A two-dimensional computational model of a loaded used nuclear fuel canister filled with dry helium gas was constructed to predict the cladding temperature during vacuum drying conditions. The model includes distinct regions for the fuel pellets, cladding and helium within each basket opening, and it calculates conduction heat transfer within all solid components, heat generation within the fuel pellets, and conduction and surface-to-surface radiation across the gas-filled regions. First steady-state simulations are performed to determine peak clad temperatures as a function of fuel heat generation rate, assuming the canister is filled with atmospheric-pressure helium. The allowable fuel heat generation rate, which brings the peak clad temperature to its limit is evaluated. The discrete-velocity-method is then used to calculate slip-regime rarefied-gas conduction across planar and cylindrical helium-filled gaps. These results are used to verify the Lin-Willis solid/gas interface thermal-resistance model for a range of thermal accommodation coefficients, α. The Lin-Willis model is then implemented at the solid/gas interfaces within the canister model. Finally, canister simulations with helium pressures of 100 and 400 Pa and α = 1, 0.4 and 0.2 are performed to determine how much hotter the fuel cladding is under vacuum drying conditions compared to atmospheric pressure. For α = 0.4, the fuel heat generation rates that bring the clad temperature to its allowed limit for helium pressures of 400 and 100 Pa are reduced by 10% and 25%, respectively compared to atmospheric-pressure conditions. Transient simulations show that the cladding reaches it steady state temperatures roughly 20 to 30 hours after water is removed from the canister.
This paper presents preliminary results of heat transfer simulations performed in geometrically-accurate-three-dimensional model of nuclear fuel canister filled with helium. The numerical model represents a vertical canister, which relies on natural convection as its primary heat transfer mechanism, containing 24 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 filled with helium at atmospheric pressure. A constant temperature of 101.7°C is employed on the canister outer surfaces, assuming the canister to be surrounded with water. These conditions of pressure and temperature were considered, in this paper, for comparison purpose with previous work. The effects of buoyancy-induced gas motion and natural convection, along with radiation and conduction through gas regions and solid are considered. Steady state simulations using ANSYS/Fluent were performed for different heat generation rates in the fuel regions. Simulations that include the effect of natural convection and others that do not include this effect are conducted. 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 were compared to two dimensional model simulations for the same heat generation rate. The results showed that the two-dimensional simulations overestimate the temperature in the canister by almost 70°C.
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