Key Engineering Materials Vols. 321-323

Abstract: A laser ultrasound technique (LUT) is reported for nondestructive characterization of hydrogen concentration (HC) in Zr-4 cladding tubes. With the LUT, ultrasonic waves propagating in the Zircaloy tubes with different HC are generated and detected remotely by optical means. By measuring the dispersion spectra with the LUT, relations between the dispersion spectra and the HC of the Zircaloy tubes are established. HC ranging from 0 to 1200 ppm in the Zircaloy tubes are successfully discriminated by the LUT with a resolution of 200 ppm.

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, absolute and differential eddy current signals from various shapes and sizes of defects in the steam generator tube are numerically predicted and their slope characteristics are investigated. The signal variations due to frequency increase are also observed. Then, the analysis of mixed defect signals affected by a ferromagnetic support plate is attempted. Axisymmetric finite element modeling is used for signal prediction and the slope angle of signal is used importantly in analyzing the signal. Results show that differential signals are useful for locating the position of a defect inside the support plate hole, while absolute signals are easy to presume and interpret even though the effect of support plate is mixed in the signal. Understanding of respective signal characteristics and relationship between them will be helpful for more reliable defect characterization.

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: Tubes in nuclear steam generators are held up by supports because the tubes are long and slender. Fluid flows of high-pressure and high-temperature in the tubes cause oscillating motions between tubes and supports. This is called as FIV (flow induced vibration), which causes fretting wear in contact parts of tube-support. The fretting wear of tube-support can threaten the safety of nuclear power plant. Therefore, a research about the fretting wear characteristics of tube-support is required. This work is focused on fretting wear transitions from mild wear to severe wear of tube-support materials by various loads and relative displacements. The transition is defined on the basis of the changes in wear amount. To investigate the transition, the fretting wear tester was contrived to prevent the reduction of relative displacement between tube and support by increasing the load. The tube and support materials were Inconel 690 and 409 SS, respectively. The results show that the transition of tube-support wear is caused by the changes of the dominant wear mechanism depending on the applied load and the relative displacement.

Abstract: A direct-current potential-drop NDE technique has been developed for the evaluation of a 3-D back-wall crack in thick-walled structures. The measuring system contains two pairs of probes – one for current supply and the other for the measurement of potential-drop, where the currentsupply and measuring probes are in close proximity to each other. The optimum arrangement of the four-point-probes system is determined from the finite-element simulation of the three-dimensional electric crack problem. The predicted potential-drop across the crack has been verified experimentally using the newly developed measuring sensor. Measured results of semi-elliptical cracks in stainless steel plates establish the practicality as well as reliability of the present potentialdrop method of testing for the evaluation of 3-D back-wall cracks in thick-walled structures.

Abstract: This study was aimed at developing an effective method and a system for on-line health monitoring of pipes in nuclear power plants by using ultrasonic guided waves. For this purpose we developed a multi-channel ultrasonic guided wave system for a long-range inspection of pipes and a few techniques which can effectively find defects in pipes. To validate the developed system we performed a series of experiments and analyzed the results.

Abstract: There are several efforts for an advanced on-line health monitoring of a piping system by using a chemical condition monitoring, a X-ray radiography and a ultrasonic techniques. In order to increase the confidence of the understanding on the health condition of the underlying piping system, an integrated and multi-disciplinary approach has been adopted in this paper. Flow-Accelerated Corrosion (FAC) under an undesirable combination of the water chemistry, flow velocity and material composition has usually caused a pipe wall thinning. The condition monitoring tests on a piping elbow of a piping system have been conducted by using several advanced sensors such as an accelerometer, ultrasonic device, and chemical sensors.

Abstract: In the present study, the synthetic signals from the combo tube are simulated by using commercial electromagnetic numerical analysis software which has been developed based on a volume integral method. A comparison of the simulated signals to the experiments is made for the verification of accuracy, and then evaluation of five deliberated single circumferential indication signals is performed to explore a possibility of using a numerical simulation as a practical calibration tool. The good agreement between the evaluation results for two cases (calibration done by experiments and calibration made by simulation) demonstrates such a high possibility.

Abstract: This study was aimed at developing a new method for detecting the sodium-water reaction as a result of a water leakage into the liquid sodium boundary for the liquid metal reactor. In the case of a passive acoustic method, to develop the leak detection algorithm, several signal processing methods have been evaluated. When the acoustic emission signal has a relatively high signal-to-noise ratio for the acoustic noises the spectral estimation method could be used to detect the sodium water reaction. In the case of a low signal-to-noise ratio within -5 dB to -15 dB, the system modeling and the identification methods using an autoregressive and an adaptive algorithm could be used to detect the sodium-water reaction. Regarding the active acoustic method, the basic attenuation factors of the ultrasonic beam were evaluated in the case of normal and abnormal plant conditions.