Timeline for the Fukushima Daiichi nuclear power plant accident

Friday, 11 March 2011 (Day 1)
An earthquake of magnitude 9.0 occurs off the eastern coast of Japan causing the Fukushima Daiichi nuclear power plant (NPP) units 1, 2 and 3 to shut down automatically. Units 4, 5 and 6 have been previously shut down for outages, with unit 4 having been defueled in November 2010. Offsite power is lost. Emergency diesel generators (EDGs) provide power for the emergency core cooling systems for a short time. A tsunami strikes the Fukushima facility. Onsite EDGs stop working. The steam-driven reactor core isolation cooling (RCIC) systems and high-pressure coolant injection (HPCI) system (unit 3) provide cooling to units 1, 2 and 3. An evacuation order is issued for persons within 3 km of the Fukushima Daiichi NPP.

Saturday, 12 March 2011 (Day 2)
Back-up battery supplies are depleted. The ability to cool the reactors of units 1, 2 and 3 is significantly degraded or unavailable. Discharges to suppression chambers designed to control pressure within the reactor coolant system cause pressure within the primary containments to increase. Venting of the unit 1 primary containment begins. Evacuation of residents within 10 km of the Fukushima Daiichi NPP is underway. A hydrogen explosion occurs in the unit 1 reactor building destroying the upper structure of the building where the spent fuel pool is housed. The unit 1 spent fuel pool is exposed to the atmosphere. The evacuation zone is extended to 20 km around Fukushima Daiichi.  Seawater is injected into the unit 1 reactor.

Sunday, 13 March 2011 (Day 3)
Venting of the unit 3 primary containment begins, and seawater is injected into the unit 3 reactor.

Monday, 14 March 2011 (Day 4)
A hydrogen explosion occurs at unit 3. The upper structure of the unit 3 reactor building is significantly damaged. The unit 3 spent fuel pool is exposed to the atmosphere. It is reported that the water level for the reactors of units 1, 2 and 3 is below the top of active fuel. Fuel damage is suspected in all three units. The containments at the three units remain intact. Seawater is injected into the unit 2 reactor.

Tuesday, 15 March 2011 (Day 5)
A fire is reported at unit 4. Damage to the top levels of the unit 4 reactor building is confirmed. Venting of the unit 2 primary containment begins. A hydrogen explosion occurs within the unit 2 reactor building. The suppression chamber (wetwell) of the primary containment is suspected to have been damaged. The unit 2 reactor building appears to remain intact. A fourth explosion occurs at the site: unit 4 sustains additional damage to the upper portion of the reactor building. The risk of water boiling in the unit 4 spent fuel pool is reported. The water level in the unit 5 reactor decreases to about 200 cm above the top of active fuel. The operational unit 6 EDG begins supplying power to the cooling systems at both units 5 and 6.

Wednesday, 16 March 2011 (Day 6)
A fire is again reported in the unit 4 reactor building in the area of the spent fuel pool. Evacuation of the 20 km zone surrounding the Fukushima Daiichi NPP is completed. Steam is observed coming from the unit 3 reactor building, indicating that the water in the spent fuel pool is likely boiling.

Thursday, 17 March 2011 (Day 7)
Helicopters are used to dump seawater into the unit 3 spent fuel pool for the first time. In addition, emergency crews begin spraying seawater into the unit 3 spent fuel pool using specialised fire fighting and riot control water cannons.

Friday, 18 March 2011 (Day 8)
Seawater continues to be sprayed into the unit 3 spent fuel pool. Adding cool water to the spent fuel pools of units 1, 2, 3 and 4 becomes the highest priority.

Saturday, 19 March 2011 (Day 9)
Seawater continues to be sprayed into the unit 3 spent fuel pool. Units 5 and 6 continue to be powered by an operational unit 6 EDG. Both unit 6 EDGs are operational and provide power to units 5 and 6. Spent fuel pool cooling at units 5 and 6 begins. It is reported that milk and spinach from areas around the plant have radiation levels that exceed Japanese standards. There are reports of higher than normal levels (though below allowable levels) of radioactive iodine and caesium-137 in water supplies away from the site in regions including Tokyo (traces of iodine). Tap water in Fukushima is found to have higher than allowed levels of radioactive iodine.

