The designers apparently could not imagine a tsunami of these proportions and the backup power — remember, the plants themselves produce power, power is brought in by multiple outside power lines, there are banks of diesels to produce backup power, and finally, banks of batteries to back that up, all were disabled.
It is a classic case of a common-mode failure and this will be the repercussion in the industry, try to enhance the availability of emergency power in cases where this kind of event can happen.
Here, we had two things happen from the same initiating event. Station blackout and loss of emergency diesel electricity. Station blackout from loss of offsite power is one of the most troublesome events nuclear plant operators have to deal with. Then you are totally reliant on emergency power. That means diesel generators and, for a short period, batteries. All indications at this point are that all the safety systems were working up to the point of the loss of diesel power. The plant shutdown safely when the seismic event was detected. Cooling systems were operable on backup power when offsite power was lost. When the diesels quit when the tsunami came ashore, things went south.
If you lose AC power on the plant safety buses, there really isn't much you can do other than work your darndest to get power back on. This is what they did. But the accident had evolved at that point to include other complications, as we've seen. So my guess is we'll see added emphasis on assuring that emergency power is available in this kind of common-mode failure scenario.
These plants need the ability to shut down on their own, to continue the process for days without human intervention until any residual heat or water loss is no longer a problem and the sucker is stone cold dead ...
There's some other issues that need addressing as well ...