Assessment of Doses around Nuclear Installations: For What? C. Ringeard J. Brenot D. Laurier A. Morin

Transcription

1 Assessment of Doses around Nuclear Installations: For What? C. Ringeard J. Brenot D. Laurier A. Morin Institut de radioprotection et de sûreté nucléaire (IRSN) ABSTRACT: There is not one methodology to assess doses to people living in the vicinity of nuclear installations. The methodology used depends on the aim - regulatory, knowledge oriented, comparison with an observed radiological impact of the study. Doses to critical groups (i.e. reference groups) around nuclear sites are calculated to check the compliance with the limit of dose to the public. Doses around sites can be assessed in order to estimate the population exposure to artificial sources of ionising radiation; in this situation, areas of homogeneous exposure are searched for. Doses around a site may also be estimated to answer to a question, as for example: are the observed incidence cases of childhood leukaemia [Laurier 2002] associated with radiation exposure? The doses mentioned are effective doses or doses to a target organ (as the red bone marrow is for the leukaemia risk). Interactions between releases, environment and people living near nuclear sites appear in figure 1. DIETARY HABITS Quantity Origin PEOPLE LOCALITY Life - Work Bq/L Ingestion.National origin MILK Consumption Time spent External exposure Bq/m 3 air water Bq/kg Local origin Ingestion Inhalation Bq/m 2 soil Figure 1: interactions between releases, environment and people 19

2 The main difference between dose assessment methodologies lies in the modelling. Three contexts are developed and illustrated by results from studies performed by IRSN: dose assessment for regulatory purpose, calculus of doses for the knowledge of population exposure, and estimation of doses and risks for epidemiological response. 1 DOSE ASSESSMENT FOR REGULATORY PURPOSE Nuclear plants in normal operation produce radioactive effluents. The discharge of these effluents to the environment is subject to stringent conditions stipulated in an administrative licence. The licence defines liquid and gaseous radioactive discharges in terms of activity levels and chemical characteristics. Impacts of discharges on human health must be estimated for the licensing [Thomassin 2001]. Figure 2 presents the French licensing procedure. Operator Proposal of liquid and gaseous discharges DGSNR IRSN Technical reports Administrative conference Recommendations of the organizations concerned (CM, CDH, ) DGSNR Prefect DGSNR Operator Public Inquiry Decree authorizing the liquid and gaseous discharges CM : Town councils CDH : Departmental council for health DGSNR: Directorate General for Nuclear Safety and Radiation Protection IRSN : Institute for Radiation Protection and Nuclear Safety Figure 2: the licensing procedure in France 20

3 In France IRSN performs the technical analysis of the discharge authorization document presented by the operator. It calculates effective doses received by members of the public as the result of radioactive discharges. Any dose must be lower than the 1mSv limit for the public mentioned in the Euratom Directive 96/29. For compliance, it is enough to ensure that groups of people who are likely to be the most exposed receive doses below the limit. This means that a conservative approach is necessarily privileged. 1.1 The methodology The methodology [Rommens 1999] is presented for the La Hague site where operates the Cogema reprocessing plant. The activities discharged are those presented to obtain the authorization. It must be underlined that the authorized discharges are larger than the actual discharges. Air concentrations, deposition to ground and crops, and activity concentrations in terrestrial and sea foods are calculated using models. The exposure pathways taken into account are: external exposure by the plume, deposition on the ground, sand or sediments, and bathing; internal exposure by inhalation of the plume and re-suspended deposits, by ingestion of terrestrial food and sea food. When the goal is the control to a limit, a simplified dose assessment modelling is often sufficient. Hypotheses retained all lead to an overestimation of doses, as for example: radionuclide surface activities are the same for deposits onto soil and leafy vegetables; soil deposits do not migrate; or a class of radionuclides is represented by the radionuclide with the highest dose coefficient. Overestimation means dose upperbounds. The drawback with such a simplified modelling is that we do not know how large is the margin between the result obtained and the result which would be obtained from a more in depth modelling or in other words a less approximate description of radionuclide transfers to biosphere. The advantage is that we do not need any sensitivity analysis and even less an uncertainty analysis. Only reference groups, i.e. groups of individuals who receive the highest doses from discharges, are considered. For the La Hague site, reference groups are two: the Digulleville individuals who live between 2 to 3 kilometres from the site under the dominating winds and receive the highest doses from gazeous discharges to atmosphere; the Goury families of fishermen who live at 7 km from the marine outlet of the reprocessing plant in the direction of the main marine stream and who receive the highest doses from liquid discharges to sea. Data on food consumption come from a local survey performed in April-May Regarding food, consumptions often come from national food surveys that are less representative than local surveys, and sometimes are fixed by the regulator at extreme amounts which cover all the possible diets but not any individual diet. 1.2 Example of results The individual annual effective doses calculated by IRSN are presented in figure 3 [Ringeard 1999]. 21

