In line with the Swiss legal and regulatory framework as well as national and international principles and practices, the deep geological repository provides safety by means of multiple geo­logical and engineered barriers, i.e. a multi-barrier system (Nagra 2024r).

Regarding post-closure safety, the geological barrier has to provide the following functions (Nagra 2024r):

• Isolation of radioactive waste from humans and the environment

• Immobilisation, retention and slow release of radionuclides

• Contribution to the stability of the repository system with regard to long-term geological and climate evolution

Furthermore, the geological and the engineered barriers must be compatible. This means that the conditions in the host rock should not influence the long-term stability of the engineered barriers in a critical way and the engineered barrier system should not adversely affect the natural barrier afforded by the host rock.

For the three siting regions, the Opalinus Clay host rock provides the primary geological barrier characterised by excellent barrier properties mainly attributed to its high clay-mineral content. The repository will be constructed in this roughly 100 m thick unit. Directly above and below the Opalinus Clay, low-permeability confining units with a partially lower and more heterogeneous clay-mineral content, further contribute to the retardation and slow transport of radionuclides.

The Opalinus Clay host rock and adjacent overlying and underlying low-permeability confining units comprise the containment-providing rock zone (CRZ)⁵. The safety concept requires the CRZ to be located at sufficient depth over the period under consideration to limit any detrimental impact of surface processes, such as erosion, on the safety-related properties of the Opalinus Clay itself and its confining units.