Heat Transfer Characteristics Of Cooling Thimbles In A Molten Salt Reactor Passive Residual Heat Removal System

The key to ensuring the safety of nuclear reactors is to remove the remaining decay heat of the core adequately and promptly when the molten salt reactor is normal or accidental shutdown.The passive residual heat removal system(PRHRS)of the molten salt reactor(MSR)is exactly an important part for ensuring this function.In the PRHRS of MSR,one way to remove the decay heat of the core fuel salt is to utilize the thimble type heat transfer element.In this paper,based on the study of PRHRS for MSR,a simulation experimental loop of PRHRS for MSR was designed and constructed.The experimental loop contains six thimble type heat transfer elements and uses molten carbonate to simulate the high temperature environment of core fuel salt.The study of the two-phase boiling flow process in the thimble-type heat transfer element was carried out using the experimental loop containing molten salt,and the two-phase flow characteristics of the thimble-type heat transfer element under natural circulation were analyzed.What’s more,in order to deeper understand the heat transfer characteristics and the flow conditions of the thimble type heat transfer element used in the PRHRS of MSR,numerical calculation is also used to study the heat transfer process.Comparing the numerical calculation results with the experimental results,this paper analyzes the two-phase flow characteristics and boiling heat transfer capacity of the thimble type heat transfer element under natural circulation condition.This study can provide some reference for the design of heat transfer element and the heat removal of high temperature molten salt in the PRHRS of MSR.According to the purpose and content of this study,a heat removal experiment under steady state conditions for a certain furnace temperature is conducted,and the influences of heat flux and mass flow rate on the heat transfer and natural circulation characteristics of two-phase flow are studied.The experimental results are compared with many saturated boiling equations such as Chen,Shah,Sun and so on.The results show that the Sun formula is in good agreement with the experimental results,and the maximum error is 6%.Based on the results,the influences of convection term and boiling term on the saturated heat transfer coefficient are analyzed.In addition,the experimental data of the two-phase resistance in rising section are compared with that of Chisholm-B,Tran and so on.The results show that the experimental data are in good agreement with that of Chisholm-B formula,and the maximum error is less than 10%.In this paper,two-dimensional and three-dimensional models are used to simulate the twophase flow boiling numerical calculation.The calculated boundary conditions are given by the experimental data.The results show that the vapor fraction and mass flow rate of numerical calculation at the outlet of the heat transfer element are in good agreement with the experimental data,the average error is about 5%.According to the numerical calculation results of the temperature field,under furnace temperature of 700 °C,the single-phase fluid is heated by the annulus fluid,and the water temperature will rise about 2 °C from the inlet to the bottom.The water in the annulus will start boiling around 0.5m from the bottom.The water which the location is less than 0.5m has not yet reached saturation and is in a single-phase state.In addition,the calculation results also show that the water temperature in the annulus of the thimble type heat transfer element increases first and then decreases along the axial direction,and the water temperatures of the rising channel of the annulus are in good agreement with that of the experimental values.The water temperature results of numerical calculation can be used to supply the experimental water temperature data.The differences between the calculation heat transfer coefficients and the experimental values are less than 5%.What’s more,the difference between the numerical calculation and the experiment was also analyzed.During the numerical calculation of the low-pressure two-phase boiling flow process,it was found that when the numerical calculation was performed under the experimental conditions of the furnace temperature 500 °C,the boiling did not appear.But in fact there is indeed a boiling phenomenon in the experiment,indicating that the model for simulation of two-phase saturated boiling heat transfer in numerical calculation software needs to be further improved.