Analytical model of eddy current bobbin coil probe responses at support plates in steam generator and heat exchanger tubes

Abstract

The steam generators (SGs) of CANadian Deuterium Uranium nuclear reactors require periodic inspections to ensure their safe operation. Eddy current (EC) testing is the primary method by which the SG tubes are inspected. Many conditions in the SG tubes affect the EC response, such as fretting, pitting, cracking, as well as tube expansion, and the presence of tubesheet and support structures. When two or more of these parameters overlap, the EC signals from degradation induced wall losses are frequently difficult to interpret because these wall loss signals are often distorted due to interference with background signal variations. In this paper, a novel analytical model that describes the impedance of a bobbin coil with a plate encircling an arbitrary number of cylindrical conductors is developed. The model is validated against finite element method modelling and experiment. This model can be used to simulate the main probe response, while inspecting SG tubes near the tubesheet and support structures and thereby, provides the potential to separate out smaller flaw responses.