| Nuclear power valves are an important part of the pressure-bearing boundary of the valve piping system in nuclear power plants.In order to ensure the smooth realization of the expected functions of the systems in nuclear power plants during earthquakes,their structural and functional integrity must be ensured during earthquakes.In evaluating its seismic capability,it is required to be evaluated according to both the design code and the seismic safety margin.The design specifications require the structural integrity and operability of the valve in the face of seismic hazards,and the operability of nuclear power valves is mainly identified through tests,but the identification tests have long design cycles and high costs,and there is a lack of research on the mechanism of the impact of seismic loads on valve operability.There are few studies in the field of seismic safety margin research based on the evaluation of seismic margin of nuclear power valve operability.Therefore,this paper takes a nuclear-grade gate valve as an example to clarify the mechanism of seismic influence on valve operability,summarizes the process of nuclear valve operability seismic qualification based on analytical method,and carries out seismic calibration of the valve based on operability and structural integrity respectively,and finally evaluates the seismic capability of the valve under seismic conditions beyond the design benchmark based on the above research contents.The research content of the paper is important for the improvement and enhancement of nuclear power valve design and its reliability during earthquakes,and provides a reference for seismic margin analysis of nuclear-grade energetic equipment.The main conclusions of the thesis are as follows:(1)In order to investigate the mechanism of earthquake load hazard to the operability of nuclear power valves,a gate valve finite element model is established,normal and accident conditions with and without earthquake load are designed,and the finite element method is applied to study the stress and contact pressure changes in various parts of the valve when it is subjected to earthquake load during opening and closing.Conclusion: The hazard of earthquake on the valve operability is to amplify the contact pressure of the valve sliding friction parts,resulting in an increase in the friction force to be overcome to close the valve,delaying the valve closing action.The valve stem collides with the sleeve and is more susceptible to seismic excitation than other parts of the valve.(2)In order to compare the difference between operability and structural integrity seismic identification,the fourth strength theory and the same boundary conditions were used to seismically calibrate the valve based on the above two perspectives,and the valve could not complete the expected closing function when the seismic load was greater than 4g;when the seismic load was greater than 6g,the seismic identification results based on structural integrity exceeded the stress limit.The seismic qualification process based on operational analysis is more comprehensive and demanding.The seismic weak points in the two different angles of analysis are shifted,and there are limitations in the static seismic identification,and the stress changes in each component during the execution of the function also need to be considered.(3)In order to evaluate the seismic margin of nuclear power valves under different analysis angles,the ultimate seismic capacity of the studied valves was calculated and the valve failure modes under different analysis angles were determined.The SFA method was applied to evaluate the seismic capability of the two failure modes with high confidence and low failure probability,and the fragility curves were plotted separately.This provides a reference for predicting the failure location and probability of nuclear valves under over-design basis earthquakes. |
