Comparison of Decay Time Constraints Affecting Transport of Spent Nuclear Fuel - 21079

Transportation of commercial spent nuclear fuel (SNF) requires that the transportation package satisfy design-neutral regulatory requirements established by the Nuclear Regulatory Commission (NRC) in 10 CFR Part 71, including dose rate limits and thermal limits. To do so, SNF typically must undergo radioactive decay over several years prior to shipment to reduce the thermal and radiation source terms. The regulatory limits are taken into account by shipping cask vendors when designing casks and preparing safety analysis reports for review by the NRC. During this process, vendors calculate necessary cask design-specific thermal limits and cask design-specific radiation shielding source term limits to satisfy 10 CFR Part 71 requirements. The cask design-specific thermal limits are often discussed in terms of kW of heat load, and the cask design-specific shielding source term limits are often expressed in terms of allowable combinations of SNF assembly burnup, enrichment and decay time. Upon successfully completing the NRC review process, these design-specific values are incorporated directly or by reference into a Certificate of Compliance (CoC) issued by the NRC for the cask design. For a given SNF payload, a determination of the required decay times to satisfy the conditions of the CoC can be performed to identify how long the SNF must decay before it can be shipped. This paper compares the decay times needed to meet cask design-specific thermal limits with the decay times needed to meet cask-specific shielding source term related limits using the HI-STAR 190 transport cask as an example. Decay times were examined for the casks when loaded with SNF having moderate burnup values and also when loaded with SNF having higher burnup values. The decay times needed to satisfy thermal limits were generally found to be more constraining (longer in duration) than those needed to satisfy shielding source term related limits. These results suggest that the decay times needed to meet cask thermal limits might tend to drive transportation readiness dates for normal, zirconium-clad SNF having moderate to high burnup values overall. Notably, lower burnup SNF was not examined here, and may behave differently. Furthermore, additional modeling detail would be necessary to capture special restrictions on damaged fuel and casks containing certain non-fuel hardware, which were not examined in this paper. (authors)