Oxidation of Micro- and Nanograined UO 2 Pellets by In Situ Synchrotron X-ray Diffraction.

When in contact with oxidizing media, UO ₂ pellets used as nuclear fuel may transform into U ₄ O ₉ , U ₃ O ₇ , and U ₃ O ₈ . The latter starts forming by stress-induced phase transformation only upon cracking of the pristine U ₃ O ₇ and is associated with a 36% volumetric expansion with respect to the initial UO ₂ . 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, UO ₂ 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 U ₃ O ₈ was detected only in UO ₂ pellets with larger grains (3.08 ± 0.06 μm and 478 ± 17 nm), while U ₃ O ₈ did not develop in sintered UO ₂ with a grain size of 163 ± 9 nm. This result shows that, in dense materials, a sufficiently fine microstructure inhibits both the cracking of U ₃ O ₇ and the subsequent formation of U ₃ O ₈ . 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.