Screenshot from AP Youtube video via Creative Commons showing the location of the pool of spent fuel in relation to the core of the reactor.

The optimism of Japanese officials surveying efforts to halt a nuclear crisis in Japan on Thursday contrasted with dire warnings from U.S. experts about the possibility of dangerous levels of radiation spreading across a large area.

The Fukushima nuclear power plant's reactors have sat idle for the past five days, but not without tremendous action around them. Japanese officials said they are close to returning a power supply to the reactors, which would allow them to restart the regular cooling process. When an earthquake and tsunami leveled much of the nearby seaboard last week, other measures had to be taken to keep the fuel roads cooled.

But as that problem approached a resolution, a new problem confronted the plant. Old nuclear fuel roads resting in pools of sixty feet of water may be sitting in empty baths. Exposure to air would cause the fuel rods to heat up, burn and send a steady, sustained amount of radiation into the atmosphere.

"We believe that around the reactor site there are high levels of radiation," Gregory Jaczko, head of the Nuclear Regulatory Commission, told a House Energy and Commerce subcommittee hearing.  "It would be very difficult for emergency workers to get near the reactors. The doses they could experience would potentially be lethal doses in a very short period of time."

Reported the New York Times:

On Thursday morning a spokesman for Tokyo Electric Power, the Daiichi plant operator, and a spokesman for Japan’s nuclear regulatory agency, denied Mr. Jaczko’s account, saying the situation at reactor No. 4 had not changed and that water remained in the spent fuel storage pool. But both officials said the situation was changing and that the reactor had not been inspected in recent hours.

"We can’t get inside to check, but we’ve been carefully watching the building’s environs, and there has not been any particular problem," said Hajime Motojuku, the spokesman for Tokyo Electric.

Japan's regulators also said early Thursday that radiation levels had decreased steadily overnight. Meanwhile, they continued to evacuate residents within 12 miles of the reactors and provide them with potassium iodide pills to counteract any negative effects of radiation. U.S. military personnel around Japan were among those taking pills after they were recorded with higher than normal radiation levels.

After high radiation levels prevented plant workers from air-dropping water into reactor three on Wednesday, officials hoped the military could make an attempt later Thursday. Reactor four, where a fire broke out earlier in the week, also remained a top priority. The drying pools with spent fuel roads from the two reactors appeared to be the greatest concern, however, because they are not encased as deeply as the active fuel roads in the reactors. Spent fuel at reactors five and six have raised concerns, but the danger doesn't appear as imminent because the reactors are more intact.

Reactors one and two are both seriously damaged, and it's unclear what kind of undertaking will be needed to rehabilitate them or completely seal them off.

The 9.0-earthquake that hit last week has caused at least $200 billion in damge, scared off stock investors across the world and left more than 1,400 people dead and tens of thousands more missing. More than two millions lacked either running water or electricity. From India to France, dozens of countries have told their citizens to flee Japan.

Even China, a ferocious builder of infrastructure, said he would suspend its approvals of nuclear power plants until they can be deemed absolutely safe.

Experts from the U.S. Energy Department and the International Atomic Energy Agency were expected to arrive in Japan later Thursday to provide help in assessing the situation at the power plant. Working with the power plant operators, the agencies must decide whether to continue stopgap cooling measures, let the fuel burn and contain the radiation or work to restore broader cooling measures.

Reach deputy editor Paresh Dave here. Follow him on Twitter: @peard33.