After emplacement of the HLW disposal canisters, as well as the buffer and V1-HLW seals in the emplacement drifts, the partially saturated EDZ and rock support around the tunnels resaturate with water, followed by saturation of the buffer and seals. EDZ fractures re-seal. The compacted clay structures (the buffer and the sealing elements of the V1-HLW seals) saturate relatively quickly due to their high capillary pressure (suction), though the rate of saturation is limited by the low permeability of the rock and impacted by decay heat generated by the waste. Full satu­ration is expected to occur some hundreds of years after emplacement (Chapter 7 n NAB 24-20 Rev. 1, Nagra 2024m). Saturation is sufficiently regular and any initial inhomogeneities in the buffer density are reduced over time, thus no potentially damaging stresses will be exerted on the canisters. The buffer density around the canister is sufficiently high to prevent, when saturated, microbial activity that might otherwise increase the rate of canister corrosion (Section 5.6 in NTB 19-03, Nagra 2019).

Anaerobic and reducing conditions develop due to the consumption of O₂, e.g., by canister corrosion. Radiation shielding provided by the disposal canisters is sufficient to protect the barriers from radiation-induced effects. Slow gas generation takes place due to the gradual anaerobic corrosion of the disposal canisters. Much of this gas dissolves in the porewater of the saturated bentonite buffer surrounding the canisters and diffuses through the tunnel support system into the Opalinus Clay porewater. However, some gas also migrates in the gas phase through the buffer and into the rock, where it dissolves (Chapter 6 in NAB 24-20 Rev. 1, Nagra 2024m). Gas migration causes no irreversible changes to either the buffer or the rock (Section 4.4.2 in NTB 24-22 Rev. 1,  Nagra 2024u).

In the first few thousands of years following repository closure, repository-generated heat, primarily decay heat from the waste packages, leads to a transient increase in temperature and pore pressure within and around the HLW disposal area, including within the CRZ (Chapter 6 in NAB 24-20 Rev. 1,  Nagra 2024m). However, these elevated temperatures and pore pressures have no long-term impact on the performance of the barriers. The tunnel support system around the emplacement drifts degrades and loses its mechanical strength (Section 6.7.2 in NTB 23-02 Rev. 1, Nagra 2024v), but the resulting stress redistribution has no impact on the safety-rele­vant properties of the barriers (any reactivation of EDZ fractures will be temporary, as the fractures will re-seal, see Appendix A-2 in NTB 19-03, Nagra 2019).

Porewater composition in the buffer becomes anoxic and reducing, gradually equilibrating with that in the rock (Chapter 7 in NTB 23-02 Rev. 1, Nagra 2024v). Chemical interactions, including interactions between canister corrosion products (Section 8.2.3 in NTB 23-02 Rev. 1, Nagra 2024v) or the cementitious lining and the clay barriers (Section 6.7.2 in NTB 23-02 Rev. 1, Nagra 2024v), are of limited spatial extent and do not affect the safety-relevant properties of the barriers.

In the post-closure period, but after at least ten thousand years, disposal canisters become locally breached due to mechanical failure following weakening by corrosion (Section 4.3 in NTB 24-20, Nagra 2024d). The exposure of internal metal surfaces to water results in corrosion of these surfaces and locally increased gas production rates. The evolution of the waste packages following canister breaching is described in the context of radionuclide release, retention, and transport in Section 5.4.1.