The explosion occurred on April 26, 1986, at 1:23 a.m. during system test at the reactor #4 of the Chornobyl plant. It destroyed the reactor completely. The building of the power plant unit and the roof of the turbine island were partially demolished. More than 30 fire beds appeared; during the first hour, the majority of them were put out, and the fire was liquidated by 5 a.m. April 26.
The fire sent a plume of highly radioactive fallout into the atmosphere and over an extensive geographical area, including Prypiat, the home town of the plant. The plume drifted over large parts of the western Soviet Union and Europe.
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From 1986 to 2000, 350,400 people were evacuated and resettled from the most severely contaminated areas of Belarus, Russia, and Ukraine. According to official post-Soviet data, about 60 percent of the fallout landed in Belarus.
The Chornobyl disaster was the worst nuclear power plant accident in history in terms of cost and casualties. It is one of only two classified as a level 7 events (the maximum classification) on the International Nuclear Event Scale, the other being the Fukushima Daiichi nuclear disaster in Japan in 2011.
During the explosion itself, one person died in the fourth power plant unit. Another one died in the morning. On April 27, 104 persons who suffered from the disaster were evacuated to Moscow. Subsequently, 134 employees of the power plant, fire workers, and rescuers were diagnosed with radiation disease. 28 of them died within next months.
About 200,000 persons were evacuated from the disaster area. The situation was aggravated by the fact that Soviet leadership concealed information of the disaster - on May 1, people took to streets for traditional Labor Day parades.
According to Soiuz Chornobyl organization's data, 10 percent of 600,000 liquidators died, and 165,000 became disabled.