Study On The Characteristics Of Passive Residual Heat Removal System For Molten Salt Reactor

Primary consideration should be given to safety in reactor design.Passive residual heat removal system,whose principal function is to provide long term decay heat removal of fuel rods by transferring heat using natural circulation,have been widely employed into advanced reactor designs in order to improve the safety performance.At present,China is designing and developing the liquid fuel molten salt reactor,and it is a key problem that must be solved to ensure the passive decay heat removal of the fuel salt after reactor shutdown;however,relevant research work is still insufficient.Therefore,the heat transfer characteristics of the cooling thimble,flow characteristics of natural circulation,thermal stratification of molten salt and system operation characteristics are studied in this paper in detail.First of all,on the basis of solving the problem of large temperature difference heat transfer between the molten salt and the water,an experimental facility for passive residual heat removal of 2 MW molten salt reactor was designed and constructed.In order to avoid the operation risk of beryllium and its compound in the fuel salt,a mixture of carbonate salt was found by experimental measurement,whose main thermal properties were similar to that of the fuel salt.A method for conducting thermal hydraulic tests using the carbonate salt in place of the fuel salt is presented.The heat transfer characteristics of the passive residual heat removal system under different boundary temperature were analyzed with experimental results.The passive cooling system exhibited a heat removal capacity of 16 kW at salt temperature of 643?,and the heat transfer rate around solidifying point of the molten salt was 5.1 kW.It was also found that the material emissivity has a great influence on the heat transfer capacity of the cooling thimble.In the normal operating temperature range,radiation heat transfer played a leading role in heat transfer between the thimble and the bayonet tube.The contributions of several constituent thermal resistances in the heat flow path were analyzed,with the thermal resistance of the air gap being dominant.The large heat resistance of the air gap was the main factor that limited the heat carrying rate of the system,and also the main factor that the system could operate under low temperature and pressure.Through the experimental study of the transient flow characteristics of natural circulation in the cooling thimble,it was found that geysering and flashing instability would occur with decreasing the inlet subcooling,and the instability map was also obtained.RELAP5/MOD4.0 was used to analyze the influence of structural parameters of the cooling thimble on natural circulation flow capacity.The correlations between the steady state circulation flow rate and the heat transfer rate in the cooling thimble under single-phase and two-phase conditions were theoretically derived.The derived correlations agreed well with the experimental results.The natural convection of molten salt with high viscosity and low flow rate was numerically simulated by CFD software Fluent.The development process of molten salt thermal stratification was analyzed based on the variation of flow field and temperature field.The differences of natural convection and temperature distributions in the drain tank between internal heating condition and external heating condition were compared,and it was clear that the results obtained from external heating condition were more conservative.After analyzing the dimensionless numbers of natural convection,it showed that thermal stratification number could be used to evaluate the degree of thermal stratification in the drain tank,mixing number could be used to predict the development speed of thermal stratification in the entire transient process,and Rayleigh number could be used to predict the development speed of thermal stratification after the temperature gradient of mainstream was formed.Finally,the transient performance of the system during the start-up and the decay heat removal process were determined by experiments.It was found that the fluid in the cooling thimble remained stationary in the first 3800 s due to the heat transfer between fluid in downcomer and in heating section.In the decay heat removal process,the axial temperature gradient near the cooling thimble increased rapidly when molten salt near the wall of the thimble tube started to freeze,and the axial temperature gradient peaked when molten salt near the wall of the thimble tube were completely frozen.Combining the methods of experimental research,numerical simulation and theoretical analysis,the feasibility of passive heat removal from high temperature molten salt is determined in this paper.It lays a foundation for the application of the passive principle in molten salt reactor and provides a basis for the design of passive residual heat removal system of molten salt reactor in China.