Qualification of clay barriers in underground repository systems

After closure of radioactive waste repositories all residual openings like boreholes, entrances to emplacement chambers, galleries, and shafts are to be sealed to avoid a release of radionuclides by the migration of fluids into the biosphere. Among others, compacted clay and clay-sand mixtures are currently investigated as major seal materials. When considering fluid migration, two-phase flow effects have to be taken into account because the pore volume of the seal materials is filled with air or gases generated by anoxic corrosion, and while in the long-term stage water or brine might flow into the repository. Within a R and D programme the parameters of single- and two-phase flow for clay-sand mixtures are determined at the GRS laboratory in Braunschweig/Germany. In order to analyse the important fluid distribution in the seal materials or geotechnical barriers, respectively, GRS performs geoelectrical monitoring in the laboratory as well as in situ. In-situ measurements are currently performed or prepared at different test sites, as for instance at Tournemire (France), Mont Terri (Switzerland), and AeSPOe(Sweden), where saturation and desaturation effects in various barriers or host rocks are of primary interest. This paper summarises the results of the laboratory and in-situ measurements obtained so far and describes the further envisaged in-situ investigations. (orig.)