Simulations of Heat Transfer within the Fuelassembly/backfill Gas Region of Transport Packages

A two-dimensional computational model of a spent 7x7 Boiling Water Reactor assembly in a horizontal support basket was developed using the Fluent computational fluid dynamics package. Heat transfer simulations were performed to predict the maximum cladding temperature for assembly heat generation rates between 100 and 600W, uniform basket wall temperatures of 25 and 4000C, and with helium and nitrogen backfill gases. Different sets of simulations modeled conduction/radiation and natural convection/radiation transport across the gas filled regions to assess the importance of different transport processes. Simulations that included natural convection exhibited measurably lower cladding temperatures than those that did not only for nitrogen, at the lower basket wall temperature, and within an intermediate range of heat generation rates. Outside these conditions and for helium, conduction and radiation transport are sufficiently large so that natural convection has no measurable effect. Finally, the maximum cladding temperature is more sensitive to the assumed value of the fuel cladding emissivities when nitrogen is the backfill gas than when helium is used. INTRODUCTION Spent nuclear fuel (SNF) assemblies are placed in thick-wall packages during transport [1]. These packages are constructed of multiple metal layers with external plastic or water-tank neutron shields. The assemblies are placed in a sealed containment vessel at the center of the package. Each assembly is supported horizontally in individual cells of basket structure. The containment region of the package is typically evacuated and then backfilled with a non-oxidizing, high thermal conductivity gas. Heat generated within the fuel elevates the package temperatures above its surroundings. Package designers must accurately predict the temperature of the fuel cladding that surrounds the individual rod pellets to assure that it does not exceed 350°C during normal transport [2]. The temperature distribution within the solid regions of a package can be accurately calculated using standard conduction methods. However, heat generated within the fuel is transferred to the basket structure by thermal radiation and natural convection through the backfill gas. The transport in this region and resulting temperature distribution are not well understood and are the subject of the current study. Package analysts typically employ “smeared” models of the fuel/back-fill-gas region [1,3,4]. These models employ effective thermal conductivities to predict the fuel cladding temperature distribution under steady state conditions. This effective conductivity is derived based on conduction, natural convection and radiation heat transfer analysis of the region [2,3] or based on experimental measurements [5,6]. However, it is not currently known if the temperature distribution is strongly dependent on assumptions regarding unknown properties of fuel assembly components, such as the fuel cladding emissivity. Moreover, the radiation and natural convection components of heat transfer in the fuel region are non-linear, and may not be readily represented by linear conduction models. Furthermore, it is not known how much