10. Demonstration of post-closure safety (NTB 24-10)

This chapter integrates the arguments and evidence elaborated in the earlier chapters of this report in a synthesis of the claims that together constitute a demonstration of the post-closure safety of a repository for both HLW and L/ILW constructed in the Opalinus Clay at the proposed site. As illustrated in Fig. ‎10‑1, these arguments justify claims concerning:

• the strength of the safety-driven processes for repository siting, design, and implementation,

• the favourable qualities of the site, the CRZ and the engineered barrier system,

• the quality and comprehensiveness of the safety assessment, including the quality of the assessment basis and its proper application when carrying out the main safety assessment processes, including performance assessment, scenario development, and the analysis of radio­logical consequences

• the favourable results of the safety assessment, and

• complementary lines of argument, including those based on natural and archaeological analogues and complementary performance and safety indicators.

Fig. ‎10‑2 illustrates how these arguments combine to provide the level of confidence in post-closure needed in order to proceed with the disposal programme, while specific aspects are given in the following sections.

The safety demonstration is further supported by more general arguments for the strength and feasibility of deep geological disposal as a waste management option as presented in Section ‎2.1.

Fig. ‎10‑1:Workflow for the post-closure safety case, highlighting the main elements of the demonstration of post-closure safety

Fig. ‎10‑2:Lines of argument in the present post-closure safety case

Nagra has proposed a repository site and developed a provisional design and implementation plan resulting from transparent, safety-driven processes that have spanned over many years; the repository siting, design and implementation processes generate confidence in the post-closure safety because they are safety-driven. Specific evidence for this argument is summarised in Fig. 10‑3 and elaborated in the following paragraphs.

1a: A safety-driven multi-stage siting process has been followed under the oversight of the regulatory authorities

The Sectoral Plan for Deep Geological Repositories (SGT) regulates the search for suitable siting regions for deep geological repositories in Switzerland. It is conducted in three stages (BFE 2008), Stages 1 and 2 have already been completed, and the general licence application, to which the present post-closure safety case belongs, is a result of the ongoing Stage 3. Clarifications on the safety-related specifications for this stage are provided in ENSI 33/649 (ENSI 2018), and have been fully followed by Nagra.

Fig. 10‑3:Lines of argument for the claim that the repository siting, design and implementation processes favour confidence in the post-closure safety because they are safety-driven

Siting regions have only progressed from one stage to the next, once they have been shown to possess multiple attributes that favour post-closure safety (as well as engineering feasibility), based on criteria developed by the Swiss Federal Office of Energy (SFOE; BFE 2008), together with other agencies and organisations that are independent of Nagra. Provisional safety assess­ments have been used throughout the siting process to support the narrowing down of potential host rocks and siting regions. The siting process has thus been safety driven from start.

The methodology adopted by Nagra for the comparison of the three siting regions in Stage 3 of the Sectoral Plan is consistent with the safety assessment methodology used for the post-closure safety case, which is summarised in Chapter ‎4 and described in Chapter 6 of NTB 24‑19 (Nagra 2024t). For a more detailed description of the methodology for site comparison and for the selection of the proposed site, the reader is referred to NTB 24‑03 (Nagra 2025a) and its supporting reports.

1b: A safety and repository concept has been developed that is mature and based on sound safety principles

The safety concept explains how the repository system ensures the protection of humans and the environment through the safety barriers it provides and the safety functions they perform. The repository concept describes these repository safety barriers and their properties. Nagra developed the safety and repository concept for the disposal of all types of radioactive waste more than two decades ago, and the main features have remained largely unchanged throughout the intervening years. This concept, for example, formed the basis for Project Entsorgungsnachweis (Nagra 2002), which successfully demonstrated the feasibility of disposing of HLW and long-lived ILW in the Opalinus Clay of Northern Switzerland, as well as for the subsequent repository siting programme. The current safety and repository concept, though still provisional, is thus considered mature. It conforms with design principles and requirements for a deep geological repository set out in the Nuclear Energy Ordinance of 2004 (KEV 2004), including that the repository ensures post-closure safety through multiple passive safety barriers and that it is designed in such a way that, during operational phase, it can, if necessary, be closed within a period of a few years. It also conforms with international principles of passive safety, optimisation of protection, and defence-in-depth.

