Intelligent Measurement and Diagnostic Techniques for Non-destructive Inspections

Transcription

1 Intelligent Measurement and Diagnostic Techniques for Non-destructive Inspections 296 Intelligent Measurement and Diagnostic Techniques for Non-destructive Inspections Fuminobu Takahashi, D. Eng. Shigeru Izumi, D. Eng. Akira Nishimura OVERVIEW: As today s nuclear power and thermal power plants play a key role in supplying electric power, it is necessary to improve their utilization factor while ensuring the reliability of their equipment and components. In the past six years, the factor of unscheduled shutdowns of nuclear power plants employing Hitachi-made boiling water reactors has been as low as 0.24 per unit per year, while the average utilization factor has been as high as 82.3%. To further improve the plant utilization, Hitachi has been making efforts not only to strengthen its administrative management for promoting plant construction and maintenance, but also to develop new advanced measurement and diagnostic techniques for confirming the reliability of plant equipment and components. For improving the plant utilization factor the reliability of equipment and components must be maintained or improved and the in-service inspection time of each plant must be shortened by using efficient inspection procedures. These requirements can be met by Hitachi s new advanced measurement and diagnostic techniques. Hitachi has been applying these techniques to all production stages, from material acceptance inspection to in-service inspections or product disposal inspections, while successfully maintaining high reliability of various products. INTRODUCTION DURING in-service inspections of power plants, various types of equipment and components of the plant undergo start-up tests, disassemble/clean-up tests, and performance tests, and aged parts are repaired or replaced. A considerable number of man-hours are spent on non-destructive inspections of the structural components to check if they have been damaged or deformed. Non-destructive inspection techniques are utilized for determining internal conditions of equipment and components without having to disassemble them. They generally include ultrasonic testing, eddy current testing, and radiographic testing to detect precise inner flaws and deformations that may be invisible to the naked eye. If non-destructive inspections can be done quickly and easily, it will be possible to reduce the in-service inspection time considerably. In addition, advanced measurement and diagnostic techniques will be able to increase product reliability through their application to material reception tests and manufacturing process tests. This paper describes measurement and diagnostic techniques for ensuring the reliability of equipment and components in power plants. MEASUREMENT AND DIAGNOSTIC TECHNIQUES AND PRODUCT RELIABILITY For checking the reliability of a product, various types of tests and inspections are carried out during design, manufacturing, and maintenance service stages of a product. At the design stage, the performance, degradation mechanism, and lifetime of the prototype of the product are confirmed. At the manufacturing stage, which starts with the reception of materials and parts and lasts until the products are shipped, the materials, parts, and assembled products are carefully examined for flaws. At the stage of maintenance service, not only the adjustments of delivered product but also the replacement of worn parts and the performance tests and non-destructive inspections of repair regions are carried out during in-service inspections. Thus, efforts to ensure a product s reliability continue until the lifetime of the product ends and the product is

