Collective Dose. Indications and Contraindications

COLLECTIVE DOSE indications and contraindications

SCIENCES 1 7, avenue du Hoggar Parc d'Activites de Courtaboeuf 91944 Les Ulis Cedex A, France

ISBN: 2-86883-598-8 Tous droits de traduction, d'adaptation et de reproduction par tous procedes, reserves pour tous pays. La loi du 11 mars 1957 n'autorisant, aux termes des alineas 2 et 3 de 1'article 41, d'une part, que les « copies ou reproductions strictement reservees a 1'usage prive du copiste et non destinees a une utilisation collective », et d'autre part, que les analyses et les courtes citations dans un but d'exemple et d'illustration, « toute representation integrale, ou partielle, faite sans le consentement de 1'auteur ou de ses ayants droit ou ayants cause est illicite » (alinea 1er de 1'article 40). Cette representation ou reproduction, par quelque procede que ce soit, constituerait donc une contrefacon sanctionnee par les articles 425 et suivants du code penal. © 2002, IPSN. Publie par EDP Sciences Tous droits reserves.

Working Group MME BRETHEAU FRANCOISE M. CHAMPION DIDIER MME CONTE DOROTHEE M. GOURMELON PATRICK M. HUBERT PHILIPPE M. LAURIER DOMINIQUE M. LECOMTE JEAN FRANCOIS M. LOMBARD JACQUES M. METIVIER HENRI M. NENOT JEAN-CLAUDE, chairman M. OUDIZ ANDRE M. QUENIART DANIEL MME RANCILLAC FRANCOISE MME ROMMENS CATHERINE MME SUGIER ANNIE MME SUPERVIL SYLVIE, secretary MME VIALA MICHELE

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Contents 1. Introduction 2. Bases of the Collective Dose 2.1. Historical Reminder 2.2. Concepts, quantities and units 2.2.1. The Risk 2.2.2. The Detriment 2.2.3. The Collective Dose 2.2.4. The Quantities 2.2.5. Critical Group - Reference Group 2.3. Relationships between quantities and concepts 2.3.1. The relationship collective dose/effective dose/detriment 2.3.2. The relationship collective dose/equivalent dose/effect on an organ 2.3.3. The relationship collective dose/ad hoc dose/ad hoc effect 3. Three criteria for use of the collective dose 3.1. Relevance of the "collective dose" indicator 3.2. Reliability of the "collective dose" indicator 3.3. Adequacy of the "collective dose" indicator for the estimated risk 4. Admitted use and contested use of the collective dose 4.1. Some generally admitted uses of the collective dose 4.1.1. Application of the collective dose to an homogenous group 4.1.2. Distribution of the collective dose with regards to individual doses 4.1.3. Use of a dose to an organ to calculate a specific risk 4.2. Some contested uses of the collective dose 4.2.1. The collective dose and the radiological impact of releases from nuclear installations 4.2.2. The collective dose and the exposure related to the disposal of high level and/or long-lived waste 4.2.3. The collective dose and the estimate of the risk to a given organ 4.2.4. Introduction of a de minimis dose in the calculation of the collective dose 5. The IPSN position 5.1. Methods of calculating the collective dose depending on its use 5.2. Confidence attributed to the calculation of the collective dose 5.3. Interpretation of the results of the calculation of the collective dose References

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Collective dose: indications and contraindications 1. Introduction This report serves as a guide relating to the use of the collective dose. It is devoted to radiological protection practitioners, whatever their level of responsibility is (operators, employers, authorities). The concept of collective protection appeared in the first report of the International Commission on Radiological Protection (ICRP, 1959): limitation of exposure must apply not only to persons but also to all individuals of a population. ICRP added that, in order to satisfy this requirement, two parameters should be considered: the exposure of individuals that make up the population and the number of individuals exposed. During the course of the next decades, ICRP gave details on the use of the collective dose, in particular on its role in the process of optimisation of protection. On this matter, ICRP Publication 22 (ICRP, 1973) specifies that, in order to express the health effects of radiation in a population, the collective dose can only legitimately be used if the dose-effect relationship is linear without threshold and independent of the dose rate. Paragraph 34 of Publication 60 (ICRP, 1991) outlines that the collective dose takes account of the number of people exposed to a source by multiplying the average dose to the exposed group by the number of individuals in the group. The operators, concerned with improving the protection of their workers, quickly became interested in using the collective dose. A current application of the collective dose within the framework of optimisation consists in comparing exposures, in time and space, due to operations or operating procedures. This comparison allows the selection of the radiological protection option that best meets the optimisation requirements. Some experts have gone further by using the equation: collective dose x probability of death by cancer per unit dose = number of radiation-induced cancers, to quantify the risk linked to radiation exposure. However, several examples, such as the Chernobyl accident (a mixture of high doses received by some and low doses received by large populations), discourage the use of the collective dose for the estimation of the health impact on very large populations that are heterogeneous in their demographic structure and their levels of exposure (Nenot, 1994). Since the end of the 80s, the collective dose has been more widely criticised (Dunster, 2000; Fairlie and Sumner, 2000; Lindell, 2000). Two radically different

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Collective dose: indications and contraindications

criticisms are emerging from the batch. The first, ethical in nature, underlines that this indicator does not reflect the inequities between the exposed individuals. The second, scientific in nature, applies to the linear-nonthreshold dose-effect relationship hypothesis between the collective dose and the risk of the occurrence of stochastic effects. Another, less radical and more operational criticism is held by those who doubt the feasibility and the reliability of some collective dose calculations. Currently, the world of radiological protection is shared between two extreme positions: the first considers that the collective dose is a panacea and that its operational use justifies the extension of its mathematical usage; the second bluntly rejects the use of the collective dose for the reason of deviations in its use contrary to common sense. The aim of this document is to provide a reasonable guide for the use of the collective dose. One of the criticisms formulated against the collective dose relating to the linear-nonthreshold dose-effect relationship shall not be raised here. In fact, the linear-nonthreshold relationship is a hypothesis upon which rests not only the collective dose but also the entire current radiological protection system. Therefore, it is irrelevant in this document to question the scientific bases and hypotheses upon which the entire system is constructed. Chapter 2 deals with the relationships between the various concepts, which include collective dose, risk, detriment and reference group. It shows that the collective dose can be expressed by different quantities. Chapter 3 discusses the criteria for use of the collective dose: relevance for risk management, calculation reliability and relevance of its link with the risk. Chapter 4 provides examples for use of the collective dose and discusses the reasons for accepting or contesting them. The final chapter gives the position of IPSN; it explains why it may be necessary to segment the collective dose and indicates the way to proceed.

