Image Recognition of Instrumentation Panels in a Nuclear Power Plant

Transcription

1 Image Recognition of Instrumentation Panels in a Nuclear Power Plant Abstract Inside a RCB (reactor containment building), there are many various systems, structures, and components for the safe operation of the reactor. We describe a recognition method of onsite instrumentation gauges shown in investigation images of the isolation condenser (IC) system of the unit 1 reactor building at the Fukushima Daiichi nuclear power plant in Japan. At first, we recognized the scales on the instrumentation gauge using the geometric shape of the gauge. And we could read the values of the instrumentation gauge by calculating the slope of the indicator needle on the same gauge. If a robot were to enter the reactor building instead of a person owing to the risk of high radioactivity exposure (as in the case of the Fukushima nuclear accident), the proposed image recognition approach could be useful in reading the onsite instrumentation gauges of the major systems, structures, and components. Keywords Image Recognition, IC (Isolation Condenser), Reactor Building, Fukushima Daiichi Nuclear Power Plant, Water Level Gauge. I. INTRODUCTION Tokyo Electric Power Company, Inc. (TEPCO) has released investigation images of the isolation condenser (IC) system located on the 4th floor of the unit 1 reactor building of the Fukushima Daiichi nuclear power plant [1],[2]. An IC is an alternative core cooling system, provided for the operation failure of an emergency core cooling system (ECCS) when a severe accident (reactor core meltdown) occurred. An ECCS consists of four trains (the flow of reactor coolant is defined as train). Each train is installed independently, and is fully capable of cooling the reactor core with a one-pump start-up. The four independent pumps that perform the ECCS function were located 10 m underground at sea level. These four ECCS pumps were all submerged under water and lost their functions due to a common cause failure (CCF) - the tsunami (15 m at sea level) that struck the Fukushima Nuclear Plant [3]. In order to make up for the loss of core cooling function as a result of losing the ECCS due to a CCF, two trains of ICs are installed as a diverse core cooling system that operates on a completely different principle from the core cooling system that depends on a pump start-up. Jai Wan Cho 1, Kyung Min Jeong Daeduk-Daero, Yuseong-Gu, Daejeon, , Korea 302 Each IC is equipped with enough capacity to cool the reactor core. The Nuclear Regulatory Commission and the National Diet & Civilian Accident Investigation Bureau of Japan have raised some issues that require further investigation (including the aforementioned IC problem). These issues are being re-examined and investigated [4]. The Investigation Committee of the Accident Investigation Bureau of the National Diet of Japan has determined that the loss of IC function was due to the impact from the massive earthquake. TEPCO claims that the IC system was operating normally even after the massive earthquake, and the loss of IC function was caused by the tsunami. The operation problem of the IC system of reactor unit 1 was the primary cause of the Fukushima Daiichi nuclear power plant accident escalating into a catastrophe. Once the earthquake-induced tsunami settled down, the person on duty in the main control room requested that the power plant headquarters have to inspect the steam exhaust visually in order to ensure the operation of the IC system. For the onsite inspection of the IC water level, a MCR operator went to the reactor building. However, the operator withdrew from the building entrance because the radiation level was unusually high. Nuclear emergency robots were developed as countermeasures, provided for accidents in the nuclear facilities in early 2001, following the criticality accident of the JCO (uranium refinery facility) at the Tokaimura, Japan in If these robots had undergone continuous maintenance, repair, and design enhancement, they could have been readily deployed to the Fukushima Daiichi nuclear power plant. Based on the technical specifications of these developed robotic systems (JAEA RaBOT and MARS-A of Mitsubishi Heavy Industry), these robots could have performed image readings of the IC water level gauge located on the 4th floor of the unit 1 reactor building [5],[6],[7]. Taking into account the radiation exposure dose limit for nuclear facility workers (100mSv / 5 years) and the radiation hardness design feature of these robots developed after the JCO criticality accident (10Sv/h x 2 h), these robots could have adequately performed the task at hand in the environment of the reactor building, which at that time had a gamma ray dose rate of approximately 100 msv/h.

