Influence Of Biosphere Modelling On The Results Of Performance Assessments Of Deep Geological Disposal: Lessons Learnt From The SPA Project.

Transcription

1 Influence Of Biosphere Modelling On The Results Of Performance Assessments Of Deep Geological Disposal: Lessons Learnt From The SPA Project. D. Gay, F. Besnus, P. Baudoin Institute of Protection and Nuclear Safety (IPSN/DES), Fontenay-aux-Roses, FRANCE INTRODUCTION The SPA project () is a direct continuation of the efforts made by the European Community since 982 to build a common understanding of the methods applicable to performance assessment of a deep geological disposal. Carried out from 996 to 999, SPA involved the participation of six national research institutions representing implementing as well as technical safety organisations: ENRESA for Spain, GRS for Germany, IPSN for France, NRG for The Netherlands, SCK.CEN for Belgium and VTT for Finland. One of the particular interests of the project was to constitute a practical framework to develop and implement methods and tools for integrated performance assessment. In this framework, the influence of some of the main assumptions used in the modelling were studied using the experiences and practices accumulated by each participant. Among other aspects, the possible influence of biosphere modelling was notably assessed. In the framework of the project, the aim of biosphere modelling was to convert the flux of activity reaching the environment into a dose value incurred by a member of a hypothetical critical group. Thus it provided an indicator of the radiological impact associated to a potential geological repository. The many uncertainties linked to biosphere evolution in time and the ensuing difficulties to define hypothetical critical groups, lead the participants to acknowledge that several different modelling assumptions were reasonably thinkable and that each could possibly result in a different calculated dose. In order to clarify this aspect and precise the extent to which biosphere could influence performance assessment results, an intercomparison of the biosphere modelling approaches was carried out by IPSN using the information made available by the other participants (2, 3, 4, 5, 6). The main results are presented in the following. The variety of approaches represented in the project comprised very simplified biosphere modelling based on water drinking only but also more detailed and classical ones based on reference biospheres and multiple exposure pathways. A particular interest of the intercomparison was therefore to evaluate how far the choice of a biosphere approach can influence the doses calculated in the framework of long-term performance assessments for a deep geological disposal and possibly modify the hierarchy of radionuclides. BIOSPHERE GENERAL DESCRIPTION For all the SPA participants, the source (or input) considered in the dose calculations was the contaminated groundwater entering the biosphere and the receptor was man. As regards transfer pathways, two distinct approaches were used: the first one assumed an exposure restricted to the ingestion of water only, while the other considered a more thorough contamination of the environment leading to multiple exposure pathways. Source description To go further into details, two different kinds of sources were distinguished depending on the nature of the interface between geosphere and biosphere: - In the first case, the contaminated water was directly abstracted from the ground through a well. The source was then expressed as a radionuclide concentration in the abstracted water. Biospheres of this kind were called «well type biosphere». - In the second case, the contaminated water entered the biosphere at a natural outlet for groundwater flow and eventually diluted in a river. In this case, the source was expressed as the amount of radionuclides annually pouring in the considered river. Biospheres of this kind were called «river type biosphere». In both cases, the relevant input data needed for biosphere calculations was the activity concentration in water. In the case of a natural outlet, this data was derived from the annual amount of radionuclides reaching the considered outlet using a volume of dilution corresponding to the annual river flow (in m 3.year - ). Choices made by each participant in terms of source type are shown in table. ENRESA GRS IPSN NRG SCK.CEN VTT River Type Biosphere annual river flow 0 8 m 3.year - ~ m 3.year - ~ 0 8 m 3.year - Well Type Biosphere Table : Source type for biosphere modelling

