Resistivity Profiling of the Graphite Bricks in an Advanced Gas-Cooled Reactor Core using a Non-linear Inverse Eddy Current Method

Tesfalem, Henok; The University of Manchester; United Kingdom

Tesfalem, H.; University of Manchester; United Kingdom
Peyton, A.J.; University of Manchester; United Kingdom
Fletcher, A.D.; EDF Energy; United Kingdom
Brown, M.; EDF Energy; United Kingdom
Chapman, B.; NDT Consultant; United Kingdom

ID: ECNDT-0321-2018
Download: PDF
Session: Civil Infrastructure - ET, UT
Room: J1
Date: 2018-06-12
Time: 15:20 - 15:40

Advanced Gas-Cooled Reactors (AGR) are the most dominant reactors in the United Kingdom, which contribute approximately 19 % of the total annual electricity supplied to the country. The AGRs use carbon dioxide as a cooling mechanism, and graphite core to moderate the fast-moving neutrons and maintain the lattice spacing between fuel assemblies and control rods. During the course of the AGRs lifetime, the graphite bricks in the core suffer from radiolytic oxidation, which leads to a localised reduction of the graphite density (weight loss) with time. As the results, the oxidation could affect the core structural integrity margins and potentially reduce the operating lifetime of the reactors. For this reason, the assessment of AGR graphite properties is becoming increasingly important.
Eddy current based non-destructive testing is now becoming routinely used to assess the conditions of the graphite bricks in the AGRs, in particular to inspect crack formation and density mapping. Many studies have shown an inverse relationship between graphite density and its electrical resistivity, which means that by knowing the graphite resistivity profiles, one can estimate the density profile as a function of depth.
This paper presents the results of the reconstructed resistivity profiles of the AGR graphite bricks as a function of depth from the individual graphite brick bore. The resistivity profiles were reconstructed using a multi-frequency eddy current data measured from the actual AGR core fuel channels and Regularised Levenberg Marquardt algorithm. The reconstructed resistivity profiles were validated against the profiles of the trepanning samples, which were taken out of the core at the locations where the eddy current data was obtained. Comparison between the reconstructed and measured resistivity values show reasonable agreement between one another in multiple cases, suggesting this method could be a viable tool for accessing the conditions of the graphite bricks in the AGR core.