the surface. A stainless steel cathode was placed in the middle of the cylindrical tank and the space was filled with an 80 % solution of phosphoric acid preheated to 358 K. Electropolishing was carried out for 15 min at 1.6 kA, current density 291 A . m-’. The residual exposure rate measured after the termination of the procedure was 0.03 Gy .h-I. It was not even necessary to preheat the electrolyte, since it was resistance-heated up to the required temperature by the electric current. The action of the electric current must in that case be extended to 45 min to achieve comparable results. The electrolyte can be used repeatedly; because the solution is too viscous, it cannot be pumped, but must be transferred by means of a vacuum siphon system [ 1091. Another decontamination method [ 1361 involves alternating square wave current electrolysis, employing sodium sulphate solution as the electrolyte. The oxide layer on the metal surface was selectively removed by the electrolytic process. Because the removal of the oxide layer depends on the diffusion of O2 and Fez+ within the layer, the optimum alternating square wave current was estimated to be 90 s. The decontamination factor achieved for stainless steel structures of BWR amounted to D, = 103-104 [136]. A system of continuous draining of the sedimented sludge, and regeneration of the electrolyte by freezing has been proposed [ 1371 to allow a prolonged electrolytic decontamination of metal wastes in an electrolytic vat. Hot cells withdrown from operation were successfully decontaminated by using two techniques of electrochemical removal of the surface film from small metal items : electrolytic etching and “in situ” electropolishing [ 1381. The “Cerium-Redox” method is suitable for remotely controlled decontamination of heavily contaminated tanks. The same electropolishing facility working with 3.2 A is used as that described for the in situ modification [109]. The electrolyte is an aqueous solution of 4 M H N 0 3 0.1 M Ce(NO,), . 6 HzO. The Ce4+ resulting from an oxidation by the electric current of Ce3+ attacks stainless steel in a liquid environment. In doing so, it becomes reduced back to Ce3+, and is again oxidized by the current and so on. The process made it possible to decontaminate the inner surfaces of a tank with detected exposure rates amounting to 1 Gy .h-’. Chemical decontamination methods were ineffective. Since the electric current is needed just to maintain the chemical reaction, its amperage is low, for instance 59A for an 8-hour period. The exposure rate within the tank could be reduced by a factor of 10 [109]. A special device has been developed, based on the “Cerium-Redox” method. An electrochemical cell containing liquid electrolyte consisting of a HNO,

+

23 1

+

solution with Ce3+ Ce4+ is attached to a recycle circuit supplying the electrolyte to the decontamination vat. The contaminated electrolyte is collected into draining pipes at the bottom of the vat and driven by a pump through a filter system. Nitric acid fumes are diverted to a condenser and returned to the electrolytic cell [ 1391. It is also possible to provide the decontamination vat with an ultrasound generator [140]. Various parts of a PWR treated for 6 h by the “Cerium-Redox” method could be decontaminated with an efficiency expressed by the decontamination factor exceeding 300 [141].

2.4.5.2 Chemical decontamination methods Chemical methods are often intensified by ultrasonication, or combined with electrochemical methods, or methods employing gels and foams. Gels and foams offer an important advantage in that they reduce the volume of liquid RAW. Gels saturated with acids and other chemical agents are applied by jet spraying in an amount of 100-200 ml . m-’. At a concentration of 1.6 mol HCl per 1 1 gel, the steel corrosion rate ranges between 0.1 and 0.3 pm .h-I, which is about 15 times less than when applying aqueous HCl. The corrosion rate is reduced because the gel inhibits the acid, contains only a small amount of the acid, dries up quickly, and prevents a natural convection of the acid. As an example, the following is a procedure which has been applied to the decontamination of BWR tubing system exhibiting an area activity of 2.37 Bq .cm-* due to 59 % 6oCoand 40 YO%Mn: 1. Immersing in a 2 M solution of H,S04 (this step reduces the radioactivity to about 1/100); 2. Spraying with a gel containing 2 M HNO, 2 M H F (this step further reduces the radioactivity by a factor of 10 to 15). Decontamination of pipes of a total weight of 6 . lo3kg gave rise to 5430 1 of liquid RAW (average activity concentration 740 Bq .l-’), 180 kg of solid RAW (with specific activity of 11 Bq .g-I) and 15 kg of solid RAW (with high specific activity of 580 Bq. g-I). The mean residual specific activity of the decontaminated tubes was 1.4 Bq . g-’ [133]. Foams are generated from aqueous solutions containing 2-5 wt. YOfoam-forming tensides (foaming agents) and chemical decontamination agents, mostly complexing substances, added in a similra concentration of 2-5 wt.%. Foams are stable for about 3 0 - 4 0 min. Residues of used foam are removed by vacuuming, sorption on various materials with large specific surfaces, mopping up with cloths or wads, or rinsing with water. Water however is used only sporadically in order to keep the volume of liquid RAW as low as possible. Any

+

232

substance which decreases the surface tension, such as silicone oil, methanol etc., is capable of destroying the foam. Even organic solvent-base solutions, such as ethyleneglycol,can be induced to produce foam, but the foaming power and also the decontaminating efficiency are low. As to the efficacy, foams are comparable with gels, but the handling of foams is much simpler. The corrosive attack due to the foams is milder than that caused by the base liquid constituent itself [ 1421. In addition, decontamination procedures using foam save much of the labour needed. In a reported attempt [143] vacuum cleaning followed by foam application reduced the surface contamination with PuO, by more than 80% and the airborne fraction of the contaminant by more than 99 YO. One of the limitations of the electrochemical methods is the requirement that the contaminated surface be readily accessible to the portable electrodes. Large machinery parts must therefore be first cut apart, or electrodes must be installed inside the equipment system. These preparatory operations are however bound to be associated with a greatly increased risk of occupational exposure. Such limitation is absent with chemical decontamination, but the large volume of the resulting liquid RAW and the ensuing immense costs of their safe management usually more than offset this advantage. As a possible solution to this problem, a procedure has been proposed [144] for decontamination of large machinery parts which first uses chemical decontamination, and the resulting liquid wastes are subjected to very effective electrolytic decontamination to reduce the volume. The solution used for the chemical decontamination step contains both a neutral salt, such as sodium sulphate or nitrate, and citric or formic acid, or possibly a mixture of these. For the electrolytic procedure, the metal vat serves as the cathode, and an insoluble electrode is used as the anode. This arrangement makes it possible to carry out effective electrolysis without decreasing the electrolyte concentration. Moreover, the liquid wastes arising from the initial chemical decontamination can be processed in the same electrolytic vat in which the decontamination took place. Since the processed solution can be utilized as the decontaminating electrolyte, the volume of the secondary wastes is substantially reduced [1441. 2.4.5.3 Wet sand-blasting

Abrasive treatment of contaminated surfaces by wet sandblasting was carried out from a distance of 7 cm at an impact angle of 40° using pressurized water as the carrying medium and 10 YO alumina of 65 pm average grain diameter. The amount of the grit was such that it would normally cover the decontaminated area with a layer 1 m thick. The corrosion rate of mild steel 233

treated by this method was 5-10 pm . h-' [133]. A vacuum abrasive sand-blast technique was applied to remove a fixed contaminant off the painted surface of steel and concrete [145]. Abrasion by means of crushed ice carried by a stream of compressed air and projected at a high speed onto the surface offers considerable advantages: it does not produce any secondary solid wastes, the volume of liquid wastes is reduced, and the decontaminated surface is not damaged. The method was successfully used to decontaminate large valves (6B-28B) of the BWR primary circuit; the efficiency was measured as D F= 10 [146]. An electrolytic-abrasive finishing method [ 1391 employs the same solution of a neutral salt as both the electrolyte and the abrasive-carrying medium. An oxide layer is generated by electrolysis, and is immediately removed by abrasion. The method is rapid, highly efficient, and has no restriction as to the geometrical shape of the object. A robot has been designed [147] enabling to perform the operation to be performed by remote control of its working tool.

2.4.5.4 Steam decontamination Steam is used at a high pressure (up to 400 MPa). An ingenious method has been proposed for decontamination of items with a complex shape. The object is submersed in a water-filled tank and treated with a jet of steam emitted from a nozzle. The steam condenses on the solid surface and a decontaminating effect is exerted by the impact vibrations without any bubble formation. As soon as the water temperature in the tank increases above the condensation point, a cooling facility is put into effect. The excess water resulting from the steam condensation is drained through an overflow [ 1481.

2.4.5.5 Pressurized water This is a highly efficient means for removal of surface contamination. A disadvantage of the method is the resulting large volume of liquid RAW, but this can be partly overcome by making the used water of the decontaminating solution recycle through a loop in which it is continuously cleared of the suspended impurities [149]. The pressure used may be up to 400 MPa. Good results have been reported with the application of pressurized water for various decontamination purposes during the accident management at the TMI-2 event. Fuel debris-was removed by jetting onto the surface water under a pressure of 34 MPa at a distance of 15 and 120 cm, or 240 MPa at a distance range of 2.5-19 cm and a traverse rate 47-91 cm .min-' [150]. The maximum efficiency ( D F =144) was achieved with 240 MPa pressure, 19cm distance and 234

41 cm .min-’ traverse rate. The method was also successfully combined with honing by, means of a rotating hone [ 1511. 2.4.6 Decontamination by means of solvents Glove-boxes which had been used for the production of nuclear fuel based on plutonium containing mixed oxides and intended to refuel fast reactors had to be decontaminated at the plant of the British Nuclear Fuel plant, Sellafield (U.K.). The decontamination operation, carried out in 1984-85, made use of freon (Arklon) and other non aqueous solvents. Radioactive dust was flushed out with these solvents as a pretreatment procedure prior to disassembly and decontamination. The used freon was filtered and reused [133]. 2.4.7 Decommissioning decontamination of waste metal scrap Total decommissioning of a nuclear plant of the PWR type produces a remarkable amount of contaminated scrap of the order of lo7kg. The prevailing radionuclides in surface contamination are 137Cs and %r. The structural materials of the core also contain induced radioactivity, mostly T o , which pervades the entire depth of the matrix. Approximately one half of the scrap may be regenerated and reused [1521. Low-activity solid radioactive wastes have a specific activity of 7.4. lo4Bq . kg-’ and higher. The natural radioactivity of steels is of the order of l0’Bq. kg-’ [152]. The following are the recommended limiting values of “clean malerials” for low-activity waste scrap metal [ 153, 1541: a) for beta and gamma emitters 1 Bq .g-’ as a mean of a maximum weight of lo3kg, with no single piece exceeding 10 Bq .g- I ; 0.4 Bq . for loose contamination determined over an area of 300 cm2, or over the entire area if smaller than 300 cm2; for fixed contamination; b) for alpha emitters 0.04 Bq .cm-2 determined over any point of an area of 300 cm2. If there are any doubts about the reliability of measurements over inaccessible areas, it is prudent to assume that the activity exceeds the “clean level” limits listed above. Properly selected decontamination techniques make it possible to reach the For isntance, stainless steel pipes were required area activity of 0.4 Bq . decontaminated by as many as 14 steps before the limiting activity was attained from the starting activity level of 3.1 . lo4 Bq .cm-2; the efficiency coresponded to a D , = 9.4. lo4. The decontamination procedure was selected with repect to 235

the lowest volume of secondary wastes, but it was applicable just to dismantled parts [153]. The objectives of remelting the contaminated scrap metal are the following: 1. To reduce the bulk volume of metal wastes. 2. To reduce the radioactivity level, mainly because the caesium radionuclides are extracted from the molten metal and transferred to the liquid acid slag. 3. To recycle the material. There are three possible pathways for processing the contaminated metal : 1. Decontamination - melting - reutilization; depending on the initial level of contamination, the costs of this procedure have been estimated to between 2.5 and 8 DM per 1 kg. 2. Immediate melting (without decontamination)-reutilization; this can be considered only if the radioactivity does not exceed 74 Bq .kg-I. This mode does not seem to have any substantial advantage over (I), since the costs of cutting, transporting and monitoring the material are reportedly about 3-9 DM per 1 kg. 3. Compressing - direct disposal as solid RAW, costing between 2.55.5 DM per 1 kg. This procedure cannot be applied to large homogeneous structures [ 1331. The wastes resulting from metal scrap remelting amount to about 3-5 YO of the initial weight of the metal [155, 1561. When remelting the scrap metal for the purpose of decontamination, it is necessary to cover the alkaline MgO lining of the melting arc furnace, which as such is unsuitable for the trapping of metal radionuclides, by an approximately 5 cm thick infusorial earth (SO,) lining which allows the use of acid slag capable of trapping the metal radionuclides, particularly those of caesium. The slag-forming minerals, e.i. quartz sand and lime, are added in a weight ratio corresponding to SO, :CaO = 3 :1 . With a slag amount reaching approximately 5 YOof the molten metal weight, about 80 YOof the caesium present is entrained in the slag, whereas more than 90 % of 6oCois retained in the melt 133. Scrap iron covered with a magnetite layer yields a higher slag activity than contaminated austenitic steel because the radioactivity of the latter is bound to the 1 metal matrix. The 'lomAgradionuclide behaves in the same manner as %o: 90 YO is retained in the melt, 10 % entrained in the slag and the aerosol filters. Together with I3'Cs, the radionuclides of l4Ce and '"Eu appear predominantly in the slag, and partly also in the lining. Because their boiling points are higher, they are absent on the aerosol filters, The "Mn is found in the slag and filters, whereas 65Zn,characterized by a low boiling point, escapes from the melt and partly from the slag, and accumulates on the filters. A few weeks after the shutdown of a nuclear reactor, the radioactivity of the stainless steel lining of the pressure vessel is almost exclusively due to @'Co. 236

Attempts have been made to separate the @-‘Co from Fe, Ni and Cr by means of a vapour phase transport using iodine as the carrier gas. Droplets of the metal, metal melted by means of a plasma arc, were allowed to cool on the steel surface, and formed a fine powder suitable for gasification. The operating temperature of the vapour phase with iodine ranged between 1300 and 1600 K. The separation could be achieved in several subsequent transport steps. It was found easy to separate @-‘Cofrom Fe and Ni, whereas the separation from Cr is still questionable. The process, if successful also on an operational scale, would offer two essential advantages: it produces no secondary wastes, and it can be optionally repeated, each time increasing the degree of the achieved separation [ 1331. Contaminated lead bricks and other parts of lead shieldings, lead pipes or the laboratory waste system etc. may be decontaminated by two alternative methods: by abrasion and, more efficiently, by remelting. The lead in an amount of several hundred kg is molten by electric heating in a large crucible. Slag-forming oxides with a high melting point, for example a mixture of 30 wt.% A1,03 + 30 wt.% SiO, + 4 wt.?” CaO, are added to the melt and left for 20 min. The floating slag, which contains almost all the radioative substances, is skimmed from the melt and disposed as solid RAW. The decontaminated lead is drained through the outlet near the bottom of the crucible and cast into an appropriate shape by pouring it into molds. The amount of lead lost by the procedure is only about 2 YO;the working temperature is close to 673 K. Since the atmosphere above the molten lead may be contaminated, it is necessary to exhaust the air and pass it through a filtering system provided with an “absolute” filter. The decontaminated lead may be released to unrestricted use, except that it is not suitable as shielding material for high-precision radioactivity measurements and similar activities requiring very low background radiation. By the melt refining of scrap metal resulting from decommissioning of a nuclear reactor, it was possible to reduce the uranium content in all metals (with the exception of Al) to a mass concentration of lO-’-lO-*; for aluminium it was between 1 and 2 . The degree of contamination depended largely upon the slag composition. While borosilicate the alkali-oxidative slags are more effective for iron and copper (and their alloys), aluminium is more effectively decontaminated with the fluoride-type slag [1571. A eutectic of sodium and potassium (“NaK”) contaminated with various radionuclides including the transuranics can be disposed at RAW repositories after transforming the alloy by means of gaseous chlorine to a mixture of inert salts NaCl KCl [158].

