Effects of Gas Rarefaction on Used Nuclear Fuel Cladding Temperatures during Vacuum Drying

Abstract

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.

Heat Transfer Modeling of Spent Nuclear Fuel Using Uncertainty Quantification and Polynomial Chaos Expansion

“…It is clear from Table 2 that uncertainty in the boundary wall temperature (Wall T) and uncertainty in the power released by the uranium dioxide (power) have the largest effect on the maximum temperature. Similar studies have shown that heat generation rate (power) is the most influential factor in peak temperatures [25].…”

Validated Simulations of Heat Transfer From a Vertical Heated-Rod Array to a Helium-Filled Isothermal Enclosure

“…Based on simulations for a canister that contains 24 fuel assemblies [6], the Grashof number is of the order Gr $ 10 9 . After an assembly is removed from a reactor, its cladding must not exceed temperatures of approximately 400 C [7].…”

“…These models are used to predict cladding temperatures relative to different canister environments for a range of fuel heat generation rates and helium pressures. Some of these simulations accurately model the geometry of fuel rods within each basket opening [6], while others use a "smeared" fuel blocks with an effective thermal conductivity to model each assembly [12,13]. Internal temperatures at a limited number of locations within loaded canisters have been measured in a variety of packaging designs and pressures [14,15].…”

“…These validated whole-canister simulations are useful for predicting the total temperature difference between the cladding and the canister environment, and the relative magnitude of temperature differences across certain canister components. For example, the temperature difference between the environment and canister surface, as well as those across helium-fill regions within the canister, is generally larger than the differences across metal components, whose thermal conductivities are relatively high [6]. However, loaded canisters are very large and complex structures.…”

“…The temperature differences across the helium-filled regions between the basket and cladding surfaces are significantly larger than those across metal components [6]. Heat transfer across those regions is affected by conduction, natural convection, and radiation, and its geometry is somewhat complex.…”