| In the field of nuclear industry,special vessels and pipelines are needed to store,dissolve and transport a large number of nuclear fuel / spent fuel in the closed-cycle process of nuclear fuel.In the service of such vesels and pipelines,corrosion defects from inside to outside will occur in weak parts,such as welds.With the initiation,growth and expansion of these defects,it may penetrate vessels and pipelines,resulting in leakage of radioactive materials,causing long-term serious harm to surrounding organisms and the environment.Post-processing work is dangerous,complex and costly.Therefore,it is necessary to use advanced non-destructive testing methods to monitor typical nuclear vessels and pipelines in service to ensure service safety.The method of eddy current testing has the advantages of no additional medium,easy to realize automation,and etc.,which can meet the special testing requirements of typical nuclear vessels.However,the eddy current testing method has limited detection depth and is suitable for the testing of surface or near-surface defects,and the testing of inner wall defects of vessel is relatively less studied.Therefore,this paper studies the adaptability and accuracy of the improved eddy current testing probe for the inner wall defect of medium thickness nuclear vessels,and studies the method of defect location and reconstruction according to signal characteristics.The specific research contents are as follows:Firstly,COMSOL Multiphysics multi-physics coupling simulation analysis software was used to simulate and analyze the excitation magnetic field of the rectangular exciting coil used in this paper,the distribution law of the induced eddy current field and the induced magnetic field in the tested sample,and the influence of coil voltage,lift-off distance and excitation frequency on the eddy current penetration depth.Secondly,the formation process of impedance diagram of eddy current detection is analyzed,and the noise reduction and feature extraction are performed on the detection signal.The process experiments of different testing parameters,different defect parameters and different material parameters were carried out,and the influence of different parameters on the characteristic value of eddy current testing signal was analyzed.Finally,a defect reconstruction and localization method based on eddy current testing signal characteristics and neural network prediction model was proposed,and the designed artificial defect samples were reconstructed and localized.And,the effectiveness and accuracy of defect reconstruction and localization method are verified.The results show that the eddy current field induced by the designed rectangular excitation coil in the tested sample is uniform and concentrated,and the penetration depth is large,which is suitable for the detection of inner wall defects of medium and thick vessels.For the simulation sample of typical nuclear vessel,the optimal detection parameters are as follows: detection frequency is 5 k Hz,excitation coil voltage is 9.6V,probe lift-off distance is 1 mm,detection speed is 44.8 mm/s,and scanning direction perpendicular to the defect.By analyzing the impedance diagram obtained by eddy current testing,the change of defect width and depth can be accurately obtained.For the non-ferromagnetic materials,such as stainless steel and zirconium-based alloy used in the vessels and pipelines,the main factor affecting the eddy current testing effect is the difference in the conductivity of the sample.For the ferromagnetic materials used in the nuclear fuel vessels,it is necessary to design a magnetic saturation device to improve the detection depth and accuracy.A variety of defect depth and width prediction models are designed,the prediction results of the defect depth and width prediction model based on BP neural network are the most accurate.The localization and reconstruction analysis of multiple artificial defects was carried out,and the relative errors are within the allowable range. |
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