Validation of COBRA-SFS with Measured Temperature Data from Spent-Fuel Storage Casks

The COBRA-SFS thermal-hydraulic code has been incorporated into the Used Nuclear Fuel-Storage, Transportation & Disposal Analysis Resource and Data System tool as a module devoted to spent-fuel-package thermal analysis. COBRA-SFS has been extensively validated and widely applied to thermal-hydraulic analysis of a large range of spent-fuel storage systems. Instead of recapping that long and detailed history, this paper summarizes the most significant and unique verification and validation of COBRA-SFS, which consists of comparison of code temperature predictions to experimental data obtained in the Test Area North Facility at the Idaho National Laboratory in the 1980s and early 1990s. These data were obtained as part of a program undertaken by the U.S. Department of Energy Office of Civilian Radioactive Waste Management for thermal performance testing of commercial spent-fuel storage cask designs. In total, four casks were tested, and all tests were performed with Westinghouse 15×15 pressurized water reactor spent fuel from the Surry or Turkey Point reactors. COBRA-SFS code results and experimental data comparisons are shown only for the CASTOR-V/21 and the TN-24P casks. CASTOR-V/21 was loaded with the highest decay heat load tested in this program, with individual assembly decay heat values up to 1.83 kW. This effectively bounds storage conditions currently contemplated for high-heat-load systems with test conditions reaching fuel cladding temperatures that approached and in some cases exceeded 400°C, the current regulatory limit for peak cladding temperature in dry storage. TN-24P, with a decay heat load of 20.5 kW, provides comparisons with experimental data that represent a realistic upper bound on typical dry storage initial conditions in independent spent fuel storage installations around the country. The consistency and accuracy of the COBRA-SFS temperature predictions in comparison to the measured data from these casks show that the code appropriately predicts the thermal-hydraulic and heat transfer behavior of these systems. The results presented here provide an excellent illustration of the capability of the COBRA-SFS code to correctly capture all three modes of heat transfer (thermal radiation, conduction, and convection) and the internal circulation of the backfill gas within a spent-fuel package in horizontal or vertical orientation.