Editor's Note: This timeline is no longer kept up to date. For the latest coverage on the aftermath of the earthquake, view our interactive timeline.
Since March 11th's record earthquake, IEEE Spectrum has continued to report on Japan's ongoing nuclear emergency. In the following timeline, we've compiled the recent happenings from information released by the Tokyo Electric Power Company (TEPCO) and the International Atomic Energy Agency (IAEA), as well as stories published by IEEE Spectrum. We will continue to update this post as we learn more.
Friday, 11 March 2011
2:46 p.m. A 9.0-magnitude earthquake, 130 kilometers off the coast of Miyagi Prefecture in northeastern Japan, shakes the country. A tsunami with waves as high as 10 meters follows within the hour.
-- Tokyo Electric Power Company (TEPCO) reports that turbines and reactors in three units of the Fukushima Dai-1 Nuclear Power Station, which includes six reactor units in total, automatically shut down.
--Japanese authorities report a fire at the Onagawa nuclear power plant, later extinguished.
3:41 p.m. Emergency diesel generators at Fukushima Dai-1 shut down, a result of the tsunami, forcing cooling pumps to start using battery backup. TEPCO uses mobile electric generators to help power to the cooling system.
6:00 p.m. TEPCO announces increase in reactor containment vessel pressure in Fukushima Dai-1 reactor No. 1. Officials decide to vent the reactor to lower pressure.
7:00 p.m. Japanese government issues a state of emergency, and officials evacuate thousands of people living within a 3-kilometer radius of the plant.
10:00 p.m. TEPCO reports blackout for about 2.97 million households.
Saturday, 12 March 2011
7:00 a.m. Government announces that officials will evacuate residents within 10 km radius of Fukushima Dai-1 and Fukushima Dai-2 Nuclear Power Stations.
9:00 a.m. Preparations for air venting from Fukushima Dai-1 reactor No. 1 begin.
3:36 p.m. Blast and white smoke come from Fukushima Dai-1, reactor No.1--blowing off the roof and the upper part of the outer walls of the structure containing the reactor. TEPCO says that the primary containment vessel remains intact. Four plant safety workers are injured.
--TEPCO begins venting steam to reduce the growing pressure inside reactor No. 1. Radiation density around the structure rises to the level of 1015 microsieverts. By 6:48 p.m. it falls to 70.5 microsieverts.
7:11 p.m. Japanese government evacuates local residents within 20 km of plants. Japanese authorities prepare to distribute iodine to residents.
8:00 p.m. TEPCO announces radiation exposure of one employee, who was working inside Fukushima Dai-1 reactor building, exceeded 100 microsieverts (106.3 microsieverts).
8:20 p.m. TEPCO begins injecting sea water mixed with boron to cool reactor No. 1.
--Japan's Nuclear and Industrial Safety Agency (NISA) detects caesium-137 and iodine-131 near Fukushima Dai-1 plant.
11:00 p.m. TEPCO announces that they have been injecting sea water into the reactor core of Fukushima Dai-1's reactor No. 1 and then boric acid to absorb radiation. Current reactor water level is lower than normal in reactor No. 2.
Sunday, 13 March 2011
9:20 a.m. TEPCO completes air venting in reactor No. 3 to reduce internal pressure, then begins injecting water containing boric acid into the reactor via fire pump, given failure of high pressure injection system.
--At Fukushima Dai-1, one worker is exposed to higher-than-normal radiation, and four injured and one dies in crane accident.
Monday, 14 March 2011
6:15 a.m. TEPCO reports that Fukushima Dai-1 reactor No. 3's pressure increases to 530 kilopascals.
--In reactor No. 2 in Fukushima Dai-1 core coolant levels decreasing and sea water pumping begins.
9:05 a.m. Pressure decreases in reactor Fukushima Dai-1 No. 3 to 450 kilopascals.
11:01 a.m. Hydrogen explosion from Fukushima Dai-1 reactor No. 3 injures seven workers. TEPCO believes that the reactor containment vessel remains intact.
