Did you know that in 1957, an explosion occurred at a plutonium plant in the USSR, resulting in the third worst nuclear accident in history. The government kept it secret for 19 years.
The Kyshtym disaster was a radiation contamination incident that occurred on 29 September 1957 at Mayak, a nuclear fuel reprocessing plant in the Soviet Union. It measured as a Level 6 disaster on the International Nuclear Event Scale, making it the third most serious nuclear accident ever recorded behind the Chernobyl disaster, and Fukushima Daiichi nuclear disaster, both Level 7 on the INES. 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.
On 29 September 1957, the cooling system in one of the tanks 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 (see ammonium nitrate bomb). The explosion, estimated to have a force of about 70–100 tons of TNT, threw the 160-ton concrete lid into the air. There were no immediate casualties as a result of the explosion, but it released an estimated 20 MCi (800 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 (80 PBq) of radionuclides spread out over hundreds of kilometers. The affected area was not virgin – the Techa river had previously received 2.75 MCi (100 PBq) of deliberately dumped waste, and Lake Karachay had received 120 MCi (4000 PBq).
In the next 10 to 11 hours, the radioactive cloud moved towards the north-east, 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 to 20,000 square kilometers (depending on what contamination level is considered significant), primarily with caesium-137 and strontium-90. This area is usually referred to as the East-Ural Radioactive Trace (EURT).