CFD Simulations of Natural Convection/Radiation Heat Transfer Within the Fuel Regions of a Truck Cask for Normal Transport

A two-dimensional finite volume mesh is constructed that accurately represents the geometry of a legal weight truck cask, including four PWR fuel assemblies inside. CFD simulations calculate buoyancy driven gas motion as well as natural convection and radiation heat transfer in the gas filled fuel regions. They also calculate conduction within the cask solid components. The cask is in a normal transportation environment. The fuel and cask temperatures are calculated for ranges of fuel heat generation rate and cladding emissivity, for both helium and nitrogen backfill gas. The cask thermal capacity, which is the fuel heat generation rate that brings the peak fuel cladding to its temperature limit, is also determined. The results are compared to simulations in which the gas speed is set to zero, to determine the effect of buoyancy induced motion. The allowable heat generation rate is 23% higher for helium than for nitrogen due to helium’s higher thermal conductivity. Increasing the cladding emissivity by 10% increases the allowed fuel heating rate by 4% for nitrogen, but only 2% for helium. The higher value for nitrogen is caused by the larger fraction of heat transported by radiation when it is the backfill gas compared to helium. The stagnant-gas calculations give only slightly higher cladding temperatures than the gas-motion simulations. This is because buoyancy induced gas motion does not greatly enhance heat transfer compared to conduction and radiation for this configuration. The cask thermal capacity from the stagnant-CFD calculation is therefore essentially the same as that from the CFD simulation. This suggests that future cask thermal calculations may not need to include gas motion. These results must be experimentally benchmarked before the CFD methods can be used with confidence for designing transport casks. Basket surface temperatures calculated in this work can be used as the basis for boundary condition in those experiments.