2 Assuming that a nuclear emergency robot is deployed in a severe accident of the nuclear power plant, the image reading using an observation camera on the robot is important. We analyzed investigation videos from the unit 1 reactor building on the 4th floor released by TEPCO. We inspected the four water level gauges that measure the water level of the two trains of ICs, and the open/close indicator of the valve that controls the IC flow. The water level gauges and indicators were of the mechanical type, with graduated lines and scales on a circular panel. The water level gauge had a black needle. And the valve indicator, which represents the degree of open (or close) status of the valve, had a red needle. We proposed an image processing technique to read an instrumentation value of the water level gauge. A geometrical shape of the mechanical-typed gauges (water level and indicator) was used in the proposed image processing technique. Using the proposed technique, we deciphered instrumentation values of the four water level gauges and indicators shown on the IC inspection video released by TEPCO and Japanese Nuclear Regulatory Commission of Japan. Figure 1. A water level gauge of an Isolation Condenser (Train A) II. ANALYSIS OF FUKUSHIMA UNIT 1 REACTOR IC VIDEO TEPCO has released three videos related to the investigation of the IC system located on the 4th floor of the Fukushima Unit 1 Reactor Building. We analyzed two of these videos that displayed the pathway from the building entrance on the 1st floor, approaching the target location on the 4th floor, from the viewpoint of a robot. The first video was recorded on October 18, A video recorded one year later (November 30, 2012) was released subsequently. Based on frame-by-frame review of the October video, it took 4 min for an operator to go from the reactor building entrance to the location of the IC instrumentation panel. Taking into consideration the walking speed (6 km/h) of a person and the driving speed (2 km/h) of a robot (JAEA RaBot and Mitsubishi Heavy Industry MARS-A), if a robot had been deployed, it would have taken 12 min. We assumed that full skill for the operation of the robot system and the information about access route and structures inside the reactor building, are available through sufficient training. Figures 1 and 2 show instrumentation panels located on the 4th floor of the unit 1 reactor building. Figure 2 A valve status (opening and closing) indicator panel of the Isolation Condenser The unit 1 reactor building was littered with debris from a hydrogen explosion due to the core meltdown; therefore, it was not easy for a person to enter the reactor building. It was a little difficult to understand the available information since the released video was not recorded in a stable position. We used relatively clear image, as shown in Fig. 1, for testing the proposed image processing technology to read the indicator value of the instrumentation panel. This is based on the assumption that the image quality would be relatively good if a robot were to enter the reactor building because the video recording of the systems, structures, and components would be done remotely under stable conditions. III. RECOGNITION OF INSTRUMENTATION PANEL As shown in Fig. 1, the instrumentation gauge of the IC system is an analog type. 303

3 The instrumentation gauge (water level) is graduated with thin lines. In this paper, to read an indication value of the analog-typed instrumentation (water level of the IC system) panel, following assumptions were applied. Supposition 1. A center point of the instrumentation gauge is the center coordinates of an ellipse, represented in a 2-D image plane acquired by CCD (or CMOS) camera. Supposition 2. The minimum and maximum scales of the instrumentation gauge are positioned below the center point of the circular (or elliptical) shaped gauge image, in the vertical (Y-axis) direction. Supposition 3. The minimum scale of the instrumentation gauge is located on the minimum point of the gauge image, in the horizontal (X-axis) direction (leftmost side). Supposition 4. The maximum scale of the instrumentation gauge is located on the maximum point of the gauge image, in the X-axis, namely positioned at the rightmost side. Supposition 5. The indicator needle of the instrumentation gauge is positioned between the minimum and maximum scale of the gauge. Image processing sequences to read instrumentation gauge, shown in Fig. 1, are divided into three parts. First, central point of the gauge, mostly represented as an ellipse in the 2-D image plane, is derived. Second, we look for the minimum and maximum scales of the gauge. If the minimum and maximum scales of the instrumentation gauge are found, the full scale of the gauge is determined. Third, a slope of the indicator needle of the instrumentation gauge is extracted. The indication value of the instrumentation gauge is calculated by the proportion of scale of the indicator needle with respect to the full scale of the instrumentation gauge. In order to obtain the central point of the instrumentation gauge, a ROI (region-of-interest) of the gauge is extracted through the pre-processing sequence such as segmentation (threshold) process. And the contour of the gauge region is derived from the segmentation image. Then, the extracted contour coordinates are fitted onto an ellipse. In the supposition 1, we assumed that the central point of the ellipse is the central point of the instrumentation gauge. Generally, if the aspect ratio between the long axis and the short axis of an ellipse is 1, then the ellipse is observed as a true circle. However, if the instrumentation gauge is observed at a certain angle by the camera, a true circle typed shape of the instrumentation gauge is viewed as an ellipse pattern in the 2-D image plane. And the aspect ratio will be less than 1.0. This is shown in Fig. 3. Figure 3 shows the central point of the instrumentation gauge (IC system water level meter, shown in Fig. 1. In Fig. 3, the central point coordinates of the ellipse are (384, 143) and the aspect ratio is Figure 3 Extraction of central point of the elliptical-shaped water level gauge, shown in Fig. 1. In this paper, to extract a region of scale of the instrumentation gauge and an area of indicator needle of the gauge, donut-shaped masks are used. A donut mask is created with a fixed width in reference to the central point of the obtained ellipse (see Fig. 3). By subtracting this donut-shaped mask from the segmentation image (separation gauge area from the background) of the gauge image, shown in Fig. 1, the scale area and the indicator needle region are extracted. Then, line segments are drawn connecting the scales to the central point of the elliptical panel, as shown in Fig. 3. The slopes and locations of these line segments are calculated. Using suppositions 1, 2, 3, and 4, the minimum and maximum scale coordinates can easily be extracted. This is shown in Fig. 4. We assume that the scales of the instrumentation gauge are evenly distributed. Among the scales, the most important scales are the minimum scale (i.e., the least significant scale (LSS)) and the maximum scale (i.e., the maximum significant scale (MSS)). The minimum and maximum scales are indicated with dotted circles in Fig. 4. In Fig. 4, the angles of the minimum scale segment and maximum scale segment, each connected to the central point of the ellipse, fall within the valid measurement range of the instrumentation gauge. 304

