High-temperature oxidation behavior of the SiC layer of TRISO particles in low-pressure oxygen

Surrogate tristructural-isotropic (TRISO)-coated fuel particles were oxidized in 0.2 kPa O-2 at 1200-1600 degrees C to examine the behavior of the SiC layer and understand the mechanisms. The thickness and microstructure of the resultant SiO2 layers were analyzed using scanning electron microscopy, focused ion beam, and transmission electron microscopy. The majority of the surface comprised smooth, amorphous SiO2 with a constant thickness indicative of passive oxidation. The apparent activation energy for oxide growth was 188 +/- 8 kJ/mol and consistent across all temperatures in 0.2 kPa O-2. The relationship between activation energy and oxidation mechanism is discussed. Raised nodules of porous, crystalline SiO2 were dispersed across the surface, suggesting that active oxidation and redeposition occurred in those locations. These nodules were correlated with clusters of nanocrystalline SiC grains, which may facilitate active oxidation. These findings suggest that microstructural inhomogeneities such as irregular grain size influence the oxidation response of the SiC layer of TRISO particles and may influence their accident tolerance.