The risk of partial meltdown at a stricken nuclear power plant in Japan increased on Monday as cooling systems failed at a third reactor, possibly exposing its fuel rods, only hours after a second explosion at a separate reactor blew the roof off a containment building.
The widening problems underscore the difficulties 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 meltdowns, which would release catastrophic amounts of radiation.
It was unclear if radiation was released by Monday's explosion, but a similar explosion at another reactor at the plant over the weekend did release radioactive material.
Live footage on public broadcaster NHK showed the skeletal remains of the reactor building and thick smoke rising from the building. Eleven people had been injured in the blast, one seriously, officials said.
Chief Cabinet Secretary Yukio Edano said 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 were ordered temporarily to take cover indoors, 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.”
In screenings, 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.
Technicians had been scrambling most of Sunday to fix a mechanical failure that left the reactor far more vulnerable to explosions.
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 a full meltdown.
Later on Monday, Mr. Edano said cooling systems at a third reactor at Fukushima Daiichi had failed. The water level inside the reactor had fallen, exposing the fuel rods at its core despite emergency efforts to pump seawater into the reactor, he said.
“The pump ran out of fuel, and the process of inserting water took longer than expected, so the fuel rods were exposed from the water for a while,” he said.
Plant workers then renewed efforts to flood the reactor with seawater, and readings showed that some of the water had started to accumulate within the reactor, Mr. Edano said.
Exposure for too long a period of time can damage the fuel rods, raise the risk of overheating and possible meltdown.
Hidehiko Nishiyama, deputy director-general at Japan's Nuclear and Industrial Safety Agency, said workers were also battling rising pressure within the reactor. They have opened vents in the reactor's containment vessel, which houses the fuel rods, a measure that could release small amounts of radiation, he said.
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 Power Company, 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.
“I'm not aware that we've ever had more than one reactor troubled at a time,” said Frank N. von Hippel, a physicist and professor at Princeton, explaining the difficulties faced by the Japanese.
“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 U.S. 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 on 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 on 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 partially 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 Power said that the valve had been fixed, but that water levels had not yet begun rising. — New York Times News Service