The Chernobyl diisaster was a horrible nuclear accident that occured on April 26 1986 at the Chernobbyl Nuclear Power Plant in Ukraine.It is marked as the worst nuclear disaster in history, it is one of the only two classified as a level 7 event on the international Nuclear Event Scale. The disaster began during a systeme test at the number four reactor of the plant. There was a sudden and unexpecter surge of power and when an emergency shutdown was attempted a larger spike in power occured, which led to a reactor vessel rupture and a series of steam explosions. This exposed the graphite moderator of the reactor to air, causeing it to ignite. The resulting fire sent a cloud of highly radioactine fallout into the atmosphere and over a great scale of places. There is 56 deaths directly connected to the accident, though there could have been 9,300 deaths caused from exposure to the radiation resulting in cancer. The Chernobyl Forum predicts the total death toll could reach 9,500 among those who were exposed to the highest levels of radiation.
https://en.m.wikipedia.org/wiki/Chernobyl_disaster

Chernobyl Nuclear Disaster (youtube video)