Fukushima I Nuclear Reactor Accidents ~ Nuclear Power Plants

The Fukushima I nuclear reactor accidents are a series of ongoing events at the Fukushima I Nuclear Power Plant, following the 11 March 2011 Sendai earthquake and tsunami. As of 13 March, other incidents are ongoing at the Fukushima II plant 11.5 kilometres (7.1 mi) to the south.

On 11 March 2011, the Japanese government declared a "nuclear power emergency" due to a loss of coolant and evacuated thousands of residents living close to Fukushima I. The next day, while evidence for partial meltdown of the fuel rods in Unit 1 was growing, a hydrogen explosion destroyed the upper story of the building housing Reactor Unit 1 and injured four workers, but the container of the reactor remained intact.

On 13 March 2011, that a partial meltdown at Unit 3 has occurred appeared also possible. As of 1pm 13 March, JST, both reactors 1 and 3 had been vented and were being filled with water and boric acid to both cool and inhibit further nuclear reactions. Unit 2 was reported to have lower than normal water level but to be stable, although pressure inside the containment vessel is high. According to a Reuters report of 3:05pm EDT of 13 March 2011, all three reactors were being cooled with seawater.

On 13 March 2011, the Japan Atomic Energy Agency announced that it was rating the Fukushima accidents at 4 (accident with local consequences) on the International Nuclear Event Scale (INES). 170,000–200,000 people were evacuated after officials voiced the possibility of a meltdown.

On 14 March, the reactor building for Unit 3 exploded injuring eleven people. There was no release of radioactive material beyond that already being vented but blast damage affected water supply to Unit 2. 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. TEPCO shares dropped 24% in this first day of trading after the tsunami.

On 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. An explosion in Unit 2 occurred at 06:14 JST in the "pressure suppression room", causing some damage to the reactor’s containment system. 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.

16 March: at 5:45 a.m. JST, Kyodo News reported that a worker spotted new flames on the fourth story of Unit 4, where the spent fuel pool is located. This cast into doubt the earlier hope that the Tuesday blaze in the Unit 4 housing was caused by lubricating oil pumps; instead TEPCO officials acknowledged it was possible the spent fuel rods are uncovered and overheating, remarking that "the possibility of a re-criticality is not zero." 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 of remaining workers at the plant had been placed on standby because of the dangerously rising levels of radioactivity up to 1000 mSv/h. Later reports stated that TEPCO had temporarily suspended operations at the facility due to radiation spikes and had pulled all their employees out. 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. Late in the evening, Reuters reported that water was being poured into reactors 5 and 6.

17 March: During the morning, Self-Defense Force helicopters dropped four containers of water on the spent fuel pools of Units 3 and 4. In the afternoon it was reported that the Unit 4 spent fuel pool is full with water and none of the fuel rods are exposed. Construction work was started to supply a working external electrical power source to all six units of Fukushima I.

18 March: Pursuant to a request from Tokyo Governor Shintaro Ishihara, Tokyo Fire Department dispatched thirty fire engines with 139 fire-fighters and trained rescue team at approximately 03:00 JST. These include a fire truck with a 22 m water tower; all units will join Japan Defense Forces fire equipment which is already deployed.JDF anticipated utilizing TFD equipment to address low water which was confirmed to exist in unit 4 and also an emergent concern with unit 3, which appeared to be more problematic than previously believed. W inds were forecast to shift to the northeast, which would continue to be toward the sea.For the second consecutive day, high radiation levels have been detected in an area 30 kilometers (18.6 miles) northwest of the damaged Fukushima Daiichi nuclear plant. The reading was 150 microsieverts per hour. Human exposure to that level of radiation for six to seven hours would result in absorption of what is considered safe in a year.

Fukushima 1 Reactor unit 1

Cooling problems at unit 1

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. Potentially radioactive steam was released from the primary circuit into the secondary containment area to reduce mounting pressure.

