Complexation of Thorium with Phosphates and Silicates under Repository Conditions

Abstract: In order to make reliable performance assessments of the safety for planed underground repositories for spent nuclear fuel large amounts of chemical thermodynamical data is needed. In some cases the databases used for modeling the migration behavior of radionuclides in groundwater is missing data. This is the case for thermodynamic complexation data of thorium with aqueous phosphates and silicates under repository conditions. The published data for thorium-phosphate complexation is doubtful and the associated uncertainties with reported constants are not well documented. What phosphate species that take part in the complexation is unclear. For aqueous complexation of thorium with silicates there are no data available at all. Studies of the complexation behavior of aqueous silicates in the near neutral pH range under repository conditions are complicated due to the effects of silicate polymerization and it is still unclear which aqueous silicate species that takes part in the complexation reactions. In this work the complexation behavior of thorium with aqueous phosphates and silicates has been studied using liquid-liquid extraction and experiments by AKUFVE technique. The thorium-phosphate complexation has been studied at different pH and temperatures. It is shown that monohydrogen phosphate (HPO42-) is involved in the complexation and stability constants, enthalpies and entropies for formation of the thorium-phosphate complexes were determined. The complexation of thorium with silicates has been studied at one pH and one temperature. Stability constants for the complexation have been tentatively determined.