A Reconstruction Scheme Using an Average Conductivity Element for Pitting Corrosion Defects in Steam Generator Tubes From Eddy Current Testing Signals

Pitting corrosion is a typical defect in steam generator tubes of nuclear power plants (NPPs) which may cause leak of coolant water in the first circulation loop of an NPP. Different from a crack, the pitting defect has small hole-like geometrical feature, which makes the conventional inversion scheme for crack profile reconstruction using eddy current testing (ECT) signals unsuitable. In this article, a numerical scheme to reconstruct the pitting corrosion defect from ECT signals is proposed and validated, including a new efficient ECT signal forward solver and an iterative inversion algorithm. For efficient calculation of the ECT signals due to a pitting hole, defect model based on a finite element of average conductivity is proposed. The model is validated by comparing its results with those of the conventional finite element method and boundary element method (FEM-BEM) hybrid code of A-phi formulation using a detailed mesh for a flat-bottom hole. To reconstruct the defect profile such as the depth and diameter, the particle swarm optimization algorithm was adopted to solve the inversion problem iteratively. Finally, ECT experiments were conducted for test piece tubes with artificial pitting defects of different sizes and the defect profile parameters were reconstructed from the measured ECT signals with the proposed numerical schemes. Both the depths and the diameters of the artificial pitting defects, i.e., flat-bottom holes, were properly reconstructed referring to the true defect sizes.