WASHINGTON — Japanese workers, who are risking their lives attempting to cool a half dozen crippled nuclear reactors, managed Saturday to stabilize a storage pool that holds some of the deadliest spent fuel, halting its release of radiation, the Japanese government said.
However, while progress was made in stabilizing reactor No. 3 at the Fukushima Daiichi Nuclear Power Station, reactor No. 4 could pose a more difficult problem if, as feared, water is leaking through a breach in its pool filled with both live and spent fuel assemblies, U.S. experts said.
Without 30 to 40 feet of water, that pool still could heat up quickly, triggering a hydrogen explosion that could shower the entire plant with deadly radioactive particles. That would further hamper efforts to prevent the one-two punch of the March 11 earthquake and tsunami from causing a nuclear disaster on the scale of the Chernobyl plant in Ukraine in 1986.
The workers, many of whom have absorbed radiation levels widely considered unsafe, labored to restore electric power to four of the reactors' critical water pumps — equipment whose disabling has led to most of the significant radiation releases over the past week.
The improved effectiveness of spraying water on the reactors came as the Japanese government disclosed that radiation levels exceeding permissible levels have been detected in milk and spinach collected in the northeast region of the country near the plant.
Japan's chief cabinet secretary, Yukiyo Edano, said that the elevated levels would only be a health threat if consumed throughout a person's life. However, he said, the government will continue to monitor food samples and, if appropriate, ban shipments of food where radiation can be linked to emissions from the plant.
Fukushima's reactor No. 3 has been of special concern because it uses reprocessed nuclear weapons fuel containing traces of plutonium, far more lethal than the deadly radiation in other reactor waste.
Emergency workers used a truck with a crane-like arm to pump some 750 gallons of water a minute through holes in the building holding reactor No. 3, refilling its spent fuel tank, according to the industry newsletter World Nuclear News.
Top U.S. officials have expressed deep consternation about the spent fuel pool in Fukushima's No. 4 reactor since midweek, when Nuclear Regulatory Commission Chairman Gregory Jaczko told Congress that his agency believed the pool's water had boiled away.
Since then, there've been reports that the pool may be leaking, apparently due to damage from the earthquake, the tsunami or from ensuing explosions in the building.
"The worst case is, you dump more water on that spent fuel pool, and it drains," said Najmedin Meshkati, a nuclear safety expert who teaches at the University of Southern California. "And then the zirconium cladding of that fuel bundle will interact with this hot water, and it will release hydrogen. Then there may be fire or an explosion."
Because part of the roof of that building has been blown off by an earlier hydrogen explosion, he said, "all of that radioactive cloud ... may get released into the environment. It's very potent."
Earlier attempts to use helicopters to dump 3,000 tons of seawater on that reactor and others proved ineffective.
Arjun Makhijani, the president of the Institute for Energy and Environmental Research, said that "there appears to be some damage in spent fuel pool No. 4. They've had a struggle to keep water in there.
"Obviously, if there's a crack in the No. 4 pool, then the injection of water has to be faster to keep up with the leak."
Makhijani expressed optimism that progress is being made to avoid "serial meltdowns (of the reactors) and fires in these spent fuel pools and a serious release of radiation."
However, he said, even if meltdowns occurred, it seems clear that containment efforts will continue. Makhijani, a longtime watchdog over the nuclear power industry, recalled that workers at Chernobyl were forced to shovel chunks of radioactive material into dumpsters to reduce radiation emissions and save the lives of others.
"Most of the workers are dead," he said, "but you know there'd be a lot more people dead if they hadn't done it."
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