Minimization of Lift-off Effects on Gap Measurements Using Optimal Frequencies

Abstract

Fuel channel inspection in CANDU reactors employs an eddy current probe to monitor gap between the pressure tube and calandria tube. However, the probe is sensitive to lift-off variations, which can adversely affect estimated gap. In this study a flat-plate model of eddy current response to gap was used to examine the effect of drive coil frequency on response to lift-off and gap. Frequencies were determined where probe response to gap is insensitive to liftoff. Measurement at these frequencies could improve gap estimates by reducing the error introduced by lift-off variations as may occur near garter spring locations.