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One Year On

In October 2011, the Fukushima Daiichi Nuclear Power Plant's Unit 1 reactor building was covered with panels and a ventilation system was activated to control the emission of radioactive materials from the damaged structure beneath the cover. The cover itself is 54 metres high, 47 metres wide and 42 metres deep. (Photo: TEPCO)

Beneath the Pacific Ocean's floor deep in the Japanese trench, tectonic plate boundaries slipped repeatedly, triggering one of the most severe earthquakes in recent history. The earthquake's epicenter lay off the eastern coastline of Japan near the Fukushima Prefecture. The plates' movements generated a tsunami that swiftly drove forward ranks of waves towards the earthquake-battered Japanese coast.

It was 11 March 2011.

Along the coastline, sensors detected violent ground motion and automatically shut down all operating reactors at Tokai Daini, Onagawa, Fukushima Daiichi and Fukushima Daini Nuclear Power Stations.

Off-site Power Lost

The reactors at TEPCO's Fukushima Daiichi Nuclear Power Plant require an electric power supply to safely remain in "shutdown" condition. If power fails, back-up generators supply electrical power for the equipment needed to shut down the reactor safely and maintain the security of the plant.

The "Great East Japan Earthquake" toppled pylons supporting the overland electrical power lines. As soon as off-site power was lost at Fukushima Daiichi, on-site emergency diesel generators started automatically, supplying electricity to the reactors' "essential loads", namely emergency core cooling pumps, valves, monitoring instruments and controls.

Vital Cooling

Minutes after the earthquake hit the plant, the specially trained operation teams confirmed that the chain reaction in the three operating reactors at Fukushima Daiichi had stopped and that the emergency diesel generators were operating.

In an operational nuclear power plant, the fission process in the reactor core produces heat. Cooling water continuously absorbs and carries off the heat. In a "boiling water reactor" like those operating at Fukushima Daiichi, the cooling water boils in the core. A mixture of steam and water then flows through separators where steam is extracted and diverted to spin the power plants' turbine-generators to produce electricity. When the steam is cooled, it condenses into water, which is returned to the reactor core that heats it again, producing steam and restarting the cycle. Powerful pumps drive the primary cooling fluid, in this closed loop, maintaining the reactor core at operating temperatures.

After the emergency shutdown, the nuclear fuel in the reactors nonetheless continues to release a considerable amount of decay heat. Continuous and reliable cooling, provided by the emergency diesel generators, is vitally important to prevent overheating and fuel damage.

IAEA Emergency Response

In Vienna, Austria, just after sunrise, the IAEA's International Seismic Safety Centre (ISSC) received a notification from a US Geological Survey system that an earthquake in Japan had occurred. The on-call ISSC staff determined that the earthquake had the potential to affect nuclear power plants along the Japanese coast. They immediately alerted the emergency response staff at the IAEA's Incident and Emergency Centre (IEC) that nuclear power plants could be damaged and a tsunami was expected.

Tsunami Strikes

About 40 minutes after the Fukushima Daiichi reactors were shut down, several massive tsunami waves crashed over the six-Unit plant's protective wall, forcing seawater deep into the plant. Emergency generators and some of the associated electrical equipment were flooded. Thirteen generators ceased operating. Only one emergency generator survived the onslaught, providing power to Units five and six.

The earthquake had already disrupted mobile and landline communication and now the on-rushing tsunami waters hurled debris into the plant, damaging equipment and buildings and blocking access.

The combined effects of the earthquake and tsunami devastated the coastal area, exacting a dreadful toll: almost 16 000 lives were lost, over 8 000 people remain missing, and more than 679 000 homes were destroyed or damaged.

International Response

The IEC went into "full response mode" and offered assistance to the Japanese Government. Experts in nuclear safety, plant and nuclear engineering, emergency response, and radiation protection from throughout the Agency converged to support the IEC response. Member States offered technical support, which the IEC coordinated. From the early hours of the accident, the Agency provided briefings and updates to IAEA Member States, international organizations, the media and the public.

Catastrophic Conditions

Within 20 minutes of the tsunami, the operation teams at the main control rooms for Units one, two and three reported that their stations were in total electrical "black out" condition. The plant operator, Tokyo Electrical Power Company (TEPCO), was faced with catastrophic conditions: no light, no reactor control, almost no instrumentation and severely hampered communications.

Literally in the dark, engineers now worked to secure the safety of five reactors, six spent fuel pools located in the reactor Units, one common fuel pool and a dry cask storage facility.

