Efficacy of argillaceous minerals used as a migration barrier in a geological waste repository

The current strategy for isolating nuclear wastes in an underground geological repository relies on a series of multiple barriers to restrict the release of radionuclides to the biosphere. Waste form and canister as well as the backfill material used to fill the space between the waste canister and the host rock, provide engineered barriers, whereas the repository itself and the overlying geologic medium represent natural barriers. A suitable backfill material, preferentially argillaceous minerals, exhibits the potential for providing a multifunctional role in the overall performance of the waste package, significantly retarding radionuclide migration via solution flow. The backfill barrier will serve both as a physical barrier, preventing convective solution flow and allowing radionuclide penetration only by diffusion, and as a chemical barrier capable of chemically interacting with the radionuclides via sorption, ion exchange or precipitation. The paper presents measurements of the hydraulic properties and the diffusion and sorption abilities of various bentonites and clays. They include investigations dealing with the sorption and retardation behaviour of the two most important fission product radionuclides, strontium and cesium. The Ksub(d)-values determined by batch distribution measurements show a strong dependence from the salt concentration in the solutions. Brine solutions bring about a displacement action upon sorbed ions. In highly compacted bentonites the transport of radionuclides through the backfill may be dominated by diffusion through the interstitial water. The rate of diffusion will thus depend on normal diffusion and on the sorption properties of the clay minerals. (author)