Small,long-life high temperature gas-cooled reactor free from prompt supercritical accidents by particle-type burnable poisons

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 subcritical operation, a greater decrease of core temperature was required. Remarkable decreases in the core temperatures, approximately 347 K for the B₄C + Gd₂O₃ case and approximately 280 K for the B₄C + CdO case, resulted in the drop of thermal efficiency to only 35.9% and 38.2%, respectively. Therefore, the relative importance of the increase in passive safety and the decrease in thermal efficiency must be considered with regard to their importance in nuclear reactor design.