1:25 p.m. TEPCO confirms that Fukushima Dai-1 reactor No. 2's cooling system has failed.
--IAEA reports 230 000 units of stable iodine distributed to evacuation centers.
5:17 p.m. TEPCO reports that water level in the Fukushima Dai-1 reactor No. 2 remains above fuel rods and that the company has restarted water injection.
Tuesday, 15 March 2011
6:00 a.m. TEPCO initiates rolling blackouts, given reduced power supply. The company reports an explosive sound in 5th floor of Fukushima Dai-1 reactor No. 4.
6:14 a.m. Blast from No. 2 reactor building of Fukushima Dai-1 nuclear power plant.
9:38 a.m. A fire breaks out in the Fukushima Dai-1 No. 4 reactor building. IAEA reports that Japanese authorities believe that spent fuel storage pool is burning, releasing radiation into atmosphere.
10:22 a.m. Radiation readings taken near the reactors show that emissions range from 30 millisieverts per hour to 400 millisieverts per hour--higher than the microsievert numbers reported when steam was vented. Officials ask Japanese residents up to 30 km from the Fukushima Dai-1 to stay indoors.
10:30 p.m. One of many aftershocks from Friday's earthquake, a 6.0-magnitude earthquake strikes Shizuoka Prefecture southwest of Tokyo.
Wednesday, 16 March 2011
5:45 a.m. TEPCO employee discovers another fire in reactor No. 4 of Fukushima Dai-1.
6:15 a.m. TEPCO reports that, upon inspection, no fire is present in Fukushima Dai-1 reactor No. 4, but will keep under surveillance.
8:30 a.m. TEPCO reports "fog like steam" from Fukushima Dai-1 reactor No. 3
4:00 p.m. Temperature in spent fuel pool of Fukushima Dai-1 reactor No. 4 rises. Self Defense Force considers spraying water from helicopters but attempt cancelled.
--Japan's Nuclear Safety Commission recommends that local authorities instruct evacuees leaving the 20-kilometre area to ingest stable iodine, available at evacuation centers.
Thursday, 17 March 2011
9:48 A Self Defense Force helicopter flying from the west drops the first load of water almost directly over the Fukushima Dai-1 No. 3 reactor. A second helicopter flying from the east then dumps its water over reactors No. 4 and No. 3. Two more passes are made before the operation is suspended around 10 a.m.
--TEPCO patrols reactor No. 4 building to inspect dry casks--longer term storage for spent fuel--and notices nothing abnormal visually. Prepares for later detailed inspection. Also monitors common spent fuel pool located in a separate building from reactor units to confirm that water levels in pool are secure.
10:00 a.m. Radiation levels measured by TEPCO read 400mSv/h at the inland side of the No. 3 reactor building and 100mSv/h at the inland side of the No. 4 reactor building.
2:00 p.m. Self Defense Force fire engines spray water on reactor No. 3.
5:30 p.m. IAEA reports that engineers have begun to lay an external grid power line cable to No. 2 reactor.
--IAEA reports that operators able to start one of the diesel generators to reactor No. 6.
7:00 p.m. Riot police's high-pressure water cannon trucks and Self-Defense
Force's fire engines spray more water on reactor No. 3.
Friday, 18 March 2011
2:00 p.m. TEPCO reports that Self Defense Forces spray reactor No. 3 with water from trucks.
--TEPCO reports that water level in spent fuel pool in reactor No. 4 is secure.
Saturday, 19 March 2011
12:30 a.m. Tokyo Fire Department's Hyper Rescue sprays more water on spent fuel pools in reactor No. 3
5:00 a.m. TEPCO reports that they have started the Residual Heat Removal System pump in reactor No. 5 to ensure that spent fuel is cooling in pool.
--TEPCO reports that it has made three holes in the roofs of reactors No. 5 and 6 to prevent hydrogen gas from accumulating within the buildings
10:14 p.m. TEPCO starts another diesel generator in reactor No. 6.
