Fukushima nuclear accident

"Fukushima nuclear incident" redirects here. For the incidents at Fukushima II, see Fukushima II Nuclear Power Plant.

Fukushima nuclear accident

File:Fukushima I by Digital Globe 2.jpg Satellite image on March 16 of the four damaged reactor buildings
Date	11 March 2011 (2011-03-11)
Time	14:46 JST (UTC+09:00)
Location	Ōkuma, Fukushima, Japan
Coordinates	37°25′17″N 141°1′57″E
Outcome	Level 4 (Accident with local consequences) preliminary INES rating by Japanese authorities as of 12 March, unofficial level 6 (Serious Accident) rating by French and Finnish nuclear authorities.
File:Japan Nuclear power plants map.gif

The Fukushima I nuclear accidents (福島第一原子力発電所事故, fukushima daiichi genshiryoku hatsudensho jiko) are a series of ongoing equipment failures and releases of radioactivity at the Fukushima I Nuclear Power Plant, following the 2011 Tōhoku earthquake and tsunami on 11 March 2011. 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 maintenance. The remaining reactors were shut down automatically after the earthquake, but the subsequent tsunami flooded the plant, knocking out emergency generators needed to run pumps which 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 buildings housing reactors 1, 3 and 4; an explosion damaged reactor 2's containment; and multiple fires broke out at reactor 4. Fears of radiation leaks led to a 20 km (12-mile) radius evacuation around the plant.

TEPCO employees and workers from other companies not involved in essential work were temporarily evacuated after an explosion was heard in the supression chamber of reactor building 2. Employees returned after it was confirmed that there had not been a containment breach but evacuated again on 16 March following a spike in radiation. Staff returned but the status of the reactors remains extremely hazardous.

To date, radiation leaks beyond the plant's boundaries have not been high enough to constitute any significant danger to the public, but there remains a risk such leaks may occur. The gravity of the disaster is such that many international leaders have expressed concerns. The Japanese Government and TEPCO have been criticized for poor communications over the incident.

Summarised daily events

Main article: Timeline of the Fukushima nuclear accidents

• 11 March: 14:46 JST (05:46 UTC): Tōhoku earthquake and tsunami. 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 20 km, affecting 170,000–200,000 people, and residents within a further 10 km 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, but only one worker was confirmed to be ill by the Tokyo Electric Power Company (TEPCO). 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 Fukushima I and Fukushima II.

• 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 unit 1. The explosion injured four workers, but the reactor containment inside the building remained intact. 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. Hydrogen is formed when overheated zircaloy reactor fuel rods oxidize with water. Operators of the plant were authorised to commence using sea water for emergency cooling, which will permanently damage the 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. Unit 2 was reported to have lower than normal water level but to be stable, although pressure inside the containment vessel was high. 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).

• 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 ( accident with wider consequences) or even a 6 (serious accident) on INES.

• 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 the "pressure suppression room" caused some damage to unit 2’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. A radiation equivalent dose rate was observed at one location in the vicinity of unit 3 of 400 millisievert per hour (400 mSv/h).

• 16 March: At approximately 14:30 on 16 March, TEPCO announced its belief that the fuel rod storage pool—which is located outside the containment area—may have begun boiling, raising the possibility that exposed rods would reach criticality. 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 10 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. Starting at 7 p.m., police and fire water trucks with high pressure hoses attempted to spray water into the unit 3 reactor, with unknown results. Japanese authorities have informed the IAEA that engineers were able to lay an external grid power line cable to unit 2. The operation was completed at 08:30 UTC. TEPCO estimated fifteen hours before power could be established, during which time they will put more water onto the site.

• 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. Winds were forecast to shift to the NE, which would continue to be toward the sea.

Fukushima I Nuclear Power Plant

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. Excess steam from the drywell enters the wetwell water pool via downcomer pipes.

SF

Spent fuel pool area.

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, about 210 kilometers (130 miles) north of Tokyo. 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. 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. 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. 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.

Units 1–5 have a Mark 1 type (light bulb torus) containment structure, unit 6 has Mark 2 type (over/under) containment structure. From September 2010, unit 3 has been fueled by mixed-oxide (MOX) fuel.

Cooling requirements

Diagrammatic representation of the cooling systems of a BWR

See also: Decay heat § Power reactors in shutdown

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 (~100 MW for reactors 2 and 3) and then decrease over several days before reaching cold shutdown levels. 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.

