Demonstration of remote field eddy current testing of double wall tube with wire mesh layer

A helical-coil-type double wall tube filled with a wire mesh layer is proposed for a steam generator (SG) of a new-type fast-breeder reactor in Japan in order to further increase reliability in operation. This double wall tube consists of inner and outer tubes and a wire mesh layer between inner and outer tubes. An inservice inspection (ISI) technology is required to put this double wall tube SG to practical use. We need to detect small defects on an outer surface of an outer tube during an insert of a sensor into an inner tube. A major candidate for this inspection is a remote field eddy current testing (RFECT). An objective of this development is to demonstrate a defect detection of a double wall tube with a wire mesh layer using a RFECT. A RFECT was required increase of sensitivity and spatial resolution to detect a smaller defect. As a high sensitivity technique, to increase an indirect magnetic field intensity, we focused attention on increasing a direct magnetic field intensity in vicinity of an exciter coil and devised a method to concentrate a direct magnetic flux into an intense field by the use of an exciter coil with a magnetic material (flux guide), additionally, applied a flux guide to a detector coil. As a high spatial resolution technique, we adopted a multiple detector coil arrayed circumferentially. We optimized a shape of a flux guide made of high magnetic permeability material by a magnetic analysis and made prototype coils. Two experiments, a measurement of a magnetic field (a voltage of a detector coil)distribution in a double wall tube with a wire mesh layer and defect detection tests, were performed to verify effects of flux guides and a multiple detector coil. According to the experimental results, the indirect magnetic field intensity ( the voltage of the detector coil in the region of the indirect magnetic field) increased more than 100 times by the application of the exciter and detector coils with flux guides. Finally, we detected a smaller defect over the wire mesh layer by the adoption of the multiple detector coil. We designed the exciter and multiple detector coils with the flux guides to increase the sensitivity and spatial resolution of the RFECT. It was confirmed the indirect magnetic field intensity increased more than 100 times. We demonstrated a smaller defect detection of a double wall tube with a wire mesh layer using a RFECT and verified flux guides and a multiple detector coil were effective for high-sensitivity and high spatial resolution of a RFECT. We considered that this RFECT also was available for inspections of other thick heat exchanger tubes