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.