NATURAL CONVECTION/RADIATION HEAT TRANSFER SIMULATIONS WITHIN THE FUEL REGIONS OF A TRUCK CASK UNDER 10CFR71- FORMAT FIRE CONDITIONS

Light water reactor nuclear fuel assemblies consist primarily of fuel rods held in square arrays by periodic spacer plates. The rods consist of heat-generating spent fuel pellets within zircaloy cladding. Spent fuel is transported away from reactors in thick wall casks. Individual assemblies are supported in square cross-section basket openings that are filled with nonoxidizing gas. Risk analysts must determine the fuel cladding temperature during and after events in which the cask is engulfed in large, long duration fires. Finite element cask models are used for that purpose. Those models typically employ Effective Thermal Conductivities (ETC) in the fuel regions. These ETC’s have been developed to model heat transfer for normal conditions of transport. However, they have not been shown to be conservative for the high temperature and transient conditions that are caused by fires. In the current work, a two-dimensional cross section model of a Legal Weight Truck (LWT) package, designed for four Pressurized Water Reactor (PWR) assemblies, is developed. Each PWR assembly consists of a 15x15 square array of heat-generating fuel rods. The Fluent computational fluid dynamics (CFD) package models three processes: 1) buoyancy-induced gas motion within the fuel regions; 2) the convective/radiation heat transfer within the fuel regions; and 3) the conduction in the solid regions. These simulations are used to determine the fuel cladding temperature during and after 10CFR71 regulatory format fires with different durations. They are also used to determine the minimum fire durations that bring the fuel cladding to its initial creep deformation and its burst rupture temperatures. The results are compared to those from finite element models that employ ETC’s in the fuel regions.