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.

The main criterion concerning the planned deep geological repository comprises its long-term safety, which must be ensured for a period of hundreds of thousands of years. Therefore, particular attention is being devoted to the selection of a suitable site for the facility. As far as the Czech Republic is concerned, it is planned that the DGR will be constructed in a suitable crystalline rock mass. Nine potential sites were originally selected for consideration, all of which were subjected to detailed geological survey assessment. The DGR site selection process was preceded by the extensive collection and evaluation of the relevant data covering the detailed site description and assessment of local conditions. Recently, SÚRAO has conducted a huge amount of research at the sites via e.g. field research, terrain reconnaissance, geological mapping, hydrogeological mapping and sampling, geophysical surveys, the measurement of seismic phenomena, etc. As a result of the research, 3D geoscientific models of the rock conditions were created, which allowed for the assessment of the sites. The nine sites were compared based on safety, feasibility and environmental impact studies following which, on 21 December 2020, the government approved the selection of the four sites recommended by SÚRAO – Březový potok, Horka, Hrádek and Janoch. Further research and exploration work aimed at the eventual selection of the final and backup sites will be conducted up to 2028. The result of this complex characterization will be compiled into a set of site descriptive models describing both – geological setting and long-term evolution of the sites.

The site selection process for deep geological repository is a long-term project based on numerous disciplines. In the Czech Republic the geological characterisation was performed on nine selected sites in crystalline rocks, from which four were taken into account for ongoing geological survey and detailed site description.

This contribution will present the approach to the geological characterisation, which was, is and will be used for selection of final deep geological repository site on an example of Hrádek locality. The Hrádek locality is based in Moldanubian granitoid and metamorphic rocks of Bohemian massif in the central part of Czech Republic. In the previous phase, mostly archive data were used in combination with local field studies, in order to present a 3D geological model as an input for other models (e.g. hydraulic, transport, discrete fracture network model). The ongoing phase of geological characterisation includes mainly field studies, geological and hydrogeological mapping in combination with geophysical research and future borehole research, geophysical profiling and hydrogeological monitoring. All the data collected will be included in 3D geological model with uncertainty assessment with the aim to compare and evaluate the best site for future deep geological repository.

The upper crust of the Earth is used more and more to transport and extract raw materials and energy. It is also used for the final disposal of radioactive waste in deep geological repositories. An accurate knowledge of the hosting rock and surrounding formation coupled to comprehensive modelling are fundamental to demonstrate the geological site has the required properties for safe and long-term underground storage. Besides other criteria, geomechanics plays an important role. Especially, the estimation of the contemporary stress state in the upper crust is a challenge. In-situ data of maximum and minimum horizontal stress magnitudes (S_Hmax and S_hmin) are required to calibrate 3-D geomechanical models.

During the recent exploration phase for a deep geological repository for radioactive waste in Switzerland, a unique dataset of stress magnitude data has been acquired from eight cored boreholes. Rock mechanical properties were constrained from geophysical logging and laboratory testing. The empirically correlated rock properties were not simply averaged, but a probability distribution was provided. The stress field was explored by conducting more than 120 tests in different stratigraphic units, to estimate the magnitudes of S_Hmax and S_hmin.

We present the results of a 3‑D geomechanical-numerical model that shows the best-fit with respect to the measured stress magnitudes. Considering the uncertainties of the tests and the ones resulting from rock property variability, the model can reproduce most of the measurements. However, we do show not only the best-fit result, but a bandwidth of individual stress components within a P₀₅-P₉₅ probability range.