Several nuclear power reactors in Japan shut as designed following the massive earthquake off the nation's northwest coast, but the failure of a back-up power generator at one unit risks a radioactive leak.
Operators at Tokyo Electric Power Co (Tepco) are still working to control the situation at one of their plants.
Tepco said at about 2.46 pm local time Units 1, 2 and 3 at its Fukushima Daiichi plant -- three of six at the plant -- shut following the earthquake. All are boiling water reactors.
Tepco said the reactors shut due to the loss of offsite power due to the malfunction of one of two off-site power systems. That triggered the emergency diesel generators to startup and provide backup power for plant systems.
About an hour after the plant shut down, however, the emergency diesel generators stopped, leaving Units 1, 2 and 3 with no power for important cooling functions.
Nuclear plants need power to operate motors, valves and instruments that control the systems that provide cooling water to the radioactive core.
Tepco declared an emergency and the government ordered thousands to evacuate the area, while engineers worked to restore power. The company is bringing in mobile generators to restore the power supply, but pressure inside the containment of Unit 1 continued to increase.
Early on Saturday Tepco said it had lost the ability to control pressure at the No. 1 and 2 reactors.
NORMAL OPERATIONS
* In a reactor operating normally, pumps circulate water through the reactor core to keep the rods from overheating. The temperature inside a reactor operating normally is about 550 degrees F (285 C).
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