In 1986, in Pripet, Russia, one of the four reactors at the Chernobyl nuclear plant exploded in the world's worst civil nuclear catastrophe. It sent a cloud of radioactive dust over Europe. The cause was an experiment that went wrong, causing the fourth reactor to explode and melt down. It was not announced until two days later, on 28 Apr 1986. Thirty-one people, mostly firemen, were killed immediately after the explosion, and several thousand more - those involved in the clean-up and children - have since died from radiation-related illnesses. Ukraine says the health of millions of its people have been affected by the disaster. The final shutdown of the undamaged last reactor on the site took place ceremoniously on 15 Dec 2000.