## 1 Introduction  
The Fukushima Daiichi Nuclear Power Plant (FDNPP) disaster that occurred in 2011 was caused by earthquakes and tsunamis. Due to the nuclear meltdown, the nuclear fuels in the reactor pressure vessel (RPV) were melted and mixed with other materials, and a large amount of fuel debris was formed. Some fuel debris fell into the pedestal below the RPV, and the rest remained in the RPV. Fuel debris removal is an inevitable mission to complete the decommissioning of the FDNPP. Numerous decommissioning activities have been conducted thus far using robot technology because of the high-dose radiation environment, where it is dangerous for humans to stay for long [1, 2]. Some inspections conducted in the primary containment vessel (PCV) of Units 1, 2, and 3 of the FDNPP [3, 4] revealed that waterdrops were dripping from the bottom of the RPV into the pedestal. Water was injected into the RPV to cool the remaining fuel debris that generated decay heat, and the water leaked from the damage at the bottom of the RPV. Determining the leakage position of the cooling water helped us estimate the location of the damaged area at the bottom of the RPV, which should be beneficial for estimating the rough distribution of the fuel debris in the pedestal because the fuel debris fell through the damage to the bottom of the RPV. In this study, we propose a method of estimating the leakage position of cooling water using a stereo camera for the FDNPP. A stereo camera mounted on an inspection system with a rotating base is inserted through X-6 penetration into the pedestal below the RPV, as shown in Figure 1. Figure 1a illustrates the RPV, pedestal, and inspection system inserted into the pedestal. Water leaks from the damaged area to the pedestal. Figure 1b depicts the inspection system with a stereo camera, where the orientation of the stereo camera can be changed using the rotating base. It is assumed that the inspection system includes a sensor that detects the direction of gravity such that the pan axis of the stereo camera can be aligned with gravity. In the proposed method, we capture the waterdrops coming from the leakage position using a stereo camera, and the leakage positions are estimated by triangulating the waterdrop trajectory lines in the stereo image. The main contribution of this study is the extraction and matching of the waterdrop trajectories in a stereo image in the FDNPP. In particular, high radiation should be considered in the pedestal because of a severe high-dose environment owing to the fuel debris. Approximately 10 Gy/h was measured in the pedestal of Unit 2 according to an investigation [5]. Radiation affects the inspection system in two ways. First, radiation energy is accumulated in electronic devices and causes malfunctions. However, the proposed method only requires a few seconds to capture stereo frames; therefore, malfunctions are not important in this study because ordinary electronic devices can survive at least a total dose of 100 Gy [6,7]. Second, strong radiation, especially gamma rays, causes image noise. Gamma rays are a form of light that has a shorter wavelength than visible light; therefore, image sensors capture gamma rays along with visible light. Moreover, gamma rays can penetrate matter easily, making it difficult to prevent gamma rays from being captured by image sensors. Gamma-ray noise appears in images as salt-and-pepper noise and line-shaped noise; the noise hinders the detection and matching of waterdrop trajectories from images because waterdrops do not have a strong color and texture. We propose a leakage position estimation method that is robust against gamma-ray image noise. The challenge is to distinguish and match waterdrop trajectories from  
Figure 1: The inspection system inserted into the pedestal below the RPV of Unit 2. (**a**) Pedestal where water leaks from the damaged area. The inspection system is inserted into the pedestal through the X-6 penetration. (**b**) The stereo camera mounted on the inspection system with a rotating base.