2. Bases of the collective dose 2.1. Historical reminder The concept of collective dose was constructed between the mid-50s and the 1970s. During the fifties, radiological risk management took distance from the search for a harmlessness threshold below which there would be no health effect related to radiation exposure. The hypothesis of linear-nonthreshold relationship and the recommendation to reduce the doses to a level as low as reasonably achievable was developed in a parallel manner. In paragraph 29 of its Publication 1 of 1958 (ICRP, 1959), ICRP recommends "authorised doses" that must not involve an unacceptable risk neither for the individual nor for the population as a whole. The collective risk is thus introduced amongst the criteria that set the limits. The principle has been applied to the definition of a limit on the genetic dose, that is, in the population the average dose received by the gonads. The hypothesis at the time was that on one hand the genetic effects were proportional to the dose (paragraph 61), and on the other hand, the main consideration for controlling genetic damage was the burden for society

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due to the increase in the proportion of individuals with pathogenic mutations. This same ICRP publication (paragraph 8) says explicitly that it is not important to know whether the defective genes originate from a few individuals exposed to high levels or from many individuals exposed to low doses; it is therefore really a concept of collective exposure that is used for the genetic risk to populations. For the somatic risk, this Publication suggests no explicit limitation based on collective exposure. ICRP estimates that the individual authorised doses are sufficient to maintain the collective risk at an acceptable level. In the mid-60s, the systematic use of the linear-nonthreshold hypothesis constituted one of the bases for radiological protection (Lindell, 1996a, 1996b). In the 1970s, this hypothesis was linked to the development of the principle of optimisation. The collective dose became a determining criterion in the decision analysis, in the form developed in ICRP Publication 26, which related to a proposal from UNSCEAR (UNSCEAR, 1972, 1977; ICRP, 1977). At this time, more than they are today, the approaches towards optimisation were largely based on cost-benefit analysis. From the 80s onwards this logic resulted in numerous developments in the monetary value of the man sievert (IAEA, 1985; ICRP, 1983,1989). To date, in the French regulations for radiological protection, the focus is put on the individual risk and its acceptability and not on the collective risk. The collective dose is not subject to any limitation; nevertheless the principle of optimisation must be applied.

2.2. Concepts, quantities and units 2.2.1. The risk The word risk evokes a danger or a more or less foreseeable drawback. It covers two different ideas: the probability of the occurrence of an event and the potential consequences of this event. Risk assessment includes the estimation of the probability and the corresponding consequences. As there are generally several possible consequences, each with its own probability and level of severity, assessment of the risk is multi-dimensional (Lindell and Mallfors, 1994). Furthermore, the risk may be individual or collective; ICRP focuses on the individual risk. The International Atomic Energy Agency (IAEA) has defined the risk concept1; it specifies that uncertainties related to possible scenarios must be taken into account (IAEA, 1996, 1999). In the light of these considerations, IPSN has chosen the following definition: The risk associated with an event expresses both the probability of the occurrence of this event and the extent as well as the nature of its deleterious consequences on the health of individuals. 1

Risk: a multiattribute quantity expressing hazard, danger or chance of harmful or injurious consequences associated with actual or potential exposures. It relates to quantities such as the probability that specific deleterious consequences may arise and the magnitude and character of such consequences.

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2.2.2.

Collective dose: indications and contraindications

The detriment

The detriment is a way of expressing the risk. In paragraph 16 of its Publication 26 (ICRP, 1977) ICRP introduced the concept of detriment: the detriment in a population is defined as the mathematical "expectation" of the harm incurred from an exposure to radiation, taking into account not only the probability of each type of deleterious effect, but also the seventy of the effect. This definition is very close to that of the risk chosen by IPSN. ICRP prefers to use this terminology, as the word risk can be understood according to various meanings. In paragraph 24 of this same Publication, ICRP directly links the detriment to the collective dose: the detriment to health is proportional to the collective dose equivalent. In its Publication 60 (ICRP, 1991) ICRP retains the concept of detriment and recommends its use in some optimisation processes, such as the comparison of detriments to workers and population from a given installation. ICRP does not recommend a single approach in the calculation of the detriment. It quotes three uses of the detriment (paragraph 50): • determination of dose limits, • construction of a quantity: the effective dose, • creation of a damage value related to this effective dose with regard to optimisation. In a general manner, the detriment expresses the total harmful effects incurred by a group of persons (or a person) due to exposure to ionising radiation. The detriment therefore includes value judgements related to the associated level of severity and the consequences of fatal cancers, non-fatal cancers, sequelae, handicaps and suffering, reduction of life expectancy... ICRP (ICRP, 1991) uses two different approaches to express the detriment when fixing the dose limits for workers (paragraphs 153 and 154) or selecting the tissue weighting factors to assess the effective dose (paragraph 94). It should be reminded ourselves that, in order to set the dose limits, ICRP uses the probability of death, the contribution of non-fatal cancers (weighted by their seriousness), the weighted contribution of hereditary effects, the years of life lost by death and the loss of life expectancy averaged at the age of 18. These criteria are not aggregated and the detriment remains multi-dimensional. The individual effective dose constitutes the basic parameter for the calculation of the collective dose as defined by ICRP. For the definition of the effective dose, ICRP proposes an indicator (ICRP 1977, paragraph 16): where G represents the health detriment, P the number of persons concerned, pi the probability (provided that it is small) of incurring the effect i and gi the weighted severity factor. IPSN notes that several uses of the detriment exist and considers that it is not necessary to recommend any other definition than that mentioned above.

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2.2.3. The collective dose In 1973, ICRP recommended the use of the collective dose introduced the previous year by UNSCEAR to evaluate the global detriment in the procedure of optimisation of protection (UNSCEAR, 1972; ICRP, 1973). In 1977, it limited its use and emphasised that it is often not necessary to assess the contributions from small values of the dose equivalent provided that an upper estimate shows that they would not add significantly to the total integral (ICRP, 1977, paragraph 23). In 1997, ICRP formulated four recommendations (ICRP, 1997) on: • the expression of the collective dose: a certain level of flexibility must prevail as far as selection of the groups of individual doses and the periods of integration is concerned. Practically speaking, these latter parameters must not differ by more than one or two orders of magnitude; • the exposures received over long periods of time: the assessment of collective doses over several thousands of years and their consequences on health over several hundreds of years must be considered with great care, due to the uncertainties, growing with time, on the evaluation of individual doses and on the size of the populations exposed, as well as forecasts on the relationship between dose and detriment (for example, increased effectiveness of anti-cancer treatments); • the disparity of individual doses: it is necessary to deal only with relatively homogeneous doses, as much in terms of dose levels as in terms of projection over time, such that the relationship between dose and detriment is nearly constant in the range of doses and the period of time considered (see paragraph 2.3.1); • low individual doses: these must not be systematically ignored because they are low, as their summation may give rise to large collective doses (it is however possible to ignore them if the sources are numerous and dispersed, since, in this situation, the cost of the reduction in the collective dose becomes out of proportion with the benefit). In addition, the existence of high doses does not allow to disregard low individual risks. A definition of the so-called collective dose is provided by ICRP and IAEA: it is the product of the number of individuals exposed to a source by their average dose1. In fact, the collective dose is the sum of all of the individual doses, if necessary over a given duration. This sum allows the calculation of the average individual dose (by dividing the collective dose obtained by the number of individuals). Under these conditions, the definition used by IPSN is: The collective dose is the total dose received by a population, that is, the sum of all individual doses of the members of this population. These individual doses may be expressed in terms of effective dose or equivalent dose to an organ or a tissue. When a time upper bound and/or lower bound is set, the collective dose is qualified as a collective dose truncated in time.

1

Collective dose: an expression for the total radiation dose incurred by a population, defined as the product of the number of individuals exposed to a source and their average radiation dose.

