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Fukushima I nuclear accidents||||= Satellite image of Unit 3 moments after the second explosion on 14 March, 11:04 JSTSatellite image of Unit 3 moments after the second explosion on 14 March, 11:04 JST
Satellite image of Unit 3 moments after the second explosion on 14 March, 11:04 JST
11 March 2011
14:46 JST (UTC+9)
Okuma, Fukushima, Japan
37°25'17?N 141°1'57?E? / ?37.42139°N 141.0325°E
Level 4 (Accident with local consequences) preliminary INES rating by Japanese authorities as of March 12,[1] unofficial level 6 (Serious Accident) rating by French and Finnish nuclear authorities.
external image Locator_Dot2.gif
external image 260px-Japan_Nuclear_power_plants_map.gif
The Fukushima I nuclear accidents are a series of ongoing equipment failures and releases of radioactivity at the Fukushima I Nuclear Power Plant, caused by the2011 Sendai earthquake and tsunami on 11 March. The plant comprises six separate boiling water reactors maintained by the Tokyo Electric Power Company(TEPCO). Reactors 4, 5 and 6 had been shut down prior to the earthquake for planned maintanance.[5] The remaining reactors were shut down automatically after theearthquake, but the subsequent tsunami flooded the plant, knocking out emergency generators needed to cool and control the reactors. The flooding and earthquake damage prevented assistance being brought from elsewhere.
Over the following days there was evidence of partial nuclear meltdowns in reactors 1, 2 and 3; hydrogen explosions destroyed the upper cladding of the building housing reactors 1 and 3; an explosion damaged reactor 2's containment; and fires broke out in the spent fuel pool at reactor 4. Radiation leaks led to a 20 km-radius evacuation around the plant. On 16 March TEPCO pulled all employees out of the complex due to a radiation spike, suspending efforts to control overheating.[6][7]However they later returned after the levels dropped.[8][9]


[edit] Day-by-day events

See also: Timeline of the Fukushima nuclear accidents

  • 11 March 2011: the Japanese government declared a nuclear power emergency due to the failure of the reactor cooling systems in one of the reactors of Fukushima I and evacuated thousands of residents living close to the reactor. After officials later voiced the possibility of core damage, the evacuation zone was extended to 20km, affecting 170,000–200,000 people,[10][11] and residents within a further 10km have been advised to stay indoors. Twenty-two residents near the plant showed signs of radioactive contamination exposure, and three workers from the plant reported symptoms of radiation sickness,[12] but only one worker was confirmed to be ill by the Tokyo Electric Power Company (TEPCO).[13] The release of fission products from the damaged nuclear reactor core, notably radioactive iodine-131, led Japanese officials to distribute prophylactic iodine to the people living around Daiichi and Daini.[12]
  • 12 March: while evidence of partial meltdown of the fuel rods in Unit 1 was growing, a hydrogen explosion destroyed the upper cladding of the building housing Reactor Unit 1. The explosion injured four workers, but the reactor containment inside the building remained intact.[14][15] The explosion is believed to be the result of a buildup of hydrogen within the building after it was vented along with steam to reduce pressure within the containment vessel.[16][17] Hydrogen is formed when overheated zircaloy reactor fuel rods oxidize with water.[18] Operators of the plant were authorised to commence using sea water for emergency cooling, which would have the likely effect of contaminating a reactor beyond further use.
  • 13 March: a partial meltdown also appeared possible at Unit 3. As of 13:00 JST, both reactors 1 and 3 were being vented and re-filled with water and boric acid to reduce temperatures and inhibit further nuclear reactions.[19] Unit 2 was reported to have lower than normal water level but to be stable, although pressure inside the containment vessel was high.[19] The Japan Atomic Energy Agency announced that it was rating the situation at Units 1 and 3 as Level 4 (accident with local consequences) on the International Nuclear Event Scale (INES).[20]
  • 14 March: the reactor building for Unit 3 exploded[21] injuring eleven people. There was no release of radioactive material beyond that already being vented but blast damage affected water supply to Unit 2.[22] The president of the French nuclear safety authority, Autorité de sûreté nucléaire (ASN), said that the accident should be rated as a 5 or even a 6 on INES.[23]
  • 15 March: problems with the vents on Unit 2 apparently meant that pressure in its containment vessel had prevented adding water, to the extent that Unit 2 was in the most severe condition of the three reactors.[24] An explosion in the "pressure suppression room" caused some damage to Unit 2’s containment system.[24][25] A fire broke out at Unit 4 involving spent fuel rods from the reactor, which are normally kept in the water-filled spent fuel pool to prevent overheating. Radiation levels at the plant rose significantly but have since fallen back.[26] A radiation equivalent dose rate was observed at one location between Units 3 and 4 of 400 millisievert per hour (400 mSv/h).[27][28][29]
  • 16 March: By midday, NHK TV was reporting white smoke rising from the Fukushima I plant, which officials suggested was likely coming from Reactor 3. Shortly afterwards, reports surfaced that all but a small group[30] of remaining workers at the plant had been placed on standby because of the dangerously rising levels of radioactivity.[31] Later reports stated that TEPCO had temporarily suspended operations at the facility due to radiation spikes and had pulled all their employees out.[32] A TEPCO press release stated that workers had been withdrawn at 06:00 JST because of abnormal noises coming from one of the reactor pressure suppression chambers.[33] Late in the evening Reuters was reporting that water was being poured into Reactors 5 & 6.[34]

