Orientation-Dependent Surface Faceting in UO₂

The surface behavior of UO₂ plays an important role throughout the lifetime of this widely-used commercial nuclear fuel, from manufacturing to waste storage. UO₂ surface properties affect fuel swelling behavior by determining the morphology, size and distribution of fission gas bubbles. Additionally, UO₂ surface characteristics influence fracture behavior, which is controlled by the thermodynamics of fracture surfaces. Therefore, detailed understanding of the fundamental surface behavior will help advance prediction of the fuel performance of UO₂. Due to significant anisotropy in surface tension, faceting features form after annealing in both UO₂ and many other crystalline materials. Knowledge of the correlation between crystallographic orientation and surface faceting features will expand the fundamental knowledge of UO₂ surface characteristics, leading to a better understanding of fission gas swelling and fracture mechanisms. Inherent characterization technique limited previous studies on the crystallographic orientation of UO₂ surface facets to low Miller index surfaces in single crystal specimens. In this study, synchrotron Laue microdiffraction was utilized to examine the crystallographic orientation of grains on the coarse surface of a polycrystalline UO₂ specimen. The results were then compared with the surface morphology, obtained from scanning electron microscopy (SEM), so that a general correlation between the surface faceting features and crystallographic orientation in UO₂ can be established. Both the fundamental surface characteristics and the crystallographic orientation measurements obtained in this study help advance the experiment validation of MARMOT, a mesoscale multi-physics code that predicts the coevolution of microstructure and properties within reactor fuel during its lifetime in the reactor. The MARMOT code is an indispensable component of the MOOSE-BISON-MARMOT (MBM) code suite that has been developed by Idaho National Laboratory to enable next generation fuel performance modeling capability as a part of the Nuclear Energy Advanced Modeling and Simulation (NEAMS) Program Fuels Product Line (FPL). In this study, the surface-faceting features of UO₂ were found to be dominated by the triple-plane structure and its variants. With this knowledge, by measuring the directions of the <110> edges, the crystallographic orientation of a surface UO₂ grain can be precisely deduced in most regions of the standard stereogram triangle (SST) of the surface normal. (authors)