2 Transfer pathways description The transfer pathways considered by the SPA participants all belonged to the following list :. direct intake of contaminated water 2. intake of vegetables irrigated with contaminated water 3. intake of animal produce from cattle fed on an area contaminated by irrigation 4. intake of fish bred in contaminated water (river or artificial pond) 5. external exposure due to a contaminated soil 6. inhalation of contaminated soil particles For every participant but VTT, transfer pathways were selected so that no potential exposure pathway was unduly neglected. According to this principle of reasonable caution, every pathway was taken into account except those considered not relevant to the characteristics of the source and more precisely to the amount and the availability of water at the considered outlet. For instance, in the case of IPSN well type biosphere, pathways 3 and 4 were not taken into account because the large volume of water they required was not considered compatible with the relatively low productivity of the aquifer. In the case of ENRESA well type biosphere, the well was considered to be located at a relatively great depth. Because of the relatively high financial cost likely to be associated to water abstraction under these conditions, the use of water was assumed to be limited to drinking purpose only. For VTT, transfer pathways were conventionally limited to direct ingestion of contaminated water (pathway ) in order to minimise the number of parameter values to be set and thus limit to the maximum the sources of uncertainties. Under these conditions, the only parameter values used were the ICRP dose coefficients for ingestion and the annual intake of contaminated water by a member of the hypothetical critical group. Table 2 shows the list of transfer pathways selected for each biospheres used in the SPA calculations. ENRESA GRS IPSN NRG SCK.CEN VTT well well river well river river well well Drink. Water Vegetable Farm animal Fish External Inhalation Table 2: Transfer pathways selected Receptor (man) description Description of man as the receptor of radionuclides transfer consisted in the definition of the diet of a member of the hypothetical critical group (annual intake of drinking water, vegetables, farm animal products, fish) and of his behaviour (agricultural practices, time annually spent on site, volume of air annually breathed). For each biosphere, the precise list of required parameters of course widely varied notably depending on the list of transfer pathways involved. However, from a general point of view, choices made by the different participants were rather similar in the sense they all relied on the same general assumptions: they were notably all defined in relevance with the current agricultural practices in developed countries, under the assumption of a temperate climate and according to a relatively high level of self-subsistence for food. COMPARISON OF BIOSPHERE CONVERSION FACTORS Definition and use of biosphere conversion factors For the purpose of a geological disposal performance assessment, biospheric transfers can be taken into account either through the use of a set of biosphere conversion factors or by coupling geosphere model with a dynamic biosphere model. Definition of biosphere conversion factors enable to run independently biosphere and geosphere calculations. They are used to convert any geosphere modelling output into a dose value. Their use relies on the assumption that the different compartments of the biosphere have a quasi-static behaviour. At every calculation time step, an equilibrium is then reached between the source and the biosphere and the contamination state of the compartments can be considered independent from the history of past releases. This assumption can be considered to be true as long as the source-term variation remains slow as compared to the time required by each compartment to reach an equilibrium. In the context of long-term performance of geological disposal, this assumption is much reasonable for river, plant or animal compartments for which dwelling time of radionuclides are usually shorter than a year. However it can be more controversial for soil because of possibly much longer dwelling time. 2