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Collective dose: indications and contraindications

2.2.4. The quantities To quantify exposure to ionising radiation, the dose is used, which depends on the deposits of energy in the body that originate from radiation. These deposits are the origin of physical and chemical changes, likely to cause biological damage. The extent of this damage depends not only on the quantity of energy absorbed, but also on the type of radiation, the exposed tissue susceptibility and the type of exposure. The fundamental quantity is the absorbed dose (DTR), which quantifies the interaction of the radiation with the material. It is the energy absorbed per mass unit exposed to radiation. The absorbed dose is expressed in joule per kilogram (J/kg); its unit is the gray (Gy). The absorbed dose is an indicator of the severity of the deterministic effects, that is, the effects of exposure to high doses at high dose rates. The absorbed dose cannot be directly used in the daily practice of radiological protection which deals with relatively low doses, as it neither accounts for the different effects (according to the nature and energy of the radiation), nor the variations in radio-susceptibility of the exposed living tissues. Experience shows that the absorbed dose poorly conveys the stochastic effects. This is why it is necessary to apply weighting factors to it, in order to define the equivalent dose that concerns the tissues and the effective dose that concerns the whole body. The equivalent dose (HT) in tissue (T) or in organ is the product of the absorbed dose (DT) averaged over the tissue T or the organ with a dimensionless weighting factor for the radiation R (WR), which depends on the density of the ionisation along the track of the ionising particle, expressed by the linear energy transfer (LET)1. This weighting factor reflects the harmfulness of the radiation. For radiological protection purposes, it is rounded up and is equal to the unit for photons and electrons, to 20 for alpha particles and 10 for neutrons whose energy is between 10 and 100 keV. The equivalent dose then enables the stochastic effects of a radiation received by a tissue or an organ to be expressed. It is expressed in joules per kilogram; but, to avoid any confusion with the absorbed dose, it has a special unit, the sievert (Sv). As organs and tissues are not affected in the same way by the various radiations, their own susceptibility must be taken into account. The resulting quantity is the effective dose, which is the sum of the weighted equivalent doses for each tissue or organ by a tissue weighting factor (WT). This dimensionless factor represents the relative contribution of the tissue or organ to the total detriment to the individual resulting from homogenous exposure of the whole body, in order to take into consideration the susceptibility of each of these tissues or organs. The effective dose is therefore a quantity weighted twice: by the radiation weighting factor WR and by the tissue weighting factor WT: Like the equivalent dose, it is expressed in sievert. 1

LET (Linear Energy Transfer) is the energy lost in a micrometer by the primary ionising

particle in its trajectory. It is expressed in keV/um. For a given material it is variable depen-

ding on the type of radiation.

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This quantity is representative of the detriment. By construction, the dose-effect relationship can be applied to evaluate the detriment on the basis of the effective dose. The weighting factors WT and WR are calculated in such a way that a given effective dose always leads to the same value of the detriment, whatever are the organ or the combination of organs exposed. But, if the value of the detriment is always the same, the respective proportions of the types of damage (fatal cancers, non-fatal cancers, hereditary effects...) vary. In order to evaluate the detriment, ICRP (ICRP, 1991) has used average values for each of the considered parameters (age, sex, type of population). For global and homogenous exposure of all the organs at 1 Sv, the total detriment (probability of occurrence of severe effects) for a worker is estimated at 5.6% (fatal cancer: 4%, nonfatal cancer: 0.8% and severe hereditary effects: 0.8%) and for an individual representative of the population at 7.3% (fatal cancer: 5%, non-fatal cancer: 1% and severe hereditary effects: 1.3%). These values of the detriment imply that the doses received by the various tissues and organs are sufficiently low: absorbed dose below 0.2 Gy or dose rates less than 0.1 Gy/h for higher doses (ICRP, 1991, paragraph 74). The use of such a dose-effect relationship is thus subjected to two important restrictions: • the probability of death by cancer (4% for workers; 5% for individual members of the public) only applies to the effective dose in the case of homogenous irradiation of the whole body, • the dose-effect relationship is only valid for an average individual, representative of the reference population for which the dose-effect relationships have been constructed. The collective dose (equivalent or effective), obtained by the sum of the individual doses (equivalent or effective), is expressed in man sievert (man Sv).

2.2.5. Critical group - reference group The objective of the dose limit for members of the public is to guarantee, as much at the design stage of an installation as for during its operation, that the radiation sources are not responsible for the exposure of individuals of the public beyond the specified values. (ICRP, 1977, paragraph 84). As it is not possible to verify the compliance with these values for each individual of the public, it is necessary to identify the group(s) of the population whose characteristics are the cause of a much higher exposure than that for the population as a whole. The definition of the critical group(s) has developed over the course of time. It is simplified in ICRP Publication 60 (ICRP, 1991, paragraph 186), which indicates: it is often convenient to class together individuals who form a homogenous group with respect to their exposures to a single source. When such a group is typical of those most highly exposed by that source, it is known as a critical group. Paragraph 27 b of Publication 77 (ICRP, 1997) expands the definition: a critical group may refer to one or several sources and to all exposure pathways1. The IAEA definition (IAEA, 1999) is more 1

At present, it is presumed that, in many situations, the critical group may be exposed to more than one source.

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Collective dose: indications and contraindications

detailed but more restrictive than that of ICRP: a critical group relates to a source and to an exposure pathway1. IAEA completes its definition by detailing that its reference to a given exposure pathway implies that several critical groups may relate to a given source. The European Directive 96/29 (C.E., 1996) adopts almost the same definition, but changes the denomination of this group to become reference group of the population. From a radiation protection standpoint, it seems logical to use the term reference group and to consider it according to exposure originating from one single source and not according to its total exposure to numerous sources, even to all sources (including natural and medical sources). To do this, it is necessary to suitably define the source. In these conditions, the definition used by IPSN is: A reference group is a group of persons of the population for which the exposures from a given source are relatively homogenous and which is representative of individuals receiving the highest doses from this source.

2.3. Relationships between quantities and concepts The fundamental quantities (see paragraph 2.2.4) - absorbed dose, equivalent dose to an organ, effective dose, collective dose - are linked according to precise relationships; likewise, the relationships between these quantities and the biological effects resulting from the exposure are specific. To express the collective dose it is useful to refer to the quantities or parameters that are particularly targeted, for example, an organ, a period of time, a category of individuals, etc. As a result, it is necessary to suitably define the relationships that exist between the quantities, the concepts and the effects. Just as several quantities exist to designate a dose received by an individual, there are several concepts of collective dose. Furthermore, each type of individual dose has a different link with the risk. It is therefore necessary to consider three types of relationship: • the relationship of the collective dose, sum of effective doses, with the detriment, • the relationship of the collective dose, sum of equivalent doses to an organ, with the prediction of effects specific to this organ, • the relationship of the collective dose, sum of particular dose indicators, with the prediction of effects over periods of time and on given populations (for example, doses effectively received by an organ over a given period2).

1

A group of members of the public which is reasonably homogeneous with respect to exposure for a given radiation source and given exposure pathway and its typical of individuals receiving the highest effective dose or equivalent dose (as applicable) by the given exposure pathway from the given source. 2 Sometimes a truncated dose is used, but this concept is ambiguous as it is used both for individual doses during a restricted period (a few years instead of the 50 and 70 years usually recommended by the ICRP for the committed dose) and for the collective dose calculated over particular periods of time (for example over 1000 years instead of millions of years).

