The place where a train called nuclear power generation goes to
==================================================================
原発爆発と定点カメラNuclear Reactor Huge Explosions and Fixed
公開日: 2012/03/24
a Camera that records huge hydrogen explosions of Fukushima Daiichi Nuclear Power Plant. and relatively long reel of each explosion.
福島第一原発の映像を捉えたカメラが紹介されています。
福島第一原発の映像を捉えたカメラが紹介されています。
==================================================================
Fukushima Daiichi nuclear disaster
http://en.wikipedia.org/wiki/Fukushima_Daiichi_nuclear_disaster
The Fukushima Daiichi nuclear disaster (福島第一原子力発電所事故 Fukushima Dai-ichi (About this sound pronunciation) genshiryoku hatsudensho jiko?) 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.[5][6] It is the largest nuclear disaster since the Chernobyl disaster of 1986 and only the second disaster (along with Chernobyl) to measure Level 7 on the International Nuclear Event Scale.[7]
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.[8] Immediately after the earthquake, the remaining reactors 1–3 shut down automatically and emergency generators came online to power 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).
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 melt down. In the hours and days that followed, Reactors 1, 2 and 3 experienced full meltdown.[9][10]
In the high heat and pressure of the reactors, a reaction between the nuclear fuel metal cladding, and the water surrounding them, produced explosive hydrogen gas. As workers struggled to cool and shut down the reactors, several hydrogen-air chemical explosions occurred.[11][12] It is estimated that the hot cladding-water reaction in each reactor produced 800 to 1000 kilograms of hydrogen gas, 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 unit 3 and 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.[13][14]
A few of the plant's workers were severely injured or killed by the disaster conditions (drowning, falling equipment damage etc.) resulting from the earthquake.[15][better source needed] Predicted future cancer deaths due to accumulated radiation exposures in the population living near Fukushima are predicted to be quite low. However, the researchers emphasized that the uncertainties in the calculations is high, suggesting further research is required.[16] 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.[17] 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,[18] 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.[19] 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.[20][21][22][23]
The Fukushima I Nuclear Power Plant consists of six light water, boiling water reactors (BWR) designed by General Electric driving electrical generators with a combined power of 4.7 gigawatts, making Fukushima I one of the 25 largest nuclear power stations in the world. Fukushima I was the first GE designed nuclear plant to be constructed and run entirely by the Tokyo Electric Power Company (TEPCO).
Unit 1 is a 439 MWe type (BWR-3) reactor constructed in July 1967. It commenced commercial electrical production on 26 March 1971.[24] It was designed for a peak ground acceleration of 0.18 g (1.74 m/s2) and a response spectrum based on the 1952 Kern County earthquake.[25] Units 2 and 3 are both 784 MWe type BWR-4 reactors, Unit 2 commenced operating in July 1974 and Unit 3 in March 1976. The earthquake design basis for all units ranged from 0.42 g (4.12 m/s2) to 0.46 g (4.52 m/s2).[26][27] All units were inspected after the 1978 Miyagi earthquake when the ground acceleration was 0.125 g (1.22 m/s2) for 30 seconds, but no damage to the critical parts of the reactor was discovered.[25]
Units 1–5 have a Mark 1 type (light bulb torus) containment structure, Unit 6 has Mark 2 type (over/under) containment structure.[25] From September 2010, Unit 3 has been partially fuelled by mixed-oxide (MOX) fuel.[28]
At the time of the accident, the units and central storage facility contained the following numbers of fuel assemblies:[29]
Power reactors work by splitting atoms, typically uranium, in a chain reaction. The reactor continues to generate heat after the chain reaction is stopped because of the radioactive decay of unstable isotopes, fission products, created by this process. This decay of unstable isotopes, and the decay heat that results, cannot be stopped.[32][33] Immediately after shutdown, this decay heat amounts to approximately 6% of full thermal heat production of the reactor.[32] The decay heat in the reactor core decreases over several days before reaching cold shutdown levels.[34] Nuclear fuel rods that have reached cold shutdown temperatures typically require another several years of water cooling in a spent fuel pool before decay heat production reduces to the point that they can be safely transferred to dry storage casks.[35]
To safely remove this decay heat, reactor operators must continue to circulate cooling water over fuel rods in the reactor core and spent fuel pond.[32][36] In the reactor core, circulation is accomplished by use of high pressure systems that pump water through the reactor pressure vessel and into heat exchangers. These systems transfer heat to a secondary heat exchanger via the essential service water system, taking away the heat which is pumped out to the sea or site cooling towers.[37]
To circulate cooling water when the reactor is shut down and not producing electricity, cooling pumps can be powered by other units on-site, by other units off-site through the grid, or by diesel generators.[36][38] In addition, boiling water reactors have steam-turbine driven emergency core cooling systems that can be directly operated by steam still being produced after a reactor shutdown, which can inject water directly into the reactor.[39] Steam turbines results in less dependence on emergency generators, but steam turbines only operate so long as the reactor is producing steam. Some electrical power, provided by batteries, is needed to operate the valves and monitoring systems.
