Makoto Ochiai

Determination of residual stresses using laser-generated surface skimming longitudinal waves

A laser-ultrasonic technique is described to non-destructively determine residual stresses in metals such as those produced by shot peening. The method is based on monitoring the small ultrasonic velocity change of the laser-generated surface skimming longitudinal wave (LSSLW) propagating just below the surface. The main advantage of using LSSLW is that the effect of surface roughness induced by shot peening is greatly reduced compared to using surface acoustic waves (SAW). To improve resolution in the measurement of small velocity changes, a cross-correlation technique is used with a reference signal taken on the same but unstressed material in similar conditions. Also, the low-frequency SAW can be used to correct the LSSLW results when affected by minute changes in the path length during the measurements. The validity of the approach is demonstrated by measuring quantitatively the near surface stress in a four-point bending experiment with different levels of surface roughness. Then, scanning results on properly and improperly laser shock peened samples are reported. In particular, the LSSLW velocity variations for the properly peened samples clearly show an increase in the laser-peened area well indicative of a compressive stress.

Visualization of Surface‐Breaking Tight Cracks by Laser‐Ultrasonic F‐SAFT

A new method to obtain a detailed image of surface‐breaking tight cracks by using laser‐ultrasonics is proposed. The surface opposite the cracking is scanned by the generation and detection lasers. Detected signals are processed by the Fourier‐domain Synthetic Aperture Focusing Technique (F‐SAFT). The point of this study is to use the laser‐generated shear waves, which interacts very effectively with the crack roots. A detailed image of the stress corrosion cracks, having depths of typically 0.5 mm and widths of less than 0.05 mm, is successfully reconstructed.

Development and Application of Laser Peening System for PWR Power Plants

Laser peening is a process to improve residual stress from tensile to compressive in surface layer of materials by irradiating high-power laser pulses on the material in water. Toshiba has developed a laser peening system composed of Q-switched Nd:YAG laser oscillators, laser delivery equipment and underwater remote handling equipment. We have applied the system for Japanese operating BWR power plants as a preventive maintenance measure for stress corrosion cracking (SCC) on reactor internals like core shrouds or control rod drive (CRD) penetrations since 1999. As for PWRs, alloy 600 or 182 can be susceptible to primary water stress corrosion cracking (PWSCC), and some cracks or leakages caused by the PWSCC have been discovered on penetrations of reactor vessel heads (RVHs), reactor bottom-mounted instrumentation (BMI) nozzles, and others. Taking measures to meet the unconformity of the RVH penetrations, RVHs themselves have been replaced in many PWRs. On the other hand, it’s too time-consuming and expensive to replace BMI nozzles, therefore, any other convenient and less expensive measures are required instead of the replacement. In Toshiba, we carried out various tests for laser-peened nickel base alloys and confirmed the effectiveness of laser peening as a preventive maintenance measure for PWSCC. We have developed a laser peening system for PWRs as well after the one for BWRs, and applied it for BMI nozzles, core deluge line nozzles and primary water inlet nozzles of Ikata Unit 1 and 2 of Shikoku Electric Power Company since 2004, which are Japanese operating PWR power plants. In this system, laser oscillators and control devices were packed into two containers placed on the operating floor inside the reactor containment vessel. Laser pulses were delivered through twin optical fibers and irradiated on two portions in parallel to reduce operation time. For BMI nozzles, we developed a tiny irradiation head for small tubes and we peened the inner surface around J-groove welds after laser ultrasonic testing (LUT) as the remote inspection, and we peened the outer surface and the weld for Ikata Unit 2 supplementary. For core deluge line nozzles and primary water inlet nozzles, we peened the inner surface of the dissimilar metal welding, which is of nickel base alloy, joining a safe end and a low alloy metal nozzle. In this paper, the development and the actual application of the laser peening system for PWR power plants will be described.Copyright

Laser-ultrasonic inspection of surface-breaking tight cracks in metals using SAFT processing

A method is presented for imaging the detailed structure of surface-breaking tight cracks from the surface opposite to the cracks. The technique combines laser-ultrasonics and data processing with an improved version of the Fourier-domain Synthetic Aperture Focusing Technique (F-SAFT). The key point of this study is to use laser-generated shear waves, which interact very effectively with the crack roots. Ultrasonic images of the stress corrosion cracks, having depths of typically 0.5 mm. and widths of less than 0.05 mm, are successfully reconstructed and compared with results from liquid penetrant testing.

Laser Ultrasonic Study of Crack Tip Diffraction

Ultrasonic diffraction pattern at a crack tip and crack depth sizing based on the diffraction are studied by using laser‐generated bulk waves. Directivity patterns oriented essentially along the normal for the diffracted longitudinal wave and essentially at 45 (deg) for the diffracted shear wave for any incident angles is obtained. A laser‐ultrasonic system combined with time‐of‐flight (TOFD) analysis is produced and demonstrates accurate crack sizing on an artificial slot having a variable depth from 0 mm to 10 mm. An improvement of signal‐to‐noise ratio by split spectrum processing (SSP) is also suggested.

