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This timeline has been pieced together from numerous sources; there may be inaccuracies and many of the times are only approximate. All times given are in Japan Standard Time (UTC+0900). The most recent information is at the bottom of this post.
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Friday March 11
A moment magnitude 9.0 earthquake occurs 32km underground, 177 km south-west of Fukushima. The three operating reactors, Unit 1, Unit 2 and Unit 3 automatically shutdown (“scram”), stopping fission reactions and power generation. The ‘quake severs the plant’s link to the Japanese electrical grid and emergency diesel generators kick in to power the emergency coolant system. This coolant system is required to disperse the heat produced inside the reactor by the radioactive decay of fission fragments left over from the power production process.
Units 1 and 2 are observed not to be operating properly and the plant’s operators notify the Japanese government.
The Fukushima I plant, located on the coast of Okuma is struck by a tsunami, probably around 10m in height. This damages the diesel generators.
The plant was required by regulators to be able to withstand a 5.7m tsunami.
By now the diesel generators have failed and the plant is running on battery backup. This is probably only enough power to run the control room systems, and not enough to run the emergency cooling system. The failure to circulate coolant leads to an increase in the temperature and pressure inside the reactors; and some coolant is released into the containment structure to ease the pressure. At this point the pressure inside Unit 1 is more than twice the normal level.
Saturday March 12
Inside Unit 1, steam containing a small amount of radioactive tritium and nitrogen-16, is vented into the reactor building to relieve pressure. Later in the day this radioactive steam is vented into the atmosphere.
TEPCO, the operators of the plant, confirm that radioactive steam has been vented from Unit 1 into the atmosphere.
Radioactive steam is vented from Unit 2 into the atmosphere.
Another earthquake triggers an explosion inside the reactor building of Unit 1. The explosion is due to a buildup of hydrogen produced by the reaction of the fuel rods’ zirconium cladding with steam from the coolant water. The walls and roof of the building collapse away as they are designed to do and the reactor containment structure is not compromised. Four workers are injured.
Sunday March 13
The Japanese nuclear authorities declare the incident at Fukushima I as an INES Level 4 “accident with wider consequences”.
Japanese authorities announces that hydrogen is building up inside the Unit 3 reactor building.
Monday March 14
The reactor building of Unit 3 explodes, due to a buildup of hydrogen. Again, the reactor vessel and the containment structure are undamaged as the building’s walls and roof are blown off. The explosion at Unit 3 appears – on video, at least – more powerful than the previous explosion at Unit 1, one worker is injured and seven more are reported as missing. It is not yet noticed, but the explosion at Unit 3 damages the roof of Unit 4.
Cooling for Unit 1 is temporarily restored.
Fuel rods in Unit 2 have been completely exposed to air (i.e. with no coolant water surrounding them), but reactor vessel and containment structure maintain integrity. Seawater is injected into the unit in order to cool it.
Fuel rods in Unit 2 reactor have been fully exposed to air again, due to an error that prevented coolant water entering the unit.
Tuesday March 15
A fire breaks out at Unit 4, a reactor that was already in cold shutdown at the time of the quake (the IAEA states that this fire started at 2354 the previous day). The problem with Unit 4 is that it contains a spent fuel pond, where fuel that has been through the reactor is stored as it cools down. This fuel is not inside a protective containment structure or reactor vessel as the fuel in the operating units is, and this fire probably resulted in a release of radioactive material.
An explosion occurs inside the Unit 2 reactor building. This is not a hydrogen explosion as with the previous two explosions, and damages the pressure supression system at the base of the reactor, leading to the most serious release of radioactivity yet, with readings of over 8000 µSv/hour. It is suggested that the fuel rods in Unit 2 may be melting at this point.
Non-essential workers begin evacuation, leaving behind the “Fukushima 50” to get the plant under control. It is at this point that the damage to the roof of Unit 4 caused by the explosion at Unit 3 is noticed. If the damage is severe enough then the surface of the spent fuel pond will be exposed to the air.
Another explosion occurs at Unit 4, which starts another fire.
The fire at Unit 4 is extinguished.
