# Electromagnetic Testing of Rod Cluster Control Assemblies in Pressured-Water Reactor Power Plants
## 4 Experimental Results and Discussions
The processed signal \(\mathrm{V_{Pro}}\) of all the defects is shown in Figure 9. The defect signal was improved so that the 30% depth SW (#1 and #4), 40% depth PCC (#2), and 30% W depth FCC (#3) could be detected. The shapes of the defect were determined as follows: the signal from the SCGs was widely distributed in the axial direction and covers full (or almost) 360\({}^{\circ}\); the signal from the SWs was concentrated in a group, and the signal from the CCs was shortly distributed in the axial direction. The results show that all the SCGs and SWs were detected. However, the small depth CCs could not be observed clearly. For instance, the 30% depth FCC (Figure 9b) looks like a PCC; this could be a variant lift-off during the scan. Figure 8: Signal processing diagram. Figure 7: Differential scan signal of the (**a**) PCCs, (**b**) FCCs, (**c**) SWs and (**d**) SCGs on the control rods. For quantitative evaluation of the defect shape and size, two factors are extracted: the amplitude of the signal (_h_) and the width of the defect signal (_WD_), as described in Figure 10a. Here, \(h\) is the average of all the signal amplitudes in the defect area, which is above the threshold, and _WD_ is the average of all the distances between the first (\(x_{1}\)) and last (\(x_{n}\)) signal, which is above the threshold. \(h\) and _WD_ are expressed in Equation (6) and (7), respectively, where, \(s\) is the scan step (1.724 mm) and n is the number of data above the threshold. By using the _WD_, the shape of the defect can be distinguished, as shown in Figure 10b. Figure 5:0  
Figure 5:1  
The relationship between defect signal (log(h)) and defect depth (d) is shown in Figure 11a and is expressed in a linear term in Equation (8). Due to a lack of information, only the depth is taken into account. For the SCGs, SWs, and CCs, the factors (a, b) in Equation (8) are (302.976, 47.1698), (578.4903, 64.5161), and (206.0808, 20.2020), respectively. Then, the depth of the defect was as shown in Figure 11b. The standard deviations of depth estimation are 18%, 5.8%, and 6.0% for the CCs (PCCs + FCCs), SCGs, and SWs. The total estimation of the depth has a standard deviation of 12.1% W.T (0.057 mm). The SCGs and SWs have a small and similar estimation error. The highest error of estimation is for the CCs. This could be the reason that the CCs have a small width of 0.2 mm and the scan step is large at 1.72 mm. It may need to supply more power for the magnetic source to produce a better signal quality of the CCs. The variation of lift-offs, including sensor lift-off and bobbin coil lift-off, is also an important factor that affects the quality of the signal. The current sensor and bobbin coil lift-offs are 0.8 mm and 2.0 mm, which is fine for large defects like the SCGs and SWs but not satisfactory for the small cracks like the PCCs and FCCs, which areonly 0.2 mm in width. Recently, researchers in [12; 13] have developed unique algorithms and sensor constructions to compensate the lift-off for the sensing coils. In the future development of the system, it is necessary to minimize the lift-offs and keep less variation during the scan to obtain a higher quality of the signal, especially for the CCs. It is also necessary to develop a method for lift-off compensation using the Hall sensor. Figure 5:2  
Figure 5:3 (**a**) The Hall sensor scan signal of a 10% depth short-circumferential groove and (**b**) determination of defect shape using the width of the defect signal.