Laser-Based Maintenance and Repair Technologies for Reactor Components

Stress corrosion cracking (SCC) is the major factor to reduce the reliability of aged reactor components. Toshiba has developed various laser-based maintenance and repair technologies and applied them to existing nuclear power plants. Laser-based technology is considered to be the best tool for remote processing in nuclear power plants, and particularly so for the maintenance and repair of reactor core components. Accessibility could be drastically improved by a simple handling system owing to the absence of reactive force against laser irradiation and the flexible optical fiber. For the preventive maintenance, laser peening (LP) technology was developed and applied to reactor components in operating BWR plants. LP is a novel process to improve residual stress from tensile to compressive on material surface layer by irradiating focused high-power laser pulses in water. We have developed a fiber-delivered LP system as a preventive maintenance measure against SCC. Laser ultrasonic testing (LUT) has a great potential to be applied to the remote inspection of reactor components. Laser-induced surface acoustic wave (SAW) inspection system was developed using a compact probe with a multi-mode optical fiber and an interferometer. The developed system successfully detected a micro slit of 0.5mm depth on weld metal and heat-affected zone (HAZ). An artificial SCC was also detected by the system. We are developing a new LP system combined with LUT to treat the inner surface of bottom-mounted instruments (BMI) of PWR plants. Underwater laser seal welding (LSW) technology was also developed to apply surface crack. LSW is expected to isolate the crack tip from corrosive water environment and to stop the propagation of the crack. Rapid heating and cooling of the process minimize the heat effect, which extends the applicability to neutron-irradiated material. This paper describes recent advances in the development and application of such laser-based technologies.© 2004 ASME

DEVELOPMENT OF A WELDING MONITORING SYSTEM FOR IN-PROCESS QUALITY CONTROL OF THICK WALLED PIPE

The authors have developed a systematized in-process quality control system for arc welding of thick walled pipe utilizing visual sensors and computers. This system consisted of four main subsystems that monitor the welding conditions, monitor the weld pool shape, visualize the weld bead shape, and visualize inner defects, respectively. The welding condition monitoring system detects abnormal welding conditions using an electric signal. The weld pool shape monitoring system assesses the position among weld pool, the electrode, wire, and groove using a charge coupled device (CCD) camera and filters. The weld bead shape visualization system detects abnormal bead surface conditions using a three-dimensional (3D) visual technique. The inner defect visualization system was achieved using a laser-ultrasonic in-process testing (LUT) technique and synthesis aperture focus technique (SAFT). Each subsystem informs workers of unsuitable welding conditions to allow them to reestablish the suitable conditions. Furthermore, the in-process quality control system was applied to thick walled pipe welding to verify the practicality of this system for improving the quality and efficiency of welding operations.

LASER‐ULTRASONIC TESTING AND ITS APPLICATIONS TO NUCLEAR REACTOR INTERNALS

A new nondestructive testing technique for surface‐breaking microcracks in nuclear reactor components based on laser‐ultrasonics is developed. Surface acoustic wave generated by Q‐switched Nd:YAG laser and detected by frequency‐stabilized long pulse laser coupled with confocal Fabry‐Perot interferometer is used to detect and size the cracks. A frequency‐domain signal processing is developed to realize accurate sizing capability. The laser‐ultrasonic testing allows the detection of surface‐breaking microcrack having a depth of less than 0.1 mm, and the measurement of their depth with an accuracy of 0.2 mm when the depth exceeds 0.5 mm including stress corrosion cracking. The laser‐ultrasonic testing system combined with laser peening system, which is another laser‐based maintenance technology to improve surface stress, for inner surface of small diameter tube is developed. The generation laser in the laser‐ultrasonic testing system can be identical to the laser source of the laser peening. As an example op...

A Remote Operated Quadruped Robot System for Investigation of Reactor Building

A remote operated quadruped robot has been developed for disaster site which can move on stairs, slopes, and uneven floor under the radiation-polluted environment, such as TEPCO Fukushima Daiichi nuclear power plants [1][2].In particular, the control method for stable walking and the remote operation system have been developed to move on stairs in the reactor building.We applied this robot to investigation of suspicious water leakage points in reactor building at Fukushima Daiichi nuclear power plants unit2[3]. In this investigation, a small vehicle equipped with camera and a manipulator which is connected the vehicle with cable were mounted on the robot and were carried to near the target by the quadruped robot and the investigation was carried out with the small vehicle.© 2014 ASME

Basic Characteristics of Eddy Current Testing Using Resonant Coupling

It has been demonstrated that magnetic resonant coupling is effective for improving the characteristics of ECT sensor, especially the lift-off-dependent reduced sensitivity and noise. The lift-off-dependent reduced sensitivity and noise are caused by the decrease of voltage gain from the exciter coil to detector coil. Magnetic resonant coupling is generally usable to increase the voltage gain from the transmitter coil to the receiver coil. Magnetic resonant coupling was applied to ECT in these experiments to investigate the phenomena of magnetic resonant coupling in the ECT. In the ECT setup, the voltage gain G increased more than 6 times by magnetic resonant coupling at a frequency of 105 kHz. The voltage gain ratio (GR/GNR) was rising 1.12 times when the lift-off length increased from 1mm to 2 mm. This result verified that magnetic resonant coupling has the potential for improving the characteristics of ECT. The EDM slit signal and the lift-off noise were calculated from the experimental results, respectively. Using magnetic resonant coupling, the EDM slit signal increased 1.5 times and the lift-off noise was reduced by 34 % at a lift-off length of 2mm. these results confirmed that magnetic resonant coupling is effective for improving the characteristics of ECT. Additional experiments were performed in order to verify the effect of magnetic resonant coupling for the wobbling (lift-off) noise in the tube inspection. In the vibration test of the ECT sensor inside the tube, the lift-off noise decreased by 28 % on average by magnetic resonant coupling. In the scanning test of the ECT sensor inside the tube, the wobbling and other noises were obviously reduced by magnetic resonance coupling.Copyright