16 April 1986. Accident on Chernobyl Nuclear Power Plant, Ukraine
Amount of 137Cs released: 85 PBq or 24 kg.
13 September 1987. Stolen radiotherapy device in Goiania, Brazil
Amount of 137Cs released: 50.9 TBq or 14 g.
25 May 1998. Accident on Acerinox metal reprocessing plant in Cádiz, Spain
A mass of radioactive 137Cs that came from a γ-ray generator accidentally melted down releasing a radioactive cloud. The Acerinox chimney detectors failed to detect it, but it was eventually detected in France, Italy, Switzerland, Germany, and Austria. The radioactive levels measured were up to 1000 times higher than normal.
11 March 2011. Accident on Fukushima Nuclear Power Plant, Japan
Amount of 137Cs released: 11 PBq or 3.1 kg. The significantly contaminated area was 10-12% that of Chernobyl.