Neptunium: Radiation Protection Guidelines (N C R P Report)

NCRP REPORT NO. 90

Neptunium: Radiation Protection Guidelines Recommendations of the NATIONAL COUNCIL O N RADIATION PROTEC1-ION AND MEASUREMENTS

Issued February 28, 1988 National Council on Radiation Protection and Measurements 7910 WOODMONT AVENUE/BETHESDA, MD. 20814

LEGAL NOTICE This report was prepared by the National Council on Radiation Protection and Measurements (NCRP). The Council strives to provide accurate, complete and useful information in its reports. However, neither the NCRP, the members of NCRP, other persons contributing to or assisting in the preparation of this report, nor any person acting on the behalf of any of these parties (a) makes any warranty or representation, express or implied, with respect t o the accuracy, completeness or usefulness of the information contained in this report, or that the use of any information, method or process disclosed in this report may not infringe on privately owned rights; or (b) assumes any liability with respect to the use of, or for damages resulting from the use of, any information, method or process disclosed in this report, under the Civil Rights Act of 1964, Section 701 et seg. as amended 42 U.S.C. Section 2000e et seg. (Title VII) or any other statutory or common law theory governing liability.

Library o f Congress Cataloging-in-Publication D a t a National Council on Radiation Protection and Measurements. Neptunium: radiation protection guidelines. (NCRP report, no. 90) Bibliography: p. Includes index. 1. Neptunium-Toxicology. 2. Neptunium-Metabolism. mental aspects. 1. Title. 11. Series. RA1231.N37N38 1987 616.9'897 87-11207 ISBN 0-913392-87-1

3. ,Neptunium-Environ-

Copyright O National Council on Radiation Protection and Measurements 1988 All rights reserved. This publication isprotected by copyright. No part of this publication may be reproduced in any form or by any means, including photocopying, or utilized by any information storage and retrieval system without written permission from the copyright owner, except for brief quotation in critical articles or reviews.

Preface The NCRP has considered from time to time specific problems associated with individual radionuclides and devoted entire reports to one radionuclide. Examples of this are NCRP Report No. 44 on 85Kr, Report No. 52 on '37Cs, Report No. 62 on 3H, Report No. 75 on '''I, and Report No. 81 on I4C. The present report reviews the current knowledge of neptunium in areas which are important to radiation protection and makes recommendations on radiation protection guidelines. The report briefly reviews the chemical and physical properties of neptunium, the sources of neptunium in the environment, and potential pathways to man. The limited animal data on health effects are also examined. In addition, animal studies that may serve as a basis for predicting the metabolic behavior of neptunium in humans are considered. On the basis of the information examined, recommendations are made on the fraction of ingested neptunium absorbed from the gastrointestinal tract into blood (fi). Finally, the report applies the available data to recommendations on radiation protection guidelines for neptunium. The units used in this report are those of the Systeme International #Unites (SI) but they are followed by the conventional units in parenthesis in accordance with the procedure set forth in NCRP Report No. 82, SI Units in Radiation Protection and Measurements. After this report was completed but before its publication, the ICRP issued its Publication 48, "The Metabolism of Plutonium and Related Elements", which considers neptunium and arrives at conclusions that are generally similar to those set forth in this NCRP report. The two reports agree on the most significant change, i.e., from a value of to a value of lop3for the fraction absorbed from the gastrointestinal tract. Furthermore, values for ALI and DAC which the ICRP might in the future base on the metabolic parameters recommended in ICRP Publication 48 would not differ significantly from those recommended in this NCRP report. This report was prepared by Task Group 13 (Neptunium) of Scientific Committee 57 on Internal Emitter Standards jointly with Scientific Committee 38 on Waste Management. Serving on the Task Group were: Roy C. Thompson. Chairman Battelle Pacific Northwest Laboratory Richland, Washington iii

iv

/

Preface

Maryka H. Bhattacbaryya Argonne National Laboratory Argonne, Illinois

Norman Cohen New York University New York, New York

Maurice F. Sullivan Battelle Pacific Northwest Laboratory Richland, Washington

Robert P. Larsen New Smyrna Beach, Florida

Patricia W. Durbin, Consultant University of California Berkeley, California

Serving as the Chairman of Scientific Committee 38 on Waste Management was Merril Eisenbud. Serving on Scientific Committee 57 on Internal Emitter Standards were: J. Newel1 Stannard, Chairman University of California at San Diego San Diego, California

