Development of a technique for inspection of the graphite bricks in an advanced gas-cooled reactor core

Eddy current (EC) based NDT is now becoming routinely used in the civil nuclear power industry to assess the conditions of the graphite bricks in Advanced Gas-cooled Reactor (AGR) cores. This paper presents a Finite Element (FE) model study of an EC probe, which could be used to inspect the graphite bricks in an AGR core. This work was initially concerned with the investigation and optimisation of EC probes in terms of their sensitivity to the conducting graphite, particularly with depth, followed by the experimental implementation of the optimised probe prototype, which was used to validate the FE model. Two different types of inverse problems (constrained and unconstrained) are solved using the measured data from a sample graphite brick and adapting an iterative inversion technique. The FE-based sensitivity study results show an average of 43.7% sensitivity improvement compared with the existing probe and reasonable agreement between the simulated and measured data. The inverse solutions in this work yield average profile errors of 11.6% and 1.3% compared with the experimentally-determined graphite electrical conductivity values as a function of depth for unconstrained and constrained approaches respectively. The accuracy of the constrained inverse solutions in this paper suggests that this method could be used in practice to estimate through-wall density variations within AGR graphite bricks.