Fukushima nuclear accident

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"Fukushima nuclear incident" redirects here. For the incidents at Fukushima II, see Fukushima II Nuclear Power Plant.
Fukushima nuclear accident
File:2011-03-12 1800 NHK Sōgō channel news program screen shot.jpg
Before and after images of the hydrogen explosion at Fukushima I Unit 1 reactor
Date11 March 2011 (2011-03-11)
Time14:46 JST (UTC+9)
LocationŌkuma, Fukushima, Japan
Coordinates37°25′17″N 141°1′57″E / 37.42139°N 141.03250°E / 37.42139; 141.03250
OutcomeLevel 4 (accident with local consequences) on the International Nuclear Event Scale[1]
Non-fatal injuries15 (injuries), 3 (radiation sickness)[2][3]
File:Japan Nuclear power plants map.gif

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

On 11 March 2011, the Japanese government declared a "nuclear power emergency" due to a loss of coolant and evacuated thousands of residents living close to Fukushima I. About 170,000–200,000 people were evacuated after officials voiced the possibility of core damage.[4][5] Twenty-two residents near the plant showed signs of radioactive contamination exposure, and three workers from the plant are experiencing symptoms of radiation sickness, according to a statement in the New York Times,[6] but only one worker was confirmed by TEPCO as ill.[7] The release of fission products from the damaged nuclear reactor core, notably radioactive iodine-131, led Japanese officials to distribute iodine to the people living around Daiichi and Daini, because taking a dose of iodine can help protect the thyroid gland.[6]

The next day, while evidence for partial meltdown of the fuel rods in Unit 1 was growing, a hydrogen explosion destroyed the upper story of the building housing Reactor Unit 1. The explosion injured four workers, but the container of the reactor remained intact.[8][9] The explosion is believed to be the result of hydrogen reacting with atmospheric oxygen. Hydrogen is formed when the zircaloy cladding of the fuel rods oxidizes with water under high temperatures,[10] which was vented along with steam from the containment vessel into the building in order to reduce pressure within the containment.[11][12]

On 13 March 2011, a partial meltdown at Unit 3 appeared also possible. As of 1pm 13 March, JST, both reactors 1 and 3 had been vented and were being filled with water and boric acid to both cool and inhibit further nuclear reactions.[13] Unit 2 was reported to have lower than normal water level but to be stable, although pressure inside the containment vessel was high.[13]

On 13 March 2011, the Japan Atomic Energy Agency announced that it was rating the Fukushima accidents at 4 (accident with local consequences) on the International Nuclear Event Scale (INES).[14] 170,000–200,000 people were evacuated after officials voiced the possibility of a meltdown.[4][15]

On 14 March 2011, the reactor building for Unit 3 exploded as well,[16] injuring eleven people. It appears there was no release of radioactive material and TEPCO has said it was contained within the reactor bunker.[17] According to the New York Times, as of March 15 (Japan time) problems with the vents on Unit 2 meant pressure in its containment vessel had impeded water flow into it to the point that Unit 2 was in the most critical condition of the three reactors, later exploding at around 8:10 local time. Initial indications suggest that the explosion was not same scale as the previous two explosions at Unit 1 and 3.[18] [19]

Fukushima I Nuclear Power Plant

Main article: Fukushima I Nuclear Power Plant
Cutaway drawing of a typical BWR Mark I Containment, as used in units 1 to 5

The Fukushima I Nuclear Power Plant is located in the town of Okuma in the Futaba District of Fukushima Prefecture, Japan. It consists of six light water, boiling water reactors with a combined power of 4.7 GW, 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.[20] 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.[21] 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.[21]

Units 1–5 had/have a Mark 1 type (light bulb torus) containment structure, unit 6 has Mark 2 type (over/under) containment structure.[21] From September 2010, unit 3 has been fueled by mixed-oxide (MOX) fuel.[22]

Earthquake and tsunami

Diagrammatic representation of the cooling systems of a BWR
Main article: 2011 Sendai earthquake and tsunami
Further information: Timeline of the Fukushima nuclear accidents

An earthquake categorised as 9.0MW on the moment magnitude scale occurred on 11 March 2011, at 14:46 Japan Standard Time (JST) off the northeast coast 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.[23] When the earthquake was detected, units 1, 2 and 3 underwent an automatic shutdown (called scram).[24]