2 Hitachi Review Vol. 48 (1999), No discarded. Non-destructive methods are very important during the various tests and inspections of these stages to ensure that the products are not damaged. The non-destructive inspections are often required to measure and evaluate more speedily and more accurately than the capability of destructive inspections by sampling tests. Therefore, the authors have developed and applied measurement and diagnostic techniques to non-destructive inspections as follows: (1) full inspection of processed materials in reception tests without sampling and non-destructive evaluation of aging structural components for in-situ tests, (2) rapid inspection through simplified operating procedures using remote operating tools, and (3) highly accurate and high-speed fluoroscopic inspections through the development of high-sensitivity sensors. NON-DESTRUCTIVE INSPECTION OF WEAR-RESISTING MATERIALS The sliding parts and friction parts of valve shafts, wheel axles, and so forth are subjected to a hardening treatment to increase their surface hardness. For example, in treating a steam valve, nitrogen atoms are caused to penetrate the surface of the low-alloy steel valve to a depth of 200 µm to increase the hardness by a factor of two or more. The thickness of the hardened layer has usually been checked by first cutting the steel material and then measuring the hardness at intervals of 20 µm from the surface downward using a micro- Vickers hardness tester. Although this destructive method is very accurate, it takes a lot of time and cannot be carried out in-situ. This is why the authors have been successfully developing a new measurement and diagnostic technique that allows for not only non-destructive evaluation of the thickness of the hardened layer in a short time, but also measurement of the thickness of the residual hardened layer while the steel materials are in use. The method is based on the phenomenon that the velocity of ultrasonic waves being propagated through a materials such as the steel surface layer (called a Rayleigh wave) changes slightly according to the hardness of the steel material. The method is to set a transmitting probe and a receiving probe at a certain interval (minimum 6 mm) on the surface of a test piece and measure the velocity of the Rayleigh wave propagating between the two probes (to within an accuracy of 2 ns). The Rayleigh wave velocity changes in proportion to the thickness of the layer (Fig. 1), so by comparing the velocities of Velocity change rate(%) Ultrasonic transducers Different symbols in data mean specimens under different nitrided conditions Nitrided layer thickness (destructive tests, mm) Steel material Rayleigh wave Standard deviation σ=20 µm Nitrided layer Fig. 1 Relationship Between Rayleigh Wave Velocity and Hardened Layer Thickness. High accuracy velocity measurement of the ultrasonic Rayleigh waves reveals that there is a proportional relationship between the thickness of the hardened (nitrified) surface layer of steel materials and the velocity of Rayleigh waves propagating through the surface layer. the Rayleigh waves between hardened steel material and the untreated steel material, it is possible to quantitatively evaluate the hardened layer thickness to within an accuracy of 20 µm. Non-destructive evaluation methods such as this easy procedure makes it possible to inspect all of hardened processed materials. However, the steam valves present another problem: an oxide layer forms on the surface as they are used, so the thickness of the hardened layer cannot be evaluated by the Rayleigh wave velocity measurement only. Because the oxide layer and hardened processed steel materials have slightly different electronic and magnetic properties, the authors improved the method to be able to evaluate the thickness of the oxide layer and the hardened layer at the same time by combining the Rayleigh wave velocity measurement and an eddy current method. Fig. 2 shows the thicknesses obtained by the new method for the oxide layers and hardened layers in

