Measurements of magnetite piles on steam generator tubing with eddy current techniques

During in-service inspection the steam generator tubes are generally inspected using single probe eddy current techniques. Magnetite deposit built up on the outer surface of the tubes can be detected with the applied techniques. As deposits grow thicker they tend to split as flakes from the surface of the tubes. These flakes often build magnetite piles between the tubes. Under magnetite piles the conditions are also favourable for corrosion induced defect initiation and growth on the outer surface of the steam generator tubing. This study is a part of the SAFIR2014 project MAKOMON. The experiments were conducted during the years 2012 - 2014 at VTT by using eddy current tube inspection techniques. A small mock-up simulating the horizontal steam generator tubing was used. The goal of the experiment was to develop a technique to size the volume of the detected magnetite piles within the steam generator tubing. The conducted experiments showed that the existence of the magnetic piles can be detected with the applied one and two probe techniques. Additionally the location and extension of the magnetite piles can me mapped with these techniques. The thickness of a solid magnetite layer attached symmetrically to the outer surface of the tubes can be measured more precisely than the thickness of the layer consisting of loose magnetite flakes. The conducted tests showed that if the tube is embedded into unsymmetrical pile, the thickness of the layer of magnetite flakes between the neighbouring tubes cannot be deduced straightforwardly from single measurement, especially when using single probe technique. However, the estimation of the extension of the pile can be mapped.