Chapter 38: Nuclear Energy: Safety – Accidents – Comparison of Risks – Edited by Dr. Mir F. Ali
Perhaps one nuclear accident could be regarded as too many from a safety perspective simply because nuclear energy continued to be considered as an antagonistic technology not only by some politicians but also by many members of the public. Nevertheless, from a realistic point of view, there were only four nuclear accidents in the world history of nuclear energy which covers a period of more than fifty years. It’s important to recognize that each nuclear accident was treated as a unique opportunity by the regulatory organizations around the world to improve means of preventing and mitigating the consequences of accidents by the application of the concept of defence-in-depth, in which consecutive and independent levels of protection are used to minimize or eliminate harmful effects that could be caused to people and the environment.
The objectives of defence in depth, according to the International Atomic Energy Agency (IAEA), are to:
- Compensate for potential human and component failures:
- Maintain the effectiveness of the barriers by averting damage to the plant and to the barriers themselves; and
- Protect the public and the environment from harm in the event that these barriers are not fully effective.
Here is a brief summary of the nuclear accidents:
The Three Mile Island Accident (USA):
The very first nuclear accident, Three Mile Island, was occurred on March 28, 1979 due to the fact that the operators were unable to diagnose properly to the unplanned automatic shutdown of the reactor. Some radioactive gas was released a couple of days after the accident but not enough to cause any dose above background levels to local residents.
There were no injuries or adverse health effects from the Three Mile Island.
The Chernobyl Daiichi Accident (USSR):
The Chernobyl accident was occurred on April 25, 1986 and it was the result of a flawed reactor design that was operated with inadequately trained personnel. The resulting steam explosion and fires released at least 5 percent of the radioactive reactor core into the atmosphere and downwind. Two Chernobyl plant workers died on the night of the accident, one after, and a further 28 people died within a few weeks as a result of acute radiation poisoning.
The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCLEAR) says that apart from increased thyroid cancers, “There is no evidence of a major public health impact attributable to radiation exposure 20 years after the accident.” Resettlement of areas from which people were relocated is ongoing.
The Tokaimura Criticality Accident (Japan):
The Tokaimura Criticality accident was occurred on September 30, 1999 and according to the IAEA, the cause of the accident was “Human error and serious breaches of safety principles”. As a result of this accident, three workers received high doses of radiation in a small Japanese plant preparing fuel for an experimental reactor. The accident was caused by bringing together too much uranium enriched to a relatively high level, causing a “criticality” (a limited uncontrolled nuclear chain reaction), which continued intermittently for 20 hours.
A total of 119 people received a radiation dose over 1 mSv from the accident, but only the three operators’ doses were above permissible limits. Two of the doses proved fatal; and
The Fukushima Daiichi Accident (Japan):
Following a major earthquake, a 15-metre tsunami disabled the power supply and cooling of three Fukushima Daiichi reactors, causing a nuclear accident on March 11, 2011. All three cores largely melted in the first three days. The accident was rated 7 on the INES scale, due to high radioactive releases in the first few days. Four reactors are written off – 2719 MWe net. After two weeks the three reactors (units 1-3) were stable with water addition but no proper heat sink for removal of decay heat from fuel. By July they were being cooled with recycled water from the new treatment plant. Reactor temperatures had fallen to below 80C at the end of October, and official “Cold Shutdown Condition” was announced in mid-December. Apart from cooling, the basic ongoing task is to prevent release of radioactive materials, particularly in contaminated water leaked from the three units.
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