Site selection for the best clay-hosted repository in Switzerland

Abstract. The site selection process for the deep geological repositories in Switzerland is in its final phase. All three remaining sites fulfil the requirements of the repositories for low level waste (LLW) as well as for high level waste (HLW) and spent fuel. Using surface-based exploration methods, including 3D seismic studies and deep bore holes, Nagra has recently collected the necessary data to select the most suitable site. The sites will be compared based on 13 technical criteria defined in 2008 and already used in the 2 previous phases of the selection process. The criteria comprise safety-related aspects including the barrier properties and their long-term stability as well as the construction suitability of the repository and its access facilities. If one site offers optimal conditions for both HLW and LLW and the space requirements can be met at that site, a co-disposal facility will be proposed. This facility will then comprise separate emplacement areas with specific safety concepts for the different waste categories. In our contribution we present the overall approach for the surface-based exploration in the different stages of the site selection process. We show how we integrated third party data, seismic surveys, surface mapping as well as deep and shallow bore holes to drive the site selection process. Example data sets from the ongoing deep drilling campaign (clay content, hydrotest data, porewater isotope profiles etc.) and recent 3D seismic surveys are used to illustrate our approach to distinguish the remaining sites according to long-term safety and to underline the large safety margin of the selected clay host rock in long-term evolution scenarios.