John A. Auxier Evaluation Research Corporation Oak Ridge. Tennessee

Roger 0.McClellan Lovelace Biomedical and Environment Research Institute Albuquerque, New Mexico

William J. Bair Batklle Pacific Northwest Laboratory Richland, Washington

Chester E. Richmond Oak Ridge National Laboratory Oak Ridge, Tennessee

Bruce B. Boecker Lovelace Biomedical and Environmental Research Institute Albuquerque, New Mexico

Robert A. Schlenker Argonne National Laboratory Argonne, Illinois

Keith J. Eckerman Oak Ridge National Laboratory Oak Ridge, Tennessee

Roy C. Thompson Battelle Pacific Northwest Laboratory

Richland, Washington

NCRP Secretarid-E. Ivan White

The Council wishes to express its gratitude to the members of the Task Group and Scientific Committee for the time and effort devoted to the preparation of this report. Especial thanks are due to Keith Eckerman, Oak Ridge National Laboratory and the dosimetry group at the Laboratory for their calculation of the Annual Limits on Intake and the Derived Air Concentrations given in this report. Warren K. Sinclair President, NCRP Bethesda, Maryland Nou. 20, 1987

Contents Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2. Chemical and Physical Properties of Neptunium . . . . . 2.1 Oxidation-Fteduction Equilibria . . . . . . . . . . . . . . . . . . . . 2.2 Neptunium (111) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 Neptunium (IV) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4 Neptunium (V) and Neptunium (VI) . . . . . . . . . . . . . . . . . 2.5 Biological Inferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 Neptunium in the Environment . . . . . . . . . . . . . . . . . . . . . 3.1 Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Pathways to Man . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Metabolism of Neptunium in Mammals . . . . . . . . . . . . . . 4.1 Ingestion (Gastrointestinal Absorption . . . . . . . . . . . . . . . 4.2 Inhalation (Pulmonary Retention and Absorption . . . . . . 4.3 Systemic Distribution. Retention. and Excretion . . . . . . . 5 Health Effects of Neptunium . . . . . . . . . . . . . . . . . . . . . . . . 6 Radiation Protection Guidelines . . . . . . . . . . . . . . . . . . . . 6.1History . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Appendix A Supplemental Dosimetric Information . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The NCRP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . NCRP Publications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

. . .

1. Introduction Neptunium, element number 93, is less well understood than some other transuranic elements such as plutonium and americium. It has become of particular recent interest, however, because of the presence in nuclear waste of its long-lived isotope 237Np(half-life, 2.14 x lo6 y), and because of its presumed environmental mobility. This report summarizes present knowledge of neptunium in areas pertinent to the establishment of radiation protection guidelines. Following the Introduction, the second section of the report briefly reviews the chemical and physical properties of neptunium, particularly as these relate to its biological behavior. The third section briefly describes sources of neptunium in the environment, and potential pathways to people. The fourth section reviews the results of animal studies that may serve as a basis for predicting the metabolic behavior of neptunium in humans. The fifth section reviews the limited animal data on health effects. The final section applies these data to the development of radiation protection guidelines for neptunium. This report includes numerous citations of the more recent neptunium literature most pertinent to radiation protection considerations, but does not purport to be an exhaustive review of the subject. A recent review, by R. C. Thompson (1982),has been drawn upon heavily in the preparation of this report and should be consulted for references to the older literature.

