The Impact of Magnetite Deposits on Steam Generator Inspections

Abstract

A successful steam generator (SG) health-monitoring program requires flaw detection, depth prediction and efficient analysis tools. It also needs reliable and timely input from advanced inspection, monitoring and diagnostic tools to assess other parameters that can impact the performance of the SG. One of these parameters is tube fouling that can affect heat transfer, flow velocity and distribution, and corrosion susceptibility.

The deposition of primary and secondary side magnetite deposits in SGs of CANDU nuclear power stations also poses a challenge during SG eddy current testing (ET) inspection campaigns. Inner diameter (ID) magnetite deposits mechanically restrict, and in extreme cases prevent inspection with ET probes. Furthermore, even thin layers of ID deposits can diminish probe sensitivity to flaws since magnetite is a ferromagnetic compound that produces a shielding effect for alternating electromagnetic fields. This has an impact on both the probability of detection (POD) and the ability to size flaws. Patchy ID magnetite, such as that which can occur after cleaning, can also cause excessive noise of the eddy current signals. This also affects the POD and the inspectability of the tubes. Monitoring and quantification of outer-wall magnetite-loaded sludge deposits is also an inspection requirement, especially at the defect prone top-of-tubesheet and tube support locations.

This paper will detail these issues from an inspection perspective, and present the results of studies evaluating the ability to quantify magnetite deposits and manage its impact on flaw detection and sizing techniques.