Nagra’s current safety and repository concept is described in Chapter ‎3 of the present report and, in more detail, in NAB 24‑18 Rev. 1 (Nagra 2024s). Design principles are described as part of the contextual and regulatory framework in Chapter ‎2.

1c: A provisional repository design and implementation plan has been developed iteratively over many years in a safety-driven process supported by a wealth of evidence from RD&D

The present post-closure safety case is based on a provisional repository design and implemen­tation plan that is in line with the regularly updated Waste Management Programme NTB 21‑01 (Nagra 2021c).

The development is safety-driven, meaning that safety is the prime concern when considering refinements or larger changes to the design and implementation plan. An important aspect of this are the system analyses that are performed to establish safety-related design requirements as illustrated in Fig. 10‑4. For example, thermal analyses are performed to formulate provisional requirements on the loading of the HLW disposal canisters, their spatial separation along the HLW emplacement drifts and the lateral separation between the drifts, thus ensuring that the emplacement of heat-generating waste does not compromise the safety functions of the engineered barriers.

Fig. 10‑4:Illustration of the iterative design development process, showing the interactions with system analysis and the development of design requirements

1d: The technology for constructing, operating and closing the repository is established, available and affordable

Based on national and international experience in radioactive waste management and in other industries, the state-of-the-art technology needed for constructing, operating and closing the repos­i­tory (e.g., for further exploration of the site, excavation, handling of the waste, manu­facturing / emplacing the engineered barriers, backfilling) is established, available and affordable. Furthermore, significant technological progress, e.g., in robotics and control, can be expected in near future.

The technology for constructing, operating and closing the repository is described in NAB 23‑01 (Nagra 2023a) and NTB 24‑11 (Nagra 2024b).

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).

The quality of the safety assessment rests on a sound assessment basis, including a sound scienti­fic understanding of all relevant phenomena, a safety assessment methodology that respects national and international requirements, principles, guidance, and best practice, and models, codes and data to support both performance assessment and the analysis of radiological consequences that are fit for purpose. Specific lines of argument for the claim that a sound assessment basis, including an appropriate safety assessment methodology, has been developed and applied are summarised in Fig. ‎10‑6 and elaborated in the following paragraphs.

Fig. ‎10‑6:Lines of argument for the claim that a sound safety assessment methodology and a sound assessment basis have been developed and applied

3a: The safety assessment methodology is systematic, structured, traceable, and consistent with national and international requirements, principles, guidance and best practice

The safety assessment methodology is described in NTB 24‑19 (Nagra 2024t) and summarised in Chapter 4 of the present report. The safety assessment methodology comprises a set of distinct processes, each with its own systematic, structured, and traceable workflow; see Fig. ‎6‑1, Fig. ‎7‑1 and Fig. ‎8‑1, which present the workflows for performance assessment, for scenario development and for the analysis of radiological consequences, respectively. The methodology conforms with a set of safety assessment principles, which are themselves based on Swiss legal and regulatory requirements, international developments and guidance, and Nagra’s own experience from past safety assessments (see Section 4.1 and Chapter 3 of NTB 24‑19, Nagra 2024t). Among these principles are the systematic and rigorous consideration and treatment of uncertainty, and the assurance of comprehensiveness, as described further below.

3b: A sound scientific understanding of relevant phenomena and interactions has been developed

A sound scientific understanding of the features, events, and processes (FEPs) relevant to the disposal system and its evolution, and the interactions between these FEPs, has been developed through Nagra’s site characterisation and RD&D programmes over many years. The quality of this scientific understanding is ensured, e.g., by publication in peer-reviewed reports and journals and participation in relevant international initiatives.

