# Electromagnetic Testing of Rod Cluster Control Assemblies in Pressured-Water Reactor Power Plants
## 4 Experimental Results and Discussions  
Figure 6 shows the scan results of the SCGs on a control rod. The offset signal was removed by back data process V\({}_{\text{b}}\), as described by Equation (4) and results are displayed in Figure 6a. The signal of each Hall sensor element has a different trend due to the inhomogeneous permeability and conductivity of the specimen, and the variation of lift-off between the sensors and the specimen during the scan. Thus, it is difficult to detect the defects using this processed data. Another approach using the  
Figure 4: Prototype of an Encircling Hall sensor (EHaS) probe. Figure 5: Block diagram and prototype of the electromagnetic testing system for a 17\(\times\)17 type RCCA. differential signal in the scan direction is described by Equation (5), and the result is shown in Figure 6b. The defect's signal could be observed clearly, with a high contrast against the background noise. The average signal of all the sensor channels in Figure 6c effectively shows the defect signal and almost the same background noise level in the differential process, but it is a strong trend signal and thus difficult to detect the defects (e.g., #1, 10%). All the SCGs have been detected, and the intensity signal of the defect increases proportionally to the depth. \[V_{b}(i,j)=V_{RMS}(i+1,j)-V_{RMS}(0,j) \tag{4}\]  
\[\Delta V_{RMS}(i,j)=V_{RMS}(i+1,j)-V_{RMS}(i,j) \tag{5}\]  
Here, \(i\) and \(j\) are the scan data index (Z-direction) and sensor element index (\(\theta\)-direction). The differential processed signal \(\Delta\)V\({}_{\text{RMS}}\) of all the defects is shown in Figure 7. The PCCs from #2-#5 in Figure 7a could be observed but with low intensity compared to the background noise. To better observe a defect, a larger angle is beneficial because of the higher concentration of the electromagnetic field around the defect and, in addition, an increased number of Hall sensor elements enhances the sensitivity. The current EHaS probe had an interval angle of 22\({}^{\circ}\), so it was difficult to detect the 45\({}^{\circ}\) angle of the PCC #1. The PCC #5 (270\({}^{\circ}\)) was easier to detect than the PCC #4 (180\({}^{\circ}\)), PCC #3(90\({}^{\circ}\)), and PCC #2(90\({}^{\circ}\)). The influence of defect depth could be observed in the signal of the FCCs and SCGs. Small depths such as FCC #1 (10% W.T, 0.047 mm) and #2 (20% W.T, 0.094 mm) could not be detected. FCC #3 and #4 were also difficult to detect, which had depths of 30% W.T (0.141 mm) and 40% W.T (0.188 mm), respectively. FCC #5, #6, and #7 could be observed, which had depths of 50% W.T (0.235 mm). All the SCGs with depths of 10% W.T were detected, and the intensity of the defect increased as the depth increased, as commented in the previous paragraph. The SCGs were much easier to detect than the PCCs and FCCs because their width was much larger (10 mm >> 0.2 mm). With the SW defects, the depth and angle had a relationship with the special shape of the SW: as the depth increased, the angle increased. So, the signal intensity of the SW was significantly increased as the depth increased. An SW with a depth of 50% W.T (0.235 mm, #2, #3, #5, and #6) could be observed, but the small depth (e.g. 30%, 0.141 mm, #1, and #4) was difficult to observe. The large signal inbetween the SW #3 and #4 was from the gap of the absorbers of the control rod. Figure 6: Experimental results of short-circumferential grooves (SCGs) with (**a**) back data process V\({}_{\text{b}}\), (**b**) differential process \(\Delta\)V\({}_{\text{RMS}}\), and (**c**) average sensor channels’ signal. For improvement of signal quality, the signal from all Hall sensor elements is taken into account in the fusion approach, as described in Figure 8. The square rule of the differential signal is used in the wavelet filter (denoise) and weighted by their average value.