Self-sealing of argillaceous media in the context of geological disposal of radioactive waste – a perspective from past and ongoing studies

A review of discontinuity self-sealing mechanisms in argillaceous media is presented. Argillaceous rocks cover a range of siliceous materials generally defined by their particle size (<0.063 mm), composition, and the quantity and type of the clay minerals present. These properties have a strong influence on their material, rock mass and hydrogeological behaviour. In the context of geological disposal of radioactive waste in England and Wales currently (2024), the Mercia Mudstone Group and Ancholme Group are being considered as potential host rocks. These rocks are potentially suitable for the isolation and containment of radioactive waste due to their low primary permeability and potential for discontinuities to self-seal, thereby reducing secondary permeability. The ability of discontinuities to self-seal is a function of lithology, mineralogy, strength, stiffness, groundwater, groundwater chemistry and the in situ state of stress. These properties are strongly affected by depositional provenance, geological history and depth of burial. The processes and mechanisms that lead to self-sealing include swelling, creep, shear, slaking and precipitation of minerals. Self-sealing, therefore, is an important factor influencing the suitability of argillaceous media as a potential host rock for geological disposal of radioactive waste as part of the multi-barrier system.