3 In the framework of the SPA project, every participants but ENRESA chose to define and use a set of biosphere conversion factors. For NRG, GRS, IPSN and SCK.CEN, this set of values was derived from dedicated biosphere modelling calculations using a constant unit source-term ( Bq.l - ). In the case of VTT, biosphere conversion factors were directly calculated as the product of the ICRP dose coefficients for ingestion by the volume of contaminated water annually drunk by a member of the critical group (0.5 m 3.year - ). For ENRESA, soil equilibrium was not a priori assumed and a real coupling was carried out between geosphere and biosphere modelling in order to take into account the kinetic of the biosphere response to geosphere output variation. For comparison purpose, ENRESA biosphere model was run with a constant unit input in order to derive biosphere conversion factors that could be compared to those used by the other participants. Comparison of values Table 3 shows the biosphere conversion factors for every participants and for a set of radionuclides considered important when assessing the long-term performance of a deep geological disposal. Values are expressed in (Sv.year - )/(Bq.m -3 ). In the case of a river type biosphere, values are converted in this unit using the annual river flow indicated in table. For every institute and every radionuclide, these values were used to evaluate the ratio between the biosphere conversion factor defined by the institute and the VTT one. Results are shown in figure. Well type biosphere River type biosphere VTT GRS IPSN ENRESA SCK.CEN NRG IPSN SCK.CEN 4 C 2.9E-0 9.7E-8 2.8E-9 3.E-8 5.5E-0.3E-8 2.E-8.8E-8 36 Cl 4.7E-0 2.6E-8 7.E-9.8E-8 2.3E-9 4.7E-0.2E-8 3.E-9 59 Ni 3.2E-.7E-9 2.6E-0 6.7E-0 3.5E- 8.4E- 3.9E-8 7.5E- 79 Se.5E-9 2.3E-7 8.9E-9 6.9E-7 3.0E-7.E-7 4.2E-8 3.7E-7 93 Zr 6.E-0 8.2E-9.8E-9 2.7E-8 6.8E-0 6.2E-0.8E-9.9E-9 94 Nb 8.5E-0 9.2E-8.4E-7 2.E-6 2.E-8 2.8E-7 9.0E-8 3.0E-8 99 Tc 3.2E-0 4.9E-9 5.9E-9 2.4E-9 3.E-0.E-9 9.5E-9 4.3E-0 07 Pd.9E- 3.0E-0.7E-0.2E-8 2.4E- 4.3E-0 4.0E-9 2.3E- 26 Sn 2.5E-9 8.7E-6 2.2E-7.8E-6 5.0E-8.5E-6 3.2E-7 3.2E-7 29 I 5.5E-8 3.7E-7 3.5E-7 3.8E-7 6.E-8 6.5E-8 4.8E-7.4E-7 35 Cs.0E-9 8.6E-8 6.6E-9.6E-8 2.3E-9 5.5E-8 2.8E-8.5E Pu.3E-7 2.2E E-7 9.6E-7 N.C..5E-7 9.0E-7 N.C. 236 U 2.4E-8 2.2E-07.4E-7.2E-7 2.E-8 6.6E-8.8E-7 2.5E Th 5.3E-7.6E E-6.2E-4 2.5E-6 5.2E-6 9.8E-6 3.7E Cm.E-7 3.0E-6.6E-6 4.E-6 N.C. 6.0E-7 2.E-6 N.C. 24 Pu 2.4E-9 5.6E-8 N.C..2E-8 N.C. 2.9E-9 N.C. N.C. 24 Am.0E-7 2.7E-6 N.C..0E-6 N.C. 3.4E-7 N.C. N.C. 237 Np 5.5E-8 6.2E-6 3.4E-7 3.E-7 4.9E-8.0E-6 4.4E-7 6.6E U 2.6E-8 3.0E-7.5E-7.3E-7 2.3E-8 8.E-8 2.0E-7 2.7E Th 3.E-7 5.4E-6.9E-6.9E-5 5.9E-7 2.7E-6 3.5E-6 9.E Cm.E-7 N.C. 8.2E-7 2.5E-6 N.C. 4.6E-7.0E-6 N.C. 242 Pu.2E-7 N.C. 7.E-7 9.4E-7.E-7.4E-7 8.6E-7 5.8E U 2.4E-8 3.E-7.5E-7.4E-7 2.E-8 6.2E-8.9E-7 2.6E U 2.5E-8 2.4E-7.5E-7.3E-7 2.2E-8 7.E-8 2.0E-7 2.6E Th.E-7 2.8E-6 6.6E-7 4.0E-6 2.9E-7.8E-6.E-6 4.0E Ra.E-6.5E-5 3.9E-6.2E-4 2.5E-7.3E-6.6E-5 4.E Am.0E-7 3.5E-6 8.3E-7 4.9E-6 N.C. 5.3E-7.2E-6 N.C. 239 Pu.3E-7 2.2E-6 7.4E-7 9.8E-7 N.C..6E-7 9.0E-7 N.C. 235 U 2.4E-8 9.5E-7.5E-7.6E-7 2.E-8 8.E-8.9E-7 2.6E-8 23 Pa 9.6E-7.4E-5 5.9E-6 4.9E-5 8.8E-7 7.6E-6 9.6E-6 7.9E-7 Table 3: Biosphere coefficient factors [in (Sv.year - )/(Bq.m -3 )] N. C. : not considered in the calculations VTT value is used 3

