Earthquake Shakes Nuclear Power Plants in Japan

A (pre-earthquake) aerial view of the Fukushima nuclear power plant.

The massive 8.9 magnitude earthquake in Japan reportedly forced the automatic shutdown of 11 nuclear reactors at four plants in the northeastern part of Japan near the earthquake's epicenter. There have been no reports of radiation leaks, although Japanese news organizations are reporting that the radiation level within one of the plants is rising, and that radioactive steam may be released to reduce pressure buildup in the facility. [Update: Japan's nuclear safety agency reportely says the amount of radiation would not threaten human health.]
The earthquake cut off power to at least one of the nuclear plants, the Fukushima plant, and backup generators failed after an hour, forcing local governments to bring in mobile generators. Even after an automatic shutdown, the reactors must be cooled—and without power, cooling water cannot be circulated. [Update: A backup cooling system that uses batteries can temporarily delay overheating.] There have been reports that enough water remains in the reactors for cooling, and to prevent the nuclear fuel from being exposed. The U.S. State Department has said that the U.S. is bringing coolant to the plant, though Jasmina Vujic, a professor of nuclear engineering at the University of California at Berkeley, says it's not clear why that would be necessary, unless ordinary clean water—which is all that's needed for coolant--is unavailable. [Update: reports that the U.S. sent coolant may have been incorrect.]
A fire in one of the non-nuclear buildings at another power plant has reportedly been extinguished.
Nuclear power plants in Japan are engineered to withstand "extremely large earthquakes," and have survived major earthquakes in recent years, Vujic says. She says they have "several layers of safety." These include concrete walls comprising a total of nearly 20 feet of concrete between the reactor and the outside, and five and a half inches of steel.

The reactors shut down automatically by inserting control rods into the reactor; the rods absorb neutrons and immediately stop the fission chain reactions inside the reactor.
Although no radiation leak has been confirmed, nearly 3,000 people near the Fukushima plant have been evacuated as a precaution.

source : here