Experimental Study On Flow And Heat Transfer Of Passive Residual Heat Removal System For Molten Salt Reactor

As the fourth generation of advanced nuclear power reactor,molten salt reactor has been put on the agenda again in China recently because of its many advantages.For any reactor,the safety issue is always important.As one of the safety barriers,the safe operation of the residual heat removal system is very important to the reactor.At present,the advanced reactor adopts passive residual heat removal system,which leads to the core decay heat through natural circulation and greatly increases its safety threshold.Passive safety design should also be introduced into the molten salt reactor residual heat export after shutdown to improve the safety.However,the research on molten salt reactor in China is still in the primary stage,so the research on thermal and hydraulic problems related to passive residual heat removal system is still insufficient.Therefore,in this paper,the flow characteristics in the natural circulation heat exchanger,the heat transfer characteristics outside the condenser tube and the thermal stratification characteristics inside the condensate tank are studied in detail.First of all,based on a set of full-scale water-cooled passive residual heat removal system for 2MW molten salt reactor designed and built by ourselves,this paper studies the transient and final steady-state flow and heat transfer characteristics of the relevant components of the system under different startup modes.Through the experimental study on the preheating and startup process of natural circulation thimble heat exchange elements,it is found that the natural circulation startup process will go through four stages: static heat storage stage,natural circulation startup stage caused by subcooled boiling,single-phase natural circulation stage and two-phase natural circulation stage.The two-phase natural circulation stage can be divided into two stages: the geysering stage and the flash-induced flow instability stage.Finally,it is found that when the heating power is over 1200 W,the corresponding boundary temperature is over 600 ?,and the system can finally enter the two-phase stable flow stage.According to the single-phase natural circulation stage of the thimble heat exchanger,the calculation formula of the laminar flow friction pressure drop in the annulus is in good agreement with the experimental results,and the relationship between the heating power of the thimble heat exchanger and the single-phase flow rate in the natural circulation loop is obtained.According to the unstable stage of the two-phase natural circulation,the two-phase flow instability is analyzed.For the stable two-phase natural circulation stage,the total pressure drop distribution of two-phase flow in the riser of the annulus is analyzed.Based on the established void fraction model of drift flow,this paper analyzes the corresponding relationship between the two-phase friction pressure drop in the riser and the flow parameters of the system,creatively considers the influence of flash and local pressure on the gas-phase parameters in the system,analyzes the main factors affecting the two-phase friction pressure drop from the perspective of mechanism,and gives a new two-phase friction pressure drop model by referring to Lee-Mudawar correlation The error range of prediction correlation is + 2.18%-4.70%.The temperature distribution and thermal stratification of the system in two different startup modes are analyzed and compared.On the whole,there is obvious thermal stratification along the vertical direction of the condensate tank;there is almost no temperature difference between the various parts in the horizontal direction.By introducing the dimensionless criterion,the prediction of the formation of thermal stratification,the development of thermal stratification and the development rate of thermal stratification in the condensate tank are made,and the mechanism of the formation of thermal stratification in the two modes is effectively described.Finally,through the study of the overall and local heat transfer characteristics of the condenser,the heat transfer mechanism of three different stages of the external heat transfer of the condenser tube,i.e.natural convection stage,subcooled boiling stage and saturated pool boiling stage,is analyzed.The differences and error sources of several different single-phase free convective heat transfer prediction models and two-phase boiling heat transfer prediction models are analyzed and evaluated.Considering the effects of tube bundle effect and slender cylinder,the hydraulic diameter is taken as the characteristic length of single-phase free convection to obtain better prediction,and the causes of special heat transfer coefficient distribution of two-phase are analyzed and evaluated.