If these were PWRs, they couldve possibly been steaming away on natural circ all this time.
The station blackout was not the issue. The EDGs ran for an hour as they were supposed to do. The Tsunami took out the EDGs. Knowing now that it was a 30 ft tall wave, it would be hard for any nuke plant to survive. If a wave like that had hit my plant, it would have taken out all of the EFW pumps. I don't know how long the turbine driven pumps could run under water, but I bet it is not long.
You also have to consider the suction source. If your CST is gone, your pumps will not matter anyway. The pumps do have backup sources like fire water or Service Water but those would probably be gone too. This was really a worst case situation.
These plants were designed for a Tsunami hit. This tsunami was just too big for them to handle. Like any nuke plant currently in operation, if you break enough emergency systems, you will damage/melt the core.
In a PWR you've also got the issue of making up to the primary for reactor coolant pump seal bleedoff. While this is typically small during normal operation (say 6 gpm), over a long period of time it starts to become becomes significant (over 8500 gallons lost from the reactor coolant system per day).
Depending on the seal design, the leakage could increase fairly dramatically once AC powered seal cooling is lost. At 25 gpm, the leakage is up to 36,000 gallons per day.
Total RCS volume is probably on the order of 50,000 gallons to 100,000 gallons depending on the specific PWR plant design involved.
Seal Leakage drops significantly when the RCPs are stopped. That would not be my concern. I would however be nervous about the shrink of the RCS water after a long time.