4 Figure 4. Minimum and maximum scale of the instrumentation gauge. And MSS is the slope of the line segment that connects the central point of the ellipse and the MSS (most significant scale as a maximum scale) point. A needle position, needle, designated as Needle Pos shown in the left side of Fig. 5 is the slope of the indicator needle line, extracted by line-fitting process. An angle of the indicator needle is degree. A real indicator angle of the instrumentation gauge, by Eq. (3). LSS needle, is given (3) (4) A real indication value of the instrumentation gauge, GuessNeedle, designated as Guess Needle shown in the left side of Fig. 5, is based on Eqs. (1) and (3), and can be obtained as follows: GuessNeedle (5) fullscale Figure 5. Recognition result of the water level meter, shown in Fig. 1 Also, using supposition 5, a ROI (region of interest) of indicator needle can be easily extracted, and a slope (angle) of the needle can be calculated by using a line fitting process. An indication value of the instrumentation gauge can be obtained from the ratio between the full measurement range (angle) of the gauge and the slope (angle) of the indicator needle, as shown in Fig. 4. A recognition result of the IC water level gauge is shown in Fig 5. A full measurement range of the instrumentation gauge, fullscale as follows., as shown in Figs. 4 and 5, can be obtained fullscale LSS MSS (1) GuessNeedle 65.2% (6) Figures 6-7 show the recognition results of the instrumentation gauges calculated by the proposed image processing techniques (2) fullscale In Eq. (1), LSS is a slope (angle) of the line segment that connects the central point of the ellipse and the LSS (least significant scale as a minimum scale) point. Figure 6. Image recognition result of the IC (defined as Train 1 for convenience) water level gauge, channel B. 305

5 The onsite instrumentation gauges were analog types. We were able to recognize the scales on the instrumentation gauges using their geometrical shape, and estimate the indication values of the instrumentation panels by calculating the slope of the indicator needle. If assumed that nuclear emergency response robot system should enter the reactor building to mitigate (or manage) an abnormal (severe) accident in the nuclear power plant, the proposed image recognition approach could be useful in reading onsite instrumentation gauges of the SSC (systems, structures, and components) located inside the reactor building under high radioactivity exposure environments. Figure 7. Image recognition result of the IC (Train 1) water level gauge, channel A. The image recognition results of the water level gauges from the IC videos released by TEPCO in October 2011 are shown in Figs. 5 and 6. In May 2013, the Nuclear Regulatory Commission of Japan independently conducted an investigation of the IC system, located at 4th floor in the unit 1 reactor building of Fukushima Daiichi nuclear power plant. Based on the released data (PDF) related to this investigation, the IC water level meter data was converted to an image, and the recognition result by proposed image processing technique is shown in Fig. 7. A slight increase in the water level of the IC (Train 1) can be seen by comparing Figs. 5 (investigated in October 2011) and Fig. 7 (investigated in May 2013). IV. CONCLUSION We described the image recognition of instrumentation gauge of the IC system from the investigation images of the unit 1 reactor building at Fukushima Daiichi nuclear power plant, as released by TEPCO in Japan. REFERENCES [1] Tokyo Electric Power Company, Inc., The image of the situation inside the Unit 1 Reactor Building of Fukushima shown to the National Diet of Japan Fukushima Nuclear Accident Independent Investigation Commission (recorded on October 11, 2011), [2] Tokyo Electric Power Company, Inc., Investigation image of the Unit 1 Reactor Building on the 4th floor at Fukushima Daiichi Nuclear Power Plant (recorded on November 30, 2012), [3] TEPCO, Responses immediately after the Great Earthquake at the Fukushima Daiichi Nuclear Power Station (Japanese), Jun 18, 2011 [4] TEPCO, Estimations of the Reactor core and the PCV status, and Review of Unidentified Problems in the units 1~3 reactors of the Fukushima Daiichi nuclear power station, - 1st Progress Report (Japanese), Attachments 2-24, Dec 13, 2013 [5] T. Kobayashi, K. Miyajima, and S. Yanagihara, Development of Robots for Nuclear Accident in Japan Atomic Energy Research Institute Part (1) Development of Remote Surveillance Squad - (Japanese), Journal of Robotics Society of Japan, Vol. 19, No.6, pp , 2001 [6] K. Shibanuma, Development of Rescue Robot at JAERI Part 2 Development of Radiation-Proof Robot -(Japanese), Journal of Robotics Society of Japan, Vol. 19, No.6, pp , 2001 [7] T. Mano and S. Hamada, Development of Robotic System for Nuclear Facility Emergency Preparedness, Journal of Robotics Society of Japan, Vol. 19, No.6, pp ,