If the water in the Unit 4 spent fuel pool had been heated to boiling temperature, the decay heat has the capacity to boil off about 70 tonnes of water per day (12 gallons per minute), which puts the requirement for cooling water in context.[40] On 16 April 2011, TEPCO declared that Reactors 1–4's cooling systems were beyond repair and would have to be replaced.[41]
The reason that cooling is so essential for a nuclear reactor, is that many of the internal components and fuel assembly cladding is made from zircaloy. At normal operating temperatures (of approximately 300 degrees Celsius), zircaloy is inert. However, when heated to above 500 degrees celsius in the presence of steam,[42] zircaloy undergoes an exothermic reaction where the zircaloy oxidises, and produces free hydrogen gas. The reaction between the zirconium cladding and the fuel can also lower the melting point of the fuel and thus speed up a core melt.[43]
The reactor's emergency diesel generators and DC batteries, crucial components in powering the reactors' cooling systems in the event of a power loss, were located in the basements of the reactor turbine buildings. The reactor design plans provided by General Electric specified placing the generators and batteries in that location, but mid-level engineers working on the construction of the plant were concerned that this made the back-up power systems vulnerable to flooding. TEPCO elected to strictly follow General Electric's design in the construction of the reactors.[44]
Contamination
Main article: Radiation effects from Fukushima Daiichi nuclear disaster
Sub article: Comparison of Fukushima and Chernobyl nuclear accident with detailed tables inside
Map of contaminated areas around the plant (22 March – 3 April).
Fukushima dose rate comparison to other incidents and standards, with graph of recorded radiation levels and specific accident events from 11 to 30 March.
Radiation measurements from Fukushima Prefecture, March 2011
Seawater-contamination along coast with Caesium-137, from 21 March until 5 May (Source: GRS) Radioactive material has been released from the Fukushima containment vessels as the result of deliberate venting to reduce gaseous pressure, deliberate discharge of coolant water into the sea, and accidental or uncontrolled events. Concerns about the possibility of a large scale release of radioactivity resulted in 20 km exclusion zone being set up around the power plant and people within the 20–30 km zone being advised to stay indoors. Later, the UK, France and some other countries told their nationals to consider leaving Tokyo, in response to fears of spreading radioactive contamination.[90] The Fukushima accident has led to trace amounts of radiation, including iodine-131, caesium-134 and caesium-137, being observed around the world (New York State, Alaska, Hawaii, Oregon, California, Montreal, and Austria).[91][92][93] Small amounts of radioactive isotopes have also been released into the Pacific Ocean.
