SOMA, Japan — 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.

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