March 13, 2011
By HIROKO TABUCHI and MATTHEW L. WALD
TOKYO — A second explosion rocked a troubled nuclear power plant Monday, blowing the roof off a containment building but not harming the reactor, Japanese nuclear officials announced on public television.
The explosion underscores the difficulties Japanese authorities are having in bringing several stricken reactors under control three days after a massive 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 would release catastrophic amounts of radiation.
It was unclear if radiation was released by the 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 thick smoke rising from the building.
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 600 people who remained within a 12 mile radius have been ordered 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,” he said. “I understand that there is little possibility that radioactive materials are being released in large amounts.”
Twenty-two people who live near the plant are already showing signs of radiation exposure from earlier radiation releases at the plant, but it is not clear if they received dangerous doses.
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.
The reactors are both at the Fukushima Daiichi Nuclear Power Station, where another reactor is also having difficulties.
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.
“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 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. A partial meltdown can occur when radioactive fuel rods, which normally are covered in water, remain partially uncovered for too long. The more 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.
Hiroko Tabuchi reported from Tokyo, and Matthew L. Wald from Washington. Michael Wines contributed reporting from Koriyama, Japan, and Ellen Barry from Moscow.
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