The repository system is robust, meaning that the site, the containment-providing rock zone and the engineered barrier system have many qualities that are favourable to safety, are well-understood, and that detrimental phenomena are, as far as possible, reduced or avoided. Specific evidence for this argument is summarised in Fig. ‎10‑5 and elaborated in the following paragraphs.

2a: The site is geologically stable and provides high flexibility for repository placement

The site is located in a tectonically quiet and stable area with limited deformation. It is located well away from large-scale tectonic zones characterised by high uplift, enhanced seismicity or high tectonic complexity, such as the Alps. The occurrence of large flat-lying and undisturbed host-rock zones within the site has also been conclusively demonstrated, providing a high degree of flexibility regarding the repository location and layout.

The characteristics of the site are described briefly in Section ‎5.2 of the present report, while all aspects are fully described in the Geosynthesis of Northern Switzerland report NTB 24‑17 (Nagra 2024i).

Fig. ‎10‑5:Lines of argument for the claim that the repository system is robust

2b: The CRZ has numerous favourable characteristics, including its low permeability, its capacity to retain radionuclides and its self-sealing capacity

The geological barrier provided by the CRZ makes a powerful contribution to the robustness of the current safety concept due to its excellent qualities. The Opalinus Clay host rock is characterised by a fine and homogeneous pore structure, extremely low hydraulic permeability, a homogeneous and high clay-mineral content and the associated high sorption capacity for many radionuclides and high self-sealing capacity. Due to high clay contents, the confining geological units have similarly favourable properties and ensure a sufficiently large distance between the Opalinus Clay and the nearest water-bearing rock layers, contributing to the retention of any radionuclides released from the repository. The CRZ is also characterised by favourable properties for long-term stability, especially its self-sealing capacity, thus forming an effective and stable barrier against radionuclide transport and a suitable physical and chemical environment for the engineered barriers. This is explicitly demonstrated by the extremely long residence time for Opalinus Clay porewater, which indicates highly effective containment.

The properties of the CRZ are described briefly in Section ‎5.2 of the present report, while details are found in NTB 24‑17 (Nagra 2024i).

2c: The depth of the repository protects it from disturbances at the surface

Host rock depth at the site is located within an ideal depth window, sufficient to protect the repository from future erosion and other events and processes at the surface, including most conceivable future human actions, and shallow enough to ensure technical feasibility. Excavation of the repository within the next one million years, due, for example, to glacial and/or fluvial erosion, has been comprehensively assessed and is not expected to occur within this time period.

Isolation from geological and climatic processes at the surface is described in Chapter 6 ofNTB 24‑17 (Nagra 2024i). The negligible impact of most conceivable future human actions is discussed briefly in Sections ‎7.5 and ‎8.5 and in detail in NAB 24‑09 (Nagra 2024r).

2d: The site lacks resources that could attract future human exploitation with potential impact on the barriers

No relevant natural mineral resources are located in the host rock that could attract future exploration and exploitation. Future exploration and exploitation of surface and shallow sub­surface mineral and other resources (e.g., open limestone quarries) will have no impact on a deep geological repository within the Opalinus Clay layer as these resources are well above any confining geological units and thus have no impact on the barrier system of the repository. Although the deep Permo-Carboniferous troughs could potentially contain some hydrocarbons, there is no indication of concentrations that could justify drilling and exploitation. The potential for geothermal energy production is not significantly enhanced at the site.

At a depth of around 900 m below ground, the repository is also unlikely to be affected by shallow engineering or geotechnical construction projects, by surface water management activities, by surface waste disposal activities, by surface water and groundwater pollution, by archaeological studies, or by the exploitation of shallow groundwater systems in the future. The only exception to this is the potential disturbance of the uppermost part of the access shaft. However, this part of the shaft does not contribute to post-closure safety as multiple barriers isolate it from the main repository system. The absence of resources that could attract future human exploitation is described in Dossier VII of NTB 14‑02 (Nagra 2014).

2e: The repository system includes a robust, mutually compatible and complementary set of engineered barriers

The geological barrier is complemented by a mutually compatible set of engineered barriers. A key role of the engineered barriers is the minimisation and mitigation of disturbances to the CRZ inevitably caused by the waste and its emplacement, including, for example, the effects of heat and gas generated by the waste and the disturbance to the rock caused by the excavation and ventilation of underground openings.

Engineered materials, such as steel, bentonite, and concrete are used, for which there is already a substantial knowledge base and experience in their application in many different fields, and which are chemically compatible with each other and with the surrounding rock, or for which inter­actions have limited impact. Furthermore, an appropriate distance between HLW emplacement drifts and L/ILW emplacement caverns counters possible thermal, hydraulic and chemical inter­actions between these two parts of the repository.

Key features of the engineered barriers that contribute to the safety functions and that are considered well understood are robust, and are thus classified as pillars of safety, including:

• the HLW disposal canisters, which are mechanically stable and corrosion-resistant in the expected environment and ensure complete containment of the radionuclides for a significant period of time,

• the bentonite buffer inside the HLW emplacement drifts, which provides a stable barrier against radionuclide transport, a suitable physical and chemical environment that enhances the longevity of the HLW disposal canisters and mechanical support to the host rock after the degradation of the tunnel support, and it reduces the potential impact of the heat release by the HLW on the radionuclide retention properties of the CRZ,

• the cementitious L/ILW near field, which promotes geochemical immobilisation and sorption of radionuclides, favours low rates of metal corrosion for metals and low rates of degradation of organic compounds in the repository; the low compressibility of the backfill also favours the mechanical stability of the host rock, even after the degradation of the tunnel support, and

• the closure system, which forms a stable barrier against radionuclide transport, provides mechanical stabilisation of the host rock and, together with the L/ILW near field, serves as a gas transport and gas storage system that prevents gas pressure build-up.

The performance of these pillars of safety over time and their robustness with respect to uncertainties and detrimental phenomena is evaluated in the performance assessment as summarised in Chapter 6 and in more detail in NTB 24‑22 Rev. 1 (Nagra 2024u).

The design of the repository also ensures that, in the event of intrusion by drilling, only a small part of the repository is affected. This is achieved by compartmentalisation, e.g., each HLW disposal canister is completely surrounded by massive amounts of bentonite and thus forms an isolated compartment, with no shortcut from one to the next. The limited size of the L/ILW emplacement caverns has a similar effect. Solidification of the wastes ensures that only a small fraction of radionuclides released from the waste package are in solution and can be transported to the surface rapidly along any boreholes created by humans in the post-closure period, once records of the repository are lost.

Design aspects of the engineered barriers contributing to the repository safety functions are outlined in Chapter 3 and presented in detail in NAB 24‑18 Rev.1 (Nagra 2024s).