Analysis And Optimization Of Flow Distribution For The CiADS Lead-bismuth Reactor Core

China Initiative Accelerator Driven System(CiADS)choses lead-bismuth cooling reactor as the research reactor model.As the fourth generation reactor,the leadbismuth cooled fast reactor has a stiffer neutron energy spectrum and higher neutron economy.In addition,liquid lead bismuth coolant has high thermal conductivity,and does not react with water.In order to ensure the safe operation of the reactor and improve the service life of the reactor,it is necessary to reasonably optimize the core flow distribution scheme of the CiADS lead-bismuth cooling reactor,so that the power share of the assemblies can match the flow share and the temperature distribution of coolant at the core outlet can be balanced.At present,there are few studies on the flow distribution of liquid metal fast reactor.Because it is difficult to carry out hydraulics experiments with liquid metal,the flow distribution characteristics of CiADS are studied by using the numerical simulation method with mature technical route,and the optimal design scheme of core flow distribution is given according to the power distribution of reactor core.This paper is based on the conceptual design of fuel assembly and reactor core of CiADS reactor.In order to improve the computational efficiency,the porous medium model is used to simplify the various components in the reactor.During the flow distribution,the input of the resistance characteristics of the complex geometric area will have a direct impact on the calculation results.However,there is no empirical relation that can better simulate the resistance characteristics of the bottom and top nozzles of the fuel assembly.Therefore,this paper first establishes a detailed threedimensional model for the bottom and top nozzles of the fuel assembly which cannot be better described by the empirical relation.The computational fluid dynamics software Fluent was used to analyze the resistance characteristics of the bottom nozzle under different coolant opening areas and different flow rates.The resistance characteristic coefficient of the bottom nozzle under each coolant inlet area was calculated,and the fitting relationship between the coolant inlet area and the resistance characteristic coefficient of the bottom nozzle was obtained.In addition,the resistance characteristics of the top nozzle at different flow rates are analyzed,and the resistance characteristic coefficient of the top nozzle is determined.Compared with using the empirical formula to describe the resistance characteristics,the resistance characteristics of the bottom and top nozzles are more accurate after careful hydraulics simulation calculation.Based on the above calculation results,a three-dimensional modeling of the CiADS reactor 1/4 structure was carried out to calculate the flow distribution.The calculation model covers pump,heat exchanger,flow distributor,cold and heat pool baffle,dummy assemblies,bottom and top nozzle of fuel assembly,reactor core assemblies,counterweight section and the inner and outer shroud of the reactor core.Through calculation,this paper analyses the coolant flow distribution characteristics of the reactor core.Then,according to the heat distribution of the core,optimizing the coolant inlet area of each assembly by using the fitting correlation between the coolant inlet area and the resistance characteristic coefficient of the bottom nozzle,thus optimizing the coolant flow distribution in the reactor core to flatten the outlet of the core outlet temperature of the coolant.The results show some several valuable consequences.Before optimizing the coolant flow distribution scheme,the maximum temperature of the coolant in the core outlet is 670.56 K,the minimum temperature is 632.61 K,and the maximum temperature difference is 37.95 K.After optimizing the coolant flow distribution scheme,the maximum temperature of the coolant in the core outlet is 654.27 K,the minimum temperature is 651.74 K,and the maximum temperature difference is 2.53 K.The optimization results show that the optimal flow distribution scheme flattens the coolant temperature distribution at the export of the reactor core and cool the assemblies in accordance with design standards.