Quantitative non-destructive evaluation of wall thinning defect in double-layer pipe of nuclear power plants using pulsed ECT method

•Formulae for gradient calculation are deduced based on adjoint field for sizing complicated shape defect from PECT signals•Wall thinning is modeled as planar defects with its edge approximated by a piece-wise line, position of edge point is sized•The PECT experiments for a double-layer region of a typical coolant pipe of nuclear power plants are conducted•The artificial wall thinning defects in double-layer tube are reconstructed with these measured signals•Efficiency/robustness of proposed algorithm against measurement noise and model error are demonstrated

In this paper, an inversion algorithm for three-dimensional profile reconstruction of wall thinning defect in a double-layer region of a typical coolant pipe of nuclear power plants from pulsed eddy current testing (PECT) signals has been proposed and experimentally validated, based on a fast simulator of PECT signals and a deterministic optimization strategy. First, the fast simulator developed by authors for PECT signal prediction based on a Fourier-series scheme in addition with interpolation and database approaches is briefly described as a base for the inversion of PECT signals. Then, the formula of the conjugate graduate inversion algorithm for sizing three-dimensional wall thinning from PECT signals is deduced in detail based on that for crack like reconstruction using single frequency ECT signals. The three-dimensional local wall thinning is modeled as a group of planar defects with different length and depth which are reconstructed from two-dimensional scanning PECT signals through inverse analysis. Through conducting PECT experiment for double-layer coolant tube test-piece and reconstructing wall thinning profile from the measured signals, the efficiency and the robustness of the proposed inversion algorithm are demonstrated.