The Kyshtym disaster was a radiation contamination incident that occurred on 29 September 1957 at Mayak, a nuclear fuel reprocessing plant in Russia (then a part of the Soviet Union). It measured as a Level 6 disaster on the International Nuclear Event Scale, making it the second most serious nuclear accident ever recorded (after the Chernobyl disaster). The event occurred in the town of Ozyorsk, a closed city built around the Mayak plant. 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 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 an estimated 2 to 50 MCi (74 to 1850 PBq ) 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.
A documentary filmed in Muslyomovo, near Mayak.
Lake Karachay (Russian: Карача́й), sometimes spelled Karachai, is a small lake in the southern Ural mountains in western Russia. Starting in 1951 the Soviet Union used Karachay as a dumping site for radioactive waste from Mayak, the nearby nuclear waste storage and reprocessing facility, located near the town of Ozyorsk (then called Chelyabinsk-40). According to a report by the Washington, D.C.-based Worldwatch Institute on nuclear waste, Karachay is the most polluted spot on Earth. The lake accumulated some 4.44 exabecquerels (EBq) of radioactivity, including 3.6 EBq of Caesium-137 and 0.74 EBq of Strontium-90. For comparison, the Chernobyl disaster released from 5 to 12 EBq of radioactivity, but this radiation is not concentrated in one location.
The radiation level in the region near where radioactive effluent is discharged into the lake was 600 röntgens per hour in 1990, according to the Washington, D.C.-based Natural Resources Defense Council, more than sufficient to give a lethal dose to a human within an hour. Wikipedia
During the first minutes on the video below, you can see concrete being dumped into the Lake Karachay. They dumped concrete prevent the highly radioactive sediments from shifting.