Fukushima Daiichi Nuclear Disaster (2011): Meltdowns in Japan

The Fukushima Daiichi nuclear disaster was a series of reactor meltdowns, hydrogen explosions and radioactive releases at a nuclear power plant on the north-east coast of Japan in March 2011. It began on 11 March, when the magnitude 9.0 Tohoku earthquake cut the plant’s power and a tsunami flooded its emergency systems, leaving three reactors without cooling. Rated level 7 on the international scale, the same as Chernobyl, it forced the evacuation of well over 100,000 people and changed the debate on nuclear power around the world.

Background: the plant on the coast

Fukushima Daiichi (“Fukushima No. 1”) stands on the Pacific coast in the towns of Ōkuma and Futaba, about 220 kilometres north-east of Tokyo. Owned by the Tokyo Electric Power Company (TEPCO), it had six boiling-water reactors, mostly of American General Electric design, which began operating between 1971 and 1979. Unit 1 was one of the oldest commercial reactors in Japan.

Japan had relied heavily on nuclear power since the 1970s, and by 2010 it supplied roughly 30 percent of the country’s electricity. Because Japan is so prone to earthquakes, its reactors were built to withstand strong shaking. The defences against tsunamis, however, were based on modest assumptions: the Fukushima Daiichi site was protected by a sea wall designed for a tsunami of about 5.7 metres, and many key emergency generators and electrical switchboards were in basements or low-lying buildings.

In the 2000s, some researchers and internal TEPCO studies had suggested that much larger tsunamis were possible on this coast, as had happened in the Jogan tsunami of AD 869. These warnings were not turned into improved defences before 2011.

The earthquake and tsunami of 11 March 2011

At 2:46 p.m. on 11 March 2011, the Tohoku earthquake, the most powerful ever recorded in Japan, struck off the north-east coast. Units 1, 2 and 3 at Fukushima Daiichi were operating and shut down automatically, as designed, by inserting control rods to stop the chain reaction. Units 4, 5 and 6 were already shut down for routine inspection.

A shut-down reactor still produces a large amount of heat from the decay of radioactive material in its fuel, so it must be cooled for days to prevent damage. The earthquake cut the plant’s connection to the outside electricity grid, but emergency diesel generators started up and powered the cooling systems.

Around 50 minutes after the earthquake, a series of tsunami waves struck the site. The largest was roughly 13 to 15 metres high, far above the sea wall. Seawater flooded the turbine buildings, disabling most of the diesel generators, batteries and electrical switchgear. Units 1 to 4 lost almost all power, a situation known as a station blackout, as well as instruments and lighting. Workers were left trying to understand and control the reactors in darkness, with portable lamps and car batteries.

Meltdowns and explosions, 12-15 March

Without cooling, water in the reactor cores boiled away, the fuel was exposed, and it overheated and melted. TEPCO later concluded that the core of unit 1 began to melt within hours of the tsunami, followed by units 3 and 2 over the next three days. Melted fuel burned through the bottom of the reactor vessels into the containment structures below.

Superheated fuel cladding reacted with steam to produce hydrogen, which leaked into the reactor buildings. On 12 March a hydrogen explosion destroyed the upper part of the unit 1 building. Unit 3’s building exploded on 14 March, and on 15 March an explosion damaged the unit 4 building, whose reactor was empty but which had received hydrogen from unit 3 through shared pipes. On the same day, damage at unit 2 led to the largest release of radioactive material of the whole accident.

The site superintendent, Masao Yoshida, and hundreds of workers stayed at the plant to fight the crisis, injecting seawater with fire engines and later concrete pumps. Helicopters and fire trucks from the Self-Defense Forces, police and fire services sprayed water onto the spent-fuel pool of unit 4, whose fate was a major concern. Prime Minister Naoto Kan, who flew to the plant on 12 March, clashed with TEPCO over its handling of the crisis, while Chief Cabinet Secretary Yukio Edano became the government’s public face.

The reactors were gradually stabilised with improvised cooling systems. On 16 December 2011 the government announced that they had reached a state of “cold shutdown”, with temperatures kept below boiling point.

Evacuation and contamination

On the evening of 11 March, the government ordered people within 3 kilometres of the plant to evacuate, extending this to 10 and then 20 kilometres on 12 March. Residents between 20 and 30 kilometres were told to stay indoors and later encouraged to leave. In April, some areas further north-west, such as the village of Iitate, were added to the evacuation zone because winds and rain had carried contamination there.

