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Nuclear Science User Facilities 120 Microstructure Evolution in Ion-Irradiated Uranium Dioxide Mahima Gupta University of Wisconsin Madison mahimawisc.edu Understanding irradiation damage evolution in uranium dioxide UO2 is crucial to protecting the worlds fleet of current and future nuclear power plants. Since all of the phenomena caused by radia- tion damage originate at point defects understanding the effects of irradiation at the atomic scale is crucial. However because the irradiation defects are aperi- odic standard approaches such as trans- mission electron microscopy TEM and x-ray diffraction XRD are ineffective necessitating the use of techniques that are sensitive to short-range order. X-ray absorption fine-structure spectroscopy XAFS measures the population- weighted local structures and chemical speciation of the examined elements making it perhaps the most incisive method for determining the local-range order in irradiated materials. TEM measurements are crucial to relating the short-range changes observed using extended X-ray absorp- tion fine-structure spectroscopy EXAFS to the complicated long-range micro- structures created through irradiation. Understanding these defects at various length scales is necessary for accurately predicting fuel degradation under reactor conditions. Project Description The goal of this experiment is to study irradiation-induced damage structure evolution in UO2 on multiple-length scales.The main objective is to combine spatially resolved synchro- tron-based X-ray absorption spectros- copy XAS withTEM observations of ion-irradiated depleted UO2. This study uses XAFS observations combined with microscopy tech- niques performed at CAES to examine lamellae having spatially varied irradiation damage. Ion irradiations that produced spatially varied micro- structures were performed at 150C on depleted UO2 samples at the University of Wisconsin Madisons Ion Beam Laboratory.The damage profiles in the helium He-implanted samples were about 10 m and were as flat as possible for the first 15 m. With the completion of these experiments the targeted research has increased our understanding of fuel degradation and added to the knowl- edge base of the nations nuclear infra- structure.The benefits of knowing how the local structure of ion-irradiated UO2 evolves extend beyond nuclear fuel operations into long-term storage. Being able to predict the stability of The first demonstration of micro-focused x-ray absorption fine-structure measurements on FIB UO2 lamellae was successful clearing the way for analysis of highly radioactive neutron-irradiated samples at synchrotron facilities. reactor-irradiated UO2 is crucial to determining what chemical reactions will occur while a fuel is being stored. Accomplishments All the milestones described in the ATR-NSUF rapid-turn-around proposal have been met.A technique to perform -EXAFS measurements has been developed at the Stanford Synchrotron Radiation Lab SSRL and a focused ion beam FIB lamella consisting of the damage layer created by injecting He ions into bulk UO2 samples was extracted at CAES. Figure 1 compares the extracted FIB lamellae Figure 1.1 to the overlay of the ion-damage profile created using Stopping Range of Ions in Matter Figure 1.2. Five such lamellae two each from the two irradiated samples and one from the pristine sample were mounted on an FIB grid.The grid was then inserted into a -EXAFS holder specially designed for the SSRL.Two of these samples are shown in Figure 1.2.They were measured at a 45-degree angle from the incident beam in fluorescence geometry. Further bulk EXAFS analysis was performed on krypton-implanted samples at an angle of 10 degrees from the incident beam which interrogated the irradiated region to a depth of 1.