Sunday, 20 March 2011 (Day 10)
The spent fuel pool temperatures at units 5 and 6 are reported to be decreasing. Units 5 and 6 reach cold shutdown conditions. Crews continue to spray seawater into the unit 3 spent fuel pool. Forty tonnes of seawater have been injected into the unit 2 spent fuel pool. The unit 2 temporary power centre is powered by offsite sources.

Monday, 21 March 2011 (Day 11)
Offsite power is available to units 1, 2, 5 and 6. Power from units 1 and 2 is diverted to a temporary distribution system. Testing of equipment affected by the earthquake and tsunami begins at units 1 and 2. Power to unit 5 is switched from the unit 6 EDG to offsite power. A government directive is issued requesting relevant businesses and individuals to suspend shipment of spinach, kakina (a green vegetable) and raw milk for the time being.

Tuesday, 22 March 2011 (Day 12)
Offsite electrical power is currently available at units 3 and 4, so all six units now have external power. Testing of components continues before reconnecting power at units 1 and 2. Units 5 and 6 are in cold shutdown with cooling of the spent fuel pools continuing. Sampling of seawater downstream of the units 1, 2, 3 and 4 discharge canal detects levels of radioactive iodine (131) and caesium (134 and 137) that exceed regulatory limits.

Wednesday, 23 March 2011 (Day 13)
Crews continue to spray water into the spent fuel pools of units 3 and 4. Work to recover power for units 1 through 6 is in progress. Integrity checks of electrical equipment is ongoing in each unit and must be completed before restoring power. Lighting is restored in the main control room of unit 3. External electrical power is replaced by an emergency diesel generator in units 5 and 6.

Thursday, 24 March 2011 (Day 14)
The unit 1 reactor almost reaches 400°C, exceeding its design value of 302°C; core cooling is thus increased. Work is temporarily suspended when black smoke is observed at unit 3.  No increase in radiation levels are observed. Vapour or steam is observed coming from units 1, 2, 3 and 4, marking the first time that steam is observed coming from unit 1. Three workers installing electrical cables in the unit 3 turbine building are exposed to high levels of radiation and contamination. Two workers are sent to the hospital and are suspected to have received burns from high-level beta radiation. Three workers are exposed to radiation doses between 170mSv/hr and 180mSv/hr. The Tokyo electric Power COmpany (TEPCO) now reports that 17 workers have received a dose of 100mSv or more.

Friday, 25 March 2011 (Day 15)
External electrical power to the main control room at unit 2 will be available today. Unit 1 reactor temperature decreases from about 400°C to 204.5°C as of 06:00. TEPCO suspects that nuclear fuel in the reactor or spent nuclear fuel in the pool has been damaged and that water contaminated with high radioactivity has leaked to the workspace. Surface temperatures of units 1, 2, 3 and 4 are below 20°C.  The surface temperature of the spent fuel pool at unit 3 has dropped to 31°C from 56°C on the previous day.

Saturday, 26 March 2011 (Day 16)
Crews switch from spraying seawater to spraying fresh water with a boric acid injection in the unit 2 and 3 reactor pressure vessels, and from using fire engine pumps to electrical pumps. Lights in the control rooms of units 2 and 3 are restored, with lights now available in the control rooms of units 1, 2 and 3.

Sunday, 27 March 2011 (Day 17)
Periodic water spraying using a concrete pump truck continues for the unit 4 spent fuel pool. TEPCO reduces the amount of water injected into the unit 2 reactor to avoid leakages to the turbine building.

Monday, 28 March 2011 (Day 18)
Plutonium is detected in the soil of the Fukushima Daiichi NPP site. The detected level poses no threat to public health. TEPCO faces challenges in cooling the reactor cores as crews try to prevent leakages to the turbine buildings. Water found in the turbine buildings of units 1, 2 and 3 contains radioactive substances. The level of radiation on the surface of water puddles is more than 1 000 mSv/h in unit 2, 750 mSv/h in unit 3 and 60 mSv/h in unit 1. High levels of radiation are reported in water in a trench outside the turbine building near unit 2.

