Electromagnetic methods of detecting magnetite in PWR steam generators

Corrosion products consisting primarily of magnetite (Fe3O4) in the crevice gap between steam generator tubes and support plates can eventually lead to tube denting and support plate fractures. In order to mitigate this problem, chemical flushing is used to remove the magnetite deposit from the crevice gaps. NDE techniques are needed to measure the effectiveness of this procedure this paper describes the development and testing of two electromagnetic probes for this purpose. A simple dc variable reluctance probe has been developed under EPRI Project SGPO-S125-1 which gives a measure of crevice gap clearance at eight discrete points around the periphery of the tube. Hall elements are used as the active sensors and the probe drive, and eight-channel signal acquisition is accomplished with ALN 4000 equipment. The alternative electromagnetic approach to the detection of magnetite buildup is to use a conventional differential eddy current probe. Work carried out under EPRI project RP1395-2 has shown that such a probe is also sensitive to the presence of magnetite in the crevice gap. In both cases, a finite element model has been used to develop ''training data'' for a variety of buildup profiles, and these theoretical results are compared with pre- and post-chemical flushing measurements on a model boiler