Very simply, when a nuclear reactor stops, the fission products in the fuel rods continue to decay and therefore continue to generate heat. A lot of heat. An ex

Very simply, when a nuclear reactor stops, the fission products in the fuel rods continue to decay and therefore continue to generate heat. A lot of heat. An external power source is needed to take that heat away from the fuel. Failing to keep the reactor cool can potentially be rather catastrophic. There is lots of information online. Why not start here: http://en.wikipedia.org/wiki/Nuclear_safety#Failure_modes_of_nuclear_power_plants http://en.wikipedia.org/wiki/Nuclear_safety#Failure_modes_of...