When a reactor is shut down, decay heat is usually removed from the fuel by circulating water over it. High pressure systems circulate water through the reactor pressure vessel and pipework. The heated water is cooled in water-water heat exchangers, the heat passing to sea water circulated through the secondary side of the heat exchangers. In this way the decay heat is pumped out to sea and disperses. The systems which do this are typically called residual heat removal, for normal removal of decay heat during a planned shutdown, or emergency core cooling systems for core cooling after an accident. They may be the same systems, or share pumps, valves, heat exchangers etc.

At the time of the earthquake on March 11, units 1, 2, and 3 were operating and units 4, 5, and 6 were out of service for scheduled maintenance and refueling. When the earthquake occurred, units 1, 2, and 3 were shut down as a standard response to an earthquake, stopping on-site generation of electricity. A few minutes later, incoming electrical supplies from the grid were also lost as a result of the earthquake. Initially on site diesel generators supplied power to the residual heat removal system, cooling the reactor cores as designed. The diesel generators failed shortly after the tsunami, and alternative power arrangements were insufficient to support the control systems for the normal or emergency cooling systems. Had these systems continued to operate, the core temperature would have remained under control, with no damage to the reactor or its fuel, and even with some fuel damaged and partially melted the reactor could have been brought under control. The first attempts to restore electrical connections to the grid to enable the cooling systems to be operated were reported on 17th March. This effort was impeded initially by the tsunami damage in the area of the plant and then by the damage caused by the explosions at the plant.

Merely pumping water into the reactor containment does not get the water into the pressurised reactor vessel and pipework and into contact with the fuel. Fire trucks and water cannon cannot achieve the required pressures. It appears that the operators are trying to fill the reactor containment with water to cool the molten core if it melts through the steel reactor pressure vessel (or to keep the exterior of the steel reactor pressure vessel cool enough to prevent it from melting, if molten fuel reaches the bottom of the steel pressure vessel). This would generate large amounts of steam within the containment. If the Fukushima reactors are fitted with containment coolers - heat exchangers like the residual heat removal heat exchangers - then this steam could be be condensed and recirculated. If there are no containment coolers, or they are not working, then the steam would have to be vented and replaced with more water from outside the containment.

A decay heat output of 10 MW (typical of a power reactor a few days after shutdown) has the capacity to boil off over three hundred tonnes of water per day, which puts the requirement for cooling water in context.

Direct effect of the 2011 earthquake and tsunami

See also: 2011 Tōhoku earthquake and tsunami

The 2011 Tōhoku earthquake and tsunami, categorised as 9.0 M_W on the moment magnitude scale occurred on 11 March 2011, at 14:46 Japan Standard Time (JST) off the northeast coast of Japan. 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. When the earthquake was detected, units 1, 2 and 3 underwent an automatic shutdown (called scram).

After the reactors shut down, electricity generation stopped. Normally the plant could use the external electrical supply to power cooling and control systems, but the earthquake had caused major damage to the power grid. Emergency diesel generators started but stopped abruptly at 15:41, ending all AC power supply to the reactors. The plant was protected by a sea wall, but the tsunami topped this sea wall, flooding the low lying generator building. 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".

After the failure of the diesels, emergency power for control systems was supplied by batteries that would last about eight hours. Batteries from other nuclear plants were sent to the site and mobile generators arrived within 13 hours, but work to connect portable generating equipment to power water pumps was still continuing as of 15:04 on 12 March. Generators would normally be connected through switching equipment in a basement area of the buildings, but this basement area had been flooded. It had been conjectured that a new powerline to the complex would revive some of the electric cooling system pumps that have failed. After subsequent efforts to bring water to the plant, efforts would shift to that strategy eventually resulting in cable emplacement reported at approximately 08:30 UTC.

Reactor unit 1

Cooling problems at unit 1

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 temporarily cleared when water level monitoring 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 unit 1 and that it was considering venting some of the mounting pressure into the atmosphere, which could result in the release of some 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 were 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 gases through heat exchangers within the containment dependent on electricity. 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. 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." 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. At 13:30 JST, radioactive caesium-137 and iodine-131 was detected near reactor 1, which indicates that some of the core was exposed 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 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. On 16 March the maximum readings peaked at 10850 μSv/hour.

Explosion of reactor 1 building

Visualization of before and after damage to Fukushima power plant, Unit 1

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. 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.