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2.3.1. The relationship collective dose/effective dose/detriment Here the collective dose concept is linked with that of the effective dose, which is constructed on that of the detriment. For a collective dose defined as the sum of effective doses, it is possible to determine the detriment based on the global doseeffect relationship (5.6% per Sv for workers, 7.3% per Sv for individuals from the general population). These values are only valid for low LET radiation and for equivalent doses resulting from absorbed doses less than 0.2 Gy and from higher doses when dose rates are lower than 0.1 Gy per hour. In addition, the two restrictions associated with the use of the effective dose, homogenous exposure of the body and average individual, have practical implications (paragraph 2.2.4). Use of the effective collective dose does not permit to know the exact composition of the detriment in terms of death, diseases and hereditary effects. The probability of occurrence of fatal cancers (4% for workers, 5% for members of the public for an exposure of 1 Sv) is not applicable in the case of heterogeneous exposure of the organs. In such a situation, it is then necessary to consider the variety of damages: for example, fatal lung cancer for irradiation limited to this organ, curable cancer after irradiation of the thyroid or even effects on the progeny after irradiation of the gonads. Nor does use of the effective dose allow some average parameters used by ICRP to be considered to construct the effective dose and the dose-effect relationships. In fact, to determine the weighting factors used in the calculation of the effective dose, ICRP Publication 60 retains (ICRP, 1991, Appendix B): • an average over various populations (5 population types), • an average on sex and age at time of exposure, • a projection on age at occurrence of cancer, • a weighted average for the various tissues and organs, • a weighted average for the detriment components, • a weighted average for the various radiations. In addition, the effective dose and the collective dose may be committed doses (doses integrated over time following the radionuclide intake). The time distribution of the doses effectively received by organs no longer needs to be taken into consideration.

2.3.2. The relationship collective dose/equivalent dose/effect on an organ Here the collective dose is the sum of the equivalent doses to a given organ within a population group. This collective dose allows the single damages linked to the exposure of this organ to be estimated, based on the specific dose-effect relationships1. On the other hand, the use of the equivalent dose does not allow the social and demographic characteristics of the exposed group as well as the age at which the damage occurs to be taken into account. 1

For this three way relationship "collective dose - equivalent dose on the organ - effect on the organ", dose-effect relationships are available in the Annexes of ICRP Publication 60.

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Collective dose: indications and contraindications

2.3.3. The relationship collective dose/ad hoc dose/ad hoc effect The collective dose may be calculated in a much more accurate way by the sum of the equivalent doses to the organs, restricted to given age ranges, accurate calendar periods, even particular volumic concentrations. Specific populations may also be distinguished. The collective dose may then be a sum of various exposure indicators: for example, equivalent doses to the thyroid, the breast, the bone marrow, within a given age range. This is how the number of thyroid cancers attributable to the Chernobyl accident was evaluated (Cardis et al, 1996; UNSCEAR, 2000a), the number of breast cancers related to mammographies (UNSCEAR, 2000b) and the number of leukaemias within the framework of the works carried out by the Groupe Radioecologie Nord-Cotentin (IPSN, 1999; Laurier, 2000; Rommens, 2000), etc. Predictions of varied risks may be made, for example by taking account of life expectancy. The indicators used are specific for each case, which prevents any generalisation of the results and involves defining the conditions of use for each application.

3. Three criteria for use of the collective dose In order to examine the relevance of the collective dose as an indicator with a view to clarifying a decision, it is necessary to examine the following three aspects: • the relevance of this indicator in the risk assessment; • the feasibility, the nature and reliability of calculation; • the relevance of the link between collective dose and risk. Confusion between these three aspects has been the cause of numerous misunderstandings in discussions on the collective dose. Consequently these three aspects must be considered separately.

3.1. Relevance of the "collective dose" indicator The use of the collective dose is particularly relevant in the following situations: • to compare the various protection options in order to prepare a concrete decision. The question is generally quite well formulated (how much a protection option reduces the doses likely to be received?), the answer is satisfactory and the collective dose indicator naturally finds its place amongst the other selection parameters (individual doses, time spent, effluents produced...); • to estimate the number of expected effects, either to appreciate the feasibility of an epidemiological study, even to avoid carrying out such a study (in the situation where the calculation of the collective dose appears to be reliable (paragraph 3.2), or in order to compare them with the results obtained from field observations (epidemiological study or simple observations); • to evaluate the global impact (collective detriment) of an activity or a source (nuclear fuel cycle, radon, fallout from nuclear weapons atmospheric tests, waste, etc.). When the initial question is too vague, the collective dose often

Collective dose: indications and contraindications

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provides an unadapted answer. Underlying questions must then be asked (for example, replace "what was the impact of the accident of Chernobyl?" with "what is its impact in the areas where the exposures were well reconstructed?"). The evaluation of the global impact of an activity or a source may also be used with an objective of comparison (for example, to compare the effects of radon to those due to other sources of radiation). The use of the collective dose indicator presents the same limitations as any other collective indicator. As a matter of fact, it should be highlighted that collective indicators: • mask the transfer of risk: when, for example, a decision increases the exposure of a group of workers in order to lower that of a group of members of the public, the total collective dose does not take into account the transfer of the doses from one group to another. In this situation, the collective dose must be calculated for the various groups concerned (segmentation of the collective dose). For example, in 1998, surface contaminations clearly exceeding the regulatory limits had been noted on the containers and wagons transporting irradiated fuel between the Centres Nationaux de Production d'Electricite (CNPE: Nuclear Power Plants) of EDF and the COGEMA reprocessing plant at La Hague. Within the framework of the technical instruction of this affair, the collective dose had been calculated by distinguishing between that of the nuclear plant workers, that of the SNCF employees (railway workers) and that of the public. The level of confidence to be granted to these doses is different according to the groups used. The result is reliable for the nuclear workers (1.91 man mSv per transported container, which is approximately 0.4 man Sv per year). It is uncertain for the SNCF workforce whose collective dose is difficult to evaluate as the individual doses are inaccurate (between 0.01 and 0.06 mSv per hour of exposure) and the number of exposed employees is difficult to determine. The reliability of the calculation has not been demonstrated for the public, as the collective dose calculated at 20 man mSv per year is very likely to be over-estimated. This calculation has enabled the dosimetric cost to be evaluated with regard to the regulations but the lack of accurate data concerning the public has not enabled the risk transfer to be quantified; • mask the inequalities in individual risk distribution. In relation to risk management, it is usually necessary to determine the individual impacts more than the collective impact. The dose cannot escape this rule. When the collective dose masks disparities, it is necessary to make them appear and above all not to restrict oneself to an average individual dose which is nothing else but a disguised collective dose. For example, with regard to radiodiagnosis in France, an average dose of 1.6 mSv per year gives no more information that an annual collective dose of 100 000 man Sv, which clearly does not reveal possible higher individual doses; • mask the disparities in various conditions of exposure: the calculation of collective doses applied to very varying situations hides the specific nature of each situation (lifestyle, age range, number of deaths...). These specific points may be related to subjective valuations; for example, neither the public nor the decisionmakers may consider as equivalent immediate deaths and deaths in some ten years time, the deaths of children and the deaths of aged persons...

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Collective dose: indications and contraindications

3.2. Reliability of the "collective dose" indicator In which cases is the estimate of a collective dose reliable enough to convey a decision? The following questions must be examined: • Is it possible to make the calculation? • What does it rest upon? On direct exhaustive observations (dosimetric evaluation for workers) or on random sampling (radiodiagnosis, domestic radon), on complete models (considering releases, radionuclide transfers in the environment, behaviour of the population under consideration, dose calculation around a site) or on an approach mixing measurements (environment) and models (lifestyles...)? • What are the big uncertainty factors relating to the method used? (uncertainties on the long term, on the models and hypotheses). With regard to the validity of the hypotheses, special attention must be given to duration and distance. The answers to the above questions depend on each situation and should be guarded against any improper generalisations. Reliability of the calculation cannot be determined absolutely, but in relation to the question posed. It is the components of the collective dose rather than its value that are the issue. For example, it is the case of making a distinction between doses in the short term and those likely to be received in the future, of the distinction between doses received with regard to the transport of irradiated fuel, by the public, by agents responsible for the transport, by agents responsible for cleaning the packages and by those responsible for receiving them.

