Clay-rich rocks are being investigated as host rocks for radioactive waste disposal in various countries (e.g. Belgium, Canada, France, Germany, Hungary, UK). The rock types range from soft, plastic and poorly indurated clays or claystones to brittle, hard mudstones and shales, i.e. very weak to very strong rocks according to the ISRM (1981) classification. The most recent compilation of characteristics of such argillaceous rocks was published by the Clay Club of the OECD Nuclear Energy Agency (OECD/NEA 2022). Comparisons of the properties of the different clay-rich rocks are discussed e.g. by Mazurek et al. (2003; features, events and pro­cesses), Mazurek et al. (2009; natural tracer profiles) and Bock et al. (2010; self-sealing properties). Figures comparing the mineralogy, porosity, hydraulic conductivity and diffusion coefficients of the Opalinus Clay with other clay-rich rocks are included in Chapter 5.

The clay-rich rocks with the closest similarities to the Opalinus Clay are two formations under investigation in France:

1. The Callovo-Oxfordian clay host rocks for the planned Cigéo (acronym for Centre Industriel de stockage géologique), a repository for high- and intermediate-level radioactive waste in the eastern Paris basin. Detailed multidisciplinary site investigations of the Callovo-Oxfordian rocks have been conducted nearby in the Meuse/Haute Marne Underground Rock Laboratory (Delay et al. 2007, Leverd 2023).

2. Toarcian – Domerian clay-rich rocks are investigated by a multi-disciplinary research pro­gramme in an abandoned railway tunnel in the Causses Basin (see Annex III of OECD/NEA 2022).

Clay-rich rocks much weaker than the Opalinus Clay are investigated in the HADES underground research facility in the Oligocene Boom Clay in Mol (Belgium). The research programme was launched already in the 1970s and included a detailed host rock characterisation programme (see e.g. De Craen et al. 2023 as an example for geochemical aspects). Besides the Boom Clay, the Eocene Ypresian Clay (also a very weak clay formation) is considered as an alternative host rock in the Belgian programme.

Clay-rich rocks much stronger than the Opalinus Clay also play an important role as a radio­nuclide transport barrier. In the Canadian programme, the potential sedimentary host rock⁹ for the spent fuel repository is a 30 m thick very low-permeability argillaceous limestone (the Ordovician Cobourg Formation). This formation is sandwiched between more than 200 m of strong clay-rich rocks (shales) above and more than 150 m of argillaceous limestones below (NWMO 2011, 2023), all in all an excellent geological barrier system.

The host rock for the German repository project Konrad for low- and intermediate-level radio­active waste is an Oxfordian oolithic limestone succession. The geological barrier is formed by overlying Lower Cretaceous claystones and marls. Below the host rock low-permeability clay-rich rocks of the Lower Dogger and Lias Groups form a geological barrier (e.g. Rübel et al. 2019). Moreover, a variety of clay-rich formations is currently investigated as potential host rocks for the German repository for high-level radioactive waste (BGE 2020). Among them is also the Opalinus Clay in Southern Germany.

The Permian Boda Claystone Formation – an albitic clay-rich formation deposited in a saline playa lake – is being investigated in Hungary with respect to host-rock properties (see Annex III of OECD/NEA 2022, with detailed references and illustrations). The formation is highly over­consolidated due to a maximum burial of 3.5 to 4.5 km. The hydraulic characteristics of the Boda Claystone Formation partly resemble those of crystalline rocks (large difference in hydraulic conductivities between intact rock matrix and fractured rock).

In the UK programme, two mudstone successions are at early stages of investigation to assess their potential to host higher activity radioactive waste. In the inshore adjacent to the East Coast of England, the focus is on the Ancholme Group, a Middle to Upper Jurassic aged, mudstone-dominated succession. In this area, the Ancholme Group comprises a ca. 180 to 450 m thick succession dominated by clay-rich units, gently inclined towards the offshore within which areas largely unaffected by significant faults are anticipated. In the inshore adjacent to the northwest of England, a mixed succession dominated by Triassic mudrocks and evaporites of the Mercia Mudstone Group is under investigation. Here the group comprises a ca. 200 to 1'600 m thick succession preserved in an extensional sub-basin, part of the East Irish Sea Basin, at depths between ca. 50 m and > 1'600 m.

Clay-rich rocks have a known ability to act as a barrier to fluid flow in the subsurface and have therefore been investigated extensively by the hydrocarbon industry and, in recent years, for carbon capture and storage, as a common caprock and seal hindering upward migration of fluid. Data and understanding from such studies (e.g. Fisher et al. 2023) provide independent evidence for the effectiveness of the clay-rich rocks as a transport barrier (Section 5.7).