One of the worst nuclear accidents took place on Sep. 29, 1957, but was only made public years later. On that day, a tank containing 80 tons of highly-radioactive liquid waste exploded at the Mayak plutonium plant in the southern Urals, 15 kilometers east of the Russian city of Kyshtym. The blast produced a radioactive cloud that was about 300 kilometers long and 40 kilometers wide, and which traveled northeast. The radiation did not reach Europe, but was at the same level of that released during the Chernobyl disaster in 1986. About 15,000 people who lived in the area were evacuated, and the houses located in a 25-kilometer zone surrounding the location were destroyed. No one was allowed to go back. The plutonium production at the plant, which also delivered the material for the Soviet Union’s first atomic bomb, was not discontinued.