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. Exposed Zircaloy metal clad 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, although an increase in radiation levels was confirmed following the explosion. 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. 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.

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.

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."

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.

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 circulated 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 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 the reactor would cool down in 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. Circulating seawater in the reactor will contaminate the unit with salt, and the reactor will have to be decommissioned.

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.

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. Some power was achieved by mobile power units, while preparations were made to perform pressure venting. A report in the New York Times suggested that plant officials initially concentrated efforts on a damaged fuel storage pool at unit 2, distracting attention from problems arising at the other reactors.

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. 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, 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 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, but had fallen back to 0.326 millisieverts per hour by 22:35.

It was believed that around 23:00 JST the 4 m 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. 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.

As of 04:11 JST, 15 March, water was being pumped into the reactor of unit 2 again. At 10:38 JST, 15 March, water level was reported to be at 1.20 meters and rising.

Explosion in reactor 2 building

An explosion was heard after 06:14 JST on 15 March in unit 2, possibly damaging the pressure-suppression system, which is at the bottom part of the containment vessel. 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. Only a skeleton crew of 50 men, also referred to as the Fukushima 50, was left at the site. Soon after, radiation equivalent dose rates had risen to 8.2 mSv/h around two hours after the explosion and again down to 2.4 mSv/h, shortly after. Three hours after the explosion, the rates had risen to 11.9 mSv/h.

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.

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". The Nuclear and Industrial Safety Agency stated 33% of the fuel rods were damaged, in news reports the morning of 16 March.

Work was to continue through the night aimed at reconnecting mains power to the reactor from the transmission grid once water spraying of unit 3 ceased at 20:09 on 17 March.

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. Units 3 and 4 have a shared control room.

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 not covered by coolant.

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 primary containment vessel (PCV) 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 2 m below the top of reactor core. Other readings suggested that this could not be the case and the gauge was malfunctioning.

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. 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 an equivalent dose rate of 400 mSv/h was detected, but this might have been due to debris from the explosion in unit 4.

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. 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. 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 40 kilometers 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. Eleven people were reported injured in the blast.

Spent fuel pool

Around 10:00 JST, 16 March, NHK helicopters flying 30 km away videotaped white fumes 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. The control room for reactors 3 and 4 was evacuated at 10:45 JST but staff were cleared to return and resume water injection into the reactor at 11:30 JST. At 16:12 JST Self Defence Force (SDF) Chinook helicopters were preparing to spray water on unit 3, where white fumes 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 50 mSv. At 21:06pm JST government reported that major damage to reactor 3 was unlikely but that it nonetheless remained their highest priority. Early on 17 March, TEPCO requested another attempt by the military to put water on the reactor using a helicopter and four helicopter drops of seawater took place around 10:00 JST. The riot police used a water cannon to spray water onto the top of the reactor building.

Reactor unit 4

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 an upper floor of the reactor building where they were held in racks containing boron to damp down any nuclear reaction. These recently active fuel rods produced more decay heat and thus required more cooling than the spent fuel in units 5 and 6. As of November 2010, the Daiichi NPS stored approximately 3,450 spent fuel assemblies in the 6 spent fuel pools. 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. IAEA was advised that temperature remained 84°C at 19:00 JST on 15 March, but as of 17 March, no information was reported for 16 March.

At approximately 06:00 JST on 15 March, an explosion—probably caused by hydrogen accumulating near the spent fuel pond—damaged the 4th floor rooftop area of the Unit 4 reactor as well as part of the adjacent Unit 3. At 09:40 JST, 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.

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 "high radiation".

Japan's nuclear safety agency reported two holes, each 8 meters square (64 m² or 689 sq. feet) 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. 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 delayed using the option of helicopters because of concerns over safety and effectiveness. 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

Japanese police water cannon

At approximately 14:30 on 16 March, TEPCO announced its belief that the storage pool—which is located outside the containment area—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, but a sustained release of radioactive materials would be a possible scenario. Around 20:00 JST it was planned to use a police water cannon to spray water on unit 4.

On 16 March the chairman of United States Nuclear Regulatory Commission (NRC), Gregory Jaczko, said in Congressional testimony that the NRC believed all of the water in the spent fuel pool had boiled dry. "1,097 tons of spent fuel rods are stored there." 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 claimed that helicopter observation indicated that the pool had not boiled off. The French Institut de radioprotection et de sûreté nucléaire agreed with this assessment, stating that helicopter crews decided to divert their water dumps to Unit 3 on the basis of their visual inspection of Unit 4.

