In-Service Inspection of Steam-Generator Tubing Using Multiple-Frequency Eddy-Current Techniques

Although a rapid, accurate, and easy-to-use inspection method for steam-generator tubes is clearly needed, it is not entirely provided by our present eddy-current tests. The signals from eddy-current tests are affected by changes in tube wall thickness, the location and size of defects, the electrical conductivity, the magnetic permeability, and the tube-to-tube support spacing and denting (which changes the probe-to-tube spacing). Simultaneous variations in these test properties produce signals that are ambiguous in present commercial eddy-current instruments. However, this type of multiple-property eddy-current testing can be done using pulsed techniques or, as in this example, multiple frequencies. A systematic, automated process has been developed at Oak Ridge National Laboratory (ORNL) to allow rapid development of accurate multiple-property eddy-current tests. It consists of the following steps: 1. Using a computer to design the coil, instrument, and operating conditions that give the best determination of the desired properties from a nonlinear combination of instrument readings. 2. Constructing the coil and instrument and testing with calibration specimens. 3. Using a large digital computer to take on-line instrument readings and perform the nonlinear property calculations. 4. Programming the on-board microcomputer in the instrument to perform the property calculations in the field. The system has demonstrated a considerable improvement in the ability to measure wall thickness, tubing inside diameter, defect size, and defect location even in the presence of severe denting using laboratory standards. We are now proceeding with field testing of the instrumentation.