+

+

237

2.4.8 Decontamination and demolition of concrete Several modes of concrete decontamination and demolition are available: 1 . Chisel hammering 2. Spike hammering 3. Grinding 4. Surface concrete finishing 5. Scabbling 6. Wire brushing 7. Abrasion 8. Mechanical scarifying 9. Flame scarifying 10. Flame cutting 1 1 . Laser beam cutting 12. Paint removal (for painted concrete) 13. Controlled blasting [1591 14. Diamond sawing and core stitch drilling 15. Diamond wire-sawing of reinforced concrete [1601 All techniques of concrete demolition and surface decontamination (with the exception of those listed under 11-12) are likely to release radioactive particulates which must be effectively entrapped to safeguard the working personnel from occupational inhalation radiation exposure. Exhaustion by means of a ventilation system with inserted effective filters (mechanical, HEPA) or electrostatic precipitators, as well as the use of half-face respirators, are the obligatory precautionary safety measures. In the course of the TMI-2 decommissioning, some 3000 m2 of contaminated concrete floors covered with epoxy paints had to be treated. The techniques listed above under the items 3 to 8, and 13, were tried. The best results were obtained with scarification by means of a specially designed scabbler as well as with mechanical scarifying. The scabbler produced more compact concrete debris and consequently less dust and aerosols. The original epoxy coatings were effective in preventing the diffusion of the contaminant into the concrete. The paint layer itself, however, was pervaded with the contaminants [159]. Concrete cutting and scarifying by means of high pressure water jetting can be well adapted to remote handling. The pressure used is 240 MPa (35 000 psi). Either a cutting head or a rotating scarification head is used [109, 1331. Laser beam cutting is being developed in Japan for demolition of a 6080 cm thick reinforced concrete wall of the biological shield contaminated to a depth of about 10 cm. The following are the obvious advantages of the laser beam : 238

The kerf is narrow and clean, thus the amount of particulates expelled is small; - The remote control technology can be easily adapted; - The volume of radioactive wastes is small and easily manageable; - No use of water is involved, a fact which facilitates the collection and disposal of RAW [129]. So far, the laser equipment is too expensive for the method to be widely used, though it is conceivable that the price will go down in the future. -

2.4.9 Preventive measures likely to facilitate the decommissioning of nuclear facilities 1 . The reactor core construction material should contain no cobalt. 2. The concrete or brickwork walls, floors and ceilings are to be covered with protective paints or strippable coatings. 3. Alterations to the design concept of the shielding should be considered in order to facilitate decommissioning. 4. Baseline data should be available on all aspects that may prolong the operating lifetime and defer the decommissioning of nuclear facilities. 5 . Surveys and catalogues of design innovations aiming at making the subsequent decommissioning safer and easier should be set up and maintained [ 1331. 6. In order to reduce the bulk of contaminated scrap metal resulting from decommissioning, it has been suggested that all exposed metal should be provided surfaces with a thin cladding made of a metal either identical to, or different from, the protected base metal. Only the cladding would have to be handled as RAW at the subsequent decommissioning [161]. 2.4.10 Experience with some decommissioning projects The spent nuclear fuel reprocessing plant at West Valley (NY), U.S.A. operated from 1966 through 1972. Decommisioning began in 1982 with decontamination involving 43 % of the surface area, i.e. 33000 m2 [162] and removal of 5 . 10’ stainless steel parts. The total costs will have reached 400 million US$ [163]. A part of the facility will not be demolished, but will be adapted to other industrial use. An advanced robot system, the ROD-robot (Remotely Operated and Driven) was developed for decontamination of massively contaminated boxes. The robot is able to decontaminate rooms, lift objects which are placed in its 239

field of view, measure the exposure rate of ionizing radiation and collect samples to be measured. The kinematic functions are ensured by six subsystems: - Manipulators with a total of six degrees of freedom and a gripper; - A vacuum cleaner enabling the cleaning of floors, walls etc.; - A shielded TV camera which makes it possible to check the functions of the manipulators ; - A six-wheeled locomotive system carrying the manipulators, the vacuum cleaner, the camera and the control unit; - Auxiliary systems, energy and control cable lines; - A control desk including the.manual control and a computer. During the decommissioning, the robot was used to inspect the facility and then to dry clean and cut apart the process equipment to be dismantled. The resulting scrap was placed in airtight containers. A continuous radiation monitoring was maintained to prevent any uncontrolled spread of radioactivity and cross contamination. The containers were temporarily transferred to a storage room and provided with a polymer coating to seal them before final disposal. The resulting liquid decommissioning wastes were solidified, usually in a borosilicate glass melt and poured into steel drums as the ultimate disposal containers. When the radiation level so allowed, the final decontamination of some sections, such as corridors, passages, galleries or boxes already repeatedly decontaminated could be performed manually. Access to some hot cells, windowless and with no manipulators or lifting jacks, could only be gained after breaking a hole through the ceiling. Appropriate measures had to be taken to preclude an uncontrolled escape of radioactivity through the hole. In other cases, remotely operated “spiders” were used [ 1621. A chemical extraction cell, 5 x 5 meter in size and 20 m high, contained over 2 km of pipes, 13 tanks, columns, platforms etc. The mean exposure rate amounted to 0.05 mGy .h-’, with spots of up to 1.5 mGy . h-’. The quantities of dispersed plutonium, americium and 235U were, respectively, 104 g, 2 g and 40 g [163, 1651. The piping system was dismantled first. The contaminated pipes were filled with polyurethane foam to immobilize the radioactive substances, cut into pieces, stoppered and sealed. After wrapping and taping, the segmented pipes were disposed into low-activity waste drums. Large components, such as tanks, were provided inside and outside with a polyurethane foam layer, wrapped, sealed and transferred out of the room. Concrete surfaces were epoxy sealed and coated with a polyurethane paint. In this way, the exposure rate in the chemical extraction cell could be reduced by a factor of 9 [163-1651. The Garigliano NPP with a 160 MWe BWR was in operation from 1964 through 1978 when it was shut down because of damage to the steam generator. The radioactivity determined by measurements in the building housing the 240

turbine was just a small fraction of the total radioactive inventory of the plant. The area activity of the internal surfaces ranged from 3-45. lo3Bq .cm-2, that of external surfaces was between 0.4 and 4 Bq . The preheater made of AISI 304 L steel was contaminated with an estimated total activity of lo3MBq. A bunch of preheater pipes were decontaminated by ultrasound applied in a bath containing 4 vol.% HCl and a mixture composed of 0.35 vol.% H F 5 vol. % HNO,. For aluminium decontamination, the solution was replaced by 0.8 wt.% NaOH. Demineralized water was used for rinsing. The experience with this operation brought out two interesting conclusions: First, the H F HNO, mixture was found superior to HCl; and secondly, the total decontamination factor attained with ultrasonication in a decontaminating solution was higher than was the product of the decontamination factors for the ultrasound ( D F u ) and chemical solution (DFc)alone:

+

+

+

DF

> DFu

*

(2.12)

DFc

Another noteworthy example is the decommissioning, in 1985, of the BWR primary circuit piping system of the NPP at Lingen (FRG) [133, 1631. The mean are activity of internal surfaces due to 6oCo was 31 kBq.cm-2. The specific activity of the construction material to a depth of 40 pm was below 4 Bq .g-'. For decontamination, the segmented tubes were connected to a special decontamination loop. The decontamination procedures employed [ 1331are given in the following three tables (Tub. 2.9, 2.10 and 2.11). TABLE 2.9

TREATMENT TO REMOVE THE OXIDE LAYER Solution

Chemical agents

vso,

Duration (h)

Temperature (K)

H2S04 formic acid picolinic acid hydrazine 15 % PH

0.75g.l-' 0.3g.l-' 1 .o g .12.5 g .I - ' 1.2g.1-' 2.5

0.5

363

Oxidation

KMnO, HNO, - pH

1.0g.Ir' 2.5

1.5

363

Acid

oxalic acid

1.0 g . I - '

1.5

36 1

L

'

Among the decontamination procedures used, the LOMI process was found to have been the most effective one. It facilitated the removal of the surface oxide layer (Cr20,). Upon termination of the decontamination step, the

24 1

TABLE 2.10 TREATMENT TO REMOVE THE BASE MATERIAL

Acid HCI HNO, duration temperature dissolved material (Fe) removed layer Oxidation KMnO, NaOH duration temperature Oxalic acid oxalic acid duration temperature

2 1 3

1

Treatment g.1-' g.1-1

h K g Pm

g.1-1 g.1-1

h K g.1-1

h K

13 4.7 6.5 333 552 15.3

22 4.7 7 333 592 16.4

0.5 0.5 2 363

36:

0.5 0.5 306

0.5 303

1

4

6.5 2.35 8 333 194 5.4

36;

2 2 2 368

0.5 305

2 1 333

residual surface activity amounted to 0.33 Bq . cm-2, a level which is only about 0.1 % of the starting activity due to @Co and which is below the maximum permissible limit of 0.37 Bq . cmP2.Operating tests revealed the appearance of pitting corrosion after the first application of an acid solution. The dismantling of the heat exchanger of the advanced gas-cooled reactor at Windscale was performed by the UK AEA Establishment, Winfrith. The system was filled with an aqueous solution of 0.5 M HCl 0.0025 M citric acid (160 m3 in any single step) and left to act for 90 min at temperatures of 293 K and 323 K. The maximum D , achieved was 85; it was found possible to obtain the minimum required D, (equal to 30) with a solution composed of 0.1 M HCl +0.0025 M citric acid at 293 K. With D , = 30, the residual maximum dose equivalent rate was 0.1 mSv . h-' at the working distance and 0.5 mSv .h-' at close contact. Altogether, 1 1 . lo3kg of frame debris and 6 . lo3kg of parts of the BWR piping system were decontaminated by newly developed methods using gels. The initial activity of the material, accounted for by @Co(59 %) and 54Mn (40 %) ranged between 2 and 37 kBq .cmP2. The NUKEM Company, Hanau (FRG), developed in the years 1985-88 a complex technique for decommissioning the Material Test Reactor (MTR) and the Thorium High Temperature Reactor (THTR). The initial area activity

+

+

242

TABLE 2.11 DECONTAMINATION COURSE

Treatment steps

Dissolved activity (Bq)

Activity determined by electrolytic removal (Bq .cm-2)

Before treatment Removal of t h e oxide layer LOMI solution oxidation solution oxalic acid solution

2.0.108 3.1. lo8 4.1. 10’

R e m o v a l f r o m t h e base m a t e r i a l 1. acid treatment acid solution oxidation solution oxalic acid solution

7.4.106 1.8. lo6 1.4. lo6

16

2. acid treatment acid solution oxidation solution oxalic acid solution

2.1.105 I .2.105 1.0.105

5

3. acid treatment acid solution oxidation solution oxalic acid solution

1.9.104 3.4.104 3.3. lo4

4. subsequent solution oxidation solution oxalic acid solution

6.5. lo4

0. 3

of walls, floors and common equipment was determined to have been 26 Bq .cm-’. Sweeping and simple cleaning up was found sufficient to decontaminate metallic and ceramic surfaces, PVC and flooring material with a contamination not in excess of 0.1 Bq .cm-2. More strongly contaminated metals, ceramics, polymers, methacrylate glass, walls, floors and concrete were decontaminated by means of physicochemical methods, i.e. rinsing with acids, scrubbing, sandblasting etc. Uranium could be reclaimed from combustible wastes, scrap metal, terrazzo and concrete after an appropriate treatment, i.e. incineration or .melting, dissolution and extraction. The turbine piping system was decontaminated by wire brushing followed by electropolishing. The duration of the mechanical procedure ranged between 146 and 253 minutes per sample, depending upon the pipe’s thickness and the 243

type of the corrosive attack (caused by the corrosive effects of steam, feed water or condensate). Similarly, the electropolishing process lasted for 220 to 370 minutes per sample. The 40 wt.% solution of an acid used as the electrolyte was recycled [ 1331. Hot cells at the Irradiated Fuel Reprocessing Plant at Dounreay (UK) were decontaminated and converted to a new facility on the same site. The original design (completed in the late 1950’s) made the cells completely sealed and inaccessible, since access of the personnel was not necessary for routine operations. Jetting with high-pressure water was very effective, but the volume of the resulting arising liquid wastes was high. All cutting was done by means of a remotely handled electric plasma arc. The tank and pipes were decontaminated by nitric acid and steam. The demolition of the structural concrete shielding was done by diamond drilling of 100 mm stitch holes and then breaking out large pieces by hydraulic jack wedging, and/or pneumatic drilling [95].

2.5 Decontamination of buildings and work places 2.5.1 Decontamination of buildings The objective for which the decontamination of a building is required may vary [12]; it may be 1. to allow continuing operation at lower radioactivity levels; 2. to release the building for other types of working activities; or 3. to facilitate its demolishing. Depending upon the ultimate goal, the requirements as to the degree of decontamination, the planned time schedule of the action, and the methods and means applied will differ. The initial step necessarily preceding the decontamination procedure consists in measurements which aim at determining the level and nature of contamination, separately for each sector or storey of the building including also large machinery systems and similar technological facilities. it is important to ascertain not only the amount of radioactivity or the intensity of the radiation field, but also other relevant parameters, such as the radionuclide composition of the contaminant, the penetration depth, and nature of the interaction between the surface and the contaminant: whether wet or dry, loose or strongly adherent. Only a comprehensive radiometric survey of the actual situation will make it possible to decide rationally on the most effective method, as well as on the adequacy and the likely costs of available alternatives. A categorization of particular sectors of the building to be decontaminated with respect to the level and nature of contamination may help in planning the decontamination operation. The important criteria are shown in Table 2.22.

244

TABLE 2.12 POSSIBLE CRITERIA APPLIED TO CATEGORIZATION OF CONTAMINATED BUILDING SECTORS FOR THE PURPOSE OF DECONTAMINATION PLANNING Criterion

Categorization I

Area activity or radiation dose rate

coniaminated noncon taminated

/I\

moderately medial strong11

Binding to the surface

loosely bound contaminat

firmly bound contaminant

Depth of penetration

surface contamination

contamination

Type of contamination

dry state

wet state

Toxicity of prevailing radionuclides

moderate

medium

high

2.5.1.1 General considerations related to decontamination of buildings The following general aspects have to be weighed and decided upon during the preparatory stage: - Clarification of the objective to be achieved; - Carrying out a detailed survey of the building providing all necessary technical information concerning the contamination; - Evaluation of the overall conditions in the building, including how the building is geometrically and functionally related to other structures of the plant complex ; - Planning an economic analysis of the operation; - Compilation of a decontamination plan and its time schedule. Past experience with decontamination of buildings leads to the following practical rules : a) Large volumes of RAW are generated; it is therefore convenient to monitor continually the radioactivity of wastes as they are collected, and segregate them immediately to reduce to a minimum the volume of those that have to be further processed. b) -The decontamination proceeds from the roof (or ceiling) downwards. c) Machinery, equipment and furniture which are either non-contaminated or are designated to be decontaminated in special Decontamination Centres are moved out of the building. 245

d) The benefit of spraying the contaminated area with water or water solutions should be carefully examined agaist the disadvantage : On the one hand, it reduces the risk of radioactive aerosols but, on the other, increases the adsorption and binding of the contaminant, and results in contamination of the sewage system. e) In some cases, it may be appropriate to leave the contaminant on the surface and bury it by spreading an added layer of concrete, plaster, tiles etc., particularly if the contaminant is a beta or low-energy gamma emitter. f ) Highly toxic radionuclides, such as Pu, require special precautionary measures to protect the working personnel. Regular monitoring of the protective cloting contamination, changing the work teams at short intervals and periodic functional testing of the dependability of protective devices (particularly air tightness) are parts of such measures. g) If work has to be done at high temperatures, the working staff must be safeguarded against overheating (sprinkling with cold water, continuous monitoring of the health state). h) In cold weather, care must be taken to prevent freezing of the water supply and the waste effluents. Decontaminating solutions may have to be warmed.

2.5.1.2 Technical approach to decontamination of buildings The objective of decontamination and the future destination of the building are the main factors determining which of the following alternatives will be applied : 1. Decontamination is performed by appropriate methods, on a part by part basis; it may sometimes require a partial or total disassembling of the equipment. All surfaces are systematically decontaminated and the rooms are refurnished to allow resumption of the activity. 2. Technological systems are completely dismantled and moved out to be appropriately decontaminated elsewhere. The building is then decontaminated to the level which is required for the intended new use. 3. The building is completely cleared of all content and demolished by suitable methods (see Sect.2.4.5). The debris is taken away and disposed of apropriately. Before dismantling brickwork structures or breaking up concrete floors, and then again when loading the rubble for transport, it may be necessary to moisten the material in order to reduce dust formation. Water or a mixture of water and glycerol in a volume ratio 1 : 1 is used for that purpose. A simple decontamination procedure is effective for flat areas contaminated in the dry state: spreading the surface with fine-grain particulate material and 246

sweeping it off after a while. The following mixture has been recommended: 0.5 wt.% detergent, - 0.5 wt.% sodium hexametaphosphate, - 0.5 wt.% disodium EDTA, - 0.3-1 wt.% carboxymethyl cellulose, - fine saw-dust to 100%. The mixture is moistened with ethyleneglycol to such an extent that the particles still remain loose. -

2.5.2 Decontamination of laboratories The classification category of the laboratory in question largely predetermines the prevailing type of surfaces in it, and consequently also the ease of decontamination. Sealed floors, smooth walls, stainless steel lining, tiles, as well as an effective ventilation system simplify the decontamination process. It is essential that the exposed surfaces (bench tops, floors) be made of materials that are non-porous, nonadsorbing and easily decontaminable. A survey of methods and decontaminating solutions suitable for various types of surfaces is presented in Table 2.13 [166].

2.6 Removal of tritium from laboratories and other work places Tritium, :H = T, or “superheavy hydrogen”, is pure beta emitter with a half-life of 12.26a. Because of the low energy of the emitted beta particles 2.98 fJ (18.6 keV) - detection of tritium requires a liquid scintillation counter; the energy is below the detectability threshold of a G-M counter. Moreover, if tritiated contaminants become overlaid with a thin film of other material, or if it penetrates into the base material even to a minimum depth, it is no longer detectable by external measurements, because the radiation is completely absorbed in the covering layer of the material. For these reasons, the dependability of monitoring contamination with tritium in common laboratory practice is questionable. Tritium is a negligible hazard factor as an external radiation source; it is however hazardous as an internal contaminant. It displaces the light hydrogen in water and hydrogen-containing organic compounds that form the main constituents of living matter. The biological half-life of tritium in a human organism is 8-10d for a major part of the incorporated amount, though a small fraction (some tenths of a per cent) is bound to biological structures characterized by a low turnover rate and is eliminated from the organism with a half-life of 25&300 d. The maximumum permissible yearly amount of tritium incorporated in 247

TABLE 2.13 DECONTAMINATION METHODS AND SOLUTIONS USED FOR VARIOUS SURFACE

248

Method

Principle, application

1. Dry:

sweeping off the floor with moist saw dust

A mixture of fine saw dust (possibly with some additives) is moistened and swept across the entire area

2. Semidry: application of foam

Foam is applied to dustfree surfaces. After a specified reaction time, foam residues are removed by vacuuming, the surface is rinsed with water or wiped off with rags, swabs

3. Wet: (i) floor cleaning by means of washing and cleaning agents (ii) decontamination with special solutions

Simultaneous removal of mechanical impurities, pigmented soil and the contaminant by washing and polishing Swabbing or washing with solutions and mechanical means (brushes etc.)

(i) Desorption

Rinsing (spraying) of surfaces or swabbing with wads soaked in solutions or proprietary decontaminating agents. Small parts may be submersed in solutions

(ii) Dissolution

Dissolution of the paint in organic solvents, removal of the paint layer by KOH or organic solventbase paint removing agents

(iii) Mechanical

Paint removal by scorching, abrasion, sandblasting etc.

TYPES OCCURRING IN LABORATORIES Composition of decontaminating solution

Technical means Mostly manual procedures using fine rice-straw brooms, brushes, carpet sweepers Foam generators and vacuum cleaners

Floor cleaners, vacuum cleaners and polishing wheels with cloth or fibre pads, rags Floor cleaners, rags, brushes etc.

Spray guns, sprinklers, ultrasonic cleaners

Brushes, spray guns, sprinklers Special burners, pneumatic blasting devices

Various option, see. Chapter 1.7.2.1

- 1-5 wt. YOdetergent 1-5 wt. YO chelating agent - I wt.% citric acid -

Evaluation Effective for surfaces contaminated in dry state; used as a precleaning method before floor decontamination with foam or solutions Suitable for drycontaminated surfaces particularly in combination with subsequent wet methods

Commercial household cleaners, possibly with additives to increase the decontaminating effect

Efficiency depends on the amount of soil, on the degree of previous wear and previous treatment of the floor.

