Experimental Study On The Thermal-hydraulic Characteristics Of Heat Transfer Element Of Fuel Drain Tank

The passive residual heat removal system(PRHRS)of the molten salt reactor(MSR)transport the residual heat to the atmosphere by three natural circulation processes,which is a very important measure to guarantee the safety of MSRs.The natural circulation of the heat transfer element(HTE)is the most fundamental process of the PRHRS,in which the decay heat is removed and the drain tank’s safety is guaranteed.In this paper,the thermodynamic and hydraulic behaviors of the HTE are studied by experimental measures,while would provide new insight into the design and optimization of the PRHRS of MSRs.The HTE of the salt drain tank consists of three layers,namely the inner pipe,the middle pipe,and the casing pipe,which is the outermost layer and touches the salt.There is an air gap between the middle pipe and the casing pipe.The water in the steam dome goes down in the inner pipe and then goes up between the inner pipe and the middle pipe when it comes to the bottom,in which process water is boiled.Finally water flows back to the steam dome.The water density difference between the outer and inner sides of the middle pipe results in the natural circulation in the HTE.In our experiment,a vertical casing type electric stove is used to simulate the temperature field of molten salt,water and gas flow rate are measured by weight method.The start up of the natural circulation,saturation boiling at different heat temperature and water level,the influence of sizes of the inner pipe and middle pipe on heat and flow are studied in the experiment.According to the fluctuation condition in the start-up process,the natural circulation is divided into seven stages.In the start-up process,with the increase of water temperature in the tank,water flow rate at the outlet first increases and then decreases.When the driving head of the natural circulation is very small,the bubble congestion phenomenon is easy to occur,at which saturation boiling occurs at the inner wall of the middle pipe.In this process,the void fraction is so great that it reduces the flow rate at the outlet and increases the heat transfer rate.The increase of the diameter of middle pipe increases flow stability and shortens the duration time of bubble congestion and thus increase the heat transfer.The heat transfer rate of the HTE is very sensitive to middle pipe diameter.Consequently,in the design process of the PRHRS of MSRs,the air gap between the middle pipe and casing pipe should be selected according to the heat transfer rate.In order to guarantee the system stability and safety,the system should be at heated condition before the starting up,and water gap in the HTE should be small enough.