After the apocalypse ??

[Tartar Horde]

The control rods need to be cooled by the water surrounding them, even if they are retracted they remain highly radioactive, but they are still surrounded by water to cool them. The back up systems all work off Diesel/petrol etc in the case of a total electrical breakdown to make electricity to power the pumps for the cooling system to work. If no one comes to top the fuel tanks up the generators will run out of fuel and stop, therefore stopping the water pump systems from maintaining a cool enough temperature to stop the fuel rods melting the building. This is what happened in Fukashima, once coolant pressure was lost nothing could stop the reactor from going critical. The Documentary about life after humans have vanished shows the very scenario I am talking about. I will try to get link for it folks.