Concept of prismatic high temperature gas-cooled reactor with SiC coating on graphite structures

A 100-MWth prismatic high temperature gas-cooled reactor was designed to be a long-life small reactor. To acquire a passive safety feature, the reactor was mainly improved with regard to graphite oxidation resistance. The concept of applying a silicon carbide coating layer on the surface of the graphite structures in the core was proposed to overcome any serious problem from graphite oxidation during unforeseen situations. However, there was concern that the deviation of neutronic and thermal properties of silicon carbide from graphite could affect the reactor operation and the heat transfer characteristics. Therefore, in this study we investigated the effects of applying a silicon carbide coating layer over the graphite structures from the neutronic and thermal-hydraulic points of view. Silicon carbide coating can lower the effective multiplication factor and shorten the reactor operating cycle, but not significantly. From the viewpoint of thermal-hydraulic operation, silicon carbide has lower thermal conductivity than that of graphite, so the layer of silicon carbide could act as a wall to keep the heat from moving across the layer. Under normal operation, the layer of silicon carbide coating had a less significant impact on the maximum fuel temperature, and the temperature remained lower than the maximum acceptable fuel temperature of 1495 °C for normal operation even when a thick layer of silicon carbide was applied. The silicon carbide later did have a significant impact on the increase of graphite moderating temperature. In summary, the reactor with a silicon carbide coating layer could safely operate under normal operating condition. Although the coating caused a decrease in the discharge burnup of the reactor, the improved passive safety of the reactor compensated for that disadvantage.