The Chernobyl nuclear disaster was a major nuclear accident that occurred on 26 April 1986 at the V.I. Lenin nuclear power plant, then located in the Ukrainian Soviet Socialist Republic, USSR. It is the most serious nuclear disaster of the 20th century, classified at level 7 (the highest) of the International Nuclear Event Scale (INES), surpassing, according to the Institute for Radiological Protection and Nuclear Safety (IRSN), by its immediate environmental impact the Fukushima nuclear accident of 2011, classified at the same level. For these accidents, IRSN mentions potential health effects, lasting contamination of territories and significant economic and social consequences.