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.