CFD SIMULATIONS OF FUEL CLADDING AND BASKET SURFACE TEMPERATURES IN AN MPC RAIL CASK DURING NORMAL TRANSPORT

A two-dimensional finite volume model of a 21 PWR Multi-Purpose Canister (MPC) inside a rail cask was constructed. In that cask, fuel is loaded into a basket structure inside an MPC. The MPC is sealed, filled with a non-oxidizing cover gas, and loaded into a transport over-pack. Steady state thermal simulations are performed for a range of fuel heat generation rates, for both nitrogen and helium cover gas, and different fuel cladding emissivities. Computational fluid dynamics (CFD) simulations are employed to calculate buoyancy induced fluid motion in, and natural convection and radiation heat transfer across, all gas filled regions. The results are compared to stagnant-gas CFD (S-CFD) simulations, and to simulations that employ Effective Thermal Conductivities (ETC) in the fuel regions. Two narrow gas-filled gaps, between the fuel basket and its support brackets, and between the MPC and the transport over-pack, account for a significant faction of the total thermal resistance between the hottest fuel and the environment. Those gap thermal resistances are strongly affected by the cover gas. The cask Thermal Dissipation Capacity (TDC) is defined as the fuel heat generation rate that causes the fuel cladding to reach its allowed temperature limit. The TDC predicted by stagnant-gas calculations is essentially the same as that predicted by the CFD simulation, and 6% higher than that predicted by the ETC model. The TDC is 27% larger when helium is the backfill gas than when nitrogen is used. A ten percent increase in cladding emissivity leads to less than a 1% increase in the TDC. The non-isothermal surface temperature profiles of the basket openings determined in this work will be used in future simulations and benchmark experiments.