Tuesday, 29 March 2011 (Day 19)
Crews switch from spraying seawater to spraying fresh water with a boric acid injection in the unit 1 reactor vessel, and from using fire engine pumps to electrical pumps.

Wednesday, 30 March 2011 (Day 20)
Crews switch from spraying seawater to fresh water into the reactors of units 2 and 3 in order to cool the reactor cores.  White smoke is observed at units 1, 2, 3 and 4.  Water is again sprayed into the spent fuel pool at unit 4.

Thursday, 31 March 2011 (Day 21)
Fresh water supplies are replenished from water barged to the site. This fresh water will replenish the filtered water being used to cool the reactors. Water is sprayed into the spent fuel pools at units 1 and 3.  Results of the analysis of water samples taken from the turbine building sub drains on 30 March 2011 find detectable levels of fission products.

Friday, 1 April 2011 (Day 22)
Fresh water is injected into the spent fuel pool at unit 2.  Water is sprayed into the spent fuel pool at unit 4 using a concrete pumping truck.

Saturday, 2 April 2011 (Day 23)
Water is sprayed into the spent fuel pools at units 1 and 3 using a concrete pumping truck.  A second barge arrives at the site with additional fresh water to replenish the filtered water being used to cool the reactors.  Dose levels exceeding 1 000 mSv/h are detected in the pit where supply cables are stored near the intake for unit 2. A 20 cm crack is found on the side of this pit where water is flowing out.  Efforts begin to seal the pit to minimise further leakage of water into the environment.

Sunday, 3 April 2011 (Day 24)
Fresh water is injected into the reactors at units 1, 2 and 3 using electrical pumps that are powered by an off-site source. Water is sprayed into the spent fuel pool at unit 4 using a concrete pumping truck.  Efforts to seal the crack in the pit near the unit 2 intake do not reduce the leakage. Additional actions are planned to seal the crack.

Monday, 4 April 2011 (Day 25)
TEPCO announces the decision to discharge approximately 11.5 tonnes of water with low levels of radioactivity into the sea. This discharge is necessary to allow the treatment of more highly contaminated water being collected in other locations at the plant. The estimated dose to the public from this discharge is about 0.6 mSv per year for residents eating fish and seaweed from the adjacent area. This dose is about a quarter of the annual dose received by the public from natural sources.

Tuesday, 5 April 2011 (Day 25)
Water is sprayed into the spent fuel pool at unit 4 using a concrete pumping truck. The location of the leakage from the pit near the unit 2 intake structure is identified. Crews attempt to seal the leak path to minimise the uncontrolled release of contaminated water from the plant.

Wednesday, 6 April 2011 (Day 26)
The leakage of contaminated water to the sea from the pit near the unit 2 intake structure is stopped. Crews prepare to inject nitrogen gas into the unit 1 reactor containment vessel. Nitrogen gas is used to mitigate the build-up of hydrogen and oxygen mixture in the plant that could become explosive.

Thursday, 7 April 2011 (Day 27)
A magnitude 7.1 earthquake (aftershock) occurs near the Fukushima Daiichi nuclear power plant. This is the largest aftershock since the magnitude 9.0 earthquake on 11 March 2011. The impacts to ongoing activities at the plant are minimal. No changes in radiation levels or spread of contamination are noted following the aftershock. Cooling water is injected into the spent fuel pools at units 2 and 4.

Friday, Saturday and Sunday, 8-10 April 2011 (Days 28, 29 and 30)
Water spraying continues into the spent fuel pools at units 2, 3 and 4. Approximately 1 320 tonnes of low-level contaminated water is discharged from the sub drains at unit 5 and 6. This is part of a plan to release approximately 11 500 tonnes of slightly contaminated water from the site to allow for the processing and storage of more contaminated water used to cool the reactors.