4 ENRESA GRS IPSN (w ell) IPSN (river) SC K.C EN (well) SCK.CEN (river) NRG C4 Cl36 Ni59 Se79 Zr93 Nb94 Tc99 Pd07 Sn26 I29 Cs Cm245 Cm246 Am24 Am243 Pu239 Pu240 Pu24 Pu242 Np U233 U234 U235 U236 U238 Pa23 Th229 Th230 Th232 Ra226 Figure : Biosphere conversion factors - Ratio to VTT values (The bar is absent when the radionuclide is not considered in the calculations) As VTT dose conversion factors indicate the contribution from water ingestion only, the values shown in figure can be considered as a measure of dose «amplification» resulting from the transfer and accumulation in environmental compartments and from a multiple pathway exposure. A value of 00 roughly means that the dose incurred by an individual that directly drinks contaminated water is 00 times higher if the contaminated food products are ingested (the water also contaminates soil, fish, plants or farm animals) and if external exposure occurs. From the comparison shown in figure, the following conclusions can then be drawn :. The influence of biosphere can be very high. The level of dose incurred by an individual because of direct ingestion of water can be greatly amplified if the water can also be transferred to other environmental compartments. For some of the considered radionuclides, value of the ratio can be as high as 3 orders of magnitudes. 2. The level of influence is specific of a radionuclide. Effects of biospheric transfer on the level of dose incurred vary greatly from one radionuclide to another. 3. A radionuclide classification can be proposed. Discrepancies exist from one participant to another but the values usually remain within the same order of magnitude or are at least sufficiently homogeneous to define three broad classes of radionuclides: those for which dose calculations are highly influenced by biosphere transfer (ratio greater than 00), those of medium influence (ratio between 0 and 00) and those for which dose calculations are lowly influenced (ratio lower than 0). This classification is shown in table 4. When the discrepancies between participants make the classification difficult, a «?» is indicated. 4

5 Influence Fission Products Actinides High (ratio > 00) 4 C, 79 Se, 94 Nb, 07 Pd (?), 26 Sn 226 Ra (?), 232 Th (?), 237 Np (?) Medium (0< ratio < 00) Cl, 59 Ni, 93 Zr, 99 Tc, 35 Cs Cm, 24 Pu, 24 Am, 229 Th, 246 Cm, 230 Th, Am, 235 U, 23 Pa Low (ratio < 0) 63 Ni, 90 Sr, 29 I 240 Pu, 236 U, 233 U, 242 Pu, 238 U, 234 U, 239 Pu Table 4: Classification of the radionuclides according to the influence of environmental transfer on their biosphere conversion factors COMPARISON BY EXPOSURE PATHWAY Depending on the modelling assumptions, dose calculations can require the definition of as much as several dozens to several hundreds of parameters that in addition sometimes greatly differ in nature from one model to another. In order to further interpret the previous comparison results, a simplified analysis of biosphere modelling was however carried about. This analysis mainly consisted in defining and quantifying a short list of aggregated parameters. These parameters were used as indicators to follow the behaviour of the radionuclides in each compartment. For the selected radionuclides, it notably helped to identify the exposure pathways likely to significantly enhance the calculated dose if taken into account in addition to water uptake. For the need of the study, the comparison focused on the five following pathways:. exposure by ingestion of contaminated water, 2. exposure by ingestion of contaminated plants, 3. exposure by ingestion of meat and milk from contaminated animals, 4. exposure by ingestion of contaminated fish, 5. external exposure resulting from dwelling on a contaminated area, Other exposure pathways such as inhalation of contaminated soil particles or ingestion of other animal products (liver in the case of ECN and eggs and chicken meat in the case of IPSN) were considered by some of the participants but were never found to lead to a decisive contribution to dose. They were therefore ignored in the comparison. For each radionuclide, the contribution of an exposure pathway to the total dose was first described as depending on two factors: the concentration in the compartment directly involved in the exposure and the relative weight of the pathway that resulted from chosen occupational behaviour. Exposure related to water drinking was for example proportional to activity concentration in water and to annual water intake. In the same way, exposures related to plant, animal products or fish ingestion were proportional to concentrations and annual intakes associated to each of these food products. Though external exposure differed significantly from ingestion, the same kind of description could also be adopted by writing down the dose contribution as follows: Fexternal Dose external = Cground Fingestion, Fingestion where F external was