After the reactors shut down, electricity generation stopped. Normally the plant could use the external electrical supply to power cooling and control systems,[citation needed] 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 easily topped this, flooding the low lying generator building.[25][26] 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".[24]

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 take several days to entirely cease releasing heat at above the shutdown level (typically 5% of operating power, here 20MW.). Boiling water reactors have steam-turbine driven emergency core cooling systems that can be directly operated by steam produced after a reactor shutdown and can inject water directly into the reactor. Using these pumps, boiling water reactors can provide water without electrically driven pumps but only while the reactor is at pressure. This results in less dependence on emergency generators but only operates so long as the reactor is safely producing steam, and some power is still needed to operate the valves and monitoring systems. Achieving cold shutdown still requires motor-driven pumps to remove decay heat.

After the failure of the diesels, emergency power for control systems was supplied by batteries that would last about eight hours.[27] Batteries from other nuclear plants were sent to the site and mobile generators arrived within 13 hours,[28] but work to connect portable generating equipment to power water pumps was still continuing as of 15:04 on 12 March.[29] Generators would normally be connected through switching equipment in a basement area of the buildings, but this had been flooded by the tsunami.[25]

Reactor unit 1

Cooling problems at unit 1

Aerial view of the plant area before accident. Unit 1 is rightmost of block of four reactors centre left. Unit 3 is second from left. (north is to the right)

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.[30] Potentially radioactive steam was released from the primary circuit into the secondary containment area to reduce mounting pressure.[31]

In the early hours of 12 March TEPCO reported that radiation levels were rising in the turbine building for Reactor Unit 1[32] and that it was considering venting some of the mounting pressure into the atmosphere, which could result in the release of some radiation.[33] 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.[34] 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.[26] At 05:30 JST the pressure inside Reactor 1 was reported to be 2.1 times the "design capacity",[35] 820 kPa.[8] Rising heat within the containment area would have led to increasing pressure, with both cooling water pumps and ventilation fans for driving gases through heat exchangers within containment dependant on electricity.[36]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.[25] 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."[37] The gamma ray radiation recorded on the main gate was increased from 69 nanogray/hour (nGy/h) (04:00 JST, 12 March) to 866 nGy/h 40 minutes later and reached the peak of 385.5 μSv/h at 10:30 JST.[38][37][39][40] At 13:30 JST, radioactive caesium-137 and iodine-131 was detected near reactor 1,[41] which indicates that some of the core was exposed to air due to a partial-meltdown or other damage of the nuclear fuel.[42] The NHK website reported that cooling water had lowered so much that parts of the nuclear fuel rods were exposed.[43] Radiation levels at the site boundary exceeded the regulatory limits.[44] Kyodo News Service later reported that partial melting may have occurred.[45][46][47][48] On 14 March 2011, Kyodo News reported radiation levels had continued to increase on the premises, measuring at 2:20 AM an intensity of 751 μSv/hour on one location and at 2:40 AM an intensity of 650 μSv/hour at another location on the premises.[49]

Explosion of reactor building

File:Fukushima explosion.jpg
Explosion at 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.[50][51] 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.[8][52]

Experts soon agreed that the cause was a hydrogen explosion.[9][53][54] Almost certainly the hydrogen was formed inside the reactor vessel[9] because of falling water levels, and this hydrogen then leaked into the containment building.[9] Exposed metal fuel rods become very hot and can then react with steam oxidising the metal and releasing hydrogen. Safety devices should ignite 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,[8][9] although an increase in radiation levels was confirmed following the explosion.[55][56] ABC news reported that according to the Fukushima prefectural government, the hourly radiation from the plant reached 1,015 µSv.[57] Two independent nuclear experts cited design differences between the Chernobyl Nuclear Power Plant and the Fukushima I Nuclear Power Plant,[58][59] one of them saying he did not believe that a Chernobyl-style disaster will occur.[58]

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.[29] At 21:00 JST TEPCO announced that they planned to cool the leaking reactor with seawater (which started at 20:20 JST), then using boric acid to act as a neutron absorber to prevent a criticality accident.[60][61] 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.[9] At 23:00 JST TEPCO announced that due to the quake at 22:15[62] the filling of the reactor had been temporarily stopped but has been resumed after a short while.[8][63] Filling the reactor with seawater will contaminate the reactor with impure water, a substance not usually allowed in reactors, meaning the reactor will likely be decommissioned, since it is not cost effective to decontaminate.[64]

