Eddy Current Testing of Thin Layers Using Co-planar Coils

A new eddy current technique using a co-planar coil configuration (ECCPC technique) has been developed and tested quantitatively in the laboratory and under real conditions in a nuclear reactor tank to detect thin resistive and/ or magnetic layers on fuel rods. First a short description of the theoretical model is given, followed by a number of model studies comparing conventional co-axial loops with co-planar loops with respect to sensitivity to layer parameters. Laboratory measurements indicate that the simple model describes the physics of the induction phenomena adequately. First measurements in Switzerland prove that the new method successfully predicts the presence of thin magnetic crud layers that were erroneously interpreted using standard techniques to correspond to thick insulating oxide layers.