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 both reactors were still fueled, unlike reactor 4 where the fuel rods had been removed prior to the earthquake.

Government spokesman Edano stated on 15 March that reactors 5 and 6 were being closely monitored, as cooling processes were not functioning well. At 09:16 JST the removal of roof panels from reactor buildings 5 and 6 was being considered in order to allow any hydrogen build-up to escape. 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. On 17 March, Unit 6 was reported to have operational diesel-generated power and this was to be used to power pumps in unit 5 to run the Make up Water Condensate System (MUWC) to supply more water. Preparations were made to inject water into the reactor pressure vessel once mains power could be restored to the plant, but water levels in the reactors were said to be declining.

Information provided to the IAEA indicated that storage pool temperatures at both units 5 and 6 remained steady around 60°C between 19:00 JST 14 March and 14:00 JST 16 March.

Reactor status summary

The Japan Atomic Industrial Forum (JAIF) has developed a status summary table for the Fukushima nuclear power plants and is publishing semi-regular updates.

Status of Fukushima I at 18 March 10:00 JST (18 March 01:00 UTC)	Unit 1	Unit 2	Unit 3	Unit 4	Unit 5	Unit 6
Power output (MWe)	460	784	784	784	784	1,100
Type of reactor	BWR-3	BWR-4	BWR-4	BWR-4	BWR-4	BWR-5
Core fuel assemblies	400	548	548	0	548	764
Spent fuel assemblies	292	587	514	1,479	826	1,136
Fuel type	Low-enriched uranium	Low-enriched uranium	Mixed-oxide (MOX)	Low-enriched uranium	Low-enriched uranium	Low-enriched uranium
Status at earthquake	In service	In service	In service	Outage (scheduled)	Outage (scheduled)	Outage (scheduled)
Fuel integrity	70% damaged (estimated)	33% damaged (estimated)	Damaged	Spent fuel damaged	Not damaged	Not damaged
Reactor pressure vessel integrity	Unknown	Unknown (Damage suspected)	Unknown (Damage suspected)	Not damaged (defueled)	Not damaged	Not damaged
Containment integrity	Not damaged	Damage suspected	Damage suspected	Not damaged	Not damaged	Not damaged
Core cooling system 1 (ECCS/RHR)	Not functional	Not functional	Not functional	Not necessary (defueled)	Not necessary	Not necessary
Core cooling system 2 (RCIC/MUWC)	Not functional	Not functional	Not functional	Not necessary (defueled)	Not necessary	Not necessary
Building integrity	Severely damaged	Slightly damaged	Severely damaged	Severely damaged	Not damaged	Not damaged
Pressure vessel, water level	Around half of the fuel	Higher than half of the Fuel	Around half of the fuel	Safe (defueled)	Safe but dropping	Safe
Pressure vessel, pressure	Stable	unknown; battery dead	Stable	Safe (defueled)	Safe	Safe
Containment pressure	Unknown	Drywell: Unknown, Suppression pool: Atmosphere	Fluctuating	Safe	Safe	Safe
Seawater injection into core	Continuing	Continuing	Continuing	Not necessary (defueled)	Not necessary	Not necessary
Seawater injection into containment vessel	Continuing	To be decided	Continuing	Not necessary	Not necessary	Not necessary
Containment venting	Continuing	Preparing	Continuing	Not necessary	Not necessary	Not necessary
Integrity of fuel in Spent Fuel Pool (SFP)	(no data)	(no data)	SFP water level low, starting water injection	SFP water level low, preparing water injection, Damage to fuel rods suspected	SFP temperature increasing, 63 °C on 16 March 12:00 JST	SFP temperature increasing, 60 °C on 16 March 12:00 JST
Environmental effect (NPS border)	646 μSv/hour (0.646 mSv/h) at 11:10 on 17 March
Evacuation radius	20 km from Nuclear Power Station (NPS). People who live between 20 km to 30 km from the Fukushima I Nuclear Power Station are to stay indoors. USA, Australia and South Korea have instructed their citizens to evacuate a radius of minimum 80 km. Spain advises to leave an area of 120 km, Germany advises to leave even the metropolitan area of Tokyo. Sweden recommends evacuation of 80 km and cancelling of all travel to Japan.
INES	Level 4 (estimated by Japanese NISA and accepted by the international IAEA); Level 6 (estimated by the French nuclear authority and the Finnish nuclear authorities)

