Tokyo (CNN) -- 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 quickly fell back to levels where it posed no immediate public health threat, Chief Cabinet Secretary Yukio Edano said.
But the deteriorating situation and concerns about a potential shift in wind direction that could loft radiation toward populated areas prompted authorities to warn people as far as 18.6 miles (30 kilometers) 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 living within a 12.4-mile radius of the plant already had been 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 seawater to keep them cool and prevent a wider environmental and public health catastrophe.
The beleaguered crew had to abandon the plant control room Tuesday night because of high radiation levels, Kyodo News reported, citing plant owner Tokyo Electric Power Company.
"Their situation is not great," said David Brenner, director of the Center for Radiological Research at Columbia University. "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 eventually were able to restore backup power, but problems keeping the reactors cool eventually forced plant officials to take the drastic step of flooding them with seawater in a bid to keep the temperatures down. Still, pressure buildups, problems with valves and even a failure to fill 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.
Radiation levels also spiked Monday, after workers vented steam to release pressure, but the levels quickly dropped, officials said.
Also Monday, an explosion in the building housing the plant's No. 3 reactor apparently damaged both a water-filled chamber at the base of the reactor and the reactor containment unit itself, IAEA Director General Yukiya Amano said at a news conference Tuesday.
Damage to the core involved about 5% of the core's nuclear fuel, Amano said.
There is still a very high risk of further radioactive material coming out
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.
The central concern Tuesday seemed to be the cooling pool for used nuclear fuel that caught fire Tuesday, as well as others like it.
"The fire there yesterday dramatized quite clearly the dangers there," said Dr. Ira Helfand, a member of Physicians for Social Responsibility.
"There are enormous quantities of radiation," Helfand said. "The containment is not nearly as good as around the reactor core themselves. The potential for a major release of radiation from those sites is very real, as we saw last night with the fire while it was burning."
Japanese officials 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.
Crews put that fire out, and by Tuesday afternoon, Edano said radiation readings -- which had reached dangerously high levels at the plant earlier -- had decreased.
Still, concerns about hot radioactive fuel boiling off cooling water and catching on fire continued into Wednesday, with plant operators and government officials considering a plan to use helicopters to drop water into the cooling pond through the damaged roof of the reactor building, according to a Kyodo News report.
And 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, were "not functioning well."
Plant managers were said to be considering removing panels from the buildings housing those reactors in an effort to prevent the hydrogen buildup that officials believe caused the other explosions, according to the IAEA.
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. 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.
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.
The U.S. Department of Energy has sent a team and a package of monitoring equipment to help detect any movement of dangerous concentrations of radiation toward population centers, Energy Secretary Steven Chu said Tuesday.
The U.S. Nuclear Regulatory Commission also announced that it was sending nine more experts to Japan.
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, 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, about 225 kilometers (140 miles) southwest of the plant, were twice the usual level on Tuesday. The concentration -- 0.809 microsieverts per hour -- still was too negligible to pose a health threat, the Tokyo Metropolitan Government said.
We are all going to hell. It was about time.