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Biosphere models for Safety Assessment of radioactive waste disposal based on the application of the Reference Biosphere Methodology

Deliverables

The project aimed at the improvement of the scientific basis for the application of biosphere models in the framework of long-term safety studies of nuclear waste disposals. Site-specific biosphere models were derived following the Reference Biosphere Methodology that was developed within the international project BIOMASS in order to ensure the development of consistent and scientifically justified biosphere models for application in long-term performance assessments for nuclear waste disposals. For these purposes, five European locations, covering a wide range of environmental and agricultural conditions were described and characterised. In the BioMoSA study the radionuclides 36Cl, 79Se, 99Tc, 129I, 135Cs, 226Ra, 231Pa, 230Th, 237Np, 239Pu, and 238U were considered which are usually most relevant in performance assessment studies of nuclear waste disposals. For each of the sites, a biosphere model has been developed specifically. As far as possible, site-specific parameters have been selected for element-independent parameters (e.g. irrigation rates, agricultural practices, human consumption rates, potential critical groups) and element-dependent parameters (e.g. transfer factors soil-plant, distribution coefficients, migration of radionuclides in soil). Site-specific calculations have been performed for each of the sites considered. Annual individual doses were calculated, important processes and parameters were identified and the uncertainties of the results were estimated by means of stochastic calculations. To enable a comparison, all results were normalized to an activity concentration in groundwater of 1Bq/m³ for each of the radionuclides considered.
The project aimed at the improvement of the scientific basis for the application of biosphere models in the framework of long-term safety studies of nuclear waste disposals. From the BioMoSA study, the following main conclusions can be drawn that relevant for the development of site-specific assessment tools that are going to be applied in long-term safety studies: - The methodology developed within the BIOMASS project for the setup of a reference model is considered to be useful. - The variations of the normalised exposures [in Sv/a per Bq/m³] for the well scenario among the sites are in general less than a factor of 10. - For all sites, the intake of drinking water is an important or even dominating contributor to the exposure. The amount of food consumed is also constrained for physiological reasons. The consumption habits among the sites vary in terms of the food items, but not in terms of the total amount of foods. - In general, there is acceptable agreement between the results obtained with the generic and the site-specific models respectively. - The results for the lake, marine and a release to the deep soil are associated with larger uncertainty, which is due to the much higher complexity of the specific sites. - From the more than hundred parameters, which were used to run the different models, only 20 seem to be having a significant contribution to the total dose. The most important parameters are the transfer factors for soil-to-plant and soil-to-beef, food consumption, irrigation water applied and distribution coefficient for soil.
The project aimed at the improvement of the scientific basis for the application of biosphere models in the framework of long-term safety studies of nuclear waste disposals. A generic model has been developed within BioMoSA that contains all features and processes that are included in the site-specific models. The generic model is used to estimate the same endpoints as the site-specific models. It is available for use, subject to purchase of a software licence for the numerical solution method. It has been successfully used to model 5 sites with a range of climates and geosphere/biosphere interfaces. The results agree well with the site-specific runs. The calculations with the generic model allow the variability between sites to be investigated on a common basis. Both the generic model and site-specific models agree on the important parameters. As recommendation to the adaptation (simplification) of the generic model, it could be said that all pathways are potentially important if the large number of different radionuclides are kept in mind that can be released by a repository.

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