Criticality Safety Analysis of Spent Nuclear Fuel Canisters Using As-Loaded Configurations

Dual purpose canisters (DPCs) used for storage and transportation of spent nuclear fuel (SNF) are typically designed and evaluated using bounding (enveloping) fuel characteristics such as fuel type, fuel dimensions, initial enrichment, discharge burnup, and cooling time. This is a design basis, bounding licensing approach for SNF storage and transportation systems, as licensing and supporting safety analysis reviews are performed prior to the actual fuel loading. The bounding fuel characteristics for a system are developed by fully utilizing the regulatory and regulators recommended safety limits such as neutron multiplication factor (keff) approaching 0.95 to maximize the system utilizations. In reality, there are wide variations in SNF assembly burnups, initial enrichments, and cooling times. Therefore, dry storage systems are typically loaded with assemblies that satisfy the bounding fuel characteristics defined in the certificate of compliance (CoC), with some amount of unquantified and uncredited margin. This paper presents an as-loaded canister-specific criticality analysis approach for quantifying inherent uncredited margins in already loaded DPCs. The as-loaded analysis approach has been implemented in a new SNF management and analysis tool - The Used Nuclear Fuel-Storage, Transportation, and Disposal Analysis Resource and Data System (UNF ST&DARDS). The paper presents as-loaded criticality analysis of 616 canisters at 28 US reactor sites. Additionally, the paper discusses as-loaded analysis methodology validation approach using detailed reactor operational and fuel design data.