The Chernobyl disaster was a nuclear reactor accident that occurred at the Chernobyl Nuclear Power Plant in Ukraine on April 26, 1986. An explosion during a safety test caused a massive release of radiation, resulting in 56 direct deaths and over 4,000 cancer deaths. It is considered the worst nuclear power plant disaster in history due to the large amount of radioactive contamination released into the atmosphere. The Chernobyl reactor remains encased in a large concrete structure today, with 200 tons of highly radioactive material still inside.