Modeling of Radionuclide Migration Through Fractured Rock in a HLW Repository With Multiple Canisters
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Modeling of Radionuclide Migration Through Fractured Rock in a HLW Repository With Multiple Canisters Doo-Hyun Lim1, Masahiro Uchida2, Koichiro Hatanaka3, and Atsushi Sawada4 1 Golder Associates Inc., 18300 NE Union Hill Road, Suite 200, Redmond WA 98052, USA 2 Japan Atomic Energy Agency, 959-31 Jorinji, Toki-shi, Gifu 509-5102, Japan 3 Japan Atomic Energy Agency, Horonobe-Cho, Teshio-Gun, Hokkaido 098-3224, Japan 4 Japan Atomic Energy Agency, 4-33 Muramatsu, Tokai-mura, Ibaraki 319-1194, Japan ABSTRACT An integrated numerical model for groundwater flow and radionuclide migration analyses in a water-saturated HLW repository with a multiple-canister configuration is developed by incorporating the heterogeneity of fractured host rock based on the previous multiple-canister model (MCFT2D [1, 2]). The current model incorporates i) heterogeneity of the fractured host rock represented stochastically by discrete fractures, ii) disposal-pit vertical emplacement concept, iii) representation of the waste package consisting of a waste canister and a bentonitefilled buffer, and iv) a user-determined repository configuration of multiple canisters using the repository parameters such as disposal tunnel spacing, waste package pitch, tunnel diameter, the number of tunnels in a repository, and the number of canisters in a tunnel. The current model can facilitate investigations into the effects of heterogeneous fractured host rock on water flow and nuclide migration for the different configurations of multiple canisters, as well as optimization of the repository design parameters in terms of release of nuclides from the repository. INTRODUCTION Migration of radionuclide (Cs-135) in a water-saturated high-level radioactive waste (HLW) repository was analyzed numerically by a two-dimensional numerical model (MCFT2D) incorporating both a multiple-canister configuration and a non-uniform horizontal flow field of the host rock [1, 2]. The previous studies [1, 2] showed that the migration of Cs-135 in a repository with multiple canisters is significantly influenced not only by the configuration of canisters but also by the groundwater flow conditions. For more robust analysis, heterogeneity of the fractured host rock should be taken into account explicitly in the flow and transport analyses. The objective of this study is to develop a reliable numerical flow and transport model incorporating multiple-canister configurations and heterogeneity of the fractured rock by discrete-fracture network (DFN) in two-dimensional space.
MODEL DEVELOPMENT Conceptual Model A hypothetical water-saturated HLW repository is modeled based on the disposal-pitvertical-emplacement method [3] in two-dimensional space. As shown in figure 1(a), a hypothetical repository consists of N disposal tunnels containing Ny packages in each tunnel. The total number of canisters is N⋅Ny for the repository with an area of Lx×Ly, where Lx=(N⋅xD), Ly=(Ny⋅yD+2aD). xD is the disposal tunnel spacing. yD is the waste package pitch in the disposal tunnel. aD is assume to be (xD-tD)
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