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Computational Fluid Dynamics Thermal Simulations of a Nuclear Fuel Canister During Drying
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Drying of nuclear fuel canisters is essential to ensure the long-term integrity and safety of nuclear fuel. Vacuum drying, which is one of the drying processes applied to nuclear fuel canisters, consists of lowering the gas pressure in the canister. This introduces a temperature-jump thermal resistance at the gas-solid interface that can cause the cladding temperature to rise beyond the regulated limits. In this thesis, the details of a numerical model of a TN24 PWR nuclear fuel canister filled with Westinghouse 15x15 assemblies is discussed. The model was constructed in ANSYS/Fluent to assess the peak cladding temperatures during vacuum drying and is geometrically-accurate and three-dimensional with distinct regions for the fuel, cladding, backfill, steel basket, and aluminum support. Considerations have been made for conduction in the solid and fluid regions as well as radiation in the fluid regions. A uniform temperature boundary condition of 101.7°C is used at the outside of the canister to conservatively model canister immersed in boiling water. Symmetry boundary conditions were employed such that only one-eighth of the canister was modeled.Steady-state simulations are performed for different fuel heat generation rates and helium pressures, ranging from atmospheric pressure to 100 Pa. Constant thermal accommodation coefficients, which characterize the effect of the temperature-jump thermal resistance at the gas-surface interface are employed. The peak cladding temperature and its radial and axial locations are reported. The maximum allowable heat generation that brings the cladding temperatures to the radial hydride formation limit (TRH = 400°C) is also reported. The results of the three-dimensional model simulations are compared to two-dimensional simulations for the same heat generation rate and pressures. The results show that the rarefaction condition causes the temperature of the rods to significantly increase compared to the continuum condition, which has no temperature-jump. This causes the maximum allowable heat generation for the rarefied condition to decrease. The three-dimensional model predicts temperatures that are ~15°C to 35°C lower than the two-dimensional model.