Timeline of the Fukushima I nuclear accidents

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During the 2011 Fukushima nuclear emergency in Japan, four nuclear reactors were damaged by explosions.

Fukushima Dai-ichi (dai-ichi means "number one"), is a disabled nuclear power plant. The Fukushima I nuclear accidents occurred after the 9.0 magnitude 2011 Tohoku earthquake and tsunami on 11 March. This offshore earthquake near the island of Honshu[1] produced a large tsunami in Japan, and a tsunami warning for over 20 countries. The earthquake triggered the shut down of the three active reactors at the Fukushima I Nuclear Power Plant (Fukushima Dai-Ichi). The subsequent tsunami stopped Fukushima I station's backup diesel generators, causing a station blackout. The subsequent lack of cooling led to explosions and partial meltdowns at the Fukushima I facility, with problems at all six reactor units and the central spent fuel pool.

[edit] Timeline

Times are given in Japan Standard Time (JST), unless noted, which is UTC plus nine hours.

[edit] March event tree

The nuclear accident event tree[2] developed quickly in the early weeks after the earthquake and tsunami caused a number of accident sequences to begin.

[edit] First week

[edit] 11 March

14:46: A 9.0 magnitude earthquake strikes off the coast of Honshu Island at a depth of about 24 kilometres (15 mi). The Fukushima I power plant's nuclear reactors 1, 2, and 3 are automatically shut down by the shake. Nuclear reactors 4, 5, and 6 were undergoing routine maintenance and were not operating, (reactor 4 was defueled in November 2010). The tremor has the additional effect of causing the power plant to be cut off from the Japanese electricity grid, however, backup diesel generators kick in to continue cooling. Tokyo Electric Power Company (TEPCO), the plant's operator, finds that units 1 and 2 are not operating correctly and notifies the proper officials.[3]

15:01 (approximate): A 14-metre (46 ft) tsunami unleashed by the earthquake overtops the seawall designed to protect the plant from a tsunami of 5.7 metres (19 ft), inundating the Fukushima facility and disabling the backup generators whose electricity was then required to operate controls and cool the reactors.[4]

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 Fukushima I, 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."[5]

16:00: The Nuclear and Industrial Safety Agency of Japan (NISA) initiates an emergency headquarters in an attempt to gather information on the 55 nuclear reactors in Japan.[6] There is no report that radiation was detected outside plant borders.[7]

19:03: Prime Minister Naoto Kan declares a nuclear emergency status.[8] This is announced by Yukio Edano, Chief Cabinet officer in Japan. Japanese government officials try to comfort the people of Japan by telling them that the proper procedures are being undertaken. They also announce that no radioactive leaks have been detected.[6]

21:00: An evacuation order is issued by the government to persons within a 3 kilometres (1.9 mi) radius of the Fukushima I station. Those within a 10 kilometres (6.2 mi) radius are told that they can remain in their homes and carry on with regular activities until told otherwise.[6]

At Fukushima I, the ongoing lack of electricity is causing the cooling system to fail, and the pressure within the nuclear units build up. This pressure buildup is the result of residual decay heat causing the coolant, which is not being circulated, to evaporate.[9] There is no confirmation of radiation leaking from the reactor.[10] TEPCO announces that pressure inside reactor unit 1 of Fukushima I is more than twice normal levels.[6]

[edit] 12 March
Overview map showing evacuation and other zone progression and selected radiation levels.

02:44: Emergency battery power for the High Pressure Core Flooder System for Reactor 3 runs out.

04:15: Fuel rods in Reactor 3 are exposed.

05:30: Despite the high risk of the hydrogen igniting after combining with oxygen from water or in the atmosphere, in order to release some of the pressure inside the reactor at Fukushima I unit 1, the decision is taken to vent some of the steam (which contained a small amount of radioactive material) into the air in the metal container building surrounding the unit.

10:09: TEPCO confirms that a small amount of vapor has been released into the air to release pressure in reactor unit 1 at Fukushima I.[11]

10:58: Pressure still remains too high inside reactor unit 2 at Fukushima I. In order to alleviate some of this pressure, a consensus is once more reached to vent radioactive vapor into the air.[11]

15:30: Evacuation of residents within 3 km of Fukushima II and within 10 km of Fukushima I are underway.[12]

15:36: At reactor unit 1 at Fukushima I cameras document a massive hydrogen explosion on the outer structure of one of four buildings at the plant. It also documents the outer structure collapsing.

18:36 (approximately): TEPCO announces that four people who are employed at the power plant have been injured in the unit 1 explosion.

20:00: Uncertainty surrounds the actual cause of the blast at Fukushima I (later identified as a hydrogen explosion) and the damage caused.