an aggregated dose coefficient for external exposure (in (Sv.year - )/(Bq.kg - )). This coefficient led to the dose annually received by a member of the hypothetical critical group by dwelling on a ground contaminated by irrigation. It notably integrates the dwelling time. In the previous equation, the second Fexternal term ( ) was taken as the weight for external exposure pathway. It thus played the same role as the Fingestion annual intakes for ingestion pathways. According to the previous description, the relative importance of a given pathway was investigated through the comparison of its relative weight and by assessing the possible accumulation of activity in the corresponding compartments. Relative weight of the different pathways The comparison showed that the assumptions considered by the participants in their respective modelling were rather similar and led to attribute the same weight to most of the considered pathways. For example, in a first approximation, the weight of the water ingestion pathway (annual water intake in l.year - ) was of the same order of magnitude as the weight of the plants or animal products ingestion pathways (annual intake for plants and animal products in kg.year - ). The relative importance of most of the pathways could then be considered as proportional to the relative concentration in the corresponding compartment. Only fish ingestion and external exposure had a significantly lower weight. The weight of fish ingestion pathway (fish annual intake in kg.year - ) used for dose calculations were 40 5

6 to 300 times lower than the weight of the water ingestion pathway (annual water intake in l.year - ). Accordingly, significant accumulation of activity in fish was necessary until this pathway could become significantly more important than water ingestion. For external exposure, the relative weight, expressed as explained previously, was found to be usually very low. It was also found to greatly vary from one radionuclide to another. The only radionuclides for which the weight was found similar to the one associated to water drinking pathway, were 94 Nb and 26 Sn. For most of the other radionuclides, ground concentrations several thousands times higher than concentration in water were required to reach dose contribution by external exposure similar to dose contribution by water drinking pathway. As a conclusion to this first step, it appeared that water drinking pathway gave a good indication of the total dose under the condition that a significant accumulation did not occur in one of the four compartment considered (soil, plant, farm animal products and fish). The second step then consisted in assessing the possibility of accumulation in each compartment through the definition of simplified parameters. Accumulation in soil With regard to soil concentration, calculations carried out by each participant were boiled down to the multiplication of two parameters: a time of accumulation and an annual water input. The annual water input (Input Part ) was defined as the amount of contaminated water annually used to irrigate a unit mass ( kg) of cultivated soil. The effective accumulation time in soil (t soil ) corresponded to the mean effective transit time through the cultivated layer of soil. It also led to the soil concentration that resulted from a unit input of l.year -.kg -. Depending on the modelling adopted, this effective transit time could entail a variety of processes such as diffusion, infiltration, erosion, For the annual water input, a relatively good agreement was found among the participants and values were usually close to l.kg -.year -. Differences however existed and could notably be associated to differences in irrigation rate. These differences roughly reflected the diversity of climatic conditions currently prevailing in the countries participating to the project. ENRESA (Spain) thus used irrigation rates higher than those considered by NRG (the Netherlands) and SCK.CEN (Belgium). In addition to climate, another source of difference was linked to the types of crops considered and to eventual agricultural rotation. Because pastures were usually weakly or not irrigated, a soil on which pasture and cereals were successively grown over a period of several