Irradiation of oxide fuels, specifically UO2 leads to the formation of insoluble fission products, such as helium (He) changing the path phonons travel, reducing its thermal transport behavior. The addition of insoluble fission products, which tend to cluster intragranularly with increasing irradiation dose, swell the fuel, degrading its overall fuel performance. Of interest in this proposed research is to investigate the evolution and segregation of He in UO2 with Atom Probe Tomography (APT). APT is uniquely suited due to its nanometer scale spatial resolution and chemical identification abilities to 3-dimensionally investigate the growth characteristics of the He clusters and its chemical composition. Additionally of interest in this proposal is to investigate the changes in UO2 near clusters. In order to characterize the precise chemical and spatial distributions of the He bubbles in UO2, laser-assisted atom probe tomography (APT) in conjunction with the focused ion beam (FIB) system available at CAES will be used. The FIB system will be used to prepare site-specific samples that target regions inside the grains and near grain boundaries to fabricate atom probe tips. Each sample analyzed will be reconstructed using the IVAS software (available both at CAES and PI Manuel’s laboratory at the University of Florida) to study He bubble evolution and segregation in UO2. Samples will be studied in the late Fall or Winter 2012. Final results will then be published in peer- reviewed journal articles and presented at fuels-related conferences. It is expected that these results will provide new insight into the spatial and chemical distribution fission product clusters in UO2, allowing for the correlation with atomic-level simulations and/or the ability to link with mesoscale structure-property relationships.

A detailed atomic-level experimental study is proposed to address a critical need for material-physics relationships under the Light Water Reactor Sustainability program. The proposed study uses atom probe tomography to characterize the effect of fission damage processes in nuclear fuel. The material under investigation for this study is helium (He) doped polycrystalline UO2. The element He was chosen because of its documented role in fission product damage, namely fission gas release, void formation, and grain modification. It is proposed that this physics-based description will provide deeper insight into the microstructural changes that occur in nuclear fuel under irradiation. The specific changes that are of interest to this study are the migration of fission products to the grains and the formation of solute clusters. Understanding these mechanisms will provide new fundamental understanding of void formation and grain boundary deterioration in nuclear fuel. Mitigating these behaviors can prevent the loss of thermal and mechanical performance of the fuel, extending the lifetime of fuel in Light Water Reactors.