A monitoring system designed to detect nuclear explosions, operated by the Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), tracked the dispersion of radioactivity from the crippled nuclear reactor on a global scale. Radioactive isotopes originating from Fukushima were picked up by over 40 CTBTO radionuclide monitoring stations. The CTBTO makes its monitoring data and analysis results available to all its 183 Member States. Around 1,200 scientific and academic institutions in 120 countries currently make use of this offer.[94]
On 12 March, radioactive releases first reached a CTBTO monitoring station in Takasaki, Japan, around 200 km away from the troubled power plant. The dispersion of the radioactive isotopes could then be followed to eastern Russia on 14 March and to the west coast of the United States two days later. By day 15, traces of radioactivity were detectable all across the northern hemisphere. Within one month, radioactive particles were also picked up by CTBTO stations in the southern hemisphere, located for example in Australia, Fiji, Malaysia and Papua New Guinea.[95][96]
According to one expert, the release of radioactivity is about one-tenth that from the Chernobyl disaster and the contaminated area is also about one-tenth that of Chernobyl.[97] A March 2012 report by the Ministry of Education, Culture, Sports, Science and Technology agreed that radioactive debris from the damaged reactors had dispersed about one-eighth to one-tenth of the distance as those in the Chernobyl disaster.[98][99] According to a study conducted by Norwegian Institute for Air Research, the release of the particular isotope caesium-137 was about 40 percent of the total from Chernobyl.[100][101][102]
In March 2011, Japanese officials announced that "radioactive iodine-131 exceeding safety limits for infants had been detected at 18 water-purification plants in Tokyo and five other prefectures".[103] As of July 2011, the Japanese government has been unable to control the spread of radioactive material into the nation's food. Radioactive material has been detected in a range of produce produced in 2011, including spinach, tea leaves, milk, fish and beef, up to 200 miles from the nuclear plant. Crops produced in 2012 did not show signs of radioactivity contamination, cabbage, rice[104] and beef were tested before reaching market and showed insignificant levels of radiation. A Fukushima-produced rice market in Tokyo was accepted by consumers as safe.[104]
On 24 August 2011, the Nuclear Safety Commission (NSC) of Japan published the results of the recalculation of the total amount of radioactive materials released into the air during the accident at the Fukushima Daiichi Nuclear Power Station. The total amounts released between 11 March and 5 April were revised downwards to 1.3 × 1017 Bq for iodine-131 and 1.1 × 1016 Bq for caesium-137, which is about 11% of Chernobyl emissions. Earlier estimations were 1.5 × 1017 Bq and 1.2 × 1016 Bq.[105][106]
On 8 September 2011 a group of Japanese scientists working for the Japan Atomic Energy Agency, the Kyoto University and other institutes, published the results of a recalculation of the total amount of radioactive material released into the ocean: between late March through April they found a total of 15,000 TBq for the combined amount of iodine-131 and caesium-137. This was more than triple the figure of 4,720 TBq estimated by the plant-owner. TEPCO made only a calculation about the releases from the plant in April and May into the sea. The new calculations were needed because a large portion of the airborne radioactive substances would enter the seawater when it came down as rain.[107]
In the first half of September 2011 the amount of radioactive substances released from the plant was about 200 million becquerels per hour, according to TEPCO, this was approximately one four-millionth of the level of the initial stages of the accident in March.[108] Traces of iodine-131 are still detected in several Japanese prefectures in the months of November[109] and December 2011.[110]
According to a report published in October 2011 by the French Institute for Radiological Protection and Nuclear Safety, between 21 March and mid-July around 2.7 × 1016 Bq of caesium-137 entered the ocean, about 82 percent having flowed into the sea before 8 April. This emission of radioactivity into the sea represents the most important individual emissions of artificial radioactivity into the sea ever observed. The Fukushima coast has one of the world's strongest currents and this transported the contaminated waters far into the Pacific Ocean, causing a high dispersion of the radioactive elements. The results of measurements of both the seawater and the coastal sediments lead to suppose that the consequences of the accident, for what concerns radioactivity, will be minor for marine life as of late 2011 (weak concentration of radioactivity in the water and