On 29 September 1957 a chemical explosion in the radioactive waste storage site involved some 20 million curies of material.
When the cooling system of a radioactive waste containment unit malfunctioned, a concrete barrier restrained most of this material. But some 2 million curies spilled across Chelyabinsk, Sverdlovsk, and Tyumen Oblasts covering a total area of 23,000 square kilometers inhabited by a quarter of a million people. Emergency measures including evacuation of the population were taken to limit serious health effects. Significant radioactive contamination covered an area of more than 800 square kilometers, and there are areas where the concentration of Cesium-137 and Strontium-90 are still hazardous to human health.
And in the spring of 1968 Lake Karachay began to dry up, and the wind carried away a substantial volume of radioactive dust, irradiating half a million people with five million curies.
A fault in the cooling system at the nuclear complex, near Chelyabinsk, results in a chemical explosion and the release of an estimated 70 to 80 tonnes of radioactive materials into the air. Thousands of people are exposed to radiation and thousands more are evacuated from their homes. It is categorised as Level 6 on the seven-point International Nuclear Events Scale (INES).
In September 1957 the cooling system in one of the tanks containing about 70-80 tons of radioactive waste failed, and the temperature in it started to rise, resulting in a non-nuclear explosion of the dried waste having a force estimated at about 70-100 tons of TNT, which threw the concrete lid, weighing 160 tons, into the air. There were no immediate casualties as a result of the explosion, which released some 74 PBq (2 MCi) of radioactivity.
In the next 10 to 11 hours the radioactive cloud moved towards the northeast, 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 square kilometers, primarily with caesium-137 and strontium-90. This area is usually referred to as the East-Ural Radioactive Trace (EURT).
Because of the secrecy surrounding Mayak, the population of affected areas were not initially informed of the accident. A week later (on 6 October) an operation for evacuating 10,000 people from the affected area started, still without giving an explanation of the reasons for evacuation. People "grew hysterical with fear with the incidence of unknown 'mysterious' diseases breaking out. Victims were seen with skin 'sloughing off' their faces, hands and other exposed parts of their bodies." It was Zhores Medvedev who revealed the nature and extent of the disaster to the world.
Even though the Soviet government suppressed information about the figures, it is estimated that the direct exposure to radiation caused at least 200 cases of death from cancer.
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.
Rumours of a nuclear mishap somewhere in the vicinity of Chelyabinsk had long been circulating in the West. That there had been a serious nuclear accident west of the Urals was eventually inferred from research on the effects of radioactivity on plants, animals, and ecosystems, published by Professor Leo Tumerman, former head of the Biophysics Laboratory at the Institute of Molecular Biology in Moscow, and associates.
According to Gyorgy, who invoked the Freedom of Information Act to gain access to the relevant Central Intelligence Agency (CIA) files, the CIA knew of the 1957 Mayak accident all along, but kept it secret to prevent adverse consequences for the fledgling American nuclear industry. Only in 1990 did the Soviet government declassify documents pertaining to the disaster.