Seismic Margin Analysis Of Main Steam Isolation Valve

After the Fukushima nuclear power safety accident in Japan in 2011,the human requirement for the seismic capacity of the nuclear power plant is no longer limited to the design benchmark seismic capacity,all countries around the world have evaluated seismic safety assessment of the super-design benchmark earthquake for the newly built nuclear power plant one after another.The seismic evaluation of the nuclear power plant has been re-evaluated.The main steam isolation valve acts as the pressure boundary barrier of the third reactor in the nuclear power plant.When the super-design reference accident occurs,the main steam isolation valve can maintain its structural integrity and is critical to the safety of the whole nuclear power system.Based on the research of seismic margin assessment method for nuclear power plants at home and abroad,the main steam isolation valve of domestic DN800 caliber is taken as the research object,and analyzes its seismic margin in this paper.The specific contents of the study are as follows:(1)The background and significance of the anti-seismic margin of the main steam isolation valve and the two commonly used methods of seismic margin assessment are introduced.The principle of the fragility analysis method is described,and the fragility analysis can be denominated with using the two-lognormal distribution model.The fragility analysis process of the nuclear grade valve is summarized,and the seismic analysis method is briefly introduced.(2)The modal analysis of the main steam isolation valve was carried out by using the finite element software ANSYS Workbench to understand its dynamic characteristics,and the first-order natural frequencies in the main steam isolation valve during the opening(during the design benchmark earthquake)and the closed state(during the super-design benchmark earthquake)were obtained.The frequencies are all greater than 33 Hz,so the equivalent static method is used to calculate the seismic response of the main steam isolation valve.Based on the pressure area method and the equivalent linearization method,the main steam isolation valve body is analyzed and checked.The calculation results are compared and found that the calculation results of the two methods are basically consistent,It is proved that the equivalent linearization method can be used to classify stress effectively and quickly.(3)Based on the stress classification method and the direct method,the seismic response analysis of the main steam isolation valve is analyzed.Comparing the analysis process of the two analysis design methods.The direct method can not only avoid the difficulty of stress classification,but also obtain better results,In addition,it can reflect the failure process under load combination more truthfully,so it is recommended to use direct method for analysis and design.During the analysis,it is found that the seismic weakness of the main steam isolation valve under different working conditions is different with the acceleration amplification.Finally,the seismic fragility of the main steam isolation valve under the super-design benchmark earthquake condition is determined as the support position.Because the constitutive relation of the material in the two methods is different,the ultimate anti-seismic capacity of the main steam isolation valve is also different.The results obtained by the direct method of 5% maximum principal strain are more realistic than those obtained by the stress classification method.(4)The seismic margin of the main steam isolation valve is calculated by using the fragility analysis method in the seismic margin evaluation.The concepts of confidence and failure probability are used to quantitatively express the ability of the main steam isolation valve to resist the overdesigned benchmark earthquake.Each fragility variable is quantitatively analyzed,the reliability of the main steam isolation valve is calculated at 95% confidence level,and the seismic capacity of the main steam isolation valve at 5% failure probability is about twice as much as that of the designed acceleration value.This indicates that the main steam isolation valve has a relatively large safety allowance.On this basis,the fragility curves of the main steam isolation valve at different confidence levels under the elastic failure mode are drawn,this fragility curve can be used to predict the failure probability of the main steam isolation valve in the event of a large earthquake,and it can be used as a reference for reducing the design difficulty of the drive actuator.