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The was an energy accident at the Fukushima I Nuclear Power Plant, initiated primarily by the tsunami of the Tōhoku earthquake and tsunami on 11 March 2011.〔Phillip Lipscy, Kenji Kushida, and Trevor Incerti. 2013. "(The Fukushima Disaster and Japan’s Nuclear Plant Vulnerability in Comparative Perspective )." ''Environmental Science and Technology'' 47 (May), 6082-6088.〕 The damage caused by the tsunami produced equipment failures, and without this equipment a loss-of-coolant accident followed with three nuclear meltdowns and releases of radioactive materials beginning on 12 March.〔(【引用サイトリンク】url=http://spectrum.ieee.org/tech-talk/energy/nuclear/explainer-what-went-wrong-in-japans-nuclear-reactors )〕 It is the largest nuclear disaster since the Chernobyl disaster of 1986 and the second disaster (after Chernobyl) to be given the Level 7 event classification of the International Nuclear Event Scale.〔("Analysis: A month on, Japan nuclear crisis still scarring" ) ''International Business Times'' (Australia). 9 April 2011, retrieved 12 April 2011〕 The plant comprised six separate boiling water reactors originally designed by General Electric (GE) and maintained by the Tokyo Electric Power Company (TEPCO). At the time of the earthquake, reactors 4, 5 and 6 were shut down in preparation for re-fueling. However, their spent fuel pools still required cooling.〔(IAEA press release Japanese Earthquake Update (19 March 2011, 4:30 UTC) 19 March 2011. Archive.org )〕 Immediately after the earthquake, the electricity producing reactors 1, 2 and 3 automatically shut down their sustained fission reactions, inserting control rods in what is termed a SCRAM. Following this legally mandated "safety precaution" which ceases the reactors' normal running conditions, the reactors were unable to generate power to run their own coolant pumps. Emergency diesel generators came online, as designed, to power electronics and coolant systems, all of which operated right up until the tsunami destroyed the generators for reactors 1–5 due to their location in unhardened low-lying areas. The two generators cooling reactor 6 were undamaged and were sufficient to be pressed into service to cool the neighboring reactor 5 along with their own reactor, averting the overheating issues that reactor 4 suffered.〔 The largest wave in the tsunami arrived some 50 minutes after the initial earthquake. The 13 meter tall wave overwhelmed the plant's seawall, which was only 10 m high,〔 with the moment of impact being caught on camera.〔(【引用サイトリンク】title=Recriticality, a Key Phenomenon to Investigate in Core Disruptive Accident Scenarios of Current and Future Fast Reactor Designs ) Note: See picture in the upper left corner of page 2.〕 Water quickly flooded the low-lying rooms in which the emergency generators were housed.〔(24 Hours at Fukushima A blow-by-blow account of the worst nuclear accident since Chernobyl By Eliza Strickland Posted 31 Oct 2011 )〕 The flooded diesel generators failed soon afterwards, cutting power to the critical pumps that must continuously circulate coolant water through a Generation II reactor for several days to keep the fuel rods from melting down following the SCRAM event, as the ceramic fuel pellets in the fuel rods continue to generate Decay heat even after the fission process has terminated. The fuel rods will become hot enough to melt themselves down during the fuel decay time period if no adequate cold sink is available. After the secondary emergency pumps (run by back-up electrical batteries) ran out, one day after the tsunami, 12 March, the water pumps stopped and the reactors began to overheat due to the high decay heat produced in the first few days after the SCRAM (diminishing amounts of this decay heat continue to be released for years, but with time, passive cooling through water convection in a pool is sufficient to prevent fuel rod melting). As workers struggled to supply power to the reactors' coolant systems and restore power to their control rooms, a number of hydrogen-air chemical explosions occurred, the first in Unit 1, on 12 March and the last in Unit 4, on 15 March.〔〔〔(Hydrogen explosions Fukushima nuclear plant: what happened? )〕 It is estimated that the hot zirconium fuel cladding-water reaction in reactors 1-3 produced 800 to 1000 kilograms of hydrogen gas each, which was vented out of the reactor pressure vessel, and mixed with the ambient air, eventually reaching explosive concentration limits in units 1 and 3, and due to piping connections between units 3 and 4, or alternatively from the same reaction occurring in the spent fuel pool in unit 4 itself,〔(【引用サイトリンク】title= MELCOR Model of the Spent Fuel Pool of Fukushima Dai-ichi Unit 4 )〕 unit 4 also filled with hydrogen, with the hydrogen-air explosions occurring at the top of each unit, that is in their upper secondary containment building.〔(page 6 )〕〔http://eetd-seminars.lbl.gov/sites/eetd-seminars.lbl.gov/files/Fukushima1_Technical_Perspective_LBL_EEDT_04052011-1.pdf ''What happened at Fukushima a Technical Perspective.'' Nuclear Regulatory Commission page 11, 26, 29.〕 Drone overflights on 20 March and afterwards captured clear images of the effects of each explosion on the outside structures, while the view inside was largely obscured by shadows and debris.〔 〕 There have been no fatalities linked to short term overexposure to radiation reported due to the Fukushima accident, while approximately 18,500 people died due to the earthquake and tsunami. However approximately 610 are estimated to have died due to workers' exposure and the evacuation of residents near the power plant. Estimates of the total human fatalities caused by the nuclear accident are up to 10,000, maximum cancer mortality and morbidity is calculated to be respectively 1,500 and 1,800.〔Abubakar Sadiq Aliyu et al, ''An overview of current knowledge concerning the health and environmental consequences of the Fukushima Daiichi Nuclear Power Plant (FDNPP) accident''. In: ''Environment International'' 85 (2015), 213–228, .〕 In addition, the rates of mental illnesses among evacuated people rose fivefold compared to the Japanese average.〔Hasegawa et al, ''From Hiroshima and Nagasaki to Fukushima 2. Health effects of radiation and other health problems in the aftermath of nuclear accidents, with an emphasis on Fukushima''. In: ''The Lancet'' 386, 9992 (2015), 479–488, .