Preliminary safety analysis of criticality for dual-purpose metal cask under dry storage conditions in South Korea

A dual-purpose metal cask is under development led by Korea Radioactive Waste Agency (KORAD) in Korea, for the dry interim storage and long-distance transportation. This cask comprises a main body made of carbon steel and a stainless steel Dry Shielded Canister (DSC), with stainless steel baskets inside to contain spent fuel assemblies. In this study, nuclear criticality safety analysis was conducted as a part of safety assessment of the metal cask. Analysis to show criticality safety in accordance with regulatory requirements of PWR spent fuel storage was carried out. 10CFR72.124 “Criteria for nuclear criticality safety” and the Regulatory Guide of the American Nuclear Society, ANSI/ANS-57.9 “Design Criteria for an Independent Spent Fuel” and US NRC's “Standard Review Plan for Spent Fuel Dry Storage Systems at a General License Facility” were employed as regulatory standard and criteria. This paper shows results of criticality analysis with respect to each designated criterion with modeling of a virtual nuclear fuel assembly and a cask body that induces the maximum reactivity among various design basis fuels of the metal cask. In addition, the sensitivity analysis of nuclear criticality taking into account the various modeling deviation such as manufacturing tolerance and modeling assumptions of conventional models was carried out to ensure the reliability of the analysis result. The criticality evaluation result of the metal cask and the maximum keff under normal and off-normal conditions were 0.36884 and 0.46255, respectively. The maximum keff under a postulated accident condition triggering flux trap reduction between baskets was evaluated to be 0.37854. In addition, the maximum keff under a postulated accident condition of a natural disaster in case of a flooding, was evaluated to be 0.94658 as a maximum reactivity. The criticality evaluations under the above storage conditions showed values below the regulatory requirement (0.95) and the values confirm adequate safety in terms of nuclear criticality.