Characterizing the relationship between hyperstoichiometry, defect structure and local corrosion kinetics of uranium dioxide

The ability of the UO2 fluorite structure to accommodate large amounts of interstitial oxygen in various lattice sites leads to the formation of hyper-stoichiometric phases. The defect structures occurring in hyper-stoichiometric UO2+x over the range 0.02 ≤ x ≤ 0.1 have been characterized by SEM/EDX and Raman analyses. The results demonstrate that as the nominal stoichiometry increases from 2.002 to 2.1, the diversity of defective structures existing on the UO2+ surface also increases. Scanning electrochemical microscopy (SECM) measurements combined with a theoretical model were used to determine the rate constant for the reduction of the redox mediator ferrocene methanol, acting as a cathodic oxidant to corrode the four UO2+x specimens. The rate constant was found to vary with location on the surface. Stoichiometric locations, with a well defined fluorite structure, exhibited very low corrosion rates. Higher rates were observed at more non-stoichiometric locations with the highest rates being obtained on locations exhibiting tetragonal distortions as their composition approached UO2.33. The distribution of rates increases with the degree of nominal non-stoichiometry as the diversity of microstructures existing on the UO2+x surface increases.