Numerical investigation on the effects of fundamental design parameters on freeze plug performance in molten salt reactors

Systematic numerical simulations were performed to explore the effects of the wall thickness and inner diameter of the freeze plug utilized in the molten salt reactors (MSRs). The opening time that is one of the most important factors to ensure the passive safety of the MSRs is dependent obviously on the initial shape of the frozen salt formed in the freeze plug tube. In view of this, a simulation of the solidification process was carried out first and it was followed by the melting process simulation. In the simulation of solidification, it was found that the heating and cooling powers required to close the freeze plug appropriately tend to increase with increased values of the wall thickness and tube diameter. The simulation results of melting showed that the opening time can be shortened by utilizing a thick tube wall since the heat conveyed within the tube wall by thermal conduction increases to accelerate the melting of the frozen salt. As for the effect of the tube diameter, small tube was advantageous to shorten the opening time but required a long time to drain the liquid salt from the reactor core to the drain tank after the opening. The present simulation results indicated that all the three processes of solidification, melting and drainage should be understood sufficiently to utilize the freeze plug as an effective passive safety system in the MSRs.