Electrochemical and Surface Compositional Studies on Uranium Dioxide

Author

Mayuri Razdan, The University of Western OntarioFollow

Degree

Doctor of Philosophy

Program

Chemistry

Supervisor

Dr. David W. Shoesmith

Abstract

This thesis describes electrochemical and surface compositional studies performed on a number of simulated nuclear fuel (SIMFUEL) materials under conditions relevant to permanent disposal of spent nuclear fuel in a geologic repository. This is important since a number of critical issues have been identified in the event of waste container failure. The research performed was mainly focused in three areas: (i) the influence of low pH on the surface chemistry of UO₂, since acidity could develop within corrosion product deposits and flaws in the fuel; (ii) the combined influence of dissolved H₂ and H₂O₂ (H₂ and H₂O₂ are key reducing and oxidizing agents) in the presence of HCO₃^2-/ CO₃^2- (the key ground water species) on the fuel corrosion process (iii) the influence of rare earth (RE^III) fission product doping on the fuel corrosion process (since matrix doping process with RE^III influences the fuel bulk properties, it is expected to influence both anodic and cathodic kinetics under natural corrosion conditions).

The influence of H₂O₂ on 1.5 at% SIMFUEL in acidic (pH 1-4) conditions was studied voltammetrically using a rotating disk electrode (RDE), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) to determine the composition and morphology of the oxidized UO₂ surface. The H₂O₂ reduction mechanism is shown to occur on either a U^V-containing surface layer of composition U^IV_1−2xU^V_2xO_2+x or on an adsorbed U^V surface intermediate, depending on the surface composition which is determined by solution pH and H₂O₂ concentration. The U^IV_1−2xU^V_2xO_2+x catalytic surface lattice layer, if formed is more stable and supports H₂O₂ reduction up to the diffusion-controlled limit. By contrast, the U^V adsorbed surface intermediate is unstable which prevents significant H₂O₂ reduction. The simultaneous occurrence of both reduction mechanisms demonstrates the influence of locally established surface compositions and the switch from one to the other appears to be controlled by the diffusive transport conditions at the electrode surface.

In addition to H₂O₂, the influence of the dominant reducing species, H₂, anticipated inside a failed waste container was investigated at different [H₂O₂] in the presence of the key ground water species (HCO₃^-/CO₃^2-). Their combined influence on the redox behavior of UO₂ was followed using open circuit corrosion potential measurements (E_CORR), cathodic stripping voltammetry (CSV) and XPS. The presence of HCO₃^-/CO₃^2- in solution inhibits UO₂ oxidation at lower [H₂O₂]. The influence of dissolved H₂ in suppressing surface oxidation under ambient conditions depends primarily on chemically added [H₂O₂] and was evident in the presence of carbonate for H₂O₂ concentrations ≤ 10^-5 mol L^-1.

A second goal of the thesis was to study the effect of fission products (metallic particles and rare earth (RE³⁺)) on UO₂ oxidation. These studies were conducted on 0.3 wt% Yttrium-doped UO₂ (Y-UO₂), 6 wt% Gadolinium doped UO₂ (Gd-UO₂), 12.9 wt% Dysprosium doped UO₂ (Dy-UO₂) and 1 wt% Palladium-doped UO₂ (Pd-UO₂) electrodes. The electrodes were characterized using Raman Spectroscopy and SEM/EDX and their anodic oxidation studied electrochemically and by XPS.

Voltammetric experiments on Y-doped UO₂ electrodes containing noble metal particles showed the presence of a current at sub-thermodynamic potentials consistent with a lattice containing a mixture of stoichiometric and non-stoichiometric domains. Their presence was verified by Raman and XPS analyses. Electrochemical investigations on homogeneously RE^III doped electrodes demonstrated a clear doping influence on both stages of the anodic oxidation process; i.e., on the initial matrix oxidization step (UO₂ → UO_2+x) and on its further oxidation to soluble U^VI (as UO₂²⁺). Doping appears to influence the kinetics of the second step more than that of first step. Raman spectroscopy shows that an increase in doping level leads to the formation of RE^III-Oxygen vacancy (O_V) clusters which decreases the number of the O_V sites required for oxidation.

The influence of carbonate/bicarbonate (the key groundwater constituents likely to influence fuel dissolution) on the electrochemical oxidation process of RE-doped UO₂ (Gd-UO₂) was examined using CV, potentiostatic polarization and XPS. While CV scans show that carbonate has a significant catalytic effect on the oxidative dissolution of UO₂, a stable surface layer (UO_2+x) is present irrespective of carbonate concentration. Potentiostatic experiments in the potential range -0.5 to 0.5V also show that the oxidation/dissolution currents are increased in the presence of carbonate. XPS analyses showed the electrode to be free of U^VI species. This indicates that the slow step in the overall anodic dissolution process is the electrochemical formation of U^VI not its chemical dissolution.

Recommended Citation

Razdan, Mayuri, "Electrochemical and Surface Compositional Studies on Uranium Dioxide" (2013). Electronic Thesis and Dissertation Repository. 1740.
https://ir.lib.uwo.ca/etd/1740