EVALUATION OF CONCRETES FOR HIGH LEVEL NUCLEAR WASTE REPOSITORIES

Many designs for radioactive waste confinement, particularly in deep geological underground disposal, envisage the presence of cementitious materials in conjunction with clay barriers. Two types of rocks are the main options to host a repository in Spain, granite and clay. In realistic conditions relevant to high level radioactive waste repositories, repository durability is limited by concrete interaction with clays, host-rock, and groundwater. Typical cement pore fluids have pH>12, which could induce mineralogical transformations in clay materials over long time periods, as predicted by models. This work addresses various aspects of long-term cement-based materials behavior. To assess the safety of an underground HLW repository, it is necessary to understand the behavior of the engineered barrier system within the near field. Cementitious and clay materials are used for plugging and sealing drifts and galleries and are often found in contact in existing repository designs in granitic or argillaceous rocks. During and after resaturation, highly alkaline interstitial waters from the concrete will diffuse through the clay (bentonite) barrier, producing a complex geochemical cement-bentonite system. The evolution of this system with time is of great importance to the global performance and safety assessment of the repository.