latimes.com
Although the restoration of electricity to two of the reactors at the stricken Fukushima Daiichi nuclear power plant appears to have stabilized them, a U.S. nuclear expert said Sunday that the situation in Japan "is still quite uncertain."
"It's premature to make any assessment about the most severely affected reactors," said Edwin Lyman of the Union of Concerned Scientists in Washington.
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.
Photos: Unrelenting crisis grips Japan
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 theInternational 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.
In addition, when the ziconium cladding surrounding the cores was exposed to air, it may have oxidized and become so brittle that radioactive fuel particles could have escaped through cracks. If enough of the escaped fuel has collected at the bottom of the reactor vessel, it could become hot enough to melt through the steel container and escape into the environment, Lyman said. Even if the steel was not breached, the collection of fuel at the bottom of the container would also make it more difficult to cool.
"These cores may not be as easily cooled as if they were undamaged," Lyman said.
Workmen have been spraying all three with seawater for several days in an attempt to keep temperatures down, but the water has combined with the steam and radioactivity to make it difficult for workmen who are attempting to reconnect power.
Had there been no intervention at the stricken power plant, the nuclear fuel would have completely melted within six hours, Lyman said. That would have formed a "hot pool" of fuel that would have melted through the bottom of its stainless steel shell within two hours, he said. But neither of those scenarios has come to pass.
"If the seawater pumping had not been effective, this would have ended days ago," Lyman said. But as long as workers can continue to feed water into the plant, the situation could be stabilized indefinitely, he said.
"I actually think it's an amazing thing that they have been able to maintain the cores," he added. "It is truly heroic."
"It's premature to make any assessment about the most severely affected reactors," said Edwin Lyman of the Union of Concerned Scientists in Washington.
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.
Photos: Unrelenting crisis grips Japan
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 theInternational 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.
In addition, when the ziconium cladding surrounding the cores was exposed to air, it may have oxidized and become so brittle that radioactive fuel particles could have escaped through cracks. If enough of the escaped fuel has collected at the bottom of the reactor vessel, it could become hot enough to melt through the steel container and escape into the environment, Lyman said. Even if the steel was not breached, the collection of fuel at the bottom of the container would also make it more difficult to cool.
"These cores may not be as easily cooled as if they were undamaged," Lyman said.
Workmen have been spraying all three with seawater for several days in an attempt to keep temperatures down, but the water has combined with the steam and radioactivity to make it difficult for workmen who are attempting to reconnect power.
Had there been no intervention at the stricken power plant, the nuclear fuel would have completely melted within six hours, Lyman said. That would have formed a "hot pool" of fuel that would have melted through the bottom of its stainless steel shell within two hours, he said. But neither of those scenarios has come to pass.
"If the seawater pumping had not been effective, this would have ended days ago," Lyman said. But as long as workers can continue to feed water into the plant, the situation could be stabilized indefinitely, he said.
"I actually think it's an amazing thing that they have been able to maintain the cores," he added. "It is truly heroic."
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