3.3. Adequacy of the "collective dose" indicator to the estimated risk The link between the expression of the collective dose and the risk can only exist if the calculation of the collective dose, sum of individual effective doses, has been made for sufficiently large populations of a quite general demographic structure, using doses distributed homogeneously to the various organs. Contrary to this, distortions may appear. For example, for the same collective dose, the risk of cancer is much smaller for a group of old persons than for a group of children. In addition, to compare the health effects in two population groups for which the exposed organs are not the same, whenever possible a concept of collective dose adapted to the problem must be used (a collective dose calculated based on equivalent doses, even based on more partial indicators).

4. Admitted use and contested use of the collective dose Today, the collective dose is currently used in the frame of the radiological protection of workers exposed to ionising radiation. If this use of the collective dose is not controversial, this is not the case for the radiological protection of the public. In this

Collective dose: indications and contraindications

19

latter case, the collective dose generally appears as an inadequate indicator because it enables the calculation of a probability of cancers in excess by adding together dissimilar doses, often without any precaution. For example, in the case of radioactive waste disposal or long-lived radioactive releases into the environment, the exposure of the most exposed population around the installation concerned may be mixed in with the exposure of the world population, which is much lower, without taking into consideration the distribution in time of the various doses and the associated uncertainties. Consequently, the collective dose should not be used without precaution. It is therefore of interest to review the various uses that have been made, so as to identify the conditions of correct use of such an indicator.

4.1. Some generally admitted uses of the collective dose 4.1.1. Application of the collective dose to an homogenous group The collective dose is often used as an indicator of the exposure of a group of persons in the situation where these persons have received doses showing only slight differences, within a relatively short period of time and in the same area. In this way, the collective dose concerning the workers is used by industrial operators as an indication of the dosimetric cost of operating their installations. An international database of information on the occupational exposure (ISOE, created on the initiative of OECD/NEA, with the co-operation of IAEA) allows the changes in mean collective doses to be compared for a given reactor type in various countries (see Figure below) (Croq, 2001).

Mean collective dose of workers per pressurised water reactor

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Collective dose: indications and contraindications

Other indicators are also used, such as collective dose at plant shutdown, collective dose for replacing a steam generator, collective dose per job function (for example insulation removers), or even the collective dose per unit of power produced by the various types of reactors throughout the world. The occupational exposure statistics in nuclear installations generally distinguish between the operator's own personnel and that of external contractors. In the situation where special attention is paid to the distribution and evolution in time of the exposure of these two categories of personnel, the corresponding collective doses are separately accounted for. Some operators refine the procedure and subdivide all of the workers into sub-groups responsible for particular tasks such as operation, maintenance or another work category. This method allows protection to be optimised by better management of the exposure, which would not allow the collective dose used for all of the personnel. Operators within the nuclear industry also use the collective dose to compare the impact of various protection options or various techniques, in view of choosing the best method according to pre-defined criteria (technical feasibility, deadlines, doses, effluent and waste produced, financial cost, safety...). The method of evaluation (clear segmentation) and the value of the collective dose changes progressively during the progress of the optimisation process (choice of a reference scenario, choice of various protection options, choice of a final scenario, construction of the operating mode...). Another example illustrates the usefulness of the calculation of the collective dose applied to homogenous groups of workers. Following the discovery of surface contamination exceeding the regulatory values of containers and wagons transporting irradiated fuel to the rail terminal at Valognes (paragraph 3.1), the authorities decided to stop the transports and not to authorise them to start up again until a check could be made relating to the compliance of commitments made by the various operators concerned. In order to estimate the impact of this decision on the workers affected when checking the containers, a calculation of the collective dose was made for each group of workers concerned by distinguishing between the homogenous categories: EDF workers, SGS/Qualitest1 workers, STMI/Valognes1 workers. The calculations of the collective doses were made for two scenarios: the first corresponding to the initial situation (level of surface contamination higher than the regulatory limits) and the second on the decontamination of the transport upon departure from CNPE, with, at Valognes, levels of surface contamination lower than the regulatory limits. Comparison of collective doses from the various groups showed: (i) an increase in the collective dose for two categories of workers, following the implementation of new measures: 1.15 man mSv compared to 0.09 man mSv for EDF workers, 0.68 man mSv compared with 0 man mSv for the SGS/Qualitest workers; (ii) maintenance of the collective dose at a comparable level for the STMI workers (0.08 man mSv compared with 0.1 man mSv); (iii) an increase by a factor of ten in the total collective dose: 1.91 man mSv compared to 0.19 man mSv. The decrease in the collective dose of the STMI agents at the Valognes station is very low compared to the increase for that of the EDF and

3

SGS/Qualitest and STMI are service provider companies participating in the checking and cleaning operations of the containers and transport of irradiated fuel.

Collective dose: indications and contraindications

21

SGS/Qualitest personnel. On the basis of 200 transport operations per year, the exposure due to surface contamination spots would represent less than 1% of the collective dose received by the personnel due to exposure to these irradiating containers.

4.1.2. Distribution of the collective dose in relation to individual doses When the individual doses are not homogenous within a group of workers (notably because the exposure differs significantly according to the operations carried out), it is useful to detail the collective dose in ranges of individual doses so as to concentrate the protection effort as a priority on a particular group of workers, for example, the group with the highest collective dose or the group with the highest individual doses. This is now the current method used by the operators who, in the majority, present the collective dose of their personnel in their activity reports by detailing the number of agents per range of individual doses (for example, less than 2 mSv/year, between 2 and 5 mSv/year, between 5 and 10 mSv/year, between 10 and 15 mSv/year).

4.1.3. Use of an organ dose to calculate a specific risk The calculation of the number of cancers resulting from the exposure of a particular organ requires the use of the relationships with the corresponding organ equivalent dose (for example, bone marrow for leukaemias, thyroid for thyroid cancers...). In this case, the collective dose calculated based on the effective dose is replaced by a more adequate indicator: the organ collective dose (collective dose on the bone marrow or thyroid). Thus, in their report on the evaluation of the health consequences in France from the Chernobyl accident, IPSN and the Institut national de Veille Sanitaire (InVS) estimated the excess risk of thyroid cancer in excess amongst 2 270 000 children aged under 15 years in the most contaminated area by iodine 131 deposits in 1986 at the time of the accident, by calculating the collective dose to the thyroid received by this group of children. The calculation of this dose was made by distinguishing between four age categories, between 0 and 14 years, in order to take into account, especially, the differences in food consumption (IPSN, 2000). In the same way, the Groupe Radioecologie Nord-Cotentin has estimated the leukaemia risk associated with the exposure to ionising radiation for young people aged from 0 to 24 years and having lived in the region of Beaumont-Hague between 1978 and 1996 (IPSN, 1999; Laurier, 2000; Rommens, 2000). It was a question of comparing the number of observed cases of leukaemia to the number of expected cases, basing this on the doses received and the knowledge on the effects of radiation. The evaluation was based on an "ad hoc" dose calculation (equivalent dose to bone marrow truncated at the age of 25) and "ad hoc" effects (model for the

22

Collective dose: indications and contraindications

risk of leukaemia taking account of the age attained). A cohort made up of almost 6 700 individuals was recreated and the individual equivalent dose to bone marrow from all sources of exposure to ionising radiation was calculated, based on the hypothesis of average behaviour and lifestyle for the various age groups. This estimation was made in the most realistic way possible, being based on local data when existing. The collective equivalent dose and the associated risk of leukaemia were then estimated by calculation. All of the individuals from the same generation were attributed the same dose estimate. Based on these doses estimates, the risk of radiation-induced leukaemia was estimated. In this case, the calculation of the collective dose for the whole group allowed the number of radiation-induced leukaemia cases to be estimated that were attributable to the routine releases from local nuclear installations over the period 1978-1996, to 0.001. The corresponding collective equivalent dose was 0.3 man Sv.