[edit] Fukushima I Nuclear Power Plant

external image 220px-BWR_Mark_I_Containment_sketch_with_downcomers.pngexternal image magnify-clip.png
Simplified cross-section sketch of a typical BWR Mark I containment, as used in units 1 to 5. DW = drywell enclosing reactor pressure vessel, WW = torus-shaped wetwell all around the base enclosing steam suppression pool, SF = spent fuel pool area. Excess steam from the drywell enters the wetwell water pool via downcomer pipes.
Main article: Fukushima I Nuclear Power Plant
The Fukushima I Nuclear Power Plant is located in the town of Okuma in the Futaba District of Fukushima Prefecture, Japan. It consists of six light water, boiling water reactors (BWR) 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 nuclear plant to be constructed and run entirely by the Tokyo Electric Power Company (TEPCO).
The reactors for units 1, 2, and 6 were supplied by General Electric, those for units 3 and 5 by Toshiba, and unit 4 by Hitachi. All construction was done by Kajima.[35]Unit 1 is a 439 MWe type (BWR3) reactor constructed in July 1967. It commenced commercial electrical production on March 26, 1971, and was scheduled for shutdown in March 2011.[36] 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.[37] Units 2 and 3 are both 784 MW type BWR-4 reactors, Unit 2 commenced operating in July 1974 and Unit 3 in March 1976. 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.[37]
Units 1–5 have a Mark 1 type (light bulb torus) containment structure, unit 6 has Mark 2 type (over/under) containment structure.[37] From September 2010, unit 3 has been fueled by mixed-oxide (MOX) fuel.[38]

[edit] Cooling requirements

external image 170px-Bwr-rpv.svg.pngexternal image magnify-clip.png
Diagrammatic representation of the cooling systems of a BWR
See also: Decay_heat#Power_reactors_in_shutdown
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This section does not cite any references or sources.
Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (March 2011)
Cooling is needed to remove decay heat even when a plant has been shut down. Nuclear fuel releases a small quantity of heat under all conditions, but the chain reaction when a reactor is operating creates short lived decay products which initially produce roughly 6.6% of full thermal heat (~100MW for reactors 2/3) and then decrease over several days before reaching cold shutdown levels (significantly below a permille). The nuclear fuel requires 1–3 years of constant active cooling (by flowing water) before the decay heat production gets low enough that effective passive cooling becomes sufficient to avoid excessive heating up to temperatures where the integrity of the fuel is at risk. Boiling water reactors have steam-turbine driven emergency core cooling systems that can be directly operated by steam produced after a reactor shutdown and can inject water directly into the reactor. Using these pumps, boiling water reactors can provide water without electrically driven pumps but only while the reactor is at pressure. This results in less dependence on emergency generators but only operates so long as the reactor is safely producing steam, and some power is still needed to operate the valves and monitoring systems. Achieving cold shutdown still requires motor-driven pumps to remove decay heat.

[edit] Direct effect of the 2011 earthquake and tsunami

See also: 2011 Sendai earthquake and tsunami
An earthquake categorised as 9.0MW on the moment magnitude scale occurred on 11 March 2011, at 14:46 Japan Standard Time (JST) off the northeast coast ofJapan. On that day, reactor units 1, 2, and 3 were operating, but units 4, 5, and 6 had already been shut down for periodic inspection.[39] When the earthquake was detected, units 1, 2 and 3 underwent an automatic shutdown (called scram).[40]
After the reactors shut down, electricity generation stopped. Normally the plant could use the external electrical supply to power cooling and control systems[41], but the earthquake had caused major damage to the power grid. Emergency diesel generators started correctly but stopped abruptly at 15:41, ending all AC power supply to the reactors. The plant was protected by a sea wall, but tsunami water which followed after the earthquake topped this sea wall, flooding the low lying generator building.[42][43] Article 10 of the Japanese law on Special Measures Concerning Nuclear Emergency Preparedness, heightened alert condition requires authorities to be informed of such an incident: TEPCO did so immediately and also issued a press release declaring a "First Level Emergency".[40]
After the failure of the diesels, emergency power for control systems was supplied by batteries that would last about eight hours.[44] Batteries from other nuclear plants were sent to the site and mobile generators arrived within 13 hours,[45] but work to connect portable generating equipment to power water pumps was still continuing as of 15:04 on 12 March.[46] Generators would normally be connected through switching equipment in a basement area of the buildings, but this basement area had been flooded by the tsunami.[42]

[edit] Reactor unit 1

[edit] Cooling problems at unit 1

external image 220px-Fukushima_I_NPP_1975_medium_crop_rotated_labeled.jpgexternal image magnify-clip.png
Aerial view of the plant area before the accidents. When this photograph was taken in 1975, Unit 6 was under construction.

