2018 | ANNUAL REPORT 81 The technique of ultrasonic-resonance spectroscopy (RUS) was utilized to evaluate the collective effects of microcracks on the elastic properties of the tubes .Without changes in crack microstructures, ~6% reduction inYoung’s modulus is anticipated in SiC due to neutron irradiation alone. However, as shown in Figure 5 (left), more significant and anisotropic decreases in elastic constants are observed after irradiation in the Fire Rabbit, implying more- extensive microcracking across the circumferential and axial orientations than radial. High-resolution X-ray computed tomography (XCT) exami- nation revealed the presence of radial microcracks, which were not found in unirradiated tubes.The microcrack shown in Figure 5 (right) appears to be slightly open at the inner surface and extends to the mid-plane of the tube wall, where the inverse thermal stress is anticipated to be neutral. Figure 5. Observation and characterization of microcracks present in the SiC/SiC composite tubes following neutron irradiation in Fire Rabbit: Resonant ultrasonic spectrometry showing significant decreases in circumferential and axial elastic moduli after irradiation (left) and X-ray computed tomography identifying radial micro- cracks near the inner surface of an irradiated tube (right).