The Kyshtym disaster was a radiation contamination incident that
occurred on 29 September 1957 at Mayak, a nuclear fuel
reprocessing plant in the Soviet Union. It measured as a Level 6
disaster on the International Nuclear Event Scale, making it the
third most serious nuclear accident ever recorded behind the
Chernobyl disaster, and Fukushima Daiichi nuclear disaster, both
Level 7 on the INES. The event occurred in the town of Ozyorsk,
a closed city built around the Mayak plant. Since Ozyorsk/Mayak
(also known as Chelyabinsk-40 and Chelyabinsk-65) was not marked
on maps, the disaster was named after Kyshtym, the nearest known
town. After World War II, the Soviet Union lagged behind the US
in development of nuclear weapons, so it started a rapid
research and development program to produce a sufficient amount
of weapons-grade uranium and plutonium. The Mayak plant was
built in haste between 1945 and 1948. Gaps in Soviet physicists'
knowledge about nuclear physics at the time made it difficult to
judge the safet