Frontier research of non-aqueous actinide clusters is discussed. Since the inception of the uranyl peroxide clusters over the last decade, they have only been synthesized and characterized as solid crystals and in aqueous solution. This thesis provides thorough characterization of aqueous uranyl clusters and the first demonstration of uranyl cluster transfer and characterization in organic solvents. Uranyl peroxide clusters self- assemble in mild alkaline solution in the presence of peroxide. Primary solution characterization used throughout this work is Small-Angle X-ray Scattering (SAXS), giving information on particle size, shape, and electron density contrast. SAXS characterization shows the capsule-structures of clusters are maintained, but unique behavior is observed under further characterization. Hydrophilic encapsulated counterions (i.e. alkalis, ammonium) become isolated in this hydrophobic environment. Immobile counterion environments are specific to cluster identity, allowing for Variable-temperature solution Nuclear Magnetic Resonance spectroscopy of ion exchange dynamics within clusters. This provides an unprecedented opportunity to probe structure of lithium atoms, not visible under x-ray diffraction, leading to an understanding of self-assembly and stabilization of clusters. Research of uranyl peroxide clusters in the organic phase have led to discoveries of a
new cluster structures not seen before in an aqueous environment. Study of uranyl species in an organic solution has implications for back-end nuclear fuel processing and separations within the nuclear fuel cycle. The use of polynuclear clusters for separation chemistry, and simple ion association as the mechanism of phase transfer is not currently employed in nuclear fuel reprocessing or radionuclide separations. All current processes utilize molecular complex formation or inorganic, solid-phase ion exchangers. Distinct benefits offered by the process presented here include; 1) the extractant molecules are benign, and 2) the process functions best if their concentration is sub-stoichiometric to the uranium concentration, yielding an ‘atom efficient’ process. These features are compared to the PUREX processes.
Additionally, transition and rare earth metals precipitate in the alkaline aqueous conditions in which these clusters self-assemble, which provides initial separation of many isotope decay products. As uranyl peroxide clusters are considered a molecular analogue of the uranyl mineral, studtite; other uranyl mineral compounds were explored. Solid-state studies of layered uranyl minerals theoretically allows for intercalation of organic molecules for exfoliation and eventual solution/film preparation. This could also lead to clusters with different dimensionalities, derived from the layer structural motifs. Improved synthesis, alkylamine intercalation and characterization of uranyl phosphate mineral, chernikovite, is discussed. Similar intercalation behavior can be seen in transition metal chalcogenides (TMC), revealing weak Van Der Waals interlayer interactions. This approach to non-aqueous uranyl clusters and uranium mineral compounds will stimulate much more in-depth and diverse studies in the scientific community.
Funding Statement (additional comments about funding)
This material is based upon work supported as part of the Materials Sciences of Actinides Center, an Energy Frontier Research Center funded by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001089.