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Japan Nuclear Accident

Technical Background on Fukushima Daiichi Nuclear Power Plant Accident

This brief provides some technical background on nuclear reactors and the major accident that occurred at the Fukushima Daiichi Nuclear Power Plant (NPP) on 11 March 2011.

Nuclear Fission

Currently, all nuclear power plants derive power from nuclear fission. This phenomenon occurs when certain heavy atoms are struck by a neutron - they absorb it, become unstable and split apart. It results in two lighter atoms and the ejection of two or three neutrons. If the ejected neutrons go on to strike other unstable atoms nearby, those too can break apart, releasing further neutrons in a process known as a chain reaction. When enough of these neutrons produce further fissions, the process becomes self-sustaining. Compared to chemical reaction e.g. burning, nuclear fission gives off an enormous quantity of energy. The energy generated in the nuclear fission process is harvested to produce electricity in a NPP.

Click here for an illustration of the fission process.

Radiation is given out as a by-product of the chain reaction. It is therefore important to ensure safe operation of the nuclear power plant so that there is no accident or leakage of radiation which could cause harmful effects to people and the environment. In addition, when uranium is 'split' into two during the fission process, significant quantities of highly radioactive wastes are created. More than 99% of the radioactivity produced during the fission reaction is retained in the fuel rods.

Types of Nuclear Reactors

At present, there are 7 types of reactors in operation globally. They are generally classified according to the type of coolant and moderator*, as listed below:

Type Coolant Moderator Commonly used Fuel No. of Reactors Percentage of Reactors
Boiling Water Reactor (BWR) Water Water Enriched Uranium 41 10.0%
Pressurized Water Reactor (PWR) Water Water Enriched Uranium 301 73.4%
Pressurized Heavy Water moderated Reactor (PHWR)/ (CANDU) Heavy Water Heavy Water Natural Uranium 46 11.2%
Light Water cooled Graphite moderated Reactor (LWGR)/ (RBMK) Water Graphite Enriched Uranium 11 2.7%
Gas Cooled Reactor (GCR) / (AGR) CO2 Graphite Natural / Enriched Uranium 8 2.0%
Fast Breeder Reactor (FBR) Liquid Metal None Mixed Oxides of Plutonium and Uranium 2 0.5%
High-Temperature Gas- Cooled Reactor (HTGR) Gas (Helium) GraphiteTRISO fuel (Uranium Dioxide) 0.2%
Total 410 100%
(Source: IAEA, as at 05 June 2023)

* The moderator slows the free neutrons to ensure that they will be absorbed by the nucleus upon collision, hence sustaining the fission process.

Boiling Water Reactor

The 6 reactors at Fukushima Daiichi NPP are of the type known as boiling water reactor (BWR). Fig. below how the boiling water reactor works.

Boiling Water Reactor

(Courtesy of US NRC)

The nuclear fuel used is Uranium Oxide, a ceramic with a very high melting point of about 2,800°C. The fuel is manufactured in pellets put into a long tube made of Zircaloy (an alloy of zirconium) with a failure temperature of 1,200°C (caused by the auto-catalytic oxidation of water), and sealed tight. This tube is called a fuel rod. These fuel rods are then put together to form an assembly of which several hundred make up the reactor core.

fuel rod

Courtesy of Nuclear Fuel Industries, Ltd (


The "defence-in-depth" principle is used by the nuclear industry to achieve a high level of safety. The solid fuel pellet is the first barrier that retains many of the radioactive fission products produced by the fission process. The Zircaloy casing is the second barrier to release that separates the radioactive fuel from the rest of the reactor.

The core is then placed in the reactor vessel (see link below). The reactor vessel is a thick steel vessel and is designed to withstand the high pressures that may occur during an accident. The reactor vessel is the third barrier to radioactive material release.

Click here to view Fukushima Daiichi NPP Containment Layout

The entire primary loop of the nuclear reactor - the reactor vessel, pipes, and pumps that contain the coolant (water) - are housed within a larger steel containment vessel. This vessel is the fourth barrier to radioactive material release. The primary containment structure is a hermetically (air tight) sealed, very thick structure made of steel and concrete. This structure is designed, built and tested for one single purpose: To contain, indefinitely, a complete core meltdown. To aid in this purpose, a large, thick concrete structure is poured around the containment structure and is referred to as the secondary containment. Both the primary and secondary containment structures are housed in the reactor building.

How Did The Accident Occur

The crisis at the Fukushima Daiichi plant is principally a result of the combination of earthquake, tsunami, and subsequent flooding, which knocked out the off-site power supply and the emergency power systems, disabling cooling water from being pumped to the fuel rods in the reactors.

