Geological Reasoning for the Site of the Swiss Nuclear Waste Repository

The site selection process for the deep geological repository for Swiss radioactive waste is in its final phase. Using 3D-seismics and deep boreholes, Nagra has collected the data for selecting the most suitable site. In autumn 2022, Nagra announced that a general license application would be prepared for a combined repository for high- and low-/intermediate-level waste in the Nördlich Lägern region.

The 13 technical criteria used for site comparison comprise safety-related aspects including barrier properties and their long-term stability, as well as the constructability of the repository and access facilities. Phases 1 and 2 of the process resulted in the selection of three potential sites in the external Alpine Foreland of Northern Switzerland, all with the ~100 m thick Jurassic Opalinus Clay as host rock. The sites fulfill the requirements for repositories for low- and intermediate-level waste as well as for high-level waste and spent fuel.

In the final Phase 3 of site selection, the remaining sites were investigated by means of 3D-seismics and 9 deep boreholes. The boreholes aimed at assessing the barrier properties of the clay-mineral rich rock sequence, including the bounding aquifers. To address the long-term stability of the geological barrier, the Quaternary erosion history was investigated with 11 cored boreholes.

In our contribution, we provide an overview of key results from the seismic surveying and the boreholes in the three siting regions. Based on these investigations, we present and discuss the geological differences between the regions that have led to the proposed site.