Abstract Geological disposal provides the safe long-term management solution for higher-activity radioactive waste. The development of a repository (or geological disposal facility) requires a systematic and integrated approach, taking into account the characteristics of the waste to be emplaced, the enclosing engineered barriers, and the host rock and its geological setting. Clays and clayey material are important in the development of many national geological disposal systems. Clays exhibit many interesting properties, and are proposed both as host rocks and as material for engineered barriers. Whatever their use, clays present various characteristics that make them high-quality barriers to the migration of radionuclides and chemical contaminants. As host rocks, clays are, in addition, hydrogeologically, geochemically and mechanically stable over geological timescales (i.e. millions of years).

Abstract Deep geological disposal of high-level radioactive waste (HLW) in a repository with a system of engineered and natural barriers has been recognized as an appropriate disposal concept by Chinese authorities since 2003, and both crystalline rocks and argillaceous rocks are considered as the candidate host rocks for HLW disposal repository. The 1:200 000 regional survey indicated that there are potential clay formations in Mesozoic–Cenozoic sedimentary basins in NW China. Five candidate areas have been suggested with potential clay formations including the Tamusu and Suhongtu areas with upper K 1 Bayingebi clay formations in the east Bayingebi Basin, in the Inner Mongolia Autonomous Region. On the basis of a detailed ground geological, hydrological and geophysical surveys, two test boreholes drilled to a depth of 800 m in the Tamusu area revealed that there are three lacustrine-facies clay formations (K 1 b 2-3 , K 1 b 2-2 and K 1 b 2-1 ). The thickness of the K 1 b 2-3 and K 1 b 2-2 clay formations is about 300–600 m with sandstone and siltstone interbeds, while the thickness of the K 1 b 2-1 homogeneous clay formation is more than 200 m with the depth of 450 m below the surface. The spatial extension of the clay formations could meet the fundamental criteria to ensure the long-term safety of the repository. Initial mineralogical studies on core samples indicated that the mineral assemblage is dominated by analcite, kaolinite, illite and dolomite. The homogeneous argillaceous rocks rich in analcite in Tamusu area could be a new type of host rock for a HLW disposal repository.

Abstract It is internationally accepted that the safest and most sustainable option for managing radioactive waste is geological disposal, utilizing both engineering and geology to isolate the waste and contain the radioactivity. This Special Publication contains 25 scientific studies presented at the 6th conference on ‘Clays in natural and engineered barriers for radioactive waste confinement’ held in Brussels, Belgium in 2015. The conference and this resulting volume cover many of the aspects of clay characterization and behaviour considered at various temporal and spatial scales relevant to the confinement of radionuclides in clay, from basic phenomenological process descriptions to the global understanding of performance and safety at repository and geological scales. The papers in this volume consider research into argillaceous media under the following topic areas: large-scale geological characterization; general strategy for clay-based disposal systems; geomechanics; mass transfer; bentonite evolution and gas transfer. The collection of different topics presented in this Special Publication demonstrates the diversity of geological repository research.