核电站堆芯吊篮振动频率下降研究

Drop in Vibration Frequency of the Core Support Barrel in Nuclear Power Plants

通过分析堆外中子噪声,识别堆芯吊篮振动频率并跟踪其变化,发现在核电站运行期间,同一个燃料循环周期内的堆芯吊篮梁型振动频率呈下降趋势,核燃料更换后的下一个燃料循环初期,频率恢复到前一个燃料循环初始值附近。对堆芯吊篮进行静力学分析,以确定其潜在的影响因素。建立压水堆内部构件的装配体模型并采取摩擦接触方法,以综合考虑堆芯吊篮法兰处的力学约束。采用流固耦合法将冷却剂对堆芯吊篮的作用力映射到堆芯吊篮结构上,并采取有限元法对堆芯吊篮进行了预应力模态分析,得到了堆芯吊篮梁型振动频率下降的原因和机理。结果表明:堆芯吊篮梁型振动频率的下降是由燃料组件压紧弹簧刚度的退化引起,随着燃料组件压紧弹簧刚度减少,轴向预紧力不足以补偿冷却剂作用力,堆芯吊篮法兰出现位移,接触刚度降低,导致模态频率的下降。

By analyzing the ex-core neutron noise signal,the vibration frequency of the core support barrel(CSB)is identified and its change is tracked.It is found that during the operation of the pressurized water reac⁃tor(PWR)nuclear power plant,the vibration frequency of the CSB beam mode during the fuel cycle shows a downward trend.However,after the replacement of the nuclear fuel,the vibration frequency of the CSB beam mode at the beginning of the new fuel cycle basically returns to the initial value of the previous fuel cycle.The static analysis of the CSB is carried out to determine the potential influencing factors on the modal.The assem⁃bly model of PWR reactor internals is established and the friction contact method is adopted to comprehensively deal with the constraints of the CSB flange.The fluid-structure coupling method is used to map the pressure of the coolant on the CSB to the CSB structure.Finally,the prestressed mode analysis of the CSB is carried out by the finite element method,and the causes for the drop of the vibration frequency of the CSB beam mode are ob⁃tained.The results show that the drop of the vibration frequency is caused by the stiffness degradation of the fuel assembly hold-down springs.As the stiffness of the fuel assembly hold-down springs decreases,the axial pre⁃load is insufficient to offset the coolant force,resulting in the deformation of the CSB flange,which causes the reduction of the contact stiffness,so that the modal frequency drops.