A violent explosion on 29th September 1957 in the Chelyabinsk-40 complex in the southern Urals, involving dry nitrate and acetate salts in a waste tank containing highly active waste, contaminated an area later called the \"Kyshtym footprint\".
There was a release of 40 PBq of fission products.
Cerium-144 and Zirconium-95 made up 91% of the release.
There was l PBq of Sr-90, and 13 TBq of Cs-137. An area 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 an effective half-life of 10-20 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.
The explosion resulted from failure of the cooling system in the tank.