Transportation Licensing of Storage Canisters Update and Lessons Learned

BNFL Fuel Solutions (BFS) is using a rigorous canister-specific burnup credit (BUC) evaluation approach to license the fiftyeight VSC-24 multi-assembly sealed baskets (MSBs) currently loaded and stored at three separate utility independent spent fuel storage installations (ISFSIs) for transportation in accordance with the requirements of 10 CFR Part 71. The canister-specific BUC evaluation approach rigorously models the specific characteristics of the spent nuclear fuel (SNF) assemblies in each MSB based on reactor records rather than the bounding design-basis SNF assembly characteristics typically used in generic BUC evaluations for transportation packages. It is anticipated that, by eliminating the conservative bounding assumptions typically used for a bounding generic BUC evaluation, many more of the MSBs will be shown to satisfy the transportation criticality acceptance criteria using the canister-specific BUC evaluation approach. Preliminary canister-specific BUC evaluations show that many of the fifty-eight loaded MSBs satisfy the criticality acceptance criteria for transportation, based on current U.S. Nuclear Regulatory Commission (NRC) guidelines. However, in order to license the remainder of the MSBs for transportation, it is likely that a more aggressive BUC evaluation approach will be required. This could include BUC for fission products that do not currently have adequate benchmark data and/or a reduced administrative criticality safety margin. Based upon discussions with the NRC, application of more aggressive BUC criteria may be acceptable, given that the specific SNF assembly contents in the MSBs are well-characterized. However, because the more aggressive criteria reduce the margin of safety against criticality, those MSBs qualified using the more aggressive criticality acceptance criteria may also require a probabilistic risk analysis (PRA) to demonstrate that the probability of an accidental criticality event due to misloading under-burned or fresh fuel is sufficiently low. In this paper, we provide an overview of the canister-specific BUC evaluation approach to be used to license the fifty-eight existing loaded MSBs for transportation. In addition, we discuss a proposed PRA regulatory framework for those MSBs that require more aggressive criticality acceptance criteria.