Fukushima Daiichi is 1 of 2 multi-reactor nuclear power sites in the Fukushima Prefecture of Japan. A nuclear disaster occurred there after a 9.0 magnitude earthquake and subsequent tsunami on 11 March 2011. 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. The subsequent lack of cooling led to explosions and meltdowns, with problems at three of the six reactors and in one of the six spent-fuel pools.
Times are given in Japan Standard Time (JST), unless noted, which is UTC plus nine hours.
March 2011 Monday, 7 March Tokyo Electric Power Company (TEPCO) submits a report to Japan's nuclear safety agency which predicts the possibility of a tsunami up to 10.2 metres high at the Fukushima Daiichi nuclear plant in the event of an earthquake similar to the magnitude 7.2 earthquake with accompanying tsunami that devastated the area in 1896. 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".
Friday, 11 March 14:46: A 9.0 magnitude earthquake strikes off the coast of Honshu Island at a depth of about . The Fukushima I power plant's nuclear reactors 1, 2, and 3 are automatically shut down by the tremor. Nuclear reactors 4, 5, and 6 were undergoing routine maintenance and were not operating (reactor 4 was defueled in November 2010). The tremor also cut the power plant off from the Japanese electricity grid; however, backup diesel generators kicked in to continue cooling. See extract from earthquake log below (the numbers e.g. D564, indicate the PID controller triggering the log event). 14:46:46: 400 ms D564 Seismic (sensor) Trip 14:46:46: 410 ms D534 Reactor SCRM A Trip (Reactor A SCRAM started) 14:46:58: 420 ms D563 Seismic (sensor) Trip 14:46:58: 420 ms D535 Reactor SCRM B Trip (Reactor B SCRAM started) 14:47: A553 ALL CR FULL IN (all control rods full in) ON 14:47: G002 GENERATOR VOLT 18.56 > 18.50 kV (indicates the diesel generators are running above minimum voltage) 14:47: Nuclear reaction has stopped, however decay heat generation continues. At SCRAM, 6.5% of the previous output power is still generated, falling to 1.5% after an hour, and approximately 0.4% after 24 hours. 14:52: Reactor 1's emergency cooling system, which was capable of running without external power, turned on automatically. 15:03: Reactor 1's emergency cooling system was manually shut down to avoid reactor damage (temperature was not critical at this point). The operators followed the manual, shutting down the emergency cooling system as reactor temperature dropped sharply. 15:08: 7.4 magnitude aftershock 15:15: 7.7 magnitude aftershock 15:25: 7.5 magnitude aftershock 15:27: The first tsunami wave, of a series of seven waves, struck the plant. This wave was measured by the tidal gauge at 4 m (13 ft) and did not overcome the site design of 5.7 m (18.7 ft), being mitigated by the seawall. 15:30: The emergency condenser designed to cool the steam inside the pressure vessel of the No. 1 reactor fails.The Mainichi Daily News TEPCO admits nuclear meltdown occurred at Fukushima reactor 16 hours after quake 15:35: A major wave arrives, exact height is unknown as the tidal gauge failed after providing a maximum reading of 7.5 m (24.5 ft). Estimates based on water damage on buildings indicate at least one wave arrived at the station of 14 m to 15 m approximately (46 to 49 ft). Water surged over the site to a depth of 4 m to 5 m (13 to 16 ft), Unit 1 reactor building and turbine hall were the lowest relative to sea level. Water flooded the turbine halls via generator air intakes, doors and once the surge reached the rear of the building, an access hatch. The basement housing the emergency diesel generators starts to flood, the generators being located at the seaward side (front) of the turbine halls basement, with switchgear adjacent. The battery room was located below the control room at the rear basement of the turbine hall, with switchgear adjacent. Sea water intake structures on the shoreline for all six units are unavailable due to damage to pumps, strainers and equipment and flooding caused electrical faults. The wave also washes away or damages the diesel fuel tanks, located on the floodplain. With the loss of all electrical power supply, the low-pressure core spray, the residual heat removal and low-pressure coolant injection system main pumps, and the automatic depressurization systems all failed (most of the emergency core cooling system). Only the steam-powered pump systems (isolation condenser in reactor 1, high-pressure coolant injection and reactor core isolation cooling system in reactors 2 and 3) remained available. 15:37: Unit 1 AC power lost 15:38: Unit 3 AC power lost 15:41: Unit 2 AC power lost Units 1 & 2 125-volt DC back-up batteries flooded and failed, Unit 3 batteries provide power for 30 more hours 15:46 (approximate): Reactors 1 to 4 lose both AC power (from the external grid, and internal emergency diesel generators), and DC power from the battery banks. Reactor 1 & 2 control room also loses all DC power. Electrical cooling pumps fail, electrically operated valves are disabled and instruments in the control room fail. Reactor 1 Isolation Condensor (IC) valves fail – close with loss of power, disabling the IC cooling system. Reactor decay heat is at approximately 1.5% prior output (22 MW Unit 1, 33 MW Units 2 & 3). This causes the reactor temperature and pressure to start rising.
