Twenty years ago, in April 1986 a terrible accident occurred at the Number 4 Chernobyl Plant in Ukraine, formerly within the Soviet Union. It was caused by six different operator errors in a risky design. Two of the errors were the use of "cutout switches" in safety shutdown circuits. Unlike nuclear reactors in the United States, Soviet plants had this feature, which was created in naval equipment before the use of nuclear power.
The accident released a cloud of radioactive particles and gases that created measurable increases in doses in the Northern Hemisphere. Doses were highest in European countries, but far below anything immediately harmful. The lack of clear information and different standards in each country created panic in Europe. Many livestock were needlessly destroyed and food wasted, while exposed food was consumed in Soviet republics.
Health effects and fallout distribution have been studied continuously since the accident, as they have been for atomic bomb survivors, and others who have received large doses. The latest results show that the most important effect has been psychological, while physical effects are much less severe than originally estimated. This is to be expected, since calculations and judgments about radiation and nuclear matters always have large safety factors, to make sure any error is in the direction of safety.
After the accident, the community of nations forced the Soviet government to "tell all." International evaluation showed that the accident was impossible in any other type of commercial reactor. Additionally, it showed that the design did not have safety systems adequate for that type of reactor.
The Chernobyl type reactor was designed to produce weapons grade plutonium and to generate electricity at the same time, and used graphite (similar to charcoal). A more prevalent practice is to have separate reactors for power and weapons material. The Chernobyl type reactors that continued in operation were modified to prevent a repetition of the accident.
The operators were attempting to repeat an "engineering run" to get data on the whole plant's performance during shutdown. Because they were repeating the engineering run, overconfidence and schedule change pressures led to the operator errors. The accident began with a nuclear power burst about equal to 160 pounds of TNT. This caused a chemical reaction with the graphite, resulting in a much larger explosion. The reactor vessel lid was blown aside; a hole was blown in the roof, and pieces of core and fuel material blown on to the roof. The graphite was set on fire, and the reactor was burning down and melting down at the same time.
The Soviets did not tell the world until after increasing atmospheric radiation in other countries led to questions. Control and containment efforts killed more than 36 people from direct radiation exposure. Reacting to the delay, the government appears to have quarantined an area that was excessive, when compared to the natural radioactive exposure to people and their food chain in some of the world's hot spots. Ultra conservative studies project large numbers of early fatalities, but these are only after decades, if they occur at all.
Revision 5, December 27, 2005
Reference: Nuclear News Special Report. September 11, 1986 "Chernobyl: The Soviet Report." Reports on the August 25-29 IAEA Conference on this accident. Nuclear News is the monthly magazine of the American Nuclear Society.
UNSCEAR 2000 Report to the General Assembly.
Last updated June 27, 2012, 8:50am CDT.