The reactor containment vessel of a pressurized water reactor (PWR) plant is an important structure that contains the reactor, cooling system, and other components, and functions to prevent the external release of fi ssion products in the event of an accident. The integrity of the containment vessel as a pressure-retaining boundary is confi rmed by the leak rate test on each periodic inspection. Visual inspection of the containment vessel must be performed for accessible surface areas (Japan Society of Mechanical Engineering, 2004). The lower part of the containment vessel embedded in concrete, however, cannot be accessed for visual inspection. If it is determined in the future that corrosion can occur in such an inaccessible area, then an inspection technique that can locate such defects and assess their severity will be needed. To inspect the reactor containment vessel plate embedded in concrete, a technique that can detect defects in a component from an accessible location at a distance of 20m or more is needed. We therefore chose a method that uses a transducer with piezoelectric elements, the most popular devices in practical use for ultrasonic testing, and developed a shear horizontal (SH) wave transducer with a large, low-frequency active element composed of three parallel-connected active elements with a refractive angle of 90 degrees for effective detection of surface thinning by corrosion and for minimization of dispersion of ultrasonic waves from steel plates to concrete (Ishida, 2004, 2007). Combining three active elements made possible an active element transducer of large width, which could not be made with a single element. This transducer was capable of clearly detecting echoes from 19 mm depth and 9.5 mm depth hollows at a distance of about 1.5 m as well as echoes from a 9 mm diameter stud bolt at various distances between about 0.7 and 1.7 m on the surface of a concrete-covered carbon steel plate. Furthermore, the capability of propagation of SH waves over a distance of about 12m with the newly made large transducer has been estimated by detecting multiple echoes between the front and back sidewalls of the concrete-covered steel plate 2 m in length. To improve the capability of propagation over longer distances, we constructed a multi-element transducer with a multichannel pulser/receiver (P/R).

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