Effects of Stoichiometry and High-Temperature Annealing on Zirconium Carbide Coating Layer in TRISO Particles

Very-high-temperature gas-cooled reactors (VHTR) are being developed to provide higher thermal efficiency and high-temperature process heat. Zirconium carbide (ZrC) has been proposed as a potential coating material for TRistructural-ISOtropic (TRISO) coated fuel particles because of its excellent resistance to fission products corrosion, good thermal stability and higher mechanical strength under elevated temperatures. The integrity and performance of the ZrC coating of the TRISO particles are very important as it provides the main barrier for fission product release. Therefore, the microstructure and property evolution of ZrC coating deserve to be investigated. Fluidized-bed chemical vapor deposition (FB-CVD) has been conducted to fabricate the ZrC coating in a ZrCl4−C3H6-Ar-H2 system. The stoichiometry of ZrC was changed by controlling the feeding rate of ZrCl4 and the flow rate of C3H6. The ZrC coatings were annealed from 1700 °C to 2200 °C to study the possible changes in microstructures and temperature-dependent performances. The effect of stoichiometries on ZrC coating was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy (Raman), and nanoindenter. Results showed that free carbon prevents grain growth under high-temperature annealing, and it reacts with ZrC1-x at higher temperatures to form pure phase ZrC. In addition, the microstructure evolution mechanism of ZrC at high temperatures was proposed.