Structural and Chemical Alteration of Uranium(VI) Phases Subjected to High Pressures or Added Reactive Oxygen Species

This dissertation explores the alteration of uranium(VI) phases relevant to storage of uranium ore concentrates prior to fuel fabrication and the storage of used nuclear fuel in a geological repository. The two main research foci are (i) characterizing the pressure-induced structural changes of uranyl silicate minerals relevant to storage of used nuclear fuel in a geological repository and (ii) probing the alteration of U3O8 submerged in dilute aqueous solutions of hydrogen peroxide or stored in high relative humidity atmospheres with and without added ozone. The first area of research used single-crystal X-ray diffraction (SC-XRD) and Raman spectroscopy to characterize the structural changes of natural samples of boltwoodite, K0.63Na0.37[(UO2)(SiO3OH)]·1.5H2O, and uranophane-a, Ca[(UO2)2(SiO3OH)2]·5H2O, at gigapascal (GPa)-range pressures achieved using diamond anvil cells (DACs). Boltwoodite exhibited no obvious phase transition between ambient pressure and 16.8 GPa while uranophane-a underwent two pressure-induced phase transitions between ambient pressure and 17.0 GPa. The bulk moduli of boltwoodite, uranophane-a and its high pressure phases were 26.3(4) GPa, 38.0(1.3), 27.1(7), and 12.4(1.0) GPa, respectively. This dissertation is the first to report high-pressure SC-XRD of any uranium(VI) mineral and describes the method developed to maximize crystallographic coverage in a DAC. The second area of research used partial least squares regression analysis of Raman spectra to quantify the alteration phases formed from U3O8 in the presence of reactive oxygen species. This work determined the sequence of formation of uranyl oxide hydrates and uranyl peroxides from U3O8 submerged in dilute aqueous solutions of hydrogen peroxide or stored in elevated relative humidity (RH) atmospheres with and without added ozone. Submersion experiments revealed the previously unreported rehydration of metastudtite, [(UO2)(O2)(H2O)2], to studtite, [(UO2)(O2)(H2O)2]·2H2O. High RH experiments determined that uranyl peroxides, the dominant alteration products, and uranyl oxide hydrates co-formed from U3O8 in atmospheres with 85% or 100% RH, with and without added ozone.