In the early hours of 12 March TEPCO reported that radiation levels were rising in the turbine building for Reactor Unit 1 and that it was considering venting hot gas from the Unit 1 reactor vessel into the atmosphere, which could result in the release of radiation. 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. At 02:00 JST, the pressure inside the reactor containment was reported to be 600 kPa (6 bar or 87 psi), 200 kPa higher than under normal conditions. At 05:30 JST the pressure inside Reactor 1 was reported to be 2.1 times the "design capacity", 820 kPa. Rising heat within the containment area would have led to increasing pressure, with both cooling water pumps and ventilation fans for driving air through heat exchangers within containment dependant on electricity.

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." The gamma ray radiation recorded on the main gate was increased from 69 nanogray/hour (nGy/h) (04:00 JST, 12 March) to 866 nGy/h 40 minutes later and reached the peak of 385.5 μSv/hour (1μSv = 0.1 mrem, 1 μGy = 1000 nGy) at 10:30 JST. At 13:30 JST, radioactive caesium-137 and iodine-131 was detected near reactor 1, which indicates that some of the core was exposed to air due to a partial-meltdown or other damage of the nuclear fuel. The NHK website reported that cooling water had lowered so much that parts of the nuclear fuel rods were exposed. Radiation levels at the site boundary exceeded the regulatory limits. Kyodo News Service later reported that partial melting may have occurred. On 14 March 2011, Kyodo News reported the radiation levels have continued to increase on the premises, measuring at two different locations at 2:20 AM an intensity of 751 μSv/hour on one location and at 2:40 AM an intensity of 650 μSv/hour at another location on the premises.

Government statements on possibility of meltdown

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. 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. 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. The temperature inside the reactor was not reported, but Japanese regulators said it was not dropping as quickly as they wanted. The chief spokesman of the Japanese government, Yukio Edano, confirmed that there was a "significant chance" that radioactive fuel rods had partially melted in Unit 1. "I am trying to be careful with words... This is not a situation where the whole core suffers a meltdown."

Explosion of reactor building

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. This building has been designed as a secondary containment of radioactive materials, but not to withstand the high pressure of an explosion: in the Fukushima I reactors the primary containment consists of "drywell" and "wetwell" concrete structures immediately surrounding the reactor pressure vessel.

Experts soon agreed that the cause was a hydrogen explosion. Almost certainly the hydrogen was formed inside the reactor vessel because of falling water levels, and this hydrogen then leaked into the containment building. Safety devices should ignite the hydrogen before explosive concentrations are reached but apparently these systems failed.

Officials indicated that the container of the reactor had remained intact and there had been no large leaks of radioactive material, although an increase in radiation levels was confirmed following the explosion. ABC news reported that according to the Fukushima prefectural government, the hourly radiation from the plant reached 1,015 µSv. Two independent nuclear experts cited design differences between the Chernobyl Nuclear Power Plant and the Fukushima I Nuclear Power Plant, one of them saying he did not believe that a Chernobyl-style disaster will occur.

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. 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. 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. At 23:00 JST TEPCO announced that due to the quake at 22:15 the filling of the reactor had been temporarily stopped but has been resumed after a short while. 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, since it is not cost effective to decontaminate.

Seawater used for cooling

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. Radiation levels around the plant were measured at around 0.03 µSv/h at 05:00 and 15:00 on 14 March.

Fukushima 1 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 circa 1 hour), and stable water levels were reported. Power was achieved by mobile power units, while preparations were made to perform pressure venting. According to a Reuters report of 3:05pm EDT of 13 March 2011, this reactor was also being cooled with seawater.

Cooling problems at Fukushima reactor 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. This was caused when fuel for pumps ran out. 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. Jiji later reported that according to TEPCO, a meltdown cannot be ruled out.

At 22:29 JST, NHK reported that 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. 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. At 21:37 JST the measured radiation levels at the gate of the plant had reached the a maximum of 3130 μSv per hour, which was enough to reach the annual limit for non-nuclear workers in twenty minutes, but had fallen back to 326 μSv/hr by 22:35.

