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).