10 Years of Fukushima Part 1: Accident sequence - Milestones of a disaster


Start of flooding of the plant site, recorded by a surveillance camera (Image: TEPCO)

This year marks the tenth anniversary of the accident at the Fukushima Daiichi nuclear power plant in Japan. Over the next few weeks, our series "10 Years of Fukushima" will review the accident and its consequences. We start with an over-view of the first 10 days of the accident - beginning with the earthquake and the tsunami, followed by the futile attempts to prevent core meltdowns in several reactors, and ending with the temporary stabilisation of the situation.

11 March 2011

The epicentre. On 11 March 2011 at around 14:47, the earth shakes east of the island of Honshu in Japan. Seismologists measure a magnitude of 9.0. The epicentre of the Tohoku quake is 163 kilometres away from the Fukushima nuclear power plant.

Reactor Units 1, 2 and 3, which are located directly on the coast, are in operation at the time of the earthquake. Units 5 and 6, three metres higher up, are shut down for maintenance and refuelling. The fuel assemblies of Unit 4 have been unloaded from the reactor for the same reason and are in the spent fuel pool.

The earthquake. Units 1, 2 and 3, which are still operating, shut down automatically as a result of the shocks from the earthquake, in accordance with the design of the plants. As the supply via the external power grid has failed due to earthquake-related damage to the transmission lines, 12 of a total of 13 emergency power diesels start up. All six reactors switch over to their emergency core cooling and residual-heat removal systems.
The tsunami. At around 3:30 p.m. local time, tsunami waves of up to 15 metres arrive at the Fukushima nuclear power plant. Reactor Units 1 to 4 are hit by waves almost five metres high; the higher-lying Units 5 and 6 are more sheltered and are therefore only flooded to a level of around one metre.

View of the turbine building of Unit 3 after the tsunami (Image: TEPCO)Power supply failure. The seawater runs into the buildings and damages the operating emergency power generators as well as most of the power distribution cabinets. Only one emergency diesel generator in Unit 6 is still intact. The flooding also causes the seawater pumps located on the coast to fail. The residual heat from the reactor cores of Units 1 to 3 can now no longer be removed to the seawater. About 15 minutes after the tsunami has hit, the operator of the Fukushima nuclear power plant announces a nuclear emergency. A team of around 400 TEPCO employees starts work. Less than an hour later, TEPCO reports the failure of the cooling system in Unit 1 to the Japanese supervisory authority.

Core meltdown in Unit 1. The reactor core with the fuel assemblies has not been fed with cooling water since the tsunami. From about 6 p.m., the upper edge of the fuel assemblies is uncovered. At about 7:45 p.m., the coolant level has dropped to below the core. At this point, core destruction has already begun. As a result, the reactor core heats up to over 2,000 degrees Celsius and melts down almost completely.

Evacuation areas around the Fukushima Daiichi nuclear power plant in 2011 (Photo: GRS)Nuclear emergency and evacuation. That same evening at around 7 pm, the Japanese government declares a nuclear emergency. Shortly afterwards, the population within a two-kilometre radius of the accident site is evacuated. The evacuation is necessary because TEPCO is planning to carry out so-called venting. The venting is intended to prevent the containment around the reactor in Unit 1 from being damaged by the rising internal pressure and thus from leaking. As the plants do not have appropriate filters, this pressure relief involves the release of radioactive substances.

Just one day later, the radius is extended to 20 kilometres. Initially, 62,000 people have been evacuated by 13 March. By the end of August, according to the Japanese government, a total of around 146,500 people have been evacuated, of which about 78,000 are from the 20-km zone.

12 March 2011

Radioactive releases. In the early hours of 12 March, there is a drop in pressure in the containment of Unit 1 after the pressure had exceeded the maximum permissible value during the night. Shortly afterwards, measurements on the plant site show radiation levels of 10 to 15 times the normal value. On the previous evening, a clear increase in radioactivity had already been detected on the plant site, even before venting. For this reason, access to the reactor building of Unit 1 was prohibited from about 11 p.m. onwards.

