The Chernobyl disaster was a nuclear meltdown that occurred on April 26, 1986, in the No. 4 light water graphite moderated reactor at the Chernobyl Nuclear Power Plant near Pripyat, in what was part of the Ukrainian Soviet Socialist Republic of the Soviet Union.
During a late night safety test which simulated a station power-failure in which safety systems were deliberately turned off, a combination of inherent reactor design flaws and the reactor operator negligence resulted in uncontrolled reactor conditions. Water flashed into the steam generating a steam explosion and graphite fire. The fire produced updrafts and toxic plumes were sent into the atmosphere. The radioactive material became a fall-out precipitate over large tracts of western USSR and Europe.
The Chernobyl disaster was the worst nuclear power accident in history, both in terms of cost and casualties. It is one of only two nuclear energy accidents classified as a level 7 event, the maximum classification, on the International Nuclear Event Scale (the other being the Fukushima Daiichi nuclear disaster in Japan in 2011).
The initial evidence that a major release of radioactive material was affecting other countries was announced by Sweden. Workers from the Forsmark Nuclear Power Plant, about 680 miles from the Chernobyl site, were found to have radioactive particles on their clothes. After searching for the source of the radioactivity and determining that there was no leak at the Swedish plant on April 28, the disaster was determined to have come from the Soviet Union.
It has been approximated that about four hundred times more radioactive material was released from Chernobyl than by the atomic bombing of Hiroshima. By contrast, the Chernobyl accident released about one hundredth to one thousandth of the total amount of radioactivity released during the era of nuclear weapons testing.
According to Mikhail Gorbachev, the Soviet Union spent 18 billion rubles (equivalent to 18 billion USD at the time) on containment and decontamination, virtually bankrupting itself. In Belarus alone, the total cost over 30 years is estimated at $302 billion. 5-7% of all government spending in Ukraine is still related to the Chernobyl disaster. Much of the current costs are related to the payment of Chernobyl related social benefits to over 7 million people across three different countries.