Cooling Spent Fuel

When the uranium contained in spent nuclear fuel is sufficiently depleted in the isotope U-235, it can no longer sustain a chain reaction. It is then removed from the reactor and replaced by fresh fuel. The spent fuel is moved temporarily to an adjacent, water-filled, spent fuel pool. The water in the pool cools the fuel and provides radioactive shielding. An active cooling system is needed to maintain a stable operating temperature in the spent fuel pool. Without cooling, the spent fuel can overheat.

Nuclear Emergency

One hour after the tsunami waves smashed through the Fukushima Daiichi Plant, the remaining, unpowered emergency cooling could no longer adequately remove the decay heat from reactor Units one and two. TEPCO declared a "Nuclear Emergency". In a worst-case scenario, when reactor cooling fails or proves ineffective, the uranium fuel pellets packaged inside long, hollow metal fuel rods melt along with their sheaths, and release radioactivity. Severe fuel damage was therefore a growing concern in the first days of the accident.


In Fukushima, on the day after the tsunami struck, an evacuation order was issued for people living within three kilometers of the Power Plant, while people living in a 10 kilometers radius of the Power Plant were instructed to seek shelter in their homes. By the evening, the evacuation zone was widened further to include residents within a 20-kilometer radius of the accident site.

Hydrogen Explosions

Within the first four days, nuclear experts in Japan and worldwide suspected nuclear fuel rod damage in Units one, two and three, and an increase in explosive hydrogen gas. TEPCO's engineers attempted to reduce the temperature and pressures by venting the trapped gases and by injecting seawater, the only cooling fluid available in large quantities. This improvised cooling, however, failed to prevent a series of hydrogen explosions at Units one, three and four. The explosions damaged the reactor buildings, the secondary structures that surround the reactor vessel containment, releasing radioactive material into the environment.

The spent nuclear fuel in the spent fuel pools continued to heat up.

Beyond Call of Duty

At nuclear power plants throughout the disaster zone, engineers, operators and technicians did their utmost to ensure their plants' safety, working long hours in stressful, often dangerous conditions. In the Fukushima Daiichi reactor buildings, operators manually operated valves to vent pressure, as the hot components melted their protective rubber boots and gloves. A short time later, workers connecting emergency, portable power and water supplies immediately adjacent to the Unit one and three reactor buildings barely escaped serious injury when debris rained down around them following explosions within the reactor buildings.

More Information

On Friday, 18 March 2011, IAEA Director General Yukiya Amano met the Japanese Prime Minister and other senior officials in Tokyo to assure the Japanese Government that the IAEA and the international community are "standing by Japan" at this difficult time. They discussed the nuclear emergency and the need for an accelerated flow of information to support a more effective international response.

Radiation Monitoring

Traveling with the Director General on his visit to Japan was the first of four radiological monitoring teams who helped validate the results of more extensive measurements made by the Japanese authorities.

Food Safety

Within days of the accident, the radioactive contamination led to food restrictions. On 26 March 2011, a Joint IAEA/Food and Agriculture Organization Food Safety Assessment Team reached Japan to advise and assist national and local Japanese authorities on food safety and agricultural countermeasures. The Team helped to support data collection on the radionuclides released during the accident and deposited in food produced and consumed in the affected areas. The data collected were used to provide advice on possible food restrictions.

Marine Survey

Following the accident, a significant amount of radioactively contaminated cooling water escaped from the Fukushima Daiichi reactor buildings through cracks caused by the earthquake. The contaminated water was discharged into the sea, raising concern about the radioactivity's harmful effects on marine life and on seafood destined for human and animal consumption. Japan initiated an intense programme to monitor both coastal and off-shore levels of seawater contamination at the discharge area, as well as at distances 10 and 30 kilometers from the reactors. At Japan's request, an IAEA expert in marine monitoring programs joined the Japanese team aboard the research vessel MIRAI. The IAEA expert gave advice on the further collection of seawater about 30 kilometers from the coast and on measurement strategies. This information was used later to develop a long-term marine study of the Pacific within an IAEA Regional Cooperation project.

Evacuation Extended

By April 2011, some 160 000 people living within a 30-kilometer radius of the Fukushima Daiichi Plant, as well as some outlying areas, had been evacuated from their homes.

Nuclear Power Experts

In April 2011, the IAEA's boiling water reactor expert team was the first international expert team to visit the Fukushima Daiichi accident site. The team met regulators, TEPCO and engineers working at the sites to mitigate conditions at the Daiichi and Daini Plant sites.


On 17 April, 2011, TEPCO announced a detailed Roadmap for Restoration from the Accident at Fukushima Daiichi Nuclear Power Station, describing the process to reach a condition "equivalent to cold shut down" by the end of 2011. According to TEPCO's Roadmap for the Fukushima Daiichi Plant, a condition "equivalent to cold shutdown" applies when the temperatures of all reactor pressure vessels containing damaged fuel are maintained below 100 degrees Celsius and the nuclear chain reaction is stopped, the release of radioactive materials from the primary containment vessel is under control and public radiation exposure to additional release is being significantly held down.

