Criticality Safety Implications of Extended-Enrichment and Accident-Tolerant Fuel for Fresh Fuel Storage

Presently, there is increased industry interest in extended enrichments (LEU+). This work investigated the effect of LEU+ in fresh fuel storage environments by modeling a representative pressurized water reactor (PWR) new fuel vault (NFV), boiling water reactor (BWR) NFV, and two PWR spent fuel pools (SFP). Nominal enrichments were increased to 6.5 and 8 wt % using SCALE 6.2.4 CSAS based on publicly available details of NFV and SFP dimensions and compositions. Additionally, several accident-tolerant fuel (ATF) concepts inspired by publicly demonstrated industry implementations allowed the examination of chromium-coated cladding, chromia-doped UO2, and FeCrAl cladding. Following 10 CFR § 50.68, the PWR NFV model was flooded to calculate the reactivity response to hydrogenous moderation at variable densities. As allowed by regulation, publicly available BWR NFVs were found to consistently implement “administrative controls and/or design features [to] prevent such flooding,” and therefore the BWR NFV, along with both PWR SFPs, modeled only fully flooded conditions. To counter increases in reactivity, absorber crediting was investigated with the addition of integral fuel burnable absorber, gadolinia rods, or reduced loading. In all cases, the use of standard UO2 fuel and zirconium cladding was the bounding reactivity condition relative to ATF concepts. Crediting 64 or fewer IFBA rods in PWR storage spaces was sufficient to meet 10 CFR § 50.68 requirements, as was removing ~1 in 4 assemblies for the BWR NFV.