When the earthquake hit North-east Japan on 11th March 2011, 3 of the reactors (Reactors #1 to #3) automatically shut down. The other 3 reactors (Reactors #4 to #6) were already in a state of cold shutdown for periodic inspection. However, even after shutting down, Reactors #1 to #3 were extremely hot and needed to be continuously cooled by water circulation. The lack of cooling water resulted in rising high temperatures in the reactor core, which ultimately led the operators to vent the reactor, resulting in release of hydrogen. The hydrogen built up in the reactor buildings, eventually leading to explosions.

The reactors in Fukushima Daiichi NPP have Mark I containment, the General Electric (GE) model that was popular in 1971. In this model, spent fuel rods are stored in a pool inside the concrete reactor building for at least 10 years before being transferred to long-term storage. The pool requires a constant circulation of water to remain cool. However, the circulation stops when electricity is disrupted. The heat generated by the rods is so hot that it causes the water to boil away. If the spent fuel is not cooled, it may result in spontaneous combustion. This is what happened at Reactor #4, where the fire which occurred resulted in the release of radioactive materials into the environment. 

Workers at the plant fought to prevent things from getting worse by pumping seawater mixed with boron through fire hoses into release valves in the containment vessels. Boron helps absorb neutrons, reducing radioactivity, and the seawater performs the cooling. The challenge was to bring the temperature of the reactor core under control, before rising heat and pressure cause the containment vessel to fail, thereby releasing huge amounts of radioactive material into the environment. Meltdown is believed to have happened in Reactors #1 to #3 with most of the fuel in Reactor #1 melted through to the bottom of the reactor vessel and the cores of Reactors #2 and #3 badly overheated.

Reports showed that fission products such as radioactive caesium and iodine were released from the plant. Westerly winds could have blown some of these to the Pacific Ocean. NEA carried out simulation of the radiation plume and given the distance of more than 5,000km, assessed that it is extremely unlikely for any radiation plume to reach Singapore. As a precaution, NEA carries out round-the-clock monitoring of ambient radiation levels and no abnormality has been detected to date.

Stabilising of situation

On 28 October 2011, the Tokyo Electric Power Company (TEPCO) announced that the canopy to cover the #1 reactor building is fully operational and functional. The canopy, equipped with a ventilation system, was built to further contain any release of radioactive materials from the Fukushima Daiichi plant. The ventilation system was tested and shown to remove more than 90 percent of the radioactive caesium contained in the canopy covering.  TEPCO also announced it would construct similar covers for Reactors #2 and #3.

On 16 December 2011, the Government of Japan Announced that all of the reactors at the Fukushima Daiichi Nuclear Power Station have been brought into a condition equivalent to “cold shutdown”. Cold shutdown conditions were reached when three conditions had been established: the reactor pressure vessel's temperature was less than 100 degrees Celsius, the release of radioactive materials from the primary containment vessel was under control and public radiation exposure by additional release was being significantly held down.

TEPCO said it would continue injecting water into the reactors until all the molten fuel has been removed. As of March 2021, the Nuclear Power Plant had accumulated 1.25 million tonnes of waste water in water tanks, and was running out of land for storage. In 2021, then Prime Minister Suga approved TEPCO to dump Advanced Liquid Processing System (ALPS) treated stored water to the Pacific Ocean. The IAEA, on the request of the Government of Japan, formed a Task Force to provide Japan and the international community with an objective and science-based review. As of 5 April 2023, the IAEA had published the fourth report issued by the Task Force, summarising progress made from the November 2022 mission to review TEPCO's plan to discharge ALPS-treated water at Fukushima Daiichi.

As of 2023, the removal of melted nuclear fuel at Reactors #1 and #2 was currently ongoing and expected to conclude in 2031, with Reactor #3 having completed the removal in 2021. Demolition of the reactor facilities of units 1 to 4 was expected to be completed within 30 to 40 years from the 2011 accident. 

Recovery Efforts

Efforts to contain further radioactive release of radioactive material, decontaminate affected areas and treat radioactive water at Fukushima power plants are still ongoing, while the region takes slow but steady steps on the road to recovery, several years after a catastrophic series of events resulted in the accident at Fukushima Daiichi Nuclear Power Plant.

For example, Fukushima rice passed Japan's radiation checks for the first time in 2014 since the 2011 nuclear disaster. About 360,000 tonnes of rice, nearly all of last year's harvest, had been checked and none had tested above the 100 becquerels per kilogram limit set by the government. Miniscule amounts of rice produced previously in 2012 and 2013 had failed to pass radiation checks and had to be destroyed.

Earlier assumptions about plant safety under extreme conditions have been scrutinised and revised, and defences are being strengthened, in the aftermath of the accident. The IAEA Nuclear Safety Action Plan sets out a blueprint for national and international action in 12 major areas. Among the actions performed:   
  • Developed a new methodology for assessing the safety vulnerabilities of nuclear power plants, which has already been used on an IAEA expert mission to review the approach taken by Japan in its own plant safety assessment;     
  • Sent a number of other expert technical missions to support Japan, and has advised the country as it establishes a new, more independent regulatory system; 
  • Stepped up its peer review services, incorporating lessons of Fukushima to help Member States assess and reinforce nuclear safety, and has taken steps to improve coordination with operators; and

In addition, the IAEA together with members of the international community are working to publish “The IAEA Report on the Fukushima Daiichi Accident” in 2015, which addresses the causes and consequences of the accident, as well as lessons learnt.

