Timeline of the Fukushima nuclear accident

The earthquake triggered a scram shut down of the three active reactors, and the ensuing tsunami crippled the site, stopped the backup diesel generators, and caused a station blackout.

TEPCO actually made this prediction in 2008 but delayed in submitting the report because they "did not feel the need to take prompt action on the estimates, which were still tentative calculations in the research stage".

[4] According to a report in the New York Times, "... at 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.

[27] The Japan Atomic Energy Agency announced that it was rating the situation at unit 1 as level 4 (an accident with local consequences) on the International Nuclear and Radiological Event Scale.

[28] After watching the helicopter effort on TV Kazunori Hasegawa, president of Chuo Construction, calls the government and offers the use of his two truck-mounted concrete boom pumps to spray water directly into the reactors.

[57] While joining in a generally positive assessment of progress toward overall control, Japanese chief cabinet secretary Edano confirms, for the first time, that the heavily damaged and contaminated complex will be closed once the crisis is over.

[59] Ongoing repair work is interrupted by a recurrence of grey smoke from the south-east side of unit 3 (the general area of the spent-fuel pool) seen at 15:55 and dying down by 17:55.

[67] The Japanese Chief Cabinet Secretary also advises that high levels of radioactivity have been found in Tokyo's drinking water and that it should not be used to reconstitute baby formula as it is around twice the legal limit for children.

[74][75] They were not wearing protective boots, as their employing firm's safety manuals "did not assume a scenario in which its employees would carry out work standing in water at a nuclear power plant".

[76] NISA announces a possible breach in the containment vessel of the unit 3 reactor, though radioactive water in the basement might alternatively have come from the fuel storage pool.

[107] Workers discover 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.

TEPCO then reattempts to plug up the trench that leads to the damaged storage pit with a combination of superabsorbent polymer, sawdust and shredded newspaper, which also fails.

[111][112] TEPCO confirms the first deaths at the Fukushima facility, two workers who had been missing since 11 March and appear to have died in the basement of reactor 4 from bleeding due to multiple injuries inflicted by the tsunami.

[114] 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.

[citation needed] Official measures at Fukushima I reactor unit 1, however, show a rise in temperature following the aftershock and a spiking amount of radiation in the Dry Well, which exceeds the instrument maximum of 100 Sv/h.

Japanese news wire NHK reports that workers are laying hoses to transfer water to an LLW waste processing facility, which continues to be inspected.

[184] Junichi Matsumoto, a general manager at Tokyo Electric Power Co., reports that radiation readings taken by two iRobot PackBot robots inside the reactor 1 building are as high as 1120 mSv/h, the highest level disclosed to date.

[187] T. Matsui of the University of Tokyo Institute of Physics releases a scientific paper analysing the ratio of iodine-131 to caesium-137 taken from water samples, which concludes that a recriticality may have occurred at least 10–15 days after the attempted shutdown.

[201] 16 hours after the earthquake and SCRAM,[15] the fuel rods of reactor 1 had "mostly melted and fallen into a lump at the bottom of the pressure vessel – a state that TEPCO officials have described as a 'meltdown'".

With data from the government report, the newspaper compares the March timelines provided by TEPCO and by NISA, which had performed further analysis; there were differences in the theoretical timing of events of up to 29 hours in the days following the tsunami.

[234] The Itabashi Ward Office announces that a concentration of caesium-134 at 2,700 becquerels per kilogram – in excess of the government's provisional limit – was detected in tea processed from leaves picked on 9 May in Tokyo.

This reports the results of an evaluation of the regulations to prevent and handle an occurrence of the full loss of alternating current (Station Blackout or SBO) in nuclear power plants in Japan and other countries.

[261] TEPCO retracts Wednesday's statement about a possible self-sustained fission reaction, and now claims that the xenon was a result of the normal decay of radioisotopes in the fuel.

[268] This announcement failed to lay to rest substantial concerns arising from TEPCO's inability to directly measure temperatures at the bottoms of the containment vessels, and the fact that the site is too radioactive for visual confirmation of the fuel rods' status.

[274] TEPCO begins pouring a 60-centimetre-thick layer of concrete over 70,000 square metres of the ocean seabed near the Fukushima Daiichi power plant in an effort to contain contaminated sediments.

"[286] Public trust in TEPCO further declines[287] as cooling systems for several spent-fuel pools go down for more than 29 hours[288] after a power interruption that may have been caused by a rat in a switchboard.

[290] As well, tritium levels in seawater from the port next to the plant have been increasing since May, and a water sample taken on 3 July showed 2,300 becquerels per litre, the highest measurement since the start of the nuclear crisis in March 2011.

Trade minister Toshimitsu Motegi says "We've allowed Tokyo Electric to deal with the contaminated water situation on its own and they've essentially turned it into a game of Whack-a-Mole".

[302] TEPCO begins to construct facilities to establish an ice wall within the ground around the reactors to prevent contaminated water to mix with groundwater flowing under the area.

[313] TEPCO estimates that 749,000 cubic metres of contaminated debris from Fukushima Daiichi will be produced by the year 2027, and releases a plan for incineration and storage of this waste.