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

Abstract

SummaryWe 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°, 60°C, and 90°C and pH 5.9, 7.0, and 8.5. Tables I, II, and III summarize the results for neptunium, plutonium, and americium, respectively.The results for 25°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 tempera ture dependance; 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 NNWSI-LBL-R707, Rl Page 3 of 70 Table I. Summary of results for solubility experiments on neptunium in J-13 groundwater at pH 5.9, 7.0, and 8.5, and at 25°, 60°, and 90°C. NNWSI-LBL-R707, RlPage 4 of 70 Table II. Summary of results for solubility experiments on plutonium in J-13 groundwater at pH 5.9, 7.0, and 8.5, and at 25°, 60°, and 90°C. 3. We need steady-state concentrations for the americium experiments at 60° and 90°C. We attribute the problems in die 60°C study to a-induced damage of the solubility container. This damage released particles into the solution on which americium pseudocolloids probably formed. To reduce the radiation level, we will replace the 243 Am with the stable lanthanide neodymium. Neodymium has similar properties to americium, including nearly identical ionic radii and valency.Additionally, neodymium carbonates and hydroxycarbonates are well character ized. We should be able to get steady-state concentrations from neodymium solu bility studies. Because of the similarity between the two elements, the results for neodymium also apply to americium.4. We plan to measure the solubilities of neptunium, plutonium, and americium in UE25p#l groundwater. This water has a higher ionic strength than J-13 water, which could affect the solubitities. These experiments will be performed at 25°and 60°C and pH values of 6, 7, and 8.5. 8. We recommend the expansion of solubility studies to other waste radionuclides.Important waste elements with solubility-limited concentrations are uranium, thorium, radium, nickel, and zirconium. OBJECTIVESThis study determines the solubilities and the oxidation state distributions (when ever possible the speciation) of waste elements in groundwaters of the potential reposi tory at Yucca Mountain.Radiation-induced heat can generate elevated temperatures at the repository. Pos sible chemical interaction of groundwater flowing through the repository with the backfill material and/or the canister material can alter the pH value of the groundwater.As a consequence of these conditions, this study will investigate the effects of tem perature and pH changes on the solubilities and speciation of waste element solutions.The data will be used as input to model radionuclide transport from the reposi tory.These data will also be compared with solubilities calculated by the EQ 3/6Chemical Equilibrium Code which is used to m...

Solid-Liquid Phase Equilibria of Pu(VI) and U(VI) in Aqueous Carbonate Systems. Determination of Stability Constants

“…A number of studies have shown Pu(VI) carbonate complexes to be stable and potentially mobile species [2][3][4][5][6]. The Pu(VI) carbonate system has been examined and characterized by a number of methods; UV-Visible spectroscopy [7][8][9][10][11][12][13][14][15], "C NMR spectroscopy [16,17], Raman spectroscopy [10,18], X-ray diffraction [14,19], and solubility experiments [19,20].…”