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Nuclear Science User Facilities 94 Study of the Microstructures of Krypton and Xenon In Irradiated Uranium Dioxide by Advanced Microscopy Techniques Lingfeng He Idaho National Laboratory lingfeng.heinl.gov Initiated in 2012 this project is a collaborative effort of INLArgonne National Laboratory and the University ofWisconsin.The purpose of this rapid turnaround experiment is to clarify the microstructure and stoi- chiometry of uranium dioxide UO2 under irradiations of krypton Kr and xenon Xe ions. Project Description Kr and Xe are the main fission gases in UO2 nuclear fuel.The radiation defects i.e. dislocation loops and precipitates i.e. gas bubbles induced during fission affect the fuels struc- ture and thermal transport properties. It is therefore important to understand and model the behaviors of Kr and Xe in order to develop optimized fuel microstructures that allow improved performance of the UO2 fuel. Accomplishments To simulate the fragment damage to UO2 caused by fission depleted UO2 samples were irradiated with Kr and Xe ions.To date the research team has used a focused ion beam FIB to prepare cross-section lamina of the irradiated samples and transmission electron microscopy TEM to study the extended defects including bubbles dislocation loops and dislocation lines as well as stoichiometry change. The results indicate bubble formation under irradiation at room-temperature Figure 1 with no solid precipitates forming in the bubbles Figure 2. This indicates that uranium U vacancies may be mobile at tempera- tures below room temperature. It could also imply that KrXe bubbles may directly nucleate at the vacancy clusters that are produced in cascades at room temperature. Such a process might not require that KrXe and U vacancies be diffusive. In addition electron energy loss spectroscopy EELS shows that under a vacuum the stoichiometry of UO2 is relatively stable during irradiation at room temperatures.At the same time the branching ratio of the M-edge in EELS is 0.695 which is within the range of 0.6950.720 for U4 .This implies that the stoichiometry in UO2 is unaf- fected by Xe irradiation Figure 3. We also found that the formation of dislocation-denuded zones is tempera- ture sensitive.At 800C enough U interstitials migrate toward the grain boundaries so that the concentra Figure 1. TEM images of bubbles in a UO2 crystal implanted with Xe at a dose of 5 1014 ionscm2 .