Radiation Shielding Analysis for a Spent Fuel Storage Cask under Normal Storage Conditions

In most cases, gamma radiation is the dominant dose contributor, but in specific configurations, neutron radiation can become significant for the overall dose rate. This occurs for canister storages where the amount of spent fuel is large and thick concrete shields or entry mazes are used for radiation protection. The design of the cask is based on the safety requirements for normal storage conditions under 10 CFR Part 72. A radiation shielding analysis of the spent fuel storage cask optimized for loading design basis fuels was performed for a single cask. For the single cask, dose rates at the external surface of the spent fuel storage cask, some distance away from the cask surface, were evaluated. The results of the shielding analysis for the single cask show that dose rates were considerably higher at the lower side (from the bottom of the cask to the bottom of the neutron shielding) of the cask. However, this is not considered to be a significant issue since additional shielding will be installed at the storage facility. The shielding analysis results showed exponential decrease with distance off the sources. The controlled area boundary was calculated to be approximately 280m from the array, with a dose rate of 20 mrem/yr. Actual dose rates within the controlled area boundary would be lower than 25mrem/yr, due to the decay of radioactivity of spent fuel in storage. Another finding of the study is that the burnup distribution of the spent fuel needs to be taken into account when assessing the yield of the neutron radiation source, because use of the assembly average burnup leads to underestimation of it. Keywords: disposal of spent nuclear fuel, radiation shielding, storage condition, dual-purpose cask, spent fuel assemblies DOI : 10.7176/APTA/79-05 Publication date :September 30 th 2019