Phased Array UT Application for Boiling Water Reactor In-Vessel Inspection

Abstract

This paper describes development and application of Phased Array Ultrasonic Testing (UT) and Remotely Operated Vehicles (ROV) for In-Vessel Internals Inspection. Stress Corrosion Crack (SCC) on reactor internals is one of the most important issues since 1990s, and demand to inspect the reactor internals is increasing. Instrument manufactures and inspection venders have developed and applied 1) Phased Array UT technologies and technique as one of our Non-Destructive Examination (NDE) technologies, 2) several kinds of ROVs and special tools for probe delivery and positioning. They are available and effective in In-Vessel Inspection (IVI) and maintenance, which shall be conducted in the narrow room under water. Furthermore, the UT technique for Alloy 182 weld that used to be difficult to detect and size flaws was developed and deployed in the BWR IVI. UT experiences in reactor vessels are increasing in recent years. An immersion technique by Phased Array UT is a key to perform the In-vessel UT on a complex geometric surface to be inspected, and to achieve very wide accessible range by ROVs or simple special tools efficiently. Advantages of the water immersion method and a ROV development result are shown in this paper. Particularly, TOSHIBA developed a flat type ROV for Shroud (Shroud ROV), which can be held against the surface of the shroud by thrusting propellers and scan mechanically through narrow gap within 2 inches {50mm}. The ROV’s positioning accuracy and applications for Shroud UT are shown. As the field experience, this introduces the UT results for CRD Stub tube Alloy 182 weld that is located on the vessel bottom head in Hamaoka UNIT 1 of Chubu Electric Power Company in Japan. An axial SCC flaw was detected by underwater visual testing, after the CRD stub tube leakage was detected. Then UT examination for the flaw was accomplished on the Alloy 182 weld in the vessel. We evaluated that the flaw penetrated into the weld metal of the CRD stub tube-pat weld and didn’t propagate into the low alloy of Reactor Pressure Vessel base metal. After UT sizing, the CRD stub tube was removed and replaced. The examination result was proven to have a good agreement with the actual crack depth. As a result, the efficiency of our Phased Array Technique was confirmed. As the other immersion method application, UT coverage example and accessible range for Shroud inspection are shown.