In this paper, the effect of rarefaction on the fuel cladding temperature is investigated. To do this, we apply a temperature-jump thermal-resistance to ANSYS/Fluent CFD simulations of a vacuum drying operation in geometrically-accurate two and three-dimensional models of a loaded nuclear fuel canister. The numerical model represents a vertical canister and basket loaded with 24 Westinghouse 15 × 15 PWR fuel assemblies. The model includes distinct regions for the fuel pellets, cladding and gas regions within each basket opening. Symmetry boundary conditions are employed so that only one-eighth of the package cross section is included. The canister is assumed to be fdled with helium. A uniform temperature of 10I.7°C is employed on the canister outer surfaces to conservatively model canister surrounded with boiling water. Steady-state simulations are performed for different fuel heat generation rates and helium pressures, ranging from atmospheric pressure to 100 Pa. These simulations include conduction within solid and gas regions, and surface-to-surface radiation across all gas regions. 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 normal radial hydride formation limit (T_(RH) = 400°C) is also reported. The results of the three-dimensional model simulations are compared to two-dimensional model 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 (atmospheric pressure). This causes the maximum allowable heat generation for rarefied condition to decrease. The three-dimensional model predicts temperature that are ~15 to 35°C lower than the two-dimensional model.
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