Sunday, 20 March 2011
--TEPCO reports temperatures have gone down in the spent fuel storage pools of the No. 5 and No. 6 reactors.
8:00 a.m. Radiation levels had been measured at 500 meters from the No. 3 reactor building: At 8 a. m. this morning the level was 2625 microsieverts compared to 3443 microsieverts measured at 2 p.m. yesterday, as reported by Hidehiko Nishiyama, deputy director general of the Nuclear and Industrial Safety Agency.
8:21 a.m. Self-Defense Forces uses fire engines to spray water on reactor No. 4.
3:46 p.m. TEPCO finishes laying cable to and starts providing power to reactor No. 2. IAEA reports that off-site electrical power has been connected to an auxiliary transformer and distribution panels.
5:20 p.m. TEPCO has injected some 40 tons of seawater into reactor No. 2
9:30 p.m. Fire Rescue Task Forces of Tokyo Fire Department sprays more water on reactor No. 3.
Monday, 21 March 2011
6:30 a.m. Self-Defense Force and U.S. Army fire engines spray more water into reactor No. 4, finishing at 8:40 a.m.
--According to representative from Nuclear and Industrial Safety Agencyradiation measured 500 meters from the plant at 6:30 a.m. Sunday morning was 2670 microsieverts per hour, 3346 microsieverts when measured at 2:50 p.m. and 2346 microsieverts at 6:30 a.m. Monday.
9 a.m. TEPCO reports that it has a 450-volt cable connected to the plant and is readying to supply a central control room for the No. 1 and No. 2 reactors.
11:36 a.m. TEPCO reports that power from emergency diesel generators partially restored to reactors No. 5 and 6 through transmission line.
3:55 p.m. TEPCO employee spots light gray smoke rising from the southeast side of the rooftop of the reactor No. 3 building.
--TEPCO reports that radioactive materials were detected [PDF] from the seawater around the discharge canal (south) of Fukushima Daiichi Nuclear Power Station. According to IAEA, testing samples included iodine-131, cesium-134, and cesium-137. Japan Agency for Marine-Earth Science and Technology plans to measure radioactivity around the plant from March 22 to March 23.
Tuesday, 22 March 2011
--TEPCO reports that the color of smoke from reactor No. 3 changed to somewhat white and is slowly dissipating.
3:10 p.m. Tokyo Fire Department's Hyper Rescue sprays more water into reactor No. 3.
4:00 p.m. TEPCO employees inject 18 tons of seawater into reactor No. 2.
5:17 p.m. Concrete pumping vehicle sprays more water into reactor No. 4, at rate of 50 tons per hour.
10:45 p.m. By this time, TEPCO reports that electricity supply from external source restored to reactors No. 1 to 6. Lights turn on in the mail operation room and reactor No. 3 main control room.
Wednesday, 23 March 2011
2:30 a.m. TEPCO reports injection of seawater into the reactor No. 1 from feed water system.
10:00 a.m. TEPCO sprays more water on reactor No. 4 building using the concrete pumping vehicle.
11:00 a.m. TEPCO injects more seawater into spent fuel pool in reactor No. 3 building.
4:20 p.m. TEPCO employees observe light gray smoke coming from reactor No. 3 building.
Thursday, 24 March 2011
--IAEA publishes slides with overview of each reactor's status.
--Three workers exposed to high radiation levels at the plant after stepping into water contaminated with radiation, and received between 173 and 180 millisieverts.
2:36 p.m. Concrete pump truck starts to spray reactor No. 4's spent fuel pool with around 150 tons of water using concrete pump truck.
3:37 p.m. Power supply restored to common spent fuel's cooling system.
4:14 p.m. Temporary pump for Residual Heat Removal repaired for reactors No. 5 and 6.
7:30 p.m. Workers continue to inject seawater into the reactor pressure vessels of Units 1, 2 and 3 and are preparing to inject pure water.
8:30 p.m. Off-site electricity and lighting in the reactor No. 1's main control room recovered.
9:00 p.m. 120 tons of seawater injected into reactor No. 3's spent fuel pool via the cooling and purification line.
