Prediction of Cladding Temperatures Within a Used Nuclear Fuel Transfer Cask Filled With Rarefied Helium

Abstract

During the used nuclear fuel vacuum drying process, helium is evacuated to pressures as low as 70 Pa, to promote water vaporization and removal. At these low pressures the gas is rarefied to the extent that there is a temperature jump thermal resistance between the surface and gas. This occurs when the mean free path of a molecule becomes a comparable to the characteristic length of a system. In order to correctly apply this jump model to a nuclear transfer cask, a two dimensional model of parallel plates and concentric cylinders were created using ANSYS/Fluent package. Heat generation was plotted against a variety of relevant pressures. The results in these simple geometries are compared to kinetic model calculations, performed by other investigators, to determine the appropriate collision diameters to use in rarefied helium gas simulations within complex geometries. A two dimensional mesh of a transfer cask containing 24 pressurized water reactor used fuel assemblies is then constructed, and the rarefied gas model was implemented in the helium-filled regions between the fuel and basket support structures. Steady state simulations with a fuel heat generation rate of 710 W/m/assemble shows that the cladding is measurably hotter when the helium gas pressure is reduced from atmospheric conditions ∼105 Pa to 500 Pa. The heat generation rate that brings the peak cladding temperature to a hydride dissolution temperature of 400°C is as much as 10% lower when the gas is at 500 Pa than under atmospheric conditions.

Design of an Experiment to Measure the Thermal Accommodation Coefficient Between Helium and Stainless-Steel in Concentric Cylinders

“…In equation (5) ζT is the temperature jump coefficient. It was demonstrated in previous work [27] that the expression of temperature jump coefficient provided by Lin and Willis [28] is in very good agreement with the kinetic simulations carried out using the Shakhov model (S-model) kinetic equation. This expression is…”

Simulation of Heat Transfer Across Rarefied Gas in Annular and Planar Geometries: Comparison of Navier-Stokes, S-Model and DSMC Methods Results

“…The heat fluxes are obtained for different values of the thermal accommodation coefficient in the range [0.2, 1], with α 1 = α 2 . The horizontal dotted lines represent the analytical expressions (11) and (14) of the free molecular limit (δ = 0). Figure 4 shows that as the value of the accommodation coefficient α decreases the dimensionless heat flux q also decreases.…”

“…One of the practical application of these geometries is the heat transfer from used nuclear fuel assemblies within canister subjected to vacuum drying operation. The gas regions that exist in the fuel transfer cask are mainly between fuels rods and between aluminum supports and enclosure (see Ref [11]). These gas regions have the size of few millimeters (∼ 3mm) and can be modeled as the gap between two coaxial cylinders and two parallel plates, respectively.…”

Geometrically-Accurate-Three-Dimensional Simulations of a Used Nuclear Fuel Canister Filled With Helium

“…Most of the geometrically-accurate models for the simulation of fuel assemblies loaded in a canister in the literature are two dimensional models [15][16][17] or represents only one fuel assembly with an isothermal enclosure [18][19][20][21][22][23][24][25]. Only few works have considered the three dimensional-geometrically-accurate simulations of transfer cask [26,27]. Current computational resources allow the use of three-dimensional (3D) models that accurately include the many fuel rods and unheated assembly components within a cask.…”