As described in Section 3.3, the V1-HLW seal ensures the mechanical and chemical integrity of the bentonite buffer, because, by design, the seal prevents buffer erosion and saturation and is constructed mostly of materials that are in chemical equilibrium with the surroundings (Section 4.2 in NAB 21-12 Rev. 1, Nagra 2021b and Chapter 6 in NAB 23-21, Martin et al. 2023). The V1-HLW seal also contributes to a retarded release of dissolved and volatile radionuclides, being designed in such a way that gas release along the EDZ and lining is limited (see Section 5.3 in NAB 21-12 Rev. 1, Nagra 2021b).

The V2-HLW seal contributes to a retarded release of radionuclides and prevents any safety-relevant interaction between the HLW and L/ILW repository sections (Section 2.1 in NTB 24-23, Nagra 2024o and Section 1.2 in NAB 23-21, Martin et al. 2023) due to a design that limits the transport of volatile and dissolved radionuclides after full saturation of the seal. The V1-L/ILW and V2-L/ILW seals also contribute to a retarded release of dissolved radionuclides from the L/ILW emplacement caverns. Since the permeability of the seals is low, transport of dissolved radionuclides across the sealing elements predominately occurs by diffusion (Chapter 5 in NAB 23-21, Martin et al. 2023).

Both, the V1-L/ILW and V2-L/ILW seals also contribute to the compatibility of the barrier components because:

  • the low gas entry pressure of the sand-bentonite mixture used in the sealing elements ensures efficient gas release (see Section 3.5 in NTB 16-07, Manca 2016), preventing the occurrence of damaging gas overpressures (Chapter 6 in NTB 24-23, Nagra 2024o), and

  • chemical interactions between the materials used in the closure system and the surrounding Opalinus Clay are limited (Section 2.3 and Chapter 3 in NAB 22-24, Prasianakis et al. 2022; Chapter 6 in NAB 23-21, Martin et al. 2023; Chapter 6 in NTB 23-02 Rev. 1, Nagra 2024v and Section 3.3.4 in NTB 23-03, Kosakowski et al. 2023).

The V3 shaft seal contributes to the isolation of radioactive waste from the biosphere because, by design, the hydraulic conductivity is so low that radionuclides are only released by diffusion across the V3 seal. Thus, the V3 seal limits the release of dissolved and gaseous radionuclides because, by design, the transport of gaseous and dissolved radionuclides is largely prevented after full saturation of the seal (Section 7.2.2 in NAB 23-21, Martin et al. 2023 and Chapter 6 in NTB 24-23, Nagra 2024o). Furthermore, the V3 seal together with the deep geological location, makes inadvertent intrusion unlikely and also contributes to isolating the repository from events at the surface

The VF1 and VF2 backfills provide gas storage volumes that contribute to a low gas-pressure build-up, and, given their low hydraulic conductivity, delay and limit water flow into the operations tunnel. The stiff backfills also contribute to the mechanical integrity of the host rock by providing mechanical support. Finally, because of their clay content, the VF1 and VF2 backfills contribute to the retardation of radionuclide release along the tunnel system (Section 3.2 in NAB 21-12 Rev. 1, Nagra 2021b and Section 2.2.4 in NAB 23-21, Martin et al. 2023).