Smart Kd-concept based on Surface Complexation Modeling

Methodology. 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. Previously, the Kd-concept with temporally constant values was applied to describe the radionuclide retardation in the far field of a repository.
In this study, pre-calculated distribution coefficients (Kds) based on surface complexation models (SCM) are implemented in the existing 3D transport program r3t [1]. The so-called smart Kd-values are calculated as a function of important environmental parameters to reflect changing geochemical conditions. Respective multi-dimensional Kd-matrices are generated and stored a-priori to any r³t run. The calculations follow a bottom-up approach, i.e. the sorption of an element on each single mineral phase contributes to the distribution coefficient for a sediment.
Results. As an exemplary proof-of-concept, the Gorleben site (a potential repository site in Germany) was selected. Figure 1 shows the 3D plot for the logKd-matrix of UO22+ in the upper aquifer (UAF) at the Gorleben site as a function of pH, [Ca], and [DIC] (logarithmic scale). These pre-calculated logKd-values vary between -6.8 and 0.75. Comparing to the temporally constant conservative logKd of -2.7 from [2], which was previously used in r³t for the retention of UO22+ in fresh water in the upper aquifer at the Gorleben site, the resulting mean logKd of -2.74 shows a good general agreement, but account now for geochemical variations.