Friday, 11 March 2011 (Day 1)
An earthquake of magnitude 9.0 occurs off the eastern coast of Japan causing the Fukushima Dai-ichi 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 Dai-ichi 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 Dai-ichi 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 Dai-ichi. 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 Dai-ichi 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. 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 Dai-ichi 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 Dai-ichi 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.
Last reviewed: 8 April 2011
