CALCULATION OF DISTRIBUTION COEFFICIENTS FOR RADIONUCLIDES IN SOILS AND SEDIMENTS

The turnover of radionuclides in parts of the biosphere is usually mod eled by use of a sorption distribution coefficient, K-d. Its value has a large influence on calculated concentrations of long-lived radionuc lides found in reservoirs, which are important for doses to humans. So rption is due to several processes and a variety of physical and chemi cal interactions (e.g., surface complexation and ion exchange). In the commonly used K-d-methodology, however, these processes were usually not considered explicitly. Additionally, many K-d values were obtained from laboratory experiments or from the geosphere, the conditions of which differ from those prevailing in the biosphere, The main objectiv e of this work was to extend the knowledge about the theoretical backg round for calculation of K-d values, To achieve this objective, theore tical 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 complexat ion model may be used to estimate K-d values for actinides as function s of some chemical parameters, such as pH and the redox potential (E(H )). An area of application is performance assessment of radioactive wa ste repositories.