EDDY CURRENT INSPECTION OF GRAPHITE PEBBLES FOR PEBBLE BED REACTORS

This paper presents the results of eddy current (EC) inspections on a set of surrogate fuel pebbles such as those in high temperature reactors (HTRs). EC testing is a fast and economical non-destructive evaluation (NDE) technique that can easily detect surface dislocations such as flaws or manufactured grooves in the outer graphite layer of the pebbles. However, EC inspection was explored further in this work to evaluate the ability of imaging beyond the surface–to potentially identify subsurface artificial impregnations of non-homogeneous artifacts. We briefly describe the numerical modeling, the experimental setup, and the data analysis methods used to optimize the EC probe design and frequency selection. We then present data collected on a set of pebbles, demonstrating the capability to detect flaws in the outer graphite layer of the pebble and discuss limitations of the system. Furthermore, we demonstrate the potential of EC inspection to distinguish between different types of pebbles, such as solid or annular pebbles, based on their conductivity and oxide particles. With current advances in high temperature EC probes, we conclude that EC inspection is a promising technique for rapid and reliable testing of pebbles for HTR applications.