Supplemental examination of alternative materials in a model steam generator: Volume 2, Correlation of Model 10 defects with eddy-current indications: Final report

The following report compares the nondestructive examination with the destructive metallographic examination of the tube bundle alternative materials, Model 10 steam generator. The model steam generator was fabricated using state-of-the-art tube processing (Alloys 600, 690 and 800) and corrosion resistant support alloys and designs. The model steam generator was operated for approximately one year with a secondary environment that was faulted with acidified simulated cooling tower water and sludge. The tube bundle was eddy current tested at the final inspection using multiple frequency differential and absolute mode probes and state-of-the-art eddy current equipment, including a computerized system for data analysis. Various frequency mixes were designed to suppress signals from deposits and to improve signal to noise ratios. Special tests were utilized to evaluate the Alloy 800 tubing. Eddy current data correlated favorably with results from the destructive examinations. Generally eddy current tests were most successful in detecting axial cracks, circumferential cracks, defects with depths greater than 10% throughwall and tube denting at trefoil supports. It was not possible to distinguish between cracks and deep wastage. Interpretation of the eddy current indications predicted corrosion damage deeper than that found metallographically for conservative depth estimates. Numerous false defect indications, with low signalmore » to noise ratios, were produced by the presence of conductive deposits on the O.D. of the tubing. Absolute mode testing provided a reliable qualitative indication of tube corrosion despite the interference from the conductive deposits. 4 refs., 1 fig., 1 tab.« less