Possible formulations: (a) - 0.5 YOcitric acid - 0.5 YO detergent (b) - 0.05 YOcitric acid - 0.05 YO oxalic acid - 0.2 YOdetergent (c) - 0.1 YO 2Na EDTA - 0.2 YO detergent an aqueous solution (1 : 10) of a household cleaning agent

It further depends on the adequacy of the solution used

Aqueous solutions: (a) - 1 YOcitric acid - 0.5 YO oxalic acid (b) - 0.2 YO detergent - 0.1 YO2 N a EDTA - 0.1 % HMPNa (c) Proprietary agents (Radiacwash, Dekont) Solvents, diluents, acetone, butylacetate etc. 10 YO water solution of KOH

Very broad application field. Depends on paint quality and depth of penetration

Effective but very laborious methods. Some agents are toxic, explosive, inflammable. Some irritate bare human skin Requires special equipment. Can be performed only in boxes under stringent precautions to protect the workers (shields, goggles, gloves, ventilation)

249

TABLE 2.13 (continuation) DECONTAMINATION METHODS AND SOLUTIONS USED FOR VARIOUS SURFACE Surface Glass, enamel, teflon

Method

Principle, application

Rubbing or rinsing with deconta. minating solution. Submersing in solutions

These surfaces are very resistani to aggressive chemicals and mechanical wear, tolerate strong ly acid decontaminating solu tions acting for long time (1 hour). Can be vigorously scrubbed with brushes, rags, wads

Dry decontamination: removal of the contaminated layer

Dry methods: abrasion with glass paper, emery cloth, grinding with grindstones, rasping, planing

Wet decontamination: Rubbing or washing with decontaminating solution. Application of foams and special reagents

Wet methods: Rinsing, washing, scrubbing with scrub brushes etc. followed by thorough rinsing with water

Machinery and equipment

Wiping with rags or wads either dry or soaked in a decontaminating solution or organic solvent

Grease is first removed by drywiping. Residues of grease or wax are dissolved in organic solvents. Surfaces are decontaminated by rinsing or wiping with decontaminating solutions, rinsed with water and wiped dry. Preservation film is renewed

Non-ferrous metals (brass nickel silver, aluminium, copper etc.)

Methods are similar to those used for equipment. The solu:ions must contain a corrosion nhibitor to prevent corrosive lamage to surfaces

Identical with previous category

250

TYPES OCCURRING IN LABORATORIES Technical means

Composition of decontaminating solution

Evaluation Highly effective, with low consumption of decontaminating solutions

Portable sprinklers, ultrasound washers, drum-type washing machines

Possibleoptions for aqueous soh tions: (a) - 3-5 vol.% HNO,, HCI (b) - 1 wt.% oxalic or citric acid (c) - 2-10 VOI. % HIP04 0.1 wt. o/o HMPNa (d) - 0.1 wt. % 2 Na EDTA 0.05 wt. YOdetergent

Grinding, polishing and planing machines

Possible options: (a) - 0.1 wt. % detergent 0.1 wt.% 2Na EDTA (b) - 0.3 wt.% soap 0.1 wt.% HMPNa (c) - water solution of proprietary agents such as Radiacwash, Dekont etc. in a ratio of 1 : l to 1:lO

Wet methods are little effective and often undesirable, as they cause deeper penetration of the contaminant. Dry methods are laborious and give rise to radioactive aerosols. The best choice is probably Dekont which is a suspension and also has an abrasive effect

Possible options: (a) - 0.5 wt.% gasoline soap 0.5 wt.% detergent in trichlorethylene (b) - 1.5 wt.% ammonium citrate 1 wt.% oxalic acid 0.5 wt.% detergent in water (c) - emulsion formed by dissolving 5 % detergent in acetone mixing with water in a 1 : I ratio

Efficiency depends on how completely the greasy film is removed. The choice of the solvent is influenced by the surface finish of the material to be decontaminated

