## 4 Experiments  
We conducted experiments to demonstrate the effectiveness of the proposed method in a simulated environment. The pedestal in Unit 2 of the FDNPP was simulated using Blender [19], as shown in Figure 4. The pedestal was approximated as a cylinder with a diameter of 5.0 m, and a textured rusty metal was added to the cylinder. The waterdrops were dripped continuously from three leakage positions, as shown in Figure 4. The coordinate system in a simulated environment is shown in Figure 5. A stereo camera was placed near the X-6 penetration in the pedestal. The baseline of the stereo camera was set to 0.5 m, as the width of X-6 penetration is 0.6 m [20]. Specifically, the left and right cameras were located at \([-2.4\;\text{m},\ 0.25\;\text{m}]^{\top}\) and \([-2.4\;\text{m},\ -0.25\;\text{m}]^{\top}\), respectively (note that \(z\) values were omitted). Both cameras faced in the same direction as the \(x\)-axis. A single point light source was also added above the stereo camera to the environment. The stereo camera images were rendered at 12 frames per second using physically-based rendering software called Cycles [21]. Both cameras had a resolution of \(960\times 540\) pixels with a horizontal field of view of \(70^{\circ}\). Thereafter, radiation noise was added to the images using the method proposed in [22]. In particular, we added 662 keV gamma rays caused by Cesium-137, which is a common radioactive material in the FDNPP, using Monte Carlo simulation called Geant4 [23,24]. In [22], the parameters for generating radiation noise were estimated using cameras capturing 12 frames per second. Therefore, the same frame rate was chosen for rendering stereo images to generate gamma-ray radiation noise that was as realistic as possible. Three different intensities of the radiation noise, namely, low level (L), middle level (M), and high level (H), were applied to images to evaluate the robustness of the proposed method against the radiation noise levels. The intensity of the high level radiation noise was determined based on the air dose rate of approximately 10 Gy/h to simulate the actual environment [5]. We set the radiation noise levels of M and L as 33% and 17% of H, respectively. Figure 4: The simulated pedestal of Unit 2 in FDNPP for the experiment. A stereo camera was placed in the pedestal using a single light source. Waterdrops were dripping from above. Figure 5: The coordinate system of the simulated environment. The origin of the coordinates is at the center of the pedestal where the z-axis is in the direction of gravity. In this study, \(N_{\text{frame}}\) was set to 24, implying that the stereo cameras captured the scene for two seconds as the input stereo images. Moreover, \(g_{\text{win}}\) was set to 10 pixels. The evaluation was conducted using the Euclidean distance between the ground truth and the estimation from the top view, which is formulated as follows:  
\[E_{i}=\sqrt{(x_{i}-\hat{x}_{i})^{2}+(y_{i}-\hat{y}_{i})^{2}}, \tag{9}\]  
where \([x_{i},\ y_{i}]^{\top}\) and \([\hat{x}_{i},\ \hat{y}_{i}]^{\top}\) are the ground truth and the estimation of the corresponding leakage position, respectively. We conducted evaluations in three different settings, namely _three separated waterdrops_, _three crowded waterdrops_, and _multiple waterdrops_, which are described below. Please watch the video of the simulated environment ([[https://drive.google.com/drive/folders/11A_bxKxL15](https://drive.google.com/drive/folders/11A_bxKxL15) pt04 pVg3M_m2 gijNhoZCaUx?usp=sharing]([https://drive.google.com/drive/folders/11A_bxKxL15](https://drive.google.com/drive/folders/11A_bxKxL15) pt04 pVg3M_m2 gijNhoZCaUx?usp=sharing), accessed on 21 August 2021). In all settings, leakage positions were generated in the right semicircle of the pedestal in Figure 5. We limited the generated leakage positions to the right semicircle of the pedestal because we wanted to capture the waterdrops in both left and right cameras to evaluate the proposed method properly.