Smart Kd-concept for a realistic description of sorption processes in repository safety assessment

A key component of safety assessment for radioactive waste repositories in deep geological formations is the simulation of potential radionuclide release scenarios and the transport of radionuclides through the repository system. The realistic modelling of (hydro-)geochemical processes is of high relevance for assessing the migration of radionuclides in groundwater systems. There, one important retardation process is sorption onto mineral surfaces of the host rock / sediments. Most often conventional concepts with constant sorption coefficients (Kd values) are applied in reactive transport simulations. Such an approach has the advantage to be simple and computationally fast but cannot reflect changes in geochemical conditions that will occur during the evolution of the repository system, e.g. due to climatic changes. Due to the German safety criteria with an assessment period of 1 million years it is necessary to consider the impact of such geochemical changes on the radionuclide transport and retardation. For this, we developed a new approach, the smart Kd concept (www.smartkd-concept.de) [1]. It is considering competitive sorption on different minerals (bottom-up approach) based on mechanistic sorption models and has been implemented in reactive transport codes [2, 3]. Possible migration scenarios for repository-relevant radionuclides (isotopes of Am, Cm, Cs, Ni, Np, Pu, Ra, Se, Th and U) through a typical sedimentary rock system covering potential repository host rocks, namely salt and clay formations in Northern Germany as natural geological barrier, were developed. The resulting smart Kd-values (for U(VI) in Fig. 1) and their associated sensitivities and uncertainties are presented for a wide range of important geochemical input parameters / boundary conditions such as pH value, ionic strength, concentration of competing cations and complexing ligands, e.g. dissolved inorganic carbon (DIC) and calcium (Ca).