~

~~~

For brass and nickel silver:

- 2-5 wt.% 2Na EDTA - 0.5 wt.% hydrazine hydrate - 0.1-0.2 wt.% ammonia For aluminium: (a) 1 wt.% citric acid

-

- 1-2 wt. YO2 Na EDTA (b) - 5 wt.% H2O2 (30%) - 3 wt.% H$O, - 0.01 wt.% 8-oxichinoline

For copper (and brass): (a) - 0.6 wt.% acetic acid - 0.35 wt. YOcitric acid :b) - 0.79 wt.% KMnO, - 4 wt.% NaOH

When contaminated with Eu, 3r, Cs, Zr, Co, Ru When contaminated with Cs, Zoo,Ru, Zr When contaminated with Ru

25 1

'H-amino acids of human blood is set at 333 MBq (0.9 mCi). A concentration of 1 1 1 Bq . cm-2 (30 nCi . cmP2) is recommended as the permissible limit for work clothing contaminated with 'H-amino acids [ 1671. In nuclear reactors, tritium is produced by a series of nuclear reactions between neutrons and hydrogen (giving rise to deuterium in the first step), thus IH+An

-,

:H

(2.13)

:H +An

-,

:H

(2.14)

or between neutrons and lithium contained in the reactor coolant, !Li +An

-,

:H +:He

(2.15)

In addition, tritium is generated by ternary fission of uranium ';;U nuclei which gives rise to two fission products having a relative mass close to 100 and one light charged particle: 'H, 'H, 'H, 6He,'Li, 'He and others. The probability that ternary fission of '''U induced by slow neutrons in a reactor will produce a tritium atom is about lo-' and is up to several orders of magnitude higher than the probability that other light charged particles will originate [168]. Tritium readily displaces protium (or deuterium) in gaseous hydrogen molecules; consequently, HT molecules and, less frequently, T2 molecules are produced. The exchange rate of hydrogen isotopes is about 28 times faster in air than in water where HTO and partly also T20 molecules are formed [169]. Similar differences in hydrogen exchange rates are observed with other hydrogencontaining compounds. Compounds labelled with a small quantity of tritium are called tritiated compounds. Various systems of airborne tritium contamination control in laboratories have been designed [170, 1711. The principle component is usually a ventilation facility which drives the contaminated air to circulate first through an aerosol filter, then to a catalytic combustion chamber where tritiated hydrogen is burned to form tritiated water and finally through absorption exchange and exsiccation filters. It should be noted that the air in laboratories may contain as much as 37 MBq (1 mCi) T per cubic meter. Effective catalysts for oxidation of tritium are CuO or some metal catalysts, including Pt, on a support, mainly A1203.Sorption properties of a variety of rocks and minerals have been tested to find an adequate sorbent for an ion exchange of tritium for protium: sandstone, granodiorite, shale, montmorillonite, illinite, kaolinite, crasolite and clinoptillolite. Tritiated water vapour diffusing through a layer of clinoptillolite at a rate of less than 10 mm .s-' is effectively decontaminated and the efficiency of the adsorption process is practically independent of the vapour humidity. Once adsorbed, tritiated water is difficult to desorb [172]. Molecular sieves and zeolites proved to be the best exsiccating sorbents. 252

Tritium present in gaseous laboratory discharges can be oxidized on palladium dispersed in a commercial catalyst in a stream of air at 373 K. The catalyst is gradually inactivated as the duration of exposure to air is prolonged. The resulting tritiated water is adsorbed on A5 molecular sieves [ 1731. Tritium in gaseous tritiated hydrogen can be separated from the gaseous or water phase by letting it diffuse through a membrane made of palladium-25 YO silver alloy (with a separation factor H: T of 2.05). By using a simple bipolar palladium membrane and electrophoresis, separation of tritium from protium can be improved; a separation factor of 6 to 11 can be attained [174]. Hopcalites are better catalysts for tritiated hydrogen oxidation than metal oxides, because their oxidative performance is unaltered by ambient temperature, and they can be regenerated repeatedly with only a negligible reduction in efficiency. The effective temperature is 673 K for hopcalite and 573873 K for Cu oxides on a kieselguhr support [175]. An effective three-step process has been developed for scavenging tritium from the offgas and for recovering high-purity hydrogen. The process makes use of a hopcalite and a ceramic-supported nickel catalyst (I), a uranium getter bed (2), and a palladium silver membrane (3). The hopcalite at ambient temperature specifically converts the reactive CO to COz using up some oxygen from the process gas. In the following step, water is decomposed on the uranium getter to H, and U 0 2 absorbing at the same time an additional part of the oxygen. Since hopcalite does not oxidize hydrogen at room temperature, no uranium getter consumption takes place. Tritiated hydrogen is effectively entrapped in the palladium silver membrane. Tritium permeation through the filter is small, because at no point does the temperature exceed 723 K. The total volume of solid wastes is relatively small, and the spent catalyst is not likely to become appreciably contaminated because the solubility of hydrogen in nickel is limited [183]. Glove boxes designed for handling tritiated compounds must be kept at an underpressure and the gloves must be made of special polymers impermeable for tritium. Common rubber gloves do not sufficiently protect the hand and arm skin from becoming contaminated. An efficient ventilation with an integrated cleaning and scavenging system is necessary [ 1761. Despite the very low yield of tritium in the ternary fission of uranium or in nuclear reactions between neutrons and light isotopes of hydrogen, lithium, boron and other light elements, an operating reactor of the WWER type produces daily about 1.5 TBq (40-45 Ci) of tritium per 1 MW of power. Each one ton (106g) of irradiated and burnup nuclear fuel contains on the average about 10 TBq (250 Ci) of tritium, the upper limit being as much as 26TBq (700 Ci) [136]. Measurements of specific tritium activities at the three WWER-type reactors of the Beloyarsk NPP (USSR) yielded the following 253

values: about 1 MBq . I - ' in the primary circuit coolant water (in the form of HTO); between 0.005-30 Bq . I-' in air; and about 2 Bq . I-' in gaseous releases from two reactors (only 29 mBq .I-' for the reactor No. 3) [177]. Although the amounts of tritium produced in reactor operations and fuel processing are immense, the common practice in the management of tritium in the nuclear fuel cycle is almost exclusively restricted to a controlled release into the atmosphere rather than trapping and disposal. Various aspects of tritium escape mechanisms, its behaviour and management are topics of continuing intense laboratory research in many countries all over the world. Several promising methods of handling the tritiated effluents and wastes have been developed on the laboratory scale, but technological problems as well as prohibitive costs so far preclude their application in operational practice. In water-moderated slow neutron reactors, tritium produced in the fuel remains confined inside the Zr alloy cladding and does not leak into the coolant to any significant extent. However, the tritium produced by neutron reactions with deuterium, lithium and the impurities present in water appears predominantly in the coolant in the liquid phase, and is partly released with the off-gases in the gaseous phase. Because of low concentrations, the liquid and gaseous tritiated effluentsare released into the environment after proper dilution so as to keep the environmental radiation risk within safe limits in compliance with the ALARA principle (see Chapter 5). The situation is much more favourable with FSBR. Stainless steel fuel element cladding allows about 99 YOtritium to pass through into the sodium coolant. It is then effectively removed and concentrated to a small solid volume in the cold trap by means of isotopic co-precipitation with sodium hydride. Ion exchange in the cold trap would be insufficient in itself to effectively remove tritium [ 1781. As the other hydrogen isotopes, tritium at higher temperatures can diffuse through the walls of the containment and contaminate the environment. Oxide layers on the surface of construction steels and alloys can reduce the tritium diffusion rates by 2-3 orders of magnitude. The diffusion rate is also a function of pressure [179]. In order to prevent tritium from escaping, plating has been suggested [ 1801 for the internal surfaces of all three cooling circuits including the heat exchanger and the steam generator (see the scheme in Fig. 2.2), with a composite Zr-Ti-Ni alloy lining. When reprocesssing the spent fuel from light water reactors, including the PWR, the fuel elements are opened and chopped into pieces, and the fuel dissolved in HNO,. A certain fraction of tritium (about 10 YO)is bound in zirconium cladding, the bulk passes to the liquid phase. Throughout the extraction cycles, the majority of the tritium inventory is retained in the aqueous phase, only a fraction remains in the refined product. By evaporating the refined products and regenerating the HNO, , tritium becomes gradually concentrated 254

in nitric acid and in the condensate returned to the re-extraction process. This recycling of tritium results in contamination of all the process streams in the technological flowsheet [1361. When reprocessing the LWR fuel by means of the PUREX process, part of the tritium spreads to the organic solvent phase (tributylphosphate). To prevent the spreading, a second scrubbing step was introduced in which the organic product stream was subjected to additional washing with 0.8 M non-tritiated HNO,, a D , = 40 could be obtained. Because of the relatively low acidity of the scrubber, it also works as a Zr trap, even when the ratio of organic to aqueous phase was selected between 20 and 40 [181]. Tests were also performed with 30 YOtributylphosphate loaded with uranium and tritium [182]. Tritium can also be effectively removed during the regeneration process by disposing the tritium-rich highly radioactive refined product of the first extraction cycle in underground disposal wells. If the volume of fresh water fed to the extraction process is kept low, it is possible to concentrate as much as 90 % of the tritium inventory of the irradiated fuel [2]. The key process in the management of tritium present in reprocessed slow reactor fuel is a preliminary oxidation of the fuel and its degassing prior to dissolution in HNO,. Spent fuel cut into pieces is calcined in special rotary furnaces in an oxygen atmosphere at 723-1023 K for 4-12 hours. The U02is oxidized to U30, in the reaction 3U02+02

-,

U30,

(2.16)

At the same time, the crystalline structure of the fuel changes and degassing takes place, resulting in an escape of Kr, Xe and other volatile fission products along with tritium in the form of tritiated water. The presence of water vapour in the oxygen atmosphere accelerates the process and makes the removal of tritium more effective due to an isotopic exchange: HTOs

+ H20,

e

HTO,

+ H20s

(2.17)

where S means “on solid phase surface” and G means “in gas”. Since the gas contains excess water, the reaction equilibrium is shifted to the right. Waste gases are then fed to exsiccators equipped with molecular sieves or zeolite filters. The described oxidation process is irrelevant to fast reactor fuel, since less than 1 YOof the tritium inventory is retained in the fuel. The feasibility of tritium removal by degassing of the irradiated fuel is well established in laboratory tests. A 4-hour oxidation at 723-1023 K resulted in removal of 90-98 YOT. Extension to 12 hours at 1023 K increased the range to 9 6 9 9 YO.Elaboration of an operational technology however meets with serious technical problems and has not yet been realized [2]. 255

Tritium-contaminated waste gas released to the atmosphere can be rapidly oxidized as it deposits on the soil surface, and then resuspended into the air in the form of HTO. Initial evaporation rates varied between 1 and 5 YOof the tritium content in the soil [184].

2.7 Decontamination of protective clothing, footwear and other protective equipment 2.7.1 Decontamination of clothing and items of personal protection Protective clothings and other means of personal protection used by personnel working in nuclear facilities may frequently become contaminated with radioactive substances, particularly when unsealed radiation sources are handled. Whenever the results of monitoring show that the relevant permissible limits of area radioactivity have been exceeded, the contaminated items must be replaced or decontaminated. Since most means of personal protection are too expensive to be simply discarded when contaminated, an elaborate system of decontamination has been worked out and is generally used. Depending upon the contamination level, the suitable technological procedures required for washing and cleaning the contaminated items of clothing, underwear and linen may differ substantially. It is therefore appropriate to first sort the items into several categories characterized by an approximately equal contamination level. An example of such categorization is shown in Table 2.14 [185].

TABLE 2.14 AN EXAMPLE OF SEGREGATION OF CLOTHINGS AND UNDERCLOTHINGS INTO CATEGORIES CHARACTERIZED BY A SPECIFIED RANGE OF SURFACE AREA RADIOACTIVITY [ 1851 Category

I I1 I11 IV

256

Value of area activity

(Bq .m-*)

(pCi .cm-*)

up to 3.7.104 3.7.104 to 1.85.10’ 1.85.10’ to l.g5. lo6 above 1.85. lo6

up to 100 100 to 500 500 to 5000 above 5000

2.7.1.1 Possible modes of garment contamination 2.7.1.1.1 Dry contamination Dry contamination of fabrics results from a contact with dry radioactive substances (deposition of dust, aerosols, but also rubbing against a contaminated surface etc.). If the presence of moisture is entirely avoided, the solid particles of the contaminant are bound to a dry cloth surface merely by adhesive forces. This is favourable for subsequent decontamination, and the contaminated material should not even in later stages of handling come into contact with any moisture; only then will dry decontamination procedures be sufficient to cope with the situation. 2.7.1.1.2 Wet contamination

Wet contamination is caused by water solutions (or water suspensions, emulsions) of radioactive substances; an analogous situation occurs if water or water solutions of decontaminants have been used to treat dry-contaminated fabrics. In the presence of water, the binding of the contaminant to the cloth surface is mediated primarly by sorption processes (physical adsorption and chemisorption). The resulting bonds are much stronger than those due to adhesive forces; consequently, wet contamination of tissues is more resistant to decontamination attempts. 2.7.1.2 Binding of contaminants to cloth surfaces 2.7.1.2.1 Binding of contaminants to fabrics under dry conditions To some extent, dry particulate contaminant may become mechanically entrapped on the cloth surface in the ribbed or twilled texture. Particles greater than 50 pm in size are fixed in the mesh formed by adjacent threads, or inside the fibrous threads, by macro-occlusion. For smaller-size particles, the binding is further strengthened by micro-occlusion on the surface of the fibres by particle sorption inside the micropores and microcavities; the role of the latter mechanism is the greater the smaller the particle size. However, the significance of mechanical fixation of dust particles is often overestimated while the role of adhesion is underestimated, even though it is the latter process which represents the chief fixation mechanism of particulate contaminants to cloth. Particles of an insoluble component of radioactive dust that are fixed mechanically are best removed again mechanically. The contamination of solid surfaces by radioactive dust is due to the adherence of the particles to the surface or the surface film. The adhesive process is mediated by molecular, electric, capillary and Coulomb forces. The process 257

is subject to modifications by various factors, such as the kind and the texture of the fabric, the size and shape of the particles, atmospheric humidity etc. Practically all components-of the adhesive force are modified by the finishing and refining processes. Impregnation, for instance, may substantially reduce the adhesion of radioactive dust; conversely, it may enhance it, if the impregnation agent is sticky, since more radioactivity becomes fixed. Normally, an appropriate finishing method will plug up the tissue pores and microcavities, and thus reduce the possibility of micro-occlusion and sorption of tiny particles.

2.7.1.2.2 Relationship between the relative air humidity and the type and intensity of forces binding the contaminant to cloth Apart from molecular and electric forces, it is the capillary forces ellicited by moisture condensation in the contact region between the dust particles and the solid surface which modify the adhesive strength. The capillary condensation begins to be manifest when the relative humidity of ambient air exceeds 65 % [1861. When testing experimentally the removal from flat solid surfaces of glass particles varying in size between 20 and 60 pm, the proportion of particles retained on the surface was found to be constant for a broad relative air humidity range of 5 - 4 5 YO. Beyond 65 YO,the particle adhesion rate began to rise. Zimon [187] noted that it took some time before the effect of capillary condensation became marked and that no effect of capillary forces upon adhesion could be observed immediately or shortly after the contact between the solid surface and the particles. The adhesion of glass particles ranging in size from 80-100pm to a solid surface gradually rose with time for about 30 min if the relative air humidity was 100 YO. The strength of capillary forces depends on several factors: particle size, surface tension of the condensing liquid, surface roughness, wettability of the particles and the surface etc. The adhesive forces rise with the increasing surface tension of the condensing liquid, with increasing particle size and with increasing wettability of the surfaces touching each other. There are as yet no experimental data available which would allow the .quantification of the effect of capillary condensation upon the strength of adhesion of dust particles to various cloth surfaces. Hodny et al. [186] used a variety of contaminated cloth samples (different types of twills, both crude and dyed or treated with various finishing agents) to investigate how the efficiency of decontamination by beating depended on the relative air humidity and duration of contact with the contaminant. A model radioactive dust was prepared from particulate phosphate glass activated by neutron radiation in a nuclear reactor. Two experimental schemes were designed: a constant length of contact time (15 h) and a varying humidity ranging between 10 and 100 YO; or a constant humidity (100 YO) combined with a contact 258

time varying from 0 to 60 h. The results obtained showed that the kind of fabric and its surface finish could to some extent modify the decontamination efficiency even at a very low level (10 Yo) of relative air humidity. In a similar manner, the level of air humidity likewise modified the effectivity of a standardized technique of decontamination. The authors conclude that the capillary condensation of water in the contact zone between the dust particles and the cloth surface accounts for the reduction of decontamination efficiency. For a given constant contact time, the degree of capillary condensation, and hence the strength of capillary forces, is dependent on the relative air humidity. The higher the humidity, the more intense the capillary condensation, and consequently the stronger the capillary forces contributing to the total binding strength. Quantitatively this phenomenon was found most marked for a sample with a hydrenhilic finish, because the finishing agent rendered it more wettable. The wetting power affects the capillary forces, F,, in a way consistent with the equation

F, = 41~0r.coSfp

(2.18)

where c i s the surface tension of water, r -the radius of a spherical dust particle and v, is the wetting angle. The more hydrophilic the surface, the smaller is the angle v, and the greater the value of 4. As would be expected, water repellent materials, on the other hand, are affected by capillary forces proportionately less. An increase in the relative humidity to 100 YOresults in a differential decrease in the effectivity of the same decontamination procedure of fabrics with hydrophilic and hydrophobic finish by 55 O h and 30 %, respectively. The condensed moisture forms a thin water film separating the particles from the substrate. If the water layer is very thin, the intermolecular adhesive forces reinforce the capillary forces and the resultant total adhesion increases. On the contrary, a greater thickness of the water film diminishes the adhesive strength. On the curve representing the decontamination efficiency as a function of relative air humidity, the critical film thickness characterized by the lowest point.

2.7.1.2.3 Binding of the contaminant to fabrics in an environment of polar solvents (water) In this discussion, the polar environment in which contamination and decontamination take place is taken to be water. The chief process whereby textiles take up the contaminant from aqueous solutions of radioactive substances is adsorption of all kinds: ionic, molecular and colloidal. The type of adsorption depends on several factors. First of all, the physico-chemical properties of the contaminant (radionuclide), the pH value of the environment, the 259

presence and concentration of electrolytes and other components, but also the properties of the surface being decontaminated. Cotton, wool and synthetic fibres belong to materials of low ion exchange capacity; the capacity rises with an increasing pH value. Cotton is a cellulosic material, one of the most favourable hygienic attributes. It has the properties of a weak cation exchanger and is therefore capable of binding to its surface those radionuclides that in a polar environment exist as cations (e.g. radionuclides of Cs, Rb, Sr, Ba etc.). Wool, on the other hand, is a proteinaceous material. Proteins are macromolecular compounds with molecules consisting of simple amino acid residues bound to each other by means of their carboxyl and amino groups. A protein molecule may exhibit properties of either a weak cation exchanger or a weak anion exchanger, depending upon the pH of the environment (in an acid environment it behaves as an anion exchanger, in an alkaline environment as a cation exchanger). Synthetic fibres belong to a group of less reactive and chemically relatively stable materials of a very limited ion exchange capacity. Those types that are significant for textile production are polyamide, polyester and polypropylene. One of the characteristic features of synthetic fibres is that the surface resists contamination and is easily decontaminable, both in a polar and a non-polar environment. A competent desorption (decontamination) procedure requires an effective disruption of all bonds between the radionuclides in the given chemical forms and the surface of the fabric. However, this can only be achieved rationally if information is available on the composition and properties of the contaminant, the contaminated surface and the environment where the contamination and decontamination processes take place.

2.7.1.3 Methods of cloth decontamination 2.7.1.3.1 Dry methodr of decontamination “Dry” methods are those which use neither a polar solvent as such (in particular water), nor any decontaminants dissolved in polar solvents. The most common modes are beating, brushing and vacuuming. Dry methods play an important role in the system of clothing decontamination, especially where the contamination occurred under dry conditions. In such cases dry methods are highly effective; the percentage of radioactivity removed ranges from 30 to 80 %, depending on the kind of fabric, the nature of the contaminant, level of contamination etc. Dry methods are easily accessible to everyone, and practically anyone is able to carry them out without any professional training. As long as the contaminant remained dry till the beginning of the decontamination process, it is advisable to try decontamination in dry state

260

