Thermodynamic Approach for Predicting Actinide and Rare Earth Concentrations in Leachates from Radioactive Waste Glasses

Studies aimed primarily at determining leach rates of different elements from doped glasses have resulted in computerized models for predicting leachate concentrations. However, leach rate related data should be limited to predicting the stability behavior of the glass matrix; the radionuclide release data based on these studies are empirical and are highly dependant on many variables and processes which have not been systematically evaluated and thus do not provide a reliable method of predicting leachate concentrations. A better approach is available for those elements that can readily form relatively insoluble solids during preparation of glass or glass/water interactions. This alternate approach relies on the experimental solubilities of pulverized doped glasses, in a wide range of well-controlled important variables such as pH and pe, and their comparisons at the given aqueous composition to predicted solubilities of known solid phases from the thermodynamic data. These comparisons are used to indirectly identify specific solubility-controlling solids in doped glass/water systems to determine scientifically defensible aqueous concentrations of different elements for any given groundwater composition, independent of glass dissolution kinetics and independent of time. This paper summarizes data available for the application of this alternate approach to reliably predict concentrations of thorium, uranium, neptunium, plutonium, and trivalent actinides and rare earth elements leachable from the doped glasses. The thermodynamic data, in addition to that reported in recent critical reviews, includes new data that were developed for the solubility products of Th₃(PO₄)₄(s) and the solid solutions of trivalent actinides and rare earth hydroxides. Thermodynamic interpretations of the doped glass solubility data show specifically that tetravalent actinide hydrous/crystalline oxides and solid solutions of trivalent actinides and rare earths hydroxides in non-phosphate glasses and Th₃(PO₄)₄(s) and MPO₄(s), where M denotes trivalent actinides or rare earths, in phosphate-containing glasses are the dominant solubility-controlling solids. Needed future research in this area is briefly outlined.