It was believed that around 23:00 JST the 4m long fuel rods in the reactor were fully exposed for the second time. 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. To replenish the water, the contained pressure would have to be lowered first by opening a valve of the vessel. Due to an accident the unit's air flow gauge was turned off. With the gauge turned off, flow of water into the reactor was blocked, leading to full exposure of the rods.

As of 04:11 JST (March 15), water was being pumped into the reactor of unit 2 again.

At 01:38 CET (10:38 JST, March 15), water level was reported to be at 1.20 meters and rising.

Explosion in reactor unit 2 building

An explosion was heard after 6:10 JST on 15 March in unit 2, possibly damaging the pressure-suppression system, which is at the bottom part of the container. The radiation level was reported to exceed the legal limit and the plant's operator started to evacuate workers from the plant. Soon after, Kyodo News reported that radiation had risen to 8,217 μSv per hour around two hours after the explosion—about eight times what one usually is exposed to within a whole year—and again down to 2,400 μSv, shortly after. Three hours after the explosion the radiation has risen to 11,900 μSv per hour.

Japanese nuclear authorities initially said that the containment vessel had not been damaged as a result of the explosion. Later reports indicated that the containment vessel had, in fact, been damaged in the explosion, that radiation levels had spiked, and that workers were being evacuated. If all workers leave the plant, the nuclear fuel in the reactors is likely to melt down, prompting a release of radioactive material. The suppression pool beneath the reactor may have cracked.

Fukushima 1 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.

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. 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. At one point, the top three meters of mixed oxide (MOX) fuel rods were exposed to the air.

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" and manual venting took place at 08:41 and 09:20. At 09:25 JST on 13 March 2011, operators began injecting water containing boric acid into the reactor via a fire pump. When water levels continued to fall and pressure to rise, the injected water was switched to sea water at 13:12. By 15:00 it was noted that despite adding water the level in the reactor did not rise and radiation had increased. 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.

At 12:33 JST on 13 March 2011, the chief spokesman 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 just as in unit 1, or that "it was 'highly possible' a partial meltdown was underway". “I am trying to be careful with words ... This is not a situation where the whole core suffers a meltdown”. He added 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. Soon after, Edano disclaimed that a meltdown was in progress. He stated that there is no “significant chance” that radioactive fuel rods had partially melted and he emphasized that there is no danger for the health of the population. He indicated that increased radiation had been measured inside the reactor.

Fukushima I Explosion of reactor unit 3 building

At 11:15 JST on 14 March 2011, a building surrounding Reactor 3 of Fukushima 1 exploded as well, presumably due to the ignition of built up hydrogen gas. There is no health risk reported, though 600 people have been ordered to stay indoors. Within minutes, it was reported that as with reactor 1, the outer reactor building was blown apart, but the inner containment vessel was not breached. TEPCO stated that one worker was injured and seven missing.

Hydrogen Blast

A hydrogen explosion occurred Monday morning at the quake-hit Fukushima No. 1 nuclear power plant’s troubled No. 3 reactor, the government’s nuclear safety agency said.

The 11:01 a.m. incident came after a hydrogen explosion hit the No. 1 reactor at the same plant Saturday, and prompted the Nuclear and Industrial Safety Agency to urge residents within a 20-kilometer radius to take shelter inside buildings.

It also followed a report by Tokyo Electric Power Co., the plant’s operator, to the government earlier in the day that the radiation level at the plant had again exceeded the legal limit and pressure in the container of the No. 3 reactor had increased.

The Fukushima No. 1 nuclear plant has been shut down since a magnitude 9.0 quake struck northeastern and eastern Japan on Friday, but some of its reactors have lost their cooling functions, leading to brief rises in the radiation level over the weekend.

On Monday, radiation at the plant’s premises rose over the benchmark limit of 500 micro sievert per hour at two locations, measuring 751 micro sievert at the first location at 2:20 a.m. and 650 at the second at 2:40 a.m., according to the report.

