Experimental Study On Thermal-hydraulic Characteristics Of Passive Residual Heat Removal System For Molten Salt Reactor

When the molten salt reactor (MSR) is normally or accidently shut down, the passive residual heat removal system (PRHRS) is one of the important systems to ensure its safety. In this paper, the PRHRS for the MSR was presented. An experimental device of the water-cooled PRHRS with single heat transfer element for the MSR was designed and built. The flow and heat transfer behaviors of the system were studied by experimental measures. The operating characteristics of the system were analyzed and summarized, which would provide basic foundation for engineering design and optimization of the PRHRS for the MSR.While the water-cooled PRHRS for the MSR is operating, the cooling water inside the steam drum gets into the thimble heat transfer element through the inlet of center pipe and flows downward. Then it flows upward along the annular riser after it gets to the bottom. The cooling water is constantly heated and vaporized to steam in the heating section. The steam enters the condenser along the rising pipe, in which the steam is condensed to single phase water. The condensate flows back to the steam drum through condensate pipe and inlet pipe of the steam drum due to gravity,thus a complete natural circulation is accomplished. In this paper,the vertical electric furnace was used to simulate the temperature field of the high temperature molten salt in the salt drain tank. According to the purpose and content of the study, the startup and temperature drop experiments of the natural circulation in the system, the experiments of the saturated boiling heat transfer and flow under the stable natural circulation and the influence of the structure sizes of the thimble heat transfer element on the characteristics of the system were analyzed and carried out. The characteristics of the startup and temperature drop processes of the system's natural circulation were studied. The effects of parameters varieties on the thermal-hydraulic characteristics of the system were studied under the stable operation conditions, including the steam drum level, the wall surface temperature of the outer thimble tube, the condensing tank level, the outlet resistance of the steam drum, the width of the gas gap, the width of the annular riser of feed water and the emissivity of the outer thimble pipe.Furthermore, the mathematical modeling of the experimental system was established, the heat transfer process of the system was calculated by a theoretical method, and the calculated results were compared with the experimental values.The experimental results show that the startup process of natural circulation of the system includes two stages, that is, the establishment of the natural circulation within the heat transfer element and the establishment of the natural circulation of the system. In the process of startup,stable operating and temperature drop, the system meets the requirements of heat removal well.The heat generated by the vertical electric furnace is exported safely and reliably. Experiments proved that reducing the width of the gas gap will obviously increase the heat transfer capacity of the system, and appropriately reducing the width of the annular riser of feed water can enhance the ability of natural circulation within the thimble heat transfer element. In order to ensure the water-cooled PRHRS for the MSR can be quickly and safely put into operation under normal or accident shutdown conditions, it is recommended that the system should be in a long-term hot standby state with low-power operation. The theoretical calculations are consistent well with experimental results, and errors are within 13%.