The Influence of Non-Stoichiometry and Rare Earth Doping on the Oxidation and Dissolution of Uranium Dioxide

The influence of fission product doping on the structure, composition and electrochemical reactivity of uranium dioxide has been studied using X-ray diffractometry (XRD), scanning electron microscopy (SEM/EDX), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Experiments were conducted on SIMFUEL specimens with simulated burn-ups (increasing doping levels) of 1.5 at%, 3.0 at%, and 6.0 at%. As the dopant level increased, the lattice contracted, suggesting the dominant formation of dopant-oxygen vacancy clusters. The smaller than expected lattice contraction can be attributed to the segregation of Zr (one of eleven added dopants) to ABO3 perovskite-type phases which SEM/EDX shows also contain Ba, Ce, and possibly some U. Raman spectroscopy shows that doping leads to a loss of cubic symmetry possibly associated with tetragonal distortions. Raman mapping confirms this loss of cubic symmetry and suggests the specimen is not uniformly doped. Electrochemical experiments show that these distortions lead to a decrease in the oxidative dissolution rate of the UO2 with increased doping density.;The oxidation / reduction behavior of the fuel and its ability to support the cathodic reduction reaction have been studied by monitoring the oxidation / reduction of the UO2 substrate during potentiodynamic cycles. It was demonstrated that, as the degree of non-stoichiometry increases, the initial reactivity of the UO2+x surface also increases. However, depending on the nature of the defect present, this enhanced reactivity may be unsustainable as the extent of oxidation is increased. For a location with small extents of non-stoichiometry, i.e., containing only random point defects, the reactivity is initially low but increases as a consequence of the irreversible insertion of a into the many available interstitial sites in the fluorite lattice. By contrast, for a location with a high degree of non-stoichiometry (x ≥ 0.25 in UO2+x), when large oxygen enriched defect clusters are present, anodic oxidation initially proceeds very rapidly but decreases substantially as oxidation progresses. At intermediate levels of stoichiometry corresponding to Willis clusters the surface appears to be reversibly oxidizable.;Electrochemical experiments also show that the oxidation mechanism is influenced by the degree of non-stoichiometry of the UO2 electrode. Oxidation of the stoichiometric UO2 leads to an extensively oxidized shallow surface layer. By contrast, non-stoichiometric, UO2+x, experiences a deeper, but less extensive oxidation process. The oxidation mechanism for stoichiometric UO2, involves migration of O-vacancies into the bulk of the UO2 matrix, a process that results in the transfer of oxygen atoms from the bulk to the surface (vacancy assisted O-migration process). In addition, absorbed oxygen is incorporated into the fluorite-type lattice leading to the extensively oxidized and oxygen enriched discrete surface layer. Further oxidation then leads to facile dissolution of this layer as soluble UO22+species. For hyper-stoichiometric UO 2+x, oxidation involves diffusion of oxygen interstitials into the bulk of the U02+x matrix. As the x increases, it becomes progressively easier to diffuse O into the bulk of the solid leading to the formation of a thicker less oxidized surface layer with a concentration gradient extending into the matrix. The surface composition eventually approaches U4O 9/U3O7 when enough oxygen has been incorporated into the lattice. Since oxygen diffusion is rapid around a stoichiometry of UO2.12 / U2.13, the oxidation process becomes reversible around this composition. (Abstract shortened by UMI.);Scanning electrochemical microscopy (SECM) measurements combined with theoretical models have been developed to determine the distribution of corrosion rates and rate constant at individual locations with various degrees of non-stoichiometry. The results quantitatively demonstrate that corrosion rates and rate constants vary over a broad range (3x10-6 m s-1 to 10-3 m s-1), and that this range is determined by the degree of non-stoichiometry of the grains and the diversity of structures on the UO2+x surface. Compared to the overall current values obtained in the range of Ecorr in the voltammogram, corrosion rates obtained on non-stoichiometric grains were approximately 20 to 100 times larger.