Friday, 25 March 2011
--Japan's Chief Cabinet Secretary holds a press conference later on Friday to advise residents living between 20 and 30 kilometers from Fukushima Dai-1 to consider voluntary evacuation.
6:05 a.m., TEPCO injects seawater into reactor No. 4's spent fuel pool using Fuel Pool Cooling and Filtering system.
10:30 a.m. TEPCO begins injecting seawater into the spent fuel pool of reactor No. 2, using Fuel Pool Cooling and Filtering system
1:28 p.m. Kawasaki City Fire Department sprays water on reactor No. 4's spent fuel pool.
3:37 p.m. TEPCO pumps fresh water (instead of sea water) into reactor No. 1 pressure vessel.
7:05 p.m. Concrete pumping vehicle sprays water onto reactor No. 4's spent fuel pool.
Saturday, 26 March 2011
8:50 a.m. TEPCO tests radiation levels in reactor No. 2 basement's water levels. Detailed results, here [PDF].
10:10 a.m. TEPCO begins pumping fresh water with boric acid into reactor No. 2
--TEPCO reports results of radioactive material analysis in seawater around discharge canal of plant to Nuclear and Industrial Safety Agency (NISA) and Fukushima prefecture. Also reports results of nuclide analysis of radioactive materials contained in the air.
4:46 p.m. The light in the main control room of reactor No. 2 turns on.
Sunday, 27 March 2011
--TEPCO releases more results of radioactive material analysis in seawater and air.
3:30 p.m. TEPCO workers find water in the trenches of reactor buildings No. 1 to 3. The radioactive emission at the surface of the water was 0.4 mSv/h for reactor No. 1 and over 1000 mSv/h for No. 2. Could not access No. 3 trench due to rubble.
4:55 p.m. More water sprayed on reactor No. 4 using concrete pump.
Monday, 28 March 2011
--TEPCO releases updated results of analysis of radioactive materials in air surrounding plant.
--TEPCO detects plutonium in soil surrounding the plant. Company says density of detected plutonium (around .27 and .26 becquerel per kg of dry soil from Pu-239 and Pu-240 in two out of five sample sites, and .54 Bq/kg from Pu-238 from one site) is equivalent to the fallout observed in Japan from atmospheric nuclear tests and poses "no major impact on human health." IAEA notes:
Traces of plutonium are not uncommon in soil because they were deposited worldwide during the atmospheric nuclear testing era. However, the isotopic composition of the plutonium found at Fukushima Daiichi suggests the material came from the reactor site, according to TEPCO officials. Still, the quantity of plutonium found does not exceed background levels tracked by Japan's Ministry of Education, Culture, Sports, Science and Technology over the past 30 years.
Tuesday, 29 March 2011
8:20 a.m. TEPCO switches to injection of fresh water from using fire engine to temporary electrical pump in reactor No. 1
11:50 a.m. Lights in the main control room of reactor No. 4 building turn on. Now all of the control room lights are on.
12:03 p.m. Water sprays three workers while they're taking off the flange of the pipe to remove the residual heat in the seawater system. TEPCO reports that no radioactive material "adhered to their bodies."
2:17 p.m. Concrete pump starts injection of fresh water (instead of seawater) into reactor No. 3.
4:30 p.m. Motor-driven pump starts injecting fresh water into reactor No. 2 instead of seawater.
Wednesday, 30 March 2011
9:25 a.m. TEPCO delivers freshwater to reactor No. 2's spent fuel pool using a temporary motor driven pump and then (after that pump malfunctions) using a fire fighting pump.
2:04 p.m. More water sprayed on reactor No. 4 spent fuel pools using a concrete pumping vehicle.
5:56 p.m. TEPCO employee spots smoke coming from power panel in reactor No. 1 building. The power panel supplies electricity to the water pump motor. Fire department investigates. Smoke stops after TEPCO cuts power to panel.
