<human>:  When did the nuclear disaster at Chernobyl happen?
<bot>: The Chernobyl nuclear power plant accident happened on April 26, 1986. It was the worst nuclear disaster in history, and it is still considered to be one of the worst man-made disasters in history. The Chernobyl disaster was caused by a steam explosion in the reactor's fourth chamber, which caused the reactor core to melt and release radioactive material into the environment. The accident killed 31 people immediately, and hundreds of others died from radiation exposure.