Research On Heat Transfer Characteristics Of Fuel Assembly In Fuel Transfer Tube

In the background of the increasing pressure on China's energy structure transformation and increasing environmental pollution,the third-generation nuclear power plants have drawn much attention because they can provide safe,clean,and economical energy.The 2011 Fukushima nuclear accident in Japan had a major impact on the development of nuclear power worldwide.Governments and the public of all countries pay more attention to the safety of nuclear power,and put forward higher requirements for nuclear safety.The HuaLong One,HPR1000,million-kilowatt-class third-generation nuclear power technology was independently developed by China National Nuclear Corporation aiming at the enormous national and international nuclear power market.The HPR1000 design fully implements the nuclear safety defense-in-depth design principles,and innovatively adopts the 'active and passive' safety design concept to ensure the safety of nuclear power plants.The HPR1000 fuel transfer tube is an underwater tube between the fuel building and the reactor building for the transportation of fuel assemblies.All all of the spent fuel assemblies in the reactor must be transported through the fuel transfer tube.During the transportation process,loss of power and mechanical failure may cause that the spent fuel assemblies be stuck in the fuel transfer tube indefinitely.Without sufficient and timely cooling,there may be a risk of local overheating and damage to the fuel assemblies.The natural circulation cooling process during the detention of the fuel assemblies in the transfer tube involves axial natural circulation between the transfer tube and the pools,natural convection in the tube cross section,crossflow across the spent fuel assembly,and so on.Several improvements adopted by HPR1000 caused more stringent heat transfer conditions for the assemblies,and the thermal-hydraulic safety of the fuel assemblies in the transfer tube needs to be studied.Firstly,a calculational and analytical model was developed based on RELAP5.The influence on flow and heat transfer of several veriables such as assembly power and water temperature of the pools was studied under several assumed conditions.The overall flow and heat transfer law in the pools,the transfer tube and the carrier was obtained.The approximate postion with the maximum wall temperature and the corresponding condition were determined.It provided a basis for the following experimental study and CFD simulation.The calculation shows that,the maximum fuel cladding temperature is lower than the local saturation temperature,and the fuel assembly is safe.Secondly,the design of the test facility was carried out.A set of similarity criterion was developed for distortion analysis of the test facility.The analysis results show that the test facility is conservative to evaluate the thermal-hydraulic safety of the assembly in the transfer tube.On the basis of the above research,the experimental study of single-phase natural convection heat transfer characteristics and pool boiling heat transfer characteristics were carried out.The influence of reference water temperature and power of simulating assembly on the fluid temperature and fuel cladding temperature in the carrier was explored.Experimental heat transfer data of the top rod were obtained with heat flux in the scope of 3900-6800W/m2.New natural convection heat transfer correlations were presented.The results show that the heat transfer coefficient of the first rod in the top-corner area of the carrier is the lowest and the wall temperature is the highest with a certain heat flux and inlet water temperature.Following the experimental study of single-phase natural convection heat transfer characteristics,the pool boiling heat transfer characteristics of the spent fuel assembly in the top-corner area of the carrier under two-phase flow condition were explored.Boiling heat transfer coefficients of the rods in the top-corner area were obtained with heat flux in the scope of 2400-20000W/m2.New boiling heat transfer correlations were presented.The research results can provide a reference for evaluating the thermal safety status of fuel assemblies under worse conditions during the transport process in future engineering applications.On the basis of experimental research,a CFD calculation model for natural convection heat transfer of spent fuel assembly in the semi-closed space enclosed by the transfer tube and the carrier was established.The comparison of the single rod test data,simple rod bundle data and the test data obtained by this study verifies the accuracy of the calculation model.On this basis,detail calculations and analysis were carried out on several conditions assunming that the spent fuel assembly of the HPR1000 was stuck inside the fuel transfer tube,and the thermal safety performance of the spent fuel assembly was evaluated.The evaluation results show that,under all possible conditions,the maximum fuel cladding temperature does not exceed the local saturation temperature and the fuel assemblies are safe.The calculation model and the evaluation results can be used as an important reference for the overall safety performance evaluation of the HPR1000 third-generation nuclear power.