The Japanese newspaper Asahi Shimbun has provided new details on what happened with the generators at Fukushima Dai-Ichi. I've been looking for more about this problem, said to be the immediate cause of what now looks like a true meltdown in Unit 1 (fuel assembly melting through the steel walls of the RPV, and gathering somewhere at the lower reaches of the primary containment.)
The original story goes like this: "The tsunami wave was so high that it breached the seawall and flooded the basement where the emergency diesel generators were located. The emergency batteries ran the pumps for eight hours. When they expired, the reactors began overheating because the decay heat couldn't be removed from the fuel assemblies."
Given eight hours of notice, and knowledge of how serious the problem would be, why couldn't emergency diesel generators be flown in? Powerful, skid-mounted generators are all over the place, used by companies in emergencies and by motion picture productions. Surely the world's fourth biggest electric company could round up portable generators on short notice.
I've suspected there was more to the story. The Asahi Shimbun piece suggests that Tepco didn't bring in generators because there were more problems than flooded generators and their diesel fuel supplies. It compares this plant to later complexes, where generators and seawater pumps were protected inside the more stoutly-built reactor buildings. Referring to the Dai-Ichi complex as plant No. 1, the article says the flooding affected not only the generators, but pumps that brought in seawater for cooling:
“The tsunami rendered inoperable all of the No. 1 plant's pumps, which were also not located within a sealed structure.... Referring to the possibility of installing the seawater pumps inside buildings, the former TEPCO engineer said, "It would have been a major project because various pipes are laid out under the pumps, and so all of that would also have had to be moved."
The basement of which building? Most likely the turbine building. See this link on PhysicsForums, where a participant went through German documents to pinpoint the emergency generators' most likely location.