b'(Figure 2b-2c) found Cr (FigureFigure 3). A high density of cavi-2d), Zr (Figure 2e), and some Uties (white dots in Figure 3d) was (Figure2f) on the interacted Cr GBs.observed. The cavities were closely This suggests that the Cr grainassociated with Fe (Figure 3e) which boundaries have served as favorablewas possibly from the HT-9 cladding pathways diffusing Zr (and some U)by diffusion. It is not clear yet if and into the barrier. TEM characteriza- how the cavities would affect the tion found some Nd and Ce-richbarrier integrity if higher burnup particles (Figures 2g-2h) and nanowas achieved. Indexing the collected crystalline formation (Figure 2i) indiffraction patterns in Figure 3g and the interaction zone. Indexing the3h confirmed that the Cr grain has collected diffraction patterns (Figurea face centered cubic (FCC) crystal 2j and 2k which were taken from #1structure, while the as-electroplated and #2 spot, respectively, in FigureCr diffusion barrier has a body 2i) confirmed that the nanocrystal- centered cubic (BCC) crystal struc-line is intermetallic _ZrCr2 Lavesture. This suggests the occurrence phase. The formation of _ZrCr2 isof phase transformation in the Cr resulted by the reactive diffusiondiffusion barrier, which may be of Zr in the Cr diffusion barrier,induced by neutron irradiation.driving the growth of interactionComparing to the HT-9 specimens layer in the Cr diffusion barrier. The(#2-#6 in Figure 4), the higher yield brittle nature of the _ZrCr2 phase explained why microcracks formedstrength (1231 MPa) of the Cr diffu-in the interacted zone (Figure 1f).sion barrier (#1 in Figure 4) suggests Further out-of-pile diffusion coupleits higher hardness. Among the studies may be necessary to properlytested HT-9 specimens, bar #2 and #3 evaluate the growth kinetics ofshowed much lower yield strength the _ZrCr2 interaction layer, which(451 and 284 MPa, respectively) and is helpful to better understand theultimate tensile strength (754 and feasibility of the Cr diffusion barrier606 MPa, respectively) than bars to prevent/mitigate FCCI. #4-6 (799-961MPa yield strength and 984-1296 MPa ultimate tensile Figures 3a-3b show the TEM imagesstrength). This may suggest the of the tested Cr diffusion barrieroccurrence of mechanical softening micro specimen. Several Cr grains,in the HT-9 nearing the Cr barrier.with sizes ranging from hundreds of nanometers to around 2 m, were observed in the gauge area (Figure 2a-2b). Figure 3c-3d are the high magnification TEM images of a selected area in the gauge section (highlighted in green rectangle in 2023|AFC ACCOMPLISHMENTS 169'