Microstructural Characterization of Nuclear Materials Using Advanced Electron Microscopy Techniques A. Hoffman1, J. Duan1, M. Arivu1, A. Bratten1, H. Pommerenke1, H. Wen1 1Missouri University of Science and Technology. Rolla, Missouri 65409 USA The core of a nuclear reactor presents an exceptionally harsh environment for materials due to the combination of high temperature, high stresses, a chemically aggressive coolant and intense radiation fluxes [1]. In order to realize the development of new reactors, and to increase the lifetime of current light water reactors many new materials have been proposed. One approach is developing nanostructured materials which have enhanced radiation tolerance due to their high density of grain boundaries and or phase interfaces which act as radiation induced defect sinks. Another class of materials is high temperature ceramics which can retain good mechanical properties even at extreme temperatures. This work focused on both of these materials classes including nanostructured steels manufactured using severe plastic deformation (SPD) and nuclear matrix graphite. Work on nanostructured steels focused on pre-irradiation characterization after deformation. Grade 91 steel was manufactured using two SPD techniques: equal channel angular pressing (ECAP) and high-pressure torsion (HPT). Previous studies demonstrated that SPD can induce precipitation during deformation [2,3], therefore precipitate analysis before and after SPD was performed using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). Due to the small grain and precipitate size, transmission Kikuchi diffraction (TKD) was used also used to characterize samples after SPD. It was shown that after deformation, precipitate size and number density does not change significantly. Therefore, SPD may be effective at inducing grain refinement in grade 91 without significantly affecting secondary phases. Characterization of matrix graphite focused on oxidation behavior of two grades of matrix graphite: A3-3 and A3-27. Both grades contain similar flake graphite, but use different binders which create different microstructures. It was shown that A3-3 graphite has a anisotropic “flake” structure with a higher fraction of partially graphitized carbon (PCG) as compared to A3-27 which has less PCG with a more isotropic structure which can be seen in Fig. 2. The high content of PCG in the A3-3 graphite caused it to oxidize faster than A3-27 graphite. This has implications for accident tolerance of high temperature gas reactor fuel, as changing the binder material in the matrix graphite can have significant impact on oxidation behavior. Both studies are part of larger projects funded by the Department of Energy which include neutron irradiation and post irradiation examination. References: [1] S. J. Zinkle and G. S. Was, Acta Mater. 61, 735 (2013). [2] M. Arivu, A. Hoffman, J. Duan, H. Wen, et al., Mater. Lett. 253, 78 (2019). [3] A. Hoffman, H. Wen, R. Islamgaliev, and R. Valiev, Mater. Lett. 243, 116 (2019). [4] Acknowledgments: This work was financially supported by the U.S. Department of Energy, Office

of Nuclear Energy through the Nuclear Energy University Program (award number DE-NE0008753) and the NEET-NSUF (Nuclear Energy Enabling Technology - Nuclear Science User Facility) program (award number DE-NE0008524). Electron Microscopy Core at University of Missouri – Columbia is acknowledged for providing access to electron microscopes.

Fig. 1. Transmission Kikuchi Diffraction (TKD) orientation map (left) and energy dispersive X-ray spectrometry chemical map or Cr (right) of grade 91 steel after ECAP.

Fig. 2. a) Secondary electron SEM and b) bright field TEM images of A3-27 matrix graphite. c) high resolution TEM of a partially graphitized region in the A3-27 graphite. d) selected area electron diffraction showing the (2000) reflection indicating a high degree of isotropy.