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 article extends in various directions our previous studies related to gas flow in long rectangular cross-section microchannels. In the present article, the mass flow rate of various gases through microchannels with different aspect ratios and, various surface coatings (Au and SiO 2) and surface roughnesses (from 0.9 to 12nm) is measured under isothermal conditions. Previously, we developed a method to calculate the mass flow rate through rectangular microchannels that allows taking into account the real dimensions of the rectangular channel cross-section. In the present article, this method was applied to extract the velocity slip and tangential momentum accommodation coefficients in the frame of the Maxwell diffuse-specular scattering kernel. An extension of the previous approach is also proposed in the present paper. This extension allows considering the possible difference in properties (roughness or material) between the vertical and horizontal channel walls by introducing different accommodation coefficients for each wall. By applying the new method, we can extract a single accommodation coefficient for all the channel walls under the assumption of homogeneous material and roughness and two different accommodation coefficients for the horizontal and vertical walls in the case when the two walls have different properties (roughness or material).

The experimental setup based on the constant volume technique is developed to measure the mass flow rate through the microtubes under the isothermal flow conditions. Four different gases: Helium, Nitrogen, Argon and Carbon dioxide, and two surface materials (Stainless stainless Steel steel and Sulfinert) are considered. In this study the Knudsen number varies from ∼ 10 −4 to 0.3. In this range the approach based on the analytical solution of the Stokes equation subjected to the first and second order velocity slip boundary conditions is used. The tangential momentum accommodation coefficient (TMAC) is extracted from the experimental data on the mass flow rate using its analytical expression. The results are summarized in the tables representing the accommodation coefficients for the couples corresponding gas-surface material combinations. The influence of the molecular mass on the tangential momentum accommodation is discussed.

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