NISA reported that injection of sea water into the primary containment vessel through the fire extinguisher system commenced at 11:55 on 13 March. At 01:10 on 14 March injection of sea water was halted because all available water in the plant pools had run out (similarly, feed to unit 3 was halted). Water supply was restored at 03:20. Radiation levels around the plant were measure at around 0.03 µSv/hr at 05:00 and 15:00 on 14 March.[65]

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.[8][66]

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.[67][30] This was caused when fuel for pumps ran out.[68] 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.[69] Jiji later reported that according to TEPCO, a meltdown cannot be ruled out.[70]

At 22:29 JST, NHK reported that workers had succeeded in refilling half the reactor with water. However, at that time, part of the rods were still exposed, and technicians could not rule out the possibility that fuel rods had melted.[71][dead link] Work was in hand to demolish parts of the walls of reactor building 2 to allow the escape of hydrogen and hopefully prevent another explosion.[72] At 21:37 JST the measured radiation levels at the gate of the plant had reached the a maximum of 3130 μSv per hour, which was enough to reach the annual limit for non-nuclear workers in twenty minutes,[72] but had fallen back to 326 μSv/hr by 22:35.[73]

It was believed that around 23:00 JST the 4m long fuel rods in the reactor were fully exposed for the second time.[72][74] 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. To replenish the water, the contained pressure would have to be lowered first by opening a valve of the vessel. Due to a defect of the valve (caused by the explosion of the unit 3 building) this could not be done and additional water could not be added.[72][75]

As of 04:11 JST, water was being pumped into the reactor of unit 2 again.[76]

Explosion in reactor building

An explosion was heard after 6:10 JST on 15 March in unit 2, and the pressure-suppression system, which is at the bottom part of the container, may have been damaged.[77][78] The radiation level was reported to exceed the legal limit and the plant's operator has started to evacuate workers from the plant.[79]

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.[45][80][81]

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.[82] 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.[83] At one point, the top three meters of mixed oxide (MOX) fuel rods were exposed to the air.[84]

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"[85] and manual venting took place at 08:41 and 09:20.[66] At 09:25 JST on 13 March 2011, operators began injecting water containing boric acid into the primary containment vessell (PCV) via a fire pump.[86][87] When water levels continued to fall and pressure to rise, the injected water was switched to sea water at 13:12.[83] By 15:00 it was noted that despite adding water the level in the reactor did not rise and radiation had increased.[88] A rise was eventually recorded but the level stuck at 2m below the top of reactor core. Other readings suggested that this could not be the case and the gauge was malfunctioning.[66]

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.[87]

Explosion of reactor building

File:Japanese second explosion.png
Explosion at Fukushima power plant Unit 3

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.[89]

At 11:15 JST on 14 March 2011, the envisaged explosion of the building surrounding Reactor 3 of Fukushima 1 occurred, presumably due to the ignition of the built up hydrogen gas.[90][91] There is no health risk reported, though 600 people have been ordered to stay indoors. Within minutes, it was reported that as with Reactor 1, the outer reactor building was blown apart, but the inner containment vessel was not breached. Eleven people were reported injured in the blast.[92][93]

Radioactive releases at the site

Venting of the reactors has caused release of short-lived radioactive gases, as well as small amounts of radioactive cesium and iodine.

Radiation levels around the perimeter of the site are[when?] currently a few millisieverts/hour. Radiation sickness typically occurs at about 1000 millisieverts total dose, which is much higher. Normal background radiation varies from place to place but is around 2.4 millisievert per year.

The radiation carried by the winds from the site has been detected at sea by the US navy, as well as at another plant site.

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.[94] Air traffic has been restricted in a 20-kilometre (12 mi) radius around the plant, according to a NOTAM.[95] 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."[96]

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.[97]

At 22:53 JST Tokyo Broadcasting System (TBS), quoting Fukushima representatives, has reported that there was an evacuation of 30 staff members and 60 patients due to the explosion. From those evacuees three patients received a checkup for radiation exposure by the hospital staff at Futaba, a town 5.6 km (3.5 miles) from the power plant. All three[citation needed] patients required decontamination, but about 90 other evacuees may also require decontamination.[98][failed verification]

A U.S. military spokesperson had said that low-level radiation was detected both by navy ships and their accompanying aircraft, forcing a change of course of the 7th fleet, en route to Sendai.[99]USS Ronald Reagan and sailors onboard were exposed to a month's worth of radiation in an hour and the carrier was repositioned.[100] 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.[101]

