Papers by Author: Myung Ho Song

Abstract: In this paper, conservatisms of current plugging criteria on steam generator tubes are reviewed and six new failure prediction models for dual through-wall cracks are proposed. In order to determine the optimum ones among these local or global failure prediction models, a series of plastic collapse tests and corresponding finite element analyses are carried out with respect to two adjacent axial through-wall cracks in thin plates. Then, reaction force model, plastic zone contact model and COD (Crack Opening Displacement) base model were selected as the optimum ones for integrity assessment of steam generator tubes with dual cracks.

Abstract: For inspection of steam generator tubes in nuclear power plants using eddy current testing, it is a very crucial task to extract the flaw signal from signals compounded by flaws and supporting structures. To perform such an important task, the multifrequency eddy current testing method is widely adopted. In this study, we explore the performance of a linear mixing technique based on the discrete cosine transform method by experiments. The results obtained using the DCT based method agree very well to those obtained by commercial software demonstrating the effectiveness of the approach.

Abstract: Most of the corrosive degradations in steam generator tubes of nuclear power plants are closely related to the residual stress existing in the local region of a geometric change, that is, an expansion transition, u-bend, ding, dent, bulge, etc. Therefore, accurate information on a geometric anomaly in a tube is a prerequisite to the activity of a non destructive inspection for a precise and earlier detection of a defect in order to prevent a failure during an operation, and also for a root cause analysis of a failure. In this paper, a newly developed eddy current technique of a three-dimensional profilometry is introduced and the proof for the applicability of the technique to a plant inspection is provided. The quantitative profile measurement using a new eddy current probe was performed on steam generator expansion mock-up tubes with various geometric anomalies typically observed in the operating power plants, and the accuracy of the measured data was compared with those from the laser profilometry.

Abstract: Steam generator tubes are expanded inside tubesheet holes and the crevice gap between tubesheet and the tube should be minimized. To examine tube expansion area, eddy current testing is usually used. In this paper, absolute and differential signals are computed by a numerical method for several different locations of transition region and signal variations due to tubesheet, tube expansion and operating frequencies are observed. Results show that low frequency is good for identifying the location of tubesheet, and high frequency is good for the measurement of expanded tube diameter and the recognition of transition region. In general, absolute signal is good for measuring tube diameter and the differential signal has advantages of locating tubesheet and both ends of the transition region.

Abstract: In this paper, eddy current signals from various anomalous defects in the steam generator tube are numerically predicted and their signal slope characteristics are investigated. The signal variations due to increased frequencies are also observed. Based on the accumulated knowledge, the analysis of compound signal is attempted which includes the effect of support plate. Both differential and absolute bobbin probe signals are analyzed. For the signal prediction, axisymmetric finite element modeling is used and this leads us to the utilization of slope angle analysis of the signal. Results show that differential signals are useful for locating the position of defect under the support plate, while absolute signals are easy to presume and interpret even though the effect of support plate is mixed. Combined use of these two types of signals will help us accomplish a more reliable inspection.

Abstract: Eddy current testing (ECT) is widely used in in-service inspection as well as pre-service inspection of the steam generator (SG) tubes in nuclear power plant of pressurized water reactor type. The interpretation of ECT signals, however, is truly a difficult task so that the reliability enhancement of signal interpretation is strongly desired. An enhanced interpretation tools for ECT signals have been developed by the novel combination of neural networks and finite element modeling for quantitative flaw characterization SG tubes. A database was constructed using synthetic ECT signals generated by the finite element models and principal component analysis (PCA) was adopted in order to optimize the feature set of ECT signals. The improvement in the performances by the features with PCA and the excellent performance for the experimental ECT signals demonstrate the high potential of the developed inversion tools for reliable interpretation of eddy current signals. To explore the possibility of applying the developed approach in practical inspection, we developed an automated system (laboratory prototype) that can acquire experimental ECT signals from SG tubes and carry out the quantitative flaw characterization in a real time fashion by applying the approach developed in the present work.