2.

Chemical and Physical Properties of Neptunium

Neptunium is the element of atomic number 93, just beyond uranium in the periodic table. Some pertinent nuclear properties of the isotopes of neptunium are presented in Table 2.1. Because of its presence in nuclear waste and its very long half-life, 237Np is the isotope of principal concern from the point of view of radiological protection. Neptunium-237 decays (2.14 x 106y)by alpha emission (=: 4.6 MeV) and soft gamma emission (up to 0.3 MeV) to 233Pa. The chemistry of neptunium is briefly reviewed here, only as a background for radiological protection considerations. For a more detailed coverage of neptunium chemistry see Keller (1971), Bailar et al. (1973), or Burney and Harbour (1974). In the discussion which follows, the chemical behavior of neptunium will be compared with that of the better understood, and somewhat similar actinides, uranium and plutonium. While the chemistry of these elements is critical to the understanding of their overall environmental and metabolic behavior, it must also be recalled that the risk from human inhalation of their very insoluble refractory oxides (probably the predominant form of occupational exposure) will be largely determined by the physical characteristics of the aerosols inhaled. 2.1 Oxidation-Reduction Equilibria

Neptunium is a multivalent element, which, like its adjacent actinides, uranium and plutonium, may exist in one or more of four oxidation states: 111, IV, V, and VI. The particular significance of oxidation state becomes apparent when one considers the wide variation in oxidizing and reducing conditions that exist within a mammal, on both macroscopic and microscopic scales. Oxidizing conditions exist in the lungs while reducing conditions exist in the small intestine; arterial blood is more oxidizing than venous blood; and extracellular fluids are more oxidizing than intracellular fluids. The most oxidizing situation is probably that in the lungs, where neptunium might be expected to exist in the V state; the most reducing situation is probably

2.1

OXIDATION-REDUCTION EQUILIBRIA

/

3

TABLE2.1-Pertinent nuclear characteristics of neptunium isotopesn Mass Number

Half-Life

,

232 233 234 235 236 236 237 238 239 240111 240

14.7 m 36.2 m 4.4 d 396.1 d 22.5 h 1.15 x lOby 2.14 X 106y 2.117 d 2.355 d 7.4 rn

65 m

Principal Decav Mode

EC EC EC EC EC flEC fla

808/

3

!

"From ICRP (1983).

that in the small intestine, where neptunium might be expected to exist in the IV state. Neptunium absorbed to the blood from these two sources may thus differ chemically, and if oxidation or reduction does not occur rapidly, the distribution and retention pattern of neptunium from one source might not be the same as that from the other. Unfortunately, it is difficult to predict the oxidation state(s) existing in a biological system for elements such as neptunium, uranium and plutonium, because of the extensive opportunities for thermodynamically allowed complexation reactions of the various valence states and the unknown kinetics of transformation of the species potentially involved. Some characteristics of neptunium oxidation states of potential interest in biological systems are considered below. 2.2

Neptunium (111)

Neptunium, uranium, and plutonium in the 111state exhibit strongly cationic properties. Each can be precipitated from solution as the hydroxide, fluoride, carbonate, or oxalate; each can be maintained in neutral solution by an excess of a dicarboxylic acid such as citrate; and each forms a very strong complex with chelating agents such as ethylenediaminetetraacetic acid (EDTA), or diethylenetriaminepentaacetic acid (DTPA). In acid solution, e.g., 1 M hydrochloric acid, U(II1) is oxidized to U(1V) by hydrogen ions, Np(II1) to Np(1V) by a mild oxidant such as iron (111), while a comparatively strong oxidant such as elemental bromine is required to oxidize Pu(II1) to Pu(1V). In neutral media the redox potentials of the 111-IV couples are highly positive because complexing by hydroxyl ions is much stronger for the IV state than for the I11 state; and the 111 state of each element is converted to the IV state by reaction with water, producing hydrogen.

