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Abstract. Radioactive waste in Switzerland will be disposed of in a deep geological repository (DGR). Responsible for the planning and preparation of realization of this task is National Cooperative for the Disposal of Radioactive Waste (Nagra). Spent fuel assemblies (SFA) constitute the main high-level waste (HLW) stream that will be disposed in the DGR. Prior to final disposal they will be transferred or transported to an encapsulation plant, where they will be loaded into final disposal canisters. To ensure that the structural integrity of SFAs is not compromised during handling and transportation, it is desirable to characterize the expected mechanical parameters of SFAs after long-term interim storage. Experimental research activities performed at the JRC Karlsruhe include safety aspects of radioactive waste management, encompassing also spent fuel storage and spent fuel/HLW disposal activities. Nagra and JRC have established a collaboration to jointly study relevant properties and behaviours of spent fuel rods, with the support of the Gösgen nuclear power plant and of Framatome, and in collaboration with other partners in Europe and internationally. As part of this collaboration, 3-point bending and impact tests were performed at the hot-cell facilities of JRC Karlsruhe, to determine the mechanical response of spent fuel rodlets under quasi-static and dynamic loads. The structural integrity of fuel rods was also evaluated under different handling scenarios using finite element (FE) analysis. Starting with the construction of a static 3D FE model of a Pressurized Water Reactor (PWR) nuclear fuel rodlet in ANSYS Mechanical, Nagra has developed a series of FE models over the years. Mechanical properties of the original rodlet model were derived through an extensive validation process, using experimental data from the 3-point bending tests. To evaluate the mechanical response of an SFA in different loading scenarios, this model was expanded using 1D beam modeling approach. The development of the simplified 1D models is shown in this presentation. In particular, the effect of the contact formulation between the spacer grid and the rods is discussed. Finally, preliminary results of the bending response of a 15×15 PWR SFA sub-model are presented.

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.

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