Abstract
This paper describes the exploration of eddy current (EC) inspection in its application to graphite pebbles. These pebbles are being used at Argonne National Laboratory (Argonne) as simulated fuel elements in high temperature reactor (HTR) design concepts. Eddy current inspection is a fast and economical non-destructive evaluation (NDE) technique that can detect flaws, impregnations, or manufactured discontinuities in the outer layer of graphite pebbles. Rapid testing for sorting pebbles is crucial when dealing with quantities of pebbles in the hundreds of thousands. In the existing paradigm, pebble batches can be sorted using their radioactivity levels, however, conventional radioactivity level sensors typically require lower operating temperatures, and therefore time for the pebbles to cool. With the current developments in high temperature EC probes, this NDE method could considerably reduce the downtime required for the sorting of marked pebbles under harsh environments. This paper describes the numerical electromagnetic modeling results and measurements on a sample pebble, which are used to optimize the EC probe design and frequency selection. We also present experimental data collected on a sample pebble with a thin film surface array probe, demonstrating the capability to detect flaws in the outer graphite layer of the pebble. We conclude that EC inspection is a promising technique for rapid and reliable testing of graphite pebbles for HTR applications.
