Disposal of radioactive wastes in argillaceous formations

One option presently under investigation internationally for the disposal of the hazardous by-products of the nuclear fuel cycle is the construction of underground repositories in low-permeability argillaceous rock formations. The host-rock represents the most important barrier to the migration of radionuclides to the surface environment. The performance of this geological barrier will depend on a wide variety of factors, including the degree of compaction and diagenetic alteration of the rock during burial, affecting plasticity and response to deformation, stress history which determines some important structural attributes (faults, fissures, etc.), mineralogy and chemistry which are primary factors influencing radionuclide sorption and mobility, and the thickness and lithological variability of the low-permeability sequence. Our paper provides a brief overview of these considerations and identifies a number of areas of uncertainty, including our, as yet, incomplete knowledge of the precise mechanisms of groundwater movement in clay-rich media, possible departures from Darcy's Law, the occurrence of long-term transient flow, the role of 'coupled flow' phenomena, and the difficult problem of gas migration within the host-formation.