The Effects of ID Magnetite Deposits on Steam Generator Tube Eddy Current Signals

Abstract

The presence of primary-side magnetite deposits in CANDU steam generator tubes poses a significant challenge to the analysis of eddy current inspection data. In principle, magnetite shields the tubing from the eddy current probe, diminishing its sensitivity to flaws. Since a probe’s flaw detection and sizing performance is typically assessed with the use of clean laboratory samples, the shielding effect can become significant enough to affect both the probability of detection (POD) and sizing accuracy of the probe. Hence, there is a need to understand the relationship between inside diameter (ID) magnetite fouling and the actual flaw signals from typical flaws, such as pitting corrosion and fretting wear. The studies presented in this paper were performed with bobbin and X-probe data in both Inconel 600 and Incoloy 800 tubing. In comparison with clean tubes, ID deposits have a definite impact on both the detection and sizing capabilities of each probe. In all cases, the flaw signal amplitudes tend to decrease with increasing amounts of ID magnetite fouling. This also causes the probes to undersize the flaws in the fouled tubes. The resulting effect can be expressed in terms of a reduction factor versus a measured quantity with the bobbin probe, called Vshield, which can be used to determine the correction factor for sizing flaws. Given the difficulty in controlling the error sources in laboratory experiments on field-pulled tubes with deposits, the resulting scatter in the data makes the extraction of useful relationships between fouling and flaw signals difficult. This paper, therefore, presents an approach that consists of a combination of computer modelling, laboratory experiments on pulled tube samples, and field-data analysis to allow trends and relationships to be developed for a range of ID magnetite loading and thickness beyond that available in pulled tubes. A 3D electromagnetic finite element model was developed for studying the effects of ID magnetite fouling on eddy current signals from a variety of flaw types. A means for expressing the shielding effect was developed, along with a method for implementing correction factors to improve flaw-sizing accuracy.