On 11 March 2011 at 16:36 JST, a nuclear emergency situation (Article 15 of the Japanese law on Special Measures Concerning Nuclear Emergency Preparedness) was declared when "the status of reactor water coolant injection could not be confirmed for the emergency core cooling systems of Units 1 and 2". The alert was cleared "when the reactor water level monitoring function was restored for Unit 1." However, it was reinstated at 17:07 JST.[47] Potentially radioactive steam was released from the primary circuit into the secondary containment area to reduce mounting pressure.[48]
In the early hours of 12 March TEPCO reported that radiation levels were rising in the turbine building for Reactor Unit 1[49] and that it was considering venting some of the mounting pressure into the atmosphere, which could result in the release of some radiation.[50] Chief Cabinet Secretary Yukio Edano stated later in the morning that the amount of potential radiation would be small and that the prevailing winds are blowing out to sea.[51] At 02:00 JST, the pressure inside the reactor containmentwas reported to be 600 kPa (6 bar or 87 psi), 200 kPa higher than under normal conditions.[43] At 05:30 JST the pressure inside Reactor 1 was reported to be 2.1 times the "design capacity",[52] 820 kPa.[14] Rising heat within the containment area would have led to increasing pressure, with both cooling water pumps and ventilation fans for driving gases through heat exchangers within containment dependant on electricity.[53] Releasing gases from the reactor is necessary if pressure becomes too high and has the benefit of cooling the reactor as water boils off, but this also means cooling water is being lost and must be replaced.[42] Water inside the reactor should be only very slightly radioactive, but this assumes no damage to the fuel elements.
In a press release at 07:00 JST 12 March, TEPCO stated, "Measurement of radioactive material (iodine, etc.) by monitoring car indicates increasing value compared to normal level. One of the monitoring posts is also indicating higher than normal level."[54] Dose or dose-equivalent rates recorded on the main gate rose from "69 nGy/h" (for gamma radiation, equivalent to 0.069 µSv/h) at 04:00 JST, 12 March, to "866 nGy/h" (equivalent to 0.866 µSv/h) 40 minutes later, before hitting a peak of 385.5 µSv/h at 10:30 JST.[55][54][56][57] At 13:30 JST, radioactive caesium-137 and iodine-131 was detected near reactor 1,[58] which indicates that some of the core was exposed due to a partial-meltdown or other damage of the nuclear fuel.[59] The NHK website reported that cooling water had lowered so much that parts of thenuclear fuel rods were exposed.[60] Radiation levels at the site boundary exceeded the regulatory limits.[61] Kyodo News Service later reported that partial melting may have occurred.[62][63][64][65] On 14 March 2011, Kyodo News reported radiation levels had continued to increase on the premises, measuring at 02:20 an intensity of 751 µSv/hour on one location and at 02:40 an intensity of 650 µSv/hour at another location on the premises.[66]

[edit] Explosion of reactor 1 building

external image 220px-Fukushima_explosion.jpgexternal image magnify-clip.png
First explosion at Fukushima power plant, Unit 1.

external image 220px-2011-03-12_1800_NHK_S%C5%8Dg%C5%8D_channel_news_program_screen_shot.jpgexternal image magnify-clip.png
Before and after images of the Unit 1 explosion.

At 15:36 JST on 12 March 2011 there was an explosion at Unit 1. Four workers were injured, and the upper shell of the reactor building was blown away leaving in place its steel frame.[67][68] The outer building is designed to provide ordinary weather protection for the areas inside, but not to withstand the high pressure of an explosion or to act as containment for the reactor. In the Fukushima I reactors the primary containment consists of "drywell" and "wetwell" concrete structures immediately surrounding the reactor pressure vessel.[14][69]
Experts soon agreed that the cause was a hydrogen explosion.[15][70][71] Almost certainly the hydrogen was formed inside the reactor vessel[15] because of falling water levels, and this hydrogen then leaked into the containment building.[15] Exposed Zircaloy metal fuel rods become very hot and can then react with steam oxidising the metal and releasing hydrogen. Safety devices should inert the hydrogen when it is vented before explosive concentrations are reached but apparently these systems failed, or could not be operated due to the shortage of electrical power.
Officials indicated that the container of the reactor had remained intact and there had been no large leaks of radioactive material,[14][15] although an increase in radiation levels was confirmed following the explosion.[72][73] ABC News (Australia) reported that according to the Fukushima prefectural government, radiation dose rates at the plant reached 1,015 microsievert per hour (1015 µSv/h.[74] Two independent nuclear experts cited design differences between the Chernobyl Nuclear Power Plantand the Fukushima I Nuclear Power Plant,[75][76] one of them saying he did not believe that a Chernobyl-style disaster will occur.[75]
The IAEA stated on 13 March that four workers had been injured by the explosion at the Unit 1 reactor, and that three injuries were reported in other incidents at the site. They also reported one worker was exposed to higher-than-normal radiation levels but that fell below their guidance for emergency situations.[77]
The Guardian reported at 17:35 JST on 12 March that NHK advised residents of the Fukushima area "to stay inside, close doors and windows and turn off air conditioning. They were also advised to cover their mouths with masks, towels or handkerchiefs" as well as not to drink tap water.[78] Air traffic has been restricted in a 20-kilometre (12 mi) radius around the plant, according to a NOTAM.[79] The BBC has reported as of 22:49 JST (13:49 GMT) "A team from the National Institute of Radiological Sciences has been dispatched to Fukushima as a precaution, reports NHK. It was reportedly made up of doctors, nurses and other individuals with expertise in dealing with radiation exposure, and had been taken by helicopter to a base 5 km from the nuclear plant."[80]
Of 90 bedridden patients moved from a hospital in the town of Futaba-machi, a sample of three patients were tested and shown to have been exposed to radiation. The patients had been waiting outdoors for rescuers before being moved by helicopter at the time when an explosion happened.[81][82]