Cooling attempts and explosions in Unit 1. From 12 March, the reactor of Unit 1 is cooled from the outside via fire engine pumps. At around 3:30 p.m., an oxyhydrogen explosion shakes Unit 1, igniting hydrogen that was produced during the core meltdown in the upper part of the building outside the containment. Four people are reported injured. The explosion also rips off a panel on the outer wall of Unit 2, which is why no hydrogen that might cause an explosion can accumulate in this unit.

13 March 2011

Dark smoke billows after the explosion in Reactor Unit 3 (Image: TEPCO)Explosion in Unit 3. The level of cooling water in the reactor in Unit 3 also continues to drop steadily. On 13 March, the cooling system fails completely from about 5 a.m. onwards. From 9:30 a.m., the reactor pressure vessel is therefore cooled with water from a cistern via fire extinguishing pipes. After the cistern is drained, the reactor is cooled with seawater from about 1 p.m. onwards. On 14 March, another explosion destroys the upper part of the reactor building of Unit 3 at about 11 a.m. Seven people are injured. Since the explosion also damages fire pumps and hoses, cooling with seawater cannot be resumed until about 3:30 pm.

14 to 17 March 2011

Core meltdown in Unit 2 and explosion in Unit 4. On 14 March, at around 6 p.m., the core of Unit 2 is also completely uncovered and begins to melt. The following day, at about 6:15 a.m., another explosion in Unit 4 destroys the upper part of the building. The hydrogen responsible for the explosion is produced in Unit 4 and enters the reactor building through the common exhaust system from Unit 3. Shortly after the explosion, a dose rate of 12 millisieverts (mSv) per hour is measured at the plant fence - this is more than 100,000 times the legal limit of 1 millisievert per year. TEPCO then decides to evacuate most of the workers at the site.

Fuel cooling. The spent fuel pool of Unit 4 is filled with about 1,500 fuel assemblies. About 200 of them were moved from the reactor into the pool shortly before the accident and have a comparatively high decay heat. Together with the failed cooling system, they cause the water in the pool to heat up quickly. There are growing fears that the pool could dry out, the stored fuel assemblies could be damaged and large quantities of radioactive material could be released into the environment. For this reason, attempts are underway from 17 March to feed the fuel pool with water from the outside. The attempt to drop water from helicopters is abandoned because the radiation level is too high. In the evening, cooling by means of water cannons begins. The spent fuel pool of Unit 3 is also provisionally cooled using water cannons.

18 March 2011

INES classification. On 18 March, the events in Units 1, 2 and 3 are upgraded to "serious accident" on the fifth level of the International Nuclear and Radiological Event Scale (INES) because of the severe core damage. On 12 March, the accident was initially classified as "Accident with off-site risk" on level 3. It is not until 12 April 2011 that the accident is finally classified as INES 7 due to the total amount of activity released. The accident is thus classified as a "Major accident", as was the case with the nuclear disaster at Chernobyl.

20 March 2011

Establishment of power supply. From 20 March, the external emergency power supply in Units 5 and 6 can be restored and a stable operating state reestablished. Until then, the only remaining emergency diesel from Unit 6 supplied both units. TEPCO is also able to reconnect Units 1 and 2 to the power grid. Units 3 and 4 follow a day later, but many systems are defective or under water.  
Looking back from 2011 to 2021. The consequences of the accident at the Fukushima nuclear power plant are still visible today. Every day, around 4,000 people work there, for example, to prepare the dismantling of the destroyed reactors and to further limit the release of radioactive substances.

The next article in this series will deal with the radiological consequences of the accident. Further topics will be the handling of contaminated water, the dismantling of the plants, and the lessons learned from the events at Fukushima.

Find out more

>> Fukushima Daiichi 11 March 2011 – Accident sequence, radiological consequences (5th revised edition) (german)
>> International Atomic Energy Agency (IAEA): The Fukushima Daiichi Accident
>> Federal Office for Radiation Protection (BfS): The Accident at Fukushima