Oxidation of Micro- and Nanograined UO₂ Pellets by In Situ Synchrotron X-ray Diffraction

When in contact with oxidizing media, UO₂ pellets used as nuclear fuel may transform into U₄O₉, U₃O₇, and U₃O₈. The latter starts forming by stress-induced phase transformation only upon cracking of the pristine U₃O₇ and is associated with a 36% volumetric expansion with respect to the initial UO₂. This may pose a safety issue for spent nuclear fuel (SNF) management as it could imply a confinement failure and hence dispersion of radionuclides within the environment. In this work, UO₂ with different grain sizes (representative of the grain size in different radial positions in the SNF) was oxidized in air at 300 °C, and the oxidation mechanisms were investigated using in situ synchrotron X-ray diffraction. The formation of U₃O₈ was detected only in UO₂ pellets with larger grains (3.08 ± 0.06 μm and 478 ± 17 nm), while U₃O₈ did not develop in sintered UO₂ with a grain size of 163 ± 9 nm. This result shows that, in dense materials, a sufficiently fine microstructure inhibits both the cracking of U₃O₇ and the subsequent formation of U₃O₈. Hence, the nanostructure prevents the material from undergoing significant volumetric expansion. Considering that the peripheral region of SNF is constituted by the high burnup structure, characterized by 100-300 nm-sized grains and micrometric porosity, these findings are relevant for a better understanding of the spent nuclear fuel behavior and hence for the safety of the nuclear waste storage.