years, effectively received an annual effective water input by irrigation lower than if cereals were continuously cultivated on it. This was notably the case for IPSN in the case of river type biosphere. The resulting variation was found to remain within an overall range of one order of magnitude. Because the annual water input was usually found to be in the order of l.kg -.year -, t soil directly led to the accumulation factor in soil for each considered radionuclides. From the comparison of t soil values shown in figure 2, it was possible to draw two main conclusions: range of variation between high and low values were rather similar for every participants (from to a few thousands). a sufficient agreement existed among ENRESA, GRS, IPSN and SCK.CEN values to distribute the considered radionuclides in two groups corresponding to two contrasted accumulation behaviour in soil:! 59 Ni, 79 Se (despite a lower IPSN value), 93 Zr, 94 Nb, 07 Pd, 26 Sn, 35 Cs and most of the heavy nuclides strongly accumulated in soil,! 36 Cl, 99 Tc, 29 I accumulated much lesser. " Because of the existing discrepancies among participants, no classification could be easily proposed for 4 C. " For NRG, values were much lowly contrasted and only 94 Nb, 26 Sn and 35 Cs clearly belonged to the first group. From a general point of view, the classification proposed above is very similar to the one already shown in table 4. This emphasises the usually important role played by soil accumulation for environmental transfer. For 94 Nb and 26 Sn, the concentrations obtained associated to the relative weight of external exposure for example largely explained the high ratio values shown in figure. 6

7 ENRESA GRS IPSN SCK.CEN NRG C4 Cl36 Ni59 Se79 Zr93 Nb94 Tc99 Pd07 Sn26 I29 Cs Cm245 Cm246 Am24 Am243 Pu239 Pu240 Pu24 Pu242 Np U233 U234 U235 U236 U238 Pa23 Th229 Th230 Th232 Ra226 Figure 2: Comparison of t soil values [in year or (Bq.kg - )/(Bq.kg -.year - )] Accumulation in plants With regard to accumulation in plant products, transfer factor by root uptake (K soil/plant ) was selected as the main parameter of concern. According to the modelling assumptions adopted, it was indeed recognised that, though it could sometimes be predominant over root uptake, foliar interception did not lead to accumulation exceeding a factor of 0. Because the objective of the comparison was mainly to identify the pathways that could lead to exceed very significantly the dose due to water ingestion, the contribution from foliar interception was not further considered and K soil/plant was the single parameter used to characterise the possible accumulation in plant. Comparison of K soil/plant values are shown in figure 3. They indicate plant concentrations in Bq.kg - for a soil concentration of Bq.kg -. From this comparison, the same kind of conclusions as for soil was drawn: values spanned over 3 or 4 orders of magnitude from one radionuclide to another. However, contrarily to t soil values, maximum values were of the order of 5 to 0 and minimum of the order of 0-2 to 0-3. This meant that concentrations in plants were usually significantly lower than concentrations in soil and that a high concentration in plants required an even higher concentration in soil. the considered radionuclides can be distributed in two main groups:! Ni, Zr, Nb and I as well as Cs (despite a rather high NRG value) and most of the heavy nuclides are weakly transferred to plants,! C, Cl, Tc as well as Se (despite a lower IPSN value) are characterised by stronger accumulation properties in plants. For Pd and Sn, no classification could be easily proposed. 7

8 ENRESA GRS IPSN SCK.CEN NRG C Cl Ni Se Zr Nb Tc Pd Sn I Cs Pu U Th Cm Am Np Ra Pa Figure 3: Comparison of K soil/plant values [no unit or in (Bq.kg - )/(Bq.kg - )] Concentration in animal products With regard to transfer to animal products, an aggregated parameter (K milk, meat ) was defined as representative of transfer to both milk and meat. It led to the mean activity concentration in animal products corresponding to a unit activity concentration in pasture ( Bq.kg - ). Comparison of K milk, meat values are shown in figure 4. ENRESA GRS IPSN SCK.CEN NRG C Cl Ni Se Zr Nb Tc Pd Sn I Cs Pu U Th Cm Am Np Ra Pa Figure 4: Comparison of K milk, meat values [no unit or in (Bq.kg - )/(Bq.kg - )] For fission and activation products, accumulation factors in milk and meat were rather lowly contrasted and usually remained within an order of magnitude close to (except for IPSN values for Nb and Pd) in the case of ENRESA, GRS and IPSN. For NRG and SCK.CEN, values were more contrasted and sometimes significantly lower than, notably for Zr, Nb and Sn. For heavy nuclides, K milk, meat values were significantly lower than and lower than most of the values 8

