Sorption of Cs, Pu and Am on clay minerals

Performance assessment of radioactive waste disposal requires modeling of long-term migration of radionuclides through the engineered barriers and the geological environment. The chemical complexity of sorption-desorption processes is usually reduced to integrated parameter distribution coefficients (Kd). There are a great number of publications on Kd determination, however, the existing data on Kd of radionuclides on different geological materials are for general understanding only and are not very useful for performance assessment, since changes of the geological conditions result in variability of Kd values by two orders of magnitude. In order to obtain realistic sorption data sets for safety relevant radionuclides present in a cement/concrete based repository some preliminary studies were carried out. The development of sorption database for the near-surface repository was started with measurements of cesium, plutonium and americium Kd values. Several experiments were performed in order to determine the chemical composition of cement water which could originate from infiltration of precipitation and from contact of groundwater with concrete. More than 100 batch sorption experiments were conducted with two clay samples. Cs, Pu and Am Kd values were determined for rainwater, groundwater and cement-water of different chemical compositions. Cs, Pu, Am Kd values ranged from 450 to 9700, from 15000 to 21000 and 15000 to 80000 ml/g, respectively. Changes in the geochemical conditions resulted in the variability of Cs, Pu and Am Kd values.