|
The was a series of equipment failures, nuclear meltdowns, and releases of radioactive materials at the Fukushima I Nuclear Power Plant, following the Tōhoku earthquake and tsunami on 11 March 2011. It is the largest nuclear disaster since the Chernobyl disaster of 1986.〔("Analysis: A month on, Japan nuclear crisis still scarring" ) ''International Business Times'' (Australia). 9 April 2011, retrieved 12 April 2011; excerpt, According to James Acton, Associate of the Nuclear Policy Program at the Carnegie Endowment for International Peace, "Fukushima is not the worst nuclear accident ever but it is the most complicated and the most dramatic...This was a crisis that played out in real time on TV. Chernobyl did not." 〕 The plant comprises 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 quake, Reactor 4 had been de-fueled while 5 and 6 were in cold shutdown for planned maintenance. Immediately after the earthquake, the remaining reactors 1-3 shut down automatically, and emergency generators came online to control electronics and coolant systems. However the tsunami following the earthquake quickly flooded the low-lying rooms in which the emergency generators were housed. The flooded generators failed, cutting power to the critical pumps that must continuously circulate coolant water through a nuclear reactor for several days in order to keep it from melting down after being shut down. As the pumps stopped, the reactors overheated due to the normal high radioactive decay heat produced in the first few days after nuclear reactor shutdown (smaller amounts of this heat normally continue to be released for years, but are not enough to cause fuel melting). At this point, only prompt flooding of the reactors with seawater could have cooled the reactors quickly enough to prevent meltdown. Salt water flooding was delayed because it would ruin the costly reactors permanently. Flooding with seawater was finally commenced only after the government ordered that seawater be used, and at this point it was already too late to prevent meltdown.〔F. Tanabe, Journal of Nuclear Science and Technology, 2011, volume 48, issue 8, pages 1135 to 1139〕 As the water boiled away in the reactors and the water levels in the fuel rod pools dropped, the reactor fuel rods began to overheat severely, and to melt down. In the hours and days that followed, Reactors 1, 2 and 3 experienced full meltdown. In the intense heat and pressure of the melting reactors, a reaction between the nuclear fuel metal cladding and the remaining water surrounding them produced explosive hydrogen gas. As workers struggled to cool and shut down the reactors, several hydrogen-air chemical explosions occurred.〔〔(Hydrogen explosions Fukushima nuclear plant: what happened? ) 〕 Concerns about the repeated small explosions, the atmospheric venting of radioactive gasses, and the possibility of larger explosions led to a -radius evacuation around the plant. During the early days of the accident workers were temporarily evacuated at various times for radiation safety reasons. At the same time, sea water that had been exposed to the melting rods was returned to the sea heated and radioactive in large volumes for several months until recirculating units could be put in place to repeatedly cool and re-use a limited quantity of water for cooling. The earthquake damage and flooding in the wake of the tsunami hindered external assistance. Electrical power was slowly restored for some of the reactors, allowing for automated cooling.〔(Stricken reactors may get power Sunday ), ''The Wall Street Journal'', 19 March 2011 〕 Japanese officials initially assessed the accident as Level 4 on the International Nuclear Event Scale (INES) despite the views of other international agencies that it should be higher. The level was later raised to 5 and eventually to 7, the maximum scale value.〔Justin McCurry. (Japan raises nuclear alert level to seven ). The Guardian. 12 April 2011 〕〔('Now radiation in Japan is as bad as radiation level is raised to 7 for only the second time in history' ) ''Daily Mail'' 12 April 2011.〕 The Japanese government and TEPCO have been criticized in the foreign press for poor communication with the public and improvised cleanup efforts.〔 On 20 March, the Chief Cabinet Secretary Yukio Edano announced that the plant would be decommissioned once the crisis was over. The Japanese government estimates the total amount of radioactivity released into the atmosphere was approximately one-tenth as much as was released during the Chernobyl disaster.〔 Significant amounts of radioactive material have also been released into ground and ocean waters. Measurements taken by the Japanese government 30–50 km from the plant showed caesium-137 levels high enough to cause concern,〔 leading the government to ban the sale of food grown in the area. Tokyo officials temporarily recommended that tap water should not be used to prepare food for infants.〔(Japan mulls Fukushima food ban: IAEA ), Reuters, 19 March 2011 〕 In May 2012, TEPCO reported that at least 900 PBq had been released "into the atmosphere in March last year () alone" although it has been said staff may have been told to lie, and give false readings to try and cover up true levels of radiation.〔 〕 A few of the plant's workers were severely injured or killed by the disaster conditions resulting from the earthquake. There were no immediate deaths due to direct radiation exposures, but at least six workers have exceeded lifetime legal limits for radiation and more than 300 have received significant radiation doses. Predicted future cancer deaths due to accumulated radiation exposures in the population living near Fukushima have ranged from none to 100 to a non-peer-reviewed "guesstimate" of 1,000.〔 On 16 December 2011, Japanese authorities declared the plant to be stable, although it would take decades to decontaminate the surrounding areas and to decommission the plant altogether. On 5 July 2012, the parliament appointed The Fukushima Nuclear Accident Independent Investigation Commission (NAIIC) submitted its inquiry report to the Japanese parliament,〔 〕 while 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. Tepco admitted for the first time on October 12, 2012 that it had failed to take stronger measures to prevent disasters for fear of inviting lawsuits or protests against its nuclear plants.〔 〕〔 〕 ==Investigations== On 7 June 2011 a government-appointed committee of 10 people convened to investigate the accident. The panel was headed by Yotaro Hatamura, professor emeritus of the University of Tokyo, and included Yukio Takasu, Michio Furukawa, the mayor of Kawamata, Fukushima, and author Kunio Yanagida, considered an expert on crisis management.