An air of panic swept parts of Japan today as scientists continued to battle radiation leaks from earthquake-damaged reactors made worse by an explosion and fire.
Although experts said the nuclear plant at Fukushima could not turn into a Chernobyl-style disaster, more than 140,000 people in surrounding areas were warned to seal themselves indoors.
Prime Minister Naoto Kan and others urged calm, but today's developments fuelled growing fears in Japan and around the world amid widespread uncertainty over what would happen next.
In the worst case, one or more of the reactor cores would completely melt down, which could spew large amounts of radioactivity into the atmosphere.
Mr Kan admitted radiation had spread from the four reactors along Japan's north-eastern coast.
Japan 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, nuclear workers said the pool might still be boiling, although the reported levels of radiation had dropped dramatically by the end of the day.
Japan was also considering asking for military help from the US to spray water from helicopters into the pool for reactor Unit 4 which had been shut down before the quake for maintenance.
If the pool water boils and evaporates the fuel rods will be exposed.
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.
"It's not good, but I don't think it's a disaster," said Steve Crossley, a 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," he said.
Less clear were the results of the blast in Unit 2, near a suppression pool, which removes heat under a reactor vessel.
The nuclear core was not damaged but the bottom of the surrounding container may have been, said a spokesman for Japan's nuclear safety agency.
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 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.
Hundreds of aftershocks have shaken Japan's north-east and Tokyo since the original offshore quake, including one today whose epicentre was hundreds of miles south-west and inland.
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.
Japan has not seen such hardship since the Second World War. 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 in the rubble left by the tsunami that hit the north-east, 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.
Today the complex was hit by its third explosion since Friday, and then a fire in a separate reactor.
Afterward, officials in Ibaraki, a neighbouring prefecture just south of the area, said up to 100 times the normal levels of radiation were detected. 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 away.
Amid concerns about radiation, Austria moved its embassy from Tokyo to Osaka.
Meanwhile, Air China and China Eastern Airlines cancelled flights to Tokyo and two cities in the disaster area. Germany's Lufthansa is also diverting its two daily flights to Tokyo to other Japanese cities.
Closer to the stricken nuclear complex, the streets in the coastal city of Soma were empty as the few residents who remained there heeded the government's warning to stay indoors.
Mr Kan warned there was a danger of more leaks and told people living within 19 miles of the complex to stay indoors to avoid exposure that could make people sick.
The Japanese government was right to be cautious, said Donald Olander, a nuclear engineering professor at the University of California.
He believed even the heavily elevated levels of radiation were "not a health hazard." But without knowing specific dose levels, he said it was hard to make judgments.
"Right now it's worse than Three Mile Island," he said. But it's nowhere near the levels released during Chernobyl.
On Three Mile Island, the radiation leak was held inside the containment shell - thick concrete armour around the reactor. The Chernobyl reactor had no shell and was also operational when the disaster struck. The Japanese reactors automatically shut down when the quake hit and are encased in containment shells.Reuse content