FEASIBILITY OF DIRECT DISPOSAL OF DUAL-PURPOSE CANISTERS FROM A CRITICALITY PERSPECTIVE*

Utilities are currently meeting their interim storage needs on an individual basis using largecapacity dry storage cask systems. These cask systems are typically composed of a dual-purpose (i.e., storage and transportation) canister (DPC) and an overpack. Although, there is a small percentage of single-purpose (storage only) systems also being utilized to meet storage needs. These are included under the “DPC” heading. The US Department of Energy Used Fuel Disposition Campaign is studying the feasibility of direct disposal of DPCs. Direct disposal of DPCs has the potential to avoid or reduce the amount of repackaging of commercial used nuclear fuel (UNF), which must occur otherwise, in order to dispose of the existing UNF inventory in a geologic repository. However, direct disposal of the current generation of DPCs poses several engineering and scientific challenges, one of which is the potential for criticality over extended repository time frames (e.g., 10,000 years or more). The analyses presented here investigate potential degradation scenario end-states conducive to criticality and consider scenarios in which water enters a package at some point over the repository period of interest. Existing DPCs are designed and loaded to meet current storage and transportation requirements; they were not designed or loaded with consideration of disposal scenarios and may not have neutron absorbers with the corrosion resistant properties necessary to maintain their continued efficacy over disposal time periods. However, they were loaded with conservative assumptions resulting in a certain amount of unquantified, uncredited margin that may be available to offset some of the increases in reactivity that may occur due to the effects of long-term degradation (e.g., loss of the neutron absorber material from between fuel assemblies). This study investigates some of the reactivity trade-offs associated with potential degradation of the neutron absorber and basket structures in representative, loaded DPCs and assesses the use of detailed evaluations to quantify the uncredited margin that could offset potential reactivity increases. Additionally, the reactivity impact of different groundwater composition characteristics of various geologic host media is studied. The results of this study indicate that DPC disposal criticality safety demonstration could benefit from, and may require detailed canister-specific evaluation and credit for neutron absorbers present in groundwater.