Human: When did the nuclear disaster at Chernobyl happen? Assistant: 
The Chernobyl nuclear power plant accident happened on April 26, 1986. It was the worst nuclear accident in history, and it occurred at the Chernobyl Nuclear Power Plant in Ukraine. It was caused by a design flaw in the reactor, and it resulted in a large release of radioactive material into the atmosphere. The accident caused widespread contamination in the surrounding areas.