4

/

2.

CHEMICAL AND PHYSICAL PROPERTIES OF NEPTUNIUM

It is difficult to conceive of Np(III) playing a significant role in biologic systems.

2.3 Neptunium (IV) In the IV state neptunium, like uranium and plutonium, may exhibit either cationic or anionic characteristics, the latter becoming dominant when ligands with which it strongly interacts are present. In media containing sulfate, fluoride, carbonate, dicarboxylates or chelates, the stability of the complex formed with the IV state is much higher than that formed with either the I11 or V state. A particularly important reaction of the IV state is that involving hydroxyl ions, the so-called hydrolytic reactions that lead to the formation of hydrous oxide polymers. The probability of these reactions occurring increases with: (1) decreasing acidity, (2) increasing concentration of actinide, (3) decreasing concentrations of strongly complexing ligands, (4) increasing temperature, and (5) the length of time that the preceding conditions exist.

2.4

Neptunium (V) and Neptunium (VI)

The chemical properties of neptunium, uranium, and plutonium in the V and VI states differ markedly from those of the I11 and IV states. The higher states react only moderately with ligands to form complexes (those formed with the VI state being more stable than those formed with the V state), and neither the V nor VI state is prone to hydrolytic reactions. As a consequence, stable solutions of the V and VI states, with appreciable actinide concentration, can be prepared in media containing a ligand such as bicarbonate. In acid solution the chemical behavior of neptunium differs from that of uranium and plutonium, in that the V state is stable; the redox potential for the neptunium IV-V couple being lower than that for the V-VI couple. The IV and V states of neptunium are thus the oxidation states of interest in biologic systems. The oxidation of Np(1V) to Np(V) and the reduction of Np(V) to Np(1V) in inorganic media are relatively slow reactions, the chemical explanation being that one involves the formation and the other the breaking of covalent bonds. The biologically relevant questions are: How slow are these reactions in biological systems? Will ingested Np(V) be reduced to Np(1V) in the intestine before absorption can occur? Will inhaled Np(1V) be oxidized in the

2.4 NEPTUNIUM (V) AND NEPTUNIUM (VI)

/

5

lung to Np(V)? Will Np(IV), if absorbed in that state, persist in the blood and be distributed differently than Np(V)? 2.5 Biological Inferences

Experimental evidence relative to the thermodynamically stable state(s) of neptunium in natural media is meager. Pentreath and Harvey (1981) have shown that neptunium discharged to the Irish Sea from Windscale is not adsorbed on sea-bed sediments. Since it is known that tetravalent actinides such as Th(1V) and Pu(1V) adsorb strongly to such sediments, it may be inferred that neptunium in sea water is in the V state. The oxidation state of neptunium incorporated in laboratory animals can be inferred to be Np(V), since DTPA therapy is not significantly effective in its removal (Smith, 1972; Morin, et al., 1973). The increased gastrointestinal absorption of neptunium when fed with an oxidizing agent suggests that it is normally reduced in the intestine to Np(IV), except when fed in large mass quantities or when fed following a fast, in which case the intestinal reductants are probably ineffective (Sullivan, 1984a). The increased deposition in liver when relatively massive quantities of neptunium are intravenously injected, suggests that polymerized products of Np(1V) may persist in the blood under these conditions (Thompson, 1982). The much lower deposition in liver under other circumstances might support the assumption that Np(V) is the normal biological form of this element in the blood. The above observations form a reasonably consistent, albeit somewhat limited and certainly speculative picture of neptunium chemistry in biologic systems. The ability of neptunium to exist in either the IV or V state under biologic conditions, and the markedly different chemical behavior of these two states, may, when properly understood, explain many of the anornolous biological observations that have been reported for neptunium.

