However, in the event of an EMP attack, the grid will come down, and it may not come up for many months, if not years. It is likely that a substantial number of transformers that are used to link power plants (and this applies to all power plants – coal, gas, oil and nuclear) to the grid will be “fried.” There will be no way to obtain off-site power to restart the nuclear power plants. Most station blackout events are assumed to be concluded (i.e., “over”) within 24 hours. No one that I know of has seriously analyzed the effects of prolonged station blackouts.
Assuming that the emergency diesel generators will start after an EMP event (and this is up for debate), most power plants only have enough diesel fuel on site to keep them running for about one week (though some may have up to 30 days of fuel). If they don't start, or if the controls systems do not operate, then everything that I describe here will still come to pass, only much more rapidly. The power from the diesel generators is needed to operate the pumps that circulate the water in the reactor (called the “primary side”) and that also feed the steam generators with water (part of the “secondary side”). If power to the reactor coolant pumps in the primary side is lost, the reactor will likely begin what is known as “natural circulation.” However, in order to remove heat from the reactor core, water still needs to be continuously pumped through the steam generators so that the heated water in the secondary side can be cooled either via cooling towers, spray ponds or some other ultimate heat sink. If these secondary side (feed water) pumps will not operate, then the steam generators will dry out and then the cooling effect for the core is lost
