The repository is constructed around 900 m below ground, in the middle of the Opalinus Clay host rock layer, with separate repository sections for the emplacement of SF and RP-HLW (the HLW repository section) and for L/ILW and ATW (the L/ILW repository section). Regional tectonic elements and seismically mappable faults are avoided with a sufficient respect distance, which is determined by performance assessment, see Section 3.7.2 in NTB 24-22 Rev. 1 (Nagra 2024u).

The following description refers to the current repository design. Future adaptions and optimisa­tion to simplify and optimise the repository implementation will be carried out in full compliance with the regulatory requirements.

The construction of all underground structures is carried out in compliance with all relevant requirements and specifications. A respect distance to the exploratory boreholes is also observed (50 m from the disposal areas, Section 4.1.5 in NAB 24-18 Rev. 1, Nagra 2024s).

If necessary, measures are undertaken to ensure that any deviations from the planned implement­tation occurring in the pre-closure period have no impact on post-closure safety.

The HLW emplacement drifts are continuously backfilled with a bentonite buffer immediately after emplacement of the individual disposal canisters. As soon as an emplacement drift has been filled entirely, it is closed with a V1-HLW seal. The emplacement caverns for L/ILW and ATW are divided into compartments. Each compartment is backfilled with cementitious mortar within a few days after emplacement of the disposal containers. After all compartments in a cavern have been filled, the cavern is closed with a V1-L/ILW seal (Fig. 3‑9).

The period of repository monitoring starts once all waste has been emplaced, as described in the Waste Management Programme 2021 (Nagra 2021c). Early in this phase, the accesses to the HLW and L/ILW repository sections are backfilled and sealed at the repository level with V2 seals. At the end of the monitoring period, test areas and all other underground structures that are still open are backfilled, and the access structures are sealed with V3 seals. The closure of the repository is again carried out with state-of-the-art technologies and best engineering practices and is assumed to comply with all relevant requirements and specifications.