In-situ X-ray diffraction measurements of u-0.1w%cr oxidation

The extensive exploration of uranium oxidation is partly driven by the pursuit for deeper understanding of the conditions for handling and long-term disposal of spent nuclear fuels . Recent studies by Chernia et al. - uranium surface is controlled by elastic strain and by anion hoping towards the oxide-metal interface, which is directed preferentially along [110], due to chains of oxygen defects clusters along [110] and consequently, [110] becomes the principal growth direction of UO2 on uranium. During the buildup of the oxide layer, elastic strain is gradually formed due to the oxide-metal density mismatch. At a certain oxide thickness the developed cracks accelerate the oxidation towards a linear or para-linear time dependence. By means of ellipsometric measurements Lin et al. found that the initial oxidation of uranium obeys a parabolic kinetic law with activation energy of 17.6 kcal/mol. Schweke et al. utilized in-situ UV-vis reflectance measurements to conclude that the initial oxidation of U-0.1w%Cr, up to ~150 nm obeys an invers-logarithmic kinetics law with activation energy of ~21 kcal/mol. Gharagozloo and Kanouff introduced a theoretical modeling of ionic diffusion driven by an electrostatic potential. The derived invers-logarithmic and parabolic oxide growth rates were found to fit well the experimental data below 200oC and oxide thickness up to ~300 nm. At higher thickness deviation from the model was explained by spalling of the oxide. Ex-situ measurements usually involve cooling of the reacted sample down to room temperature, before measuring the oxide thickness. Due to the stress that develops across the oxide layer, such cooling triggers its exfoliation and therefore hinders the monitoring of its growth. To circumvent this limitation in-situ measurements are highly beneficial. X-ray diffraction is a powerful tool for monitoring the oxidation of uranium. In addition to the measurement of oxide thickness it may reveal the various oxide phases and their crystallographic orientations. Furthermore, it may follow the different oxidation rates that characterize the distinctive metallic planes. In the present work in-situ X-ray diffraction and scanning electron microscopy (SEM) were utilized to study the oxidation of U-0.1w%Cr under oxygen pressures of 150 Torr and within the temperature range of 90-150oC