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Nuclear Science User Facilities 70 in Row 13 Position 13D4. Over the course of the EBR-II irradiations Run 1 to Run 170B the 304SS was exposed to extended periods of high operating temperatures 450460C. Researchers analyzed samples using a using multi-length scale grain-boundary site-specific process optimized to obtain detailed chemical and structural information. Specifically they performed EBSD to determine particular grain boundary misorientations and used an FIB to extract specific grain boundaries. These samples were analyzed using bothTEM and APT. The study focused on the following grain boundary characters coinci- dence site lattice CSL three coherent twin three incoherent twin and random high-angle misorientation grain boundaries. Researchers focused on the three system due to the differ- ences observed between the atomic structures of the coherent twins having a 111 symmetric tilt grain boundary plane and the incoherent twin having a 112 symmetric tilt grain boundary plane.This grain boundary examination system allows the careful study of the effects of the grain boundary plane inclination angle with respect to irradiation- induced precipitation and segregation. The goal was accomplished and all three grain boundary types were examined using both APT and STEM- EDS.The results indicated that each grain boundary type had extensive carbide M23 C6 precipitation.The presence of carbide nucleation and growth on the coherent twin grain boundary Figure 1 highlights the fact that even low grain-boundary- energy and high atomic-fit grain boundary characters can be susceptible to carbide growth when subjected to extensive irradiation at relatively high temperatures 450460C.The APT examination showed that depletion of Cr to less than 10 at. existed in regions adjacent to the grain boundary carbide.These Cr-depleted zones are typical of grain boundary carbide formations and indicate regions that are potentially susceptible to increases in localized corrosion of aging stain- less steel alloys. The second sample condition the project focused on was a 316 hex-duct stainless steel from location U9027 Row 9 in the EBR-II. Data collection on the exact reactor temperature and fluence is ongoing but they are esti- mated to be 370500C and 2025 displacements per atom dpa respectively. Unlike the 304SS detailed aboveAPT revealed that the 316SS had extensive nickel-silicon Ni-Si clus- ters throughout the examined regions. The grain-boundary microchemistry independent of grain-boundary char- acter showed areas of both carbides and extensive W-shaped Cr depletion. Figures 2-4 show the steps researchers performed on the grain-boundary structures at CAES to fully characterize a particular grain boundary EBSD hummingbird half-grid TEM holder STEM andTEM atom probe. Overall like the 304SS the 316SS has grain-boundary-depleted regions of Cr and other minor solute elements. Unlike the examined outer blanket assembly in 304SS the 316SS has extensive Ni-Si clusters at levels consistent with the presence of Ni3Si precipitates andor possible Si enrich- ment in dislocation arrays. Future Activities The project was completed in 2014. Figure 3. Sequence of three BF-TEM diffraction conditions A-C and STEM-HAADF D of the same GB structure shown in Figure 2.