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The bobbin probe is now widely accepted as a basic and important ECT technique among various ECT techniques for steam generator tube integrity assessment that is practiced during each plant outage. The bobbin probe is also one of the essential components which consist of the whole ECT examination system, and provides us with decisive data for the evaluation of tube integrity in accordance with acceptance criteria described in specific procedures. Accordingly, the design of ECT probe is especially important to examination results because the quality of acquired ECT data is determined by the optimized probe design characteristics, such as coil geometry, electrical properties, operation frequency, and so on. In this study, the relationship between electric characteristic changes and differential coil gap variation for the optimization of the ECT probe signal was investigated. As a result, for the examination of volumetric flaws fabricated on the O.D. side of ASME type calibration standard, we propose that the best Lissajous signal for the examination will be obtained when the coil gap of differential bobbin probe is approximately 1.2 ~ 1.6 mm.

Steam Generator (SG) tube, as a barrier isolating the primary coolant system from the secondary side of nuclear power plants (NPP), must maintain the structural integrity for the public safety and their efficient power generation. So, SG tubes are subject to the periodic examination and the repairs if needed so that any defective tubes are not in service. Recently, corrosion related degradations were detected in the tubes of the domestic OPR-1000 NPP, as a form of axially oriented outer diameter stress corrosion cracking (ODSCC). According to the studies on the factors causing the heat fouling as well as developing corrosion cracking, densely scaled deposits on the secondary side of the SG tubes are mainly known to be problematic causing the adverse impacts against the soundness of the SG tubes [1]. Therefore, the processes of various cleaning methods efficiently to dissolve and remove the deposits have been applied as well as it is imperative to maintain the structural integrity of the tubes after exposing to the cleaning agent. So qualification test (QT) should be carried out to assess the perfection of the chemical cleaning and QT is to apply the processes and to do ECT. In this paper, the chemical cleaning processes to dissolve and remove the scaled deposits are introduced and results of ECT on the artificial crack specimens to determine the effectiveness of those processes are represented.

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