The use of passive shutdown systems to enhance safety is one element of next-generation reactor design. The Freeze-Valve has been proposed as a key device in the passive system to stop the chain reaction of the Molten Salt Reactor (MSR), which has been chosen by Generation IV International Forum (GIF) as one of the six Generation IV reactor concepts. During reactor normal operation, the molten salt in the valve is cooled to a solid plug. In the event that the reactor overheats under accident conditions when all other active control systems fail, the plug will melt. The liquid fuel salt will be pulled out from the reactor core by gravity into dump tanks, and criticality will cease because the reaction is no longer moderated by the graphite in the reactor core. The more accurate the Freeze-Valve’s thermal design is, the more efficient the passive shutdown system becomes. In this study, an investigation of the thermal performance of the Freeze-Valve is conducted based on finite element methods verified by experimental data, and some modified designs are presented with recommendations. For further consideration, some innovative governing techniques used to control the Freeze-Valve are discussed in detail. Here, a more critical thermal design is focused on that can make the passive system shut down the nuclear reactor quickly and reliably. The Freeze-Valve can be used in the molten salt loop rather than a mechanical valve, which may become jammed by frozen salt. Paper published with permission.

The thorium molten salt experimental reactor with solid fuels (TMSR-SF1) was one of the conceptual designs developed by Shanghai Institute of Applied Physics, Chinese Academy of Sciences (SINAP, CAS). The fuel pebbles at the reactor core and the pool type structure of the vessel increase the complexity of thermal-hydraulic (T-H) analysis of the reactor. In order to analysis the T-H feature of TMSR-SF1 in case of the postulated Loss Of Forced Cooling (LOFC) accident, and investigate whether its external air cooling system with nature convection is capable of removing the residual heat, the Computational Fluid Dynamics (CFD) method was used to model the reactor and simulate the transient. In this research, an integrated pseudo 2-D thermal-hydraulic model of the core was developed and a simulation and analysis of the LOFC accident has been conducted. The preliminary calculation results using CFD method show that the external air cooling system has the capability of removing the residual heat. The calculation results also indicate that the peak temperatures of the fuel pebbles, key components and structures of TMSR-SF1 remain under the safety limits and the temperature of the molten salt remains below boiling point.