Biosphere modeling for safety assessment of high-leveI radioactive waste geological diposal

Baba, Tomoko ; not registered; Suzuki, Yuji*; not registered ; Ishiguro, Katsuhiko; Ikeda, Takao*; Richard*

In the safety assessment of a hifh-level radioactive waste disposal system, it is required to estimate future radiological impacts on human beings. Consideration of living habits and the human environment in the future involves a larg degree of uncertainty. To avoid endless speculation aimed at reducing such uncertainty, an approach is applied for identifying and justifying a "reference biosphere" for use in safety assessment in Japan. Considering a wide rang of Japanese geological environments, some specific "reference biospheres" are developed using an approach consistent with the BIOMOVS II reference biosphere methodology. The models represent the components of the surface environment using compartments between which fluxes of materials (solid/water) and radionuclides are defined by transfer factors. A range of exposure pathways via which such radionuclides enter the food-chain, along with uptake and concentration factors, are also defined. The response to a step function of unit flux from the geosphere is determined for each model. The results show that it is reasonable to use steady-state biosphere responses to a unit-input flux to define nuclide-dependent factors for converting fluxes from the geosphere to doses. This simplifies safety assessment calculations, which then require only look-up tables for such flux to dose conversion rather than fully coupled biosphere models.