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 16 March 2011 satellite image by DigitalGlobe.jpg Satellite image of four damaged reactor buildings at the plant on 16 March
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 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. 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.

Several events occurred during the following days: there was evidence of partial nuclear meltdowns in reactors 1, 2 and 3; hydrogen explosions destroyed the upper cladding of the building housing reactors 1, 3 and 4; an explosion damaged reactor 2's containment; and fires at reactor 4. Radiation leaks led to a 20 kilometres (12 mi)-radius evacuation around the plant. On 16 March TEPCO employees and workers from other companies not in charge of injection work started tentative evacuation to a safe location. This was done after an extraordinary explosion was heard in the supression chamber of Reactor Building 2. However, they later returned after it was confirmed that there had not been a breach there. The status of the reactors remains extremely hazardous. The gravity of the accidents is such that many international leaders have expressed concerns.

Timeline

Main article: Timeline of the Fukushima nuclear accidents

• 11 March 2011: the Japanese government declared a nuclear power emergency due to the failure of the reactor cooling systems in one of the reactors of Fukushima I and evacuated thousands of residents living close to the reactor. After officials later voiced the possibility of core damage, the evacuation zone was extended to 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 Daiichi and Daini.

• 12 March: while evidence of partial meltdown of the fuel rods in Unit 1 was growing, a hydrogen explosion destroyed the upper cladding of the building housing Reactor Unit 1. The explosion injured four workers, but the reactor containment inside the building remained intact. 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 or even a 6 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 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 restore power to all six units of Fukushima I.

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, SF = spent fuel pool area. Excess steam from the drywell enters the wetwell water pool via downcomer pipes.

Main article: Fukushima I Nuclear Power Plant

The Fukushima I Nuclear Power Plant is located in the town of Okuma in the Futaba District of Fukushima Prefecture, Japan, 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. If these systems could have been operated, the core temperature could have been brought under control, even with some fuel damaged and partially melted.

At the damaged Fukushima reactors, the operators appear to have been unable to operate the residual heat removal or emergency core cooling systems. Initially this was because the diesel generators failed in the tsunami. But presumably alternative diesel generators could have been trucked to the plant fairly quickly, given the need, and used to power the residual heat removal or emergency core cooling systems. This does not appear to have happened, and it can therefore be inferred that the residual heat removal and emergency core cooling systems cannot be operated. Perhaps they were disabled by the earthquake or the tsunami.

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. 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

An earthquake categorised as 8.9 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 correctly but stopped abruptly at 15:41, ending all AC power supply to the reactors. The plant was protected by a sea wall, but tsunami water which followed after the earthquake topped this sea wall, flooding the low lying generator building. 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 by the tsunami. It is possible that a new powerline to the complex will revive some of the electric cooling system pumps that have failed.

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 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 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 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 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

File:Fukushima explosion.jpg

First explosion at Fukushima power plant, Unit 1.

File:2011-03-12 1800 NHK Sōgō channel news program screen shot.jpg

Before and after images of the Unit 1 explosion.

At 15:36 JST on 12 March 2011 there was an explosion at Unit 1. Four workers were injured, and the upper shell of the reactor building was blown away leaving in place its steel frame. 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 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 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 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.

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. Power was achieved by mobile power units, while preparations were made to perform pressure venting.

Initial instability in unit 2 spent fuel storage pool

According to a report in the New York Times, "[A]t the start of the crisis Friday, immediately after the shattering earthquake, Fukushima plant officials focused their attention on a damaged storage pool for spent nuclear fuel at the No. 2 reactor at Daiichi, said a nuclear executive who requested anonymity.... The damage prompted the plant’s management to divert much of the attention and pumping capacity to that pool, the executive added. The shutdown of the other reactors then proceeded badly, and problems began to cascade."

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.

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.

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.

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.

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 water dumps were underway by 9:45 JST.

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 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. 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 bay

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 Japan time on March 17 it was confirmed that not only way the pool not boiled dry, but that it was in fact full of water and none of the fuel rods were exposed. Japan expressed bewilderment over Jaczko's statement.

Reactor units 5 and 6

Both reactors were off line at the time the earthquake struck (reactor 5 had been shut down on 3 January 2011 and reactor 6 on 14 August 2010). Although an IAEA report indicated that the fuel rods are still in the reactor vessels of both units and not in the spent fuel pools as in Unit 4, 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.

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 17 March 16:00 JST (17 March 07:00 UTC)	Unit 1	Unit 2	Unit 3	Unit 4	Unit 5	Unit 6
Power output (MWe)	460	784	784	784	784	1100
Type of reactor	BWR-3	BWR-4	BWR-4	BWR-4	BWR-4	BWR-5
Estimated core fuel assemblies (3y / 5y rotation)	200 / 340	350 / 575	350 / 575	0	350 / 575	500 / 810
Estimated spent fuel assemblies	340	575	575	575 (storage) 350 or 575 (from core)	575	810
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	33% damaged	Damaged, MOX fuel	Spent Fuel Damaged	Not damaged	Not damaged
Containment integrity	Not damaged	Damage suspected	Damage suspected	Not damaged (defueled)	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	Stable	Safe (defueled)	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 level low, Preparing water injection	SFP level low, Preparing water injection, Damage to fuel rods suspected	SFP temperature increasing	SFP temperature increasing
Environmental effect (NPS border)	646 μSv/hour (0.646 mSv/hr) 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.
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)

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. 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.

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.

Isotopes of possible concern

See also: Fission_products § Countermeasures_against_the_worst_fission_products_found_in_accident_fallout

Radioactive isotopes of iodine and cesium 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-31 than adults. Increased risk for thyroid neoplasm remains elevated for at least 40 years after exposure. Potassium iodide tablets prevent iodine-131 absorption. However, 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 of the reactors. Officials at the International Atomic Energy Agency say the presence of MOX fuel does not add significantly to the dangers. However, 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. Although plutonium is a long-lived emitter of radiation, it is also quite heavy, so it is not likely to move very far downwind from its source.

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

The Pentagon said troops are receiving anti-radiation pills before missions to areas where possible radiation exposure is likely.

Japanese government reaction

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 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.

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

Japanese business reaction

In financial markets, the plant operator TEPCO's shares dropped 24% on Monday, 14 March, the first day of trading after the tsunami, and Japan's Nikkei 225 stock index fell 6% on the day. 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

International response has included Humanitarian response to the 2011 Tōhoku earthquake and tsunami, some of which pertains to the Fukushima crisis, Operation Tomodachi and also re-evaluation of existing national nuclear energy program, as far as shut-down of nuclear plants in Germany. Panic shopping occurred in China over the fear of shortages in unpolluted seafood and ocean salt. On Thursday afternoon, salt and soy sauce were sold out in Beijing, Shanghai, Guangzhou, Shenyang, and other major cities. Supermarkets and convenience shops put up signs "salt and soy sauce sold out". Shortages of rice, flour and food oil are imminent.

See also

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

Background

• International Nuclear Event Scale
  • Three Mile Island accident (1979, INES 5)
  • Kyshtym disaster (1957, INES 6)
  • 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

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