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 of fuel is the segregation behavior of insoluble fission products, such as krypton (Kr) in uranium dioxide (UO2) towards specific grain boundaries. Changes in the microstructure, specifically the addition of Kr, lead to a change in the efficiency of the transport of phonons or lattice vibrations. Micro level experiments are needed to help explain the role grain boundary character play on segregation behavior of Kr, which alter 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 the fission products segregation and microstructural evolution. The proposed research utilizes electron backscatter diffraction (EBSD) and atom probe tomography (APT) to study grain boundary dependent Kr segregation in polycrystalline UO2. In order to characterize grain boundary character and changes in concentration across the grain boundary of interest in UO2, electron backscatter diffraction (EBSD) in conjunction with atom probe tomography available at CAES will be used. Samples will be prepared by conventional techniques for EBSD. [1] Samples will be studied in early Spring 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 insoluble fission products (Kr) segregation in UO2, allowing for the correlation with atomic-level simulations and/or the ability to link with mesoscale structure-property relationships. 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 segregation to and away from defect structures 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 segregation in krypton-irradiated uranium dioxide. Literature has shown evidence of local segregation of krypton to different phases and a strong propensity for grain boundary segregation. The results of this study will be a significant step into predicting the behavior of fission products in fuel. 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.