<human>:  When did the nuclear disaster at Chernobyl happen?
<bot>: The Chernobyl accident happened on April 26, 1986. It was a major nuclear accident at the Chernobyl Nuclear Power Plant in Ukraine. The accident occurred during a test of the RBMK-1000 reactor, which was a Soviet design. The explosion was so big that it blew the roof off the reactor building, which was built on the site of a medieval city. The explosion caused a fire that burned for several days, and released radioactive material into the air. The accident is considered to be the worst nuclear power plant accident in history.