Multi-scale, multi-modal characterization of silicon carbide cladding following safety testing in the Transient Reactor Test facility (TREAT)

Silicon carbide fiber-reinforced silicon carbide matrix (SiCf-SiCm) composites are promising candidates for accident-tolerant fuel (ATF) cladding in nuclear reactors because of their exceptional oxidation resistance, irradiation tolerance, and structural stability under extreme conditions. This study investigates the microstructure and mechanical performance of a SiCf-SiCm composite cladding following a transient irradiation test in the Transient Reactor Test (TREAT) Facility. A multi-scale, multi-modal post-irradiation examination (PIE) characterization approach was employed to quantify radiation effects on the SiC fiber and chemical vapor-infiltrated (CVI) matrix. In-situ micro-tensile testing revealed a significantly higher average ultimate tensile strength (UTS) in the CVI region (2597 MPa) compared to the fiber region (939 MPa), highlighting the distinct impact of defects and flaws on tensile properties. Transmission electron microscopy (TEM) confirmed the retention of the 3C-SiC phase without secondary phase formation or amorphization. Four-dimensional scanning transmission electron microscopy (4D-STEM) analysis quantified significant differences in grain morphology: the CVI region exhibited a preferred orientation with grain sizes ranging from 115 to 1800 nm, while the fiber region displayed a randomized structure with fine grains between 21 and 78 nm. Atom probe tomography (APT) analysis demonstrated compositional stability with no evidence of clustering or precipitation, while identifying silicon segregation of approximately 10 at.% at a grain boundary within the CVI region. The integration of advanced characterization techniques confirms that no phase instability or microstructural degradation was detected following the short-duration transient test.