Use of thermodynamic sorption models to derive radionuclide Kd values for performance assessment: selected results and recommendations of the NEA sorption project

For the safe final disposal and/or long-term storage of radioactive wastes, deep or near-surface underground repositories are being considered worldwide. A central safety feature is the prevention, or sufficient retardation, of radionuclide (RN) migration to the biosphere. To this end, radionuclide sorption is one of the most important processes. Decreasing the uncertainty in radionuclide sorption may contribute significantly to reducing the overall uncertainty of a performance assessment (PA). For PA, sorption is typically characterized by distribution coefficients (Kd values). The conditional nature of Kd requires different estimates of this parameter for each set of geochemical conditions of potential relevance in a RN’s migration pathway. As it is not feasible to measure sorption for every set of conditions, the derivation of Kd for PA must rely on data derived from representative model systems. The recently concluded NEA Sorption Project demonstrated that thermodynamic sorption models (TSMs) are uniquely suited to derive Kd as a function of conditions, because they allow a direct coupling of sorption with variable solution chemistry and mineralogy in a thermodynamic framework. The results of the project enable assessment of the suitability of various TSM approaches for PA-relevant applications.