Japan suspended operations to prevent a stricken nuclear plant from melting down today after a surge in radiation made it too dangerous for workers to remain at the facility.
Chief Cabinet Secretary Yukio Edano said work on dousing reactors with water was disrupted by the need to withdraw.
The level of radiation at the Fukushima Dai-ichi nuclear power plant surged to 1,000 millisieverts early today before coming down to 800-600 millisieverts. Still, that was far more than the average that is considered safe.
"So the workers cannot carry out even minimal work at the plant now," Edano said. "Because of the radiation risk, we are on standby."
Experts say exposure of around 1,000 millisieverts is enough to cause radiation sickness.
Japanese officials said today that many fuel rods in the nuclear plant were damaged in a string of explosions and fires.
The latest fire broke out early today, a day after the power plant emitted a burst of radiation that panicked an already edgy Japan following Friday's massive earthquake and tsunami that is estimated to have killed more than 10,000 people.
Hajimi Motujuku, a spokesman for Tokyo Electric Power, operator of the plant, said the outer housing of the containment vessel at the No. 4 unit at the complex caught fire.
On Tuesday, a fire broke out in the same reactor's fuel storage pond — an area where used nuclear fuel is kept cool — causing radioactivity to be released into the atmosphere. The company said the new blaze erupted because the initial fire had not been fully extinguished.
About three hours after the second blaze erupted, Japan's nuclear safety agency said flames could no longer be seen at Unit 4. But it was unable to confirm that the blaze had been put out, and clouds of white smoke were billowing from the reactor.
Japan's nuclear safety agency said 70 percent of the nuclear fuel rods may have been damaged at another Fukushima Dai-ichi reactor that was first stricken last week, triggering the nuclear crisis.
"But we don't know the nature of the damage, and it could be either melting, or there might be some holes in them," said spokesman Minoru Ohgoda.
Japan's national news agency, Kyodo, said 33 percent of the fuel rods at a second reactor were also damaged.
The troubles have been caused by overheating of the reactors, which have lost their cooling ability because of damage to equipment from the earthquake and tsunami. Excessive heating will lead to a meltdown of the reactor and release hazardous radiation.
Engineers have been desperately trying to cool the reactors and spent fuel rods after the electricity was cut off in the wake of the quake, shutting down their cooling functions.
Radiation levels in areas around the nuclear plant rose early Tuesday afternoon but appeared to subside by evening, officials said.
The radiation levels caused the government to order 140,000 people living within 20 miles of the plant to seal themselves indoors to avoid exposure, and authorities declared a ban on commercial air traffic through the area. Worries about radiation rippled through Tokyo and other areas far beyond that cordon. The stock market plunged for a second straight day, dropping 10 percent Tuesday.
Before they were ordered to leave today, a small crew of technicians, braving radiation and fire, were the only people remaining at the Fukushima Dai-ichi plant.
They crawled through labyrinths of equipment in utter darkness pierced only by their flashlights, listening for periodic explosions as hydrogen gas escaping from crippled reactors ignites on contact with air.
They had to breathe through uncomfortable respirators or carry heavy oxygen tanks on their backs. They wore white, full-body jumpsuits with snug-fitting hoods that provided scant protection from the radiation sleeting through their bodies.
They were the faceless 50, the unnamed operators who stayed behind. They volunteered, or were assigned, to pump seawater on dangerously exposed nuclear fuel, already thought to be partly melting and spewing radioactive material, in hopes of preventing full meltdowns that could throw thousands of tons of radioactive dust high into the air and imperil millions.
The workers were being asked to make escalating sacrifices that so far are being only implicitly acknowledged: Japan's Health Ministry said Tuesday that it was raising the legal limit on the amount of radiation exposure to which each worker could be exposed, to 250 millisieverts from 100 millisieverts, five times the maximum exposure permitted for nuclear plant workers in the United States.
The change meant workers could remain on site longer, the ministry said.
The few details Tokyo Electric has made available paint a dire picture. Five workers have died since the quake and 22 more have been injured, while two are missing. Tokyo Electric evacuated 750 emergency staff members from the stricken plant Tuesday, leaving only about 50, when radiation levels soared.
Information from the New York Times and Associated Press was used in this report.
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