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.