4.2. Some contested uses of the collective dose 4.2.1. The collective dose and the radiological impact of emissions from nuclear installations Long half-life radionuclides, released by nuclear installations (for example iodine 129 and carbon 14), may be dispersed over the entire surface of the globe and thus expose current and future generations on a world population scale. Global exposure resulting from these discharges has sometimes given rise to calculations of the collective dose for the world population and for periods as long as 100 000 years, a million years, even infinity. These calculations make no true sense. In addition, the collective effective dose is sometimes used to calculate the excess of cancers attributable to nuclear installations in normal operation or following an accident. Thus, the National Radiological Protection Board (United Kingdom) has estimated the collective dose received within the countries of the European Community at 80 000 man sievert following the Chernobyl accident and has concluded an excess of death by cancer in the order of 1 000 in the fifty years following the accident (NRPB, 1987). Likewise, the World Health Organisation used the collective dose to evaluate the number of cancers attributable to this accident in Europe: for a population of 550 million people, the study shows an excess of 7 000 cancers, that are indiscernible amongst the 110 millions of cases of spontaneous cancers (WHO, 1989). It is however necessary to question the significance of the collective dose resulting from such a calculation as much in relation to the reliability of the calculation as to the use in a context of risk management. Indeed, the reliability of the collective dose thus calculated is dubious, given the uncertainties on the behaviour of radionuclides in the environment, on the changes in size of the population and its lifestyle habits as well as on the evaluation of the exposure levels. With regard to risk management, the decision maker needs information reliable both on short and medium terms. In the same way, the spatial distribution of the exposure to radiation is a data that may prove necessary in taking the decision.

Collective dose: indications and contraindications

23

Independently of the reliability of the calculation over very long periods, the collective dose is insufficient to help in preparing the decisions. It may be helpful to calculate partial collective doses on time and distance intervals where these collective doses make sense. As far as atmospheric and liquid discharges from nuclear fuel cycle installations are concerned, the reports from UNSCEAR present two expressions of the collective dose. The first deals with the exposure of the local and regional populations over short periods of time. The second deals with the whole of the world population and is calculated over 10 000 years. It assesses the exposure due to emissions of tritium, carbon 14, krypton 85 and iodine 129 originating from nuclear reactors and reprocessing plants (UNSCEAR, 2000c). The sum of the collective effective doses resulting from these two components relating to the dispersion of radionuclides on a global scale is full of uncertainties that question its use. Splitting the period 0 to 10 000 years into intermediate periods should have given additional useful information. Note that this calculation results in very low doses without any real significance: on a regional scale the collective dose is in the order of 200 man Sv for a population of 250 million people, i.e. an average individual dose of less than 1 uSv; however, this average dose relates to very varied individual situations. On the other hand, the report from the OECD Nuclear Energy Agency concerning "A comparison of the radiological impacts of spent fuel management options", carried out for the OSPAR convention, fixed upper bounds in time and space (OECD/NEA, 2000). The collective doses resulting from the emissions of carbon 14 and radon 222 were calculated over periods of 500 years and for populations of limited geographical areas. With regard to ore extraction, the report referred to two populations: up to 100 km and from 100 km to 2 000 km; for the other stages of the cycle, it referred to the European population. A calculation made over a much longer period would have shown excessive uncertainty and would not have provided any information useful for the comparison between the fuel cycles whether or not they included the reprocessing of the irradiated fuel.

4.2.2. The collective dose and the exposure related to the disposal of high level and/or long-lived waste The calculation of long term exposure of populations resulting from the disposal of high level and/or long-lived waste using the collective dose also meets some difficulties. As a matter of fact, a great deal of uncertainties exists over the dispersion of radionuclides into the geosphere as well as over the characteristics of the populations likely to be exposed in the far off future (number of people, lifestyles). A study based on an average individual dose received by the individuals of a reference group with various time scales is probably preferable to a study based on collective dose. A recent ICRP publication (ICRP, 1997) recommends referring to the individual dose (individuals of the reference group) rather than to the collective dose for the management of long-lived waste (paragraph 69). Indeed this step allows the detriment for future generations to be worked out in relation to what is currently accepted for the present generation (for example, the risk of occurrence of cancer per year and per individual). The use of calculated collective doses is neither

24

Collective dose: indications and contraindications

possible on numerous generations nor on a given generation, nor on future hypothetical groups for determined periods of time (a year, a lifespan...). Besides, it is this approach that has been used since 1991 with regard to the safety analysis of the disposal of these wastes in deep geological formations (J.O., 1999).

4.2.3. The collective dose and the estimate of the risk to a given organ In order to estimate the equivalent dose to a tissue T or to an individual organ exposed, a classic error consists of weighting the effective collective dose by the inverse of the weighting factor WT. For example, to evaluate the risk of thyroid cancer, the equivalent dose to the thyroid is calculated on the basis of the effective dose, when only the inverse calculation is permitted (the weighting factor WT can, under no circumstances, serve as a basis for the calculation of a equivalent dose to an organ). The quantity that allows the collective risk to a given organ to be expressed is the collective equivalent dose to the organ, obtained from the equivalent doses received by the organs of the individuals from the considered population.

4.2.4. Introduction of a de minimis dose in the calculation of the collective dose The concept of the de minimis dose returns to that of trivial risk. Experience shows that it is deceptive to seek a social consensus on a generic level of trivial risk. Opinion on what is acceptable or trivial is not independent neither from the positive or negative evaluation that individuals are carrying out activities originating the risk, nor of their capacity themselves to control the risk. This opinion also depends on the level of social and economic development of the groups concerned. Faced with the reservations quoted above by the addition of very small doses received by a very large number of people (paragraph 4.2.1), some authors recommend that the calculation of the collective dose be systematically truncated by not taking into account the contribution resulting from exposure below a de minimis level fixed a priori, such as 10 (uSv/year for example. Such an approach can however be criticised. On one hand, on an individual level, the fact that doses are low is not enough to justify that they can be neglected; on the other hand, this truncating of the collective dose masks the number of people effectively affected by these exposures.

5. The IPSN position The protection of a population exposed to ionising radiation implies that what must be considered and controlled is not only the risk run by each individual of this population but also the risk run by the whole population involved. IPSN considers

Collective dose: indications and contraindications

25

that the collective dose, sum of the individual doses of the members of a given population, constitutes a useful, whilst not exhaustive, indicator of the collective dimension of the risk, the use of which it recommends within the limits indicated below. The collective dose, effective and/or equivalent, may be used: • to assess the dosimetric impact of an activity or a source over a group of people; • to assess the number of stochastic effects that could result from the exposure of a group of people. The methods for calculation of the collective dose and its use must be adapted to the nature of the situation under consideration. There exist a wide variety of exposure conditions for a given group of people under consideration (levels of dose, durations of exposure, mean individual doses, numbers of individuals exposed, types of population, geographic distributions...) and situations for the people exposed (occupational, public or medical exposure; normal or accidental situation...). To retain the relevance and significance of the collective dose, it is necessary not to amalgamate dissimilar situations and conditions. In response to the main questions raised by the use of the collective dose related to the objectives of its use, its role in the assessment of the health effects, as well as to the confidence to be granted to its calculation and its interpretation, the guidelines set retained by IPSN are summarised below.