limited accumulation in sediments). On the other hand, significant pollution of sea water along the coast near the nuclear plant might persist, because of the continuing arrival of radioactive material transported towards the sea by surface water running over contaminated soil. Further, some coastal areas might have less favorable dilution or sedimentation characteristics than those observed so far. Finally, the possible presence of other persistent radioactive substances, such as strontium-90 or plutonium, has not been sufficiently studied. Recent measurements show persistent contamination of some marine species (mostly fish) caught along the coast of Fukushima district. Organisms that filter water and fish at the top of the food chain are, over time, the most sensitive to caesium pollution. It is thus justified to maintain surveillance of marine life that is fished in the coastal waters off Fukushima.[111]
As of March 2012, there had been no reported cases of Fukushima residents suffering ailments related to radiation exposure. Experts cautioned that insufficient data was available so far to make conclusions on the impact on residents' health. Nevertheless, Michiaki Kai, professor of radiation protection at Oita University of Nursing and Health Sciences, stated, "If the current radiation dose estimates are correct, (cancer-related deaths) likely won't increase."[112]
On 24 May 2012, TEPCO released their estimate of radiation releases due to the Fukushima Daiichi Nuclear Disaster. An estimated 538,100 terabecquerels (TBq) of iodine-131, caesium-134 and caesium-137 was released. 520,000 TBq was released into the atmosphere between 12–31 March 2011 and 18,100 TBq into the ocean from 26 March – 30 September 2011. A total of 511,000 TBq of iodine-131 was released into both the atmosphere and the ocean, 13,500 TBq of caesium-134 and 13,600 TBq of caesium-137.[113]
In May 2012, TEPCO reported that at least 900 PBq had been released "into the atmosphere in March last year [2011] alone".[114][115] In August 2012, researchers found that 10,000 people living near the plant at the time of the accident had been exposed to well less than 1 millisievert of radiation, far less than Chernobyl residents.[116]
In October 2012 an article in Science-magazine concluded, that at that time radiation was still leaking from the reactor-site into the ocean. Fishing in the waters around the site was still prohibited, and the levels of radioactive 134Cs and 137Cs in the fish caught were not lower compared with the levels found after the disaster. [117] On 26 October 2012 TEPCO admitted that it could not exclude radiation emissions into the ocean, although the radiation levels were stabilised. Undetected leaks into the ocean from the reactors, could not be ruled out, because their basements remain flooded with cooling water, and the 2,400-foot-long steel and concrete wall between the site’s reactors and the ocean, that should reach 100 feet underground, was still under construction, and would not be finished before mid-2014. Around August 2012 two greenling were caught close to the Fukushima shore. They contained more than 25,000 becquerels of cesium-137 per kilogram of fish, the highest cesium levels found in fish since the disaster and 250 times the government’s safety limit.[118][119]
A report by the World Health Organization(WHO) published in February 2013 anticipated that there would be no noticeable increases in cancer rates for the overall population, but somewhat elevated rates for particular sub-groups. For example infants of Namie Town and Iitate Village were estimated to have a 6% relative increase in female breast cancer risk and a 7% relative increase in male leukemia risk. A third of emergency workers involved in the accident would have increased cancer risks.[120]
However the WHO expressly stated that the values stated in its report were expressed as relative increases, and not representative of the absolute increase in developing cancer:[121]
These percentages represent estimated relative increases over the baseline rates and are not absolute risks for developing such cancers. Due to the low baseline rates of thyroid cancer, even a large relative increase represents a small absolute increase in risks. For example, the baseline lifetime risk of thyroid cancer for females is just (0.75%)three-quarters of one percent and the additional lifetime risk estimated in this assessment for a female infant exposed in the most affected location is (0.5%)one-half of one percent.
In 2013, two years after the incident, the World Health Organization indicated that the residents of the area were exposed to so little radiation that it probably won't be detectable. They indicated that a Japanese baby's cancer lifetime risk would increase by about 1%.[122]
This page was last modified on 23 May 2013 at 02:31.
==================================================================
0 件のコメント:
コメントを投稿