What is a meltdown?
When you get right down to it, a nuclear power plant is very sophisticated way to boil water. Controlled fission in the core generates heat, which creates steam that spins turbines. When a big earthquake hits, nuclear power plants are programmed to automatically shut down. But even after the fission reaction in the core is stopped, it takes a certain time for the radioactive byproducts to decay and cool down. If something – in this case a huge earthquake and tsunami double hit – prevents the cooling system from pumping water to the core and the emergency cooling system is prevented from kicking in, it can get hot enough to melt. In the very worst cases, part of the containment building, which envelops the reactor vessel which itself contains the fuel rods (it’s like a russian doll of buildings), can partially melt, but containment buildings are designed to withstand the high pressures and high temperatures that occur in a meltdown, so in principle they should hold and allow the fuel ro
