Three-Dimensional Characterization of Sintered UO_2+x: Effects of Oxygen Content on Microstructure and Its Evolution

The oxygen content during the intermediate and final stages of sintering can have a strong effect on the microstructural evolution of oxide fuels. Two depleted urania (d-UO_2.0 and d-UO_2.14) samples, sintered up to a theoretical density of 90%, were serial sectioned using a focused ion beam and characterized with electron backscatter diffraction (EBSD). The EBSD data were used to make three-dimensional reconstructions of the microstructures to evaluate their characteristics at an intermediate stage of sintering. The oxygen content was found to affect grain shape and grain boundary (GB) mobility, as curved and elongated grains were observed in UO_2.0, as well as stronger pore-GB interactions, which is an indication that microstructure was less evolved in UO_2.0. Both samples presented a similar fraction ([approximate]20%) of special, coincident site lattice boundaries, with larger amounts of Σ3ⁿ GBs, and a rather large fraction of Σ11 GBs for UO_2.14. Crystallographic GB planes were also determined to study the distributions of all GB parameters. The UO_2.0 sample had a large fraction of GB planes close to the Σ3 twinning planes, which suggests that lower-energy interfaces are used to minimize energy in this sample, potentially due to lower overall GB mobility as compared to UO_2.14.