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The Kyshtym disaster (Russian: Кыштымская авария), sometimes referred to as the Mayak disaster or Ozyorsk disaster in newer sources, was a radioactive contamination accident that occurred on 29 September 1957 at Mayak, a plutonium reprocessing production plant for nuclear weapons located in the closed city of Chelyabinsk-40 (now Ozyorsk) in Chelyabinsk Oblast, Russia in the Soviet Union.

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). The matter was covered up, and few either inside or outside the Soviet Union were aware of the full scope of the disaster for nearly two decades. The disaster is the second worst nuclear incident by radioactivity released, after the Chernobyl disaster, and was regarded as the worst nuclear disaster in history until Chernobyl. It is the only disaster classified as Level 6 on the International Nuclear Event Scale (INES). It is the third worst nuclear disaster by population impact after the two Level 7 events: the Chernobyl disaster, which resulted in the evacuation of 335,000 people, and the Fukushima Daiichi disaster, which resulted in the evacuation of 154,000 people. At least 22 villages were exposed to radiation from the Kyshtym disaster, with a total population of around 10,000 people evacuated. The disaster spread hot particles over more than 52,000 square kilometres (20,000 sq mi), where at least 270,000 people lived. Since Chelyabinsk-40 (later renamed Chelyabinsk-65 until 1994) was not marked on maps, the disaster was named after Kyshtym, the nearest known town.

Initially, Mayak dumped high-level radioactive waste into a nearby river, which flowed to the river Ob, flowing farther downstream to the Arctic Ocean. When Lake Kyzyltash quickly became contaminated, Lake Karachay was used for open-air storage, keeping the contamination a slight distance from the reactors but soon making Lake Karachay the "most-polluted spot on Earth". The accident involved waste from the sodium uranyl acetate process used by the early Soviet nuclear industry to recover plutonium from irradiated fuel.

The disaster is estimated to have released 20 MCi (740 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 (74 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 (102 PBq) of deliberately dumped waste, and Lake Karachay, which had received 120 MCi (4,400 PBq). The land area thus exposed to radioactive contamination was termed the "East Ural Radioactive Trace" (EURT). Of particular concern is Lake Karachay, the closest lake to the plant (now notorious as "the most contaminated place on Earth") where roughly 4.4 exabecquerels of high-level liquid waste (75–90% of the total radioactivity released by Chernobyl) was dumped and concentrated in the shallow 45-hectare (0.45 km2; 110-acre) lake over several decades.

But it was only eighteen years later, in 1976, that Soviet dissident Zhores Medvedev made the nature and extent of the disaster known to the world. 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, the former head of the Biophysics Laboratory at the Engelhardt Institute of Molecular Biology in Moscow. Starting in 1989, several years after the Chernobyl disaster, the Soviet government gradually declassified documents pertaining to the incident at Mayak.

Monument dedicated to the liquidators of the consequences of the Kyshtym disaster, installed on the initiative of the public organisation “Kyshtym-57”. Documentary film The Kyshtym Disaster: Life in the Radioactive Zone.

Official documents Archived 21 January 2013 at the Wayback Machine pertaining to the disaster (in Russian)

Quick Facts

• The disaster is the second worst nuclear incident by radioactivity released, after the Chernobyl disaster, and was regarded as the worst nuclear disaster in history until Chernobyl.
• It is the third worst nuclear disaster by population impact after the two Level 7 events: the Chernobyl disaster, which resulted in the evacuation of 335,000 people, and the Fukushima Daiichi disaster, which resulted in the evacuation of 154,000 people.
• At least 22 villages were exposed to radiation from the Kyshtym disaster, with a total population of around 10,000 people evacuated.
• It is the only disaster classified as Level 6 on the International Nuclear Event Scale (INES).
• The matter was covered up, and few either inside or outside the Soviet Union were aware of the full scope of the disaster for nearly two decades.

Source material: Wikipedia - "Kyshtym disaster". Adapted and summarized for DiscoverScroll. Original contributors are credited through the linked Wikipedia article. Read original on Wikipedia. CC BY-SA 4.0. Changes were made from the original.