Simulation of EDZ Fracture Patterns Around the HLW- and L/ILW-Emplacement Rooms of a Deep Geological Repository in the Siting Region NL (SGT-E3)

The procedure for selecting the sites for the deep geological repositories in Switzerland was specified in the conceptual part of the Sectoral Plan for Deep Geological Repositories (SGT) (BFE 2008) and consisted of three stages. In Stage 3, the three remaining potential siting regions Jura Ost (JO), Nördlich Lägern (NL) and Zürich Nordost (ZNO) were examined in detail and, where necessary, supplementary geological investigations were performed. The broadened site-specific database formed the basis for selecting the NL siting region for a repository with a section for spent fuel and high-level waste (SF/HLW) and a section for low- and intermediate-level waste (L/ILW), respectively, for which the general licence application has been prepared by the end of 2024. A provisional repository project for HLW and L/ILW at the Haberstal site was elaborated. The technical repository layout described in the engineering dossier (Nagra 2023a, Nagra 2023b) foresees the emplacement rooms for HLW and L/ILW to be placed in the Opalinus Clay formation at a depth of around 900 m.

The assessment of long-term safety of the repository requires a reliable representation of the excavation damage zone (EDZ) around the backfilled HLW and L/ILW emplacement rooms, which may form a preferential transport path for radionuclides after repository closure. The excavation and lining concept of the repository structures have been elaborated for the provisional repository project at the Haberstal site (Nagra 2023b), based on a comprehensive geomechanical database derived in the context of a dedicated site investigation program (Nagra 2024), Nagra 2024b). In a previous performance assessment study (Nagra 2014), a comprehensive numerical workflow has been developed aimed at generating plausible (“representative”) EDZ fracture networks around the emplacement rooms of a generic HLW and L/ILW repository (Geomechanica 2013). These hypothetic EDZ fracture networks were used in a subsequent step to simulate fluid flow and radionuclide transport along the backfilled underground structures, drawing on a simplified but consistent representation of transmissive features of the EDZ (Alcolea et al. 2014).

The scope of this report is to update the previous workflow (Geomechanica 2018) by considering the latest site specific and layout specific databases, associated with the combined repository for HLW and L/ILW in the siting region NL. Probable variations of the EDZ fracture patterns are to be determined for HLW emplacement tunnels, L/ILW emplacement caverns and shaft. Simulated fracture patterns are to represent the expected shape and size of the EDZ as well as deviations thereof.