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

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