APPLICATION OF SURFACE COMPLEXATION AND ION EXCHANGE MODELLING FOR THE ESTIMATION OF BENTONITE SORPTION CAPACITY ON SELECTED RADIONUCLIDES UNDER POSSIBLE GEOLOGICAL DISPOSAL CONDITIONS IN LITHUANIA
Designing a repository for radioactive waste as a multi-barrier system with predefined multiple safety functions provides for the protection of people and environment from the harmful effect of the ionising radiation of radionuclides once released from high-level radioactive waste. The multi-barrier system is comprised of the engineered barrier system (EBS) and its surrounding natural environment. Because of the high retention capacity for the radionuclides, various forms of bentonite are usually considered as a part of the EBS in the design of radioactive waste repositories. Laboratory batch experiments with natural sediments illustrate that the sorption (and Kd values) is sensitive to aqueous chemical conditions, including the pH and the dissolved carbonate concentration as well as the mineralogy and surface area of the bentonite buffer. The cations considered for the study were chosen as covering a representative range of cations behaviour: from a weakly sorbed cation Cs(I) to moderately sorbed cation the Ni(II) and strongly sorbed cation, Eu(III). In this study, a potential retardation of sorbing radionuclides (Cs, Ni, Eu) as a function of aqueous chemical conditions in the groundwater of the Lithuanian crystalline basement contacting the bentonite material was estimated for the first time. The ion exchange and surface complexation models (SPNE/CE) and the computer program PHREEQC (USA) were applied for modelling the interaction within the radionuclide/water/bentonite system. The modelling of the Kd values under different pH conditions and a given partial pressure of carbon dioxide in the gas phase were performed. The highest value of Kd in the pCO2 open system and the strongest sorption dependence on pCO2 was estimated for Eu(I) among radionuclides being studied. There was estimated only a slight change in Kd values with respect to the temperature.