Swiss perspective on post-closure criticality safety assessments in the final disposal of high-level waste

Performing criticality safety assessments for a high-level waste (HLW) deep geological disposal concept is a regulatory requirement in Switzerland and many other countries. A significant challenge is posed by the long timescales, i.e. orders of magnitude larger than in any other area of the fuel cycle, over which the safety assessment is to be performed. As criticality safety in the post-closure phase is a topic specific to geological disposal, there exist limited opportunities for sharing of experience within an individual waste management organisation. Therefore, sharing of experience and knowledge between WMOs is beneficial in understanding where the approaches are similar and where they are not, and the reasons for this. To achieve this benefit, a project on Post-Closure Criticality Safety has been established through the Implementing Geological Disposal - Technology Platform (IGD-TP). This project currently has 10 participating nations, including Switzerland. Nagra, the Swiss National Cooperative for the Disposal of Radioactive Waste, is continuously elaborating the technical and scientific foundation of the criticality safety assessment for the Swiss spent fuel (SF) and HLW repository. Nagra's safety concept relies on natural and technical barriers. For the initial phase of the repository, the foreseen carbon steel canister will assure complete containment of the SF. However, the criticality safety of the system has to be ensured for a much longer period. This contribution presents the current Swiss perspective concerning post-closure criticality safety assessments as well as the identified potential for improved understanding from the project initiative. (author)