Influence of Water Vapor Content on Oxidation of the SiC Layer of Tristructural Isotropic Particles

ABSTRACT Surrogate tristructural‐isotropic (TRISO) fuel particles were oxidized at temperatures between 1000°C and 1300°C for 4 h in 5 and 10 kPa water vapor atmosphere balanced with ultra‐high‐purity helium gas. These experiments were performed to investigate the behavior of TRISO fuel particles during an off‐normal accident scenario involving exposure to a steam‐based oxidizing environment. The oxide scale morphology and oxide nature were studied via scanning electron microscopy, focused ion beam, transmission electron microscopy, and atom probe tomography. The oxidation mechanism was consistent over the temperature range for 5 and 10 kPa H 2 O with an activation energy of 247.0 ± 8.6 and 228.7 ± 2.6 kJ/mol, respectively. No difference in the oxidation mechanisms was observed between 5 and 10 kPa H 2 O. The area fraction of the crystalline regions in the oxide layer increased with an increase in the oxidation temperature. Networks of bubbles (coalescing to form elongated fissures), formed as a result of the gaseous products during the reaction, were believed to serve as diffusion pathways for the oxidant to the reaction interface (SiC/SiO 2 ). Preferential attack of water vapor on grain boundaries was observed using atom probe tomography.