1. Introduction (NTB 24-10)

In Switzerland, legal regulations (the Nuclear Energy Act KEG 2003 and the Nuclear Energy Ordinance KEV 2004)​ and guidelines are in place for the management of radioactive waste. The Nuclear Energy Ordinance (KEV) classifies Switzerland's radioactive waste into low- and intermediate-level waste (L/ILW), alpha-toxic waste¹ (ATW) and high-level waste (HLW), the latter comprising spent fuel (SF) and vitrified HLW from reprocessing (RP-HLW). Currently, this waste is stored safely in interim storage facilities. The Nuclear Energy Act (KEG 2003, Articles 30 and 31 requires the disposal of all types of radioactive waste in deep geological repositories. The feasibility of this disposal solution has been demonstrated for L/ILW in Nagra (1985) and for long-lived intermediate-level waste and HLW in Nagra (2002) . 

The Sectoral Plan for Deep Geological Repositories (SGT) regulates the search for suitable sites for deep geological repositories in Switzerland. This site selection process is conducted in three stages (BFE 2008)​. Stages 1 and 2, which had the goals of identifying six and then of proposing at least two potential siting regions per repository type, have already been completed and Nagra is currently in Stage 3 (SGT-E3). Clarifications on the safety-related specifications for this stage are provided in ENSI 33/649 ​(ENSI 2018)​, while ENSI Guideline G03 and the corresponding explanatory report ​(ENSI 2023, ENSI 2020)​ specify the protection objective, protection criteria and design principles for deep geological repositories. 

In SGT-E3, three siting regions located in Northern Switzerland have been considered – Jura Ost (JO), Nördlich Lägern (NL) and Zürich Nordost (ZNO) – with the objective of selecting one siting region for an HLW repository and one for an L/ILW repository, or alternatively one for a repository for both HLW and L/ILW. 

Following a comparison of the siting regions, Nagra concluded that NL is the most suitable and most flexible siting region. Therefore, it is applying for a general licence for a repository for both HLW and L/ILW, with surface facilities at the Haberstal site (municipality of Stadel, ZH). 

The justification for this selection and the safety demonstration for a repository at the site are submitted as part of the general licence application (RBG²). The reports relating to post-closure safety, shown in Fig. ‎1‑1, are structured as follows: 

• The overall safety-related argumentation, including post-closure and operational safety, is summarised in an overarching safety report (“Sicherheitsbericht”) NTB 24-01 ​ (Nagra 2025b)​ shown at the top of Fig. ‎1‑1. 

• The upper left side of Fig. ‎1‑1 shows the reports supporting Nagra’s site comparison. The available options are compared with respect to safety and technical feasibility and the results are synthesised in NTB 24-03 (Nagra 2025a)​ (“Bericht zur Begründung der Stand­ort­wahl”).    

• The upper right side of Fig. ‎1‑1 shows the reports documenting the post-closure safety case for a geological repository for both HLW and L/ILW at the selected site, including its synthesis in the post-closure safety report NTB 24-10, which is the present report and highlighted with a red box. 

• The lower middle part of Fig. ‎1‑1 shows a report that presents the current safety and repository concept, including the provisional design.  

• The bottom of Fig. ‎1‑1 shows a selection of the many reports that document the assessment basis, which encompasses the methodologies, assessment tools, databases and all other evidence and knowledge developed or acquired in support of the safety assessment.

Fig. 1‑1:Structure of reports covering post-closure safety aspects of the site comparison and the post-closure safety case

The present report is highlighted with a red box.

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.

As a synthesis, the present report is underpinned by numerous supporting reports (see Fig. ‎1‑1), as described in the context of the following report structure:

Following this introduction in Chapter ‎1, the contextual and regulatory framework guiding the development of the safety case is summarised in Chapter ‎2. Chapter ‎3 provides information on the current safety and repository concept and provisional repository design, for which the safety case has been developed. These two chapters summarise report NAB 24‑18 Rev. 1 (Nagra 2024s) on the current safety and repository concept and provisional design, which describes in more detail the safety requirements related to design and how different engineered and geological barriers contribute to system safety by means of the safety functions they perform. The methodology adopted for the safety assessment, which provides the central elements of the safety case, is described in Chapter ‎4. It is based on the extensive description of the safety assessment methodology in NTB 24‑19 (Nagra 2024t) and the methodological sections within a set of reports that describe the main processes of safety assessment, together with their outcomes, in more detail.

Chapter ‎5 then summarises the remainder of the assessment basis, where the assessment basis as a whole is defined as the evidence, knowledge, assessment tools, and methodologies developed or acquired by Nagra in support of the safety assessment. Key aspects of the assessment basis, besides the methodology, are the integrated understanding of the geology of Northern Switzerland as detailed in the Geosynthesis of Northern Switzerland in NTB 24‑17 (Nagra 2024i), the phenomenological understanding of the expected evolution of the repository described in NAB 24‑20 Rev. 1 (Nagra 2024m) and the understanding of the retention, release and migration of radionuclides (several reports, see Fig. ‎1‑1). Finally, the assessment basis also contains a features, events, and processes (FEPs) database in NAB 24‑20 Rev. 1 (Nagra 2024l) and the many models, codes and databases used in safety assessment (grouped by topic and described in the corresponding reports, see Fig. ‎1‑1).

Chapters ‎6, ‎7 and ‎8 are dedicated to three of the central processes within safety assessment, namely:

• The performance assessment comprising an analysis of the thermal, hydraulic, mechanical, and chemical evolution of the repository system. Details are given in the synthesis report NTB 24‑22 (Nagra 2024u), in NAB 24‑25, which describes in more detail the modelling work carried out for the performance assessment (Nagra 2024k) and NTB 24‑23 on the production and fate of gases within the repository (Nagra 2024o).

• Safety scenario development, which entails the identification and description of a set of safety scenarios that capture uncertainty in the broad ways in which the disposal system can evolve over time, considering all relevant sources of uncertainty, as well as the identification of additional “what-if?” cases to test system robustness, as described in detail in NTB 24-21 (Nagra 2024e).

• Analysis of radiological consequences, which consists of an evaluation of radionuclide release rates to the surface environment as a function of time for the various safety scenarios and “what-if?” cases. The main results for the present safety case are detailed in Part B of NTB 24‑18 (Nagra 2024p), and complemented by analyses linked to scenarios of future human actions in NAB 24‑09 (Nagra 2024r) and the excavation of the repository by erosion processes in NAB 24‑08 (Nagra 2024q). Furthermore, the treatment of ¹⁴C, which differs in some aspects from that of other radionuclides, is set out in NAB 24‑07 (Nagra 2024w). The fate of radionuclides that reach the surface environment and can ultimately contribute to the dose rate is discussed in detail in the biosphere report NAB 24‑06 (Nagra 2024n).

In Chapter ‎9, complementary lines of argument are made, before all the evidence, arguments and analyses from the previous chapters are brought together in Chapter ‎10 to demonstrate safety.

Since the development of the safety case is a prolonged and iterative process, an outlook for future developments is given in Chapter ‎11.

Chapter 12 provides a list of references and, in Chapter 13, a glossary is presented.

App. A highlights the main commonalities and differences of the present post-closure safety case Compared with the safety case developed for Project Entsorgungsnachweis, while App. B summarises the key phenomena identified in performance assessment, their relevance to system evolution and performance, and their potential to lead to performance deviations.  Finally, App. C considers non-radioactive but potentially hazardous materials that are chemically toxic or could pose a risk to water quality, which can also be safely contained in a deep geological repository.