Overview of Post-Closure Criticality Safety – RD&D Topics in Switzerland

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Abstract. The demonstration of post-closure criticality safety of spent nuclear fuel in a deep geological repository is a regulatory requirement in Switzerland and many other countries. One of the main challenges stems from the very long timescale (1 million years in Switzerland) that has to be considered. Nagra, the Swiss National Cooperative for the Disposal of Radioactive Waste, is presently elaborating the technical and scientific foundation of the criticality safety assessment in view of the upcoming general licence application for the Swiss Spent Fuel and HLW repository. In this context, Nagra supports and pursues a focussed RD&D programme in collaboration with several renowned research institutes. Nagra's safety concept relies on natural and technical barriers. For the initial thermal phase of the repository, a steel canister assures complete containment of the spent fuel. The canisters are foreseen to remain intact for approximately 10 000 years; however, the subcriticality of the system has to be ensured for a much longer period. In this context, an important part of the research activities pursued by Nagra address the nearfield evolution and the formulation of scenarios for the corresponding evolution of the canister and spent fuel system. The role that variations in the canister design and material composition have on the system's reactivity are also investigated. Other research topics focus on developing a reliable methodology for carrying out the criticality safety assessment. This symposium contribution gives an overview of the post-closure criticality RD&D activities pursued and envisioned by Nagra. The general context and Nagra's fundamental approach to elaborating the current phase of the criticality safety assessment are presented first. Following this, the current RD&D landscape and the most important technical considerations underpinning Nagra's technical basis for the post-closure criticality safety assessment in particular are discussed. Future planned research topics and points of interest are also presented as an outlook of this presentation.