The Construction Of Green’s Function For Quick Calculation Of Thermal Stress In Safety-injection Pipeline

When the nuclear power plant is in service,in order to deal with the thermal fatigue aging mechanism of nuclear pipelines,satisfy the requirements of nuclear safety supervision and evaluate the degree of thermal fatigue damage quickly and effectively,it is necessary to actively implement effective life extension management for auxiliary pipelines of nuclear power plants.The life extension of old nuclear power plants will bring huge economic benefits.Therefore,various countries have developed the fatigue monitoring system.The focus of fatigue monitoring systems is the generation of transient temperature fields and the calculation of transient stress fields.Affected by the change of thermal transient load,transient thermal stress will be generated.Accumulation of fatigue damage occurs in areas where stress is concentrated.During online monitoring,it is necessary need to calculate the thermal stress of dangerous parts rapidly.This study firstly combined with the running status of the nuclear power plant safety-injection system,I designed with four simulation conditions for analysis and verification.Then,the computational fluid dynamics software FLUENT is used to simulate the four different conditions adopting the Transition SST turbulence calculation model.The results of theoretical analysis are verified and determined.The fluid flow state of the thermal safety injection tube segment of the pressure reactor nuclear power plant and the division of the analysis part is defined clearly.In order to determine the temperature transfer coefficient of each hot zone,I extracted the average temperature of the fluid in different areas.The ANSYS APDL software is used to establish the finite element model of thermal stress analysis that the check valve simplifies into an equivalent cylinder.In order to simulate the influence of the pipeline after the check valve on the thermal stress,I analyse the pipeline after the check valve to obtain the pipeline stiffness coefficient.It makes an elastic restraint to apply at the end of the check valve equivalent cylinder.According to the flow conditions of the safety-injection pipeline,the hot zone is divided.The Green’s function of stress is constructed by applying different step temperature to the thermal boundary.The temperature transfer coefficient based on the fluid temperature of the flow boundary is introduced to determine the fluid temperature load of each heat zone.The linear superposition of the response in each hot zone is used to realize a quick calculation by the Green’s function method to calculate the thermal stress of the complex flow boundary structure.By comparing the thermal stress calculation results of the Green’s function method with the direct calculation results of the finite element method,the step temperature is optimized.In order to avoid the hot zone division of the complex fluid temperature field in the safety-injection pipeline,the stress Green function is constructed based on the result of fluid-solid coupling method.A certain temperature step is applied under the condition of constant flow.The transient thermal stress at the evaluation site is calculated.The thermal stress calculated by the Green’s function based on the fluid-solid coupling method is compared with the result calculated by the finite element method.The result shows that this two methods can match well.And it verify that the method of constructing the Green’s function based on fluid-structure coupling is effective.In order to improve the calculation accuracy of the safety-injection pipeline ’s thermal stress calculated by hot bone division method in Green function,this study inspects the effect of the number of stratified parts on the calculation results.Compare the finite element calculation results of different stratification numbers with the fast calculation results of fluid-structure coupling by the Green ’s function.The result shows that the thermal stratified area divided into four layers can make calculation better and more accurate.