Later Tuesday, the government said radiation levels had fallen again, somewhat quelling fears. Though another 6.0-magnitude earthquake shook eastern Japan, which worsened the crisis still.
Prime Minister Naoto Kan announced that radiation was spreading from four reactors at the Fukushima Dai-ichi nuclear plant, 150 miles north of Tokyo. A 15 mile exclusion zone was enforced around the plant as engineers battled to bring overheating reactors under control.
"The level seems very high, and there is still a very high risk of more radiation coming out," Kan said in a nationally televised statement, The Associated Press reported. "We are making utmost efforts to prevent further explosions and radiation leaks."
Japanese authorities told people living within 20 miles of the plant to stay indoors with windows closed and air conditioners off. A senior Japanese official warned that the radiation levels were now high enough to pose a medical risk.
"Now we are talking about levels that can impact human health," Chief Cabinet Secretary Yukio Edano said, according to reports.
The plant's operator, Tokyo Electric Power, told the BBC that radiation levels around Fukushima for one hour's exposure rose to eight times the legal limit for exposure in a year.
A statement posted on Facebook by the commander of U.S. Naval forces in Japan said that instruments on the USS George Washington, which is docked for maintenance in Yokusuka, south of Tokyo, were picking up low levels of radiation. The statement advised Navy personnel to limit outdoor activities and secure ventilation systems.
The latest explosion was thought to be at the No. 2 reactor of the plant, and the fire at the No. 4 reactor, which had been not operational but was storing spent nuclear fuel. The fourth reactor was thought to have caused the increased radiation because of the hydrogen release that caused the fire.
Edano said the fourth reactor “did not pose an imminent threat.”
AP quoted a government official saying that waste water in a storage pool for one of the damaged reactors may be boiling. He said technicians were trying to solve the problem, but did not expand on what additional risks this poses.
Most of the 800 workers at the plant were evacuated due to fears of exposure to high levels of radiation, but about 50 stayed behind to pump seawater into the three reactors that experienced explosions to cool them.
The explosions have already injured 15 people who work at the plant and exposed up to 190 to higher radiation.
All of the explosions have happened after cooling system breakdowns at the reactors, BBC reports. To prevent complete meltdowns, engineers are trying to flood the chambers.
Meanwhile, Japan is still struggling to cope with the mass scale of human casualties and suffering caused by the earthquake -- upgraded to magnitude 9.0 by the U.S. Geological Survey -- and tsunami. The official death toll has risen to 2,500, and is expected to pass 10,000.
Some areas, like the coastal town of Minami Sanriku, have been completely devastated after the tsunami flattened virtually everything in its path.
More than 500,000 remain homeless.
The disaster has left Japan's already fragile economy reeling. There has been major damage to key industries, with electronics giants Toshiba and Sony shutting down factories and automakers Nissan, Honda and Toyota halting production lines at the cost of millions of dollars a day.
And stock markets plunged in Japan and across much of the Asia-Pacific region Tuesday.
Yet, even as hopes were fading of finding thousands of people missing since the tsunami swept ashore, there were fresh stories of survival.
A 70-year-old woman was found in her home in the town of Otsuchi, Sky News reported. It said she was suffering from hypothermia, but otherwise unhurt. A man was also rescued from the town of Ishimaki. A day earlier, a four-month-old girl was pulled, uninjured, from rubble in the town of Ishinomaki.