It is widely known that microstructural changes influence the thermal performance of nuclear fuels. One mechanism that plays a pivotal role in the microstructural evolution is fuel chemistry variation at boundaries, specifically grain boundaries. Changes in the microstructure, specifically discontinuities at grain boundaries, lead to a change in the efficiency of the transport of phonons or lattice vibrations. Focused research is needed to help explain the role grain boundary character plays fuel chemistry and thus the thermal transport properties in UO2. The objective of this proposed research is to study the effect of grain boundary character on the microstructure of nuclear fuel. The goal is to elucidate the fundamental material-physics underlying the connection between fuel chemistry variation at specific grain boundaries and their role in thermal transport properties. The proposed research utilizes electron backscatter diffraction (EBSD) and energy dispersive spectroscopy (EDS) to study grain boundary dependent chemistry change in off stoichiometric polycrystalline UO2 thin films. In order to characterize grain boundary character and changes in fuel chemistry across the grain boundary of interest in UO2, electron backscatter diffraction (EBSD) in conjunction with energy dispersive spectroscopy (EDS) available at CAES will be used. Samples will be prepared by conventional techniques for EBSD [1]. Samples will be studied in the late Summer or early Fall 2013. Final results will then be published in peer- reviewed journal articles. It is expected that these results will provide new insight into the grain boundary character dependence on chemistry variation in UO2, allowing for the correlation with atomic-level simulations and/or the ability to link with mesoscale structure-property relationships, specifically in thermal transport. References [1] P. V. Nerikar, et. al. J. Am. Ceram. Soc. 94 [6] 1893-1900 (2011)

The Department of Energy’s Light Water Reactor Sustainability program, and specifically the Advanced Nuclear Fuels program goal, is focused on understanding the role of chemical, physical and microstructural drivers on fuel performance. Due to the mechanistic complexity attributed to the multiplicative interacting chemical and microstructural parameters, state of the art models that predict fuel performance have been largely phenomenological. These phenomenological models typically represent the combined effect of various mechanisms actively involved in fuel burn-up. Since the principle goal in fuel design is to increase the thermal conductivity, isolating the individual phonon scattering contributions present during microstructural evolution of a fuel pellet is of upmost importance. Quantifying the effects of local chemistry and to and away from boundaries move current descriptions of thermal transport towards science-based models grounded in microstructure and microchemistry that can provide predictability of fuel behavior outside the common design space, into accident conditions.



The current study is designed to characterize local fuel chemistry variation off stoichiometric uranium dioxide thin films. Literature has shown evidence of local chemistry variation at grain boundaries in oxides. The results of this study will be a significant step into predicting the behavior of chemistry changes at grain boundaries. It is anticipated that the results from this study will provide the foundation and fundamental parameters for the development of robust mechanistic analytical and computational models.