US, Japan differ on danger zone, spent fuel risk

This image made available from Tokyo Electric Power Co. via Kyodo News, shows the damaged No. 4 unit of the Fukushima Dai-ichi nuclear complex in Okumamachi, northeastern Japan, on Tuesday March 15, 2011. White smoke billows from the No. 3 unit. (AP Photo/Tokyo Electric Power Co. via Kyodo News) JAPAN OUT, MANDATORY CREDIT, NO LICENSING IN CHINA, HONG KONG, JAPAN, SOUTH KOREA AND FRANCE AP Photo/Tokyo Electric Power Co. via Kyodo News

Updated 12:14 p.m. ET

As Japanese officials scramble to cool dangerously overheated nuclear reactors at the damaged Fukushima power plant in Japan, the discrepancy between U.S. concern -- expressed Wednesday by Nuclear Regulatory Commission chair Gregory Jaczko -- and the Japanese risk assessment is "pretty hard to explain," a nuclear expert told CBS Thursday.

James Acton, an associate in the Nuclear Policy Program at the Carnegie Endowment for International Peace spoke to CBS' "The Early Show" Thursday, nearly a week after a historic earthquake and tsunami set off a nuclear crisis at the Fukushima site.

Complete coverage: Disaster in Japan

While Japan has evacuated residents from a 12-mile radius around the site, and advised residents to remain indoors within an additional 7-mile circle, the U.S. has told all citizens within 50 miles to get out.

"There's a big difference between Americans in Japan and Japanese in Japan right now. And that's that the Americans in Japan can go home," Acton said. Over half a million Japanese are homeless, displaced and "Japanese authorities have to factor that into their plans," he said. "If they extend the evacuation zone and evacuate more people but they're not able to feed those people or shelter those people, then it's probably better for those people to stay where they are and inside their houses -- which is actually better radiation protection than it might first appear."

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The nations also have conflicting views on whether there is any water left in the stricken spent fuel pool at Fukushima's Unit 4.

The Japanese government issued a statement on radiation levels outside the reactors based on helicopters that were flying overhead and collected the following data: 

1.000 feet: 4.13 millisieverts/hour

300 feet: 87.7 millisieverts/hour

100 feet: 250 millisieverts/hour

Watch video shot from a Japanese Self Defense Force helicopter at the Fukushima site late Wednesday, March 16:

The data seems to contradict what the NRC's Jaczko told Congress on Wednesday. A person might absorb 6 millisieverts in a year from a natural sources of radiation as well as other sources, such as X-rays. However, Jaczko may have been referring to radiation within the containment areas of the reactors, not the outside atmosphere, and the radiation levels that the plant workers are potentially receiving. On Wednesday, Japan's Ministry of Health Labor and Welfare raised the maximum legal exposure for nuclear workers to 250 millisieverts from 100 millisieverts. It described the move as "unavoidable due to the circumstances."

"I don't know who's right," Acton said. "I'm inclined right now, if the Japanese government has made the mistake, to give them the benefit of the doubt, and to believe it was a genuine mistake rather than an attempt to conceal information," he said, but added, "These issues will clearly have to be examined very carefully afterward."

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The water in spent fuel pools has two functions. It's a shield against radiation and acts as a coolant to the keep the highly radioactive, used fuel rods cool. Losing the shield means the fuel rods will be releasing radiation, making the site difficult for engineers to access and raising the overall risk.

The cooling issue is a little bit more complicated, Acton said, and even U.S. agencies disagree about the risk of exposed rods catching fire.

"The truth here is actually, we don't seem to know right now how much of a danger is posed if you lose the cooling function of the water," he said.

Even if power is restored to the reactors; cooling system, it's unclear whether the system will be functional given the quake and tsunami damage.

"There's no guarantee that restoring power is going to bring this terrible crisis to an end," Acton said.

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