The self-sealing of fissures in the Opalinus Clay is one of the central arguments contributing to the post-closure safety of the deep geological repository (Explanatory Report on the General Licence Applications, NTB 24-12, Chapter 5).

The self-sealing capacity of the Opalinus Clay is a key property for the long-term preservation of the barrier properties, which ensures the containment and retention of radionuclides in the long term (Sicherheitsbericht (“Safety Report” (in German)), NTB 24-01, Section 4.4.5).

The swelling capacity of clay-mineral-rich rocks plays a decisive role with regard to post-closure safety: Fissures can be sealed, resulting in barrier properties that remain stable and predictable over geological timescales (Post-Closure Safety Report, NTB 24-10, Section 5.2). These properties result in large safety margins in the radiological consequence analysis (Post-Closure Safety Report, NTB 24-10, Chapter 8). Open fissures are therefore considered as alternative or hypothetical scenarios (Post-Closure Safety Report, NTB 24-10, Section 7.4). Smectites found in the iron ore mine in Kiruna were confirmed to be several hundreds of millions years old, which underpins this argument (Post-Closure Safety Report, NTB 24-10, Section 9.1.2).

Self-sealing is mainly controlled by the clay mineral content of the rock. In this context, the swelling of smectite is the key process for reducing the hydraulic permeability of discontinuities (Geosynthesis of Northern Switzerland, NTB 24-17, Section 5.7).

The other dominant parameter controlling self-sealing is the stress field. The current stress field at the site is considered favourable for robust self-sealing. With regard to the long-term evolution of the deep geological repository, a residual cover of > 200 m is also ensured to allow for the self-sealing of the Opalinus Clay at the site (Geosynthesis of Northern Switzerland, NTB 24-17, Section 5.6, Section 5.7 and Section 6.5.2.2).