The so-called smart Kd‑concept treats radionuclide sorption processes in natural systems, whereby the smart Kd‑values are mainly based on mechanistic surface complexation (SC) models and ion exchange (IEx). The distribution coefficients may thus vary in time and space depending on the actual geochemical conditions. The concept developed and introduced here is based on a feasible considering the treatment of the most relevant geochemical parameters in the transport code as well as on a pre-calculated matrix of smart Kd‑values that considers the geochemistry-dependence. Sorption is described by a so-called component-additivity approach (bottom-up strategy), i.e. sorption for each element on a particular geological unit (a mixture of a number of minerals) is described as the sum of the sorption of the element on every single mineral fraction. In the first stage this approach is developed for a typical sedimentary system covering rock salt and clay formations in Northern Germany.
The approach requires a comprehensive thermodynamic database including data for SC and IEx for the mineral phases and elements (radionuclides) considered in safety assessment calculations. By defining the most relevant mineral phases in the sedimentary systems in Northern Germany and by reviewing existing sorption data, especially via the Rossendorf Expert System for Surface and Sorption Thermodynamics (RES3T database), crucial data gaps have been identified. Specific batch sorption experiments have been planned and performed to generate lacking data and to test the component-additivity approach.