Current scientific understanding of the features, events, and processes (FEPs) relevant to the disposal system and its evolution and the interactions between these FEPs has been synthesised in a supplementary volume of NAB 24-20 Rev. 1 (Nagra 2024l) and is summarised in Sections ‎5.1 to ‎5.4 of the present report.

3c: Models, codes and databases are used that are fit-for-purpose and correctly applied

Measures taken to ensure models, codes, and databases used in performance assessment and in the analysis of radiological consequences are fit-for-purpose include comparison of model outputs with the results of experiments, covering a range of spatial and temporal scales and with observations of natural systems, verification of numerical codes, e.g., by benchmarking against analytical solutions and against other codes that address similar problems, and quality control of the data feeding into the safety assessment databases. Quality assurance and control are applied to all activities that produce or apply models, codes and databases.

Individual models, codes and databases are discussed, and their reliability described, in the specific reports that they correspondingly support; see also Section ‎5.6 of the present report.

3d: A systematic treatment of uncertainty has been adopted

The body of information that is contained within the assessment basis includes information gathered from a variety of sources, and the associated uncertainty represents a mixture of epistemic and aleatory types. Epistemic uncertainty is reduced, as far as possible, e.g., by site characterisation, research, and design optimisations. Remaining uncertainties are then identified and, if possible, quantified, e.g., by specifying either most likely values and ranges or probability density functions (PDFs) for associated parameters. The management of these uncertainties is a feature of all processes within the safety assessment methodology. Uncertainty in the broad evolution of the pillars of safety is handled by defining and evaluating a reference safety scenario, a set of alternative safety scenarios and a set of future human action (FHA) safety scenarios. Uncertainty in models used in both performance assessment and in the analysis of radiological consequences, where this may have an impact on results, is, generally, handled using simplifying, conservative assumptions. The use of conservatism (see Section 9.3 for specific examples) implies that the actual performance of the repository will, in reality, be more favourable than that evaluated in quantitative analyses. Both deterministic and probabilistic techniques are used to handle parameter uncertainty. Hypothetical performance assessment scenarios and “what-if?” cases have also been systematically defined and analysed within performance assessment and the analysis of radiological consequences. These involve extreme and hypothetical assumptions, primarily aimed at demonstrating the robustness of the repository system. Such analyses may also pre-empt potential criticism that the selected ranges of parameter values are too narrow or that some detrimental FEPs are either unknown or have been overlooked.

The treatment of uncertainty is described in detail in Chapter 3 of NTB 24‑19 (Nagra 2024t) and in Section 4.4 of the present report.

3e: Measures have been adopted to ensure the inclusion of potentially relevant phenomena

Understanding of the initial state and post-closure evolution of the repository and of the charac­teristics and evolution of the site has been developed and documented iteratively over many years, informed by Nagra’s extensive RD&D programme, and documented in peer-reviewed reports. The comprehensiveness of this understanding is further assured by effective information exchange among safety assessors, technical experts within Nagra, and the broader scientific community. In parallel, a FEP database (supplementary volume of NAB 24-20 Rev. 1, Nagra 2024l) has been developed independently of the safety assessment, and reviewed by both internal and external experts, drawing on similar databases developed internationally. A FEP audit has been conducted to verify the inclusion of all relevant FEPs in the safety assessment. The audit evaluates whether all FEPs in the database are adequately addressed in the safety assessment, either through inclusion in safety scenarios or via explicit or implicit consideration in performance assessment models. Excluded FEPs are shown as outside the assessment scope or irrelevant, ensuring none are overlooked without justification.

The assurance of comprehensiveness and the FEP audit are discussed further in Sections 4.5 and 5.5 of the present report. The FEP database and FEP audit are documented in a supplementary volume of NAB 24‑20 Rev. 1 (Nagra 2024l).

