Chernobyl disaster (1986)
The accident, which occurred in 1986, resulted when operators took actions in violation of the plant’s technical specifications. Operators ran the plant at very low power, without adequate safety precautions and without properly coordinating or communicating the procedure with safety personnel.
The four Chernobyl reactors were pressurized water reactors of the Soviet RBMK, which were designed to produce both plutonium and electric power, they were very different from standard commercial designs, employing a unique combination of a graphite moderator and water coolant. The reactors also were highly unstable at low power, primarily owing to control rod design and positive-void-coefficient factors that accelerated nuclear chain reaction and power output if the reactors lost cooling water. These factors all contributed to an uncontrollable power surge that led to Chernobyl 4’s destruction. The power surge caused a sudden increase in heat, which ruptured some of the pressure tubes containing fuel.
The hot fuel particles reacted with water and caused a steam explosion, which lifted the 1,000-metric-ton cover off the top of the reactor, rupturing the rest of the 1,660 pressure tubes, causing a second explosion and exposing the reactor core to the environment. The fire burned for 10 days, releasing a large amount of radiation into the atmosphere.
The Chernobyl plant did not have the massive containment structure common to most nuclear power plants elsewhere in the world. Without this protection, radioactive material escaped into the environment. The accident cause 44 immediate death and 9,000 to 16,000 cancer deaths in the following years from radiation exposure. (Image source: Wikipedia)