On April 25th -26th, 1986 the World's worst nuclear power accident occurred at Chernobyl in the former USSR (now Ukraine). The Chernobyl nuclear power plant located 80 miles north of Kiev had 4 reactors and whie testing reactor number 4 numerous safety procedures were disregarded. While the chain reaction in the reactor became out of control at 1:23AM, creating explosions and a fireball which blew off the reactor's heavy steel and concrete lid.
The Chernobyl accident killed more than 30 people immediately, and as a result of the high radiation levels in the surrounding 20-mile radius, 135,00 people had to be evacuated.
link:
http://www.myownlittleserver.us/chernobyl/chapter1.html