5.1. Methods of calculating the collective dose depending on its use ISPN considers that before calculating a collective dose it is necessary to clarify the purpose for which the calculation is done. As a matter of fact, the difficulties encountered, particularly in the interpretation of the results, vary in nature according to whether it is a question of assessing the level of exposure of a given population or the health effects in this population. When it is a question of assessing the level of exposure of a given population, the variability of the collective dose components (dosimetric, demographic) governs the significance of the result. Difficulties in interpreting the collective dose essentially exist for the following cases: very low levels of individual exposures, exposures prolonged over very long periods and over several generations, individual exposures of a varying nature (for example external, internal...) or comparison of collective doses of populations having different characteristics of exposure. When it is a question of assessing the health effects in a given population, uncertainties on the various risk factors available (for example the probability of the occurrence of cancer per unit dose) and the loss of information inherent to the calculation of the collective dose (for example, nature and severity of the effect) may compromise the accuracy of the result.

26

Collective dose: indications and contraindications

• Assessment of the levels of exposure The collective dose must be calculated for homogenous segments, such that each segment corresponds to parameters that are adapted to the specific context being considered. It is necessary to avoid the use of a single collective dose, resulting from the summation of several collective doses corresponding to different segments; such a dose summation prevents the variation of individual doses and their distribution from being assessed. Segmentation of the collective dose must meet three conditions: • to group together those individual doses that present no differences greater than a maximum of one or two orders of magnitude; • not to group together periods of time that differ by more than one or two orders of magnitude, for example to evaluate the development over time (10,100,1 000 years...) of collective doses related to exposures of long duration. For very long periods of time, limit this to the duration of a few centuries and, in any case, always less than a thousand years, due to the wide range of uncertainties that accompany each forecast of dose beyond this time scale, for example to assess the global dosimetric impact of long-lived waste disposal; • a priori do not neglect very low doses before ensuring that they have no substantial contribution towards the collective dose. The process of segmentation and evaluation of the segments constitutes a more realistic and more credible solution than using a generic de minimis dose.

• Assessment of the health effects The dosimetric quantities used must be adapted to the risk corresponding to the considered exposure. Thus, to assess the risk of a specific cancer, it is necessary to use the equivalent dose to the relevant organ (thyroid, bone marrow). In the absence of knowing the location of the cancer, the effective dose may be used to estimate the risk of death by cancer, all cancers being considered together. If the risk to be assessed is the occurrence of a cancer before a given age, it is necessary to use an estimation of the dose truncated at the age attained (for example, dose to the bone marrow before the age of 15 for a risk estimate of childhood leukaemia). As in the case of dosimetric assessment, the collective dose must be calculated for homogenous segments. This segmentation must meet three main conditions that are to be added to or substitute those mentioned above for a dosimetric assessment: • do not group together those individual doses that represent different risks (for example: do not amalgamate effective doses with equivalent doses; do not amalgamate fatal cancers with high cure rate cancers); • do not consider periods greater than some tens of years and in any case, periods greater than a century, because of unknowable changes in demographic and geographic conditions, developments in knowledge on the dose-effect relationship and in the progress of medicine which might reduce the severity of some harmful effects and, consequently, might modify the risk factors; • consider only those groups of population with identical biological conditions (age, condition of health...), notably due to differences in genetic susceptibility.

Collective dose: indications and contraindications

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IPSN underlines the fact that all segments of the collective dose do not have the same importance. For example, for risk management, it is necessary to separate the collective dose relating to occupational exposure within an installation from that relating to the exposure of the public resulting from the operation of this installation, for several reasons: levels of individual exposures of workers differs from those for members of the public, age distributions (and often sex) are not the same, dose limits and regulations are different. With regard to the population, it is necessary to distinguish the sub-groups according to the distance from the installation considered, the lifespan of the installation, the length of exposure (short, medium or long-term), the distribution in age of the concerned population, etc.

5.2. Confidence attributed to the calculation of the collective dose The individual doses used for the calculation of a collective dose originate either from assessments or measurements of individual exposures, or from estimates of the radionuclides transfer into the environment using a model or a code, or even from measurements in the environment. IPSN underlines that the degree of reliability of the calculation of the collective dose depends on the method used to assess individual doses. Moreover, the components used to calculate the collective dose (populations considered, types of dose, temporal, geographical, distributions), which have lead to an appropriate segmentation, confer degrees of varying uncertainty on the result (for example, disparities in the dose distribution in time and space, different organs exposed). When the calculation of some segments of the collective dose shows major uncertainties, IPSN recommends recording these uncertainties. In a general manner, it is important, in order to evaluate a collective dose result, to correctly examine the bases of its calculation.

5.3. Interpretation of the results of the calculation of the collective dose IPSN considers that, for the interpretation of the results of a collective dose calculation, it is essential to know the main components of this dose (number of people exposed, mean level of individual exposures...). It is also useful to know the individual doses of the most exposed people, in addition to the collective dose of a given population. It is the presentation of the results for the various segments that promotes the discussion, for example between operators and national authorities, and enables decisions to be elaborated. Within the framework defined previously, IPSN considers that the collective dose, calculated on the basis of individual effective doses from a large population notably mixing ages and sexes, constitutes a correct indicator for the global risk of

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Collective dose: indications and contraindications

the occurrence of stochastic effects. Furthermore, in situations where homogenous groups of population are identified (children or adults in a given age range, pregnant women...), it may be useful to detail the risk incurred by these groups of specific populations. The collective doses calculated in such cases cannot be compared without care with collective doses of other groups. It is also necessary to distinguish between collective doses leading to an estimate of a risk of death by cancer with those that lead to the estimate of a risk of non-fatal cancers or hereditary effects. IPSN reminds that the collective dose constitutes the indicator of the risk of stochastic effects following exposure to low doses of ionising radiation (lower than 0.2 Gy) or to low dose rates (lower than 0.1 Gy/h). Consequently, the collective dose cannot in any case constitute an indicator of the deterministic effects that occur following high doses delivered at high dose rates and above the dose thresholds for such effects; in this situation, doses must only be expressed in terms of individual absorbed doses.

References E. Cardis, L. Anspaugh, V.K. Ivanov, LA. Likhtarev, K. Mabuchi, A.E. Okeanov, A.E. Prisyazhniuk. Estimated long term health effects of the Chernobyl accident. In: One decade after Chernobyl, summing up the consequences of the accident. Proceedings of an International Conference, EC-IAEA-Who, Vienna, 241-279, IAEA, Vienne, 1996. M. Croq, S. Mundigl, L. d'Ascenzo, T. Lazo, C. Lefaure. Un outil peu connu mais efficient au service du principe ALARA: ISOE (Information System on Occupational Exposure). Radioprotection 36, n°2, 2001. H.J. Dunster. Collective dose: kill or cure? J. Radiol. Prot. 20, No 1 March 2000, 3-4, 2000. European Communities. Directive 96/29/Euratom du Conseil, of 13 May 1996, fixing the basic standards relating to the protection of the health of the population and workers against dangers resulting from ionising rays. Official Journal of the European Communities, L159, 29 juin 1996. I. Fairlie and D. Sumner. In defence of the collective dose. J. Radiol. Prot. 20, No 1 March 2000, 9-19, 2000. Groupe Radioecologique Nord-Cotentin. Synthese: Estimation des niveaux d'exposition aux rayonnements ionisants et des risques de leucemies associees de populations du Nord-Cotention. Fontenay-aux-Roses, France. Institut de Protection et de Surete Nucleaire, 1999. International Atomic Energy Agency. Assigning a value to transboundary radiation exposure. IAEA Safety Series n° 67, Vienna, 1985. International Atomic Energy Agency. International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources. FAO, IAEA, ILO, OECD/NEA, PAHO, WHO. Safety Series No 115, IAEA, Vienna, 1996.

