The Chernobyl disaster was the worst nuclear power plant accident in history in terms of cost and casualties.
It was occurred on 26 April 1986 in in the town of Pripyat in Ukraine, which was the Ukrainian SSR at that time.
As a result of a huge blast and fire, radioactive particles were released into the air, spreading over much of the western USSR and Europe.
The Chernobyl disaster is one of only two classified as a level 7 event (the maximum classification) on the International Nuclear Event Scale. The other was the Fukushima Daiichi nuclear disaster in Japan in 2011.
The disaster began during a systems test on April 26, 1986, at reactor number four of the Chernobyl plant. Thirty-one deaths are directly attributed to the accident, while the World Health Organization (WHO) estimated that 4,000 other fatalities will eventually be linked to the worst nuclear power plant disaster in history, according to NBC News.