Evaluating the conductivity distribution in isotropic polycrystalline graphite using spectroscopic eddy current technique for monitoring weight loss in advanced gas cooled reactors

Recently studies have shown correlation between the conductivity of graphite and its weight loss (density). This paper presents an eddy current technique for evaluating the conductivity profiles within 100 mm thick layered graphite sections. Flat graphite plates with three different grades of density for which the corresponding conductivities were measured, have been used to construct these sections. An initial test was performed to examine the capability of the eddy current technique to penetrate to a 100 mm depth in graphite. Then spectroscopic inductance measurements were acquired from different test profiles and compared with finite element method (FEM) simulated data. Conductivity profiles were reconstructed from the experimental data via the solution of an inverse eddy current problem. For an inverse solution, direct Tikhonov regularization and iterative Gauss Newton reconstruction schemes have been employed. Results show that the inverse methods are able to produce reasonable estimates of the conductivity profiles.