| Summary: | 碩士 === 國立成功大學 === 資源工程學系 === 103 === Deep geological disposal is generally adopted worldwide for high level radioactive waste (HLRW) management. The spent nuclear fuel is encapsulated in a metal canister and then placed into an engineered facility within the bedrock at 300-1000 meters depth. Such a multiple barrier system, comprising both engineered and natural barriers, will efficiently retard the migration of radionuclides, long enough for them to decay to a safe level before reaching the biosphere. However, HLRW continues to emit heat for up to 100,000 years. Thus, thermal, hydro and mechanical (T-H-M) factors all have a potential effect on the long-term safety of the deposited waste.
This thesis links TOUGH2 and FLAC3D by MATLAB to perform coupled T-H-M analyses of underground nuclear waste repositories to reproduce the TOUGH2-FLAC3D numerical procedure proposed by Lawrence Berkeley National Laboratory (2003). The proposed TOUGH2-FLAC3D coupled procedure is validated with the technical report presented by the SKB report (1999). The proposed TOUGH2-FLAC3D coupling routine can analysis coupled T-H-M case successfully. The geometry of disposal holes and disposal tunnels are complex. Consequently, a code is written to transfer the mesh produced by the ANSYS and converted to FLAC3D equivalents, transferred to a format data file, and then exported to FLAC3D.
The present study is focus on near-field isolation system of high level radioactive waste repository. The peak temperature obtained from the coupled T-H-M model at the top of the Bentonite buffer is compared with that obtained from a coupled T-H model and a Thermal model, respectively, for the case of a single disposal canister. The results show Thermal model yields the most conservative estimate of the peak temperature of the three simulation models. Hence, hydro and mechanical effects should be ignored for conservative design of the repository. However, in practice, multiple canisters may be stored within a single disposal tunnel. Moreover, a single repository may comprise multiple disposal tunnels. It is essential to determine the number of canisters which should be considered when performing numerical simulations in planning the safe layout of a real-world repository. After that the repository is assumed to have 2 parallel and symmetrical tunnels and the spacing between adjacent disposal tunnels is specified as 40 m. 5, 7, 9, 11 and 13 canisters, with 6 m spacing, are placed within each disposal tunnel for Thermal model analysis. The results show the peak temperatures at the top of the Bentonite buffer are the same for 7 or more canisters (i.e., 9, 11 and 13). In other words, when using the most conservative model (the Thermal-model), a simulation model consisting of 7 canisters in a tunnel is sufficient to investigate the effect of the deposited canisters on the local rock temperature distribution. Afterwards, T-H-M, T-H and Thermal models are performed using a domestic buffer material (Zhisin clay) to investigate the different peak temperature between foreign buffer material (MX-80) with same geometry (7 canisters emplaced within 2 parallel and symmetrical tunnels and the spacing between adjacent tunnels is 40 m). The results show the peak temperature is found to reach a value of 97.7 oC at the top of the Zhisin clay which is higher than that of the MX-80 (93.36 oC). The peak temperature at the top of the Zhisin clay is very close to the maximum permissible value of 100 oC.
The research uses the same geometry (model consisting of 7 canisters in 2 parallel and symmetrical tunnels with the disposal tunnels sapcing fixed at 40 m) and with varies thermal conductivity of the host rock and different spacing between each canister to investigate the the temperature variations. Finally, the results are plotted in the canisters pacing-peak buffer temperature chart which can be used as a reference for the layout of a nuclear waste repository.
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