After the emplacement of L/ILW and ATW and the backfilling and sealing of the emplacement caverns, the partially saturated EDZ and rock support around the L/ILW caverns largely resaturate with water and the EDZ fractures re-seal, although some gas-filled pores can remain as a result of repository-generated gas (see below). Some water can also enter the caverns and accumulate under gravity at the cavern floor. Anaerobic and reducing conditions develop due to the consump­tion of O² in trapped air and hyperalkaline conditions develop in the porewater within and around the caverns due to the presence of cementitious materials (see Chapter 6 in NAB 24-20 Rev. 1, Nagra 2024m).

Gas generation takes place due to corrosion of metals and degradation of organic materials in the caverns. Some of this gas dissolves in porewater within and around the caverns, with the remainder accumulating within the caverns, increasing the gas pressure and limiting further inflow of water. The V1-L/ILW seals that close the L/ILW caverns and the V2 seals that isolate the L/ILW repository section from the rest of the deep geological repository also start to saturate, but full saturation is prevented by the gas pressure in the caverns. The gas migrates through the seals and accumulates in the backfill of the adjoining underground structures, providing a storage volume for the gas in addition to that provided by the caverns themselves. Desaturated conditions persist in the upper parts of the L/ILW emplacement caverns and in the backfill of the connected underground structures for several tens of thousands to hundreds of thousands of years, until gas generation ceases. Some gas may enter the host rock but will migrate through it without causing damage. Pore clogging due to cement – clay interactions within the V1-L/ILW and V2-L/ILW seals that might impact overall seal permeability is not expected, as explained in Section 6.5 of NAB 23-21 (Martin et al. 2023), eespecially given that it is likely that the seals remain only partially saturated for the entire period of gas production (Section 7.2.4 in NAB 23-21, Martin et al. 2023).

Transient heat initially produced by cement hydration and, in the longer term, radiogenic heat from the L/ILW, and especially from ATW, leads to a transient short-term increase in temperature within and around the L/ILW disposal area. This heat, in combination with gas penetration, also leads to elevated pore pressures. These elevated temperatures and pore pressures, however, are small and have no long-term impact on the performance of the barriers (Appendix A.5 in NAB 24-20 Rev. 1, Nagra 2024m).

The disposal containers provide an initial resistance to water ingress, but they degrade over time, and, after a few hundred years, water encounters the waste matrices in at least some of the containers, and the radionuclides embedded within them start to be dissolved. ¹⁴C, mainly in the form of methane (¹⁴CH⁴), may also mix with the repository-generated gas in the emplacement caverns and closure system.

The tunnel support system around the emplacement caverns degrades and loses its mechanical strength over time, but the resulting stress redistribution has no impact on the safety-relevant properties of the barriers. In particular, the presence of backfill material limits cavern convergence to the extent that no previously sealed EDZ fractures are reactivated (Geomechanica Inc. 2025).

The cementitious near field evolves over time due to the mineralogical evolution of cement phases. A high pH is expected to be maintained for at least 100,000 years (Section 3.3.5 in NTB 23-03, Kosakowski et al. 2023). Interactions between the cementitious near field and the surrounding rock are of limited spatial extent and do not affect the safety-relevant properties of either barrier (Section 3.3.4 in NTB 23-03, Kosakowski et al. 2023). In particular, any pore clogging that occurs due to precipitation does not reduce the overall permeability to gas.