Development, Use, and Accuracy of a Homogenized Fuel Region Model for Thermal Analysis of a Truck Package Under Normal and Fire Accident Conditions

In the current work, a geometrically-accurate two-dimensional model is developed of an isolated fuel assembly within isothermal compartment walls. Finite difference thermal simulations are performed to determine the cladding temperature for a range of compartment wall temperatures and assembly heat generation rates. The results for zero-heat-generation-rate are used to determine a temperature-dependent effective thermal conductivity of the fuel region. The effective volumetric specific heat of the region is determined from a lumped capacity model. These effective properties are then applied to a two-dimensional model of a legal weight truck cask with homogenized (smeared) fuel regions. Steady-state normal conditions of transport simulations are performed for a range of fuel heat generation rates. The generation rate that brings the zircaloy cladding to its radial-hydride formation temperature, predicted by the homogenized model, is greater than that determined by simulations that employ an accurate-geometry fuel region model. Transient regulator fire accident simulations are then performed for a range of fire durations. The critical fire duration is defined as the minimum that brings the fuel cladding to its burst-rupture temperature. That duration is found to decrease as the fuel heat generation rate increases. The critical durations predicted by the homogenized fuel-region model are shorter than those predicted by the accurate-geometry model.