| Abstract: |
The turnover of radionuclides in parts of the biosphere is usually modeled by use of a sorption distribution coefficient, Kd. Its value has a large influence on calculated concentrations of long-lived radionuclides found in reservoirs, which are important for doses to humans. Sorption is due to several processes and a variety of physical and chemical interactions. In the commonly used Kd-methodology. however, these processes were usually not considered explicitly. Additionally, many Kd values were obtained from laboratory experiments from the geosphere, the conditions of which differ from those prevailing in the biosphere. The main objective of this work was to extend the knowledge about the theoretical background for calculation of Kd values. To achieve this objective, theoretical models for ion exchange and surface complexation were adapted to simulation under biospheric conditions. Elements studied were Cs, Ra, Np, U, and Pu. The results show that a triple-layer surface complexation model may be used to estimate Kd values for actinides as functions of some chemical parameters, such as pH and the redox potential (EH). An area of application is performance assessment of radioactive waste repositories. |
