Conceptual design of a freeze-tolerant Direct Reactor Auxiliary Cooling System for Fluoride-salt-cooled High-temperature Reactors

One of the key potential advantages of the Fluoride-salt-cooled High-temperature Reactor (FHR) is the use of passive safety systems that ensure the safe response of the reactor during anticipated operational occurrences, design basis events, and beyond design basis events. Typically, FHRs use a Direct Reactor Auxiliary Cooling System (DRACS) to remove the decay heat under a variety of accidents when the active reactor shutdown system is unavailable. Thus, it plays a key role in preventing the overheating/overcooling of the reactor in case of the failure of other primary means of heat removal. To enhance the operational reliability of DRACS, two DRACS designs are proposed in this study. The proposed DRACS designs are composed of two natural circulation salt loops and a water tank as the heat sink. This study focused on the transient behavior of DRACS of the FHRs during Loss of Heat Sink (LOHS) accident. Calculations were performed using RELAP5-3D to evaluate the design features of each concept, based on one design concept, the University of California, Berkeley's Mark-1 pebble-bed FHR (PB-FHR). The calculation shows that the Mk-1 PB-FHR with DRACS proposed in this paper can operate for a relatively long time (about 90 h for design A and 430 h for design B) for a LOHS transient without operator involvement.