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Magnetic Imaging of Sludge Outside Steam Generator Tube With Array Magnetoresistance Sensors

The water impurities and corrosion products of the nuclear power plant structure form sludge outside the steam generator tube (SGT), which decreases the energy exchange efficiency resulting in tremendous economic loss and may lead to serious defects posing a threat to the safe operation. Thus, it is critical to efficiently inspect the deposition of sludge outside SGT. This work reveals that the sludge sample of a nuclear power plant has little conductivity and relative permeability about 2.88. Based on the electromagnetic properties of the sludge, we proposed a novel probe with array tunnel magnetoresistance (TMR) sensors for inspection of magnetic field disturbed by the presence of sludge outside the SGT. The principle of the method and the parameters of the probe were studied using a 3D finite element method (FEM) model. It is seen that the signal amplitude of the offset sensor configuration is stronger than that of the probe with non-offset sensor setup and the offset design avoids the disadvantage of non-sensitive to symmetry sludge. Further, it is observed that the sludge inspection performance is better at a lower excitation frequency, i.e. 1 kHz. The TMR sensors provide superior image quality at the low excitation frequency. A prototype probe is manufactured and tested. Sludge samples prepared according to the realistic fouling scenarios in steam generator were tested by the probe. The experimental results validate the feasibility of the probe and demonstrate the advantage of enhancing signal to noise ratio (SNR) of imaging by suppressing the background signal of tube support plate structure under the low excitation frequency.