A flexible microloop magnetic sensor array has been developed for nondestructive evaluation in nuclear power plants and its fundamental characteristics have been investigated. A flexible micro-eddy current testing (micro-ECT) probe consists of a pancake-type exciting coil of the diameter of 3.2 m and a flexible microloop magnetic sensor array with an area of 14 2 14 mm 2 and a thickness of 125 m. The array has 16 microloop sensors made of copper formed on a flexible polyimide film by sophisticated photolithography. Each coil has 40 turns (8 m line and 8 m space) within an area of 2 2 2 mm 2 . The flexible sensorarray enables the micro-ECT probe to be applied to an arbitrary geometric configuration. The purpose of this work is to detect and reconstruct flaws in conductive tubing of a steam generator in a pressurized nuclear power reactor by using the flexible micro-ECT probe with higher spatial resolution than that of a conventional ECT probe. The spatial resolution of flaw detection was 3 mm and the detectability of an outer 20% flaw made by electric discharge machining was confirmed. The reconstruction of circumferential and axial flaws using the electromotive force and phase measured with the sensor array is described. Threedimensional eddy current and magnetic field analysis was also performed for comparison with measured data. Finally, a design study of a practical and robust micro-ECT for a realistic tube configuration with respect to detectability was carried out.

SUMMARYA micro-eddy current testing (ECT) probe, which consists of a planar microloop inductive magnetic sensor array and a pancake-type exciting coil, was developed to upgrade the ECT technology, especially as it is used for inspecting steam generator tubing in a pressurized nuclear power plant (PWR). Eddy currents and resultant magnetic fields are perturbed due to a flaw in a conductor. The flaw is detected via perturbation of the electromotive force (EMF) and phase from the array. Flaws manufactured by electric discharge machining in INCONEL 600 planar specimens, the thickness of which was 1.25 mm, were detected and reconstructed by using stacked database and simplified algorithm. Even an outer-edged 10 percent flaw could be detected. Measured EMF and phase signals were compared with numerical results obtained by using a three-dimensional eddy current and field analysis code and then verified. The experiment confirmed that the spatial resolution of detection of the micro-ECT probe is superior to that of the conventional pancake-type ECT probe. Finally, it was demonstrated that it is possible to establish the same measurement speed using the micro-ECT probe as it is by using the pancake-type ECT probe.

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