Yukio Edano announces that the concrete building surrounding the steel reactor vessel at unit 1 in Fukushima I has collapsed as a result of the explosion; however no damage is believed to have been sustained by the reactor itself.

21:40: The evacuation zone around Fukushima I is extended to 20 km, while the evacuation zone around Fukushima II is extended to 10 km.[12]

To release pressure within reactor unit 1 at Fukushima I, steam is released out of the unit into the air. This steam contains water vapor, hydrogen, oxygen and some radioactive material, mostly tritium and nitrogen-16.

TEPCO engineers decided to directly inject sea water inside the pressure vessel of the reactors by means of the mobile trucks of the firemen, a brilliant ideas which avoided the situation to worsen. The pressure relief was also necessary to allow the firemen to inject seawater into the reactors vessels.

[edit] 13–17 March

[edit] Second week

[edit] 18–24 March

[edit] Third week

[edit] 25–31 March
  • 25 March: NISA announced a possible breach in the containment vessel of the unit 3 reactor, though radioactive water in the basement might alternatively have come the fuel storage pool.[60][61] Highly radioactive water was also found in the turbine buildings of units 1 and 2.[62] The US Navy sent a barge with 1,890 cubic metres (500,000 USgal) of fresh water, expected to arrive in two days.[63] Japan announced transportation would be provided in a voluntary evacuation zone of 30 kilometres (19 mi). Tap water was reported to be safe for infants in Tokyo and Chiba by Japanese authorities, but still exceeded limits in Hitachi and Tokaimura.[64] Iodine-131 in the ocean nearby measured 50 Bq/ml, a "relatively high" 1,250 times normal.[65]
    • White vapour, possibly steam, emanating from the buildings of reactors 2, 3, and 4.
    • The roof of the reactor 2 building has been badly damaged but is still intact.
    • The reactor 3 building is largely uncovered, its roof blown off in a hydrogen explosion over two weeks prior.
    • The walls of the reactor 4 building have also collapsed.

    [edit] April event tree

    The nuclear accident event tree continued to evolve in the second month after the earthquake and tsunami caused the accident sequence to begin.

    [edit] Fourth week

    [edit] Saturday, 2 April

    TEPCO observed for the first time that contaminated water from the unit 2 was flowing into the sea.[88] Workers discovered a crack about 20 cm (8 inches) wide in the maintenance pit, which lies between the reactor 2 and the sea and holds cables used to power seawater pumps.

    [edit] Sunday, 3 April

    The radioactive water leaked into the sea by unit 2 continued despite concrete pumped Saturday evening. Workers injected a mixture of a water-absorbing polymer, sawdust and shredded paper.[89] Radiation levels in the water were estimated at 1 Sv/h.

    TEPCO announced that the bodies of two workers killed by the tsunami were discovered on 30 March.[90]

    On Sunday, April 3, Japanese government officials said the Daiichi plant may continue to release dangerous radiation into the air for several months.[91]

    [edit] Monday, 4 April

    TEPCO began dumping water tainted with low levels of radioactivity into the Pacific Ocean on Monday night, 4 April, so that a central waste facility could be used to store more dangerously radioactive water, officials said. The company said it could release up to 11,500 tons of radioactive water into the sea. A spokeswoman for Japan’s Nuclear and Industrial Safety Agency said the less-contaminated water must be disposed of so that workers can secure a place to store more highly contaminated water on the site.[92]

    Engineers consider plans to inject inert nitrogen gas into the containment buildings of units 1, 2 and 3 to expel atmospheric oxygen and to dilute accumulated hydrogen, which combine explosively.[93]

    [edit] Tuesday, 5 April

    It was determined that the leak in the cable storage pit by unit 2 was likely due to a faulty joint where the pit meets a duct, leading to a gravel layer beneath, resulting in highly radioactive water pouring directly into the sea.[94][95]

    [edit] Wednesday, 6 April

    TEPCO announced that an injection of 6,000 litres (1,600 USgal) of polymer coagulant into the pit mitigated the leaking;[96] however, the IAEA and others credit additional factors.[97] Sodium silicate, also known as "water glass", and additives were injected into the ground in order to stop the leakage of radioactive water.[98] The residual heat carried by the water used for cooling the damaged reactors accelerated the setting of the injected mixture.