Performance assessment analyses the evolution of individual system components and of the entire repository with respect to the various safety functions. The analysis of radiological consequences evaluates annual individual dose or risk for a range of safety scenarios, derived from the results of performance assessment. Since Project Entsorgungsnachweis (Nagra 2002) the body of knowledge has significantly expanded, and the assessment methodologies have evolved. Despite the advances made in the methodologies and tools used to build the safety case over the past 20 years, there is notable consistency in the safety-related findings. This consistency strengthens confidence in the Opalinus Clay as a suitable host rock for radioactive waste.

Specific lines of argument for the claim that the findings of these analyses confirm that the repository system provides a high margin of safety are summarised in Fig. ‎10‑7 and elaborated in the following paragraphs.

Fig. ‎10‑7:Lines of argument for the claim that the findings of the safety assessment confirm the high margin of safety provided by the repository system

4a: Performance assessment shows that the repository system will most likely perform as expected

In performance assessment, the expected performance of the multi-barrier systems is expressed in terms of a set of claims concerning the contribution of each component of the multi-barrier system and of the overall repository system to the repository safety functions, in line with the current safety concept. Sound arguments are provided for each claim, which are themselves supported by convincing evidence, such that the claims are deemed robust. Overall, the perfor­mance assessment shows that, following a rigorous evaluation of the functioning of the evolving disposal system and considering all identified sources of uncertainty, the system will perform its safety functions and conform to the reference safety scenario in most reasonably foreseeable circumstances. Deviations from the expected performance are captured in other, alternative, safety scenarios.

The performance assessment is synthesised in NTB 24-22 Rev. 1 (Nagra 2024u) and its findings are summarised in Chapter 6 of the present report. The development of safety scenarios, accounting for the results of the performance assessment, is described in Section 4.4 in NTB 24‑21 (Nagra 2024e), and the safety scenarios themselves are summarised in the present report in Chapter 7.

4b: The analysis of radiological consequences shows that large safety margins are available in all safety scenarios

The annual individual effective dose is the main “safety indicator” used in the analysis of radio­logical consequences. The maximum dose rates are summarised in Fig. ‎8‑14, where they are compared with both the regulatory guideline of 0.1 mSv/a and with the range of typical radiation exposure in Switzerland.

The coloured horizontal bars indicate individual calculations, e.g., for different variants within each safety scenario. For each of these variants, maximum dose rates are below both the regulatory guideline and the range of typical radiation exposures in Switzerland, often by several orders of magnitude. The figure also shows the results of a probabilistic uncertainty analysis carried out for the reference safety scenario, and the results of deterministic cases where the most sensitive parameters are set to their most pessimistic bounding values. Even in the most pessimistic cases, the maximum dose rate remains below the regulatory guideline, highlighting the robustness of the results with respect to remaining uncertainty.

The regulatory risk criterion has been applied when analysing excavation of the repository by erosive processes for the time period for assessment, i.e., up to one million years post-closure. The annual risk is found to be more than two orders of magnitude below the risk criterion of 10^‑5 per year and is so low that any impact during this period may be considered, in effect, hypothetical. Beyond one million years, the calculated dose rate is found to be within the range of natural background radiation exposure in Switzerland, which is again in accordance with the regulatory requirements.

The analysis of radiological consequences is documented in detail in NTB 24-18 (Nagra 2024p). Key aspects and results are presented in Chapter ‎8 and findings are summarised in Section ‎8.6 of the present report.

4c: Hypothetical and unfavourable “what-if?” cases confirm the robustness of the repository system

Fig. ‎8‑14 also shows the maximum dose rates calculated for “what-if?” cases that involve extreme and hypothetical assumptions. These include the occurrence of hypothetical events, as well as the hypothetical degradation of each of the pillars of safety without any specified cause. Even in these cases, the calculated dose rates remain below the regulatory dose criterion, thus confirming the robustness of the repository system.

The analysis of “what-if?” cases is part of the wider analysis of radiological consequences documented in NTB 24‑18 (Nagra 2024p) and is summarised in Chapter ‎8 of the present report.

