Prior to the SR-Site safety assessment, Kemakta was commissioned to develop a database of site-specific distribution coefficients (Kd values) for the sorption of radionuclides in the Forsmark geosphere.
Sorption data are typically obtained from experiments on crushed rock samples of various size fractions to enable rapid diffusion equilibrium (approximately six months or less). Since sorption is a surface-reactive process, a significant issue has been that the measured sorption is not representative of undisturbed rock. Kd values obtained from experimental data on crushed material may significantly overestimate sorption in a way that is not desirable for the purposes of safety analysis. Another issue is that Kd values are only valid for the exact groundwater composition used in the laboratory experiments. Deviations in composition can lead to significant changes in sorption for certain radionuclides, which is problematic since groundwater compositions that develop over long periods may differ significantly from the current measured compositions used as reference waters in sorption experiments.
As part of the project, Kemakta developed a methodology that enabled the correction of sorption data obtained from laboratory experiments on crushed rock to derive Kd values suitable for undisturbed in-situ rock and varying groundwater compositions. The Kd values recommended by Kemakta were used in radionuclide transport calculations for the SR-Site safety analysis. The methodology was further developed for the SR-PSU safety analysis to account for the impact of alkaline leachate from concrete and cement repository components on groundwater chemistry.
