FUKUSHIMA, Japan — Workers at a quake-damaged atomic power plant briefly suspended operations and evacuated Wednesday after a surge in radiation made it too dangerous to remain there, dealing a setback to Japan’s frantic efforts to stem a nuclear crisis.
Chief Cabinet Secretary Yukio Edano said work on dousing the overheated reactors with water was temporarily disrupted by the need to withdraw.
"The workers cannot carry out even minimal work at the plant now," Edano said. "Because of the radiation risk we are on standby."
The workers were allowed back into the plant less than an hour later after the radiation levels receded, Reuters reported.
The radiation spike was apparently the result of a release of pressure that had built up in one of the reactors, officials said, though it was not immediately clear which one. Steam and pressure build up in the reactors as workers try to cool the fuel rods, leading to controlled pressure releases through vents — as well as uncontrolled explosions.
The nuclear crisis has triggered international alarm and partly overshadowed the human tragedy caused by Friday's magnitude 9 earthquake and tsunami, which pulverized Japan's northeastern coastline, killing an estimated 10,000 people.
How much radiation is dangerous?
Authorities have tried frantically to avert an environmental catastrophe at the Fukushima Dai-ichi complex in northeastern Japan, 170 miles north Tokyo.
Edano said the government expects to ask the U.S. military for help. He did not elaborate. He said the government is still considering whether and how to take up the various offers of help from other countries.
Officials had originally planned to use helicopters and fire trucks to spray water in a desperate effort to prevent further radiation leaks and to cool down the reactors.
"It's not so simple that everything will be resolved by pouring in water. We are trying to avoid creating other problems," Edano said.
"We are actually supplying water from the ground, but supplying water from above involves pumping lots of water and that involves risk. We also have to consider the safety of the helicopters above," he said.
Some academics and U.S. nuclear experts said they feared the worst.
"It's more of a surrender," said David Lochbaum, a nuclear engineer who now heads the nuclear safety program for the Union of Concerned Scientists, an activist group that opposes the expansion of nuclear power. "It's not like you wait 10 days and the radiation goes away. In that 10 days things are going to get worse."
"It's basically a sign that there's nothing left to do but throw in the towel," Lochbaum said.
"This is a slow-moving nightmare," said Dr. Thomas Neff, a research affiliate at the Center for International Studies, which is part of the Massachusetts Institute of Technology.
The government has ordered some 140,000 people in the vicinity to stay indoors. A little radiation was also detected in Tokyo, 150 miles to the south, triggering panic buying of food and water.
France urged French nationals in Tokyo to leave the country or head to southern Japan. It said it had asked Air France to provide planes for their evacuation.
Damaged fuel rods
There are six reactors at the plant. Units 1, 2 and 3, which were operating last week, shut down automatically when the quake hit. Since then, all three have been rocked by explosions. Compounding the problems, on Tuesday a fire broke out in Unit 4's fuel storage pond, an area where used nuclear fuel is kept cool, causing radioactivity to be released into the atmosphere.
Units 4, 5 and 6 were shut at the time of the quake, but even offline reactors have nuclear fuel — either inside the reactors or in storage ponds — that need to be kept cool.
Meanwhile, the Nuclear and Industrial Safety Agency estimated that 70 percent of the rods have been damaged at the No. 1 reactor.
Japan's national news agency, Kyodo, said that 33 percent of the fuel rods at the No. 2 reactor were damaged and that the cores of both reactors were believed to have partially melted.
"We don't know the nature of the damage," said Minoru Ohgoda, spokesman for the country's nuclear safety agency. "It could be either melting, or there might be some holes in them."
Meanwhile, the outer housing of the containment vessel at the No. 4 unit erupted in flames early Wednesday, said Hajimi Motujuku, a spokesman for the plant's operator, Tokyo Electric Power Co.
Japan's nuclear safety agency said later fire could no longer be seen at Unit 4, but that it was unable to confirm that the blaze had been put out. Tokyo Electric Power was also considering spraying boric acid by helicopter to prevent spent nuclear fuel rods at No. 4 from restarting a chain reaction.
Several experts said that Japanese authorities were underplaying the severity of the incident, particularly on a scale called INES used to rank nuclear incidents. The Japanese have so far rated the accident a four on a one-to-seven scale, but that rating was issued on Saturday and since then the situation has worsened dramatically.
France's nuclear safety authority ASN said Tuesday it should be classed as a level-six incident.
The Fukushima Dai-ichi complex was due to be decommissioned in February but was given a new 10-year lease on life.
Its reactors were designed by General Electric. (Msnbc.com is a joint venture between NBC Universal and Microsoft. GE is a part owner of NBC Universal.)
Japan has a total of 55 reactors spread across 17 complexes nationwide.
The Associated Press and Reuters contributed to this report.