What caused a a major disaster in Chernobyl, Ukraine, in 1986?

The April 1986 disaster at the Chernobyl a nuclear power plant in Ukraine was the product of a flawed Soviet reactor design coupled with serious mistakes made by the plant operators b ... Furthermore, large areas of Belarus, Ukraine, Russia, and beyond were contaminated in varying degrees.

It was caused by the emergency system not turning on right away that let the nuclear fuel melt toghether causing a nuclear meltdown