Thermal Performance of Vertical Dry Cask for Storage of High-Burnup Used Fuel

Modeling of the temperature profiles of dry casks was identified as a high-priority item in a Department of Energy gap analysis. In this work, a three-dimensional model of a vertical dry cask was constructed for computer simulation by using the ANSYS/FLUENT code. The vertical storage cask contains a welded canister for 32 used fuel assemblies from a pressurized water reactor (PWR), with a total decay heat load of 34 kW. To simplify thermal calculations, we developed an effective thermal conductivity model for a 17x17 PWR used (or spent) fuel assembly and employed it in the simulation of thermal performance. The effects of canister fill gas (helium, nitrogen), internal pressure (1-6 atm), and basket material (stainless steel or aluminum alloy) were studied to determine the peak cladding temperature and the canister surface temperatures. The results showed that high thermal conductivity of the basket material greatly enhances heat transfer and reduces the peak cladding temperature. The results also showed that natural convection affects both peak cladding temperature and the canister surface temperature profile, while the latter depends strongly on the type of fill gas and canister internal pressure. Of particular interest to condition and performance monitoring is the identification of canister locations where significant temperature change occurs after a canister breach, where the fill gas changes from high-pressure helium to ambient air. This study provided insight on the thermal performance of a vertical storage cask containing high- burnup fuel, and helped advance the concept of monitoring canister surface temperatures as a means to detect helium leakage from a welded canister.