The overriding post-closure safety objective of a deep geological repository is “the long-term protection of humans and the environment from the effects of ionising radiation, without imposing undue burdens and obligations on future generations” (English version of ENSI Guideline G03 ENSI 2023). Before a disposal system can be implemented at a site, a post-closure safety case must be made that clearly demonstrates that this protection objective is met. This is in accordance with the International Atomic Energy Agency (IAEA) Joint Convention on the Safety of Spent Fuel and on the Safety of Radioactive Waste Management (IAEA 1997), to which Switzerland is a signatory.

Recently, the Swiss waste management programme has mainly been concerned with finding the most suitable site for the disposal of the nation’s radioactive waste. This followed the demonstration of the general feasibility of disposal of L/ILW (Nagra 1985) and of HLW and ILW (Nagra 2002) in Switzerland, its review by the regulatory body (HSK 1986, HSK 2005), and approval by the government (Schweizerischer Bundesrat 1988, Schweizerischer Bundesrat 2006). During the siting process, knowledge of the geology within the potential siting regions has significantly increased, design aspects of the repository have been refined, and process understanding has further improved. Safety analyses performed regularly by Nagra have continuously confirmed that the waste can safely be disposed of in Switzerland. The formal demonstration of post-closure safety of a repository in Nagra’s preferred siting region is set out in the present report as an up-to-date safety case.

A comparison of the present safety case with the most recent previous safety case in Switzerland (Nagra 2002), reveals the following similarities and differences (App. A gives a more detailed comparison):

  • The same host rock, the Opalinus Clay, a clay rock formation found in large areas of Northern Switzerland, and a broadly similar safety and repository concept are considered.

  • Significantly increased knowledge is now available for the siting region, due to an extensive geological characterisation programme (see, e.g., Chapter 7 of the Geosynthesis of Northern Switzerland, NTB 24‑17, Nagra 2024i).

    • The favourable properties of the Opalinus Clay have been confirmed and uncertainty in many of the parameter values used in safety assessment (e.g., diffusivity, sorption capacity) have been reduced.

    • The favourable properties of the confining geological units and the hydrogeological situation in the siting region have been characterised in detail and can now be relied upon in the safety case.

    • The proposed depth of the repository has been confirmed as sufficient to efficiently protect the repository from long-term surface processes such as deep glacial erosion.

  • Geomechanical properties of the host rock have been re-assessed (NAB 24-10 Rev. 1, Nagra 2024h) and the technical feasibility of constructing and operating the repository has been confirmed with ample robustness at the depth under consideration (NAB 23-01 Vol. 1 – 9, Nagra 2023a).

  • Evolution of the repository design

    • Rather than two separate repositories for HLW and L/ILW, as considered in Nagra 2002 and earlier work, a provisional design has been developed for a repository for both HLW and L/ILW, with common access structures and separate repository sections for HLW and L/ILW. A repository for both HLW and L/ILW has been shown to meet the operational and post-closure safety requirements, as would two separate repositories, but has clear advantages in terms of construction-related risks, resource consumption and greenhouse gas emission, as fewer facilities have to be constructed and operated (Nagra 2021c).

    • The provisional design of the repository and its components considers recent technical engineering developments. This allows, e.g., the emplacement drifts and the rock support to be optimised.

The aim of the present post-closure safety report, which is the synthesis of the post-closure safety case, is to demonstrate concisely the long-term (post-closure) safety of the planned deep geological repository for both HLW and L/ILW. In other words, the report presents the outcome of a safety assessment, which is the means by which the safety case is developed. In the context of this report, safety assessment is defined by Nagra as the process of gathering all relevant evidence and arguments and carrying out analyses for the safety of the disposal system during the post-closure phase.