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Nuclear Science User Facilities 86 Irradiation-Assisted Diffusion of Uranium-Iron Diffusion Couples Lin Shao Texas AM University lshaotamu.edu Irradiation of nuclear fuel results in fuel swelling and the production and transport of fission products to the cladding. This contact may allow chemical and mechanical interactions with the cladding. Since the chemical compatibility between the fuel and the cladding is imperative for safe opera- tion of a reactor it is essential to limit the interdiffusion between the two. Project Description This project is investigating fuel- cladding interactions with prototypic material systems that have traditionally been difficult to study these interac- tions under extreme conditions. Accomplishments The objective of this research is to provide an understanding of thermally activated and irradiation-enhanced multicomponent-multiphase diffu- sion and microstructural evolution in transition and rare earth metals relevant to fuel-cladding interactions. Studies to date have validated equi- librium phases predicted by phase diagrams of uranium-iron U-Fe uranium-nickel U-Ni and uranium- zirconium U-Zr binary systems. In addition researchers observed unusual microstructures in several phases which were not reported before. Discrepancies with previous publica- tions were documented for crystalline structures and lattice parameters of certain phases. Integrated diffusion coefficients at different temperatures and their activation energies were extracted for each diffusion couple. In the U-Fe system researchers found that integrated diffusion coefficients for diffusion couples prepared using polycrystalline Fe and single-crystal- line Fe are largely different. Figure 1 shows that diffusion coefficients for the polycrystalline Fe sample are systematically larger and the diffusion activation energy is lower than that of the single crystalline Fe sample. This is because atoms can use grain boundaries as quicker diffusion paths to broaden the widths of interfacial phases.This finding also points to the necessity of using single crystal diffu- sion couples for better comparisons between modeling predictions and actual results. Figure 2a-d plot the U and Fe elemental distributions in U-Fe single crystal diffusion couples obtained at different annealing temperatures. Figure 2e-f shows typical back-scattered-electron BSE images of diffusion annealed at 625C and 650C respectively. Future Activities All the focused ion beam FIB time allocated for this project has been used. Researchers plan to write another proposal for continuing support. Publications and Presentations 1. T. ChenT. Smith J. Gigax D. Chen R. Balerio B. H. Sencer J. R. Kennedy L. Shao Diffusion ki- netics and grain boundary effects in interface reactions of UFe diffusion couples Journal of Nuclear Materialsin press. 2. L. Shao D. Chen C-C.Wei M. Martin X.WangY. Park E. Dein K. R. CoffeyY. Sohn B. H. Sencer and J. R. Kennedy 2015 Radia- tion effects on interface reactions of UFe UFeCr and U FeCrNi Journal of Nuclear Materials vol. 456 pp. 302. See additional publications from other years in the Media Library on the NSUF website. Since the chemical compatibility between the fuel and the cladding is imperative for safe operation of a reactor it is essential to limit the interdiffusion between the two. Figure 1. Comparisons of integrated diffusivities forming UFe2 and U6 Fe phases in U-Fe diffusion couples using polycrystalline Fe and single crystal Fe respectively.