Finally, lines of argument have been developed that are independent of, and complementary to, the main safety assessment processes. These are summarised in Fig. ‎10‑8 and in the following paragraphs.

5a: Support from studies of natural and archaeological analogues

Indirect support for the safety of deep geological repositories comes from observations of natural and archaeological analogues, including the longevity of uranium ore deposits in many different geological environments around the world, as discussed in more detail in Section 9.1. The low mobility of many radionuclides in such environments and the demonstration that clay rocks provide robust transport barriers are of particular interest. Sedimentary uranium ore deposits and studies on natural or archaeological glasses provide information relevant to the stability of SF and RP-HLW. Indications of slow, diffusion-dominated transport in clay formations is gained from isotope exchange and diffusion studies around the globe. Moreover, numerous examples exist in the oil and gas industry where clay-based seals have been demonstrated to prevent oil and gas transport completely over timescales of thousands to millions of years.

Natural and archaeological analogues, which allow processes and their effects to be observed on large temporal and spatial scales, similar to, or even exceeding, those relevant to safety assess­ment, provide insights into the durability of system components and into the effective isolation of radioactive materials in a repository. They can showcase the resilience of chosen barrier materials to potentially detrimental repository-induced effects and to long-term degra­dation processes. Analogues have been identified that yield information on the stability and performance of the geological barrier, the individual engineered barriers, and the interfaces between these barriers.

Natural and archaeological analogues relevant to the proposed repository system are summarised in Section 9.1 of the present report.

Fig. ‎10‑8:Lines of argument that are independent of, and complementary to, the main safety assessment processes

5b: Support from complementary safety indicators, which show that radionuclide releases and concentrations are well below those found in the environment

Safety and performance indicators complementary to dose and risk provide addition insights into:

• the evolution of the hazard potential of the waste,

• how radiotoxicity (a measure of this hazard potential) is distributed within the repository system and how it diminishes over time due to radioactive decay predominantly within the repository system, with little reaching the biosphere, and

• how the fluxes and concentrations of radiotoxicity are generally much smaller than, or similar to, natural or other man-made radiotoxicity fluxes and concentrations.

Complementary safety and performance indicators are described further in Section ‎9.2 of the present report. They are computed from the results of radiological consequence analysis and are documented in more detail in Chapter 7 of NTB 24-18 (Nagra 2024p).

5c: Existence of “reserve FEPs”

Conservative assumptions and simplifications are widely used in the analysis of radiological consequences. These include the omission of several “reserve FEPs”, which have the potential, in the future, to provide additional quantitative contributions to the evaluated performance of the disposal system, although many of these would likely have only minor impacts on total dose, even if they were included. Conservative assumptions and simplifications mean that the doses calculated for each of the safety scenarios err on the side of caution, being higher than would be expected in reality. Many examples are provided in Section ‎9.3 of the present report.

The integration of the safety arguments presented above further reinforces the findings from Project Entsorgungsnachweis (Nagra 2002) and subsequent investigations. The geological barrier is of primary importance due to the excellent properties and long-term geological stability of the Opalinus Clay host rock and the wider containment-providing rock zone (CRZ). The extensive research over the last 20 years conducted by Nagra and other organisations further underlines the suitability of the Opalinus Clay as an outstanding host rock for the disposal of radioactive waste.

A key role of the engineered barriers is the minimisation and mitigation of disturbances to the CRZ. The current safety and repository concept at the chosen site has been shown to be well chosen, with the site itself providing highly favourable and stable properties, as well as significant flexibility for the placement of the repository. The remaining uncertainties have been assessed as insignificant to post-closure safety and are expected to be further reduced when additional underground information becomes available.

Post-closure safety is demonstrated based on the current mature and robust, but still provisional concept and design. In future stages of the waste management programme, up until construction and finally operation, further optimisation of the current safety and repository concept and design will be carried out as discussed in the next chapter.