before an attempt to apply wet procedures. A contact with water or moisture at any stage renders the dry methods largely ineffective. Nevertheless, dry methods of decontamination of textiles must be regarded as the basic starting procedure of partial decontamination of persons as far as protective clothing is concerned. The devices used for this purpose are beaters, brushes, vacuum cleaners and ultrasound generators. 2.7.1.3.2 Semi-dry methods'of decontamination As described for other solid surfaces, semidry methods of clothing decontamination involve the use of foams. Foam is applied by means of a sponge or a foam generator. The foam layer is left to dry up, and the dried, brittle crust -provided it is not sticky -is crumbled and got rid of by beating or brushing. Alternatively, a foam layer approximately 2 cm thick is allowed to act for several minutes and then removed by vacuuming. The decontamination efficiency may be as high as 90%, but the percentage varies greatly with the properties of the fabric, composition and nature of the contaminant and the foam, as well as with how thoroughly the foam residues have been removed. The use of foam may be preferable to other methods of clothing decontamination for several reasons. It is easy and simple, and leaves practically no wastes. An efficient and safe exhaustion system must be available when dusting off the crushed foam residues. 2.7.1.3.3 Wet methods of decontamination This category comprises soaking and washing on the one hand, and chemical cleaning on the other. 2.7.1.3.3.1 Clothing decontamination by soaking and washing This is essentially an adapted laundering process in aqueous solutions of tensides and other agents. Certain specific conditions help in fostering the adsorption reversibility, i.e. a maximum desorption of radionuclides bound to the cloth surface. Among the more important conditions are the following: - Perfect contact of the decontaminating solution with the contaminated surface ; - Prevention of redeposition, i.e. renewed fixation of the loosened contaminant, by immediately scavenging the radionuclides from the solution or binding them to a solid form; - Promotion of processes competing with adsorption. Summing up these conditions it is obvious that the soaking and washing medium must contain the following ingredients : - A tenside (a surface active agent, e.g. in the form of detergents, washing powders etc.); 26 1

A complexing agent capable of binding the radionuclide ionic forms into firm, stable complexes, and reducing the water hardness; - An electrolyte. It is further desirable that the washing (decontaminating) solutions have a low pH value, i.e. a higher concentration of hydrogen ions capable of competing with the fabric for ionic forms of radionuclides during the ion exchange processes. Data obtained in experimental studies [188, 1891 and verified in practice show that it is best to soak textiles (clothing, underclothing, garment, gear) for at least 4-6 hours in 0.5-1.0 g .1-' solutions of washing powders and 0.30.5 g .1-' of Syntron B (technical grade disodium salt of EDTA) or in sodium hexametaphosphate. Any type of household washing machine, or communal laundry is suitable for the washing process. The washing bath usually has the following composition: 3-5 g washing powder, 0.5g Syntron B and 0.6 g sodium hexametaphosphate per 1 litre. The washing temperature is 333 K, exceptionally 363 K, and the length of the washing process about 20 min. Kunz and Henning [190] examined the effectivity of decontamination by washing, using various kinds of cloth samples. They concluded that the decontaminability decreased in the order polyester > mixed staple > cotton. The nuclides 13'Cs and 54Mnwere found to be most easily removed, whereas IMRuIMRh and "Cr were among the most resistant radionuclides. The results also provided evidence that a lowering of the pH value of the washing bath resulted in a decreasing residual radioactivity of the decontaminated fabric. A new washing agent, Hanka-Dekopur, has been developed in Germany, it exhibits a high decontamination efficiency, thermal stability and low foaming power. Reiff et al. [191] confirmed the outstanding decontaminating effects of polyphosphates when decontaminating fabrics of various chemical compositions by a washing procedure using 0.25 wt.% Calgon in water for 15 to 30 min at a temperature of 353 K (except for Perlon which required only 3 13 K). Hafez et al. [192] tested, under static and dynamic conditions, the pH dependence of the decontamination efficiency of aqueous solutions of complexing agents without any additives. It was shown that an addition of detergents was indispensable for an optimal decontamination of cotton. In the concentration range between 0.06 and 5 wt.%, the effectivity decreased in the order DTPA > > EDTA > citric acid. The recommendation in practice is to use 0.5 wt.% aqueous solutions of the agents. The following table (Table 2.15) outlines a successfully tested procedure of soaking and washing protective clothing and underclothing made of cotton or synthetic fabrics based on polypropylene [ 1851. One of the drawbacks of soaking and washing as a decontamination -

262

TABLE 2.15 A POSSIBLE TECHNOLOGICAL PROCEDURE FOR WASHING EXTERNAL CLOTHINGS AND UNDERCLOTHING (CATEGORY I)

Soaking Rinsing 1st washing Rinsing 2nd washing Rinsing Rinsing Rinsing

f

Time (min) 303 low 333 303 363 333 303 303

8

2 5

2 8 3 3 3

Consumption of washing agents (€9 Alfa

&nit

Syntron B

900

-

-

-

I100

-

1 100

-

-

-

90

90

-

procedure for clothing is the fact that it produces a relatively large volume of low-activity laundry waste water. The waste water contains tensides which enhance the foaming power, and foam is a nuisance in the processing of effluents by precipitation, electrochemical coagulation or distillation. The relatively high requirements on energy, working time, water and material, as well as the high purchase costs of the equipment, make decontamination by washing an expensive affair. 2.7.1.3.3.2 Decontamination of fabrics in a non-polar environment (“dry” cleaning) Decontamination by “dry” cleaning in a medium of organic solvents (trichlorethylene, tetrachlorethylene, tetrachlormethane, heavy gasoline and others) has a number of advantages. First of all, the duration of the cleaning and drying steps is substantially shorter than with washing. Further, the less polar solvent leads to weaker transfer of radioactive substances from the solution to the tissue surface and thus reduces the possibility of radionuclides becoming re-adsorbed. Finally, the materials are damaged much less than in a washing process. In addition, a common process of washing would not remove the radioactive contaminant with a sufficient effectivity from polypropylene-base tissues which have been simultaneously contaminated with radioactive substances and soiled with fats (oil, grease). This is so because fats damage the structure of those tissues. A likely explanation was provided by Vaeck in his extensive study [193]. He drew the attention to the fact that even fibres with a seemingly smooth 263

surface, such as synthetic textile fibres, are not entirely free from a certain structural unevenness. For example, fissures reaching deep inside the fibre appear during the manufacturing and processing of polyester fibres. Although the diameter of the finest crevices does not exceed 20 nm, it is enough for oils and fats to penetrate inside the fibre. This may be one of the possible reasons explaining why oily dirt sticks so tenaciously to PE fibres, and why organic solvents, such as perchlorethylene, are so strongly adsorbed. The presence of cracks and diffusion of oily dirt into them was also confirmed by means of X-ray difraction patterns. It was further shown that alkaline solutions cause a coarsening of the PE surface. Heat applied in the course of final processing of woven fabrics made of PE fibres may alter the structure and obliterate the fissures. Also interesting are Vaeck’s conclusions concerning the interactions between pigmented dirt and greasy dirt when washing two samples of different tissues (one of pure cotton, the other made of a PE-cotton mixture, 65:35). Contrary to expectations greasy dirt reduced the deposition of pigmented dirt on cotton, whereas on the mixed cloth the deposition was slightly increased. This may be taken as an evidence that greasy dirt plays a certain role in the adhesion of hydrophobic pigmented dirt. While it diminishes the adhesive forces binding the dirt to hydrophilic cotton, with hydrophobic PE fibres the effect is slightly the reserve. Electrolytes in an increasing concentration cause a gradual increase in pigmented dirt (soot) deposition on both types of tissue samples placed in a bath containing a low concentration of a tenside (about 0.1 g .l-’). Relatively self-contained is the technique of cleaning textile materials in emulsions of organic solvents [ 1931. Unlike chemical cleaning, cleaning in emulsions allows all the auxiliary operations, such as bleaching, optical clarification etc., to be carried out without much difficulty. The proteolytic action of enzymes is not at all obstructed in water and perchlorethylene emulsions, so that proteinaceous stains (e.g. blood) can be removed as effectively as with water solution. A prerequisite of success is the use of a sufficient amount of water [193]. Wadachi et al. [194] investigated the effectivity of decontamination of cotton tissue contaminated with ‘%o, 32Pand I3’I radionuclides, and soiled either before or after the radioactive contamination with polar oils (e.g. l-octadecanol and alcaline tripalmitate) as well as with non-polar hydrocarbons. The decontamination efficiency was determined for water solutions of common detergents, either in the presence or absence of electrolytes. Other studies [195, 1961 tested the effectivity, role and significance of decontamination by chemical cleaning in various types of organic solvents. Dependence on the bath temperature and duration of the cleaning process, as well as the effect of added “amplifiers” were assessed. The following two tables summarize the results obtained with decontamina264

tion of polypropylene fabrics contaminated by routine use in a nuclear facility. The fabrics have not previously been washed or chemically cleaned. Table 2.16 lists the values of DE obtained after washing the samplex first and cleaning them in solvents afterwards, whereas in the experiment summarized in Table 2.17 the reverse sequence was used. The samples examined in the latter experiment were contaminated artificially with a solution of fission products mixture. The effects TABLE 2.16 DECONTAMINATION EFFICIENCY OF WASHING AND SUBSEQUENT CHEMICAL CLEANING (POLYPROPYLENE-BASE FABRICS CONTAMINATED WITH OPERATIONAL CONTAMINANT). DATA OF TWO PARALLEL TESTS

Chemical cleaning, type of solvent

Duration of cleaning (min)

Gasoline

10 10

Perchlorethylene

DE (Yo) after washing (20 min)

20 20

61.4 48.3 74.7 31.6

10 10 20 20

81.5 80.6 83.4 83.5

f 7.7 f 8.1

f 5.5 f 7.6 f 0.8 f 1.0

f 0.9 f 1.0

DE (Yo) after subsequent chemical cleaning 37.0 40.5 26.6 26.1

f 2.8 f 6.7 f 11.1 f 5.3

12.2 f 4.7 15.8 f 3.5 20.6 f 2.7 15.8 f 2.9

Total DE (Yo)

75.3 f 4.2 69.2 f 6.9 76.8 f 14.5 56.2 f 10.9 86.5 87.2 86.6 86.1

f 0.5

f 0.6 f 0.7 f 0.9

TABLE 2.17 DECONTAMINATION EFFICIENCY OF CHEMICAL CLEANING AND SUBSEQUENT WASHING (POLYPROPYLENE-BASEFABRICS ARTIFICIALLY CONTAMINATED WITH A MIXTURE OF FISSION PRODUCTS) DE (Yo) after chemical cleaning in gasoline 28 f 1

-

-

21 f 1 -

perchlorethylene

-

21 f 1 19 f 2

DE (Yo) after subsequent washing

Total DE (Yo)

86f 1 79f 1 91 f O 89f 1

90f 1 83f0 93 f O 91 f 1

Duration of cleaning and washing 20 min. Temperature of cleaning baths 293 f 2 K, that of washing baths 323 f 2 K. Six samples per each test.

265

TABLE 2.18 DECONTAMINATION EFFICIENCY OF CHEMICAL CLEANING IN GASOLINE SOLUTIONS OF AMPLIFIERS DE after using amplifiers in stated concentration

k.I-9

Type of amplifier

t

I Gasoline alone Leginol Kompensan AH Gasoline soap

I

19f6 ~

-

I

2 2 * 10 21 f 8 42 f 5

I

I

33 f 2 35 f I 50 f 5

‘ -* 1 45 * 3 40 6

of the kind and concentration of added amplifiers upon the DE of gasoline for the treatment of polyethylene fabrics is shown in Table 2.18. All these experimental data, considered together with the results obtained in other studies on chemical cleaning efficiency for tissues soiled with common pigmented soil and other kinds of soil allow the following conclusions to be drawn: a) Chemical cleaning is important particularly for those cases where the fabrics are soiled with fats, oils, grease etc. in addition to a radioactive contaminant: b) Chemical cleaning is more effective if it precedes decontamination by soaking and washing : c) Chemical cleaning has a number of advantages: it shortens substantially the duration of the decontamination process (there is no need for drying and, for the most part, of ironing), it leaves a small amount of easily manageable wastes, and the solvent may be recovered by distillation. If a washing process follows, it is possible to use smaller doses of detergents and washing powders, which in turn makes the decontamination of laundry waste water easier. d) Suitable solvents for chemical cleaning are perchlorethylene and gasoline. While the former is safe but more expensive, the use of gasoline is less favourable because it is highly inflammable. e) It is possible to increase the decontamination effect if the solvent is supplemented with a suitable amplifier in a concentration Qf 1-10 g .1-’. f ) Modem high productivity machinery now exists comprising also a drier and a device for recovery of the solvent. A special “dry cleaning” process has been devised and a washing machine constructed [197] enabling a continuous separation of radioactive particulate material released to the solvent into a

266

sump. As the solvent is continuously pumped during the wash cycle from the sump to be returned to the cleaning drum, it passes through a filtering unit which removes substantially all of the radioactive particles suspended in the solvent. 2.7.1.3.3.3 Intensol and Dual methods

Modern tendencies in the field of decontamination include also an effort to raise the effectivity of processes designed for decontamination of clothes, particularly by combining the merits of chemical cleaning and washing in a single procedure. The following are the main reasons that stimulate attempts to develop new procedures : - There exist fabrics that cannot be cleaned satisfactorily in a solvent or in water alone; - Since solvents may be recovered by distillation, the volume of wastes and consequently the costs of their disposal can be reduced; - Combined cleaning in solvents and washiqg in water, in whatever succession, always yields better results than any single procedure alone. The important conclusions inferred from the experience with a practical application of the described methods are as follows: a) When removing the pigmented dirt, the degree of clarification can be increased by - increasing the frequency of baths, - increasing the volume of water, - increasing the water temperature. b) Removal of water soluble dirt can be improved by - increasing the volume of water, - adding a washing powder, - using a bleaching agent, - increasing the frequency of rinses in clean water. c) The degree of tarnishing decreases with - an increase in the frequency of solvent-containing baths, - washing in a water bath subsequent to the treatment with the solvent. d) the degree of tarnishing increases with - an increase in the volume of water and the water temperature, - washing in a water bath prior to the treatment with the solvent. A new method, designated as “Dual” consists in pretreating the garment by a normal chemical cleaning process using solvents and an auxiliary agent (amplifier) as an additive. This process loosens and extracts fats, oils and the fat-bound soil. The role of the auxiliary agents is to maximally adsorb the pigmented dirt which has been set free, and thus prevent its redeposition. The 267

organic solution is then drained off and a washing process follows using a water solution of a washing powder, in order to remove the water soluble soil. Another method designated as Intensol, is a cleaning process using emulsions. In the first step, a practically pure solvent extracts most of the fats and oils and the dirt bound to them, and is then directed to a still for recovery. An emulsion bath then follows. Water is added to a load of fresh solvent (30 YOof the volume or even more) and an emulsifier added to form an emulsion. The emulsifiers may profitably be combined with an amplifier. An experimental study was set up [198] to test the decontamination efficiency of a modified Intensol method in treating samples of clothing contaminated in the course of normal use during operation of a NPP of the WWER type. The high level of area contamination, i.e. 1.85. lo6Bq .cm-2 (5000 pCi . .cm-2) made these samples, by all practical criteria, virtually non decontaminable. The best results were obtained with the following two soaking solutions: A) - perchlorethylene (69 vol.%) water (29 vol.%) + 1 wt.% gasoline soap 1 wt.% DUBAROL (DE 65.6 YO); B) - perchlorethylene 1 wt.% gasoline soap 1 wt.% SYNTAPON (DE 65.2%). The samples were then subjected to machine washing for 20 min at 333 K in a washing bath consisting of 4 g .1-' Zenit plus 1 g .1-' disodium EDTA. The resulting total DE for completed soaking and washing reached 83 % (soaking solution A) and 95 YO(soaking solution B).

+

+

+

+

2.7.2 Decontamination of protective clothing, footwear and gloves ma& of plastic It is true in general that surface decontaminability of plastics depends on their composition and structure. Chemically stable, resistant, water repellent and smooth surfaces can be decontaminated with ease. From this aspect, the best materials are teflon, polyethylene and polypropylene, followed by silicon resins, teflon resins, polyvinylchloride, polyurethans and polyamides. Polymethylmethacrylate, polyester and synthetic rubber are least favourable. The properties of all the plastics are strongly modified by additives, such as fillers, dyes, softeners etc. For example, PVC decontaminability can be classified from good to very poor, depending on which additives are present and in what amount. It is correct to say that, as regards the contaminability and the ease of decontamination, every additive affects adversely the properties of plastics. The materials which are used to manufacture protective clothings for special purposes are most frequently polyethylene (PE), polyvinylchloride (PVC), polyvinylacetate (PVAC) and their various combinations. Bar and PelEik [199] followed the course of ruthenium nitrosylnitrate 268

adsorption on teflon, PVC, PE, polycaprolactam and rubber. The ruthenium was tested at concentrations of 0.084 and 0.0084 mol. 1-’ and was dissolved either in 0.1 M HCl or in an acetate buffer, pH 5.5. Arranged in the order of increasing adsorption, the tested surfaces rank as follows: teflon < PVC < < PE < polyamide < rubber. Polansky [200] studied the adsorption and desorption of trace amounts of 91Y,143Pr, and 14’IPmon teflon and compared the properties of teflon to those of other surfaces (paper, glass). The results showed that the adsorption coefficients of yttrium and promethium adsorbed on paper were an order of magnitude higher than the coefficient for adsorption on teflon under otherwise identical conditions. For cerium, the difference was even higher and reached two orders of magnitude. The maximum values of adsorption on teflon were found for 9’Yat a pH value of between 5 and 6, for I4Ce likewise at pH 5-6 and for 14’IPrnat pH 4.5. The maximum values of desorption were found for all the tested radionuclides at a pH range between 1 and 3. This implies, therefore, that a strongly acid environment favours the suppression of adsorption and facilitates the decontamination of teflon surfaces. On the basis of experimental tests and practical experience with decontamination of plastic protective clothing, an effective decontamination procedure has been worked out (see Table 2.19).The principal operation which favourably affects the decontamination efficiency is soaking in a composite solution of a detergent and a complexing agent. Essentially equivalent is a decontaminating TABLE 2.19

POSSIBLE DECONTAMINATION PROCEDURE FOR PLASTIC PROTECTIVE CLOTHING, OVERSHOES AND GLOVES ~~

Operation Soaking in solution Rinsing with lukewann water

Composition - 0.3 wt. % detergent - 0.3 wt. YOdisodium EDTA -

Rinsing with hot water Spraying with solution Rinsing with hot water

- 1 wt. % soap - 0.5 wt. YOsodium

(K)

Duration (min)

approx. 298

60

313

1 to 3

353 to 368

1

313

3 to 5

353 to 368

1

Temperature

hexametaphosphate

269

solution consisting of 0.3 YOsurfactant plus 0.7 YOsodium polyphosphate (sodium hexametaphosphate) [201]. The experience with. decontaminatim of full plastic protective clothing (inflatable frog suits) gained in the laundry isntalled at the NPP JaslovskC Bohunice (CSFR) indicates that satisfactory results can be achieved with machine washing in water and a water solution of nitric acid. Decontaminated clothing is dried in a stream of dry air [185]. Overshoes and gloves may be decontaminated in a sprinkle washer and dried afterwards in a drier. If the contamination level of overshoes and gloves is high, it is advisable to pre-soak them in a 9 YOHNO, solution in a wash tub for 5 hours. A scheme of a decontamination procedure applicable to plastic clothings, galoshes and footwear is presented in Table 2.20. TABLE 2.20

AN EXAMPLE OF A TECHNOLOGICAL PROCEDURE FOR DECONTAMINATION OF FULL-PLASTIC PROTECTIVE CLOTHING (FROG SUIT), GALOSHES AND GLOVES Sequence

Operation ~

~

Temperature (K)

~

Flushing with water 1st washing 1st rinsing 2nd washing 2nd rinsing 3rd rinsing Drying in a drier

Cleaning agent (g)

EDTA-2Na (g)

Duration (min)

250

100 -

5 I 10

~~

303

333 333 333

333 303

500 -

250 -

1

5

as apropriate

2.7.3 Decontamination of footwear The problem of an effective decontamination procedure applicable to footwear touches upon a host of partial questions connected with decontamination of rubber, plastics, paints, tanned leather etc. Because of the diversity of involved materials possessing greatly disparate chemical and physical properties, effective decontamination of footwear is not an easy task. Furthemore, it must be taken into consideration that the footwear surface may be worn to varying degrees or soiled with preservation fats, waxes, oils etc. All these circumstances make it next to impossible to suggest a standard, generally applicable decontamination protocol. It is therefore appropriate to seriously consider all preven-

270

tive measures of an organizational nature which are likely to diminish the degree of surface contamination. Among the most obvious ones are: - Selection of the proper type of footwear (smooth surface, non-perforated shoes made of low-contaminability material that can be easily decontaminated); - Frequent partial decontamination after each shift, at the end of each working week or as appropriate; - Proper regular care of the footwear and its proper use. In practice, the type of footwear actually selected is usually a compromise between what would be regarded as an “ideal” shoe from the viewpoint of general hygiene, occupational safety and health protection on one hand, and the ease of decontamination on the other. Properly preserved footwear (e.g. treated by preservation fat) repels radioactive solutions from its surface, but large amounts of radioactive dust and other impurity particles may be bound by adhesive forces to the fatty layer.

2.7.3.1 Partial decontamination of footwear Partial decontamination is carried out by members of the staff themselves immediately after an actual or suspected inadvertant contamination or upon termination of a working task, at the end of each duty period etc. It is done directly on the spot, in a properly equipped, separated section of the work area, and in a manner which is laid down by the foreman, shift master or other any authorized person. The basic modes of partial decontamination are: - Rinsing or wiping off with a decontaminating solution; - Washing (wiping) with organic solvents, most often gasoline. The former procedure is not very effective, while the latter one deprives the leather of its preservation layer; apart from that, it is associated with a risk of poisoning or explosion. Organic solvents further increase the risk that radioactivity will penetrate deep inside the shoe material. Washing with water is only effective if the footwear is made of a smooth, non-absorptive material. However, thoroughly and frequently drenched footwear becomes deformed, unless it is put on a last for drying.

2.7.3.2 Total decontamination of footwear There is no difference in principle between a partial and a total decontamination of footwear, except that one additional method, that of sole abrasion, belongs exclusively to total decontamination. Abrasion is a highly effective 27 1

method, but its frequent application substantially shortens the useful life span of the footwear, because the sole rapidly becomes too thin. Unlike partial decontamination, total decontamination of footwear is carried out in a special facility (laundry, wash room, cleaning station) and often makes use of special devices or machinery. Each step of the procedure is monitored to determine the residual radioactivity. The most practicable methods use rinsing or spraying with solutions of decontaminating agents. One of the well-tried procedures is described as an example: 1. Rinsing or spraying with a solution consisting of 0.5 % detergent and 0.2 YOsodium polyphosphate (temperature 303 K, duration 1 min); 2. Rinsing or spraying with water (cold, 30 s); 3. Rinsing or spraying with a solution consisting of 0.1 wt.% SyntronB and 0.25 wt.% washing powder (333 K, 1 min); 4. Rinsing or spraying with water (cold, 30 s). Rubber shoes can be better decontaminated if prior to decontamination they are left for some time immersed in a decontaminating solution in a suitable tub. The composition of the soaking solution is similar to that which is recommended as the introductory step of the above described procedure. In this way, dirt is removed more effectively, particularly from the soles, and the resulting decontamination factor is higher. As can be expected, footwear decontamination is facilitated and shortened by simultanously rubbing or brushing the shoe surface with rags or brushes. In general, decontamination of footwear is less effective then decontamination of most of the other items of personal protection. Modern trends prefer therefore to protect footwear from a direct contact with the contaminant by means of simple disposable slip covers made of plastic or cloth. The covers are discarded after use and disposed of as solid radioactive wastes.

