In the early morning hours of April 26, 1986, a nuclear power plant in Ukraine (formerly part of the Soviet Union) exploded, creating what is widely considered to be the worst nuclear disaster in history. The International Scale of Nuclear and Radiological Events (INES) was established by the International Atomic Energy Agency (IAEA) in 1990 to measure the severity of nuclear accidents. This eight-point sliding scale is similar to the Richter scale. The second most serious nuclear accident occurred at the Sellafield plant in the United Kingdom on October 10, 1957.This Level 5 incident resulted in radioactive contamination across Europe and is believed to have caused several hundred cancer diagnoses.
The third most serious nuclear accident happened at the Mayak plant near Kyshtym, Russia. On this occasion, a faulty refrigeration system caused an explosion with a force equivalent to 70-100 tons of TNT.
Nuclear rainspread more than 300 kilometers away and, due to the classified nature of the plant, 10,000 inhabitants were only evacuated a week later. The Chernobyl disaster is the worst nuclear power plant accident in history in terms of death toll and cost.
It was classified as a Level 7 incident on the INES scale. The explosion released huge amounts of radioactive waste across Western Europe, killing about 30 people from acute radiation poisoning immediately afterwards and increasing long-term fear of an increase in thyroid cancer cases. Since 1989, more than 1,000 nuclear engineers from the former Soviet Union have visited Western nuclear power plants and there have been many reciprocal visits. Most of this has been under the auspices of the World Association of Nuclear Operators (WANO), a body formed in 1989 that links 130 nuclear power plant operators in more than 30 countries (see also the information page on Cooperation in the Nuclear Energy Industry).
The scientific consensus is that Chernobyl surpasses its counterparts as the most damaging nuclear accident the world has ever seen. The first British nuclear reactor, known as Windscale, was built in North West England in the late 1940s to produce plutonium and other materials for the country's burgeoning nuclear weapons program.