At the peak, roughly 150,000 to 165,000 people had left their homes, under orders or voluntarily. Many moved several times, and families were split up. Japan’s Nuclear and Industrial Safety Agency rated the accident at level 7 on the International Nuclear and Radiological Event Scale (INES) on 12 April 2011, making it the second accident after the Chernobyl disaster of 1986 to receive the highest rating.

Estimates suggest that Fukushima released roughly one-tenth to one-fifth of the radioactive iodine and caesium released by Chernobyl, and much of it was carried out over the Pacific. Contaminated water also leaked into the sea. Food from affected areas was tested and some was banned, and decontamination work removed topsoil from large areas, filling millions of bags with radioactive waste.

Health effects

No one died from acute radiation sickness. The UN Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) concluded that radiation doses to the general public were low and that no discernible increase in radiation-related cancers was expected. A large thyroid screening programme for children in Fukushima Prefecture found many small abnormalities, which most experts attribute largely to the sensitivity of the screening itself. In 2018 the government accepted that one worker’s death from lung cancer was linked to radiation exposure at the plant.

The evacuation itself took a heavy toll. Moving elderly and sick people, especially from hospitals and care homes, caused deaths in the first days, and the stress, upheaval and loss of community contributed to illness and suicide in later years. Fukushima Prefecture has officially recognised more than 2,000 such disaster-related deaths. These findings led to debate about how evacuations after nuclear accidents should be planned.

Investigations and responsibility

In July 2012 the Fukushima Nuclear Accident Independent Investigation Commission appointed by Japan’s parliament, the National Diet, concluded that the accident was “a profoundly manmade disaster” that could and should have been foreseen and prevented. It blamed collusion between TEPCO and regulators, and a culture that discouraged questioning authority. Government reports and those of the International Atomic Energy Agency (IAEA) also found that tsunami risks had been underestimated and that plans for a long station blackout had been inadequate.

Japan replaced its old nuclear regulator with an independent Nuclear Regulation Authority in September 2012 and introduced stricter safety standards. TEPCO was effectively nationalised through a state rescue in 2012. Three former TEPCO executives were tried for professional negligence and acquitted in 2019, while in a separate civil case in 2022 a Tokyo court ordered former executives to pay huge damages to the company.

Timeline

• 2011, 11 March, 2:46 p.m.: The Tohoku earthquake strikes; operating reactors shut down automatically.
• 2011, 11 March, about 3:35 p.m.: The largest tsunami waves flood the plant, causing a station blackout.
• 2011, 12 March: The evacuation zone is extended to 20 km; a hydrogen explosion destroys the unit 1 reactor building.
• 2011, 14 March: A hydrogen explosion wrecks the unit 3 building.
• 2011, 15 March: An explosion damages unit 4, and the largest radioactive release follows damage at unit 2.
• 2011, 12 April: The accident is rated level 7 on the INES scale.
• 2011, 16 December: The government declares that the reactors have reached cold shutdown.
• 2012, 5 July: The Diet commission publishes its report.
• 2023, 24 August: Release of treated and diluted water from the site into the Pacific begins.

Legacy

Fukushima was the worst nuclear accident since Chernobyl and the most serious ever to occur in a democratic, technologically advanced country. Along with the Three Mile Island accident, it became a central reference in the debate about the safety of nuclear energy.

In Japan, all reactors were gradually shut down for safety checks, and by May 2012 none was operating. Restarts under the new rules began in 2015, but many reactors have been permanently closed. Germany decided in 2011 to phase out nuclear power entirely and shut its last three reactors in April 2023, and Italian voters rejected a return to nuclear power in a referendum in June 2011. Other countries, including China, the United Kingdom and France, kept or expanded their nuclear programmes with new safety requirements.

The clean-up continues. Removing the melted fuel from units 1 to 3 is one of the hardest engineering tasks ever attempted, and decommissioning is expected to last until at least the 2050s. Some towns near the plant have partly reopened, but their populations remain far smaller than before 2011.

Why it matters

The Fukushima Daiichi disaster showed how a natural hazard larger than planned for could overwhelm a nuclear plant’s defences. It forced hundreds of thousands of people’s lives to change, reshaped nuclear safety rules around the world, and remains a defining event in the history of nuclear energy.