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Nuclear Science User Facilities 68 Multiscale Investigation of the Influence of Grain Boundary Character on Radiation-induced Segregation and Mechanical Behavior in Steels Used in Light Water Reactors Mitra Taheri Drexel University mtahericoe.drexel.edu Project Description This project is centered on under- standing the behavior of 304 and 316 stainless steels 304SS 316SS in light water reactors LWR.The primary objective is to understand and quantify any microstructural evolution taking place related to long-term aging thermal effects and any irradiation-induced or enhanced solute segregation and precipitation occurring in-grain or at the grain boundaries. A second objective is to understand the role of grain boundary character in low-stacking-fault face-centered- cubic fcc stainless steels including any dependence they exhibit on irradiation-enhanced or -induced grain boundary solute segregation under long-term irradiation. The research provides through advanced electron and atom probe microscopy direct quantification of grain boundary character- dependent stainless steel irradiation responses during long-term aging in current LWRs. Accomplishments Drexel Universitys technical objective in the project was to investigate the role of grain boundary character in radiation-induced segregation and precipitation in fcc austenitic stain- less steel.A 304SS from the Experi- mental Breeder Reactor II EBR-II outer-blanket assembly Row 13 was previously irradiated to a peak fluence of 4.5x1021 ncm2 . High-quality electro-polished bulk samples were prepared at INLs Hot Fuel Examina- tion Facility HFEF for end-user examination at CAES using advanced microscopy techniques.These tech- niques included electron backscatter diffraction EBSD transmission electron microscopy bright field TEM-BF scanning transmission elec- tron microscopy energy dispersive x-ray spectroscopy STEM-EDS and atom probe tomography APT all of which are capable of probing any grain boundary character dependence on multiple-length scales from mesoscale grain boundary structure to atomic scale solute segregation. The 304SS U1302 sample was a hexagonal duct strip from the outer-blanket assembly irradiated The research provides direct quantification of the responses of grain boundary character- dependent stainless steel under LWR irradiation.