On-power Detection of Pipe Wall-thinned Defects Using Ir Thermography in Npps

The number of aging nuclear power plants (NPPs) has increased recently. Accordingly, the number of operational interruptions has increased due to malfunctions of the NPPs secondary systems. These cases occur in the secondary systems of NPPs with a range of structures due to fatigue, wall-thinned defects, corrosion, etc. Of these problems, wall thinned defects occur in the pipes by the diffusion of the corrosion with the flow of the fluids, and the defects frequently take place in carbon steel pipes with lower Cr content. Such wall-thinned defects can lead to damage without warning signs and they can be found frequently in the base material part. Therefore, they are one of the major factors that degrade the integrity of a pipe [1], [2]. Systematic management of wall-thinned defects requires regular inspections. In particular, systematic management requires a close inspection even when the NPP is in normal operation. The secondary system of a NPP is the place to which the operators or workers gain access for their work frequently. Unexpected damage to a pipe may have significant social impacts, which highlights the importance of systematic management of wall-thinned defects. Consequently, considerable attention has been paid to nondestructive inspections to examine the integrity of major facilities. In addition, there is increasing demand for nondestructive inspection methods that are relatively safe and enable measurement in a quick and easy manner [3]. Currently, a range of non-destructive inspections are conducted, such as ultrasonic testing (UT), eddy current testing (ECT), and magnetic particle testing (MT) [4], [5]. Non-destructive inspection techniques involve infrared (IR) thermography. IR thermography is expected to help resolve the issues related to the limitations of the existing non-destructive inspection techniques because it is used to examine defects based on measurements of the temperature difference between defective parts and non-defective parts. IR thermography is also expected to be useful on a NPP site [6]. IR thermography with a cooling device is a reliable technique for detecting wall-thinned defects in the inner pipes of NPPs that are in normal operation, and is expected to facilitate the maintenance of secondary systems of NPPs. The results of this study will be used as the basic material for the inspection of wall-thinned defects. Wall-thinned defects caused by accelerated corrosion due to fluid flow in the inner pipe appear in many structures of the secondary systems in nuclear power plants (NPPs) and are a major factor in degrading the integrity of pipes. Wall-thinned defects need to be managed not only when the NPP is under maintenance but also when the NPP is in normal operation. To this end, a test technique was developed in this study to detect such wall-thinned defects based on the temperature difference on the surface of a hot pipe using infrared (IR) thermography and a cooling device. Finite element analysis (FEA) was conducted to examine the tendency and experimental conditions for the cooling experiment. Based on the FEA results, the equipment was configured before the cooling experiment was conducted. The IR camera was then used to detect defects in the inner pipe of the pipe specimen that had artificially induced defects. The IR thermography developed in this study is expected to help resolve the issues related to the limitations of non-destructive inspection techniques that are currently conducted for NPP secondary systems and is expected to be very useful on the NPPs site.