In the reference safety scenario, the engineered barriers are assumed to be implemented according to the provisional repository design, as described in Section 3.3. The geological barrier (i.e., the CRZ) has the characteristics inferred from site characterisation and understanding, as described in Section 5.2. The engineered and geological barriers are further assumed to evolve and perform according to the phenomenological description of their expected evolution given in NAB 24-20 Rev. 1 (Nagra 2024m) and summarised in the present report in Section 5.3. This phenomenological description is supported by the results of the performance assessment as described in NTB 24‑22 Rev. 1 (Nagra 2024u) and is also in accordance with the current safety and repository concept presented in Chapter 3 of NAB 24‑18 Rev. 1 (Nagra 2024s) and summarised in the present report in Section 3.2. Human intrusion and other future human actions, either deliberate or inadvertent, that could impair the functionality of the repository are assumed not to occur during the post-closure period in the reference safety scenario. This is justified by the placement of the repository in a deep underground location where human intrusion is unlikely and which is isolated from human actions occurring at or near the surface. Criticality safety in the post-closure phase is assumed, since it will be a design requirement of the repository, specifically, for the HLW final disposal canister and for the HLW canister loading plans. Criticality safety will be ensured through the implementation of technical and administrative measures (NAB 24-03, Nagra 2024j).

Based on the performance assessment, repository-induced effects associated with heat, gas and dissolved chemical species are expected to have minimal effects on the geological environment. Repository-generated heat could, however, lead to a narrow zone of altered bentonite around the HLW canisters, which is considered in the modelling carried out in the analysis of radiological consequences (see Appendix B, Fig. B-1, of NTB 24‑18, Nagra 2024p). Stress changes during the construction of the underground openings will result in the formation of Excavation Damaged Zones (EDZs) which will largely re-seal over time. However, their constant presence is conservatively accounted for in the modelling carried out in the analysis of radiological consequences. Repository-generated gas will permeate much of the closure system but will be largely retained underground by the V3 seals and slowly, over a period of tens of thousands of years or more, dissolve and diffuse into the host rock.

There will also be some changes associated with long-term climatic and geological processes. The repository is planned to be constructed deep underground, in a location where it is not susceptible to geological events and processes of relevance to post-closure safety. The future formation of permafrost and glaciers above the repository would affect hydraulic gradients, but the resulting variability in the gradients falls within the range of uncertainty covered by parameter variations in the analysis of the radiological consequences of this scenario. In addition, long-term glacial and fluviatile erosion processes acting at the surface will gradually reduce the thickness of the overburden above the repository. Studies of the future erosion processes indicate that, considering the 5 – 95% probability range, between ~ 350 and 670 m of overburden are expected to remain above the repository (see, e.g., Section 6.4.4 of the Geosynthesis of Northern Switzerland, NTB 24‑17, Nagra 2024i). It is also shown in Section 5.7.4 of NTB 24-17 (Nagra 2024i) that the minimum overburden of 200 m is sufficient for the Opalinus clay to maintain its self-sealing capacity. There may be some impact of the loss of overburden on the hydraulic conductivity of the Opalinus clay, but this again falls within the range of uncertainty covered by parameter variations in the analysis of the radiological consequences of this safety scenario. The impact of extreme erosion which leads to an excavation of the repository is discussed in the separate report NAB 24-08 Rev. 1 (Nagra 2024q).

The release of radionuclides from the repository during its expected evolution in the reference safety scenario and their retention and transport in the repository barriers are described phenomenologically in Section 5.4. Radionuclides will be retained within the HLW canisters until the canisters eventually become breached (see Section 6.1.), and most will be released only slowly thereafter, as the waste packages degrade. Radionuclides will be released more rapidly from the waste packages and containers for L/ILW.

Radionuclides that are dissolved in water migrate away from the waste packages predominantly by diffusion and, upon reaching one of the deep aquifers, by advection. Given the geometry of the CRZ with large horizontal distances from the repository to potential exfiltration pathways, this transport can be approximated by 1D vertical transport (see Section 8.1 for more details on the implementation). Radionuclides that form volatile species, principally ¹⁴C as methane, mix with, and are transported by, repository-generated gases through the engineered barriers and could also enter the CRZ. In contrast to the aqueous phase transport, the 3D characteristics of gas transport must be considered (see Section 8.1 for more details on the implementation). These radionuclides also eventually dissolve in porewater within the repository barrier system or within the deep aquifers. Radionuclides that do not decay in transit could eventually reach the biosphere.

Biosphere dose conversion factors (BDCFs) are calculated to convert the radionuclide release rate to dose rates by assuming different enveloping biospheres with respect to climate and geo­morphology. Different biospheres are considered via calculation cases as described below in Section 7.2.3. Since it is not possible to make a statement about when which biosphere will occur in the far future, all of the different biospheres are evaluated for the whole time period for assessment (i.e., assuming constant climatic conditions) within the reference safety scenario.