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Abstract
A two-dimensional finite volume model of a Multi-Purpose (MPC) in a rail cask with twenty-one pressurised water reactor (PWR) assemblies inside was constructed. Steady state thermal simulations were performed for a range of fuel heat generation rates, for both nitrogen and helium cover gas, and different fuel cladding emissivities. Geometrically accurate computational fluid dynamics (CFD) simulations were employed to calculate buoyancy induced motion in, and natural convection and radiation heat transfer across, all gas filled regions. The results are compared to stagnant-gas CFD (S-CFD) simulations in the same geometrically accurate mesh, and to simulations that employ Effective Thermal Conductivities (ETC) in a mesh with homogenized fuel/cover gas regions. The cask Thermal Dissipation Capacity (QTDC) is defined as the fuel heat generation rate that causes the fuel cladding to reach its allowed temperature limit. QTDC is 27% larger when helium is the backfill gas than for nitrogen. The QTDC predicted by the geometrically accurate CFD and S-CFD models is essentially the same, but 3-6% higher than that predicted by the homogenized ETC model. A ten percent increase in cladding emissivity leads to less than a 1% increase in QTDC. The non-isothermal temperature profiles of the fuel basket surfaces determined in this work will be used as boundary conditions in future benchmark experiments. The thermal resistance in two narrow gaps (between the fuel basket and its support brackets, and between the MPC and the transport over-pack), and between the cask outer surface and the environment, account for a significant fraction of the total thermal resistance between the hottest fuel and the environment. Uncertainty in those resistances affects the conclusions of this work.