Taiji Hirasawa

Crack detection and sizing technique by ultrasonic and electromagnetic methods

Improvements in defect detection and sizing capabilities for non-destructive inspection techniques have been required in order to ensure the reliable operation and life extension of nuclear power plants. For the volumetric inspection, the phased array UT technique has superior capabilities for beam steering and focusing to objective regions, and real-time B-scan imaging without mechanical scanning. In contrast to the conventional UT method, high-speed inspection is realized by the unique feature of the phased array technique. A 256-channel array system has developed for the inspection of weldment of BWR internal components such as core shrouds. The TOFD crack sizing technique also can be applied using this system. For the surface inspection, potential drop techniques and eddy current techniques have been improved, which combined the theoretical analysis. These techniques have the crack sizing capability for surface breaking cracks to which UT method is difficult to apply. This paper provides the recent progress of these phased array and electromagnetic inspection techniques.

Performance of non-destructive evaluation by diffracted SH ultrasonic waves in predicting degree of fatigue in cyclic bending of ferritic steel

Abstract The degree of fatigue in ferritic wrought steel during cyclic bending was determined by analysis of diffracted SH ultrasonic waves with the aid of multiple regression analysis. As the degree of fatigue increases, accompanied by residual stress, the incident waves curve to the interior region with positive stress field owing to the acoustoelastic effect. As a result, the propagation time of the launched waves lengthens, leading to a modulation of the received waveform. Multiple regression analysis for the waveform modulation produces a reliable estimation, with correlation coefficient of 0.948, for the degree of fatigue.

Computerised defect sizing using ALOK analysis in shrunk-on wheel of turbine rotor

Abstract A computerised defect evaluation system using an advanced ultrasonic technique for shrunk-on wheels of a turbine rotor has been developed. The flaw image was reconstructed by a modified ALOK method with attention to the tip diffraction echoes of stress corrosion cracking (SCC). The results of the application of this system to the mock-up wheel with artificial slits and real SCC cracks show that this system is very useful for crack sizing and locating.

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

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.Copyright

Nondestructive Evaluation of Neutron Irradiation Embrittlement for Reactor Vessel Steel by Magnetomechanical Acoustic Emission Technique

A modified magnetomechanical acoustic emission (MAE) technique denoted Pulse MAE, in which the magnetizing current has a rectangular wave form, was developed as an NDE technique. Its applicability to the radiation damage for reactor pressure vessel steel was evaluated. The reactor pressure vessel steel A533B base metal and weld metal were irradiated to the two fluence levels: 5 {times} 10{sup 22} and 3 {times} 10{sup 23} n/m{sup 2} at 288 C. One side of the specimen was electropolished after irradiation. Pulse MAE signals were measured with a 350 kHz resonance frequency AE sensor at the moment when the magnetizing voltage is applied from zero to the set-up value abruptly. The AE signals were analyzed and the peak voltage Vp was determined for the measuring parameter. The peak voltage Vp showed the tendency to increase monotonically with increasing neutron fluence. The relationship between the Vp and mechanical properties such as yield stress, tensile strength and Charpy transition temperature were also obtained. The Pulse MAE technique proved to have the possibility to detect and evaluate the neutron irradiation embrittlement. The potential of the Pulse MAE as an effective NDE technique and applicability to the actual components are discussed.