    Despite protests from the South Korean government, Russian scientists, and Japanese fishermen, Japan authorized the release of the 11,500 tonnes (12,700 tons) of less radioactive water into the ocean to make room to store more highly contaminated water.[95][99]

    Iodine-131 levels reached 7.5 million times the legal limit in a seawater sample taken near the facility.[95]

    [edit] Thursday, 7 April

    Nitrogen injection into the pressure containment vessel of unit 1 was commenced at 01:31.[100]

    A large aftershock, later downgraded from a 7.4 to a 7.1 by USGS, occurred. A tsunami warning was also issued but lifted after 90 minutes. Most of the workers at the nuclear plant were evacuated. TEPCO reported that no further damage to the nuclear plant was detected after this earthquake.[citation needed]

    Official measures at Fukushima I reactor unit 1, however, showed a rise in temperature consecutive to the aftershock and a spiking amount of radiation in the Dry Well which exceeded the instrument maximum of 100 Sv/h.[101] Gauge B, in the meantime, has recorded a steady increase of the pressure for the previous ten days, in the same reactor.[102] Reporting the rise to 100 Sv/h up from the earlier 30 Sv/h TEPCO declared that the "validity of the measurement is questioned" both for radiation levels and pressure.

    [edit] Summary

    [edit] Fifth week

    [edit] Friday, 8 April
    [edit] Saturday, 9 April

    Japan is still struggling to keep water on the reactors to cool them and prevent further meltdown. Russian Antonov An-124 cargo planes flew out of Atlanta and Los Angeles, each carrying a huge concrete boom pump. The two 95-ton boom pumps which TEPCO purchased for $2million each, can be operated from two miles away by remote control. Each boom pump can direct focused streams of water into the damaged reactors.[154]

    Currently TEPCO does not plan to take a Chernobyl approach to resolving nuclear power plant crisis by entombing the radioactive material in concrete.[155] If this decision were to change, the boom pumps could be retrofitted to deliver concrete for that purpose.[154]

    [edit] Monday, 11 April

    Coolant injection into reactors 1 and 3 was interrupted for 50 minutes due to a loss of power after a strong earthquake.[156]

    [edit] Tuesday, 12 April

    Japan officially raises Fukushima to INES Level 7, the same as Chernobyl.[157][158] This new rating considers the accidents as a single event and uses estimated total release to the atmosphere as a justification.[159] At Chernobyl, approximately 10 times the amount of radiation was released into the atmosphere as was released from Fukushima I through April 12, 2011.[160]

    After cooling efforts at spent fuel pool 4 were halted due to an erroneous warning about the pool filling up[161], the temperature of the pool rose to 90 °C and the dose rate 6 meters above the pool spiked at 84 mSv/h.[162]

    [edit] Summary

    Before the crisis evaluation was elevated by Japanese authorities to level 7, the highest level, experts already recognized that Fukushima is the most complicated nuclear accident ever.[163]

    [edit] Sixth week

    Plans were announced for a large-scale study on the environmental and health effects of radioactive contamination from the nuclear plant. Academics and researchers from across Japan will work with the Fukushima Prefectural Government starting in May.[167]

    Nuclear fuel was reported to have melted and fallen to the lower containment sections of three reactors, including reactor three. The melted material was not expected to breach a container (which might cause a massive radiation release). Instead, the melted fuel was thought to have dispersed fairly uniformly across the lower portions of the containers of reactors No. 1, No. 2 and No. 3, which would make the resumption of the fission process, to the extent of a recriticality accident, most unlikely.[168] However, it is only during future dismantling of the three damaged reactors that it would be possible to verify this hypothesis and to know what really occurred inside the reactor cores.

    [edit] Monday, 18 April

    The Associated Press is reporting that two PackBot ground robots from iRobot have entered Unit 1 and Unit 3 of the crippled Fukushima nuclear power plant and performed temperature, pressure, and radioactivity measurements. The remote-controlled robots entered the two reactors over the weekend. The devices opened closed doors and explored the insides of the reactor buildings, coming back with radioactivity readings of up to 49 millisieverts per hour inside Unit 1 and up to 57 millisieverts per hour inside Unit 3. TEPCO officials said that the radiation data from the robots don't change their plans for shutting down the plant by the end of this year. And though more robots will be used, a TEPCO official, Takeshi Makigami, said that robots are limited in what they can do and eventually "people must enter the buildings." [169]

    Test spraying of an "anti-scattering agent" on the ground to prevent further spread of radioactive materials from the site is carried out over an area of about 1200 m2.[170]

    [edit] Tuesday, 19 April

    TEPCO began transferring excess, radioactive cooling water from the reactor No. 2's basement and maintenance tunnels to a waste processing facility.[171]

    [edit] See also

    [edit] Notes

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    Coordinates: 37°25'17?N 141°1'57?E? / ?37.42139°N 141.0325°E? / 37.42139; 141.0325

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