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International Atomic Energy Agency. IAEA Safety Glossary. Terminology used in Radiation Protection and in Nuclear, Radiation, Waste and Transport Safety. Working Material, version 0.9, 22 November 1999, IAEA, Vienna, 1999. International Commission on Radiological Protection. Recommendations of the ICRP, ICRP Publication 1, Pergamon Press, Oxford, 1959. International Commission on Radiological Protection. Implications of Commission recommendations that doses be kept as low as readily achievable. ICRP Publication 22, Pergamon Press, Oxford, 1973. International Commission on Radiological Protection. Recommendations of the International Commission on Radiological Protection. ICRP Publication 26, Pergamon Press, Oxford, vol. 1, No 3,1977. International Commission on Radiological Protection. Cost-benefit analysis in the optimization of radiation protection. ICRP Publication 37, Annals of the ICRP, Pergamon Press, Oxford, vol. 10, No 2/3,1983. International Commission on Radiological Protection. Optimization and decisionmaking in radiological protection. ICRP Publication 55, Annals of the ICRP, Pergamon Press, Oxford, vol. 20, No 1,1989. International Commission on Radiological Protection. 2990 Recommandations of the ICRP. ICRP Publication 60. Pergamon Press, Oxford, vol. 21, No 1-3,1991. International Commission on Radiological Protection. Radiological protection policy for the disposal of radioactive waste. ICRP Publication 77, Annals of the ICRP, Pergamon Press, Oxford, vol. 27, Supplement, 1997. Institut de Protection et de Surete Nucleaire - Institut National de Veille Sanitaire. P. Verger, L. Cherie-Challine et al. Evaluation des consequences de I'accident de Tchernobyl en France: dispositif de surveillance epidemiologique, etat des connaissances, evaluation des risques et perspectives. Rapport IPSN-InVS, IPSN/00-15a, 2000. Journal Officiel, n°1606,1999. Regie Fondamentale de Surete (Juin 1991), RFS III.2.f.; Tome III: Production, controle et traitement des effluents et dechets; chapitre 2: dechets solides. D. Laurier, C. Rommens, C. Drombry-Ringeard, A. Merle-Szeremeta, J.P. Degrange. Evaluation du risque de leucemie radioinduite a proximite d'installations nucleaires: I'etude radioecologique Nord-Cotentin. Rev. Epidemiol. Sante Publ. 48, supl. 2: 24-36, 2000. B. Lindell, T. Mallfors. Comprehending radiation risks, in: Radiation and Society: comprehending radiation risk. A report to the IAEA, Riskkollegiet, Stockholm, IAEA, Vienna, 7-18,1994. B. Lindell. History of radiation protection. Radiation Protection Dosimetry 68, No 1/2, 83-95,1996. B. Lindell. Un siecle de protection radiologique. In: Continous conference on health and nuclear safety. Third meeting: To inform the public of the European standards of radioprotection. European Commission. Luxembourg, 26-27 November 1996.

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B. Lindell. On collective dose. J. Radiol. Prot. 20, No 1-2, 2000. National Radiological Protection Board. A preliminary assessment of the radiological impact of the Chernobyl reactor accident on the population of the European Community. NRPB Report, 1987 J.C. Nenot. Chernobyl: health management in chaos. In: International Conference on Radiation and Society: comprehending radiation risk. Paris, 24-28 October 1994, IAEA, Vienna. Organisation for Economic Co-operation and Development/Nuclear Energy Agency. A comparison of the radiological impacts of spent fuel management, 2000. C. Rommens, D. Laurier, A. Sugier. Methodology and results of the Nord-Cotentin Radioecology Study. J. Radiat. Prot. 20, n°4: 361-380, 2000. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) Ionizing Radiation: Levels and Effects. Report to the General Assembly, with Annexes. United Nations, New York, 1972. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) Sources and Effects of Ionizing Radiation. Report to the General Assembly, with Annexes. United Nations, New York, 1977. United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) Report to the General Assembly, with Annexes. Vol. 2, Annex J, Exposures and effects of the Chernobyl accident, 451-566, United Nations, New York, 2000(a). United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) Report to the General Assembly, with Annexes. Vol. 1, Annex D, Medical exposures, 293-495, United Nations, New York, 2000(b). United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) Report to the General Assembly, with Annexes. Vol. 1, Annex C, Exposures to the public from man-made sources of radiation, 157-291, United Nations, New York, 2000(c). World Health Organization (WHO) Health hazards from radiocaesium following the Chernobyl accident. Report of a Working Group, Regional Office for Europe. J. Environ. Radioactivity 10, 257-295,1989.

Book series coordinated by Henri Metivier IPSN collection Agriculture, Environnement et Nucleaire : comment reagir en cas d'accident Auteurs : R. Coulon, J. Delmas, G. Griperay, Ph. Guetat, R. Loyau, C. Madelmont, R. Maximilien, J.-C. Rottereau

Traitement de la contamination interne accidentelle des travailleurs Auteurs : M.H. Bhattacharyya, B.D. Breistenstein, H. Metivier, B.A. Muggenburg, G.N. Stradling, V. Volf

Approche de la surete des sites nucleaires Auteur : J. Faure

Circonstances et consequences de la pollution radioactive dans Vancienne Union sovietique D. Robeau, Coordinateur.

Elements de surete nucleaire (ainsi que ses traduction anglaise et russe) Auteur : J. Libmann Le Tritium - de I'environnement a I'Homme Y. Belot, M. Roy, H. Metivier, Coordinateurs.

Radionuclides in the oceans - Inputs and inventories P. Guegueniat, P. Germain, H. Metivier, Coordinators.

Le Radon - de I'environnement a I'Homme H. Metivier, M.-C. Robe, Coordinateurs.

Les installations nucleaires et I'environnement - Methode devaluation de I'impact radioecologique et dosimetrique L. Foulquier, F. Bretheau, Coordinateurs.

Les retombees en France de I'accident de Tchernobyl - Consequences radioecologicjues et dosimetricjues Auteurs : Ph. Renaud, K. Beaugelin, H. Maubert, Ph. Ledenvic

Calliope - Un outil pedagogiaue en dosimetrie interne (cederom) Auteurs : B. Le Guen, Ph. Berard, P.N. Lirsac, M.L. Perrin, M.-M. Be, J.L. Malarbet, B. Gibert, M. Roy, H. Metivier

ICRP — Historique, politiques, methodes de la CIPR J.-C. Nenot, H. Metivier, Coordinateurs.

Le Cesium — de I'environnement a I'Homme D. Robeau, F. Daburon, H. Metivier, Coordinateurs.

Publication 84 de la CIPR, grossesse et irradiation medicale

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Collective dose: indications and contraindications

Radioactive Pollutants, Impact on the environment F. Brechignac, B.J. Howard, Editors. L'Uranium - de I'environnement a I'Homme H. Metivier, Coordinateur. Catastrophes et accidents nucleaires dans I'ex-Union sovieticjue D. Robeau, Coordinateur.

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