Solutions considered or attempted

Legend

Effective

Partially effective

Not effective

Not applicable or unknown

Solution	General effectiveness	Specific effectiveness
		Reactor cores							Spent fuel pools
		1	2	3	4	5	6		1	2	3	4	5	6
Backup diesel generators: The backup diesel generators operated.	100% effective, until they were swamped by the tsunami, one half hour after the earthquake.
Backup batteries: The backup batteries kept coolant flowing for 8 hours after the generators failed.	Effective, until they ran out. The operators failed to connect portable generators before the 8 hours ran out, thereby dooming #3 to overheat and nearly melt down.
Mobile power units: Normally, the reactors’ cooling systems pump water through the core containment areas and spent fuel pools. Due to the power loss after the earthquake, mobile power units were installed to provide power to pump water. General Electric reported through spokesperson Michael Tetuan that they are sending 10 gas turbine generators to help replace lost power generating capacity but GE did not know where these turbines would be used.	Mobile power units were generally successful particularly at unit 2. However, it is unclear how many mobile power units are available locally or nationally and what fuel they require. Regardless of partial power from mobile units, without full power, all of the cooling systems are less effective which led the water to overheat and water levels dropped below safe levels. Eventually, additional sea water had to be added along with the release of a buildup of radioactive steam.
Cooling core containment areas by adding sea water: Sea water is being "manually" "injected" into the core containment areas of units 1, 2 and 3. Neutron absorbing boric acid is being injected with the sea water.	Sea water cooling has been effective for cooling core containment areas of units 1, 2 and 3 but it is unclear how this sea water is being injected and whether it is necessary to replenish the water in the damaged sea water holding areas.
Spraying water into spent fuel pools with water cannon trucks: Riot police and military used riot-control water cannons to spray 30 tonnes of water onto the roofs and into the spent fuel pools of reactor unit Number 3 although it is unclear if any water reached the spent fuel pool. Officials considered spraying water onto the roof of unit Number 4 also.	Steam emanated from the roof of unit 3 after spraying which suggested that spraying was at least partially successful in reaching overheated spent fuel rods. Radiation levels dropped slightly after the steam had dissipated suggesting the cooling may have been successful.
Helicopter dumping of sea water into spent fuel pools: Helicopters used for forest fire suppression dumped sea water onto the reactors.	Strong winds prevented effective targeting of the dumped water since high radiation levels prevented the helicopters from flying low. Furthermore, footage captured appeared to show much of the water dispersing in mid-air weakening the intensity of the water to cool the overheating reactor.
Repair power lines to restart pumps: New and repaired power lines are being completed that could provide power to the original cooling systems and pumps. The original water pumps may be inoperable due to damage from injected sea water, the earthquake, tsunami or explosions.	A power line was laid to reconnect power to unit Number 2 and attempts to restart water pumps are scheduled to begin on 18 March 2011.
Aerial heat sensing photos: To determine effective solutions and to focus ongoing efforts, infrared photos will identify "hot spots" among the reactors and spent fuel pools.
Boron carbide pellets: Insertion or targeted aerial dropping of boronated plastic beads or boron carbide pellets into the spent fuel pools to absorb neutrons.	France is flying 95 tonnes of boron to Japan on Thursday.
Tin coolant: Ukrainian engineers with Chernobyl experience suggest: ". . . the type of reactor fuel coolant needs to be changed - water, which might trigger a steam-zirconium reaction fraught with the release of hydrogen and potential blasts, should be replaced with low-melting and chemically neutral metal, for instance tin which will pull heat away from the fuel rods (molten or damaged) towards the inner walls of the reactor, while continuing to use sea water to cool down its outer walls." "The tin 'lake' inside the reactor will "reduce the discharge of heavy fission products and bring ionizing radiation levels down. Chipped tin could be pumped in through steam communications under pressure using cylinders with helium or argon."

Radiation levels and radioactive contamination

Radiation readings from the TEPCO website for 3 months before the earthquake

Radiation readings from TEPCO during the accident

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 to occur at a later time following a one hour exposure. 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 a mix short-lived isotopes (noble gases and nitrogen) and iodine-131 and cesium-137 that escaped 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. This number of 1,000 mSv/h is highly suspect, as all references to it come from a single source (CBS news, ref #8), and the maximum recorded on site measurement of radiation by TEPCO is around 11 mSv/h . 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.