11-17 April 2011 (Days 31 - 37)
The magnitude 7 earthquake on 11 April has a minimal impact on efforts to cool the reactors. On 12 April, Japan raises the INES rating of the accident from 5 to 7 based on the significant releases of radioactive material from the site. The releases are estimated at about 10% of those released from Chernobyl. Crews begin processing contaminated water from the trench near the unit 2 intake structure and transfer it to the unit 2 condenser. On 13 April, steel plates are installed in front of the unit 2 intake structure. Crews continue to spray water to cool the spent fuel pools at various units (unit 3 spent fuel pool on 14 April; unit 4 spent fuel pool on 15 and 17 April; and unit 4 spent fuel pool on 16 April). Seawater samples from the plant continue to show levels of radioactive material above regulatory limits. Efforts continue to minimise the release of radioactive material from the site. On 15 April, Japan's Nuclear and Industrial Safety Agency (NISA) instructs TEPCO to increase the number or sample points and to take marine samples in order to better characterise and assess the release of radioactive material. On 17 April, TEPCO issues its Roadmap towards Restoration from the accident. Crews begin using remote controlled robots to take radiation readings inside the units 1 and 3 reactor buildings.

18-25 April 2011 (Days 38 - 45)
Crews continue to cool the spent fuel pools at units 1 to 4 by spraying water from concrete pumping trucks or by injecting water directly into the pools with electrical pumps. Cooling of the reactors at units 1 to 3 is ongoing. Efforts continue to manage the spread of contamination from the site with steel plates installed at the unit 2 intake structure and double-layered silt screen fences installed in front of the other intake structures. Activities to prepare the centralised radioactive waste treatment facility to process the contaminated water collected on site are completed and transfers start on 19 April 2011.

26 April-2 May 2011 (Days 46 - 52)
Crews continue to cool the spent fuel pools at units 1 to 4 by spraying water from concrete pumping trucks at unit 4 and by injecting water directly into the pools with electrical pumps at units 1 to 3. Cooling of the reactors at units 1 to 3 is ongoing. Efforts continue to manage the spread of contamination from the site. Contaminated water continues to be transferred to the centralised radioactive waste treatment facility for processing to remove radioactive material. NISA asks TEPCO to analyse the effects of flooding the unit 1 primary containment until the exterior of the reactor pressure vessel is covered with water up to the level of the top of active fuel. Core damage estimates are revised on 27 April as follows: unit 1 = 55% (previously reported at 70%); unit 2 = 35% (previously reported at 30%); and unit 3 = 30% (previously reported at 25%). Visual inspections and analysis of water samples indicate that damage to the fuel in the unit 4 spent fuel pool is not as significant as previously thought.

3-9 May 2011 (Days 53-59)
Crews continue to cool the spent fuel pools at units 1 to 4 by injecting water directly into the pools with electrical pumps at units 1 to 3 and by spraying water from concrete pumping trucks at unit 4. Cooling of the reactors at units 1 to 3 is ongoing. Work begins on 2 May to set up a ventilation system in order to improve the working environment inside the reactor building of unit 1. Analyses performed by TEPCO confirm that flooding the unit 1 primary containment (drywell and wetwell) to cover the reactor vessel above the level of the top of active fuel will not challenge its structural integrity. On 6 May the injection flow rate to the unit 1 reactor is increased so that the water level in the reactor vessel can be increased to above the top of active fuel. On 8 May activities are conducted to support opening the unit 1 reactor building airlocks. The airlocks are opened on 9 May and measurements of the air dose rate confirm that there is no radiation impact to the outside environment.

10-17 May 2011 (Days 60-67)

Crews continue to cool the spent fuel pools at units 1, 2 and 4 by injecting water directly into the pools with electrical pumps and concrete pumping trucks. Water is injected into the unit 3 spent fuel pool using the spent fuel pool cooling and filtering system. Cooling continues at the reactors at units 1 to 3. The rate of water being injected into the reactors at units 1 to 3 is increased to support increased cooling as additional equipment becomes available. Crews continue to inject water through the fire extinction system and through the reactor feedwater system.  Boric acid is also injected with the water for criticality control. Work begins on 13 May to install a temporary cover over the unit 1 reactor building to minimise the further spread of radioactive material and to support recovery activities. Activities continue to transfer the radioactive water collecting in the reactor and turbine building basements and sumps for treatment. Crews continue to spray dust inhibitors around the site to minimise the spread of contamination.On 17 May, analyses performed by TEPCO indicate that significant fuel melting occurred in the unit 1 reactor core with relocation of molten fuel to the lower portion of the reactor vessel.  Further, the high-temperature molten fuel (greater than 2 800°C) appears to have caused small leaks in the lower head of the reactor pressure vessel.

