Research On Reactor Fuel Rod Deformation Monitoring Method Based On Optical Fiber Sensing

In pressurized water reactors,the bending deformation of fuel rods caused by mechanical fatigue accumulation is a major safety hazard in the operation of power stations.In view of the problem of rod bundle deformation,the bending deformation state of the rod bundle under different working conditions can be explored by building a nuclear reactor simulation experimental device,measuring the characteristic parameters such as the bending deformation of the simulated fuel rod in the device,analyzing the deformation factors of the rod bundle,and then improving the structural design of the nuclear reactor and optimizing the thermal parameters of the reactor core,so as to suppress the bending deformation of the fuel rod,extend the service life of the fuel rod,and improve the operation safety level of the nuclear power plant.However,in the past,the deformation detection mostly took spent fuel rods as the object,and used image photography technology or ultrasonic sensing to carry out poolside detection of spent fuel components,the detection process was cumbersome,the measurement accuracy was limited,and the measurement data lacked reference significance for the optimal design of the reactor.In view of this,this paper studies a real-time monitoring method for vector deformation of fuel rods that can withstand high temperature,high pressure and complex electromagnetic environment by optical fiber sensing technology for reactor thermal simulation experimental device.In this method,4×3 fiber Bragg grating(FBG)arrays are packaged with multiple micro-slots on the surface of the rod bundle to obtain the force strain at different positions when the rod bundle is bent,and the vector deformation parameters are solved with the help of convolutional neural network model,and finally the three-dimensional reconstruction of the bending pattern of the rod bundle is completed on the basis of strain interpolation.Double FBG is used to detect the temperature and strain parameters of fuel rods,which is used as the research basis for fuel rod bending detection.The calibration experiment of FBG was carried out to obtain its temperature and strain coefficient,and then a simple device for reactor simulation was built,and the fuel rod encapsulated with FBG was placed in a complex water bath environment with high temperature and multiphase flow to detect the temperature,strain and vibration parameters of the rod bundle.The test shows that the temperature and strain measurement error of FBG is small,the vibration information of the rod bundle can be obtained,and the FBG sensing performance is stable and the sensitivity is high in the core simulation environment,which verifies the reliability of using FBG sensing technology to detect the bending deformation of fuel rods in the reactor simulation equipment.Based on the FBG array sensing structure,the bending deformation detection of fuel rods is realized.Using the 4×3FBG array package structure,according to the principle of circular cross-section strain symmetry of the rod bundle,a temperature separation method is proposed,and the cross-sectional temperature of the rod bundle is extracted and the temperature compensation of the strain measurement is realized.Then,a convolutional neural network model is established to solve the vector deformation parameters of the rod bundle,including the bending position,direction and deflection,and the model solution obtains that the bending parameters are consistent with the actual values with small errors.The three-point bending strain value measured by the FBG array is interpolated by the three-point strain interpolation method to obtain the strain distribution of the bar bundle,and the curvature distribution of the bar bundle is calculated accordingly.The discrete-space coordinates of the rod bundle are solved by using the curvature distribution of the rod bundle,and then the three-dimensional reconstruction of the bending form of the rod bundle is completed,and the reconstruction curve is consistent with the ANSYS simulation results.Experiments show that the method can accurately detect the bending deformation parameters in the variable temperature environment,and the bending reconstruction curve fits well with the mechanical simulation results,and it can be seen from the test results of FBG in the core simulation environment that FBG has high withstand capacity and stable sensing characteristics in extreme environments,which can provide reliable sensing for reactor thermal experiments and meet the measurement accuracy requirements.