Kyshtym disaster
During different accidents, a large amount of radioactivity was released into the environment. The biggest of these accidents happened in 1957, and is known as Kyshtym Disaster today. It happened in a plant near Kishtim (Кыштым). There were many accidents in this secret plant.  The most serious of them happened on September 29, 1957. The cooling system of the plant failed to work. A (non-nuclear) explosion occurred. This explosion had the force of between 75 and 100 tons of TNT. It threw the lid of the container, which weighted 160 tons, into the air. The accident released about 20 MegaPCi (about 740 Petabecquerel) of radioactivity. At least 200 people died from radiation sickness. About 10,000 people were evacuated from their homes. More than 470,000 people were affected by the radiation, without knowing it.