9 for fission products. Pa(for ENRESA and GRS) and Ra (for GRS and IPSN) constituted the only relative exception to the previous pattern. Concentration in fish With regard to transfer to fish, calculations roughly boiled down to the multiplication of water concentration and a transfer factor (K fish ). This factor led to the concentration in fish (in Bq.kg - ) for a unit concentration in water ( Bq.l - ). It was naturally taken as the parameter of concern in the comparison. Comparison of K fish values are shown in figure 5. As mentioned previously, because of the relatively low annual fish intake, values as high as 40 to 300 were necessary to reach dose contributions similar to those due to water drinking. In figure 5, the horizontal lines display the corresponding domain of values. K fish values started from values of the order of 0 (Bq.kg - )/(Bql - ) and spanned over 3 orders of magnitude depending on the radionuclide. These values were sufficiently homogeneous and contrasted to propose the following classification :! C, Sn and Cs (despite a lower NRG value) highly accumulated in fish,! Cl, Tc, I (despite a higher SCK.CEN value) and the heavy nuclides relatively weakly accumulated in fish,! Ni, Se, Zr (despite a lower NRG value) and Nb had a rather intermediate behaviour. For Pd, no clear statement could be proposed but according to IPSN value strong accumulation was thinkable. GRS IPSN SCK.CEN NRG C Cl Ni Se Zr Nb Tc Pd Sn I Cs Pu U Th Cm Am Np Ra Pa Figure 5: Comparison of K fish values [in (Bq.kg - )/(Bql - )] CONCLUSIONS According to the comparison carried out within the SPA project, biosphere modelling can have a large influence on the results of performance assessment for deep geological disposal. Depending on the radionuclide considered, it involved differences up to 3 orders of magnitude on the calculated doses. Influence was thus found to be relatively limited for 29 I and most of the heavy nuclides, but could be very high for 4 C, 79 Se, 94 Nb, 07 Pd and 26 Sn. The comparison underlined showed the importance of considering multiple transfer and exposure pathways. Soil appeared to play a particular and important role because of its capacity to strongly concentrate some elements. It notably led to a very significant contribution from external exposure for 94 Nb and 26 Sn. Ingestion of fish appeared to be important for 4 C and possibly for 07 Pd. Ingestion of plants was found to be predominant for 36 Cl, 79 Se and possibly 26 Sn. In addition, the comparison results gave a first insight into the various biosphere models. For some radionuclides, they led to identify some discrepancies with regard to the respective accumulation properties of 9

10 the various environmental compartments involved but also enabled to differentiate the radionuclides depending on their behaviour. REFERENCES. SPA project. Topical Report 3 - Far-Field Data and Models - Far-Field Performance Assessment. Commission of the European Communities. DOC XII /99, ENRESA. Performance Assessment for a High Level Radioactive Waste Repository in Granite: Philosophy, Methodology, and Conceptual Model. Final Report, GRS. AVV - Allgemeine Verwaltungsvorschrift zu 45 Strahlenschutzverordnung: Ermittlung der Strahlenexposition durch die Ableitung radioaktiver Stoffe aus kerntechnischen Anlagen oder Einrichtungen. Erschienen im Bundesanzeiger, 42. Jg., Nummer 64a, IPSN. P. Santucci - Conceptual and mathematical Modelling of the Biosphere: ABRICOT version 2.0; SERGD technical report n 94/09; NRG. D. H. Dodd - Biosphere Modelling ECN Position Paper, ECN memo 7085/NUC/DD/mh/0783, VTT. T. Vieno - WELL-97 - A stylised well scenario for indicative dose assessment of deep repositories. Espoo, VTT Energy, Technical Report SPAVTT-2/97,