Geochemical controls on radionuclide releases from a nuclear waste repository in basalt: estimated solubilities for selected elements

Two basalt flows within the Grande Ronde formation at the Hanford Site in Southeastern Washington are candidates for a high-level nuclear waste repository. In order to determine the anticipated rates of release and migration of radionuclides from the repository, solubility controls must be determined for key elements. Solubilities, the solids controlling solubility and aqueous speciation for a variety of actinides and fission products in groundwater, have been determined at 250C from available thermodynamic data over a range of Eh values. Groundwater compositions used include all available analyses from the Grande Ronde formation determined by the Basalt Waste Isolation Project. Solids predicted to control solubility for the selected radionuclides include hydroxides and hydrous oxides (palladium, antimony, samarium, europium, lead, americium), oxides (nickel, tin, thorium, neptunium, plutonium), elements (selenium, palladium) and silicates (zirconium, uranium). Dominant soluble species include hydroxy complexes (zirconium, palladium, tin, antimony, samarium, europium, thorium, uranium, neptunium) and carbonate species (nickel, samarium, europium, lead, uranium, neptunium, plutonium, americium). The effect on solubility of dissolved, reduced (S2-, HS-) sulfur species is large for nickel, palladium, antimony, and lead. However, available data suggest that essentially all sulfur in Grande Ronde groundwater is in the form of sulfate (SO42-). Selenium solubility appears to be a strong function of Eh and may reach very high levels where -0.45 > Eh > -0.05 V. This result suggests that future experimental work should include solubility determinations for selenium. In addition, other elements, notably technetium and radium, with large uncertainties in calculated solubility should also have solubilities determined experimentally