Today is the 20th anniversary of Chernobyl which was considered the worst Nuclear Energy disaster in the world and It happened on April 26 1986 at 01:23 a.m. (local time) in Ripyat, Ukraine, 18 km northwest of the city of Chernobyl, 16 km from the border of Ukraine and Belarus, and about 110 km north of Kiev.
This disaster was a mix of many events and the explosion of the reactor released 400 times more radiation then the atom bomb dropped at Hirosema.
This is a keen example on why Nuclear Power (That of Fission) should not be allowed. It currently is a necessary evil but with the evolution of Fusion power other resources, I hope that fission will be outlawed, but this issue is for another time.
A good link is Chernobyl Disaster @ Wikipedia It gives some good infomation about this.