As part of a cooperative program, the United States Department of Energy (DOE) has supported analyses to determine the effect of cask loading on the thermal and shielding performance of a cask containing spent nuclear fuel. Two considerations that must be addressed in licensing spent fuel storage casks are peak fuel temperature and cask surface dose rate. Generally, storage systems are approved for uniform loading of the cask with design basis fuel. The storage system design basis typically specifies maximum assembly enrichment, maximum burnup, and minimum cooling times for the design basis fuel. Some casks specify an enrichment/burnup table. These conditions set the maximum decay heat loads and maximum radioactive source terms for the design. Supportive analysis using conservative assumptions is then used to demonstrate that acceptable fuel storage temperatures and cask dose rates are maintained. This study analyzes the effect of non.-uniform load patterns on peak fuel cladding temperatures and cask surface dose rates using previously validated analytical methods.
The study was performed using a spent fuel storage cask that was designed to hold 24 spent fuel assemblies with a decay heat load of 24 kW. The cask was assumed to have a forged steel body with an overall length of 5.0 m and a diameter of 2.3 m. The body was assumed to be surrounded by a resin layer for neutron shielding and a steel outer shell. The fuel was selected to have cooling times of 3.5 to 10 years and burnups of 20 to 60 GWd/MTU to bound the expected range of burnup for most of the fuel to be discharged from boiling water and pressurized water reactors from the mid-1970s through 2020.
Three radial power distributions were considered in the study: uniform loading, hotter assemblies in the center of the cask, and hotter assemblies near the wall of the cask. Each load pattern resulted in a total decay heat output of 24 kW from the cask. Seventeen different load patterns were selected, and the thermal analysis was repeated for three backfill gases: helium, nitrogen, and vacuum. For a given decay heat load in the cask, loading assemblies with higher decay heat output around the outside of the cask results in lower peak fuel cladding temperatures than loading hotter assemblies in the center of the cask. Several of the load patterns resulted in a peak cladding temperature that was lower than for a uniformly loaded cask. For a helium backfill with an optimum load pattern in the cask (hot assemblies near the basket wall), the peak fuel clad temperature was 17°C lower than a uniformly loaded cask. Using the same assemblies from the optimum load pattern but reversing the load pattern so the hot assemblies are moved to the inside of the cask., increased the peak fuel clad temperatures by 35°C for a helium backfill. This is 18°C greater than for a uniform load pattern.
Seven source terms were selected to provide the thermal output used in the thermal analysis. Source term calculations were completed using fuel burnups of 20 to 60 GWd/MTU and enrichments of 2.4 to 4.8%. A constant power density of 32 MW/MTU was used for all irradiation calculations. Cooling times were selected to provide the decay heat values used in the thermal analysis.
Photon dose rates are dominated by the cobalt-60 in the bottom-end fittings, top-end fittings, and plenum and are proportional to fuel burnup. For short cooling times, photon dose rates on the side of the cask are somewhat higher due to short-lived fission products. Cask loadings with high decay heat assemblies near the periphery exhibit increased photon dose rates on the side surface and top and bottom surfaces away from the centerline. Near the centerline, on the top and bottom of the cask, the dose rates are reduced substantially. Neutron dose rates increase exponentially with burnup and are nearly independent of cooling time. Cask loadings with high decay heat assemblies impact the neutron dose rates minimally. The peak dose rates (neutron plus photon) for the short-cooled, higher-burnup fuel loaded around the outside of the cask’s basket are generally less than for a uniform loading of longer-cooled, higher-burnup spent fuel.