Preparation of SiC Composite Coating on the Surface of Graphite Spheres in VHTR

Abstract Due to various characteristics such as high thermal efficiency, inherent safety, passive safety, and hydrogen production for industrial use, the high temperature gas-cooled reactor (HTR) is considered as one of the most promising technologies in the future. Fuel elements are the core part of a reactor. In the existing high-temperature gas cooled reactor HTR, spherical fuel elements are involved, with graphite accounting for more than 90% of the mass and volume of the fuel element. However, graphite is prone to oxidation in oxidizing atmospheres at a higher temperature than 400°C. Regarding the design of future HTR, accident conditions must be taken into consideration. Under the accident conditions of inlet and outlet water, the graphite in the spherical fuel element is easy to be oxidized. This causes damage to the fuel elements and increases the release of fission products, which threatens the safety of HTR. In this study, two-step pack cementation is performed to prepare SiC composite coatings on the surface of graphite spheres with a diameter of 40 mm, which improves the oxidation resistance of graphite spheres. According to research, the thickness of coating is about 150 μm after two-step pack cementation, with SiC and Si as the main phase of the coating. However, it is necessary to improve the density of SiC composite coatings needs. After 7 h of oxidation in static air at 1500°C, the mass loss reaches 22.22%. To a certain extent, the compactness of the coating can be improved by the slurry impregnation and Si sol gel method.