SOMA, Japan (AP) -- Dangerous levels of radiation leaking from a crippled nuclear plant forced Japan to order 140,000 people to seal themselves indoors Tuesday after an explosion and a fire dramatically escalated the crisis spawned by a deadly tsunami.
In a nationally televised statement, Prime Minister Naoto Kan said radiation had spread from the four stricken reactors of the Fukushima Dai-ichi nuclear plant along Japan's northeastern coast. The region was shattered by Friday's 9.0-magnitude earthquake and the ensuing tsunami that is believed to have killed more than 10,000 people, plunged millions into misery and pummeled the world's third-largest economy.
Japanese officials told the International Atomic Energy Agency that the reactor fire was in a fuel storage pond - an area where used nuclear fuel is kept cool - and that "radioactivity is being released directly into the atmosphere." Long after the fire was extinguished, a Japanese official said the pool might still be boiling, though the reported levels of radiation had dropped dramatically by the end of the day.
That reactor, Unit 4, had been shut down before the quake for maintenance.
If the water boils, it could evaporate, exposing the rods. The fuel rods are encased in safety containers meant to prevent them from resuming nuclear reactions, nuclear officials said. But they acknowledged that there could have been damage to the containers. They also confirmed that the walls of the storage pool building were damaged.
Experts noted that much of the leaking radiation was apparently in steam from boiling water. It had not been emitted directly by fuel rods, which would be far more virulent, they said.
"It's not good, but I don't think it's a disaster," said Steve Crossley, an Australia-based radiation physicist.
Even the highest detected rates were not automatically harmful for brief periods, he said.
"If you were to spend a significant amount of time - in the order of hours - that could be significant," Crossley said.
Less clear were the results of the blast in Unit 2, near a suppression pool, which removes heat under a reactor vessel, said plant owner Tokyo Electric Power Co. The nuclear core was not damaged but the bottom of the surrounding container may have been, said Shigekazu Omukai, a spokesman for Japan's nuclear safety agency.
Though Kan and other officials urged calm, Tuesday's developments fueled a growing panic in Japan and around the world amid widespread uncertainty over what would happen next. In the worst case scenario, one or more of the reactor cores would completely melt down, a disaster that could spew large amounts of radioactivity into the atmosphere.
"I worry a lot about fallout," said Yuta Tadano, a 20-year-old pump technician at the Fukushima plant, who said he was in the complex when the quake hit.
"If we could see it, we could escape, but we can't," he said, cradling his 4-month-old baby, Shoma, at an evacuation center.
The radiation fears added to the catastrophe that has been unfolding in Japan, where at least 10,000 people are believed to have been killed and millions of people were facing a fifth night with little food, water or heating in near-freezing temperatures and snow as they dealt with the loss of homes and loved ones. Up to 450,000 people are in temporary shelters.
Officials have only been able to confirm a far lower toll - about 3,300 killed - but those who were involved in the 2004 Asian tsunami said there was no question more people died and warned that, like the earlier disaster, many thousands may never be found.
Asia's richest country hasn't seen such hardship since World War II. The stock market plunged for a second day and a spate of panic buying saw stores running out of necessities, raising government fears that hoarding may hurt the delivery of emergency food aid to those who really need it.
In a rare bit of good news, rescuers found two survivors Tuesday in the rubble left by the tsunami that hit the northeast, including a 70-year-old woman whose house was tossed off its foundation.
The Fukushima Dai-ichi nuclear complex, along that battered coastline, has been the focus of the worries. Workers there have been desperately trying to use seawater to cool the fuel rods in the complex's three reactors, all of which lost their cooling ability after Friday's quake and tsunami.
On Tuesday, the complex was hit by its third explosion since Friday, and then a fire in a separate reactor.
Afterward, officials in Ibaraki, a neighboring prefecture just south of the area, said up to 100 times the normal levels of radiation were detected Tuesday. While those figures are worrying if there is prolonged exposure, they are far from fatal.
Tokyo reported slightly elevated radiation levels, but officials said the increase was too small to threaten the 39 million people in and around the capital, about 170 miles (270 kilometers) away. Closer to the stricken nuclear complex, the streets in the