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TOKYO (Reuters) - Plutonium found in soil at the crippled Fukushima nuclear complex heightened alarm on Tuesday over Japan's protracted battle to contain the world's worst atomic crisis in 25 years.
Tokyo Electric Power Co (TEPCO) said the radioactive material -- a by-product of atomic reactions and also used in nuclear bombs -- had been found in soil in five places at the plant, hit by an earthquake and tsunami on March 11.
The drama at the six-reactor facility has compounded Japan's agony after the twin disaster left more than 28,000 people dead or missing in the devastated northeast.
"I apologize for making people worried," Sakae Muto, vice-president of under-pressure TEPCO, said at a briefing around midnight in Tokyo. Yet he stressed the traces of plutonium-238, 239 and 240 were not dangerous and work would not be stopped.
"It's not at the level that's harmful to human health."
Muto said the readings were similar to those found in the past in other parts of Japan due to particles in the atmosphere from nuclear testing abroad.
TEPCO said it was unclear where the plutonium was from, though it appeared two of the five finds were related to damage from the plant rather than from the atmosphere.
Experts believe that at least some of the plutonium may have come from spent fuel rods at Fukushima or damage to reactor No. 3, the only one to use plutonium in its fuel mix.
The United Nations' International Atomic Energy Agency (IAEA), said the find was expected due to known fuel degradation.
Japan's own nuclear safety agency was concerned at the plutonium samples, whose levels of radioactive decay ranged from 0.18 to 0.54 becquerels per kg.
"While it's not the level harmful to human health, I am not optimistic. This means the containment mechanism is being breached so I think the situation is worrisome," agency official Hidehiko Nishiyama was quoted as saying by Jiji news agency.
The plutonium discovery, from samples taken a week ago, was the latest bad news from the Fukushima plant, where engineers are resigned to a struggle of weeks or possibly months to re-establish cooling systems vital to control of the reactors.
On Monday, TEPCO said highly radioactive water had been found in concrete tunnels that extend beyond one reactor.
Fires, blasts and leaks have forced engineers to stop work at times, including at the weekend when radiation spiked to 100,000 times above normal in water inside reactor No. 2.
Chief Cabinet Secretary Yukio Edano said a partial meltdown of fuel rods inside the reactor vessel was responsible.
The crisis, the worst nuclear accident since Chernobyl in 1986, has already contaminated vegetables and milk produced in the area, as well as the surrounding sea.
With Japan's towns and villages on the northeast coast reduced to apocalyptic landscapes of mud and debris, more than a quarter of a million people are homeless. The event may be the world's costliest natural disaster, with estimates of damage topping $300 billion.
The environmental group Greenpeace said its experts had confirmed dangerous radiation of up to 10 microsieverts per hour in Iitate village, 40 km (25 miles) northwest of the plant.
It called for the extension of a 20-km (12-mile) evacuation zone. "It is clearly not safe for people to remain in Iitate, especially children and pregnant women, when it could mean receiving the maximum allowed annual dose of radiation in only a few days," Greenpeace said in a statement.
In most countries the maximum permissible annual dose for radiation workers is 50 millisieverts, or 50,000 microsieverts, according to the World Nuclear Association Industry body.
Greenpeace urged Tokyo to "stop choosing politics over science."
Tens of thousands of people have been evacuated from a 20 km (12 mile) radius around the plant and people within a radius extending a further 10 km have been told by the government to stay indoors or, better still, leave too.
Beyond the evacuation zone, traces of radiation have been found in tap water in Tokyo and as far away as Iceland.
The U.S. government said tiny traces of radiation believed to be from Japan were being detected by sensors round the nation, but were "far below levels of public health concern."
Japanese officials and international experts have generally said the levels away from the plant were not dangerous for human beings, who in any case face higher radiation doses on a daily basis from natural sources, X-rays or flying.
Facing a long and uncertain operation, TEPCO sought outside help from firms including Electricite de France SA and Areva SA, a French government minister said.
Japan is also seeking American help, said Robin Grimes, head of the center for nuclear engineering at Imperial College in London. "Each country has its own expertise."
Mark Prelas, a professor of nuclear engineering at University of Missouri in the United States, warned against overreaction to the fast-moving events in Japan.
"It's worth remembering that millions of Americans in the west of the country would have been exposed to tiny traces of plutonium during the above-ground bomb tests prior to the 1960s," he said.
The risk of a core meltdown had receded, he added, and hopefully water pumps would be restarted in the next couple of weeks to get the reactors into a safe "cold shutdown" stage.
"The eventual clean-up still looks like it should be easily manageable," he added.
The crisis has put huge pressure on TEPCO, which has been criticized for safety lapses at the plant and a slow response to the disaster. Its boss Masataka Shimizu has barely been seen.
Prime Minister Naoto Kan, leading Japan during its worst crisis since World War Two, has been similarly low-profile.
Even though Japan's culture stresses group efficiency over individual charisma, many are unhappy and a weekend poll suggested two-thirds of people felt Kan was not showing leadership.
"The characters involved are too weak to take decisive actions," said Jesper Koll, analyst at JP Morgan Securities.
Additional reporting by Shravya Jain in Bangalore, Timothy Gardner in Washington, Sylvia Westall in Vienna, David Sheppard in New York, Eileen O'Grady in Houston, Alister Doyle in Oslo, Deborah Zabarenko in Washington; Writing by Andrew Cawthorne; Editing by Kevin Liffey