〕 In 2013, the World Health Organization (WHO) indicated that the residents of the area who were evacuated were exposed to low amounts of radiation and that radiation induced health impacts are likely to be low.〔(WHO report, page 92. )〕〔http://science.time.com/2013/03/01/meltdown-despite-the-fear-the-health-risks-from-the-fukushima-accident-are-minimal/#ixzz2MnbjhPmv ''Meltdown: Despite the Fear, the Health Risks from the Fukushima Accident Are Minimal'' Time magazine article which includes a link to the WHO report, and explains the report in laymans terms.〕 In particular, the 2013 WHO report predicts that for evacuated infant girls, their 0.75% pre-accident lifetime risk of developing thyroid cancer is calculated to be increased to 1.25% by being exposed to radioiodine, with the increase being slightly less for males. While the risks from a number of additional Radiation-induced cancers are also expected to be elevated due to exposure caused by the other low boiling point fission products that were released by the safety failures. The single greatest increase is for thyroid cancer, but in total, an overall 1% higher lifetime risk of developing cancers of all types, is predicted for infant females, with the risk slightly lower for males, making both some of the most radiation-sensitive goups.〔 Along with those within the womb, which the WHO predicted, depending on their gender, to have the same elevations in risk as the infant groups. A screening program a year later in 2012 found that more than a third (36%) of children in Fukushima Prefecture have abnormal growths in their thyroid glands. As of August 2013, there have been more than 40 children newly diagnosed with thyroid cancer and other cancers in Fukushima prefecture as a whole. However whether these incidences of cancer are elevated above the rate in un-contaminated areas and therefore were due to exposure to nuclear radiation is unknown at this stage. Data from the Chernobyl accident showed that an unmistakable rise in thyroid cancer rates following the disaster in 1986 only began after a cancer incubation period of 3–5 years, however whether this data can be directly compared to the Fukushima nuclear disaster is still yet to be determined. A survey by the newspaper Mainichi Shimbun computed that of some 300,000 people who evacuated the area, approximately 1,600 deaths related to the evacuation conditions, such as living in temporary housing and hospital closures have occurred as of August 2013, a number comparable to the 1,599 deaths directly caused by the earthquake and tsunami in the Fukushima Prefecture in 2011. With the exact cause of the majority of these evacuation related deaths not being specified, as according to the municipalities, that would hinder the deceased relatives' application for condolence money compensation. On 5 July 2012, the Japanese National Diet appointed The Fukushima Nuclear Accident Independent Investigation Commission (NAIIC) submitted its inquiry report to the Japanese Diet. The Commission found the nuclear disaster was "manmade", that the direct causes of the accident were all foreseeable prior to 11 March 2011. The report also found that the Fukushima Daiichi Nuclear Power Plant was incapable of withstanding the earthquake and tsunami. TEPCO, the regulatory bodies (NISA and NSC) and the government body promoting the nuclear power industry (METI), all failed to correctly develop the most basic safety requirements—such as assessing the probability of damage, preparing for containing collateral damage from such a disaster, and developing evacuation plans for the public in the case of a serious radiation release. Meanwhile, the government appointed Investigation Committee on the Accident at the Fukushima Nuclear Power Stations of Tokyo Electric Power Company submitted its final report to the Japanese government on 23 July 2012.〔 A separate study by Stanford researchers found that Japanese plants operated by the largest utility companies were particularly unprotected against potential tsunami.〔 TEPCO admitted for the first time on 12 October 2012 that it had failed to take stronger measures to prevent disasters for fear of inviting lawsuits or protests against its nuclear plants.〔〔〔〔 There are no clear plans for decommissioning the plant, but the plant management estimate is thirty or forty years.〔 A frozen soil barrier is being constructed in order to prevent ongoing exposure of running groundwater with melted down nuclear fuel. == Background == Following the 1999 Tokaimura criticality accident, there was interest in Japan for developing radiation-resistant robots for use in the event of nuclear accidents- other countries (e.g. Germany and France) already had them available. The Japanese government budgeted 3 billion yen (US $38 million) for research and development. Several companies produced state of the art prototypes in 2001, which were tested and deemed technical successes. In December 2002, a task force (which included TEPCO executives) further concluded that the robots were unnecessary: the possibility of Chernobyl-scale disasters was completely discounted and it was thus assumed that human employees- compared to whom the robots had limited speed and range- would still be able to operate in the event of an accident. The program halted, and the prototypes remained in storage until March 2006; some were subsequently donated to Tohoku University. The termination of the program left Japan without functional radiation-resistant robots to send into Fukushima when the crisis began. As the crisis unfolded, the Japanese government sent a request for robots developed by the U.S. military. The robots went into the plants, and took pictures to help assess the situation, but they couldn't perform the full range of tasks usually carried out by human workers. Following Fukushima, efforts to develop humanoid robots that could supplement relief efforts have accelerated dramatically.〔 Similarly, pre-Fukushima, Japan's Nuclear Safety Commission said in its safety guidelines for light-water nuclear facilities that "the potential for extended loss of power need not be considered."〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Fukushima Daiichi nuclear disaster」の詳細全文を読む スポンサード リンク
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