Self-sealing of Fractures in Argillaceous Formations in the Context of Geological Disposal of Radioactive Waste

Disposal of high-level radioactive waste and spent nuclear fuel in engineered facilities, or repositories, located deep underground in suitable geological formations is being developed worldwide as the reference solution to protect humans and the environment both now and in the future. Assessing the long-term safety of geological disposal requires developing a comprehensive understanding of the geological environment. The transport pathways are key to this understanding. Of particular interest are fractures in the host rock, which may be either naturally occurring or induced, for example, during the construction of engineered portions of a repository. Such fractures could provide pathways for migration of contaminants. In argillaceous (clay) formations, there is evidence that, over time, fractures can become less conductive and eventually hydraulically insignificant. This process is commonly termed 'self-sealing'. The capacity for self-sealing relates directly to the function of clay host rocks as migration barriers and, consequently, to the safety of deep repositories in those geological settings. This report - conducted under the auspices of the NEA Clay Club - reviews the evidence and mechanisms for self-sealing properties of clays and evaluates their relevance to geological disposal. Results from laboratory tests, field investigations and geological analogues are considered. The evidence shows that, for many types of argillaceous formations, the understanding of self-sealing has progressed to a level that could justify its inclusion in performance assessments for geological repositories. (authors)