--Media continue to question if there's been a breach of a pressure vessel in any of the Fukushima reactors. Richard Lahey, former chair of nuclear engineering at Rensselaer Polytechnic Institute, clarifies previous statements in an interview with IEEE Spectrum. He believes that all of the cores have melted, and that reactor No. 2's core may have melted through, but the data make it difficult to come to any firm conclusions. Japanese officials say that there is not evidence of a big breach.
Thursday, 31 March 2011
--TEPCO establishes "Fukushima Nuclear Influence Response Division," sending employees to help evacuees at emergency evacuation sites.
--Nuclear Industry and Safety Agency (NISA) says radiation in the seawater sampled on Wednesday 330 meters south of plant has risen to 4385 times maximum safety levels, a rise from the previous high of 3355 times that was detected on Tuesday.
--President Sarkozy of France, as chair of the G20 and G8, flies to Japan to show unity with the nation and to find out what additional help these countries might offer.
Friday, 1 April
--TEPCO reprimanded for sending teams of workers into the Fukushima plant without supplying the proper safety equipment. Some work groups entered with only one worker having been issued a dosimeter .
Sunday, 3 April
--TEPCO says that it has detected levels of radioactive iodine in seawater samples taken near the plant that are 5 million times the maximum concentration allowed by law. Company officials said that samples taken on 2 April showed the figure at 7.5 million times the limit.
Monday, 4 April
--TEPCO announces plans to release more than 11 500 tons of what it’s calling "low-level radioactive waste water" into the Pacific Ocean. The water, which was contaminated with radiation due to still-mysterious leaks, is being dumped to make room in the plant’s storage tanks for more highly radioactive water, which has pooled in the basements of several of the reactors’ turbine buildings.
--As TEPCO undertakes its planned release of contaminated water into the sea, it tried, without success several schemes to stem the flow of highly radioactive water from an as yet undiscovered source that is pouring out of a 20-centimeter-long crack in a concrete pit near the seawater intake area for reactor No. 2.
Tuesday, 5 April
--TEPCO has made some headway in its efforts to stop the leak from the crack near the reactor No. 2 intake area. Workers injected liquid glass (sodium silicate) into the bedrock near the concrete pit and said that the water flow has since decreased by half. Where the water still flowing from reactor buildings will go instead is still an open question.
Wednesday, 6 April
5:38 a.m. TEPCO confirms that water is no longer spilling from the crack in the concrete lateral of the pit near the seawater intake for reactor No. 2. But a TEPCO official cautioned that the stoppage of water running through the crack does not mean that the water isn’t escaping via another route.
--TEPCO injects 6000 cubic meters of nitrogen gas into reactor No. 1 in an effort to thin the concentration of hydrogen and prevent an explosion like the ones that damaged buildings housing reactors in the first few days following the 11 March earthquake and tsunami. The utility is also considering injecting nitrogen into the No. 2 and No. 3 reactors.
Thursday, 7 April
12:32 a.m. A 7.1 magnitude earthquake triggered a tsunami warning in the vicinity of the Fukushima nuclear plant. TEPCO says there was no additional damage, and its efforts at cleaning up and controlling the plant would continue as planned.
--TEPCO reports that the highly contaminated water that had been flowing through the now stoppered crack in a concrete pit near reactor No. 2’s seawater intake is pooling inside the vertical shaft of the trench carrying cables and pipes for the Unit 2 turbine building. The water level has risen 5 centimeters inside the shaft; there is about 1 meter of shaft above the water’s surface.
Friday, 8 April
--Though Fukushima was basically unscathed by the earthquake that struck Japan on 7 April, NISA reports that the quake caused trouble at several other nuclear facilities in the tregion. The Higashidori Nuclear Power Plant and the Rokkasho spent fuel reprocessing plant temporarily lost their off-site power feeds and had to rely on emergency diesel generators to keep spent fuel storage pools cool. Radioactive water spilled from a spent fuel pool at the Onagawa power plant during the quake.
Monday, 11 April
--Fukushima lost power after a 7.0-magnitude quake. During the outage, water injections into reactors 1, 2, and 3 were halted for about 50 minutes. A tsunami warning caused workers to evacuate the facility without manually switching on diesel-powered back-up systems. When the workers returned, monitoring systems showed no signs of radiation increases, and no other significant problems were found.
