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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.

A design concept for a high temperature gas-cooled reactor without the possibility of a prompt supercritical accident has been proposed by coupling the use of particle-type burnable poison (BP) and criticality control by the core temperature. The combinations of two different BPs, B4C and Gd2O3 particles and B4C and CdO particles, with the proper particle sizes and the appropriate volume ratio, showed excellent performance in controlling excess reactivity and flattening the reactivity swing. To maintain reactivity at a lower level than the prompt critical state, the reactor was designed to operate in a subcritical mode for a burnup period or for the whole operation cycle. Under subcritical operation during the partial burnup period, the core temperature had to be lowered by at least 164 K for the loading of B4C + Gd2O3 particles and by at least 178 K for the B4C + CdO particles, which in turn dropped the thermal efficiency from 48% to 42.26% and 41.77%, respectively. On the other hand, under full subcriti...