A two-dimensional computational model of a loaded used nuclear fuel canister filled with helium gas was constructed to predict the cladding temperature during vacuum drying conditions. It includes distinct regions for the fuel pellets, cladding and helium within each basket opening. Symmetry boundary conditions are employed so that only one-eighth of the package cross-section is included, and temperature boundary conditions on the canister exterior surface in contact with water is used. Thermal modeling includes heat generation within the fuel pellets, conduction heat transfer within all solid components, and conduction and surface-to-surface radiation across the gas filled regions. The peak clad temperature is determined as a function of fuel heat generation rate, assuming atmospheric pressure helium. The allowable fuel heat generation rate, which brings the peak clad temperature to its limit is determined. The Willis solid/gas interfaces thermal-resistance model is verified against discrete-velocity-method slip-region rarefied-gas heat transfer calculated across planar and cylindrical helium filled-gaps for a range of thermal accommodation coefficients, a. The Willis model is then implemented at the solid/gas interfaces within the canister model. Simulations with a helium pressure of 100 Pa and α - 1, 0.4 and 0.2 are preformed to determine how much hotter the fuel cladding is under vacuum drying conditions compared to atmospheric pressure. The results showed that the allowed fuel heat generation rate is reduced by up to 34% for a = 0.2. Transient simulations are performed, and show that the fuel cladding temperature rises for roughly 50 hours after the loaded canister is removed from the water pool.
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