Numerical investigation of behaviour of small modular molten salt reactor freeze plug under various reactor operating conditions

Advanced reactors are characterized by passive safety systems to be activated in case of an accident. Molten salt reactors are equipped with a passive emergency draining system. In case of an accident, the reactor shuts itself down, the freeze plug melts due to decay heat, and fuel salt is discharged into the draining tank. The behavior of the freeze plug during liquid-to-solid transition and reactor operation, as well as in the event of an accident was investigated taking two important safety criteria into consideration i.e. the plug shall not melt during normal operation (unwilling opening) and shall passively melt in case of an accident. To evaluate the effectiveness of the freeze plug, computational fluid dynamics method was used. The results showed that draining pipe blocking time is 900 s and 70% of the interior of the freeze plug solidifies in 2700 s. Furthermore, it was concluded that the freeze plug did not cause any unwilling opening. In addition, the freeze plug opening time was calculated as 400 s. Since this value is less than the limiting time of 1100 s which depends on material strength temperature, the freeze plug can be utilized as a passive safety system component.