Reactors convert the energy stored in nuclear fuel rods into electricity. This process generates intense heat.
Water is circulated through the reactor core to produce steam. Power stations have back-up systems in case of emergencies, to keep the reactors cool.
During Japan’s earthquake, power to the Fukushima plant was cut off and the back-up system failed.
Diesel generators should have provided emergency cooling—but they were also damaged and so coolant stopped circulating through the reactor core. As the remaining water boils away in the heat, the fuel rods become exposed.
If the cooling system isn’t restored a meltdown can occur—where the core melts and radiation escapes into the environment.