[en] During the emergency response period of the accidents that took place at Fukushima Daiichi in March of 2011, researchers at Oak Ridge National Laboratory (ORNL) conducted a number of studies using the MELCOR code to help understand what was occurring and what had occurred. During the post-accident period, the Department of Energy (DOE) and the US Nuclear Regulatory Commission (NRC) jointly sponsored a study of the Fukushima Daiichi accident with collaboration among Oak Ridge, Sandia, and Idaho national laboratories. The purpose of the study was to compile relevant data, reconstruct the accident progression using computer codes, assess the codes predictive capabilities, and identify future data needs. The current paper summarizes some of the early MELCOR simulations and analyses conducted at ORNL of the Fukushima Daiichi Unit 3 accident. Extended analysis and discussion of the Unit 3 accident is also presented taking into account new knowledge and modeling refinements made since the joint DOE/NRC study

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[en] The 4th regular inspection was carried out in fiscal 1980 on Unit 3 in the Fukushima Nuclear Power Station I for the period from January 16 to July 14, 1981. The facilities subjected to the inspection were reactor proper, cooling systems, instrumentation and control systems, radiation control systems and others. No abnormality was detected in external appearance, leakage, performance, etc. The radiation exposure of personnel concerning the inspection work was all below the legally permissible level. The improvement works conducted during the period of inspection were the replacement of the pipings for reactor recirculation and coolant purification by SCC-resisting ones, the replacement of the casing of a residual heat removal sea-water pump and a control-rod hydraulic accumulator by corrosion- resisting ones, and the replacement of power-range instrumentation detectors. (J.P.N.)

[en] The seventh regular inspection of No.3 plant in the Fukushima No.1 Nuclear Power Station was carried out from October 17, 1984, to February 28, 1985. The resumption of parallel operation was on February 13, 1985, 120 days after the parallel off. The object facilities of inspection were the reactor proper, reactor cooling system, measurement and control system, fuel facilities, radiation control facilities, waste facilities, reactor containment installation, and emergency power generation system. On these object facilities of inspection, the appearance, disassembling, leak, function, performance and other inspections were carried out, and as the results, abnormality was not found at all. The works related to this regular inspection were accomplished within the range of the allowable exposure dose based on the relevant laws. The main reconstruction works carried out during this regular inspection were the exchange of a circulating pump in the coolant purification system, the exchange of a main steam relief safety valve, and the replacement of a manual fuel exchanging machine with a remote-controlled automatic machine. (Kako, I.)

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[en] When we examined TEPCO's Fukushima Daiichi nuclear power station unit 3 reactor building basement torus room investigation video, we found dozens of speckles in the entire image frame. Generally, speckles occur in a CCD/CMOS image when the CCD/CMOS camera is exposed to high dose gamma ray source. In the above torus room investigation image by the Survey Runner robot system, the gamma ray dose rate was about 100mSv/h. The dozens of speckles in the entire image (640x480) are not obstacles to examine the unit 3 reactor building basement torus room situation closely. Analyzing other videos, as a second investigation inside the primary containment vessels (approx. 500∼1000mm inside of the internal wall) in the unit 2 reactor of the Fukushima Daiichi nuclear power station using an industrial endoscope, dense speckles were observed in the investigation image. The gamma ray dose rate was 30∼70 Sv/h at the measurement location. The overwhelming number of speckles in the investigation image are a hindrance to scrutinize the inside situation of the primary containment vessels of the unit 2 reactor. The CCD/CMOS cameras, which are loaded on the robot system, are generally used as the eye of the robot and monitoring unit. A major problem that arises when dealing with images provided by CCD/CMOS cameras under severe accident situations of a nuclear power plant is the presence of speckles owing to the high dose rate gamma irradiation fields. To use a CCD/CMOS camera as a monitoring unit in the high radiation area, the legibility of the camera image in such intense gamma radiation fields should therefore be defined. In this paper, we describe the monitoring index as a figure of merit of the camera's legibleness under a high dose rate gamma ray irradiation environment. From the low dose rate (2.11 Gy/h) to the high dose rate (200 Gy/h) level, the legible performances of the cameras owing to the speckles are evaluated. The numbers of speckles, generated by the gamma ray irradiation, in the camera image are calculated by an image processing technique. The relation between the legibility of the camera image and the numbers of speckles is also presented

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