User: When did the Chernobyl disaster happen?
Assistant: The Chernobyl disaster happened on 26 April 1986. It was caused by a reactor explosion at the Chernobyl Nuclear Power Plant in Ukraine. The explosion released radioactive material into the atmosphere, and the resulting fire burned for days. The explosion and fire released radiation into the atmosphere, and it contaminated the surrounding areas. In the aftermath of the disaster, the Soviet Union attempted to cover up the extent of the disaster. The accident led to the evacuation of thousands of people from the area, and it contributed to the fall of the Soviet Union in 1991.