18 May-17 June 2011 (Days 68 - 99)
Crews continue to cool the spent fuel pools at units 1 to 4 by injecting water directly into the pools with electrical pumps and concrete pumping trucks.  Cooling of the reactors at units 1 to 3 is ongoing.  Activities continue to transfer and treat radioactive water that is collecting in reactor and turbine building basements and sumps. Crews continue to spray dust inhibitors around the site to minimise the spread of contamination. On 7 June 2011, the Government of Japan releases a report prepared by the Government Nuclear Emergency Response Headquarters for the IAEA Ministerial Conference on Nuclear Safety (20-24 June 2011, Vienna, Austria). This report includes analyses of the damage to the cores at units 1 to 3.  The analyses show that unit 1 has suffered extensive fuel melting and that the reactor pressure vessel has been breached. Units 2 and 3 have fuel damage but it appears that the reactor pressure vessels are intact. The analyses also show that most of the fuel is located within the reactor pressure vessels and that, although the core geometry has changed by continuing the current injection of water, large releases of fission products should not occur in the future.  On 10 June 2011, NISA reports that two workers have received doses in excess of both the 250 mSv emergency exposure limits established by Japan and the internationally recommended 500 mSv emergency exposure limit. The doses received were 678 mSv (external exposure 88 mSv, internal exposure 590 mSv) and 643 mSv (external exposure 103 mSv, internal exposure 540 mSv).  Medical examinations confirm that there is no immediate health impact to the workers.

18 June -17 August 2011 (Days 100 - 160)
Cooling of the reactors at units 1 to 3 is on-going. Activities continue to confirm the functional capability of the water injection systems to provide the flow rates necessary to achieve and maintain cold shutdown conditions of all three reactors. Temperatures at the bottom of the reactor pressure vessels (RPVs) continue to decline slowly.  As of 16 August, unit 1’s RPV temperature was at slightly less than 100 °C, unit 2’s was at about 115 °C, and unit 3’s was just over 100 °C. In late July and early August the licensee initiated circulating cooling in the spent fuel pools at units 1 and 4. All four spent fuel pools continue to be adequately cooled with temperatures of 34 °C at units 1 and 3, 37 °C at unit 2, and 43 °C at unit 4 (as of August 16).  Several earthquakes of varying magnitude have occurred during this period with no reported impact on the Fukushima Daiichi site. On 30 July, installation of support structures for the spent fuel pool at unit 4 is completed. On 10 August, steel-framing for the unit 1 reactor building cover begins. Crews continue to remove debris from the site. As of 16 August, approximately 49 230 t of contaminated water have been processed. On 17 August, TEPCO issues a progress status on the “Roadmap towards Restoration from the Accident at Fukushima Daiichi Nuclear Power Station”.

16 December 2011
On 16 December 2011, Japanese Prime Minister Yoshihiko Noda announced that the three reactors at Fukushima Daiichi, damaged by the 11 March earthquake and tsunami, are in a stable state of cold shutdown. All three units are being maintained at around 70°C and dose rates at the periphery of the site are at or below 1 mSv per year.

7 March 2012
Step 2 of TEPCO’s Roadmap towards Restoration for the Accident at Fukushima Daiichi Nuclear Power Station  has been completed as indicated in the March 2012 Road to Recovery report by the Government of Japan.  The three affected reactors at the Fukushima Daiichi site remain in a stable, cold shutdown condition.  Mid- and long-term recovery actions have begun in a phased approach. Phase 1, to be conducted over the next two years, includes removing the spent fuel from the unit 4 spent fuel pool; reducing the radiation impact due to additional releases of accident-generated radioactive waste; maintaining stable reactor cooling; processing accumulated contaminated water; beginning research and development as well as decontamination activities to support removal of fuel debris; and initiating research and development activities for radioactive waste processing and disposal.  Phase 2, which will take ten years, involves removing the remaining spent fuel from all other spent fuel pools and starting the removal of the fuel debris from the damaged reactors.  Phase 3 will focus on the decommissioning of the site after the fuel debris has been removed.  Decommissioning is anticipated to be completed in 30-40 years from the date of the accident.

Last reviewed: 7 March 2012