Development and Use 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 their radial hydride formation temperature predicted by the homogenized model is greater than that determined by an accurate geometry model. Transient regulator fire accident simulations are performed for a range of fire durations. The minimum fire durations that bring the fuel cladding to its burst rupture temperatures are estimated. These results are compared to simulations which employ cask models with geometrically-accurate fuel region models.