2.8 Decontamination of environmental terrain and road systems Environmental terrain and roads are likely to become contaminated mainly by radioactive substances arising from uncontrolled technogenous sources (nuclear accidents, nuclear explosions). Global consequences of a large-scale uncontrolled escape of radioactivity may be aptly illustrated by the following observation: The total beta activity of the terrain near Munich (FRG) rose in 1986 as a result of the Chernobyl nuclear accident to approximately 2.5. lo4 Bq .m-*, i.e. an increase over the preceding five-year average by a factor of 3 , 1 0 3 12021.

272

The risk of environmental contamination must be considered from four aspects : - External irradiation of persons and animals; - Internal contamination of persons and animals (mainly due to inhalation and ingestion of radioactive substances); - Surface contamination; - Uptake of fallout radionuclides into the biocycles. Both persons and animals may become contaminated primarily, i.e. when the radioactive cloud or plume passes over and the radioactive particles fall out and deposit on the ground; or secondarily, i.e. by radioactive dust particles whirled up and made airborne as a result of wind gusts or the movements of persons, animals or vehicles across a contaminated terrain. The degree of the environmental contamination, Qter, can be determined by the relation Qter

=2*

10’ pver

(2.19)

where Qte, is expressed in dpm. and PYter is the exposure rate in cGy .h-’ at the given spot of the terrain. It follows that an area activity of 3.7. 10’’ Bq (1 Ci) per 1 cm2corresponds to an exposure rate of 11.1 cGy .h-’, and conversely. Decontamination of environmental terrain is a serious problem, very demanding on both technical means and economic resources. Though it is not always feasible to defer the decontamination action and wait until the dose rate level spontaneously decreases by decay to safe limits, decontamination of large areas of the terrain will probably be rather an exceptional venture performed only under particularly pressing circumstances. Radioactive substances which have descended from the cloud or plume do not cause a homogeneous contamination of the terrain. The inhomogeneity is due to a variety of factors, such as the differing properties of the surface, growth of vegetation, relief of the terrain, distance from the source of contamination, meteorological conditions, but also the properties of the contaminant, especially the size of the particles. The subsequent fate of the radioactive substances, once they become deposited on the ground, depends on their physical and chemical properties, the nature of the soil and the vegetation covering it, intensity of precipitations etc. For instance, some of the nuclear test explosion products gradually penetrate the soil to deeper layers and reach a depth of approximately 2.5 cm in five years [187]. With respect to human safety, it is important to arrive at a reasonably accurate estimate of how large an area of the radioactively contaminated terrain must be decontaminated to reduce the risk of radiation exposure at a given spot

273

to an acceptable level. The assessment is based on the relationship between the required decontamination factor DF and the radius of a circle r. The following relations have been derived by means of a .graphic analysis [ 1871 for an infinitely large flat area : r (m)

DF

20

40

60

80

100

3

6

8

14

20

Thus, if for instance the requirement specifies that the radiation hazard in the centre of the circle be reduced by a factor of 20, i.e. DF = 20, then it is necessary to decontaminate a circular area of a radius r equal to 100 m. Note that DF in this case is defined as the quotient of the risk reduction of human radiation exposure and is expressed by the ratio (2.20) where D and Dz are absorbed doses of ionizing radiation delivered at identical geometries to the exposed persons before and after decontamination, respectively. The quantities D and Dz in the relation (2.20) may equally be replaced by other relevant quantities, such as surface area radioactivity or dose rates before and after decontamination. The radiation dose which is derivered to unprotected (not shielded) persons within the contaminated area depends not only on the size of the contaminated terrain and the distance from the source of contamination, but also on the shape of the area (geometries) [187].

2.8.1 Modes of environmental decontamination The actual method of decontamination chosen for cleaning a particular terrain will vary greatly with the ultimate goal which is to be achieved. In some cases, the objective may be just to prevent the deposited radioactive dust particles from becoming stirred up, or to fix the contaminant in the soil and preclude thus an incorporation of the radioactive material into the biocycles and food chains; alternatively, the requirement may be to rigorously reduce the exposure (dose) rates of ionizing radiation to a safe level and make the area habitable again. The decontamination efficiency greatly depends on the characteristics of the terrain and nature of the soil. The following report may be quoted as an example [203]. A large area of rugged terrain on the Nevada test site was contaminated with spilled radioactively contaminated drilling mud. The contamination was found to consist of "'Ru and '06Ru-Rh with total estimated radioactivity at the 274

release time of 14. 10" Bq and 2.2. 10" Bq (38 Ci and 6Ci), respectively. The terrain was decontaminated by several methods and their effectiveness was assessed .by determining the fraction of radiactivity remaining (FR) following each procedure. In flat areas, the use of a front-end loader was found by far the most efficient way, removing large quantities of soil in relatively short periods were achieved. In canyon areas, flushing with water was of time. FRs of most effective on rocky surfaces with FRs of shovelling and bagging in the evaporated mud collection pond was also satisfactory and resulted in FRs of Flushing with water yielded FR of 10-I in rocky cracks. In locations where the radioactive mud and water had penetrated the ground surface to less than 5-3 cm, vacuuming was very effective achieving FRs of Unless the contaminated area was very small (e.g. droppings from front-end loading operations), vacuuming was however too slow to be of practical values. Under the supervision of an experienced radiation safety officer, the radioactive mud spill area could be safely cleaned up using, for the most part, standard earth moving equipment and personnel untrained in decontamination procedures. The basic modes of terrain decontamination are the following : 1. Removal of the contaminated surface layer; 2. Overlaying the contaminated surface with inert material; 3. Moistening of the soil with water or special solutions (emulsions); 4. Other methods. 2.8.1.1 Removal of the Contaminated surface layer This is the most common and most frequently used method. It entails removing a 3 to 5 cm thick soil layer or a layer of snow (6-10 cm of stamped snow or 20-25 cm of loose snow). The actual working approach depends on the nature of the terrain and the availability of suitable earth-moving equipment. The collected contaminated soil is transferred and deposited in a safe guarded repository; it is left there until the exposure rate decreases below the permissible limit, after which it can be returned. The method nevertheless has serious disadvantages: the need of a large number of special earth-moving machines and transport trucks, the risk of spreading the contamination, the devastation of cultivated grounds, and relocation of fertile arable soil. On the other hand, the decontamination efficiency of the method is very high. The technical means which can be utilized for the purpose are graders, bulldozers, scrapers and trucks and tractors equipped with a front-end loader, ploughshare or snow plough. The further fate of the soil depends primarily upon the degree of contamination and on the radionuclide composition of the contaminant. In a review [204], Riorgan (ed.) defined the goals for soil decontamination work. The 275

objectives which relate to the disposition of the products of a decontamination process were summarized as follows: Soil fractions which show less than 30 d,, .g-' (disintegrations per minute of transuranic nuclides per gramme of soil) can be disposed of as surface soil with unrestricted usage. Fine soil fractions with less than 500 4, .g-' and coarse fractions with less than 1000 d,, .g-' can be disposed of as subsurface soil as long as is its usage is controlled to ensure compliance with appropriate dosage guidance. Soil concentrates that have an activity greater than the above limits but less than 22 000 dpm should be interred in an approved, low level waste burial ground. If the activity exceeds 22000 dpm.g-' soil concentrates should be stored as retrievable waste. Changes in the technical and legal aspects of soil decontamination are rapid. Permissible soil decontamination levels are likely to change, as are the decontamination technology and the ability to monitor the effectiveness of the decontamination processes. As a result of these developments, frequent updates of decontamination criteria, goals, and monitoring techniques must be expected. Special conditions for terrain decontamination occur during the winter season. Soil may be frozen to various depth, or covered with snow, either loose or frozen, often with a hard ice crust. The contaminated snow can be cleared away also by means of a tow-plough. Another possibility is to spread a layer of noncontaminated snow over the contaminated surface, and to dispose subsequently of the two layers together. Snow sweepers or blower-type snow ploughs may be applied. Contaminated ice crust is best removed by hot air blowing. 2.8.1.2 Overlaying the contaminated soil surface

A 6-8 cm thick layer of earth or any other available loose material (or 8 -12cm snow, if appropriate) is spread over the contaminated surface. The method has disadvantages similar to those described above, but is fairly effective. The covering material (earth, sand, gravel, dross, ash) must of course be free of radioactive contamination. A variant of the method consists in turning up the soil by ploughing to a minimum depth of 20 cm.However, sowing or planting new crops must wait until monitoring of soil samples confirms that recultivation is safe. 2.8.1.3 Moistening of the soil with water or special solutions and emulsions

The method in fact accelerates the penetration of the radioactive substances to deeper layers of the soil, with all its negative consequences. Sorption of radionuclides takes place and the radionuclides may be taken up into the food chain. It is known that the decontamination efficiency of this method greatly varies with the soil type (particularly its ion exchange properties). One of the 276

serious negative consequences is contamination of the deeper soil layers and, from the practical aspect, a rather high consumption of utility water. Either water alone or water solutions of suitable decontaminants are used for the purpose. The reclamation of soil contaminated with "Sr was studied by Kemper [205]. Samples of various types of loam (Evesboro loamy sand, Sassafras sandy loam, and Fort Collins silt loam) were treated with a mixed calcium-strontium solution to give about 95 % saturation with Ca and 5 'YO saturation with Sr. Samples of the selected soils were leached with 0.06N CaCl, to remove Sr. Extracting the leached soil samples with 1 N HCl yielded residual Sr equal to 0.43, 0.47, and 0 % of the Sr exchange capacity, respectively, of the Evesboro, Sassafras and Fort Collins soils. There was a general agreement between the Sr concentrations measured at various depths and those calculated theoretically, especially so with heavier soils. The Evesboro and Fort Collins soils were also labelled with carrier-free 85Srand mounted in columns, leached with 0.06 N solutions of either CaCl, or SrCl, . The removal of "Sr was more complete from the Evesboro than from the Fort Collins soil, and from the centre than from the edge of the columns. Short-term leaching capability of SrCl, exceeded that of CaCl, where "Sr was present in an amount small enough to be adsorbed mostly on specific soil adsorption sites. When larger amounts of "Sr had been adsorbed, Ca was equally effective as Sr in displacing the contaminant. The feasibility of an in-situ use of the commerciallyavailable agglomerating agent RESINEX 60 to treat radioactively contaminated soils was reported [206]. The treatment process involved wetting the soil with an emulsion of submicron resin particles in an aqueous carrier. The carrier penetrates the soil and then evaporates, leaving the finer particles bound in larger agglomerates. The investigation examined a variety of treatment methods to determine the optimum technique for meeting the apropriate particle size criterion for transportation and disposal ( < 1 'YO by weight of < 10 pm particles and < 15 'YO by weight of < 200 pm particles). A soil decontamination project was initiated by Stevens et al. [207]. To remove actinides from soils at rocky flats, wet screening, attrition scrubbing at low pH, and cationic flotation were ivnestigated. Pilot plant studies were carried out and conceptual designs generated for mounting the processes in semitrailers.

2.8.1.4 Other methods An example is the removal of the vegetation cover. The practicability of the method depends primarily on the kind of vegetation. Decontamination can be effective only with dense continuous growth, such as low, densd grass. A 277

necessary prerequisite of success is that the removal is carried out as soon as possible after the contamination, which practically means within a few hours. It is obvious that the chance of effective decontamination is close to zero if the vegetation consists of tall trees and cover the terrain only sparcely. The same is true if precipitation (rain or snow) either accompany or immediately follow the fallout. It can be said in general that this approach (i.e. removal of the vegetation) only rarely brings satisfactory results. Where appropriate, vacuuming of the contaminated surface layer of sand or dust may be another possible method of decontamination.

2.8.2 Decontamination of routes Road systems include highways and auxiliary roads for vehicles, sidewalks and footpaths for pedestrians, supply routes, connecting routes, tunnels and others. The pavement is usually made of convrete with various surface finishes, less frequently asphalt, paving stones and tiles. The main effort in reducing the level of contamination will logically be focused on those sections of the route system where the traffic is heaviest. Because the decontamination process is very time-consuming and demanding on human and technical resources, it is likely that road decontamination will mostly be deferred until some later stage of the decontamination operation, and that it will then begin with those stretches where the highest risk of human radiation exposure is likely to occur. 2.8.2.1 Methods of route decontamination

Mobile technical means (such as street-cleaning machines, fire engines) and portable devices (such as garden sprinklers, watering cans) are used for road decontamination. The methods and means actually applied depend on the extent of contamination, the nature of the contaminant and type of the road (in particular its load carrying capacity and the kind of pavement). The following decontamination methods are available: - Flushing with (nonpressurized) water or decontaminating solution ; - Washing with pressurized water or decontaminating solution; - Covering with foam followed by water rinsing; - Steam cleaning or application of steam emulsions; - Sandblasting. ' Utility water, preferably with an addition of 0 . 0 3 4 . 0 5 wt.% of an optional detergent (tenside) is used for decontamination. The tenside added reduces the surface adhesion of impurities and facilitates the removal of grease.

278

Experimental testing of new methods of special cleanup using conventional washing procedures [208] revealed that the decontamination efficiency rose with the increasing kinetic energy imparted to the cleaned surface. The velocity of the water jet as it comes out of the spout is proportional to the square root of the pressure in the nozzle. The effectivity of the decontamination is the higher, the greater the volume of water impinging on a unit area of the cleaned surface per unit time, the higher the hydrodynamic pressure in the nozzle and-the longer the action of the water jet. The effectiveness does, surprisingly, not markedly depend on the incidence angle of the water stream. In addition to detergents, it is of advantage to supplement the washing solution with some amplifying additives which will charge the solid surfaces with an extra electric charge. As a consequence, the adhesive strength of the less polar impurities is reduced and affinity to the surface of the strongly polar groups of ionogenic tensides is increased, making it possible for the tenside to penetrate with greater ease between the solid surface and the impurity particles. The following activating additives are used to amplify the effect of tensides are used : - alkalies (alkaline carbonates, silicates, phosphates) enhancing the dispersing effects; - complexing agents; - protective colloids preventing the redeposition of impurity particles on the surface just cleaned. The experience gained with decontamination of various building materials was reviewed by Sandals [209]. He conducted a series of laboratory tests aimed at finding a non-destructive means of removing radioactive caesium from the surface of a range of typical urban construction materials (bricks, tiles, concrete, asphalt, painted timber etc.) which might become contaminated as a result of an accident at a nuclear power plant. Water alone was found to be ineffective except on materials of very low specific surface area (e.g. glass). Treatment with dilute acid was effective on asphalt, Welsh slate and cement products, but the last suffered surface damage. The most useful means of decontamination was found to be treatment with an aqueous solution containing ammonium ions. In no case did surface damage occur and in some cases more than 90% of the caesium was removed. This technique was scaled-up and used in tests considered to be realistic with respect to decontamination of a typical urban area. Both steam-cleaning and sand-blasting tests were also performed. Steam-cleaning was generally ineffective although there were some exceptions. Sand-blasting was always effective although of limited application on account of the surface damage caused, non-containment of the contaminated particulate waste and the expense in term of manpower. 279

2.8.2.2 Technical means for route decontamination 2.8.2.2.1 Mobile devices

Machines in this category are street-cleaning vehicles, mobile urban sanitation machinery and special technical vehicles designed specifically for military and civil defense purposes. The street-cleaning vehicles can be used to sprinkle and wash all types of routes. The sprinkling and flushing device is operated electropneumatically and controlled by the driver from his cabin. The tank can be filled either from an open water source or from any source of water under pressure (hydrants). It is equipped with an outlet allowing spontaneous draining. Certain types of sprinkling trucks can be also used with success in the decontamination of urban structures. If the washing solution is prepared from solid substances, it is important to ensure a complete and rapid dissolution of the reagents. This can best be done if a concentrated solution is.prepared first, poured into the tank and the tank then filled up with water to the desired volume. It is certainly inappropriate to simply open the hinged lid and strew the substance into the water tank. Experience shows that the substances then remain undissolved for at least one hour, even though the vehicle keeps moving. 2.8.2.2.2 Hand (portable) devices

These are all the various types of garden sprinklers or piston knapsack sprayers. They are well suited for decontamination of limited areas of all less accessible places. The sprinklers are designed far pressures of 0.6-0.8 MPa. A hand operated pump can exert a pressure of 0.8 MPa. Even very simple devices, such as watering cans of different size and shape, and water hoses equipped with a sprinkler or a spout, belong to this category. These simple means must not be underestimated they are extremely handy for a quick action in case of an accident.

2.9 Decontamination of persons The primary objective of decontamination of persons is to avert the deleterious health consequences of contamination with radioactive substances; on one hand, radiation damage to the skirr, and possible secondary internal uptake of radionuclides on the other. Decontamination of persons essentially means clearing the skin, the mucous membranes of the mouth and nasopharynx and the conjunctivae of the radioactive contaminant. 280

With its area of 1.5-1.9 m2, skin is the largest single organ of the human body. The relative weight proportion of the total body weight is also substantial and makes,up about 16 %. Skin is an organ which protects the organism against excessive effects of mechanical, chemical, thermal and radiation (UV) factors, as well as against an invasion of pathogenic bacteria and dermatophytes. The skin (dermis) consists of three layers (Fig.2.9 and 2.10): the outer ectodermal epidermis 0.1 to 2.0 mm thick ;an immediately underlying corium or cutis 0.6 to 3.0 mm thick; and the inner mesodermal connective tissue of the subcutis 0.6 to 100 mm thick. ACID PROTECTIVE LAYER EPIDERMIS

CORIUM

SUBCUTlS

1

3 4 6 8 9 1 0

12

UlS

Fig. 2.9. Structure of human skin. A schematic cross section of the skin as an organ I - Krausean corpuscle. 2 Ruffinian corpuscle, 3 - lanugo. 4 small sweat gland. 5 - papillary ledge. 6 - vessel of the first shelf (vene), 7 - vessel of the third shelf. 8 - fme nerve ending in the epithelium, 9 - hair inside a fillicle. 10 - sebaceous gland. I I - erectile smooth muscle of the hair follicle. I2 - vessel of the first shelf (artery), 13 - large sweat gland, 14 - subcutaneous artery, I5 - Vater-Pacinian corpuscle ~

~

DEsouLIMl4TNG W LAYER. STRATUM coRNEUMSTRATUM

GFwwwsW-

STRATUM SphMuM-

STRATUM BASALE COMUM

-

-

Fig. 2.10. Structure of human skin. A schematic cross section of the upper skin region

28 1

The boundary between the epidermis and corium is formed by the “basal membrane”, a double-layer membrane composed of polysaccharides and proteins. The epidermis consists of numerous layers of squamous epithelial cells progressively hornified towards the outer layers. New cells proliferate from the most inward layer, the stratum basale, each successive (more outward) sublayer being more differentiated. The outer keratinous layer (Fig.2.22), is covered by a “dermal film”, a thin coat formed by metabolic products of epidermal cells and secretions of the sweat and sebaceous glands (pH between 4.5 and 5.5) (Fig.2.22).

Fig. 2.11. Components of the horny skin layer (according to Pratzel)

2.9.1 Radiation damage to skin resulting from surface contamination Surface contamination may result in a certain, mostly localized, radiation damage to the skin and accessible mucous membranes. As a rule, only the uncovered body parts are likely to become contaminated (face, neck, hands and wrists). The exposed area makes up about 10 % of the total body surface (i.e. approximately 1500 an2).The critical tissue particularly vulnerable to radiation resulting from surface contamination is the basal layer of the epidermal cells. The radiation dose to the basal layer is mostly due to beta radiation. The skin damage inflicted by beta emitters depends on the following factors: - Degree of the contamination and hence the total radiation dose and the dose rate, particularly in the region of the basal layer; - Size of the contaminated skin area;

282

4

SKIN FILM PROPER

I

SOLUBLE

EXAMPLES : GLICIDS. UREA PROTEINS. INORGANIC SALTS

u IMPURITIES

EXAMPLES : ACIDS, FATS

ORGANIC

APRlED OELI-

BERATELY (OHT-

MENTS.cos METICS )

Fig. 2.12. Composition and properties of skin film components

EXAMPLES: LUBRICANTS, ASPHALTS, TARS, PAINTS, LACQUERS

TABLE 2.21

