TOKYO — The risk of a meltdown spread to a third reactor at a stricken nuclear power plant in Japan on Monday as its cooling systems failed, exposing its fuel rods, only hours after a second explosion at a separate reactor blew the roof off a containment building.
The widening problems underscored the difficulties the Japanese authorities are having in bringing several damaged reactors under control three days after a devastating earthquake and a tsunami hit Japan’s northeast coast and shut down the electricity that runs the crucial cooling systems for reactors.
Operators fear that if they cannot establish control, despite increasingly desperate measures to do so, the reactors could experience full meltdowns, which could release catastrophic amounts of radiation. The two reactors where the explosions occurred are both presumed to have already suffered partial meltdowns — a dangerous situation that, if unchecked, could lead to full meltdowns.
The chief cabinet secretary, Yukio Edano, said that the release of large amounts of radiation was unlikely. But traces of radiation could be released into the atmosphere, and about 500 people who remained within a 12-mile radius of the plant were ordered to take cover indoors temporarily, he said.
The country’s nuclear power watchdog said readings taken soon after the explosion showed no big change in radiation levels around the plant or any damage to the containment vessel, which protects the radioactive material in the reactor.
“I have received reports that the containment vessel is sound,” Mr. Edano said. “I understand that there is little possibility that radioactive materials are being released in large amounts.”
But later Monday Mr. Edano said cooling systems at a third reactor at the Fukushima Daiichi plant had failed. The water level inside the reactor fell, exposing the fuel rods at its core for more than two hours despite efforts to pump seawater into the reactor, he said. Exposure of the rods means they heat up, melting their outer casing and raising the risk of a meltdown.
At first, water was successfully injected into the reactor and the rods were again submerged. But new problems resulted in the rods being exposed again.
A vent that had been letting out steam from the reactor closed, leading to pent-up pressure inside the containment vessel and hampering water from being injected. Water levels then fell rapidly, leaving the fuel rods again exposed, Tokyo Electric officials said at a news conference early Tuesday.
The jury-rigged fire hose pumps being used by the workers have added to the crisis by hindering efforts to keep reactors adequately cooled. Difficulties in gauging exactly how much water remains in the containment vessel, as well as what exactly is occurring at the heart of the reactor, have also added to problems.
Earlier, Hidehiko Nishiyama, deputy director-general at Japan’s Nuclear and Industrial Safety Agency, said plant workers had renewed efforts to flood the reactor with seawater, and readings suggested that water again covered the fuel rods. Workers were also battling rising pressure within the reactor, Mr. Nishiyama said. They have opened vents in the reactor’s containment vessel, which houses the fuel rods, a measure that could release small amounts of radiation. Higher-than-normal levels of radiation have been detected from at least 22 people evacuated from near the plant, the nuclear safety watchdog said, but it is not clear if the doses they received were dangerous.
The Fukushima Daiichi plant and the Fukushima Daini power station, about 10 miles away, have been under a state of emergency.
On Monday morning, Tokyo Electric, which runs both plants, said it had restored the cooling systems at two of three reactors experiencing problems at Daini. That would leave a total of four reactors at the two plants with pumping difficulties.
Katsuhiko Ishibashi, a nuclear safety expert formerly at the Research Center for Urban Safety and Security at Kobe University, said emergencies at multiple reactors in close proximity posed particular risks. “If an incident were to happen at one reactor that released high amounts radiation, the whole area would become unapproachable,” Mr.
Ishibashi said. “Then the other reactors would have to be abandoned, and left to run their disastrous course.”
Frank N. von Hippel, a physicist and professor at Princeton, said he was not aware of any cases were more than one reactor had problems.
“The whole country was focused on Three Mile Island,” he said, referring to the Pennsylvania nuclear plant accident in 1979. “Here you have Tokyo Electric Power and the Japanese regulators focusing on multiple plants at the same time.”
In what was perhaps the clearest sign of the rising anxiety over the nuclear crisis, both the United States Nuclear Regulatory Commission and the Russian authorities issued statements on Sunday trying to allay fears, saying they did not expect harmful levels of radiation to reach their territory.
Late Sunday night, the International Atomic Energy Agency announced that Japan had added a third plant, Onagawa, to the list of those under a state of emergency because a low level of radioactive materials had been detected outside its walls. But on Monday morning, it quoted Japanese authorities as saying that the radioactivity levels at the Onagawa plant had returned to normal levels and that there appeared to be no leak there.
“The increased level may have been due to a release of radioactive material from the Fukushima Daiichi nuclear power plant,” the agency said. The Onagawa and Daiichi plants are 75 miles apart. The operator of the Onagawa plant, Tohoku Electric Power, said that levels of radiation there were twice the allowed level, but that they did not pose health risks.
Soon after that announcement, Kyodo News reported that a plant about 75 miles north of Tokyo was having at least some cooling system problems. But a plant spokesman later said a backup pump was working.
The government was testing people who lived near the Daiichi plant, with local officials saying that about 170 residents had probably been exposed. The government earlier said that three workers had radiation illness, but Tokyo Electric said Monday that only one worker was ill.
The problems at Fukushima Daiichi appeared to be the most serious involving a nuclear plant since the Chernobyl disaster in 1986. A partial meltdown can occur when radioactive fuel rods, which normally are under in water, remain partly uncovered for too long. The longer the fuel is exposed, the closer the reactor comes to a full meltdown.
Technicians are essentially fighting for time while heat generation in the fuel gradually declines, trying to keep the rods covered despite a breakdown in the normal cooling system, which runs off the electrical grid. Since that was knocked out in the earthquake, and diesel generators later failed — possibly because of the tsunami — the operators have used a makeshift system for keeping cool water on the fuel rods.
Now, they pump in new water, let it boil and then vent it to the atmosphere, releasing some radioactive material. But they are having difficulty even with that, and have sometimes allowed the water levels to drop too low, exposing the fuel to steam and air, with resulting fuel damage.
On Sunday Japanese nuclear officials said operators at the plant had suffered a setback trying to bring one of the reactors under control when a valve malfunction stopped the flow of water and left fuel rods partially uncovered. The delay raised pressure at the reactor.
At a late-night news conference, officials at Tokyo Electric said that the valve had been fixed, but that water levels had not yet begun rising.
Hiroko Tabuchi reported from Tokyo and Matthew L. Wald from Washington. Michael Wines contributed reporting from Koriyama, Japan, and Ellen Barry from Moscow. Keith Bradsher contributed from Hong Kong.