A military tank that will be used to clear rubble from the Fukushima nuclear… (Takehiko Kobayashi / Yomiuri…)
Radiation levels at the stricken Fukushima Daiichi nuclear power plant in Japan are still high but may be tapering off, a senior U.S. nuclear official said Sunday.
Indications from the plant, which houses six nuclear reactors, were levels in the range of hundreds of millisieverts per hour, said Gregory Jaczko, chairman of the U.S. Nuclear Regulatory Commission. The duration of those levels was unclear. The exposure limit for Japanese workers was recently raised to 250 millisieverts per year.
For the sake of comparison, the average American is exposed to 6.2 millisieverts of radiation per year, half of which come from natural sources, according to the commission.
Photos: Unrelenting crisis grips Japan
"We believe right now that the radiation levels at the site are high, but we have some indications that they may be coming down," Jaczko said on the C-Span program "Newsmakers."
The Nuclear Regulatory Commission is using "a variety of sources," including data from the Department of Energy, to assess radiation levels at the plant and in the surrounding area, Jaczko said. But power interruptions at the plant have knocked out some of the instruments that would normally provide reliable readings.
"It's difficult to obtain accurate information," Jaczko said.
On Monday, the five members of the commisison will begin developing a plan to glean lessons from events in Fukushima that can be applied to nuclear plants in the United States.
"We want to take a very systematic and methodical look at all the information we're getting from Japan," Jaczko said. The first conclusions based on solid information about how the plant and its operator, Tokyo Electric Power Co., have performed could be made by late summer or early fall, he said.
"If that good information tells us we need to make changes to our licensing process, then we will do that," he said.
The commission has 11 experts stationed in Tokyo, where they are providing technical assistance. There are no plans to send any of them to Fukushima, Jaczko said. "I'm not sure it's really the appropriate role for NRC to send staff actually to the site," he said.
Although the restoration of electricity to two of the reactors at the Fukushima plant appears to have stabilized them, the situation in Japan "is still quite uncertain," said Edwin Lyman of the Union of Concerned Scientists in Washington.
"It's premature to make any assessment about the most severely affected reactors," he said.
Damage already incurred to the nuclear fuel rods in the plant's other four reactors may make it more difficult for workers to cool them to a safe temperature, even after electricity is fully restored, Lyman said.
Progress restoring power to the damaged plant has apparently stalled after a full day of work Sunday, although the situation has not deteriorated any further.
Officials at Tokyo Electric Power Co., which owns the plant 140 miles north of Tokyo, said they had managed to restore power to a switchboard at the No. 2 reactor at the plant, but have not yet been able to restore coolant flow in the reactor.
Meanwhile, Japanese health authorities have banned the sale of milk and vegetables from the prefecture in which the power plant is located because they have been contaminated by radioactive fallout, although officials claim the levels are not yet high enough to present a danger to human health.
After stringing a new power line to the plant from the electric grid, company officials reported on Saturday that they had reconnected coolant pumps in reactor Nos. 5 and 6 and restored the flow of water to the spent fuel cooling pools in those buildings. In the day since, temperatures in those pools have returned to near normal.
But those two pools had not been considered a significant threat. Authorities are much more concerned about reactors No. 2 and No. 3 and the spent fuel pool at No. 4. The reactor containment vessel at No. 2 may be cracked and venting some radioactive gases into the environment. Reactor No. 3 is the only reactor at the site that contains plutonium in the fuel rods and its escape would be extremely dangerous because it is carcinogenic in even minute doses.
And the spent fuel pool at reactor building No. 4 is thought to have boiled dry, allowing the fuel rods to heat up and become damaged, also releasing radioactivity into the environment.
The nuclear cores inside the reactors are usually covered in water, but the top halves of the cores in reactors 1, 2 and 3 were exposed to air for at least several days, according to reports from the International Atomic Energy Agency and other sources. Even if those cores are resubmerged, they may have experienced permanent damage that would make them more difficult to keep cool, Lyman said.
For instance, he said, if the exposed portions of the fuel rods have swelled due to heat, the gaps between them may now be too small to pass enough water to cool them.