DOSE RATES (mGy .h-') OF BETA RADIATION. CONTACT DRY CONTAMINATION OF THE SKIN WITH 37 k3q ( 1 pCi). cm-' OF THE INDICATED RADIONUCLIDE Skin depth (mm)

Radionuclide I

0.05

0.10 0.15

0.20 0.25 0.30 0.40 0.50

0.60 0.80 1

.oo

,

l 4 c

3zp

3 5 s

s7Co

58Co

59Fe

6SGa

19.1 3.4 0.3 0.0

66.3

19.1 4.1 0.6 0.1 0.0

3.1 1.6 0.4 0.0

10.6 7.7 5.7 4.4 3.4 2.6

40.6 23.7

58.2 52.1 47.9 45.0 41.9 39.4 35.7 32.3 29.7 25.3 21.7

58.0 52.7

48.2 44.6 41.7 37.0 33.3 30.1 25.0 21.0

15.5

1.5

10.6 7.5 5.4 2.9

0.9

1.5

0.5

0.9

0.1 0.0

0.1

0.0

75%

4.7 1.6 0.7 0.4 0.3 0.3 0.2 0.1 0.0

87mSr

%Tc

113mln

1311

13.5 11.9 10.7 9.9 9.1 8.4 7.1 5.7 4.4

10.9 6.0

26.5 23.3 21.1 19.3 17.8 16.4 13.8

57.6 39.8 30.2 23.8 19.2 15.7 10.6 7.2 4.8 2.1 0.8

1.6

0.1

1.1

0.1 0.0

11.1

8.4 2.9 0.1

169yb

1 3 3 ~ ~

49.9 21.3 12.8 7.9 5.0 3.1 1.1

0.3 0.0

51.1

24.2 8.4 2.7 1.3 0.6 0.2 0.1 0.0

1 9 8 ~ ~

66.1 51.0 42.4 36.4 31.8 28.1 22.4 18.0 14.5 9.4 6.2

- Duration of the contact between the skin and the contaminant; - Aggravating effects of other damaging factors (burns, injuries, scratches etc.).. The following table (Table 2.21) presents the values of the dose rates and doses imparted to skin by monoenergetic electrons and beta particles emitted by certain radionuclides, as computed by Henson [2 101. The post-irradiation skin damage caused by beta particles usually manifests itself as radiation dermatitis. The symptomatology depends largely on the radiation dose: after 8 to 10 Gy skin erythema; after 10 to 15 Gy erythema accompanied by oedema and subsequent desquamation; after 15 to 20Gy appearance of blisters; and after 25 to 50 Gy possibility of ulceration [211]. 2.9.2 Uptake of radionuclides by skin

2.9.2.1 Intact skin A number of experimental studies have brought out the basic facts concerned with this problem. The distribution of polonium (’loPo) and transuranic elements (24’Amand 237Np)in the organism after percutaneous absorption in chronically exposed pigs was studied by Sitko and Shimakov [212]. The results showed that up to 80-90 YOof the alpha emitters were localized in the surface layer of the skin (within 100 pm). The turnover of the alpha emitters in the skin corresponded to their effective half-lives. The organ distribution confirmed that the 210Poaccumulated preferentially in the kidneys, whereas the transuranics deposited in the skeleton and liver. Pratzel et al. [213] studied the mechanism of human skin contamination and penetration of radionuclides into the organism after their application to the skin in the form of water solutions. A particularly significant uptake was noted for polyvalent cations; their absorption was found to be a function of the pH value and the valence (ionic charge) of any particular radionuclide. For all the substances studied the concentration of absorbed radionuclides decreased almost exponentially with the depth. The presence of epidermal keratin in the amount of 1 mg .cm-2 most probably accounts for the observed slow penetration rate of the studied radionuclides (ten compounds).

2.9.2.2 Damaged skin A- kinetic study comparing the uptake of radionuclides by intact and by

damaged skin was published by Inaba and Suzuki-Yasumoto [214]. They followed the absorption of 6oCo, 13’Cs and ’“Ce applied in solutions to either intact or injured rat skin. The conclusions drawn from the experimental data 285

confirmed that an intact epidermis represented an effective barrier limiting the penetration of radionuclides, while injured skin permitted absorption of 137Cs and 6oCoto a considerable extent. The implications drawn for decontamination practice emphasize the need to decontaminate skin by gentle methods and at the earliest possible moment after the contact with the contaminant. Experiments investigating the role of the keratinized layer of the skin contaminated with I3'I (NaI) led Kiyoshi et al. [215] to a similar conclusion. The presence of a horny layer slowed down and reduced the penetration of iodine; skin which was deprived of the keratinous layer made it possible for radioiodine to penetrate into the organism at a rate which was almost equivalent to an intravenous injection. The uptake of "Sr by intact or scarified skin of volunteers was studied by Ilyin et al. [216]. Six hours after the contact with the skin, only 0.26 YO of the applied amount of "Sr was absorbed by intact skin, compared to 57.4 YOwhich penetrated the damaged skin (a difference by a factor of 200). Most of the contaminant was taken up during the first 30 min after the application (38.0 YO of the total amount applied). An increase in the pH value of the "Sr solution (from pH 2.0 to pH 6.0) resulted in a decrease in the absorption to only about one third. The results further showed that the effectivity of a decontamination procedure was higher for intact skin than for scarified skin by up to two orders of magnitude. Parfenova et al. [217] examined the uptake of 137Cs,"Sr, "'I and 241Amby burned skin (burns of the first to the third degree). Relative to the intact skin area, the first degree and second degree burns caused an increase in the rate of absorption by factor of 1.5 and 3, respectively. Surprisingly, the third degree burns were accompanied by a decrease in the uptake. Tomoko et al. [218] analyzed experimentally how an injured mouse skin (damaged by abrasion, puncture, incision or chemical burns by mean of acids or alkalies) transported and absorbed W o applied in the form of CoCl,. Measurements were made 15,30 and 60 min after the application of the contaminant. Whereas intact skin practically did not absorb the contaminant at all, skin damaged by physical factors took up 20-40 YOof the applied ' T o 30 min after the contact. On the other hand, chemical dermal injury did not result in any increase in the absorption. Consequently, the absorption rate detected in skin injured by physical means exceeded by two orders of magnitude that found in skin injured by chemical means. Ilyin et al. [219] studied the uptake by rat skin of 137Csand 89Srdissolved in HNO, of varying normality (from 0.05 to 8 N). Damage to the skin caused by the acid markedly enhanced the absorption of the two radionuclides and consequently also their organ uptake. After a 24-hour contact with the radionuclides dissolved in 1 N HN03, 15.9 YO and 5.3 YOof the applied amounts of, 286

respectively, caesium and strontium became absorbed; the corresponding figures obtained with solutions in 8 N HN03rose to 26 YOand 44 %.The decontamination efficiency greatly depended on how soon the decontamination procedure had been initiated; an increasing H N 0 3 concentration made the decontamination progressively more difficult. A 3 YOsoap solution applied early enough proved to be an effective decontaminant. If the decontamination process was deferred by one hour or more after the initial contact, it was no longer possible to prevent the absorption of the two radionuclides and their ensuing accumulation in the organism. Bazhin et al. [220] measured the absorption of 238Puand 239Pu (in the form of an oxalate or nitrate dissolved in 0.1 N to 10 N nitric acid) by rat and pig skin. The absorption rates of each chemical form of the contaminant were related to its appropriate retention in the organism, and the two radionuclides were compared. The tested animal skin was decontaminated by means of a 2 % soap solution or a commercial decontamination agent Zashchita (USSR). The results led to the conclusion that the coefficient of 238Puabsorption exceeded twice that of 239Pu,a fact which could be accounted for by the difference in the degree of radiation damage to the skin. A model mixture of radionuclides (%o, '%r, 137Cs and W e , pH 3.85) was used to study the absorption by rat skin, either intact or injured by scarification to a depth of 0.5 and/or 1.O mm. The proportion of the total applied radioactivity absorbed by the intact compared to the injured skin differed by about one order of magnitude. Similarly, differences exceeding one order of magnitude were noted in the uptake of radionuclides penetrating through the scarified skin [22 I]. A model mixture of dissolved radionuclides (Wo, 137Cs and W e , pH 5.5) was used to study the dependence of the distribution of individual components on the depth of both intact and scarified rat skin. There were significant differencesbetween the intact and the injured skin concerning both the penetration rate and the depth of radionuclide deposition. It could further be concluded that the contribution to internal contamination attributable to rare earths radionuclides contaminating the epidermal surface could be neglected. An equilibrium state in the distribution of all components of the mixture was found along the entire depth of the demo-epidermal skin layer [221].

2.9.2.3 Interactions of radionuclides with the skin; types of bonds between radionuclides and skin The nature and the strength of the fixation of radioactive substances to the skin surface and inside the skin, and hence their penetration into the organism, depend on a complex set of factors. The factors may be roughly divided in two

287

categories: those characterizing the contaminant, and those determining the state of the skin surface. Among the former are such factors as chemical forms of the radionuclides and their compounds, chemical properties of radionuclides, characteristics of the solvent and other substances which modify the basic features of the contaminant. The latter group includes the presence of the layer of fat and water on the skin surface, state of the keratinized layer, secretion intensity of the sweat and sebaceous glands. A special category of intermediate factors include those that can modify the conditions of the mutual interaction between the contaminant and the skin, such as the contact time of the contaminant with the skin, secondary wetting of the contaminated skin etc. Depending on the physical and chemical properties of the radionuclides, various types of their bonds with the skin can occur differing not only in the binding mechanism, but also in the strength of the bond. The following four types are recognized : 1. Mechanical entrapping of solid radioactive particles or radionuclide solutions on the skin surface or in its pores; 2. Physical adsorption of undissociated molecules and, to some extent, colloids; 3. An ion exchange effect which may be the main mechanism contributing to the binding of radioactive substances to the skin. This effect is due to the fact that various proteinaceous skin constituents (collagen, albumin, keratin, elastin) are amphoteric. Because of their polarization in a water environment, an acidic medium will expose the alkaline protein groups of the type H3N+-RCOOH capable of forming salts with acid anions, whereas the carboxyl groups of the H,N-R-COOtype appearing in an alkaline environment are capable of reacting with cations. 4. Chemisorption, a process which takes place whenever a radionuclide forms a firm covalent bond with atoms of the substances present in the skin. This bonding type is characteristic for heavy metal salts. A particularly firm bond involves the Zr, Th, La, Ce, Fe, Cr cations because they can react with the side chain polar groups ( X O O H , -NH2 and - O H ) as well as with the peptide groups, and form insoluble multinuclear chelates. 2.9.3 Cutaneous adnexa and their role in processes of contamination and decontamination

Components of the skin are accessory organs - hairs, nails and glands (subaceous, sweat and milk glands). With respect to the processes that result in radioactive contamination (both surface contamination and penetration of radionuclides through the skin) and to decontamination as well, special attention must be paid to hair and sebaceous and sweat glands. 288

Extensive studies on the percutaneous penetration and the distribution pattern of radionuclides in skin led Ilyin [222] to conclude that the relatively high absorption rate of radionuclides is mainly due to their penetration through the sebaceous and sweat gland ducts. As soon as the radionuclides enter the gland ducts, they immediately react with the skin constituents, primarily of proteinaceous nature, and pervade the entire skin depth. 2.9.4 Prophylaxis of skin contamination and percutaneous absorption of contaminants

What is meant by prophylaxis in this connection is a set of organizational, technical and operational measures intended to prevent or reduce the surface contamination of the skin. This section will deal with means that can diminish the level of surface contamination of uncovered body parts; questions concerning the decontamination will be taken up in later paragraphs. If skin is contaminated with radionuclides, particularly with radioactive aerosols, it must be presumed that the contaminant is bound to the skin surface, and that the bond is primarily due to relatively strong adhesive forces. The magnitude of the adhesive forces will largely depend on the thickness and the composition of the dermal film. The adipose dermal film plays a particularly important role in the processes of adsorption (contamination) and desorption (decontamination) of radioactive substances. The film enhances the degree of contamination with radioactive dust particles and conversely diminishes the adsorption and percutaneous penetration of radioactive water solutions. As regards decontamination, the film will reduce the wetting effect of water and decontaminating aqueous solutions, particularly in the absence of any tensides, and thus restrict the decontamination efficiency. Numerous formulations have been proposed in the past of preparations having mostly properties of colloidal substances characterized by a high adhesive affinity towards the skin, as for instance “biological” gloves. The preparations usually contain swelling agents and additives [223, 224, 2251.

2.9.5 Theories on radionuclide penetration through the skin: models The problems concerning the relevance of models proposed for the study of percutaneous penetration of substances in animals and men are discussed in a number of publications. Wester and Noonan [226] summarized the data obtained in comparative studies with percutaneous absorption of a variety of substances. Consistent differences in the absorption rates were found when comparing the tested substances against each other; there were also differences

289

among animal species tested, as well as between experimental animals (rats, rabbits, pigs, monkeys) on one hand and man on the other. The disparities in the results were apparently due to different properties of the substances, species differences and particularly variations in experimental techniques. In general, human skin exhibited the minimum percutaneous uptake of all the biological species tested. Ivanov and Maximova [227] attempted to extrapolate to human beings the results on percutaneous absorption of radioactive substances obtained in animal experiments. With respect to the thickness of the skin as a whole and of its particular parts, laboratory rat and young pig skins are most closely similar to human skin. However, considerable differences exist in the development of skin adnexa which are often believed to have a decisive role in the penetration of radionuclides through the skin, and in their elimination. Results obtained in experiments using animals or human volunteers clearly showed that laboratory rat skin differed from human skin in several respects and could not be used as an acceptable model system for studies on cutaneous absorption of radionuclides. An useful model explaining diffusion of a radioactive solution across the horny skin layer has been proposed by Pratzel in Hennig’s work [228]; it is shown in Fig.2.13.

ICEUS OF THE HORNY LAYER

STRATUM GRANULOUlM CREVICE

Fig. 2.13. Diffusion of a solution across the horny layer of skin - a model

An attempt to summarize the available experimental data on radionuclide penetration through the skin and to interpret them in terms of a possible underlying mechanism was made by Osanov and Filatov [229]. The uptake was characterized by a series of equations derived for diffusion of water solutions across a semipermeable membrane. A model of membrane transport by diffusion was proposed, based on criteria of the theory of similarity. 290

The results implied that the skin penetration process might be divided into three separate stages: 1. A ,relatively rapid filling of hair follicles and sweat gland ducts with the substance ; 2. Penetration of the follicular walls and glandular duct walls by radionuclides and spreading of the substance radially; 3. Diffusion of the substance across the horny layer matrix and epidermis. A schematic model of the skin structure employed in the study is reproduced in Fig.2.14.

HORNY LAYER

EPIDERMIS AND CORUM

HAIR FOLLICLE

.

D, D2

SWEAT GLAND DUCT

Fig.2.14.A schematic model of skin structure ~

diffusion across the epidermis and conurn, Dn diffusion through the sweat gland dust and along the hair follicle; 1 horny layer thickness, r - depth of the microcirculation level, R - length of follicles and sweat glands ~

~

The experimentally ascertained data on the concentration of a radionuclide in various depths of live skin determined for each time interval of contact with the contaminant led to derivation of a relationship

g

- the mean values of

which calculated for the tested radionuclides were as follows: 16 f 30 YOfor 137Cs, 17 & 30 % for 89Srand 14 f 30 % for 24'Am.Thus, the same value applies to all three radionuclides. It can therefore be inferred that passive dzfusion is the main process determining the penetration rate of those radionuclides through the skin adnexa. The complexity of the calculation does not permit definite conclusions to be drawn on the penetration of epidermis by radionuclides. It can reasonably be expected that the mechanism will differ from that described above. Another series of experiments used dermo-epidermal grafts of human and pig skin. Aqueous solutions of 6oCo,137Cs and with isotopic carrier concentrations varying by three orders of magnitude were brought into contact for differenttime intervals with the grafts, and the penetration rates were measured. 29 1

Of the three radionuclides tested, caesium was found to penetrate with the highest rate (higher by a factor of 5-10 when compared to Co2+,and factors of 10 to 50 compared to Ce3+and Ce4'). .About 5 to 6 h after the first contact, a dynamic equilibrium was established at a level which depended on the chemical form of the radionuclides (the fraction of the total amounts of radionuclides in the solutions which had penetrated the skin were approximately 8 % for I3'Cs, 1.1 % for 6oCoand 0.14% for '@Ce)[221]. The feasibility of the transfollicular transfer mechanism was also verified experimentally. Grafts of intact human skin withdrawn from either dorsal or ventral side of the forearm of the same donor were simultaneously subjected to a test in which solutions of a model mixture of radionuclides (%o, I3'Cs and "'Eu) were allowed to penetrate the skin grafts. Contrary to expectations, absorption of radionuclides in hairy grafts did not significantly differ from that in hairless grafts. In fact, the fraction of the total applied radioactivity that had passed through was quite negligible in both graft types and was significantly lower than the values obtained in comparable experiments using rat skin [221].

2.9.6 Decontamination of skin As concerns the extent of the operation, decontamination of persons may be either partial, i.e. decontamination of hands and uncovered body parts (face, neck), and flushing of conjunctival pouches, mouth and nose, or total, i.e. involving the entire body surface. If a decontamination process is to be efficient, it must be started as soon as possible after a confirmed or suspected contamination, and performed with optimum methods and means. The most frequently used method is a simple washing with warm water and soap using face-cloths, brushes etc. (soap is a standard item in the emergency personal safety kit). Apart from soap, the decontaminating agents include washing pastes, shampoos and various decontamination solutions prepared specially with respect to the prevailing radionuclides in the contaminant. As a rule, it is of advantage if the solution contains both a tenside and a complexing agent. Soap in itself is not the best agent for skin decontamination for several reasons. First, soap solutions (with few exceptions) are alkaline (pH value about 9-10, even more), a reaction which tends to transform many radionuclides, particularly those of the rare earth elements, into colloidal forms. Although only a small fraction of the colloids attach to the skin, the adsorption is irreversible. The efficiency of soap is further reduced by hard water. Sharov et al. [230] proposed a formula for a thermoplastic hydrophilic gel-base skin decontamination agent supplemented, if need be, with EDTA and

292

triethanolamine. It is applied after warming up to 323 K. It has been claimed that the agent was efficient in removing the radioactive substances deposited even in deeper layers of the skin, and was therefore recommended as a generally applicable suitable skin decontamination agent. Rat skin etched by HNO, solutions (0.05 N, 1 N and 8 N) and subsequently contaminated with 241Amwas subjected to decontamination tests in a study performed by Parfenova et al. [23 I]. The highest decontamination efficiency was observed with a 3 % soap solution applied not later than 5 min after contamination. Ivanikov et al. [232] pursued this line further and reported a favourable effect of a combined use of Pentacine (trisodiumcalcium salt of DTPA) and dimethylsulfoxide to remove the 24'Amfrom burned rat skin. Moore and Mettler [233] compared the decontamination efficiency of water, soap and water, and two proprietary agents - Radiacwash and Isoclean - in an experimental study in which volunteers' skin had been contaminated with radionuclides used in medical diagnostics ~"'TC, ','I, 67Ga, "'In). The activity of most of the radionuclides could be reduced to less than 1 YO of the initial level, except for sodium pertechnetate (w"'Tc). None of the tested decontamination agents proved to be appreciably effective in removing technetium adsorbed on the skin. Experience gained with decontamination of personnel as routine procedure at the NPP Jaslovske Bohunice (Czechoslovakia) was reviewed by Herchl et a]. [234]. Personnel monitoring and decontamination (partial or total, as the case may be) is the duty of the Decontamination Unit, which is a part of the Radiation Protection Centre. Between 20 and 50 cases of accidental contamination requiring a decontamination action have been registered each year among the members of the technical staff. As to the level of surface radioactivity detected in the persons contaminated, activities below 5 Bq .cm-2 were registered in about 46 % of cases, while 34 YO exhibited activities between 5 and 10 Bq .cm-2. In almost 70 % of subjects, the contamination was limited to hands only, followed by forearms, face, conjunctivae and hair, i.e. only the uncovered body parts were involved with any appreciable frequency. In about 7 % of the treated cases, contamination was combined with injuries, most frequently cuts, pricks and burns. The prevailing radionuclides were 134Cs,I3'Cs, %Co,6oCoand IIOmAg. Merrick et al. [235] compared the effectiveness of four agents applied to decontamination of the skin contaminated with radioactive drugs tagged with %Tc, I2,I and "Cr. An abrasive-containing paste appeared to exhibit the highest decontamination efficiency, and may therefore be recommended as a universally applicable decontaminant for all kinds of skin contamination. A subsequent additional increase in decontamination efficiency could be achieved if a gauze wad soaked in the proprietary agent Decont was affixed for 293

24 h to the already decontaminated area [221]. This procedure proved to be effective in experiments made with intact and/or scarified rat skin contaminated with a solution of a model mixture of radionuclides ("Co, %r, I3'Cs and lace, pH 3.85). The main generally applicable principles to be observed when decontaminating human subjects are the following: a) Decontamination must begin as early as possible after contamination. b) In any case of confirmed internal uptake, treatment of internal contamination shall be immediately started. c) If skin contamination is complicated by an open wound, spreading of the contaminant into the wound shall be prevented (by covering it with a polyethylene foil etc.). d) Decontamination never cause a delay in emergency medical aid, particularly life-saving measures. e) Fat solvents shall not be used for skin decontamination (as they facilitate the penetration of the skin by radioactive substances). f ) Simple methods shall be used for decontamination. As a rule, the process starts with washing, using soap (preferably acid) and water. Thorough hand cleaning shall precede the washing of the body to prevent spreading of the contaminant to other body parts that are usually less severely contaminated. Particular attention shall be paid to careful hair shampooing. g) Water shall be lukewarm, never hot. Hot water induces hyperaemia and consequently increases the penetration rate of the radioactive materials through the skin. h) Decontamination procedures shall be gentle, particularly if the treatment involves more sensitive parts of the body. This holds true especially when brushes, face cloths etc. are used. i) It is advisable that each step in the decontamination procedure be accompanied by radiological monitoring, i.e. determination of the level of radioactive contamination. The monitoring is obligatory upon the termination of the procedure. j) In special cases, skin shall be decontaminated by means of specially designed solutions respecting the nature of the contaminant and the actual conditions of the skin. k) Electrolytes shall never be added to decontaminating solutions, as they enhance the percutaneous absorption of radionuclides. 1) Since prophylaxis is easier than any decontamination, it is only rational to strictly comply with all principles of radiation protection and to rigorously adhere to all safety regulations applicable to the handling of unsealed radiation sources. 294

