Transient Analysis of Depressurized Loss-of-Forced-Circulation Accident Without Scram in High-Temperature Gas-Cooled Reactor

The transient response of the high-temperature gas-cooled reactor (HTGR) to depressurized loss of forced circulation combined with failure of all reactor trip systems, a beyond-design-basis accident, is analyzed for an extended period of time during which no active core cooling is resumed. The characteristic behavior of the reactor during the long-term conduction cooldown event is found to be shaped by several parameters that are usually not considered in the safety design of the HTGR. For example, while the Doppler effect is usually relied upon to provide inherent shutdown of the reactor, the reactivity coefficient of temperature of the graphite moderator is found to be a critical parameter for determining the final settling temperature of the fuel following the recriticality. Furthermore, this study finds that the peak fuel temperature reached during this event is correlated strongly even to the initial core operating temperature prior to the initiation of the transient event. These and other results of this study are expected to provide useful input to the development of enhanced safety design guidelines for commercial HTGRs in the aftermath of the Fukushima accident.