3. Neptunium in the Environment Neptunium isotopes have not presented unusual problems in occupational radiation protection, nor have they, until recently, been of special environmental concern. Particular attention has recently been directed to the potential of environmental exposure to the long-lived 237Np,which is estimated to be the principal surviving component of high level nuclear waste after ten or twenty thousand years (Cohen, 1982). In this section the potential sources of neptunium isotopes in the environment, and the pathways by which these isotopes might reach man are briefly considered. Such information, while not essential to the derivation of radiation protection guidelines, is necessary for their proper application. Thompson (1982) provides additional information in this area, with references to the early literature. This is, however, an expanding area of much current activity.

3.1 Sources The P-emitting, 2.4-day half-life 239Npis an intermediate product in the production of plutonium. It results from the absorption of a neutron in uranium according to the following reaction.

As such, it was a prominent component of the early fallout from the atmospheric detonation of nuclear devices. It may also be present in the waste streams from nuclear power plants where it is produced according to reaction (1)either within the uranium fuel from which it may leak, or from neutron irradiation of traces of uranium on the fuel cladding or in the reactor cooling water. Although 239Npmay contribute significantly to the radiation dose from early fallout following a nuclear detonation (Perkins and Thomas, 1980), its short half-life precludes any lingering environmental contamination. The neptunium isotope of primary interest is 2"Np. It has a half-

3.1

SOURCES

/

7

life of 2.14 x lo6 years and is produced in nuclear reactors by several neutron-capture nuclear-decay routes.

Z U (n, 2n) 2:iU &'17N 6.7 d

'ZU (n, y ) ,3' ,9U

93

0- 'ZNp + 0+ 24 m 2.4 d

Reactions (2) and (3) are the primary ones involved in the production of '"Np in the fuel from plutonium production reactors; reaction (4) is the primary source of 237Npin the spent fuel from light water and fast breeder reactors, and hence in the wastes generated when these fuels are processed. Due to its formation from the decay of 241Am,the amount of 237Npin these fuels at the time of removal from a reactor is a factor of about 10 lower than it is 1000 years later (Croff et al., 1982). Neptunium-237 has been purposefully separated from the other constituents of spent fuel and irradiated with neutrons to produce ='Pu (Schultz and Benedict, 1972).

The '"Pu so produced is employed as the heat source in the thermoelectric devices that have been used as power sources in space vehicles and in carhac pacemakers. ~h~ 2 3 7 ~ ~ / 2 3 9 + 2 4Pu 0 activity ratio in fallout, as measured in lichens from Sweden, is 2.7 x 10-"Holm, 1981); similar results have been obtained from measurements in soil (Efurd et al., 1984). It is conceivable that "'Np could be accidentally dispersed from a production facility, but the greater concern is associated with its disposal in high-level nuclear waste repositories, and its possible subsequent release and environmental dispersal over future periods of hundreds-of-thousands of years. Based on some recent assumptions