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. 2 reactor at Daiichi, 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." 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. There is no report that radiation was detected outside power-plant borders. 18:00 (estimated): The falling water level in reactor 1 reaches the top of the fuel, and the core temperature starts climbing. 18:18: Operators attempted to start reactor 1's emergency cooling system, but it did not function. 19:03: Prime Minister Naoto Kan declares a nuclear emergency status 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. 19:30 (estimated): The fuel in reactor 1 becomes fully exposed above the water surface, and fuel damage in the central core begins soon after. 21:00: An evacuation order is issued by the government to persons within a radius of from the Fukushima I station. Those within a radius of are told that they can remain in their homes, and carry on with regular activities, until told otherwise. TEPCO announces that the pressure inside reactor unit 1 of Fukushima I is more than twice normal levels. 21:00: The first mobile generator sets arrive onsite, further units arriving through the night. Workers check motors and switchgear in attempt to restart cooling of the reactors, testing reveals Unit 2 standby liquid control (SLC) pumps to be undamaged, the first mobile generator is placed adjacent to Unit 2 and workers start to run cable to the SLC pump distribution panel.
Saturday, 12 March
05:30: In order to release some of the pressure inside the containment at unit 1, the decision is made to vent some of the steam (which contained a small amount of radioactive material) into the air, despite the risk of hydrogen (produced from the water in the reactor) igniting after combining with oxygen. 05:50: Fresh water injection into reactor 1 is started. 06:50: Although unknown at the time, the core of reactor 1 has now completely melted and falls to the bottom of the reactor pressure vessel. 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. 10:58: Pressure remains too high inside reactor unit 2 at Fukushima I. In order to alleviate some of this pressure, a decision is made to vent radioactive vapor into the air. 14:00: Operators open the containment venting line of unit 1 and receive confirmation of steam release to the atmosphere at 14:30. 14:50: Fresh water injection into reactor 1 is halted. 15:30: Evacuation of residents within 3 km of Fukushima II and within 10 km of Fukushima I is underway. 15:30: Workers complete connection cabling to the Unit 2 SLC pumps, powering up the mobile generator outside Unit 2. 15:36: There is a massive hydrogen explosion in the reactor building of unit 1. The primary containment is not damaged, but there is extensive damage to the secondary containment (the reactor building). Five workers are injured. The mobile generator and cabling to Unit 2 SLC pump are damaged. Hoses providing seawater injection to Unit 1 and Unit 2 are also damaged in the blast. Debris covers the site, some of which is highly radioactive. Workers are evacuated to the Emergency Response Center for headcounts and for the wounded to be treated. 19:00: Seawater injection into reactor 1 is started. TEPCO orders Daiichi to cease seawater injection at 19:25, but Daiichi plant boss Masao Yoshida orders workers to continue with the seawater injection.Kyodo News, "Disregard Tepco order, boss told plant workers", Japan Times, 30 November 2011, p. 1. 21:40: The evacuation zone around Fukushima I is extended to 20 km, and the evacuation zone around Fukushima II is extended to 10 km.
To release pressure within reactor unit 1 at Fukushima I, steam containing water vapor, hydrogen, oxygen and some radioactive material is released out of the unit into the air.
TEPCO engineers decided to directly inject seawater inside the pressure vessel of the reactors using firetrucks. The pressure relief was also necessary to allow the firemen to inject seawater into the reactors vessels.
Sunday, 13 March 02:42: The high pressure coolant injection system for reactor 3 stops and, shortly thereafter, the water level within the reactor starts falling. 05:10: Fukushima I Unit 1 is declared as an INES Level-4 "accident with local consequences" event. 07:00 (approximate): The water level in reactor 3 reaches the top of the fuel. 09:00: Core damage starts occurring in reactor 3. A partial meltdown was reported to be possible at unit 3. At 13:00 JST reactors 1 and 3 are vented to release overpressure and then re-filled with water and boric acid for cooling, and to inhibit further nuclear reactions. Unit 2 was possibly suffering a lower than normal water level, but was thought to be stable; although pressure inside the containment vessel was high. 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.
Monday, 14 March 11:01: The unit 3 reactor building explodes. According to TEPCO, there was no release of radioactive material beyond that already being vented, but blast damage affected the water supply to unit 2. 11 workers were injured in the explosion. 13:15: The reactor core isolation cooling system for reactor 2 stops and, shortly afterwards, the water level within the reactor starts falling.