There is enough fuel in those reactors that if the water level just drops low enough to expose the tops of the rods, the rods will IMMEDIATELY heat up and cause the rest of the water to boil off...
This will be followed by a nuclear inferno (fire) which will be impossible to stop...
The melted nuclear fuel will become so hot that it will literally melt through concrete, steel etc and nothing will stop it....
This will result in a release of radiation that has a high probability of being a PLANET EXTINCTION EVENT.