8

/

3. NEPTUNIUM IN T H E ENVIRONMENT

as to its biological behavior (ICRP, 1980a), it has been estimated that 237Npmay be the most hazardous remaining constituent of high-level nuclear waste during the interval from about 10,000 years to 30,000,000 years following disposal (Cohen, 1982). 3.2 Pathways to Man Environmental 237Npbecomes a significant hazard only to the extent that it is ingested, inhaled, or otherwise assimilated by man. The environmental pathways by which such assimilation can occur seem generally more permeable to neptunium than to other actinides (Thompson, 1982). In mineral soils, the extractability of neptunium is markedly greater than that of plutonium, americium, or curium in the p H range below 6, while in calcareous soils a second extraction maximum is observed at a pH of 10 (Nishita et al., 1981). Values of the distribution coefficient, Kd [concentration on solid phase (g/g) divided by concentration in liquid phase (g/ml)], for these soil systems ranged from 3 to 900. Extractability and Kd appear to be influenced by soil organic matter and the hydrous oxides of aluminum, iron, and manganese. These data indicate either a persistence of Np(V) in surface soils, or relatively easy oxidation of Np(1V). However, studies by Bondietti and Frances (1979), using suspensions of igneous rock maintained under anaerobic conditions such as might be expected to exist under waste disposal circumstances, demonstrated the rapid reduction of Np(V) to Np(IV), with the latter being strongly sorbed. The prediction of long-term movement of neptunium through geological media is, therefore, somewhat uncertain. The movement of neptunium from soil to plants occurs more freely than does the movement of other actinides. Plant availability of neptunium in soils, CR (concentration per unit dry weight of vegetation divided by concentration per unit dry weight of soil), has been reported to range from to 1 at relatively high soil concentrations of W7Np(Romney et al., 1981; Schreckhise and Cline, 1981; Watters, et al., 1980). More recent studies using 235Np,to permit evaluation of neptunium availability at environmental concentrations, showed CR values ranging from 0.5 to 4 for foliage of forage and seed crops, with seed values lower than leaves by a factor of ten (Cataldo et al., 1987). In the studies of Schreckhise and Cline (1982), five years of weathering in field lysimeters had little effect on the uptake of neptunium by plants (Cline, private communication). Limited data for marine environments indicate that neptunium

3.2 PATHWAYS TO MAN

/

9

behaves somewhat similarly to other actinides. At the very low concentrations in which it is present, it exists primarily in the (V)state in filtered waters (Pentreath et al., 1985; Fowler and Aston, 1982). Concentration factors due to uptake/adsorption from water to alga are from 8 to 56, a factor of about 100 less than for other actinides. Concentration factors for lobster tail and claw muscle, of 0.02 to 0.3 vary with season and are comparable to data for plutonium. Reliable data are not available for freshwater systems. Limited animal food chain data show little evidence for enhanced transport of neptunium as compared to other actinides. Transfer to milk by sheep, goats, and rats is generally lower for neptunium than for plutonium, americium, or curium (Thompson, 1982). While these data on environmental availability of neptunium are not essential to the derivation of intake limits, they are essential to the proper application of such limits in terms of derived, environmentally measurable, action levels. This kind of information is largely lacking in the case of neptunium and is of particular importance because neptunium does not seem to fit the environmental behavior pattern of other actinides.

4. Metabolism of Neptunium in Mammals The only measurements of 237Npin humans are those on liver and lung samples from 3 autopsies, which indicated Np/Pu ratios in both organs that are 0.01 to 0.06 times the Np/Pu ratios in weapons-test fallout (Efurd et al, 1984, and personal communication). This observation suggests that neptunium is discriminated against, relative to plutonium, but provides no clue as to where or how this discrimination may occur. In the absence of any substantial human data, our understanding of neptunium behavior in humans must be inferred from information on other actinides, modified by the results from comparative studies with neptunium and other actinides in laboratory animals. For purposes of radiation protection, one is primarily concerned with defining those human metabolic parameters required for dosimetric modeling. Such parameters include the fraction of ingested neptunium that is absorbed from the gastrointestinal tract; the fraction and rate of absorption of inhaled neptunium from the respiratory tract; and the distribution and retention of neptunium in body organs. The more pertinent data bearing on these concerns will be considered in this section.

4.1 Ingestion (Gastrointestinal Absorption) The fraction of ingested neptunium absorbed from the gastrointestinal tract into blood (fl) is currently assumed to be 0.01 by the International Commission on Radiological Protection (ICRP, 1980a). This fraction had previously been set a t 0.0001 by both the ICRP (1960) and NCRP (1959) and the change in 1980 by a factor of 100 was primarily responsible for the current renewed interest in potential health risks from neptunium. Data that formed the basis for the 1980 ICRP fl value of 0.01 are summarized in Table 4.1. The Hamilton (1947) data, derived from only 3 animals, was the basis of the earlier ICRP/NCRP fi value of 0.0001. 10

TABLE 4.1-Pre-1981 Isotope

Species (no.)

Dose' (mg/kg)