Assessment of the safety reserve offered by a concrete buffer in case of a geological repository in clay

Many safety evaluations of geological repositories located in a clay formation focus on the strong contribution of the natural clay barrier to the confinement of the radionuclides present in the disposed waste. The Belgian radioactive waste management agency, ONDRAF/NIRAS, has developed a repository concept, known as 'supercontainer concept', in which the buffer surrounding the high-level waste overpacks consists of concrete. These large amounts of cementitious materials constitute an additional sorption reserve that can contribute to the long term safety of the repository. It has been shown that some anions, e.g. Cl as chloride and I as iodide, which are not or poorly retarded by the clay, are strongly sorbed upon 14 concrete phases. Other mobile anions, e.g. bicarbonate containing the radioactive isotope C, can exchange carbon isotopes with the calcite phases in the concrete.