[edit] Seawater used for cooling

At 20:05 on 12 March 2011, according to the nuclear regulation act and to the directives of the Prime Minister, the Japanese government ordered seawater to be used in Unit 1 in an effort to cool down the degraded reactor core.[46] At 21:00 JST TEPCO announced that they planned to cool the leaking reactor with seawater (which started at 20:20 JST), then using boric acid to act as a neutron absorber to prevent a criticality accident.[83][84] The water would take five to ten hours to fill the reactor core, after which it would need to stay for cooling for around ten days.[15] At 23:00 JST TEPCO announced that due to the quake at 22:15[85] the filling of the reactor had been temporarily stopped but has been resumed after a short while.[14][86] Filling the reactor with seawater will contaminate the reactor with impure water, a substance not usually allowed in reactors, meaning the reactor will likely be decommissioned.[87]
NISA reported that injection of sea water into the primary containment vessel through the fire extinguisher system commenced at 11:55 on 13 March. At 01:10 on 14 March injection of sea water was halted because all available water in the plant pools had run out (similarly, feed to unit 3 was halted). Water supply was restored at 03:20.
NISA stated 70% of the fuel rods were damaged, in news reports the morning of 16 March.[88]

[edit] Reactor unit 2

Unit two was operational during the earthquake and experienced the same cooling procedures directly after the earthquake (power supply by diesel engine, which failed after about an hour), and stable water levels were reported. Power was achieved by mobile power units, while preparations were made to perform pressure venting.[14][89]

[edit] Cooling problems at unit 2

On Mar 14, at 15:29 JST the Jiji news agencies reported that the cooling functions at reactor unit 2 had stopped and that the cooling water levels were falling.[90][91]This was caused when fuel for pumps ran out.[92] Jiji news agencies later reported that nuclear fuel rods at reactor unit 2 were fully exposed and there was a risk of a full meltdown at reactor unit 2.[93] Jiji later reported that according to TEPCO, a meltdown cannot be ruled out.[94]
At 22:29 JST, workers had succeeded in refilling half the reactor with water. However, at that time, part of the rods were still exposed, and technicians could not rule out the possibility that fuel rods had melted.[95][dead link] Work was in hand to demolish parts of the walls of reactor building 2 to allow the escape of hydrogen and hopefully prevent another explosion.[96] At 21:37 JST the measured radiation rates at the gate of the plant had reached a maximum of 3.13 millisievert per hour, which was enough to reach the annual limit for non-nuclear workers in twenty minutes,[96] but had fallen back to 0.326 millisieverts per hour by 22:35.[97]
It was believed that around 23:00 JST the 4m long fuel rods in the reactor were fully exposed for the second time.[96][98] At 00:30 JST of 15 March, NHK ran a live press conference with TEPCO stating that the water level had sunk under the rods once again and pressure in the vessel was raised. The utility said that the hydrogen explosion at unit 3 may have caused a glitch in the cooling system of unit 2: Four out of five water pumps being used to cool unit 2 reactor had failed after the explosion at unit 3. In addition, the last pump had briefly stopped working.[99] To replenish the water, the contained pressure would have to be lowered first by opening a valve of the vessel. The unit's air flow gauge was accidentally turned off and, with the gauge turned off, flow of water into the reactor was blocked leading to full exposure of the rods.[96][100]
As of 04:11 JST, 15 March, water was being pumped into the reactor of unit 2 again.[101] At 10:38 JST, 15 March, water level was reported to be at 1.20 meters and rising.[102]

[edit] Explosion in reactor 2 building

An explosion was heard after 06:14 JST[103] on 15 March in unit 2, possibly damaging the pressure-suppression system, which is at the bottom part of the containment vessel.[104][105] The radiation level was reported to exceed the legal limit and the plant's operator started to evacuate all non-essential workers from the plant.[106] Only a skeleton crew of 50 men was left at the site.[107] Soon after, radiation equivalent dose rates had risen to 8.2 mSv/hr[108] around two hours after the explosion and again down to 2.4 mSv/hr, shortly after.[109] Three hours after the explosion, the rates had risen to 11.9 mSv/hr.[110]
While admitting that the suppression pool at the bottom of the containment vessel had been damaged in the explosion, causing a drop of pressure there, Japanese nuclear authorities emphasized that the containment had not been breached as a result of the explosion and contained no obvious holes.[111]
In a news conference on 15 March the director general of the IAEA, Yukiya Amano, said that there was a "possibility of core damage" at the No. 2 unit of the damaged Fukushima power plant. He went on to add that the damage was estimated as being "less than five percent".[112] The Nuclear and Industrial Safety Agency stated 33% of the fuel rods were damaged, in news reports the morning of 16 March.[88]