Fukushima 1 Reactor unit 4

Although the reactor unit 4 was not functioning at the time of the earthquake, fire was observed coming from the reactor on 15 March. Authorities believe that the used spent fuel is the cause of this fire, or that fallout from the explosions at units 1-3 supplied the ignition source.

At the time of the earthquake unit 4 had been shut down for a scheduled periodic inspection since 30 November 2010. All fuel rods had been transferred in December 2010 from the reactor to the spent fuel pool on the top floor of the reactor building where they were held in racks containing boron to damp down any nuclear reaction. These recently active fuel rods were hotter and required more cooling than the spent fuel in units 5 and 6. At 04:00 JST on Monday 14 March water in the pool had reached a temperature of 84°C compared to a normal value of 40-50°C.At approximately 06:00 JST on 15 March, a loud explosion was heard within the power station, and later it was confirmed that the 4th floor rooftop area of the Unit 4 reactor building had sustained damage. 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. TEPCO said workers extinguished the fire by 12:00. As radiation levels rose, some of the employees still at the plant were evacuated. The reason for the fire seems to have been a hydrogen explosion.

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. Government speaker Edano has stated that there was no continued release of radiation. 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/h dose rate for 10 hours of a day, or a 400 mSv/h dose rate for 2.5 hours of a day.

Japan's nuclear safety agency reported two holes, each 8 meters square (64 m² or 689 sq. feet -- not 8 sq. meters each) in a wall of the outer building of the number 4 reactor after an explosion there. Further, at 17:48 JST it was reported that water in the spent fuel pool might be boiling.

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. Seventy staff remained on site but 800 had been evacuated.^[158] By 22:30 JST, TEPCO was reported to be unable to pour water into No. 4 reactor's storage pool for spent fuel. At around 22:50 JST, it was reported that TEPCO was considering using helicopters to drop water on the spent fuel storage pool. However, TEPCO soon dismissed the option of helicopters because of concerns over safety and effectiveness. Chinook helicopters have been used in an attempt to dump water but have a maximum payload of around 10 tonnes TEPCO went on to consider the use of high-pressure fire hoses instead.

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. 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. 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." Technicians reportedly considered spraying boric acid on the building from a helicopter.

Possibility of criticality in the spent fuel pool

This new fire cast into doubt the earlier hope that the Tuesday blaze in the Unit 4 housing was caused by lubricating oil pumps; instead at approximately 14:30, TEPCO announced its belief that the storage pool may have begun boiling, raising the possibility that exposed rods would reach criticality. BBC commented that criticality would not mean a nuclear bomb-like explosion; however, a sustained release of radioactive materials would be a possible scenario.

Around 20:00 JST on 16 March it was planned to use a police water cannon to spray water on unit 4.

Iouli Andreev, former director of the Soviet Spetsatom clean-up agency involved in the Chernobyl clean-up, as well as Laurence Williams, professor of nuclear safety at the University of Central Lancashire, speculate that the Fukushima management could have been engaged in an unsafe industry practice of re-racking spent rods in the pool well beyond its rated capacity, in effect heightening danger of melting and pool boil-off.

On 16 March the chairman of United States Nuclear Regulatory Commission, Gregory B. Jaczko, said in Congressional testimony that the NRC believes all of the water in the spent fuel pool has boiled dry. Japanese nuclear authorities and TEPCO contradicted this report, but later in the day Jaczko stood by his claim saying it had been confirmed by sources in Japan. At 1PM TEPCO observed via helicopter the pool had not boiled off, nor were any fuel rods exposed.

Fukushima 1 Reactor unit 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, Kyodo News said that there were rods in the pools, but only one-third as many in the pools as compared to Unit 4.

Government spokesman Edano stated on 15 March that reactors 5 and 6 were being closely monitored, as cooling processes were not functioning well. At 21:00 on 15 March water levels in unit 5 were reported to be 2 m above fuel rods, but were falling at a rate of 8 cm per hour. Unit 6 was reported to have operational diesel generated power and this was to be used to power pumps in unit 5 to supply more water.