3 Intelligent Measurement and Diagnostic Techniques for Non-destructive Inspections 298 Pressure vessel Nitrided layer (mm) Destructive method 100 mm Nitrided layer Oxide layer Axial position (mm) (a) Oxide layer/hardened layer thickness obtained by destructive tests Oxide layer thickness (µm) Drain piping (diameter 60 mm) Vehicle setting position 4 5 m transport tube CRDH: control rod drive housing Rail CRDH Cable to control box to wire handling Source container Nitrided layer (mm) Combined NDE method Axial position (mm) (b) Oxide layer/hardened layer thickness evaluated from the non-destructive method Oxide layer thickness (µm) Fig. 3 Scheme of Inspection of Drain Piping Under Reactor Pressure Vessel. A remote-controlled vehicle passes through the clearance (145 mm) between control rod guide pipes to inspect the drain piping. Operation of the vehicle and insertion and withdrawal of the γ-radiation source are all remote-operated. Fig. 2 Inspection Results of Thickness of Oxide Layers and Hardened Layers of Steam Valves. The combination of the ultrasonic Rayleigh wave method and the eddy current method has made it possible to measure the thickness of oxide layers and hardened layers of steam valves non-destructively. actual steam valves in use. The figure compares the results obtained by the destructive method with the results obtained by the non-destructive method. It can be seen that the thicknesses of the oxide layer and the hardened layer obtained by the non-destructive method were more precisely evaluated than that obtained by the destructive tests. Thus, the non-destructive evaluation method will make it possible not only to examine the integrity of component materials, but also to determine optimum replacement timing. RAPID INSPECTION USING REMOTE- CONTROLLED ROBOT Power plants have several areas which are inaccessible to the inspector because: (1) they are confined by a labyrinth of piping, (2) they are exposed to high levels of radiation, (3) they are exposed to high temperatures, or (4) they are submerged in water. Therefore, the authors developed a prototype inspection system, which is composed of a small robot and an RT (radiographic testing) device for inspecting the integrity of piping in confined spaces speedily and remotely. The work area for inspecting drain piping under the pressure vessel of a boiling water reactor is very narrow because of the many control rod drive housings (Fig. 3). It takes a lot of time for the sensor to access the drain piping and check its integrity. In order to reduce the inspection time, the authors developed: (1) a vehicle that can access the drain piping through the clearance (145 mm) between the control rod drive housings, and that is remote-controlled by an extremely small CCD (charge coupled device) camera provided at the front end; and (2) an RT device that employs a high-sensitivity IP (image intensified plate) which stores γ-ray transmission images obtained by an iridium radiation source (Ir-192) as optical information (Fig. 4). The IP facilitates sophisticated digital image processing of transmission images and displays clear images (resolution: 0.3 mm) by focusing on the profiles of the outer wall, inner wall, and artificial defects fabricated in a mock-up pipe. The new vehicle and RT device have made it possible to remotely inspect the drain piping in a short time (minimum 3 minutes) and to improve the reproducibility of radiographic inspection points and image resolutions successfully. The practical use of the system in actual nuclear power plants and its capability to examine the integrity of drain piping has

