The CRZ encompasses several complementary barrier elements, addressed in PA as separate assessment objects, each of them contributing to one or several safety functions of the geological barrier. The Opalinus Clay represents the diffusion-dominated primary geological barrier and provides long-term confinement of the waste, ensuring a stable geochemical and geomechanical environment. The upper and lower confining geological units serve as additional transport barriers.

The geological barrier, in particular the Opalinus Clay, has been subjected to extensive investi­gations for more than 20 years, comprising empirical studies and dedicated site investigations, with flow and transport experiments at a borehole scale and at a core scale and including experiments within the scope of Nagra’s recent deep drilling campaign as described in the geosynthesis report NTB 24‑17 (Nagra 2024i). There is ample evidence that the intact Opalinus Clay represents a diffusion-dominated transport barrier with sufficient vertical and lateral extent and that the confining units also have favourable properties.

The diffusive properties of the Opalinus Clay and the confining units are well characterised as documented, e.g., in NTB 23-08 (Glaus et al. 2024a) and NAB 23-26 (Van Loon et al. 2024).

Excavation Damage Zone (EDZ)

A large body of evidence exists, showing that the shape, extent, and characteristics of the EDZ in the Opalinus Clay around the HLW emplacement drifts and the L/ILW emplacement caverns impairs the barrier functions of neither the CRZ nor the engineered barriers. The following points summarise the main arguments (for details, see Section 3.3.3 in NTB 24‑22 Rev. 1, Nagra 2024u):

  • the choice of the excavation method and tunnel support limits the extent of the EDZ, such that it does not significantly impact the transport distance of radionuclides through the Opalinus Clay,

  • simulations show that the resealed EDZ around the HLW drifts and L/ILW caverns does not represent a significant axial transport path for dissolved radionuclides, the possibility of which is, in any case, minimised by the dead-end architecture of the HLW drifts and L/ILW caverns,

  • the hydromechanical evolution and self-sealing of the EDZ limits microbial activity,

  • the higher porosity within the EDZ around the HLW drifts and L/ILW caverns provides an additional storage volume for repository-generated gas, further limiting gas pressure build-up,

  • the existence of an EDZ around the HLW emplacement drifts reduces the loads on the drift lining (Geomechanica Inc. 2025), and

  • the evolution of the EDZ around the backfilled emplacement rooms in response to eventual failure of the lining is limited, because the swelling pressure of the bentonite and the low compressibility of the L/ILW backfill compensates for the loss of strength of the lining.