Although it is impossible to prove beyond all possible doubt the completeness of a safety assessment, measures are taken to ensure the inclusion of a comprehensive set of potentially relevant phenomena and, of equal importance, to ensure that the safety-relevant phenomena have been represented appropriately in the safety scenarios and analyses carried out within safety assessment.

An assessment basis has been developed that includes all identified and relevant phenomena and processes, as outlined in Chapter ‎5. As part of this assessment basis, understanding of the initial state and post-closure evolution of the repository, which forms the basis for performance assess­ment, safety scenario development, and analysis of radiological consequences, is synthesised in NAB 24‑20 Rev. 1 (Nagra 2024m), while the characteristics and evolution of the site are detailed in NTB 24‑17 (Nagra 2024i). These descriptions have undergone iterative development over many years, with each version being subjected to peer review and taking into account Nagra’s extensive RD&D programme documented in NTB 21‑02 (Nagra 2021a). The comprehensiveness of these descriptions is further enhanced by effective information exchange among safety assessors, technical experts within Nagra, and the broader scientific community.

The assessment basis also includes a comprehensive catalogue of features, events and processes (FEPs), which has also been developed over many years, benefiting from reviews by both internal and external experts, and comparisons with similar international databases. A FEP audit has been conducted to verify the inclusion of all relevant FEPs in the safety assessment and to justify the omission of others. The audit evaluates whether all FEPs in the catalogue are adequately addressed in the safety assessment, either through inclusion in safety scenarios or via explicit or implicit consideration in performance assessment models and in the models used for the analysis of radiological consequences and dose assessment. It is ensured that the exclusion of any FEPs is justified by showing them to be either outside the assessment scope or irrelevant to safety. This process, documented alongside the phenomenological evolution of the repository in a supplemen­tary volume of NAB 24‑20 Rev. 1 Nagra 2024l), , guarantees a thorough assessment and justifi­cation for FEP inclusion or exclusion. Similar audits have been conducted in Project Entsorgungs­nachweis (demonstration of disposal feasibility, NTB 02-23, Nagra 2003), ensuring consistency and reliability across safety assessments. More details are also provided in Section ‎5.5.

Finally, as noted in Section ‎4.2 and discussed further in Section ‎7.4, “what-if?” cases are defined and analysed as part of the safety assessment. These “what-if?” cases 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.