  2. Time magazine, 11th March Edition   
  3.  World Nuclear News website  
  4. The IAEA website
  5. Reuters   

Japan Nuclear Accident FAQ

Q: Where can I obtain updates?
A: You may obtain the latest updates from the NEA website at or contact the NEA Call Centre at 1800-2255632.
Q: What is the current situation on the radiation leak?
A: Japan is planning to discharge Advanced Liquid Processing System (ALPS) treated water to the Pacific Ocean. NEA has been closely monitoring radioactivity levels in our sea waters following the Fukushima Nuclear accident and will continue to do so.
Q: How is the situation being monitored?
A: NEA has set up monitoring stations to scrutinize the ambient radiation levels. So far, no abnormal levels of radiation have been detected. Singapore is also keeping in touch with the International Atomic Energy Agency and overseas experts for latest information and updates.
Q: For items coming from Japan, are there plans to check for radiation?

The Agri-Food & Veterinary Authority of Singapore (AVA) had worked closely with Japanese counterparts to keep watch on food imports from Japan. As of May 2021, Singapore has lifted its requirements on food imports from Japan’s Fukushima Prefecture.

Q: What is the travel advisory to Japan and other countries (e.g. Hong Kong, Taiwan, China)?
A: Singaporeans travelling to Japan and other countries are advised to refer to MFA's website at for the latest travel advisories and to eRegister with the MFA at This will enable MFA to contact them and render the neccessary consular assistance in case of emergencies. Singaporeans may also wish to download a smartphone application (MFA@SG) from Goggle Play and Apple App Store that facilitate the eRegistration process as well as to obtain the relevant Travale Notices to various countries.
Q: Is aircraft from Japan screened for radiation?
A: The Civil Aviation Authority of Singapore (CAAS) had been monitoring the potential impact of the accident on flights and airport operations. Developments did not necessitate the screening of aircraft or passengers for radiation. Notwithstanding this, Changi Airport has contingency plans to deal with radioactive contamination.
Q: What are the precautionary measures that one can take if he has to travel in/out of Japan? How can I protect myself if I have to travel to Japan? Can someone who is exposed to radiation passed on the contamination to another person?
A: For health-related queries, please refer to MOH at or contact them at 1800 333 9999.

Let’s Learn More About Radioactivity

Radioactivity is the term used to describe disintegration of atoms. The atom can be characterized by the number of protons in the nucleus. Some natural elements are unstable. Therefore, their nuclei disintegrate or decay, thus releasing energy in the form of radiation. This physical phenomenon is called radioactivity and the radioactive atoms are called radionuclides. The radioactive decay is expressed in units called becquerels. One becquerel equals one disintegration per second.

The radionuclides decay at a characteristic rate that remains constant regardless of external influences, such as temperature or pressure. The time that it takes for half the radionuclides to disintegrate or decay is called half-life. This differs for each radionuclide, ranging from fractions of a second to billions of years. For example, the half-life of Iodine 131 is eight days, but for Uranium 238, which is present in varying amounts all over the world, it is 4.5 billion years. Potassium 40, the main source of radioactivity in our bodies, has a half-life of 1.42 billion years.

Radiation Dose

The biological effects of ionizing radiation vary with the type and energy. A measure of the risk of biological harm is the dose of radiation that the tissues receive. The unit of absorbed radiation dose is the Sievert (Sv). Since one sievert is a large quantity, radiation doses normally encountered are expressed in millisievert (mSv) or microsievert (µSv) which are one-thousandth or one millionth of a sievert, respectively. For example, one chest X-ray will give about 0.05 mSv of radiation dose. The IAEA limit for public exposure to ionising radiation is 1 mSv per year, excluding what a person normally receives form natural background radiation.

Natural Background Radiation

Human beings are exposed to natural background radiation on a daily basis. This radiation mainly comes from space (i.e. cosmic rays) and from naturally-occurring radioactive materials found in the soil. Radon, a naturally-occurring gas, is also a contributor. By far the largest source of natural radiation exposure comes from varying amounts of uranium and thorium in the soil around the world. The radiation exposure due to cosmic rays is very dependent on altitude, and slightly on latitude. The level of natural background radiation can vary considerably from one place to another on the globe, depending on the geographical location, by several hundred percent. In Singapore, the natural background radiation is about 0.1 micro-sieverts per hour.

A typical radiation dose chart is as appended below.

Typical radiation dose chart

Courtesy of National Institute of Radiological Sciences (