Additional studies of criticality safety of failed used nuclear fuel

Commercial used nuclear fuel (UNF) in the USA is expected to remain in storage for periods potentially greater than 40 years. Extended storage (ES) time and irradiation to high burnup values (≥45 GWd t^-1) may increase the potential for fuel failure during normal and accident conditions involving storage and transportation. Fuel failure, depending on the severity, could result in changes to the geometric configuration of the fuel, which has safety and regulatory implications. The likelihood and extent of fuel reconfiguration and its impact on the safety of the UNF are not well understood. The objective of this work is to assess and quantify the impact of fuel reconfiguration due to fuel failure on the criticality safety of UNF in storage and transportation casks. Criticality analyses are conducted considering representative UNF designs covering a range of enrichments and burnups in multiple cask systems. Prior work developed a set of failed fuel configuration categories, and specific configurations were evaluated to understand trends and quantify the consequences of worst case potential reconfiguration progressions. These results are summarised here and indicate that the potential impacts on subcriticality can be rather significant for certain configurations (e.g. >20% Δk_eff). However, for credible fuel failure configurations from ES or transportation following ES, the consequences are judged to be manageable (e.g. <5% Δk_eff). The current work expands on the previous efforts by including part length rods in fresh boiling water reactor fuel assemblies and studying the effect of damage in varying numbers of fuel assemblies.