Fact-Finding Mission

An IAEA International Fact-Finding Expert Mission, comprising 18 experts, began its work in Japan in May 2011. Led by Mike Weightman, the United Kingdom's Chief Inspector of Nuclear Installations, the mission produced a preliminary assessment of the accident after collecting information at the Fukushima Daiichi, Fukushima Daini and Tokai Daini Nuclear Power Plants, as well as in meetings with NISA and TEPCO. At the accident site, Weightman said, "Our entire team was humbled by the enormous damage inflicted by the tsunami on Japan. We are also profoundly impressed by the dedication of Japanese workers working to resolve this unprecedented nuclear accident."

Action Plan on Nuclear Safety

IAEA Director General Amano convened a Ministerial Conference on Nuclear Safety in June 2011. The International Fact-Finding Expert Mission's initial assessments were presented to the Conference and IAEA Member States considered the early lessons learned. In the resulting Ministerial Declaration, the IAEA Director General was requested to develop an Action Plan on Nuclear Safety, based on lessons learned from the accident in Japan.

After intensive consultation with the Member States, the Action Plan was approved by the IAEA Board of Governors and adopted by the General Conference in September 2011. The Action Plan consists of 12 main actions:

  • Assess the safety vulnerabilities of nuclear power plants in the light of lessons learned from the Fukushima accident;
  • Strengthen IAEA peer reviews in order to maximize the benefits to Member States;
  • Strengthen emergency preparedness and response;
  • Strengthen the effectiveness of national regulatory bodies;
  • Strengthen the effectiveness of operating organizations with respect to nuclear safety;
  • Review and strengthen IAEA Safety Standards and improve their implementation;
  • Improve the effectiveness of the international legal framework;
  • Facilitate the development of the infrastructure necessary for Member States embarking on a nuclear power programme;
  • Strengthen and maintain capacity building - including education, training and exercises at the national, regional and international levels;
  • Ensure the on-going protection of people and the environment from ionizing radiation following a nuclear emergency;
  • Enhance transparency and effectiveness of communication and improve dissemination of information; and
  • Effectively utilize research and development.


Large areas around the Fukushima Daiichi Nuclear Power Plant remain radioactively contaminated. An IAEA International Expert Mission was dispatched in October 2011 to help Japan to develop its remediation plans. At several demonstration sites, experts evaluated the efficiency of a number of methods and technologies that can be used in environmental remediation strategies.

Roadmap Milestone

In October 2011, TEPCO announced that the damaged reactor building surrounding Unit one had been covered by panels to slow the release of radioactive material from the building. By December 2011, TEPCO declared that the crippled reactors had achieved a condition "equivalent to cold shutdown ".

Better Protection Against Extreme Events

One of the key lessons learned from the accident is the crucial importance of preparation in protecting vital safety systems - cooling, emergency power, emergency communications and control systems - against extreme events such as tsunamis, floods, hurricanes and earthquakes. Since the accident, regulators and operators around the world have been reassessing the safety margins of nuclear power plants in their countries, conducting the so-called "stress tests" and making the necessary improvements. The IAEA's methodology for assessing the safety vulnerabilities of a nuclear power plant, which meets the IAEA Safety Standards, was provided to Member States in November 2011. It is now being used by Member States, of which Japan is the first to implement it.

Japan directed the operators of existing Japanese nuclear power plants to conduct Comprehensive Safety Assessments, or "stress tests" and to put in place emergency safety measures to protect against an earthquake or tsunami that causes the loss of all electrical power and the "ultimate heat sink".

An IAEA international expert mission in January 2012 reviewed NISA's process for reviewing the Comprehensive Safety Assessments. In addition to meetings in Tokyo with officials from NISA, the Japanese Nuclear Energy Safety Organization and the Kansai Electric Power Company, the team visited the Ohi Nuclear Power Station to see stress tests in action. "We concluded that NISA's instructions to power plants and its review process for the Comprehensive Safety Assessments are generally consistent with IAEA Safety Standards," said team leader James Lyons, director of the IAEA's Nuclear Installation Safety Division.

Studying Marine Contamination

Since the accident, the Japanese authorities and the IAEA monitored the marine radioactivity near the Plant site. Countries in the Asia and Pacific region requested the IAEA's support, worried that the considerable releases of radioactively contaminated water could reach their coasts, with possible damaging consequences for those communities and their economies. In June 2011, the IAEA Board of Governors approved an IAEA Regional Cooperation project to support countries in the Asia and Pacific region. Twenty-four countries are participating in the multi-year project Marine Benchmark Study on the Possible Impact of the Fukushima Radioactive Releases in the Asia-Pacific Region.

