Characterization of SiC Layers in TRISO Coated Particles at Various Temperatures

TRISO-coated fuel particles for high-temperature gas-cooled reactors consist of UO₂ microspheres coated with multilayers of porous pyrolytic carbon (PyC), inner dense PyC (IPyC), silicon carbide (SiC), and outer dense PyC (OPyC). For a uniform coating of the microspherical particles, a fluidized-bed chemical vapor deposition (FBCVD) method is utilized. Among the TRISO coating layers, the SiC layer is particularly important because it acts as a diffusion barrier to gaseous/metallic fission products and a miniaturized pressure vessel of the fuel particle. In order to insure the integrity of the SiC layer after a fabrication, the microstructure, mechanical properties, and chemical composition of the SiC layer should be properly controlled. Properties of the coating layer depend largely on the FBCVD conditions such as a flow rate, concentration of the coating gas, coating temperature, etc. In this study, we investigated the influence of deposition temperatures on the property of SiC layer. Microstructure, chemical composition, and other properties of the SiC layer were characterized by various techniques. TRISO coatings on ZrO₂ surrogate kernels were conducted using a FBCVD method. The influence of the deposition temperature on the properties of the SiC layer were investigated. The SiC layers coated at 1350 .deg. C contained a small amount of excessive carbon, while the stoichiometric composition of SiC coating layer was obtained at 1400 .deg. C~1500 .deg. C. Moreover, it was verified that the observed acoustic SiC bands in Raman spectra was due to an increased grain boundary caused by fine grains and stacking faults.