[edit] Reactor unit 3

Unlike the other five reactor units, reactor 3 runs on mixed uranium and plutonium oxide, or MOX fuel, making it potentially more dangerous in an incident due to the neutronic effects of plutonium on the reactor and the carcinogenic effects in the event of release to the environment.[62][113][114]

[edit] Cooling problems at unit 3

Early on 13 March 2011, an official of the Japan Nuclear and Industrial Safety Agency told a news conference that the emergency cooling system of Unit 3 had failed, spurring an urgent search for a means to supply cooling water to the reactor vessel in order to prevent a meltdown of its reactor core.[115] At 05:38 there was no means of adding coolant to the reactor due to loss of power. Work to restore power and vent pressure continued.[116] At one point, the top three meters of mixed oxide (MOX) fuel rods were exposed.[117]
At 07:30 JST, TEPCO prepared to release radioactive steam, indicating that "the amount of radiation to be released would be small and not of a level that would affect human health"[118] and manual venting took place at 08:41 and 09:20.[89] At 09:25 JST on 13 March 2011, operators began injecting water containing boric acid into the primary containment vessell (PCV) via a fire pump.[119][120] When water levels continued to fall and pressure to rise, the injected water was switched to sea water at 13:12.[116] By 15:00 it was noted that despite adding water the level in the reactor did not rise and radiation had increased.[121] A rise was eventually recorded but the level stuck at 2m below the top of reactor core. Other readings suggested that this could not be the case and the gauge was malfunctioning.[89]
Injection of sea water into the PCV was discontinued at 01:10 on 14 March because all the water in the reserve pool had been used up. Supplies were restored by 03:20 and injection of water resumed.[120]

[edit] Explosion of reactor 3 building

At 12:33 JST on 13 March 2011, the chief spokesman of the Japanese government, Yukio Edano said that hydrogen was building up inside the outer building of unit 3 just as it had in unit 1, threatening the same kind of explosion.[122] At 11:15 JST on 14 March 2011, the envisaged explosion of the building surrounding Reactor 3 of Fukushima 1 occurred, due to the ignition of built up hydrogen gas.[123][124] It was reported that as with unit 1, the top section of the reactor building was blown apart, but the inner containment vessel was not breached. The explosion was larger than that in unit 1 and felt 25 miles away. Pressure readings within the reactor remained steady at around 380 kPa at 11:13 and 360 kPa at 11:55 compared to nominal levels of 400 kPa and a maximum recorded of 840 kPa. Water injection continued. Radiation rates of 0.05 mSv/h were recorded in the service hall and of 0.02 mSv/h at the plant entrance.[125] Eleven people were reported injured in the blast.[126][127]
On the morning of 15 March 2011 (JST), Secretary Edano announced that according to the TEPCO, at one location near reactor Units 3 and 4, radiation at anequivalent dose rate of 400 mSv/hr was detected.[27][28][29]
Around 10:00 JST, 16 March, NHK helicopters flying 30 km away videotaped white smoke rising from the Fukushima I facility. Officials suggested that the reactor 3 building was the most likely source, and said that its containment systems may have been breached.[128] At 16:12 JST Self Defence Force (SDF) Chinook helicopters were preparing to spray water on unit 3, where white smoke rising from the building was believed to be water boiling away from the fuel rod cooling pond on the top floor of the reactor building, and on unit 4 where the cooling pool was also short of water. The mission was cancelled when helicopter measurements reported radiation levels of 50mSv.[129][130] At 21:06pm JST government reported that a containment breech at unit 3 was unlikely.[131]

[edit] Reactor unit 4

At approximately 06:00 JST on March 15, a loud explosion was heard within the power station, and later was confirmed that the 4th floor roof top area of the Unit 4 reactor building had sustained damage.[132]
external image 40px-Wikinews-logo.svg.png
Wikinews has related news: Fukushima reactor suffers multiple fires, radiation leak confirmed
At 09:40 JST on 15 March 2011, the Unit 4 spent fuel pool caught fire, likely releasing radioactive contamination from the fuel stored there.[133][134] TEPCO said workers extinguished the fire by 12:00. As radiation levels rose, some of the employees still at the plant were evacuated.[135][136] The fire has been put out.[137][138]Reason for the fire seems to have been a hydrogen explosion.[139]
On the morning of 15 March 2011 (JST), Secretary Edano announced that according to the Tokyo Electric Power Company, radiation dose equivalent rates measured from the reactor unit 4 reached 100 mSv per hour[27][28]. Government speaker Edano has stated that there was no continued release of radiation.[140] The dose after which the symptoms of acute radiation poisoning typically appear is approximately 1000 mSv, or 1 Sv, received over one day. An exposed worker would be expected to begin experiencing radiation sickness soon after receiving a 100 mSv/hr dose rate for 10 hours of a day, or a 400 mSv/hr dose rate for 2.5 hours of a day.
Japan's nuclear safety agency has said there are two holes, each 8 meters square (64 m2 or 689 sq. feet — not 8 sq. meters each as sometimes incorrectly reported) in a wall of the outer building of the number 4 reactor after an explosion there.[141] Further, Reuters reported on 17:48 JST that the water of the pool containing the spent fuel might be boiling.[142][143]
As of 15 March 2011 21:13 JST, radiation inside unit 4 had increased so much inside the control room that employees could not stay there permanently any more.[144]Seventy staff remained on site but 800 had been evacuated.[145] By 22:30 JST, TEPCO was reported to be unable to pour water into No. 4 reactor's storage pool for spent fuel.[146] At around 22:50 JST, it was reported that TEPCO was considering using helicopters to drop water on the spent fuel storage pool.[145][147][148]However, TEPCO soon dismissed the option of helicopters because of concerns over safety and effectiveness. TEPCO went on to consider the use of high-pressure fire hoses instead.[149]
A fire was discovered at 05:45 JST on 16 March in the north west corner of the reactor building by a worker taking batteries to the central control room of unit 4.[150][151] This was reported to the authorities, but on further inspection at 06:15 no fire was found. Other reports stated that the fire was under control.[152] At 11:57 JST, TEPCO released a photograph of No.4 reactor showing that "a large portion of the building's outer wall has collapsed."[153] Technicians were reported to be considering spraying boric acid on the building from a helicopter.[154][155]

