Analysis of radionuclide migration with consideration of spatial and temporal change of migration parameters due to upli
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Analysis of radionuclide migration with consideration of spatial and temporal change of migration parameters due to uplift and denudation Taro Shimada, Seiji Takeda, Masayuki Mukai, Masahiro Munakata and Tadao Tanaka Nuclear Safety Research Center, Japan Atomic Energy Agency, 2-4 Shirakata-shirane, Tokaimura, Ibaraki, 3191195, JAPAN. ABSTRACT Integrated safety assessment methodology that analyzes radionuclide migration reflecting the spatial and temporal changes of disposal systems was developed for a geological disposal site with uplift and denudation, and then some case analyses for an assumed site were carried out. The combination of uniform uplift and denudation has the largest effect on the radionuclide migration because the ground water flow velocity increases with decreasing depth from the ground surface. In the case without denudation, tilted uplift has more effect than uniform uplift because flow velocity in tilted uplift increases with increasing hydraulic gradient. The long-term change of the geological structures including the uplift and denudation, the hydraulic conditions, and the recharge and outlet of the ground water around a candidate site should be carefully investigated to determine the appropriate the place, depth and layout of the repository. INTRODUCTION In long-term safety assessment of geological disposal system for high level radioactive wastes, it is necessary to evaluate the impact on the radionuclide migration where ground water flow and water composition are changed with decreasing depth of the repository from the ground surface by uplift and denudation. When the position of interest is up-warped mountain, a block of strike-slip fault or reverse dip-slip fault, the ground surface and geological layers in the entire area of interest are uplifted uniformly with maintaining the original shape of the ground surface (uniform uplift). On the other hand, when the position of interest is on a wing part of fold structure at the compression field, the elevation gradient of the ground surface and the geological layers in east-west or north-south direction will increase gradually over time (tilted uplift )[1]. The difference of the type of the uplifts may affect ground water hydraulics on and around disposal site, which may result in chemical phenomena within engineered barrier and along the migration path from the repository to the outlet of natural barrier. It is necessary to understand the quantitative effects of uplift and denudation to radionuclide migration, taking into account the linkage between hydraulics and chemicals. When denudation is taken into consideration for long-term evaluation, it had been often assumed that the uplifted part at the surface was deleted by denudation to keep the current shape and elevation of ground surface. The depth of repository from the ground surface is decreased while the shape and elevation of ground surface is not changed in the combination of uplift and denudation. Therefore, it is possible that the migration path and the flow velocity along the path are
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