The accidents at Fukushima Daiichi reactor units 1, 2 and 3 were the combined result of the loss of offsite power to the plant, which was caused by the earthquake, and the loss of both onsite power and the ultimate heat sink because of the tsunami. Without a source of electrical power, the systems and components used to cool the fuel in the reactors were not able to function. Although the plant's operator, the Tokyo Electric Power Company (TEPCO) attempted other measures to cool the fuel, they were unsuccessful in preventing the fuel from overheating and melting. In addition, hydrogen generated during the accidents collected within the reactor buildings and caused explosions in the upper portions of the unit 1, 3 and 4 reactor buildings.
The reactors at the Fukushima Daiichi nuclear power plant are boiling water reactors (see BWR Design Basics). The combined magnitude of the earthquake and subsequent tsunami make it difficult to determine how other reactor designs and even other BWRs with different design features would have responded in similar circumstances. Following the earthquake, the units in operation on 11 March 2011 (units 1 to 3) shut down automatically as designed. After offsite power was lost, the determining factors for the accident were the loss of emergency power supplies and the loss of the plant’s “ultimate heat sink”, which were caused by the tsunami. Ultimate heat sink refers to a virtually inexhaustible supply of water such as a river or sea, in this case the Pacific Ocean, used to cool vital systems in the event of a worst-case scenario accident. The external hazards at Fukushima Daiichi are described as “design neutral”, and would have severely impacted the plant regardless of reactor type.
In simple terms, the accident at Chernobyl was caused by conditions inside the plant, as opposed to conditions outside the plant as in the case of Fukushima. The events also differ in terms of public exposure to radiation.
The Chernobyl accident, which occurred on 26 April 1986, was caused by a power surge during a test being carried out by operators in violation of safety regulations (important control systems had been switched off). The accident was also in part due to the weaknesses of the RBMK reactor design (which only exists in the former Soviet Union). The power surge led to a rapid increase in the heat of the nuclear fuel which, combined with a steam explosion and limited containment, resulted in large amounts of solid and gaseous radioactive materials being widely distributed across Europe. Whereas Chernobyl can be accounted for by human error and reactor-design weaknesses, Fukushima was caused by external events that overwhelmed the engineering features designed to protect the plant.
After Fukushima, varying levels of radioactive contamination were detected across the northern hemisphere. According to current figures from the Japanese authorities, the total amount of contamination released is estimated to be 12% of that released by the Chernobyl accident. However, the exposure of the population at Fukushima has been far less than at Chernobyl, mainly because of rapid governmental action and because local inhabitants were evacuated and sheltered immediately. The public was also kept informed of the consumption restrictions of fresh milk or certain vegetables.
Before the Fukushima accident, there was a 20% risk of death from cancer among members of the Japanese public (it should be noted that almost all populations around the world are subject to similar levels of risk). After Fukushima, the risk will remain at about 20%. Based on the radiation releases and the evacuation and other precautions taken, the number of people who will contract cancer in Japan is not going to change as a result of the accident. The number of fatal cancers does, however, fluctuate naturally and quite largely year on year. Thus, even after Fukushima, in a given population (for example of 100 000 people), the potential extra risk from the doses received are so small that any extra cancers that could be caused by these exposures would be indistinguishable from the natural variation in the annual number of cancers. In other words, at these low doses it is impossible to distinguish between a population which has been exposed to radiation and one which has not – unless the individuals in the exposed population received a dose of radiation greater than 100 millisieverts (mSv) The population around Fukushima has received far lower doses than this.
This does not mean that the local population should not be followed medically. The Japanese government is working on an extensive medical surveillance programme in which 2 million people will be followed medically for a long period of time. This is to ensure that if a cancer were detected, it would be caught as early as possible. The expectation is that the level of cancers will not be statistically significant or outside normal levels.
In some areas around the Fukushima Daiichi plant, there will still be measurable contamination in the environment 300 years after the accident. This is because the caesium-137 radionuclide has a half-life of 30 years, and it is assumed that after 10 half-lives this radionuclide will no longer be detectable. Such contamination comes in the form of radioactive particles like dust which settle on the ground, plants, houses and roads.
The Japanese government has stated that the people who were evacuated from their communities following the accident may return to their homes once the expected dose rate is equal to or less than 20 mSv per year. The Japanese authorities will decide when it is acceptable for people to return home. Through ongoing decontamination work, the intention is to cut dose rates by 50% over the next two years. However, in some communities there will be parts of villages or towns which will be off limits. This is because at present certain buildings may be too difficult to decontaminate.
There are two manners in which people can become exposed to radiation. The first “exposure pathway” is by eating foods that are contaminated or inhaling dust that is contaminated. The second exposure pathway is simply being in an area which is contaminated and being exposed to the radiation. For the first exposure pathway, the Japanese authorities are doing their best to continue to monitor and control all food that is produced in the Fukushima area. If the dose that would result from people eating contaminated food is higher than a given level (1 mSv), it is not sent to market. The Japanese authorities are also keeping the public informed of the ingestion risks so that they are knowledgeable about how to protect themselves. The second exposure pathway is more difficult to control because it involves removing contamination from where people are living. This is done by decontaminating affected areas, using such methods as washing walls and roads, taking off the top 5 cm of affected soil and by removing the surface of concrete around homes or schools in affected areas.
Every country with an operating nuclear power plant has undertaken a national review to identify immediate safety concerns and areas in which to enhance safety. To date, no reactor has been shut down on the basis of technical safety. However, areas in which safety can be improved in nuclear power plants have been identified.
Several measures have been identified to improve the ability of nuclear power plants to withstand multiple external hazards such as combined earthquakes and tsunamis, no matter how small the probability of this occurring. Lessons are being learnt regarding the layout of a nuclear power plant, and the positioning of back-up power systems so that they are not lost in the event of flooding, as was the case at the Fukushima Daiichi nuclear power plant. In the area of emergency response, provisions have already been made for operators of multi-unit power plants to have access to enough back-up generators for several reactor units in the event of an accident.
In general, while four countries (Belgium, Germany, Italy and Switzerland) have opted to phase out or not to re-introduce nuclear power, a larger number have reaffirmed their commitment to continuing or increasing its use. These countries include China, the Czech Republic, Finland, France, Hungary, India, the Netherlands, Poland, Russia, Korea, the Slovak Republic, Turkey, the United Arab Emirates, the United Kingdom, the United States and Vietnam.
Most countries have realised that, despite the significance of the Fukushima Daiichi accident, it has not brought into question nuclear power as such but rather specific sites and reactor designs for these sites. In addition, the fundamental drivers for nuclear power remain unchanged, including the growing demand for energy worldwide, countries’ needs for energy security and limiting greenhouse gas emissions.
The accident at Fukushima Daiichi is likely to delay nuclear power plant construction plans in the short term, but overall, its effects will probably be less than initially thought from the medium and long-term perspectives. In the coming years, growth in nuclear capacity is expected to continue strongly in China, Korea, India and Russia.
A millisievert (mSv) is the unit which describes the amount of radiation, or dose, received by an individual. For an introduction to radiation issues, see the IAEA guide to Radiation Safety.
Last modified: 9 March 2012