--The Japanese government says it will order residents of some towns in the areas just beyond the original 20-kilometer evacuation zone around Fukushima to leave. The new evacuation orders will apply to municipalities in the 20-to-30-km zone and beyond.
Tuesday, 12 April
--The Japanese government raises the severity rating of the Fukushima disaster to a 7 on the International Atomic Energy Agency's severity scale--the highest rating possible. The change puts the incident on par with the 1986 accident at Chernobyl.
Monday, 18 April
--TEPCO officials outline a plan to achieve a cold shutdown of Fukushima within nine months. The utility’s first-phase efforts will include removing 60 000 tons of contaminated water, installing so called stable cooling circulation systems, and halting the release of radioactive dust and water to the air and sea. TEPCO says that once all this has been achieved (supposedly in 3 months), it will spend the ensuing 6 months trying to further stabilize the reactors so it can reliably keep the cooling water at 100 degrees Celsius.
Tuesday, 26 April
--TEPCO sends a remote controlled robot to check the No. 1 reactor for water leaks. Finding none, nor a significant rise in radiation levels, the company says it will try to increase the amount of cooling water it is injecting into the reactor from 6 metric tons a day to about 14 metric tons. The aim is to eventually fill the suppression pool until the water covers the reactor’s pressure vessel and can cool it down from the outside.
Tuesday, 3 May
--TEPCO reports that workers will enter the damaged No. 1 reactor building to install filtering equipment for removing 95 percent of the radioactive dust from the air inside the building. The aim is to create a working environment safe enough for workers to put in place a cooling system to replace the one that was damaged by the tsunami that followed the 11 March earthquake.
--A TEPCO official told IEEE Spectrum that the company also plans to construct a temporary wall or dyke to protect the No. 3 and 4 reactors, which are considered the most vulnerable to another tsunami. The 1- to 2-meter-high wall, which would be built on ground some 10 meters above sea level, would stretch for about 500 meters. He added that the design was still under consideration.
Monday, 9 May
--TEPCO workers and government safety officials enter the No. 1 reactor building to carry out a survey of conditions inside. A TEPCO official radiation levels in the building ranged are as high as 700 millisieverts per hour—nearly three times the 250-millisievert annual exposure limit for workers prescribed by law.
--Chubu Electric Power Co. says it will suspend operation of its Hamaoka nuclear power plant until construction of a concrete seawall to protect the facility against tsunami is completed. The plant will likely be off line for 3 years, making the 3.5 gigawatts supplied by its three active reactors (roughly 10 percent of the region’s power supply) unavailable. Chubu had been providing make-up power for the areas normally served by the Fukushima plant, but will now have to suspend that service.
Thursday, 12 May
--TEPCO reports that nuclear fuel rods in the No. 1 reactor have suffered at least a partial meltdown. Apparently, the water level in the reactor vessel fell below the fuel rods, leaving them at least partially exposed. TEPCO suspects that the molten fuel has damaged the bottom of the reactor’s pressure vessel. For now, TEPCO has halted plans to flood the reactor's containment vessel—a prerequisite to installing a new cooling system for the damaged reactor.
Sunday, 15 May
--The Chubu Electric Power Co. completes the cold shutdown of its Hamaoka nuclear plant.
Tuesday, 17 May
--TEPCO reveals its revised plan for bringing Fukushima to a cold shut down. Despite having discovered problems such as a partial meltdown and a suspected pressure vessel breach inside reactor No. 1, TEPCO still expects to shut the plant down by January. But the company has changed how it plans to carry it out. For example, instead of pumping freshwater into the damaged reactors, the water already pooled in the reactor buildings will be decontaminated and used for cooling.
--TEPCO releases the most detailed description to date of what happened in the hours following the 11 March earthquake.
Note: All times are local. Some times are approximate (based on time of TEPCO press releases) and subject to change as news develops.