Aqueous Alkaline Actinide Chemistry for Advance Nuclear Materials Processing. MELISSA ENSOR (The University of the South, Sewanee Sewanee, TN 37383) WOLFGANG RUNDE (Los Alamos National Laboratory, Los Alamos, NM, 87545)

As an alternative to the acidic PUREX reprocessing of spent nuclear fuel, SNF, oxidative dissolution of UO2 into alkaline aqueous solutions and subsequent separation of fission products is considered. The efficiency and kinetics of dissolution depends heavily on the solution and solid-state chemistry of actinides and fission products in carbonate media. Peroxide has been shown to be a primer oxidizing agent in carbonate solutions. However, the addition of peroxide increases the complexity of actinide solution chemistry, creating a dynamic system of multiple ligands. Uranium in carbonate solution forms the anionic complex UO2(CO3)34- in solution with ternary uranyl(VI) phases of the general formula M4UO2(CO3)3onH2O as the solid equilibrium phase. We determined the solubility of these solids using M = NH4, K, Na, and Cs in 0.1 M, 0.5 M, 1 M, and 2 M M2CO3 solutions. To establish an accurate interpretation of the experimental data the solids were characterized using X-ray diffraction, Raman spectroscopy, and diffuse reflectance spectroscopy. The uranyl(VI) concentration in the carbonate solutions was determined spectroscopically using the characteristic absorbance of the triscarbonate complex between 380 and 450 nm. The data from the pure carbonate system serves as a foundation to study the competing complexation behavior that occurs upon the addition of peroxide. Spectroscopic titrations of U(VI) carbonate in Na2CO3, NaHCO3, and (NH4)2CO3 were performed with varying peroxide molar ratios, and the solution speciation was characterized. We identified the predominant solution species, UO2(O2)(CO3)24-, and determined its formation constant. For comparison, we also performed spectroscopic titrations in the mixed hydroxide/peroxide system, where the data indicates a significantly different solution chemistry and complexation behavior. In this poster, the structure and solubility of the relevant solid U(VI) carbonate phases will be discussed in terms of their dependence on the nature of the metal cation and the carbonate concentration. Also the results of solution speciation of U(VI) in mixed peroxide/carbonate (or hydroxide) solutions will be presented and, for the first time, thermodynamic data for ternary U(VI) carbonate solution complexes.