Kyshtym disaster, explosion of buried nuclear waste from a plutonium-processing plant near Kyshtym, Chelyabinsk oblast, Russia (then in U.S.S.R.), on September 29, 1957. Until 1989 the Soviet government refused to acknowlege that the event had occurred, even though about 9,000 square miles (23,000 square km) of land were contaminated, more than 10,000 people were evacuated, and probably hundreds died from the effects of radioactivity. After details became known, the International Atomic Energy Agency classed the Kyshtym disaster as a Level 6 accident on the International Nuclear and Radiological Event Scale. Only the subsequent nuclear disasters at Chernobyl and Fukushima have been classed at the seventh and highest level of severity.

The nuclear reactors and plutonium-processing plant of the Kyshtym industrial complex were built during the late 1940s in the Soviet program to develop nuclear weapons. The secret nuclear facility was called Mayak but was more widely known by the code name Chelyabinsk-40, because mail to the plant and its workers had to be addressed to Post Office Box 40 in Chelyabinsk, a large city 55 miles (90 km) distant from Kyshtym. (The nuclear site was known later as Chelyabinsk-65 and still later as Ozersk.) The facility was located on the eastern slopes of the central Ural Mountains; nearby lakes provided a water supply for reactor cooling and also served as repositories for nuclear waste. The pace of the Soviet nuclear program was so hurried and its technology so new that conditions were chronically unsafe for both workers and neighbours.

It was eventually revealed that the Kyshtym disaster was a consequence of the failure to repair a malfunctioning cooling system in a buried tank where liquid reactor waste was stored. For more than a year the tank’s contents grew steadily hotter from radioactive decay, reaching a temperature of about 660 °F (350 °C) by September 29, 1957, when the tank exploded with a force equivalent to at least 70 tons of TNT. The nonnuclear explosion blew off the tank’s one-metre-thick concrete lid and sent a plume of radioactive fallout, including large quantities of long-lasting cesium-137 and strontium-90, into the air. About two-fifths as much radioactivity was released at Kyshtym as was later released at Chernobyl. The plume drifted hundreds of miles, generally northeast, through a region that had hundreds of thousands of inhabitants, but authorities were slow to order evacuation. In the ensuing months, area hospitals were filled with sufferers of radiation sickness.

Scattered reports of a nuclear accident in Russia appeared in the Western press as early as 1958. But the Kyshtym disaster was not widely known until 1976, when the exiled Soviet biologist Zhores A. Medvedev reported on the incident in the British journal New Scientist. Lev Tumerman, an émigré scientist, corroborated Medvedev’s story with his own account of having driven between Sverdlovsk (now Yekaterinburg) and Chelyabinsk through a dead zone where there were no houses or farms, and where road signs cautioned drivers not to stop but to proceed at maximum speed. Even so, some Western authorities doubted that a storage accident could have had such severe consequences, and others offered an alternative theory wherein a distant nuclear weapons test had produced the radioactivity.

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Medvedev then undertook a study of Soviet scientific papers on the ecological effects of experimental discharges of radiation. Even though the authors and censors had withheld or fudged numerous details, Medvedev was able to discover many cases in which there was simply too much radiation covering too big an area over too long a period to have been intentionally released for experimental purposes. His detective work also showed him that the questionable “experiments” had taken place in the Ural region, and that the contamination must have occurred in 1957 or 1958. At about the same time, an antinuclear group organized by the American consumer advocate Ralph Nader made a request under the Freedom of Information Act for the findings of the U.S. Central Intelligence Agency, which was known to have overflown the Urals in a U-2 spy plane. The agency seemed to confirm Medvedev’s assertion but provided few details. It was later suggested that the U.S. government kept silent about the accident for so long, and remained uncommunicative even after others had called attention to it, for fear of planting seeds of doubt in the minds of Americans about the safety of their country’s own nuclear program. Despite the evidence of a disaster, the Soviet Union denied its occurrence until 1989, and even then, officials downplayed the extent of damage.

The long-term effects of the Kyshtym disaster were difficult to assess, partly because of Soviet secrecy and partly because Chelyabinsk-40 routinely released dangerous quantities of radioactive waste into the environment for many years. Inhabitants of the region have suffered increased rates of cancer, deformities, and other major health problems.