Kyshtym disaster
During the next ten to eleven hours, the radioactive cloud moved towards the northeast, reaching between 300 and 350 kilometers from the accident.  The fallout of the cloud resulted in a long-term contamination of an area of more than 800 square kilometers, mostly with caesium-137 and strontium-90.  Today, the area is known as the East-Ural Radioactive Trace (EURT).