One of the worst accidents in history took place on April 26 1986.
The nuclear reactor exploded at the Chernobyl nuclear power plant in Ukraine.
To give you some background, this is how a nuclear power plant works.
A reactor emits the radiation. The radiation heats the water, the steam then turns the turbine and it's generator produces electricity.
The nuclear reactor itself is generally just a big kettle filled with water with
the rods mounted in it.
One of the types of rods is the fuel rod, which contains fissile elements that heat up enormously.