Pulsed Eddy-Current Detection of Loose Parts in Steam Generators

Abstract

Flow-induced vibration of loose parts located on steam generator (SG) tube sheets can result in SG tube damage and eventual loss of SG efficiency. Regular inspection of SG tubes at support structures can help extend SG life by detecting and monitoring flaws as well as identifying accumulation of loose parts. Application of conventional eddy-current technologies in this case, however, is limited when magnetite fouling is present or when loose parts are resting directly on the SG tube sheet. Pulsed eddy current (PEC) combined with a modified principal components analysis (MPCA) was investigated as a method of detecting and identifying a variety of loose part materials in close proximity to 15.9-mm outer diameter Alloy-800 SG tubes and surrounding steel support structures. Loose parts were found to be detectable for varying axial positions along the SG tube and when in contact with the tube sheet. When offset radially from the SG tube, 1.6-mm-diameter carbon steel parts with a strong magnetic component allowed detection up to 3.2 mm away, while nonferromagnetic stainless steel parts of the same size could only be detected up to 1.5 mm away. Signal variation in the presence of loose parts was attributed to differences in amplitude of transient response and in relaxation times for diffusion of transient magnetic fields through various materials. The ability to accurately detect and identify loose parts located in close proximity to SG tubes establishes PEC combined with MPCA as a potential candidate for SG inspection.