[edit] Possibility of re-criticality

At approximately 14:30[citation needed], TEPCO announced its belief that the storage pool may have begun boiling, which increased the possibility that exposed rods would reach criticality.[156] At 12:09[citation needed] It was planned to use a police water cannon to spray water on unit 4.[157]

[edit] Reactor units 5 and 6

Both reactors were off line at the time the earthquake struck (reactor 5 had been shut down on 3 January 2011 and reactor 6 on 14 August 2010). Although an IAEA report indicated that the fuel rods are still in the reactor vessels of both units and not in the spent fuel pools as in Unit 4,[158] Kyodo News said that there were rods in the pools, but only one-third as many in the pools as compared to Unit 4.[159]
Government spokesman Edano stated on 15 March that reactors 5 and 6 were being closely monitored, as cooling processes were not functioning well.[140][160] The removal of roof panels from reactor buildings 5 and 6 was being considered in order to allow any hydrogen build-up to escape.[158] The BBC later reported that units 5 and 6 were believed to be heating up.[161] At 18:31 on 16 March, Tepco was reported to be pouring water into both reactors.[162]

[edit] Radioactive contamination

Chief Cabinet Secretary, Yukio Edano, said that on 15 March 2011 radiation rates had been measured as high as 30 mSv/h between the Units 2 and 3, as high as 400 mSv/h[29][27] near Unit 3 between it and Unit 4, and 100 mSv/h near Unit 4. He indicated that "There is no doubt that unlike in the past, the figures are the level at which human health can be affected,"[163] Prime Minister Naoto Kan urged people living between 20 and 30 kilometers of the plant to stay indoors, "The danger of further radiation leaks (from the plant) is increasing," Kan warned the public at a press conference, while asking people to "act calmly".[164][165]
A spokesman for Japan's nuclear safety agency said TEPCO had told it that radiation levels in Ibaraki, between Fukushima and Tokyo, had risen. "The level does not pose health risks," the spokesman said. The Tokyo metropolitan government said it has detected radioactive material, such as iodine and cesium, up to 40 times normal levels in Saitama, near Tokyo.[164][163] Radiation levels in Tokyo were at one point measured at 0.8 µSv/hour although they were later measured at "about twice the normal level".[166] Later, on 15 March 2011, Edano reported that radiation levels were lower. A changed wind direction dispersed radiation away from the land and back over the Pacific Ocean.[167] Thousands of Tokyo residents are reported to have left for cities further south, although Edano insisted that levels in Greater Tokyo were not hazardous.[168]
On 16 March Japan's ministry of science measured radiation levels of up to 0.33 millisieverts per hour 20 kilometers northwest of the power plant.[169]
International commentators were divided in their analysis of the scale of the danger, with French Foreign Minister, Alain Juppe, saying that the threat was "extremely high" while others said it was too early to make comparisons to the 1986 Chernobyl disaster.[168]

[edit] U.S. Navy

The U.S. Navy dispatched aircraft carrier USS //Ronald Reagan// and other vessels flew a series of helicopter operations.[170][171] A U.S. military spokesperson had said that low-level radiation was detected both by navy ships and their accompanying aircraft, forcing a change of course of the 7th fleet, en route to Sendai.[172] USSRonald Reagan and sailors onboard were exposed to a month's worth[clarification needed] of radiation in an hour and the carrier was repositioned.[173] 17 U.S. sailors were decontaminated after they and the 3 helicopters they were on were found to have been contaminated with low-levels of radioactive particulates.[174]
The USS //George Washington// was docked for maintenance in Yokosuka Naval Base, about 280 km (175 mi) from the plant, when instruments detected the radiation at 07:00 JST, 15 March 2011, the Navy said in a statement.[175] At 320 km (200 mi) from Fukushima, Rear Admiral Richard Wren stated that the nuclear crisis in Fukushima is happening too far from Yokosuka to even warrant a discussion about evacuating base residents.[176] However, daily monitoring and limited precautionary measures were recommended for Yokosuka and Atsugi bases: limiting outdoor activities and securing external ventilation systems.[177]

