Development of computational methodology in simulating thermal responses of spent fuel in deep geological disposal

Deep geological repository with multiple barriers has been recognized as the safety management of nuclear wastes in the world. The decay heat from the spent fuel can elevate the temperature values of canisters, buffers, backfills, and host rock, which may challenge repository integrity. It is crucial to investigate the thermal response of in order to maintain the performance and the integrity of repository system. A gap of millimeter order in general exists between the canister/buffer interface. This gap effect would strongly influence the temperature distribution in the buffer and its peak temperature. A large amount of mesh size and computing power is needed to realistically model this small gap for numerically simulating the temperature responses in the entire disposal site. A simplified computational methodology with the effective conductivity of buffer (lambda eff) is proposed in this paper, which can reasonably predict the peak buffer temperature with dramatic reduction in the mesh size and numerical computation. In addition, under the uniform heat source distribution in the whole repository system, a geometric simplified model with 1/4 deposited hole + 4-side symmetric boundary conditions can also precisely capture the peak buffer temperature that is close to the value predicted using the entire disposal-site simulation. In the engineering point of view, the lambda eff model and the geometric simplified model can be applied in the numerical simulation of temperature responses for the whole repository system.