TOKYO (Reuters) - Japan pumped nitrogen gas into a crippled nuclear reactor on Thursday, trying to prevent an explosive buildup of hydrogen gas as the world's worst nuclear disaster in 25 years stirred atomic safety debate and inspections in the United States.
Engineers worked through the night injecting nitrogen into the containment vessel of reactor No.1 at Fukushima Daiichi power plant, following success in stopping highly radioactive water leaking into the sea at another reactor in the complex.
"It is necessary to inject nitrogen gas into the containment vessel and eliminate the potential for a hydrogen explosion," an official of plant operator Tokyo Electric Power (TEPCO) told a news briefing.
The possibility of another hydrogen explosion like those that ripped through reactors No.1 and No.3 early in the crisis, spreading high levels of radiation into the air, was "extremely low," he said.
But TEPCO suspected that the outside casing of the reactor vessel was damaged, said the official.
"Under these conditions, if we continue cooling the reactors with water, the hydrogen leaking from the reactor vessel to the containment vessel could accumulate and could reach a point where it could explode," he added.
A second TEPCO official said 6,000 cubic metres of hydrogen gas would be pumped into reactor No.1 and the utility was preparing nitrogen gas injections for reactors No.2 and No.3 in the six-rector plant as a safety precaution.
Although TEPCO succeeded after days of desperate efforts to plug the leak at reactor No.2, they still need to pump 11.5 million liters (11,500 tonnes) of contaminated water back into the ocean because they have run out of storage space at the facility. The water was used to cool overheated fuel rods.
Nuclear experts said the damaged reactors were far from being under control almost a month after they were hit by a massive earthquake and tsunami on March 11.
In Vienna, the head of a U.N. scientific body said the Fukushima accident is not expected to have any serious impact on people's health, based on the information available now.
Wolfgang Weiss, chairman of the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), also said the Fukushima disaster was less dramatic than Chernobyl in 1986 but "much more serious" than Three Mile Island in 1979.
Asked what health consequences he expected from Fukushima, he said: "From what I know now, nothing, because levels are so low."
"We have seen traces of iodine in the air all over the world now but they are much, much, much lower than traces we have seen at similar distances after Chernobyl," Weiss added.
Growing concerns in nearby South Korea and China over radioactive fallout from Japan were underscored when China's health ministry reported trace amounts of radioactive iodine in spinach in three Chinese provinces.
The government is preparing to revise guidelines for legal radiation levels, designed for brief exposure to high levels of radiation in emergencies and not cumulative absorption, for people living near the damaged plant.
Workers are struggling to restart cooling pumps -- which recycle the water -- in four damaged reactors.
Until those are fixed, they must pump in water to prevent overheating and meltdowns, but have run out of storage capacity for the seawater when it becomes contaminated.
Radioactive iodine detected in the sea has been recorded at 4,800 times the legal limit, but has since fallen to about 600 times the limit. The water remaining in the reactors has radiation five million times legal limits.
Martin Virgilio, a top official for the U.S. Nuclear Regulatory Commission, said at a House of Representatives hearing that the NRC did not believe that the core of Fukushima's reactor No. 2 had melted down.
Earlier, a Democratic lawmaker had said the NRC informed him the core had become so hot it had probably melted through the reactor pressure vessel. Lawmakers also grilled the NRC on whether the U.S. nuclear power industry was doing enough to ensure American reactors can withstand worst-case scenarios.
The NRC is conducting special inspections at two Illinois nuclear plants operated by Exelon Corp after routine checks in February found a problem with backup pumps that would be used to remove heat from the reactors in case of an accident.
COOLING REACTORS KEY
Japan is facing its worst crisis since World War Two after the 9.0 magnitude earthquake and tsunami left nearly 28,000 people dead or missing and thousands homeless, and rocked the world's third-largest economy.
It will likely take months to finally cool down the reactors and years to dismantle those that have been damaged. TEPCO has said it will decommission four of the six reactors.
Two Fukushima plants together provide 4 percent of Japan's electric power and local politicians warn that reopening them will be politically difficult.
The key to bringing the reactors under control is the extent of damage to the plant's cooling system, said analysts.
The Sankei newspaper reported that the government and TEPCO were considering building new cooling systems for three reactors to operate from outside the reactor buildings.
Japan's fishermen, who are part of the politically powerful agricultural lobby, made clear they were not assuaged by assurances that ocean radioactivity levels were low and safe.
"From now on, our fishermen will never cooperate with or accept nuclear power generation. I would like them to stop even those reactors that are now in operation right away," Ikuhiro Hattori, chairman of the Japan Fisheries Cooperatives, told NHK state television.