Modeling of Groundwater Flow and Radionuclide Transport in a High-Level Radioactive Waste Repository with Multiple Canis
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Modeling of Groundwater Flow and Radionuclide Transport in a High-Level Radioactive Waste Repository with Multiple Canisters
Doo-Hyun Lim Japan Nuclear Cycle Development Institute, 4-33 Muramatsu, Tokai, Ibaraki, 319-1194, JAPAN.
ABSTRACT
A two-dimensional numerical model incorporating a multiple-canister configuration and a non-uniform flow field of the host rock is developed for more reliable performance assessment of a water-saturated high-level radioactive waste repository. A space-dependent flow velocity distribution is established numerically using the finite-element method. Transport of Cs-135 is simulated using the random-walk method. Numerical results for a source condition of instantaneous-pulse-input show that (i) migration of Cs-135 is influenced significantly not only by the canister configurations but also by flow boundary conditions, ii) effect of applied hydraulic gradient direction on Cs-135 migration becomes more significant as the number of canisters increases, while the effect is negligible for a single-canister configuration, (iii) the direction of the hydraulic gradient orthogonal to the orientation of the disposal tunnel is most advantageous with respect to the isolation of the radionuclide, and (iv) the single-canister configuration yields conservative results compared with multiple-canister configurations.
INTRODUCTION
A potential water-saturated high-level radioactive waste (HLW) repository is considered to contain thousands of waste canisters (e.g., 40000 [1]) emplaced in deep geologic formation (i.e., a hard rock in the current study) below the water table. HLW repository performance assessments stand on conservative assumptions and/or simplifications, such as the independent (single) canister configuration [1-3] and/or the uniform water flow in the host rock [1-5]. The independent canister models are based on the assumption that the release of radionuclides from one canister is independent of the release from other canisters [1-3], while the connected-canister (compartment) models assumed that radionuclides released from one compartment must be transported into the next compartment through the homogenized rock represented by the uniform water flow (i.e., perfectly dependent canister) [4,5]. A model incorporating not only a multiplecanister configuration but also a non-uniform flow field of the host rock is needed because in previous models the dependency of canister models for nuclide migration has been assumed to be either independent [1-3] or perfectly dependent [4,5], while the dependency varies due to the repository design and the hydraulic (and transport) conditions in the repository area.
MODEL DEVELOPMENT
Conceptual model
A two-dimensional numerical model incorporating a multiple-canister configuration for groundwater flow and radionuclide transport is developed for a water-saturated HLW repository based on the disposal pit vertical emplacement concept (see Figure 1). A repository consists of N disposal tunnels containing Ny waste packages in each tunnel (see Figure 1(b))
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