The project will harmonize the analyses of various radioisotopes in marine waters, marine plant and animal life, sediments and suspended matter. The uniform measurement of the radioisotopes in the ocean will ensure that any impact assessment is comparable and verifiable across the enormous volume of the Pacific Ocean.

Action to Strengthen Nuclear Safety

The lessons from the Fukushima accident are now being used to strengthen international nuclear safety. In implementing the Nuclear Safety Action Plan, particular attention is focused on strengthening regulatory institutions by increasing their independence and effectiveness, enhancing emergency preparedness, response, and communications, strengthening nuclear safety peer reviews and nuclear safety standards.

Speaking to journalists in Vienna in March 2012, IAEA Director General Yukiya Amano said, "We know what went wrong at Fukushima Daiichi. The accident was caused by a huge earthquake and tsunami, but there were also human and managerial failings. These are being addressed in Japan, which has reorganised its nuclear regulatory system."

Throughout the world, he added, "The right lessons are being learned. Stress tests are being carried out at all of the world's nuclear plants. Practical steps are being taken to strengthen defences against powerful natural disasters. All aspects of safety are being reviewed."

Stronger Regulators

Following the accident, the role of national regulatory authorities has received increased attention. An independent authority, strongly enforcing regulations based on IAEA Safety Standards, is essential in establishing a safety culture that can effectively detect and correct safety issues before they can impinge on the safe operation of a nuclear power plant. Japan is planning to establish a new, more independent regulatory institution that unifies national regulatory functions, improves crisis management, transparency and training, while developing new regulations and standards. The IAEA will support Japan in implementing its new nuclear safety regulatory system.

Emergency Communications

Effective communication during a nuclear emergency ensures that the affected populations are better protected and informed, while the accident is resolved more quickly and with less damage. The IAEA introduced an enhanced web-based communication platform, the Unified System for Information Exchange in Incidents and Emergencies (USIE), to improve communication with Member States and other international organizations.

Updating Peer Reviews and Safety Standards

The IAEA is expanding and updating its programme of peer reviews, in which the IAEA gathers and dispatches teams of international experts to assess the effectiveness of a Member State's nuclear regulatory system, nuclear power plant operation safety, nuclear power plant site seismological safety, nuclear emergency preparedness, and the systematic development of nuclear power infrastructure in countries planning to embark on a nuclear programme. The IAEA conducted over 30 peer review missions between March 2011 and March 2012. Such missions will continue to be undertaken regularly. The IAEA's Safety Standards, based on best international practices and accumulated experience, are used as benchmarks to determine compliance performance. The IAEA Safety Standards, focusing on the regulatory framework, as well as site evaluation of nuclear installations, design, commissioning and operation, have also been updated following the accident.

Safer Nuclear Power

With 435 nuclear power plants in operation around the world and a further 90 expected to become operational by 2030, Director General Amano said: "Nuclear power is now safer than it was a year ago. But nuclear safety is something that must be worked at every day and we must never become complacent. The major causes of this severe accident can be addressed effectively if the Action Plan on Nuclear Safety is implemented diligently by the Member States." Although the accident has triggered a global effort to re-examine nuclear safety, the Director General warned, "We cannot return to business as usual if we wish to prevent nuclear accidents in the future. Fukushima teaches us that nuclear facilities, operators and regulators must be adequately prepared to survive extreme events without significant damage. Through the focused action and unwavering commitment all nations, international organizations and other stakeholders, nuclear safety will continue to be further strengthened."

Upcoming Nuclear Safety Events

Nuclear safety experts around the world are studying the Fukushima Daiichi nuclear accident. At international conferences, workshops and meetings, organized or co-organized by the IAEA, these experts will gather to discuss a broad array of issues and lessons learned from the accident. In March 2012, experts will discuss reactor and spent fuel safety in the light of the accident. The Contracting Parties to the Joint Convention on the Safety of Spent Fuel Management and on the Safety of Radioactive Waste Management will convene for the Convention's fourth review in May 2012. In Vienna, experts will consider best practices in enhancing transparency and communication effectiveness in the event of a nuclear or radiological emergency in June 2012. Seismologists and nuclear safety experts will meet in Vienna to consider protection against extreme earthquakes and tsunamis in August 2012. An extraordinary Meeting of the Contracting Parties to the Convention on Nuclear Safety will also meet in Vienna, in August 2012. In Fukushima Prefecture, Japan, a Ministerial Conference on Nuclear Safety will be held in December 2012. In the following spring, international experts will consider remediation and decommissioning issues in March 2013. An International Conference on Effective Nuclear Regulatory Systems will be held in Canada in April 2013.

Last update: 26 Jul 2017

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