4 Figure 3: individual annual effective doses (µsv) The highest annual dose is obtained for an adult living in Digulleville. This dose is lower than the limit of 1mSv, by a factor of KNOWLEDGE OF DOSES TO POPULATIONS LIVING IN THE VICINITY OF NUCLEAR SITES Among artificial sources of ionising radiation, radioactive discharges from nuclear installations expose a large number of people to a gradient of doses decreasing with the distance to nuclear sites. Knowledge of the spatial distributions of doses is useful to compare the radiological impacts at various sites and to evaluate the importance of that source of ionising radiation relatively to the other sources. That knowledge is useful too in a public health perspective. Indeed it brings some elements of response regarding the relation between health risk indicators and exposure to ionising radiation, in terms of plausibility and strength of the relationship. An example of such a research is the one carried out by IRSN and the national institute of medical research (INSERM) which proposed a study of the hypothesis of an association between environmental exposure to ionizing radiation and incidence of childhood leukaemia in France with particular emphasis on leukaemias observed in the vicinity of nuclear sites [Laurier 2003]. To do so doses need to be estimated around the 19 French nuclear power plants and the other large nuclear installations of the uranium cycle. This study, a systematic assessment of doses around nuclear installations out of the regulatory context, is the first of that type performed in France. 22

5 2.1 The methodology The approach is necessarily more realistic than the one followed for the regulatory context. The activities discharged are actual discharges measured by the operators. They are lower than the authorized discharges. They are considered not on annual basis but during the period of operation of each installation. Two major problems arise for which there are no obvious responses. The first one is the continuous decrease of actual discharges over the operation period for most of the installations. The second one is the changes in discharges reporting that occurred over the operation period, an example of which is the absence of measurements of 14 C and 129 I for a long time at many plants. The long duration of the operation periods in general makes the collection of the discharges data uneasy. These difficulties have been identified quite well by the European Commission experts (in charge of designing the European data base on actual discharges of nuclear installations) to the point of giving up the collection of such data retrospectively. Consequently it means that reasonable assumptions are needed to sum up the doses received by the exposed populations over years and that these assumptions concern not only the assessment of actual discharges in the past but also the evolution of life habits in time. Air concentrations, deposition to ground and crops, and activity concentrations in foodstuffs (terrestrial, from the river or from sea) are calculated using models. The exposure pathways taken into account are: external exposure by the plume, deposition on the ground, sand or sediments, and bathing; internal exposure by inhalation of the plume and re-suspended deposits, and by ingestion of terrestrial food and of river or sea food. With respect to liquid discharges, not all sites are equal; if doses due to gaseous discharges concern all populations surrounding a site, doses due to liquid discharges are received by a few communities; then the summation of the two types of doses is not obvious. Because the objective is to perform a dose assessment as realistic as possible, the modelling is more achieved in terms of phenomenology description and values for the model parameters are chosen when they constitute good compromises. Doses are calculated for a yearly exposure at locations situated on a polar or square grid. Then isodoses can be mapped and they define zones of homogeneous exposure which provide a reasonable information about the levels of doses. All people living near nuclear sites are considered. The consumption of food can be that of the average French individual, or a little more detailed if diets are regional; all data come from food surveys published by this French institute of statistics. 2.2 Example of results Figures 4 and 5 present two maps drawn for a nuclear power plant. The first map, the centre of which is the nuclear plant, gives informations about exposure zones by means of isodoses. The second map presents the level of doses for each village around the site. 23

6 Figure 4: zones of exposure defined by isodoses (effective dose µsv/year) Figure 5: doses by village around the site (effective dose µsv/year) 24

7 3 DOSE ASSESSMENT FOR COMPARISON WITH OBSERVED RISK A precise assessment of doses is needed when the results of a calculation procedure must be compared with observed data. The context in which data have been observed frames the assessment process and involves the search of best dose estimates. The Nord-Cotentin Radioecological Study [Nord Cotentin 2000] is a good example of what was necessary to do in such a context. In 1997 epidemiological studies suggested the existence of a cluster of leukaemia, related to people aged 0 to 24 years and living in the French canton of Beaumont-Hague between 1978 and Four cases of leukaemia were observed compared with 2 cases expected, and the excess was suspected to be due to radioactive discharges of the nuclear facilities - Cogema La Hague reprocessing plants, ANDRA s Shallow-land radioactive waste disposal center, EDF s nuclear power station and the French navy arsenal - located in the canton. Reprocessing plant Waste disposal Naval dockyards 10 km Nuclear power station 20 km 35 km Figure 6: locations of the Nord-Cotentin nuclear facilities A working Group (acronym GRNC) including nuclear operators, authorities, international experts, members of environmental organisations and IRSN experts was set up to establish as precisely as possible the radioactive discharges during the operation periods, to calculate the doses received over years by these people, to estimate the risk of radiation induced leukaemia in that population, and finally to discuss the plausibility of the supposed relationship. 25

