April 26, 1986, 40 years ago: The Number 4 reactor explodes at the Chernobyl Nuclear Power Plant in Pripyat, Ukraine. It is the biggest nuclear accident in human history.
The accident occurred during a test of the steam turbine's ability to power the emergency feedwater pumps in the event of a simultaneous loss of external power and coolant pipe rupture. Following an accidental drop in reactor power to near-zero, the operators restarted the reactor in preparation for the turbine test with a prohibited control rod configuration.
Upon successful completion of the test, the reactor was then shut down for maintenance. Due to a variety of factors, this action resulted in a power surge at the base of the reactor which brought about the rupture of reactor components and the loss of coolant. This process led to steam explosions and a meltdown, which destroyed the containment building.
This was followed by a reactor core fire which lasted until May 4, during which airborne radioactive contaminants were spread throughout the Soviet Union and Europe.
In response to the initial accident, a 10-kilometer (6.2-mile) radius exclusion zone was created 36 hours after the accident, from which approximately 49,000 people were evacuated, primarily from Pripyat. The exclusion zone was later increased to a radius of 30 kilometers (19 miles), from which an additional 68,000 people were evacuated.
Following the reactor explosion, which killed two engineers and severely burned two more, an emergency operation to put out the fires and stabilize the surviving reactor began, during which 237 workers were hospitalized, of whom 134 exhibited symptoms of acute radiation syndrome (ARS).
A United Nations committee found that to date fewer than 100 deaths have resulted from the fallout. Model predictions of the eventual total death toll in the coming decades vary. The most widely cited study conducted by the World Health Organization in 2006 predicted 9,000 cancer-related fatalities in Ukraine, Belarus and Russia.
North America was lucky with Three Mile Island in 1979. Eastern Europe was not so lucky with Chernobyl in 1986.
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