4 Hitachi Review Vol. 48 (1999), No Image intensifier plate CCD camera Drain pipe (Mock-up) Outer surface Heat insulator Positioning plate transport tube Inner surface Magnet wheel holder 10 cm Diameter: 60.5 mm Thickness: 5.5 mm (a) Remote-controlled vehicle operation procedure CCD camera (b) RT image of drain piping mock-up Image intensifier plate CRDH holder hold position (1) Access (2) Arm opening (3) insertion (4) RT inspection CCD: charge coupled device Fig. 4 Remote-controlled Vehicle for Measuring Piping Wall Thickness. The vehicle is controlled remotely by an image-based navigation system to pass through the narrow clearance between the control rod drive housings, access to the drain piping under the pressure vessel, and inspect the drain piping by RT. Object Array detector Signal processing unit LAN Rotation Vertical motion Data acquisition system (PC) X-ray source (Electron linear accelerator) Scanner Image processing system (EWS) Fig. 5 X-ray CT System Configuration. A high-performance X-ray CT system (inspection time: 10 seconds) was developed using a linear accelerator to improve the intensity of the X-ray source and a channel array detector to improve the efficiency of radiography.

5 Intelligent Measurement and Diagnostic Techniques for Non-destructive Inspections 300 (a) Solid image (b) CT image Fig. 6 Stereoscopic Solid Image and CT Image of Turbo-motor. The CT image is a high-resolution image consisting of 1,000 1,000 picture elements. By superimposing high-resolution CT images and selecting any surface of an internal part, it is also possible to display a solid image. confirmed its capability. In the near future, the system will be widely used to inspect pipes, valves, and so forth, covered with heat-insulators. HIGH SPEED AND HIGH RESOLUTION OF X-RAY COMPUTER TOMOGRAPHY IMAGING OF INTERNAL STRUCTURES In product and in-service inspections, it is becoming more and more important to be able not only to detect flaws but also to examine deformations of internal structures and identify foreign materials. For example, checking and identifying waste or other material packed in a pail can is becoming an indispensable technology for environmental management and recycling of resources. The authors have developed a high-energy X-ray CT (computer tomography) system that can obtain fluoroscopic images of large products in a short time and can identify shapes and contents of materials accurately. In this system, a high X-ray source, which generates X-rays by an electron linear accelerator (6 MeV), and a one-dimensional X-ray array detector are set up opposite each other as shown in Fig. 5. A large product is placed in the center between them, and is scanned as it is rotated and moved vertically. The system represents tomographic and stereoscopic images of the product interior through computer reconstruction of the detected X-ray data. The electron accelerator can generate an extremely intense dose rate, 10,000 to 100,000 times stronger than that of a radioisotope. The array detector consists of strips of Si-SSD (silicon semiconductor detector) arranged one-dimensionally. It can detect incident high-energy, X-ray photons, and is 10,000 times more sensitive than ordinary X-ray film. Thus, the new system has an X-ray source intensity and sensitivity that are much higher than those of conventional CT systems. The new system is both high speed and high resolution, and has a radiographic inspection time for 1,000 1,000 picture elements of 10 seconds and sizing accuracy of 0.1 mm. This system permits not only computer tomography, but also digital radiography (DR), simply by rectilinear scanning. Images of a turbo-motor obtained by the CT system are shown in Fig. 6. By superimposing precise CT images, it is even possible to display a stereoscopic solid image that permits a part of the internal structure to be observed in more detail. The system not only aids in inspecting interior shapes and deformations, but also helps in visualizing chemical reactions that occur in electronic cells or other similar devices and investigating causes of product deterioration based on the visual images.

6 Hitachi Review Vol. 48 (1999), No CONCLUSIONS The authors have introduced intelligent measurement and diagnostic techniques to ensure the reliability of various types of equipment and devices of power generation plants. Hitachi is not only carrying out inspections for detecting flaws and nonconformances, but also developing new nondestructive evaluation and diagnostic methods, which make the inspections more efficient. The efforts to ensure high reliability of products should be continued by applying these simple, efficient measurement and diagnostic techniques to all stages, from material reception, manufacturing and shipping, to maintenance and discarding. REFERENCES (1) Y. Nagashima et al., Non-Destructive Evaluation on of Nitrided Layer Integrity of Turbine Components by a Combined Method of Ultrasonic and Eddy Current Techniques, 1st Int. Con. on NDE in Relation to Structural Integrity for Nuclear & Pressurized, Vol. 1 (Oct. 1998), pp (2) F. Takahashi et al., Development of an Ultrasonic Inspection Vehicle for BWR Core Shrouds, 14th Int. Con. on NDE in the Nuclear & Pressure Vessel Industries (Sep. 1996), pp (3) S. Izumi et al., High Energy X-ray Computed Tomography for Industrial Applications, IEEE Transaction of Nuclear Science, 40 (Apr. 1993), pp ABOUT THE AUTHORS Fuminobu Takahashi Joined Hitachi, Ltd. in 1971, and now works at the Nuclear Chemistry & Reactor Preventive Maintenance Department of the Power & Industrial Systems R&D Laboratory. He is currently engaged in R&D of non-destructive inspection/diagnostic techniques and inspection robots. Dr. Takahashi is a member of the Japan Atomic Energy Society, the Applied Physics Society and the Japan Non- Destructive Inspection Corporation, and can be reached by at takahasi@erl.hitachi.co.jp. Shigeru Izumi Joined Hitachi, Ltd. in 1967, and now works at the Measurement Technology Research Center of the Power & Industrial Systems R&D Laboratory. He is currently engaged in R&D of radiation and electromagnetic measurement techniques. Dr. Izumi is a member of the Japan Atomic Energy Society, the Japan Electrical Society, the Japan Mechanical Society and the Japan Non-Destructive Inspection Corporation, and can be reached by at sizumi@erl.hitachi.co.jp. Akira Nishimura Joined Hitachi, Ltd. In 1971, and now works at the Nuclear Power Plant Service Department of the Nuclear Power Plant Division. He is currently engaged in coordination of preventive maintenance of nuclear power plants. Mr. Nishimura is a member of the Japan Atomic Energy Society, and can be reached by at nishimura@cm.hitachi.hitachi.co.jp.