The favourable sorption properties of the Opalinus Clay are 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).

Many radionuclides are bound by sorption to the negatively charged clay mineral surfaces in the Opalinus Clay (Sicherheitsbericht (“Safety Report” (in German)), NTB 24-01, Section 4.4.4). The Opalinus Clay thus makes an important contribution to the retention of radionuclides through sorption.

The favourable sorption properties of the Opalinus Clay resulting from the high content of clay minerals are a central argument for the post-closure safety of the deep geological repository (Post-Closure Safety Report, NTB 24-10, Section 5.2). Most of the radionuclides that are released after canister failure will be bound by the clay minerals, thereby retaining them in the near-field. Due to this high retention capacity, the strict protection criteria of the Swiss Federal Nuclear Safety Inspectorate can be clearly met – even taking into account highly unlikely or hypothetical assumptions (Post-Closure Safety Report, NTB 24-10, Chapter 8.6). Natural analogues such as Cigar Lake confirm that the sorption properties have remained constant over temporal and spatial dimensions similar to the time period for assessment for the post-closure period of a deep geological repository (Post-Closure Safety Report, NTB 24-10, Section 9.1.2).

The high clay mineral content and the pore structure of the Opalinus Clay (Geosynthesis of Northern Switzerland, NTB 24-17, Section 5.2 and Section 5.3) ensure the sorption of the radionuclides and the spatially constant sorption properties.

The binding of radionuclides by sorption is quantitatively described over a wide spectrum of geochemical conditions. The sorption coefficients for many safety-relevant radionuclides are comprehensibly documented in a sorption database (Sorption Databases for Opalinus Clay, Confining Geological Units, and Bentonite: Methods, Concepts, and Upscaling of Data, pdf NTB 23-06, chapter 4 and 5(5.02 MB) ).