Feasibility Study of Application of MFL to Monitoring of Wall Thinning Under Reinforcing Plates in Nuclear Power Plants

This paper discusses a feasibility of magnetic flux leakage (MFL) method for estimation of wall thinning on pipes under reinforcing plates in nuclear power plants. The layered steel plates having a slit on its underlayer were prepared and the estimation of slit size by MFL method was examined. A single magnetic yoke is fixed on a layered specimen, and then a magnetic field sensor scans the leakage flux between the yoke legs for condition monitoring. The single magnetic yoke with a field sensor fixed at the center of the yoke is also scanned over layered specimens. It is clarified that estimation of the size of slit fabricated on underlayer of the layered specimen using profile of leakage field is possible: this indicates monitoring conditions of wall thinning to be occurred under reinforcing plates will be possible by MFL methods. Additionally, the effect of an air gap between yoke and first layer and a gap between first and under layer were also investigated experimentally.