## 6 Conclusions  
In this study, we proposed a method for estimating the leakage positions in the pedestal of the FDNPP using a stereo camera. The leakage positions are estimated by detecting, matching, and triangulating waterdrop trajectories in the stereo images. The leakage position estimation is important because the estimation would give the rough distribution of the unknown fuel debris on the floor of the pedestal, which should be removed for decommissioning. The challenge was to distinguish and match waterdrop trajectories from images with severe radiation noise owing to fuel debris in the pedestal. We assumed that the waterdrops fell vertically without wind; therefore, the waterdrop trajectories could be captured as vertical lines in the images by aligning the pan axis of the stereo camera with gravity. As a waterdrop appeared at a certain column index in an image, a vector of the column pixels was utilized as an image feature. In this study, the aggregated value of the column pixels was employed to detect the waterdrop trajectory lines in the images, and the vector of the column pixels was used to match the waterdrop trajectories between the left and right images. We simulated the pedestal in Unit 2 of the FDNP, and the proposed method succeeded in estimating the leakage positions from stereo images with severe radiation noise. During the experiments, our method only failed to detect waterdrops due to two conditions: if the waterdrop only appeared to only one of the stereo cameras or none  
Figure 8: Ground truth and estimated leakage positions. The first row and second row are the results with five and seven leakage positions, respectively. The number of detections and mean error of the estimated leakage positions are indicated above each figure. The red and blue dots indicate the ground truth and estimated leakage positions, respectively. The yellow lines illustrate the correspondence of the ground truth with the estimated leakage positions. The leakage positions that were not detected are represented as the purple dots. For reference, the camera positions and the fields of the view are illustrated as the greed dots and the cyan lines, respectively. of them; second, if the waterdrops were occluded in the stereo images. Therefore, the radiation noise did not affect the detection rate, which demonstrates the robustness of the proposed method against radiation noise. The worst mean error of the estimated leakage position was 0.37 m, and 0.37 m is 7.4% of 5 m, which is the diameter of the pedestal. In the current FDNPP situation with a lot of uncertainty, we believe errors lower than 10% would be beneficial for decommissioning. Furthermore, the experimental results suggested that shadows degrade the accuracy of the proposed method. Thus, there is room for improvement in deciding on the locations of multiple light sources. Therefore, we believe that the proposed method would be beneficial for estimating the rough distribution of the fuel debris that fell through the damage to the bottom of the RPV. We hope that our proposed system can be applied to the actual pedestal in the FDNPP and help with decommissioning. The proposed method was specifically designed for scenarios with leakages and high radiation noise. At the moment, the FDNPP is the only applicable situation for the proposed method. However, in the future, there might be a similar nuclear accident where a hydrogen explosion occurs, followed by the nuclear meltdown; therefore, the proposed method might be useful in the future. As future work, in addition to using multiple light sources, investigating the optimal frame rate for the stereo cameras is worth exploring. On one hand, capturing at a lower frame rate produces longer waterdrop trajectories, which would be helpful for detecting and matching the trajectories using our method. However, the stereo images would have more accumulated radiation noise.