8 3.1 The methodology The GRNC carried out a very exhaustive, systematic and critical analysis [Rommens 2000]. The Group analysed operators actual measurements of liquid and gaseous discharges in order to validate, correct or reconstruct them for all the operation periods. The objective was to have the most exhaustive and realistic source terms. Two site specific dispersion models, for respectively the gaseous and the liquid discharges, were developed. Air concentrations, deposition to ground and crops and activity concentrations in terrestrial and marine foodstuffs were assessed. The parameters values best suited to local characteristics were chosen. Wherever possible, results of the models were compared with existing environmental measurements and for few radionuclides, correction factors were introduced in the modelling. All possible exposure pathways were taken into account: external exposure by the plume, deposition on the ground, sand or sediments, and bathing; internal exposure by inhalation of the plume, re-suspended deposits and sea spray, by ingestion of foodstuffs from terrestrial and marine environments, by accidental ingestion of sea water while bathing, of soil, and of sand during time spent on the beach. Consumptions of food were obtained from local food surveys. This modelling considered more exposure pathways than those retained in chapters 1 and 2. Doses to the red bone marrow were assessed for people of 0-24 year old living between 1978 and 1996 in the Beaumont-Hague canton composed of 19 villages (figure 7). For each individual of the cohort defined above, doses were not annual but cumulated over life up to Leukaemia risks were calculated from dose-risk relations. Individual risks were all summed to obtain the expected number of leukaemia cases. At end, this estimate was compared with the number of cases in excess. The whole assessment process was designed to provide estimates of doses and therefore of risks as precise as possible, in other words as unbiased as possible. In that case, it is interesting and even recommended to consider the uncertainties existing at the various stages of the process. Therefore an uncertainty analysis was performed to establish confidence and probability intervals which have been used to test the significance of riskexposure relationships [Merle-Szeremeta 2002] [Rommens 2002]. One recognizes the common statistical approach based on both estimation as first step and testing as second step. 26

9 Figure 7: geographic location of the Beaumont-Hague canton 27

10 3.2 Example of results People living in Digulleville Dose (µsv) Years Figure 8: effective individual dose for an adult living in Digulleville 4 CONCLUSION This short paper points out three contexts of dose assessment that cover most of the situations encountered in practice. It underlines the fact that methodological choices and dose calculation codes are context dependent. In a regulatory context, a conservative approach is acceptable and even well suited in general. It is no longer applicable when knowledge of exposures is the main goal; more realistic approaches are then required. At end, when results from calculation must be compared with observed data, a realistic approach is necessary and it must go with analysis of uncertainties. 28

11 5 REFERENCES [1] Laurier D, Grosche B, Hall P, Risk of Childhood Leukemia in the Vicinity of Nuclear Installations, Findings and Recent Controversies, Acta Oncologica Vol. 41, n 1, p , [2] Laurier D, Billon S, Morin A, CLavel J, Exposition environnementale aux rayonnements ionisants et risque de leucémie chez les enfants, Programme Environnement et Santé 1999, Rapport scientifique final, Institut de Radioprotection et Sûreté Nucléaire, [3] Merle-Széréméta A, Brenot J, Chojnacki E, Uncertainties in Risk Assessment, Experience from the Nord-Cotentin Radiological Study, Stakeholders' Conference on Approaches to the Management of Environmental Radioactivity, Luxembourg, December [4] Nord-Cotentin Radioecology group, Synthesis, Estimation of exposure levels to ionizing radiation and associated risks of leukemia for populations in the Nord-Cotentin, Institut de Radioprotection et de Sûreté Nucléaire, July [5] Ringeard C, Chartier M, Radiological impact of the limits of radioactive discharges of the La Hague reprocessing plant and comparison with impact of the Sellafield plant discharges, Preliminary report, Institut de Radioprotection et de Sûreté Nucléaire, October [6] Rommens C, Morin A, Merle-Szeremeta A, Le modèle FOCON d'évaluation de l'impact dosimétrique des rejets radioactifs atmosphériques des installations nucléaires en fonctionnement normal, Radioprotection V. 34, n o 2 p , [7] Rommens C, Laurier D, Sugier A, Methodology and results of the Nord-Cotentin radioecological study, Journal of Radiological Protection v. 20(4) p , [8] Rommens C et al, La nécessaire prise en compte des incertitudes dans les évaluations de risque, L'exemple du Groupe radioécologie Nord-Cotentin, Environnement Risques & Santé - Vol. 1, n 5-6, p , [9] Thomassin A, Morin A, Using the FOCON96 computer program for the analysis of release authorization requests, 7th International Conference on Harmonisation within Atmospheric Dispersion Modelling for Regulatory Purposes, Proceedings p , Belgirate, Italy, May