Solubility and speciation studies of waste radionuclides pertinent to geologic disposal at Yucca Mountain: Results on neptunium, plutonium and americium in J-13 groundwater: Letter report (R707): Reporting period, October 1, 1985--September 30, 1987

We have studied the solubilities of neptunium, plutonium, and americium in J-13 groundwater from Yucca Mountain (Nevada) at three temperatures and hydrogen ion concentrations. They are 25/degree/, 60/degree/C, and 90/degree/C and pH 5.9, 7.0, and 8.5. The results for 25/degree/C are from a study which we did during FY 1984. We included these previous results in the tables to give more information on the solubility temperature dependence; they were, however, done at only one pH (7.0). The solubilities were studied from oversaturation. The nuclides were added at the beginning of each experiment as NpO/sub 2//sup +/, Pu/sup 4+/, and Am/sup 3+/. The neptunium solubility decreased with increasing temperature and with increasing pH. The soluble neptunium did not change oxidation state at steady state. The pentavalent neptunium was increasingly complexed by carbonate with increasing pH. All solids were crystalline and contained carbonate, except the solid formed at 90/degree/C and pH 5.9. We identified this solid as crystalline Np/sub 2/P/sub 5/. The 25/degree/C, pH 7 solid was Na/sub 3/NpO/sub 2/(CO/sub 3/)/sub 2/ /center dot/ nH/sub 2/O. Plutonium concentrations decreased with increasing temperature and showed no trend with pH. Pu(V) and Pu(VI) were the dominant oxidation states in the supernatant solution; as the amount of Pu(V) increased with pH, Pu(VI) decreed. The steady-state solids were mostly amorphous, although some contained a crystalline component. They contained Pu(IV) polymer and unknown carbonates