New Methodology for Realistic Integration of Sorption Processes Safety Assessments

Sorption on mineral surfaces of sediments is one important retardation process for radionuclides to be considered in long-term safety assessments for radioactive waste repositories. In the past the Kd-concept with temporally constant values was applied to describe radionuclide retardation in the far field of a repository. This modeling approach has the advantage of being simple and computationally fast but it does not take into account the spatial and temporal changes of geochemical conditions that will occur during the evolution of the repository system, e.g. due to long-term climatic changes. One option to describe the impact of such geochemical changes on radionuclide transport and retardation is the so-called smart Kd-concept. In the present study this new modeling approach is developed allowing the consideration of geochemical changes over time without strongly affecting the computational times of transport calculations. The state-of-the-art transport program r3t, used for large model areas and very long time scales, was modified to incorporate the smart Kd-concept. The methodology is based on a description of the radionuclide sorption as a function of selected, important environmental parameters (e.g. pH, pCO2, ionic strength and Ca2+, DIC and the respective radionuclide concentration) using mechanistic sorption models.

First, this approach is developed for a typical sedimentary system covering rock salt and clay formations in Northern Germany. This system mainly consists of tertiary and quaternary sands and clays. However, the approach is applicable to any site of interest. Thermodynamic data were missing for some major minerals present in the model area, such as feldspars and mica, and for relevant elements. To fill these data gaps, an experimental program including batch and column experiments has been conducted. By prior calculations of distribution coefficients for each radionuclide and sediment as functions of the environmental parameters, multidimensional Kd-matrixes are established combining surface complexation and ion exchange. The calculations are performed coupling the geochemical speciation code PHREEQC with the parameter estimation tool UCODE. After implementation of the methodology in r3t, transport calculations considering long-term changes of geochemical conditions have been performed for selected test cases. The results of the concept applied here are very promising. The concept allows the description of radionuclide sorption and transport through large model areas over very long time frames in dependence of variable geochemical conditions.