[12] The accident involved waste from the sodium uranyl acetate process used by the early Soviet nuclear industry to recover plutonium from irradiated fuel.

[citation needed] An improperly stored underground tank of high-level liquid nuclear waste exploded, contaminating thousands of square kilometers of land, now known as the Eastern Ural Radioactive Trace (EURT).

On 29 September 1957, Sunday, 4:22 pm, an explosion occurred within stainless steel containers located in a concrete canyon 8.2 m (27 feet) deep used to store high-level waste.

The explosion was caused because the cooling system in one of the tanks at Mayak, 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.

[citation needed] The workers at Ozyorsk and the Mayak plant did not immediately notice the contaminated streets, canteens, shops, schools, and kindergartens.

In the first hours after the explosion, radioactive substances were brought into the city on the wheels of cars and buses, as well as on the clothes and shoes of industrial workers.

The city was intentionally constructed to be upwind from the Mayak plant given the prevailing winds, so most of the radioactive material drifted away from, rather than towards, Ozyorsk.

[19] Even as late as 1982, Los Alamos published a report investigating claims that the release was actually caused by a weapons test gone awry.

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.

Slavsky, it was noted that the reason for the explosion was insufficient cooling of the container, which allowed it to increase in temperature to the point its contents reacted with each other and exploded.

[23][24] Medvedev's description of the disaster in the New Scientist was initially derided by Western nuclear industry sources, but the core of his story was soon confirmed by Professor Lev Tumerman, former head of the Biophysics Laboratory at the Engelhardt Institute of Molecular Biology in Moscow.

[27] The area closest to the accident produced 66 diagnosed cases of chronic radiation syndrome, providing the bulk of the data about this condition.

[30] Starting in 1989, several years after the Chernobyl disaster, the Soviet government gradually declassified documents pertaining to the incident at Mayak.

[31][32] The level of radiation in Ozyorsk, at about 0.1 mSv a year,[33] is harmless,[34] but a 2002 study showed the Mayak nuclear workers and the Techa riverside population are still affected.