Use of Geometrically-Accurate Models to Predict Spent Nuclear Fuel Cladding Temperatures Within a Truck Cask Under Normal and Fire Accident Conditions

Abstract

The temperature of spent nuclear fuel cladding within transport casks must be determined for both normal conditions of transport and hypothetical fire accident conditions to assure that it does not exceed certain limit conditions. In the current work a two-dimensional finite-element thermal model of a legal-weight truck cask is constructed that accurately models the geometry of the fuel rods and cover gas. Computational fluid dynamics (CFD) simulations are performed that include buoyancy induced motion in, and radiation and natural convection heat transfer across the cover gas, as well as conduction in all solid components. Separate simulations are performed using helium or nitrogen cover gas. Stagnant-gas CFD (SCFD) simulations are preformed and compared to CFD simulations to determine the effect of gas motion. For normal conditions of transport, the peak clad temperature is determined for a range of fuel heat generation rates to determine the thermal dissipation capacity based on peak cladding and surface temperature, QC and QS. These are respectively, the fuel heat generation rates that bring the peak cladding temperature to 400°C, or the peak surface temperature to 85°C (their allowed limits for normal transport). Transient fire/post fire simulations are then performed for a range of fire durations to determine the critical durations for cladding Creep Deformation or Burst Rupture, DCD or DBR. These are the fire durations that bring the cladding temperature to 570°C or 750°C, respectively. When the cladding temperature is used to select the fuel heat generation rate, the thermal dissipation capacity is 3265 W/assembly when helium is the cover gas, which is 30% higher when nitrogen is used (due to helium’s higher thermal conductivity). When nitrogen is the cover gas, the critical fire durations for creep deformation and burst rupture are, respectively, 3.3 and 7.2 hours. These durations are 18% and 14% shorter for helium (because the allowed fuel heat generation rate is higher for helium). When the fuel heat generation is chosen based on the package surface temperature, for helium, the thermal dissipation capacity is 1040 W/assembly, and the critical fire durations for creed deformation and burst rupture are, respectively, 4.7 and 11.6 hours. The values for nitrogen are all within 4% of these values. The CFD and SCFD simulations give essentially the same results. This indicates that gas motion does not significantly affect the cladding temperature, and the future calculations may not need to incur the increased computation expense required to model that motion.