The action indicates that the reactor's normal backup cooling system has failed and is no longer able to supply fresh water to the core. Officials at Tokyo Electric Power Co., which owns the plant, have been struggling to keep six shut-down nuclear reactors cooled because seawater from the tsunami that followed Friday's magnitude 9.0 earthquake damaged the diesel generators that power the circulating pumps.
That action at the No. 1 reactor seems to have been successful so far. Power company officials have reported that radiation levels at the plant have fallen and that the core seems to be cooling effectively.
Now they have been forced into the same decision at the No. 3 reactor at the site.
Power company officials say hydrogen gas has also been building up inside the reactor building at the No. 3 reactor. They have vented some of the gas, but fear that could lead to an explosion similar to the one that destroyed the building at reactor No. 1.
The International Atomic Energy Agency said Sunday evening that Japan had informed it that an emergency had been declared at the nearby Onagawa power plant because excessive radiation had been detected outside the plant.
Power company officials later said, however, that all three reactors at Onagawa had been safely shut down and were under control and that wind had blown the radiation there from the Fukushima plant.
On Monday morning, Japan Atomic Power said that two of the three backup diesel generators at its Tokai No. 2 facility about 75 miles north of Tokyo in Ibaraki prefecture had failed, but that the remaining generator was providing sufficient electricity to power the circulating pumps to keep the three shut-down nuclear reactors there controlled.