Government reaction

Prime Minister visits plant

The Prime Minister of Japan, Naoto Kan, visited the plant for a briefing on 12 March 2011.[102]

Government statements on possibility of meltdown

In a press conference, the chief spokesman of the Japanese nuclear authorities was translated into English as having said that a nuclear meltdown may be a possibility at Unit 1.[103] 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.[104][103] 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.[103] The temperature inside the reactor was not reported, but Japanese regulators said it was not dropping as quickly as they wanted.[105] At 12:33 JST on 13 March 2011, the chief spokesman of the Japanese government, Yukio Edano, was reported to have confirmed that there was a “significant chance” that radioactive fuel rods had partially melted in unit 3 and unit 1, or that "it was 'highly possible' a partial meltdown was underway".[5] “I am trying to be careful with words ... This is not a situation where the whole core suffers a meltdown”.[89] 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.[106][107] Chief government spokesman said around 17 UTC on March 14 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". The government does not use the term "meltdown", because that term could be misunderstood to mean radioactive material had breached containment.[108][109]

Evacuations

After the declaration of a nuclear emergency by the Government at 19:03 on 11 March, the Fukushima prefecture ordered the evacuation of an estimated 1,864 people within a distance of 2km from the plant. This was extended to 3 kilometres (1.9 mi) and 5,800 people at 21:23 by a directive to the local governor from the Prime Minister, together with instructions for residents within 10 kilometres (6.2 mi) of the plant to stay indoors.[29][27] 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.[29][110]

Evacuations were also ordered around the nearby Fukushima II (Daini) plant. Residents within 3 kilometres (1.9 mi) were ordered to evacuate at 7:45 on 12 March, again with instructions for those within 10km to stay indoors. Evacuation was extended to 10km by 17:39.[29] A journalistic investigation was stopped 60 kilometres (37 mi) from the plants by police.[98] Over 50,000 people were evacuated during 12 March.[111] The figure increased to 170,000–200,000 people on 13 March, after officials voiced the possibility of a meltdown.[4][5]

Nuclear authorities reactions

At 01:17 JST on Sunday 13 March 2011, the Japan Atomic Energy Agency announced that it was rating the Fukushima accidents at 4 (accident with local consequences) on the 0–7 International Nuclear Event Scale (INES), below the Three Mile Island (TMI) accident in seriousness[14] which was at 5, a rating that would make the severity of the Fukushima event comparable to Sellafield accidents between 1955 and 1979 that were also at 4.

This has been questioned by the French ASN nuclear safety authority. They say the accident can be classed as a 5 or 6, which would be comparable to or worse than the Three Mile Island accident.[112]

International reaction

As a result of the incident, many energy companies reliant on non-renewable sources have seen drops in their stock prices, while, conversely, renewable energy companies have had dramatic increases in value.[113] In response, the World Council for Renewable Energy (WCRE) stated: "The WCRE insists on the global and coordinated move to finally outlaw all nuclear power. After Harrisburg, Three Mile Island, Chernobyl and Fukushima it is time to wake up and terminate reliance on and trade in this incredibly dangerous technology. No matter how small the risk of a similar event may be in any country – it can never be excluded. It is high time for the immediate and unwavering turn to a fully renewable world: there is no country that cannot be provided with renewable energy alone." A press briefing promoting nuclear energy to be held in the United Kingdom on Tuesday, 15th March, was postponed in light of the incident.[113]

Germany

During the chancellorship of Gerhard Schröder, the social democratic-green government had decreed Germany's final retreat from using nuclear power by 2022, but the phase-out plan was delayed in late 2010, when during the chancellorship of Angela Merkel the conservative-libertarian government decreed a 12-year delay of the schedule.[114] This delay provoked protests, including a human chain of 50,000 from Stuttgart to the nearby nuclear plant in Neckarwestheim. This protest had long been scheduled for March 12, which now happened to be the day of the explosion of reactor block 1.[115] Merkel on 14 March 2011 about-faced and declared a 3-months moratorium on the reactor lifespan extension passed in 2010.[116] It is not yet clear what Merkel's moratorium will mean in practice, but there is a possibility that older nuclear plants like Neckarwestheim I might be shut down - for just three months or for good.[116]

See also

References

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External links

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