2, Idaho National Laboratory, Idaho Falls, Idaho, United States
Microstructural evolution (e.g. grain growth) in uranium dioxide (UO2) nuclear fuels significantly affects nuclear fuel performance. The anisotropy of both the energy and mobility of grain boundaries determines to a large extent how the microstructure evolves. These boundary properties are also critical input parameters for mesoscale modeling of microstructural evolution. To better understand the anisotropy of these interfacial properties, circular grain boundaries in UO2 were studied using molecular dynamics simulations. Grain boundary energies and mobilities were extracted for the high-symmetry rotation axes (<100>, <110>, and <111>). The calculated energies for the high-symmetry axes follow the five-dimensional grain boundary energy model well. The mobilities of grain boundaries were calculated across the three axes using the shrinking circular grain method. The <100> mobilities follow typical Arrhenius behavior across a range of high temperatures, followed by an abrupt shift to another Arrhenius-like trend with a much higher activation energy. Mobilities for the <111> axis were found to be the fastest boundaries. Mobilities for the <110> axis were found to be the slowest. The atomic transfer mechanisms across grain boundaries during grain growth are studied to explain the anisotropic mobilities of grain boundaries of different rotation axes.