Fukushima Nuclear Power Plant

At this point in the Fukushima disaster, Wilmshurst said, the tsunami appears to be the immediate culprit, since the plants shut down as they were designed to do following the earthquake. When the tsunami hit an hour later, it damaged the site infrastructure, he said. So while the earthquake had cut the reactors' external power supply, which is needed to keep coolant pumps doing their job, the tsunami killed the diesel backup generators needed to provide power for the cooling system. Batteries provided power for only up to eight hours. Mobile generators were brought in to take over.
- http://news.nationalgeographic.com