On 17 March IAEA radiation monitoring over 47 cities, showed that levels of radiation in Tokyo had not risen. They had received no new radiation level information from the Fukushima power plant site. Although at some locations around 30km from the Fukushima plant, the dose rates rose significantly in the preceding 24 hours (in one location from 80 to 170 μSv/h and in another from 26 to 95  μSv/h), but levels varied according to the direction from the 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.

Effects on human health

An acute exposure to radiation is much more harmful than the same dose spread out over a longer period of time, making comparisons between short-term and yearly doses problematic. Cells can repair damage from radiation, but these pathways can be overwhelmed by a high dose. Therefore, exposure to penetrating radiation is less harmful than long term exposure to ingested or inhaled radioactive dust. People can mitigate their exposure to radiation through a variety of protection techniques.

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 International Commission on Radiological Protection. 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.

It should be noted that the reported value of 400 mSv/h is for a highly localized point between Units 3 and 4 . Radiation decreases with distance as an inverse square law, so the amount of radiation exposure will be much less even several meters away from the center of radiation. At the north edge of the Fukushima plant, radiation has barely exceeded 10 mSv/h according to site data from Tepco .

Isotopes of possible concern

See also: Fission products § Countermeasures against the worst fission products found in accident fallout

Radioactive isotopes of iodine and caesium have been detected at Saitama, near Tokyo. These isotopes can have serious effects on health, depending on the quantity inhaled or ingested.

The isotope iodine-131 is easily absorbed by the thyroid. Persons exposed to releases of I-131 involving melted fuel at nuclear power plants have a higher risk for developing thyroid cancer or thyroid disease, or both. Children are particularly more vulnerable to I-131 than adults. Increased risk for thyroid neoplasm remains elevated for at least 40 years after exposure. Potassium iodide tablets prevent iodine-131 absorption. CBS news reported that the number of doses of potassium iodide available to the public in Japan was inadequate to meet the perceived needs for an extensive radioactive contamination event.

Caesium-137 is also a particular threat because it behaves like potassium and is taken-up by the cells throughout the body. Prussian blue helps the body secrete caesium-137. Cs-137 can cause acute radiation sickness, and increase the risk for cancer because of exposure to high-energy gamma radiation. Internal exposure to Cs-137, through ingestion or inhalation, allows the radioactive material to be distributed in the soft tissues, especially muscle tissue, exposing these tissues to the beta particles and gamma radiation and increasing cancer risk.

Strontium-90 behaves like calcium, and tends to deposit in bone and blood-forming tissue (bone marrow). 20-30% of ingested Sr-90 is absorbed and deposited in the bone. Internal exposure to Sr-90 is linked to bone cancer, cancer of the soft tissue near the bone, and leukemia. Risk of cancer increases with increased exposure to Sr-90.

Plutonium is also present in the MOX fuel of the Unit 3 reactor and in spent fuel rods, although there has been no official indication that plutonium has been detected outside the reactors. Officials at the International Atomic Energy Agency say the presence of MOX fuel does not add significantly to the dangers. Plutonium-239 is particularly long-lived and toxic with a half-life of 24,000 years, and if it escaped in smoke from a burning reactor and contaminated soil downwind, it would remain hazardous for tens of thousands of years. The primary danger from plutonium is that small particles will become airborne and be inhaled, with estimates that 0.08 milligrams inhaled will have 100% probability of causing a fatal cancer.

Rush for iodine

Fear of radiation from Japan prompted a global rush for iodine pills, including in the United States, Canada, Russia, Korea, China, and Malaysia. There is a rush for iodized salt in China. A rush for iodine antiseptic solution appeared in Malaysia. WHO warned against consumption of iodine pills without consulting a doctor and also warned against drinking iodine antiseptic solution. The United States Pentagon said troops are receiving potassium iodide before missions to areas where possible radiation exposure is likely.

U.S. military

The U.S. Navy dispatched aircraft carrier USS Ronald Reagan and other vessels flew a series of helicopter operations. A US 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. USS Ronald Reagan and sailors onboard were exposed to "a month's worth of natural background radiation from the sun, rocks or soil" in an hour and the carrier was repositioned. 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.

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. 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. Daily monitoring and limited precautionary measures were recommended for Yokosuka and Atsugi bases: limiting outdoor activities and securing external ventilation systems.