[edit] Effects on human health

Normal background radiation varies from place to place but delivers a dose equivalent in the vicinity of 2.4 mSv/year annually, or about 0.3 µSv/h.[178][179] The international limit for radiation exposure for nuclear workers is 20 mSv per year, averaged over five years, with a limit of 50 mSv in any one year.[179] Chronic radiation exposure of 100 mSv per year is the "lowest level at which any increase in cancer is clearly evident," according to the World Nuclear Association.[179] Symptoms ofradiation poisoning typically emerge with a 1000 mSv total dose over a day.[180]
Radiation dose rates at one location between reactor Units 3 and 4 was measured at 400 mSv/h at 10:22 JST, 13 March 2011, causing experts to urge rapid rotation of emergency crews as a method of limiting exposure to radiation.[181][182] At these levels, the 50 workers remaining on site risked developing radiation sickness.[181]Prior to the accident, the maximum permissible dose for Japanese nuclear workers was 100 mSv in any one year, but on 15 March 2011, the Japanese Health and Labor Ministry increased that annual limit to 250 mSv, for emergency situations.[183][184] The general population faced separate risks from chronic exposure to lower-level contaminants released into the environment.[181]
The isotope iodine-131 is easily absorbed by the thyroid. However, taking potassium iodide tablets prevents this absorption. Caesium-137 is also a particular threat because it behaves like potassium and is taken-up by the cells throughout the body. Strontium-90 behaves like calcium, leading to accumulation in bones and teeth.[181]

[edit] Japanese government reaction

[edit] Evacuations

After the declaration of a nuclear emergency by the Government at 19:03 on 11 March, the Fukushima prefecture ordered the evacuation of an estimated 1,864 people within a distance of 2km from the plant. This was extended to 3 kilometres (1.9 mi) and 5,800 people at 21:23 by a directive to the local governor from the Prime Minister, together with instructions for residents within 10 kilometres (6.2 mi) of the plant to stay indoors.[46][44] The evacuation was expanded to a 10 kilometres (6.2 mi) radius at 5:44 on 12 March, and then to 20 kilometres (12 mi) at 18:25, shortly before ordering use of sea water for emergency cooling.[46][185]
Evacuations were also ordered around the nearby Fukushima II (Daini) plant. Residents within 3 kilometres (1.9 mi) were ordered to evacuate at 07:45 on 12 March, again with instructions for those within 10km to stay indoors. Evacuation was extended to 10km by 17:39.[46] A journalistic investigation was stopped 60 kilometres (37 mi) from the plants by police.[186] Over 50,000 people were evacuated during 12 March.[187] The figure increased to 170,000–200,000 people on 13 March, after officials voiced the possibility of a meltdown.[10][11]
On the morning of 15 March, the evacuation area was again extended. Prime Minister Naoto Kan issued instructions that any remaining people within a 20km (12 mile) zone around the plant must leave, and urged that those living between 20km and 30km from the site should stay indoors.[188][189] A 30km no-fly zone has been introduced around the plant.

[edit] Statements on meltdown possibility

In a press conference, the chief spokesman of the Japanese nuclear authorities was translated into English as having said that a nuclear meltdown may be a possibility at Unit 1.[190] Toshihiro Bannai, director of the international affairs office of Japan's Nuclear and Industrial Safety, in a telephone interview with CNN, stated that a meltdown was possible.[191][190] However, the Japanese prime minister soon indicated that a nuclear meltdown was not in progress and emphasized that the containment of Unit 1 was still intact. After the statement, the government added that the claim of a meltdown had been mistranslated.[190] The temperature inside the reactor was not reported, but Japanese regulators said it was not dropping as quickly as they wanted.[192] At 12:33 JST on 13 March 2011, the Chief Cabinet Secretary of the Japanese government, Yukio Edano, was reported to have confirmed that there was a “significant chance” that radioactive fuel rods had partially melted in Unit 3 and Unit 1, or that "it was 'highly possible' a partial meltdown was underway".[11] “I am trying to be careful with words … This is not a situation where the whole core suffers a meltdown”.[122] Soon after, Edano disclaimed that a meltdown was in progress. He stated that the radioactive fuel rods had not partially melted and he emphasized that there was no danger for the health of the population.[193][194] Cabinet Secretary Edano later said that there were signs that the fuel rods were melting in all three reactors. "Although we cannot directly check it, it's highly likely happening".[195]

[edit] Prime Minister visits plant

The Prime Minister of Japan, Naoto Kan, visited the plant for a briefing on 12 March 2011.[196] He has been frequently quoted in the press, calling for calm and minimizing exaggerated reports of danger. [197]

[edit] Accident rating

At 01:17 JST on Sunday 13 March 2011, the Japan Atomic Energy Agency announced that it was rating the Fukushima accidents at Level 4 (accident with local consequences) on the 0–7 International Nuclear Event Scale (INES), below the Three Mile Island (TMI) accident in seriousness[20] which was at Level 5. A rating of four would make the severity of the Fukushima event comparable to Sellafield accidents between 1955 and 1979.
On Monday, March 14, 2011, three Russian experts stated that the nuclear accident should be classified at Level 5, perhaps even Level 6.[198] One day later, the French nuclear safety authority ASN said that the Fukushima plant could be classified as a Level 6.[199]

