The alternative safety scenarios relate to uncertainty in the features of the CRZ, namely the possibility that there is either:

• a hydraulically active confining geological unit, or

• an undetected fault intersecting the repository.

It is expected that further site investigations for future licensing milestones will reduce the uncertainty concerning these features, and it may then be possible to exclude these alternative scenarios and their variants in future iterations of the post-closure safety case. It is also likely that hydraulically active faults that are currently undetected would be identified during repository construction, and design measures would then be implemented, so as to avoid waste emplacement in their vicinity. However, this countermeasure is disregarded in the current definition of an alternative safety scenario.

The creation of new faults, or the re-activation of existing faults, in the post-closure period by repository-induced effects, i.e., by the effects of repository heat and repository-generated gas, or by geological phenomena, has been shown in the performance assessment in Section 6.3 of NTB 24‑22 Rev. 1 (Nagra 2024u) and in the assessment of earthquake scenarios in NAB 24‑28 (Nagra 2024g) to be either implausible or exceedingly unlikely. It has also been shown that the excavation of the repository by erosive processes is hypothetical within a one-million-year time frame. These effects are, nonetheless, considered in the “what-if?” cases in Section 7.4.