Handful of 'heroes' battles to keep nuclear plant under control

After three explosions and a fire in four days, the situation at Japan's earthquake-stricken Fukushima Daiichi nuclear plant grew more serious Tuesday, chasing all but a handful of workers from the site and raising fears of a far more dangerous radiation threat.

The latest incidents, an explosion Tuesday at the plant's No. 2 reactor and a fire in a cooling pond used for nuclear fuel at the No. 4 reactor, briefly pushed radiation levels at the plant to about 167 times the average annual dose of radiation, according to details released by the International Atomic Energy Agency.

That dose would quickly dissipate with distance from the plant, and radiation levels quickly fell back to levels where they would be no immediate public health threat, Chief Cabinet Secretary Yukio Edano said.

But the deteriorating situation at the plant and concerns about a potential shift in winds that could loft radiation toward populated areas nevertheless prompted authorities to warn people as far as 30 kilometers (18.6 miles) away from the plant to stay inside.

"There is still a very high risk of further radioactive material coming out," Prime Minister Naoto Kan said, asking people to remain calm.

About 200,000 people within a 20-kilometer (12.4 mile) radius of the plant had been previously evacuated.

Authorities also banned flights over the area and evacuated most workers from the plant. Those who remained behind continued a seesaw, last-ditch effort to flood reactors with sea water to keep them cool and prevent a wider environmental and public health catastrophe.

"Their situation is not great," said David Brenner, director of the Center for Radiological Research at Columbia University in reference to the remaining workers. "It's pretty clear that they will be getting very high doses of radiation. There's certainly the potential for lethal doses of radiation. They know it, and I think you have to call these people heroes."

Troubles at the plant began shortly after the 9.0-magnitude earthquake that struck Friday off the shore of northeast Japan. Although the plant's three functioning reactors shut down automatically when they detected the quake, the tsunami that followed swamped the diesel generators that provided backup power to the reactor cooling systems.

Crews were eventually able to restore backup power, but problems keeping the reactors cool enough eventually forced plant officials to take the drastic step of flooding the reactors with sea water in a bid to keep the temperatures down. Still, pressure buildups, problems with valves and even a failure to fill up a generator's gas tank have led to explosions and other problems with keeping the reactors under control.

Tuesday's incidents appeared to escalate the situation: Edano said the radiation releases from the explosion and fire were the first that appeared to pose a threat to human health, if only briefly.

But it was still unclear how much radioactive material may have been emitted, what kind of health threat that could pose or when the danger would end.

Japanese officials earlier told the International Atomic Energy Agency that radioactivity was "being released directly into the atmosphere" during the fire, according to a statement from the U.N. watchdog organization.

High temperatures inside the building that houses the plant's No. 4 reactor may have caused fuel rods sitting in a pool to ignite or explode, the plant's owner said.

By Tuesday afternoon, Edano said radiation readings -- which had reached dangerously high levels at the plant earlier -- had decreased.

But as if the suddenly minimal crew at the plant did not already have enough to worry about, Edano said cooling systems at two other reactors, No. 5 and No. 6, now were "not functioning well."

Tuesday's announcement "points to something different, something more serious" after the explosion at the No. 2 reactor, CNN analyst James Walsh said. "But we don't have the definitive evidence yet."

Edano said earlier that he could not rule out the possibility of a meltdown at all three troubled reactors at the plant.

A meltdown occurs when nuclear fuel rods cannot be cooled and melt the steel and concrete structure containing them. In the worst-case scenario, the fuel can spill out of the containment unit and spread toxic radioactivity through the air and water. That, public health officials say, can cause both immediate and long-term health problems, including radiation poisoning and cancer.

If fuel rods inside the reactors are melting, Walsh said a key detail is whether the melted material stays inside the reactor.

"The Japanese plants and all modern plants have a containment vessel. Essentially the reactor is inside of a vault. And that vault is made of thick concrete and steel," Walsh said. "The million-dollar question is whether that melting will be contained... We'll know within 24 hours. That's the key thing people should be paying attention to."

The long-term impact on public health from the crisis at the plant remains unclear. But at the moment, it appears minimal, Brenner said.

"I think at this point in time there's no real evidence that there are health risks to the general population," he said.

But until crews are able to bring the situation under full control, it's impossible to say how much radiation may be released from the plants or for how long.

A fresh concern was the weather. Wind patterns that had been blowing the thin plume of radiation out to sea appeared to be shifting towards more populated areas, CNN meteorologist Jennifer Delgado said. The IAEA said it believed the winds would continue to blow the radiation out over the ocean, where it would pose little risk to overseas populations.

U.S. Navy personnel in Japan began limiting outdoor activities and securing external ventilation systems after instruments aboard an aircraft carrier docked in Yokosuka detected low levels of radioactivity from the Fukushima Daiichi nuclear plant, the Navy said.

The USS George Washington was docked for maintenance about 175 miles (280 kilometers) from the plant when instruments detected the radiation at 7 a.m. Tuesday (6 p.m. ET Monday), the Navy said in a statement.

"These measures are strictly precautionary in nature. We do not expect that any United States federal radiation exposure limits will be exceeded even if no precautionary measures are taken," the Navy said.

On Monday, defense officials said the Navy had repositioned the USS Ronald Reagan after detecting low-level radiation on some sailors and equipment.

Radiation levels in Tokyo -- which is 223 kilometers (138 miles) southwest of the plant -- were twice the usual level on Tuesday. But the reading -- 0.809 microsieverts per hour -- was too negligible to pose a health threat, the Tokyo Metropolitan Government said.