[edit] Requests for help

The Japanese government has asked the United States to provide cooling equipment to the Dai-ichi plant to help resolve the crisis. As of 15 March 2011, the United States had provided Japan with 7,200 pounds of special equipment to help monitor and assess the situation at the plant.[200][201]

[edit] Japanese business reaction

In financial markets, the plant operator TEPCO's shares dropped 24% on Monday, March 14, the first day of trading after the tsunami,[202] and Japan's Nikkei 225stock index fell 6% on the day. The Nikkei then plunged another 11% on March 15 after the government warned of elevated radiation risks.[203] On 16 March the index recovered 5.7%.[204]

[edit] International reaction

[edit] Assessments by scientists

David A. Lochbaum of the Union of Concerned Scientists pointed to the fact that pools holding spent fuel rods in the top level of reactor buildings at the Fukushima I complex could release even larger quantities of hazardous substances than a reactor meltdown. This is thought to be possible if the pools should dry out so that the old fuel rods might catch fire.[205] The New York Times reported that such pools and the rods they contain had to be kept cooled at reactor buildings nearby those probably affected by meltdowns, and that some of them had very probably already been exposed to the atmosphere, particularly "on a damaged storage pool for spent nuclear fuel at the No. 2 reactor at Daiichi",[206][207] as well.
Masashi Goto, a former Toshiba reactor researcher and designer, warned that mixed oxide (MOX) fuel used in reactor 3 of the Fukushima I (Daiichi) plant containedplutonium, which is more toxic than the fuel used in the other reactors.[208]

[edit] Business

As a result of the incident, stock prices of many energy companies reliant on nuclear sources and listed on stock exchanges have dropped, while renewable energy companies have increased dramatically in value.[209] Stock in some health and life insurance companies have also dropped.[210]

[edit] European Union

In an interview on ORF2, Austrian Environment Minister Nikolaus Berlakovich said that he would enter a request at the environmental meeting in Brussels on 14 March 2011 for a review of reactor safety. He emphasized both coolant and containment and compared these measures to reviews of the financial system after the banking crisis of 2008.[211]

[edit] Germany

During the chancellorship of Gerhard Schröder, the social democraticgreen government had decreed Germany's final retreat from using nuclear power by 2022, but the phase-out plan was delayed in late 2010, when during the chancellorship of Angela Merkel the conservativeliberal government decreed a 12-year delay of the schedule.[212] This delay provoked protests, including a human chain of 50,000 from Stuttgart to the nearby nuclear plant in Neckarwestheim. This protest had long been scheduled for March 12, which now happened to be the day of the explosion of reactor block 1.[213] On 14 March 2011, in response to the renewed grave concern about the use of nuclear energy the Fukushima incident raised in the German public and in light of upcoming elections in several German states, Merkel declared a 3-months moratorium on the reactor lifespan extension passed in 2010.[214] It is not yet clear what Merkel's moratorium will mean in practice, but there is a possibility that older nuclear plants like Neckarwestheim I could be shut down for good.[214] On March 15, the German government announced that it would temporarily shut down 7 of its 17 reactors, i.e. all reactors that went online before 1981.[215]

[edit] China

During a News Conference of the Annual Meeting of National People’s Congress of China, The Deputy Chief of Environmental Protection Authority of China, Mr Lijun Zhang said: "We are concerned about the damage of the Nuclear Facility of Japan, concerned about the further development of this issue, we will learn from it, and will take it into account when we make strategic planning for the nuclear energy development in the future. However, our decision on development of more nuclear power plants and current arrangement on nuclear energy development will not be changed."[216]

[edit] India

The Prime Minister of India, Dr. Manmohan Singh, has ordered the Nuclear Power Corporation of India Limited to review the safety systems and designs of all the nuclear power reactors. The Government of India is also looking to put in place additional environmental safeguards to ensure safety of newly proposed nuclear reactors. [217]

[edit] Pakistan

The Government of Pakistan has ordered the Pakistan Atomic Energy Commission (PAEC) to check and review the power plants security, system, and designs of all the atomic power reactors in the country.[218] The Pakistan Nuclear Regulatory Authority (PNRA) issued safety guidelines in plants, and ordered to re-evaluate the designs of KANUPP-II and Chashman nuclear power complex in Chashman city.[219] The PAEC is continuously monitoring the flow of events in nuclear plants in Japan in the wake of the recent earthquake and tsunami there, APP concluded.[220]

[edit] Russia

Russian authorities were reported ready to evacuate the Kuril Islands and Sakhalin, if needed.[221]

[edit] United States

The New York Times editorialized that "the unfolding Japanese tragedy also should prompt Americans to closely study our own plans for coping with natural disasters and with potential nuclear plant accidents to make sure they are, indeed, strong enough. We've already seen how poor defenses left New Orleans vulnerable toHurricane Katrina and how industrial folly and hubris led to a devastating blowout and oil spill in the Gulf of Mexico".[222]
Mark Hibbs, a senior associate at the Carnegie Endowment's Nuclear Policy Program, reiterated the theme that "This was a wake-up call for anyone who believed that, after 50 years of nuclear power in this world, we have figured it out and can go back to business as usual."[223]

[edit] See also

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