The removal of roof panels from reactor buildings 5 and 6 was being considered in order to allow any hydrogen build-up to escape. The BBC later reported that units 5 and 6 were believed to be heating up. At 18:31 on 16 March, TEPCO was reported to be pouring water into both reactors.

Radioactive levels and radioactive contamination

Radiation levels at the stricken Fukushima I power plant have varied up to 1,000 mSv/h(millisievert per hour), which is a level that can cause radiation sickness. The level of radiation within the 20 km exclusion zone surrounding the power plant is such that people have been advised to evacuate, and people within the 20-30km zone are being advised to stay indoors. The radiation is believed to come from short-lived isotopes (noble gases and nitrogen) that escape mixed with steam through venting, or with the hydrogen explosions.

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 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," 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".

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. 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". 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. Thousands of Tokyo residents are reported to have left for cities further south, although Edano insisted that levels in Greater Tokyo were not hazardous.

On 16 March power plant staff were briefly evacuated after smoke rose above the plant and radiation levels surged to 1,000 mSv/h before coming down to 800–600 mSv/h, and staff returned. Japan's defence ministry criticized the nuclear safety agency and TEPCO after some of its troops were possibly exposed to radiation when working on the site. The Japan's ministry of science measured radiation levels of up to 0.33 millisieverts per hour 20 kilometers northwest of the power plant.

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.

The United Nations are predicting that a radiation plume from the stricken Japanese reactors will reach the USA by Friday March 18. Health and nuclear experts emphasize that radiation in the plume will be diluted and, at worst, would have extremely minor health consequences in the United States.

Government reaction

The Prime Minister of Japan, Naoto Kan, visited the plant for a briefing on 12 March 2011.

At 01:17 JST on Sunday 13 March 2011, the Japan Atomic Energy Agency announced that it was rating the Fukushima accidents at 4 (accident with local consequences) on the 0–7 International Nuclear Event Scale (INES), below the Three Mile Island accident in seriousness which was at 5, a rating that would make the severity of the Fukushima event comparable to Sellafield accidents between 1955 and 1979 that were also at 4.

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. Prime Minister Naoto Kan also issued instructions that140.000 peoples in fukushima to seal up themself and 800 workers must be evacuated.

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. 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.

Evacuations were also ordered around the nearby Fukushima II (Daini) plant. Residents within 3 kilometres (1.9 mi) were ordered to evacuate at 7:45 on 12 March, again with instructions for those within 10km to stay indoors. Evacuation was extended to 10km by 17:39. BBC correspondent Nick Ravenscroft was stopped 60 kilometres (37 mi) from the plants by police. Over 50,000 people were evacuated during 12 March. The figure increased to 170,000–200,000 people on 13 March, after officials voiced the possibility of a meltdown.

Fukushima I Nuclear Accidents Effect on employees and residents

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. Air traffic has been restricted in a 20-kilometre (12 mi) radius around the plant, according to a NOTAM. 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."

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.

At 22:53 JST Tokyo Broadcasting System (TBS), quoting Fukushima representatives, has reported that there was an evacuation of 30 staff members and 60 patients due to the explosion. From those evacuees three patients received a checkup for radiation exposure by the hospital staff at Futaba, a town 5.6 km (3.5 miles) from the power plant. All three patients required decontamination, but about 90 other evacuees may also require decontamination.

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. 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, however for workers performing emergency services EPA guidance on dose limits is 100 mSv when "protecting valuable property" and 250 mSv when the activity is "life saving or protection of large populations." A 250 mSv dose is estimated to increase one's lifetime risk of developing fatal cancer from about 20% to about 21%, and chronic 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. Symptoms of radiation poisoning typically emerge with a 1000 mSv total dose over a day.

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. 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. The general population faced separate risks from chronic exposure to lower-level contaminants released into the environment.