Evaluation of Effect of Fuel Assembly Loading Patterns on Thermal and Shielding Performance of a Spent Fuel Storage/Transportation Cask

Abstract

Executive SummaryThe licensing of spent fuel storage casks is generally based on conservative analyses that assume a storage system uniformly loaded with design basis fuel. The design basis fuel typically assumes a maximum assembly enrichment, maximum burnup, and minimum cooling time. These conditions set the maximum decay heat loads and radioactive source terms for the design. Recognizing that reactor spent fuel pools hold spent fuel with an array of initial enrichments, burnups, and cooling times, this study was performed to evaluate the effect of load pattern on peak cladding temperature and cask surface dose rate.In 1991, DOE pursued a cooperative program with the Sacramento Municipal Utility District (SMUD). Part of the cooperative program was to demonstrate the effect of cask loading on the thermal and shielding performance of a cask containing spent nuclear fuel. At the time the cooperative agreement was established between DOE and SMUD, SMUD had fuel with cooling times ranging from 5 to 17 years, burnups of 8 to 38 GWd/MTU, and enrichments of 2.01 to 3.43% at its Rancho Seco nuclear power plant. It was thought that there was enough variability in the fuel to determine the effect of load pattern on dose rates. The effect of load pattern on the thermal performance of the casks was to be determined analytically using a computer code that had been validated extensively. With the termination of the DOE/SMUD cooperative agreement prior to cask loading, an analytical method was chosen to determine the effect of load pattern on dose rates. The codes were validated using data generated in other DOE cooperative efforts.The cask used in the analysis was similar to the TN-24P spent fuel storage cask. The TN-24P spent fuel storage cask is designed to hold 24 spent fuel assemblies with a total heat load of 24 kW. The cask body is forged steel surrounded by a resin layer for neutron shielding and a steel outer shell. The overall cask length is 16 ft (5.0 m), and the outer diameter is 7. The influence of load pattern on the thermal performance of the cask was determined through analysis with the COBRA-SFS code. The same COBRA-SFS model previously developed for analyzing the TN-24P cask was used in this parametric study. Three radial power distributions were considered. Each load pattern had an average decay heat output of 1 kW per fuel assembly for a total output of 24 kW from the fully loaded cask. Seventeen different load patterns were selected, and three backfill conditions were considered for each load pattern. The backfill media selected were helium, nitrogen, and vacuum. Except for the backfill medium and the radial power distribution, nothing was changed in the COBRA-SFS input between cases. This combination of load patterns and backfill gases provided a total of 51 runs in the test matrix.iv