The explosion resulted from a failure of the cooling system of the tank. There was a release of 740 PBq of fission products, approximately 10% of which was dispersed into the atmosphere. Cerium-144 and Zirconium-95 (both relatively short lived isotopes with a half life of 285 and 64 days respectively) made up 91% of the release. There was 1 PBq of Sr-90, and 13 TBq of Cs-137. The contaminated zone, called East Urals Radioactive Trace (EURT), measuring 300 x 50 km was contaminated by more than 4 kBq/m2 of Sr-90. The global fallout of Sr-90 was about 2 kBq/m2. An area measuring 17 km2 was contaminated by about 100 MBq Sr-90/m2. There were 270,000 inhabitants of the area. Mass evacuation was carried out as the critical contamination resulted from Sr-90 with a half-life of 28.8 years. About 800 km2 of land were taken out of use, and 82% of this area has now been taken into use again for forestry and farming. However, evacuation was limited to the nearest settlements leading to more than 1000 acknowledged victims. It was estimated in 1990 that at this time, around 10,000 people lived in areas where the level of ambient radiation was more than quadruple that of the average in Chernobyl's restricted area after 1986. The Kyshtym accident was largely concealed by the Soviet government until 1980, when the Soviet biologist Zhores Medvedev revealed its existence. References Notes Sources External links Photo gallery of Kyshtym Brief history of Kyshtym Cities and towns in Chelyabinsk Oblast Yekaterinburgsky Uyezd