Japanese authorities state that the water inside the spent fuel pond at Unit 4 may be boiling. This suggests that the coolant in which the fuel rods are submerged is not being properly circulated and that the rods are beginning to overheat.
TEPCO inform the Japanese government that the coolant water level in Unit 5 is decreasing.
Wednesday March 16
Unit 4 is reported to be on fire again. TEPCO officials state that the spent fuel pond may now be empty of coolant and that the rods are overheating. They state that there is a possibility that the rods may restart fission reactions spontaneously, raising doubts as to whether they have been properly stored.
The fire in Unit 4 is no longer visible and is thought to be extinguished.
“White smoke”, later revealed to be steam, is seen issuing from the Unit 3 building. This is thought to be the result of water in the empty spent fuel pond boiling away.
The fire in Unit 4 is confirmed to be extinguished. TEPCO considers spraying boric acid into the fuel ponds in order to prevent the spent fuel rods from starting a new nuclear chain reaction but this is ruled out due to high levels of radiation.
Unit 3 reactor begins issuing “white smoke”. This is thought to be radioactive steam from inside the reactor.
TEPCO evacuates the Fukushima 50 due to a surge in the level of radioactivity. A reading of 3391 microsieverts per hour is recorded at the boundary of the plant site.
The Fukushima 50 return to the site after radiation levels decrease.
The Fukushima 50 are again evacuated due to another surge in radiation levels.
The Fukushima 50, and further additional workers return to the site.
Thursday March 17
US regulators claim that the spent fuel pond at Unit 4 is empty. Japanese authorities deny this and claim that the condition of Unit 4 is “stable”.
There is a spike in the pressure of the supression system of Unit 3, indicating that it is overheating. TEPCO express concern about the status of the spent fuel pond at Unit 3.
The Japanese Self Defense Force use a modified lead-shielded Chinook helicopter to drop water directly onto Unit 3.
The use of helicopters is discontinued and spraying operations using fire-fighting equipment begin from the ground instead. These trucks only carry only limited supplies of water and thus their effect is limited.
Yukio Edano, a spokesman for the Japanese governement, declares that Unit 3 is the top priority as far as cooling efforts are concerned. Spraying by water cannon from the ground by Japanese Self Defense Force continues.
Work begins to reconnect the power station to the electrical grid. This is very significant as power would therefore be available to run the normal coolant systems, but it is not clear what state these coolant systems would be in and whether or not they are fully functional.
Friday March 18
Spraying on Unit 3 resumes with assistance from Tokyo Fire Department.
Saturday March 19
The plant is reconnected to the electrical grid, but the new power supply is not yet activated.
Sunday March 20
The power supply to Unit 2 was re-established.
The pace of news coming from the plant has slowed considerably. This is a reflection of the increasingly stability of the situation there. Reactors 5 and 6 are in cold shutdown, which means they are safe, and power has been reconnected to some of the other units.
Wednesday March 23
Power cables were connected to all Units.
Thursday March 24
Three workers are hospitalised after standing in radioactive water in the turbine building of Unit 3.
Friday March 25
Radioactive water similar to that found in Unit 3 is found in Units 1 and 2.
Saturday March 26
Due to concern that salt is accumulating inside the reactor, seawater used for emergency cooling is replaced with fresh water. This is possible after a US Navy barge containing two million litres of fresh water arrives at the plant.
Sunday March 27
Video from a Self Defense Force helicopter shows steam coming from Units 1, 2 and 3.
Monday March 28 – Sunday April 3
Airborne monitoring reveals low dose rates in the air around the plant, whilst plutonium was detected in the soil. A 20cm crack in the base of the Unit 2 building is allowing radioactive water to seep into the sea. The bodies of two workers, killed by the tsunami that struck the plant, were recovered on Sunday.
Monday April 4 – Sunday April 10
TEPCO dumped low level radioactive water from holding tanks into the ocean in order to make room for high level radioactive water from the Unit 1, 2 and 3 reactors. Nitrogen gas is being injected into the reactor in order to prevent any explosion and “liquid glass” was used to seal the reactor and harden the ground to prevent the release of any more radioactive water.
Monday April 11 – Sunday April 17
Two substantial (MW>7) earthquakes disrupted recovery work.