〔Ito, Masami, "(Official probe begins into nuclear disaster )", ''Japan Times'', 8 June 2011, p. 1. 〕〔(Investigation Committee on the Accident at the Fukushima Nuclear Power Stations of Tokyo Electric Power Company ) 〕 As part of the government inquiry, the House of Representatives of Japan's special science committee directed TEPCO to submit to them its manuals and procedures for dealing with reactor accidents. TEPCO responded by submitting manuals with most of the text blotted out. In response, the Nuclear and Industrial Safety Agency ordered TEPCO to resubmit the manuals by 28 September 2011 without hiding any of the content. TEPCO replied that it would comply with the order.〔Jiji Press, "Tepco must submit reactor accident manuals without deleted portions", ''Japan Times'', 28 September 2011, p. 2.〕 On 24 October NISA published a large portion of Tokyo Electric Power Company's procedural manuals for nuclear accidents. These were the manuals that the operator of the Fukushima Daiichi nuclear power plant earlier did send to the Lower House with most of the contents blacked out, saying that this information should be kept secret to protect its intellectual property rights, and that disclosure would offer information to possible terrorists. NISA ordered TEPCO to send the manuals without any redaction, as the law orders. 200 pages were released from the accident procedural manuals used for Fukushima Daiichi nuclear power plant. All their contents were published, only the names of individuals were left out. From these documents could be concluded: * TEPCO did not make sufficient preparations to cope with critical nuclear accidents. * After the batteries and power supply boards were inundated on 11 March, almost all electricity sources were lost * TEPCO did not envision such a power failure or any kind of prolonged power loss. * TEPCO thought that in a serious incident, venting pressure in the reactor containment vessels or carrying out other safety procedures would still be possible, because emergency power sources would still be available. The agency said, the decision to publish the manuals was taken, for transparency in the search what caused the nuclear accident in Fukushima and also to establish better safety measures for the future.〔NHK-world (24 October 2011) (Parts of TEPCO's accident manuals made public ). 〕 On 24 October 2011 the first meeting was held by a group of 6 nuclear energy specialists invited by NISA to discuss the lessons to be learned from the accidents in Fukushima. Their first remarks were: * Japanese nuclear power plants should have multiple power sources * plants should be able to maintain electricity during an earthquake or other emergencies * TEPCO should examine why the equipment failed to work and should take appropriate actions to prevent such failures in the future According to professor Tadashi Narabayashi of the Hokkaido University Graduate School, plant operators should arrange emergency power supplies with other utilities. These discussion should be completed in March 2012, to be able to implement their conclusions into the new safety-regulations by the new nuclear safety agency to be launched in April 2012.〔NKH-world (24 October 2011)(Nuke energy experts discuss technological issues ). 〕 The Investigation Committee on the Accident at the Fukushima Nuclear Power Stations of Tokyo Electric Power Company was formed 7 June 2011 by the Japanese government as an independent body to investigate the Fukushima Daiichi disaster.〔Ito, Masami, ("Official probe begins into nuclear disaster" ), ''Japan Times'', 8 June 2011, p. 1. 〕 The Investigation Committee issued an interim report in December 2011, and is expected to issue its final report summer, 2012. The interim report was "a scathing assessment of the response to the Fukushima disaster", in which the investigative panel "blamed the central government and the Tokyo Electric Power Co., saying both seemed incapable of making decisions to stem radiation leaks as the situation at the coastal plant worsened in the days and weeks after the disaster". In February 2012, an independent investigation into the accident by the Rebuild Japan Initiative Foundation said that "In the darkest moments of last year's nuclear accident, Japanese leaders did not know the actual extent of damage at the plant and secretly considered the possibility of evacuating Tokyo, even as they tried to play down the risks in public". The government was preparing for the possibility of having to evacuate Tokyo while assuring its millions of residents that everything was under control.〔 〕
A 2012 report in ''The Economist'' said that the response to Fukushima has, so far, been inadequate, as many questions remain. One of the more worrying is how much damage the earthquake did to the reactors:〔 〕
Oregon's United States Senator Ron Wyden toured the plant and issued a statement that the situation was "worse than reported." He sent a letter to Japanese Ambassador Ichiro Fujisaki urging Japan to seek international help to relocate spent fuel rods stored in unsound structures and prevent leakage of dangerous nuclear material.〔Phred Dvorak, (Fukushima Daiichi’s Achilles Heel: Unit 4′s Spent Fuel? ), Wall Street Journal, 17 April 2012. 〕〔(After Tour of Fukushima Nuclear Power Station, Wyden Says Situation Worse than Reported ), from Office of United States Senator Ron Wyden, 16 April 2012. 〕 TEPCO released its final internal investigation report on 20 June 2012. In the report, TEPCO complained that top politicians, including the prime minister, interfered with recovery efforts during the initial stages of the disaster by making specific requests that were out of touch with what was actually taking place at the plant. TEPCO concluded that the direct cause of the accident was the tsunami which knocked out the reactors' cooling system. TEPCO also admitted that it was at fault in not being prepared for the situation, but said that its workers did the best they could "amid unprecedented circumstances."〔Nagata, Kazuaki, "(Tepco lashes prime minister's office )", ''Japan Times'', 21 June 2012, p. 1 〕 On 5 July 2012, the National Diet of Japan Fukushima Nuclear Accident Independent Investigation Commission released an executive summary report〔 〕 of The Fukushima Nuclear Accident. The report "blames Japanese culture for the fundamental causes of the disaster."〔 〕 The panel is due to deliver its final report at the end of July.〔 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「Investigations into the Fukushima Daiichi nuclear disaster」の詳細全文を読む スポンサード リンク
|