Reaction in Japan

Evacuations

Fukushima I and II Nuclear Accidents Overview Map showing evacuation and other zone progression and selected radiation levels.

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 2 km 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 07:45 on 12 March, again with instructions for those within 10 km to stay indoors. Evacuation was extended to 10 km by 17:39. A journalistic investigation 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.

On the morning of 15 March, the evacuation area was again extended. Prime Minister Naoto Kan issued instructions that any remaining people within a 20 km (12 mile) zone around the plant must leave, and urged that those living between 20 km and 30 km from the site should stay indoors. A 30 km no-fly zone has been introduced around the plant.

On 16 March, the U.S. Embassy advised Americans in Japan to leave areas within "approximately 50 miles" (80 km) from the plant. Gregory Jaczko, the chairman of the United States Nuclear Regulatory Commission, said before the United States Congress, believing the Japanese government was not telling the full story, "We would recommend an evacuation to a much larger radius than has currently been provided by Japan."

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. 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. 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". “I am trying to be careful with words ... This is not a situation where the whole core suffers a meltdown”. 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. 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".

Prime Minister visits plant

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

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 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, 14 March 2011, three Russian experts stated that the nuclear accident should be classified at Level 5, perhaps even Level 6. One day later, the French nuclear safety authority ASN said that the Fukushima plant could be classified as a Level 6.

Requests for help

Japan has not been able to formulate and express requests for assistance as rapidly and specifically as aid agencies would hope was the case. Even at the highest level, Elisabeth Byrs, of the U.N. Office for the Coordination of Humanitarian Affairs (OCHA), stated that "Japan has requested international search and rescue teams, but only a handful." Nevertheless, search and rescue teams have been dispatched on an ad hoc basis, based upon their own assessment and ability to operate.

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 3,265 kg (7,200 lb) of special equipment to help monitor and assess the situation at the plant.

Dearth of information

Prime Minister Qupasdfghjklz met with Tokyo Electric Power Company on 15 March and lamented the lack of information in terms. According to press accounts, he bluntly said, "What the hell is going on?"

Business reaction

In financial markets, the plant operator TEPCO's shares halved in value as it dropped over four consecutive trading days, before rising again on Thursday. Japan's Nikkei 225 stock index fell 6% on Monday. The Nikkei then plunged another 11% on 15 March after the government warned of elevated radiation risks. On 16 March the index recovered 5.7%.

International reaction

Main article: International reaction to Fukushima I nuclear accidents

The International reaction to the nuclear accidents has been a humanitarian response to the 2011 Tōhoku earthquake and tsunami, also to those people affected by the events at Fukushima I. The response has also included the expression of concern over the developments at the reactors and the risk of escalation. The accidents have furthermore prompted re-evaluation of existing and planned national nuclear energy programs, with some commentators questioning the future of the nuclear renaissance.

Pr. Robin Grimes, Director of the Centre for Nuclear Engineering, Imperial College, London has said the leaking plant is a 40 year old Generation 1 design which has far too much fuel outside the containment vessel and would not be allowed to be built today. There are a number of Generation 2 reactors in the same region, which survived the earthquake well and were back up and running before some conventional power stations.

See also

• Timeline of the Fukushima nuclear accidents
• Fukushima 50
• 2011 Japanese nuclear incidents

Background

• International Nuclear Event Scale
  • Kyshtym disaster (1957, INES 6)
  • Three Mile Island accident (1979, INES 5)
  • Chernobyl disaster (1986, INES 7)
• Orders of magnitude (radiation)

Lists

• List of civilian nuclear accidents
• List of civilian nuclear incidents
• List of civilian radiation accidents
• Lists of nuclear disasters and radioactive incidents

Major earthquakes of Japan

• Tokai earthquakes
  • Hamaoka Nuclear Power Plant（Nuclear plant in source region）

External links

• Webcam Fukushima nuclear power plant I
• TEPCO News Releases, Tokyo Electric Power Company
• Official web site, Nuclear and Industrial Safety Agency (NISA), the nuclear safety authority of Japan
• Official web site, Japan Atomic Energy Agency (JAEA)
• Update on Japan Earthquake, International Atomic Energy Agency (IAEA)
• Second "Explosion" at Reactor No. 3 on YouTube
• Anatomy of a meltdown, Nature
• Weather forecast for the area