2.9.7 Partial decontamination of persons The primary objective of partial decontamination of persons (PDP) is to remove to the highest possible degree the radiactive contaminant from the surface of uncovered body parts. PDP is carried out upon any discontinuation of the working activity inside the contaminated zone (in the course of a continuing decontamination operation etc.), prior to any food intake, and immediately upon leaving the contaminated area. The process requires removing the contaminant not just from the skin surface, but also from the clothing and protective equipment. The extent of decontamination and the procedure chosen depend on whether the PDP is performed inside or outside the contaminated zone. PDP usually precedes total decontamination. If the contaminated subject is injured, decontamination is usually performed in the Health Centure, and life-saving medical aid always has priority. The simplest method of PDP is a rinsing of the uncovered skin surface with water using the nearest accessible source of noncontaminated water. Mucous membranes (conjunctivae, mouth and nasopharynx cavities) are preferably rinsed with saline (0.9% NaCl) or weak solutions of disinfectants, such as 0.3 wt.% KMnO, or 0.3 wt.% boric acid.

2.9.8 Complete decontamination of persons The complete decontamination of persons (CDP) is performed outside the contaminated zone upon termination of a decontamination operation; it is also indicated immediately in all human subjects who have been, or are suspected to have been, contaminated accidentally. It is obligatory also after PDP, whenever the values of residual skin contamination determined by monitoring exceed the permissible levels. It is advisable to carry out CDP upon concluding the duty period in a contaminated zone, at the end of each working shift, after operating contaminated technological system, and when leaving the area affected by an uncontrolled spread of radioactive substances. CDP can best be done in suitably equipped specialized facilities, the Facilities for Complete Decontamination of Persons (FCDP) or at least in “sanitary loops”. In essence, the facility or the loop must have a separate space (an undressing room) for taking off the contaminated clothing, a wash room with showers and a dressing room for putting on clean clothing. Figure 2.15 shows a schematic lay-out of such a facility (a sanitary loop). In case of an accident involving a radioactivity release, the number of contaminated individuals is likely to be much higher than that expected from normal operation of a nuclear facility. In such an exceptional situation, all 295

IMMOBILE CONTAMINATED INJURED PERSONS

I----------

t ADMISSION OF PERSONS INVOLMD IN AN ACCIDENT (CONTAMINATED1

CHANGE ROOM

----- - - - _ -La

PERMNS WITH RESlSTENT CCNTAMINATKX-4 AND WITH INTERNAL CONTAMINATON

WASH ROOM SHOWERS

1

I

A

I I I I I

CHANGE ROOM

-

.

i

I

I I

I OF CLEAN CLOTHING

CONTAMINATED WATER OUTLET

UNAFFECTED

PERSONS

TO APPROPRIATE DESTINATION

Fig. 2.15. A schematic lay-out of a facility for complete decontamination of persons HC - the plant's health care centre. HP duty station of the health physics officer, MO duty station of the medical officer (physician), SP - sorting post, S - containers with a supply of decontaminating solutions, B - bin for disposal of soiled swabs and washcloths ~

~

suspected subjects first pass through a sorting post to eliminate the noncontaminated persons. Those with detectable contamination take off their clothing, underwear and footwear; personal valuables are deposited in a bag labelled with the owner’s name and kept in safety in a depository. A routine medical examination ascertains whether or not an injury would interfere with the decontamination procedure. The wound is covered with a sterile gauze pad and a PE foil to make complete decontamination possible. If the injury cannot be dealt with on the spot, the injured subject is referred to further treatment in the plant’s health care centre. The monitoring devices and personal dosimeters are either evaluated immediately and the readings are recorded, or are transmitted for prompt evaluation to the health physics unit. If the permissible limits of radiation exposure are exceeded, the head of the health care centre must be notified of the situation without any undue delay. Each incoming person is registered and his basic data entered in a special form (personal data, date, radiation monitoring results indicative of the level of body surface contamination). The health physics officer identifies the spots of the highest contamination and advises on appropriate procedures of decontamination. Next, the affected persons pass to the shower room. Here they wash with warm water and soap, using scrubbing with soft brushes and washcloths. Particular care must be given to washing thoroughly the hands (especially the interdigital folds), neck, face and hairy body parts. Nails are clipped, if possible. In particularly serious cases hair must be cut or even shaved, yet careful shampooing and washing of hair is normally sufficient, but is obligatory. The decontamination procedures must be chosen so as to avoid strong skin irritation and mechanical roughening or damage to its surface. It is therefore preferable to select a gentle procedure which does not damage skin, and apply it repeatedly until the residual contamination level is reduced to a minimum without injuring the skin. The spots that had been identified as the sites of the strongest contamination are treated with special decontaminating agents and means. Used washcloths and gauze swabs are disposed of in collecting bins. The spots decontaminated by special means and procedures are ultimately washed again with soap and water. The treatment of mucous membranes has been already described in the section dealing with PDP. It is desirable that the shower faucets be pedal-operated and that the water have a constant automatically regulated temperature. Soft hair brushes attached directly to the water hose are very convenient. When leaving the shower room (before entering the dressing room), all subjects are again subjected to personal radiation monitoring supervised by the health physics officer. Those who still show a Contamination level exceeding the permissible limits are returned to repeat the washing procedures. If these again fail to achieve the expected results, cases of persistent contamination are refer-

297

red to the health care centre for treatment. Refractory contamination (particularly if higher exposure rates of gamma radiation are recorded over the thoracic and abdominal regions) are always indicate of internal contamination. The plant's health care centre also admits for treatment individuals who suffered minor contaminated injuries requiring professional attendance. All persons who have successfully completed decontamination may get dressed and leave for the place of their destination.

2.9.9 Composition of solutions used for skin decontamination The reagents which have proved to be effective in skin decontamination are mixtures of the following constituents: surface active compounds, complexing agents, ion exchangers, various types of fillings and skin protective agents. The nature, function and properties of all these components have been described in Chapter I of this book. Whereas pastes may be composed of all the listed components, solutions usually do not consist of more than three components. If solutions are used, the treated skin is then covered by some of the soothing reagents.

2.9.9.1 Formulations of solutions and pastes applicable to skin decontamination Citric acid Solutions of 0.5-3 wt.% concentration, mostly in distilled or demineralized water, are used. The citrate anion forms stable complexes with a number of cations. The main advantage of citric acid is an easy availability, as well as the fact that the compound at this concentration does not attack the skin. Oxalic acid Aqueous solutions of 0.5-1 wt. YOare used with advantage in the treatment of skin contaminated with Sr, Ba and Zr actinides. Polyaminopolycarboxylic acids and their salts The most common representative is EDTA (disodium salt of ethylenediaminetetraacetic acid). This compound forms complexes of a relatively high stability with a great number of cations. It is sufficiently soluble in water at ambient temperature (10.8 g .1-' at 295 K). At a pH below 2, it separates out of the solution in the form of the white, water-insoluble, free ethylenediaminetetraacetic acid. Solutions of 0.5-1.5 wt.% are used for decontamination. Another important agent of this type is DTPA (diethylenetriaminepentaacetic acid) used in decontamination practice mainly as calcium trisodium or pentasodium salts. The compounds are water soluble and are used as 0.51.5 wt.% solutions. Since they are stable, the solutions may be kept as stock solutions. 298

Sodium polyphosphates Among the best known and most common decontaminants of this type is sodium hexametaphosphate, a mixture of sodium polyphosphates. The agent combines properties of complexing agents and liquid cation exchangers. It deflocculates and peptizes colloids. Solutions of 0.25-1 wt.% (pH about 5 ) are used. It dissolves easily, but lacks stability while in solution and undergoes hydrolysis, though its efficiency is retained. It is often combined with sulfonated higher fatty acids. Soap, caustic soda, detergents Caustic soda is used as a 0.5-1 wt.% solution, and soap of any kind as a 1-3 wt.% solution. A well-tested combination suitable for cleaning heavily soiled hands is a solution of 0.2-0.5 wt.% soap plus 2 wt.% soda. Detergents are used as solutions in concentrations that are specified by the appropriate manufacturers; the concentration does not usually exceed 0 . 1 - 0 . 3 wt.%. Special formulation of soaps with an increased decontaminating efficiency are also available [236]. Multicomponent solutions The following may be given as examples: - Aqueous solution of soda or soap (0.1-0.3 wt. %) supplemented with 0.5 wt. % disodium EDTA; - Aqueous solution of a detergent or soap (0.3 wt. YO)supplemented with 0.7 wt.% sodium hexametaphosphate; - 1 wt.% citric acid supplemented with 0.5 wt.% DTPA plus 0.3 wt.% of a detergent . Pastes Pastes are stable, can be prepared in advance and kept in stock. Being applied preferentially to partial decontamination of hands, they can be of particular benefit when shortage of noncontaminated water makes washing impossible. The following formulations are quoted as illustrative examples of decontamination pastes (the quantity of the component is given in wt.%): A - Disodium salt of sulfosuccinic acid monoester Diethylethanolamide Sodium hexametaphosphate Pumice Clay (dried, ground) Carboxymethylcellulose

7 8 5 15 63 2

B - Alkylsulphate Sodium hexametaphosphate Clay (dried, ground) Lanolin (or glycerin)

5 20 73 2 299

C

-

Detergent (powderized) Sodium hexametaphospha te Kaolin Carboxymethylcellulose Glycerin

5 5

76 3 11

2.9.9.2 Special reagents

All commercially distributed proprietary reagents for skin decontamination belong in this group, such as Radiacwash (U.S.A.), Dekontacoll (FRG), Dekont (CSFR) and others.

2.10 Decontamination of domestic animals All of what has been said in Sect. 2.9 on processes resulting in surface contamination of persons holds true also to the full extent for contamination of animals. Similarly, the degree of radiation damage to animal skin depends on the same factors as those dealt with in connection with human skin contamination. As a characteristic feature of surface contamination of animals, one would expect rather more frequently contamination of the entire body with a higher level of contamination of legs and paws, lower parts of the body and regions around the snout. On the other hand, the uptake of radionuclides via the skin is more likely to differ in animals from that known for man because of the differences in human and animal skin histology, and due to the fact that the animal body surface is covered with a hairy layer of variable density and thickness. The mechanisms of processes that lead to internal contamination are different not only in men and in animals, but they may also significantly differ among various animal species. Since meat and milk of some animal species are an important source of food for human populations, it is necessary to take into consideration also all aspects connected with the transfer of the contaminant not just to the critical organs, but also to all other tissues that are either directly consumed or used to prepare derived consumable products. The basic objective of animal decontamination is to reduce to a minimum the risk of radiation injury to skin and to prevent as much as possible the danger of internal contamination of the animal organism. Here again the fundamental rule is that it is far better and less costly to protect animals from the radioactive fallout and prevent external and internal contamination than to have to perform a time-consuming and expensive decontamination of affected animals. The principles of minimizing the incorporation of radioactive substances into a 300

human organism via the food and drinking water, described in the appropriate chapter (Sect. 2.11) apply fully to the uptake of contaminants by animals via contaminated food and water. The time period critical for external and internal animal contamination appears to be a short interval immediately following the release of radioactivity at any given place and time. The secondary contamination with deposited radioactive fallout components made airborne secondarily by wind gusts and movement of animals and vehicles is always considered less intensive than the primary contamination occurring at the time of the primary descent of radioactive particles from the radioactive cloud. It is therefore advisable to shelter domestic animals for some time after the initial release of radioactivity, restrict their free movement and feed to them only preserved forage. The animals may be turned out to pasture again about 10 to 20 days later, preferably after the first rain has washed out the atmosphere. 2.10.1 Contamination of animals

In a radioactively contaminated terrain, animals may become contaminated both on the body surface and internally. Internal contamination can occur even in animals kept in clean stables unexposed to direct radioactive fallout, if they are fed with contaminated fodder or watered with contaminated water. Depending on the absorbed radiation dose, radiation damage to the skin can become manifest in various clinical forms: 1. Epidermal atrophy - the mildest form occurring after absorption of a low dose of ionizing radiation. First signs appear several weeks after the exposure as an almost imperceptible loss of hair pigment. The underlying skin is seemingly intact, slight atrophic changes may be detected histologically in a microscope. 2. Exfoliative dyskeratosis - induced by medium doses of beta radiation. The skin becomes squamous and peels off. Chronic dermatitis almost regularly develops. The epidermis shows atypical cellular formations, hair follicles are usually destroyed and the surrounding tissue grows depigmented hairs. 3. Trans-epidermal necrosis - the most serious beta radiation “burns”, similar to thermal burns, accompanied with oedema and bullous desquamation. Hair is lost permanently. The bordering regions show signs of the two previous stages of skin damage. The radiation dermatitis is characterized by the following typical clinical symptomatology : - Hyperaemia, apparent especially in pigmentless skin regions, oedema and pain; hyperaemia and oedema of visible mucous membranes.

30 1

Itching of the affected skin regions (forcing the animal to scratch vigorously over the affected regions). - Epilation and desquamation of the. upper epidermal layers. - Slow peeling off of radiation “burns”, often aggravated by ulceration and other complications (formation of infiltrates and cavities, sepsis). - Very slow advancing of healing and regeneration processes, frequent relapses with progressing skin necroses and haemorrhages. When assessing the consequences of an internal exposure of animals to radiation emitted by incorporated fallout radionuclides, it is assumed that - the mechanisms of entry of radioactive substances into an animal organism are the same as those operative in man (inhalation, ingestion, transcutaneous absorption); and - the biological effects are determined primarily by the magnitude of the dose equivalent commitment in the relevant critical organ or tissue, and depend on the same factors as those specified in Section 1.2. -

t

2.10.2 Decontamination of animals The decontamination of animals, called also veterinary cleanup, may be either partial or complete. Partial decontamination consists in cleaning the external openings of alimentary and respiratory channels and their immediate vicinity, with the basic aim at preventing the uptake of radioactive substances into the animal body. It is done particularly when lack of time, decontamination means (especially clean noncontaminated water) or skilled personnel preclude the performance of a complete decontamination. Complete veterinary cleanup requires decontamination of the entire animal body surface. Either dry or wet decontamination procedures may be applied. The complete decontamination necessarily includes also cleaning of the entire space in the stable and the paddock to reduce the specific activity (Bq .m-2) to levels which would make a serious recontamination of the animals unlikely. It is obvious that the same goal can be achieved if enough time is allowed for a spontaneous decrease in radioactivity due to natural decay. The chief criterion which decides whether or not a complete veterinary cleanup is required is an objectively assessed degree of surface contamination determined by measuring the exposure (dose) rate or the pcount rate over a unit area of body surface. The measurement is conventionally taken by placing the detector about 1-2 cm above the body surface. It must necessarily be carried out in a place with a low radiation background, in to avoid the interference of the radiation emitted from the adjacent contaminated terrain or objects. Skin damage depends primarily on the absorbed skin dose of beta radiation. Since it is not feasible to measure the absorbed dose directly, a permissible 302

level of surface contamination is used instead as a derived limit. The critical value has been conventionally set to 2 . los p disintegrations per 1 cm2 body surface per 1 min. It is important to determine whether or not, and if so then to what extent, the animal are internally contaminated, and to differentiate the component of the measured exposure (dose) rate attributable to radiation emitted by the incorporated radionuclides. The differentiation is facilitated if the body surface contamination is independently assessed by the smear technique and the internal contamination by measuring the radioactivity in urine and faeces specimens. It is an inevitable ethical requirement that the animals be not injured nor excessively scared by the decontamination procedures. All spent solid material as well as water used in veterinary cleanup processes must be handled appropriately as radioactive wastes. It has to be realized that any animal decontaminating operation is a costly and laborious procedure and the balance between the expected benefit and the costs must be carefully weighed. The same reagents, solutions, utensils and means are used for animal decontamination as in the case of decontamination of human subjects. The following dry methods of a complete veterinary cleanup are used: - Rubbing off the body surface with a cloth or a bunch of straw or hay; - Brushing the hairy coat with a body brush or a curry comb; - Wiping the accessible body cavities and their external openings (nostrils, mouth, snout) with gauze or cloth swabs. Suitable equipment includes all types of sprinklers, washing ramps or platforms, together with a simple source of pressurized clean water. The fleece of animals covered with a thick fur is to be removed by shearing before starting the decontamination procedure. It is preferable to use lukewarm or warm water (up to 303 K); tensides (surface active agents) are added to enhance the cleaning effect, and chelating compounds are used to prevent redeposition of the removed radionuclides. When large-scale veterinary cleanup activity is envisaged, special veterinary decontamination stations may be set up, as shown in Fig.2.26. In their experiments with pigs, sheep and cattle, HavliCek et al. [237, 2381 investigated the decontamination efficiency of various agents - liquid toilet soap, soft potash soap, washing detergent, cleansing detergent, EDTA disodium salt, sodium thiosulphate, oxalic acid, and thiourea. Surface contamination was effected by spraying a water solution of the contaminant on a delimited area (25 by 20 cm) on the animal's back. The following radionuclides were tested: "Sr, 95Zr-9?Nb, IMRu, I3'I, I3'Cs and '"Ce. Water alone showed only a very poor decontamination efficiency for all possible combinations of animal species and nuclides. Without exception, the best performance was given by a water solution of liquid washing detergents (1 YOby volume) supplemented with 1 YOby weight

303

-

, THE

THE "DIRTY '* SECTION

"CLEAN" SECTION

Fig. 2.16. Lay-out of a special veterinary cleanup unit [239] The "dirty" section: I pen for contaminated animals. 2 distribution, 3 frame sprinklers for spraying the animals as they pass through. 4 special cleanup process, 5 -- frame sprinklers. 6 - monitoring post. 7 .- detaining pen for animals requiring repeated cleaning and animals treated with decontaminating reagents; a high pressure pump. b - solution tank. c water supply. d veterinary kit. e special equipment. f drains. g decontamination of protective clothing and harness. h sump. The "clean" section: 8 pen for decontaminated animals. 9 fixing cage. 10 distribution, I I sorting post, I2 ramp; i pen for sick animals. k cattle lorries. m - veterinary kit ~

~

~~

~

~

~

~

~

~

~

~~

~~

~

of the EDTA disodium salt. Over 90% of the initial radioactivity could be removed in this way. Whereas the removal of 137Cs was practically complete, the relatively most resistant radionuclides were 95Zr-95Nb, Io6Ru and lace. As regards the animal species, the best results were achieved with pigs. As could be expected, shorn animal skin was decontaminated more effectively than hairy skin. It must be again emphasized that it is important to avoid an undue solubilizing of a dry surface contaminant. Animals that were contaminated with dry radioactive substances (dry fallout) and kept dry until the start of the decontamination process are to be subjected to a dry decontamination procedure first before applying any wet method. The following table (Table 2.22) lists the approximate standards of consumption of the decontamination agent (1 vol. % detergent plus 1 wt. % complexing agent) and the rinse water, as well as the required duration of action [239].

2.11 Decontamination of human.and animal food The uptakes of radionuclides via the air and the foodchain represent the two most important pathways of internal contamination of organisms. The 304

TABLE 2.22 CONSUMPTION OF DECONTAMINATING SOLUTION AND RINSING WATER DURING ,DECONTAMINATION OF ANIMALS Consumption (dm’) per I animal Animal species

decontaminating solution

rinsing water

8 8

30

5 2

20

cattle Pig sheep dog

Duration of action (min) 10 6 5 5

15 10

radionuclides introduced into the environment originate from both controllable and incontrollable radiation sources. The main objective of food and forage decontamination is to reduce to a reasonably achievable minimum the risk of radioactivity intake, so that the effective dose equivalent resulting from the internal radiation exposure attains only a very small fraction of the total radiation dose imparted to the organism. Shortly after the radioactivity is released (in a nuclear accident or a nuclear weapon explosion), external radiation contributes by far the largest component of the total absorbed dose; however, the ratio of external to internal radiation DIRECT IRRADIATION

f i DEPOSITION

DEPOSITION

l=iiil SUBST

INHALATION

INGESTION -

u)IL

t

.

INHALATON

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