An In-Tube Radar for Detecting Defects in Thin-Walled Metal Tubes

摘要

A major cause of failures in heat exchangers and steam generators in nuclear power plants is degradation of the tubesrnwithin them. The tube failure is often caused by the development of cracks that begin on the outer surface of the tube and propagaternboth inwards and laterally. A new technique will be described for detection of defects using a continuous-wave radarrnmethod within metal tubing. The technique is 100% volumetric, and may find smaller defects, find them more rapidly, and findrnthem less expensively than present methods. The In-Tube Radar is in the first year of a three-year joint project between SandiarnNational Laboratories (SNL) and New Mexico State University (NMSU). The project is funded under the Department ofrnEnergy (DOE) Nuclear Energy Research Initiative. The radar consists of two orthogonal magnetic dipoles that are centered onrnthe tube axis and perpendicular to it. One of the electric field components from each dipole is parallel to but vanishes on therntube axis. Hence, an electric dipole located upon, and parallel to, the axis is not coupled to the electric fields of the magneticrndipoles, and should be able to detect the backscatter from a defect anywhere in the tube wall. This favorable situation isrndegraded when the antenna geometry is non-symmetric with respect to the tube axis, and cross coupling from transmitter-toreceiver,rnor ’self-clutter’, will occur. The degree of asymmetry that can be tolerated with respect to a useful signal-to-clutterrnratio is being investigated with results from two different 3D forward-modeling codes. The geometric asymmetry is set by thernability to keep the probe centered in the tube and parallel to the axis as the probe is moved along the tube during use. Therndesign and performance of this important feature will be described. The probe will be battery-powered and linked by fiberrnoptics to the outside of the tube to avoid unwanted reflections, and some of the components will be shown together with packagingrnand assembly details. The schedule for further development and testing for both the scaled-up prototype and the finalrnradar that will lead to planned public demonstrations in 2002 will be described in terms of the major program milestones.