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 identify potential radionuclide-bearing precipitates. Spent fuel was assumed to dissolve congruently into a static mass of J-13 groundwater at 25°C and 90°C. No inhibitions to the precipitation and dissolution of secondary phases were assumed to exist. The elements Ac, Zr, Nb, Pd, Sm, Mo, Sb and Cm were not considered in the simulations because of a lack of thermodynamic data.
Simulation results indicate that haiweeite, soddyite, Na2U2O7(c) and schoepite are potential U-bearing precipitates. Na2U2O7(c) is only predicted to occur at 90°C. U concentrations in solution and the identity of the U-bearing precipitate depend on the activity of SiO2(aq) in solution. U concentrations are limited to < 1 mg/kg when sufficient SiO2(aq) exists in solution to precipitate uranyl silicates. Depletion of SiO2(aq) in solution by the precipitation of silicates results in predicted increases of U concentrations to 87 and 619 mg/kg at 25°C and 90°C, respectively. Subsequent reaction and precipitation of schoepite cause U concentrations to decrease.
Radionuclides 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. Concentrations of other elements are held constant (Sn) or are alternately held constant and then increase (Am, Ni, Np) as various solid phases precipitate and pH decreases from 8.5 to 6.5 at 25°C and 8.7 to 8 at 90°C. No solid phases containing Cs or Tc are predicted to form. Increasing the temperature from 25°C to 90°C does not impact greatly the identity of precipitated phases or solution composition, except in the case of U.
A technique involving isotope dilution measurements may allow determination of the rates of spent fuel dissolution in future experiments.