The Kyshtym disaster was a radioactive contamination accident that occurred on 29 September 1957 at Mayak, a plutonium production site for nuclear weapons andnuclear fuel reprocessing plant in the Soviet Union. It measured as a Level 6 disaster on the International Nuclear Event Scale (INES), making it the third most serious nuclear accident ever recorded, behind the Fukushima Daiichi nuclear disaster and the Chernobyl disaster (both Level 7 on the INES). The event occurred in the town of Ozyorsk, Chelyabinsk Oblast, a closed city built around the Mayakplant. 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.

In 1956 the cooling system in one of the tanks containing about 70–80 tons of liquid radioactive waste failed and was not repaired. The temperature in it started to rise, resulting in evaporation and a chemical explosion of the dried waste, consisting mainly of ammonium nitrate and acetates (see ammonium nitrate/fuel oil bomb).

The explosion, on 29 Sept 1957, estimated to have a force of about 70–100 tons of TNT, threw the 160-ton concrete lid into the air. There were no immediate casualties as a result of the explosion, but it released an estimated 20 MCi (800 PBq) of radioactivity. Most of this contamination settled out near the site of the accident and contributed to the pollution of the Techa River, but a plume containing 2 MCi (80 PBq) of radionuclides spread out over hundreds of kilometers.

 Previously contaminated areas within the affected area include the Techa river which had previously received 2.75 MCi (100 PBq) of deliberately dumped waste, and Lake Karachay which had received 120 MCi (4,000 PBq).

In the next 10 to 11 hours, the radioactive cloud moved towards the north-east, reaching 300–350 kilometers from the accident. The fallout of the cloud resulted in a long-term contamination of an area of more than 800 to 20,000 square kilometers (depending on what contamination level is considered significant), primarily with caesium-137 and strontium-90.[3] This area is usually referred to as the East-Ural Radioactive Trace (EURT)

At least 22 villages exposed to radiation from the disaster, with a total population of around 10,000, were evacuated. Some were evacuated after a week but it took almost 2 years for evacuations to occur at other sites.

The true number of fatalities remains uncertain because radiation-induced cancer is clinically indistinguishable from any other cancer, and its incidence rate can only be measured through epidemiological studies. One book claims that "in 1992, a study conducted by the Institute of Biophysics at the former Soviet Health Ministry in Chelyabinsk found that 8,015 people had died within the preceding 32 years as a result of the accident."

 By contrast, only 6,000 death certificates have been found for residents of the Techa riverside between 1950 and 1982 from all causes of death, though perhaps the Soviet study considered a larger geographic area affected by the airborne plume. The most commonly quoted estimate is 200 deaths due to cancer, but the origin of this number is not clear. More recent epidemiological studies suggest that around 49 to 55 cancer deaths among riverside residents can be associated to radiation exposure.

 This would include the effects of all radioactive releases into the river, 98% of which happened long before the 1957 accident, but it would not include the effects of the airborne plume that was carried north-east. The area closest to the accident produced 66 diagnosed cases of chronic radiation syndrome, providing the bulk of the data about this condition.

To reduce the spread of radioactive contamination after the accident, contaminated soil was excavated and stockpiled in fenced enclosures that were called "graveyards of the earth". The Soviet government in 1968 disguised the EURT area by creating the East Ural Nature Reserve, which prohibited any unauthorised access to the affected area.