Electrochemical Investigations of Redox Reactions of Uranyl(VI) on Magnetite and Computational Modeling of the UO2-HfO2 Solid Solution.

Abstract

Uranium has a unique chemical behavior because of the presence of localized 5f electrons. The redox chemistry of uranium influences its mobility in the aqueous environment. This thesis investigates the redox processes of aqueous uranium (uranyl) in order to understand and predict its behavior in the environment. In addition, the behavior of the UO2-HfO2 solid-solution (Hf being a neutron absorber) is modeled to study the conditions under which the mixture forms a solid solution or exsolves, which is essential for its in-reactor performance. Soluble uranyl(VI) can be reduced on surfaces of Fe(II)-bearing minerals to solid U(IV)O2, resulting in the decrease of its mobility in the environment. However, the previously considered one-step two-electron reduction pathway from U(VI) to U(IV) has been challenged by the presence of stable pentavalent U(V). The experiments here investigate the mechanism of uranium reduction by reducing uranyl(VI) electrochemically on powdered and bulk magnetite electrodes. The number of electrons transferred per redox change is found to be one, which confirms the one-electron reduction from U(VI) to U(V). Nano-size uranium precipitates were found on the surface of magnetite by electrochemical AFM. Further spectroscopic evidence (XPS, AES, XANES, and EXAFS) suggests these precipitates are poorly crystallized mixed-valence state U(V)/U(VI) solids, which stabilize U(V) by preventing its disproportionation. In contrast, the catalytic properties of the surface of powdered magnetite facilitates the disproportionation of U(V), which is attributed to the adsorption/desorption kinetics of protons on the particulate magnetite. In order to better control the power distribution in a nuclear reactor, UO2, a nuclear fuel material, is mechanically mixed with the neutron absorber HfO2. The thermodynamic mixing properties of the UO2-HfO2 were simulated using DFT and Monte Carlo simulations. The calculated binary forms extensive solid solution at high temperatures across the entire compositional range, with a variety of exsolution phenomena associated with the different HfO2 polymorphs upon cooling. Close to the UO2 end member, which is relevant for nuclear fuel fabrication, the isometric uranium-rich solid solutions exsolve as the fuel cools. There is a tendency to form the monoclinic hafnium-rich phase in the matrix of the isometric, uranium-rich solid solution phase.