Geochemical simulation of reaction between spent fuel waste form and J-13 water at 25{sup 0} and 90{sup 0}C.

Geochemical simulations of the degradation of spent fuel waste form in the presence of groundwater at the candidate Yucca Mountain, Nevada repository have been carried out to attempt to predict elemental concentrations in solution and to identity potential radionuclide-bearing precipitates. Spent fuel was assumed to dissolve congruently into a static mass of J-13 groundwater at 25{sup 0}C and 90{sup 0}C. Simulation results inidcate that haiweeite, soddyite, Na{sub 2}U{sub 2}O{sub 7}(c) and schoepite are potential U-bearing precipitates. Na{sub 2}U{sub 2}O{sub 7}(c) is only predicted to occur at 90{sup 0}C. U concentrations in solution and the identity of the U-bearing precipitate depend on the activity of SiO{sub 2}(aq) in solution. U concentrations are limited to < 1 mg/kg when sufficient SiO{sub 2}(aq) exists in solution to precipitate uranyl silicates. Depletion of SiO{sub 2}(aq) in solution by the precipitation of silicates results in predicted increases of U concentrations to 87 and 619 mg/kg at 25{sup 0}C and 90{sup 0}C, respectively. Subsequent reaction and precipitation of schoepite cause U concentrations to decrease. Radionuclide other than U commonly precipitate as oxides in the simulations. The precipitation of solid phases appears to be extremely effective in limiting the concentrations of some radionuclides, such as Pu and Th, in solution. Increasing the temperature from 25{sup 0}C to 90{sup 0}C does not impact greatly the identify of precipitated phases or solution composition, except in the case of U. 7 refs., 7 figs., 2 tabs