CVR-SiC coating of graphite pebbles for fusion blanket application

The Helium Cooled Ceramic Reflector (HCCR) Test Blanket Module (TBM), recently planned for installation and testing in the nuclear fusion energy project of International Thermonuclear Experimental Reactor (ITER), uses graphite in the form of pebbles as a neutron reflector, opening the possibility of avoiding the use of a beryllium neutron multiplier. A silicon carbide (SiC) coating should be formed on the graphite pebbles to prohibit the reaction of graphite with steam and/or air and to reinforce the surface strength. In this work, a dense β-SiC coating was formed over the surface of the graphite pebbles by a chemical vapor reaction (CVR) process of the solid–solid (SS) and vapor–solid (VS) reaction of the SiO2(s)–C(s)–SiO(v)–CO(v) system. The microstructural features, XRD patterns, pore size distribution, porosity and oxidation behavior of the SiC-coated graphite pebbles were investigated. A SiC coating with a thickness of approximately 30 μm remarkably improved the oxidation resistance and the density of the graphite pebbles. In an isothermal oxidation test conducted at 700 °C, the SiC-coated graphite pebbles showed strong oxidation resistance and a weight loss of approximately 2 wt% over the course of 2 h. This paper describes the major results obtained from our experiments on the CVR-SiC coating of nuclear graphite pebbles, recently conducted in the Korean HCCR TBM team of the ITER project.