TOKYO (Reuters) - Plutonium found in soil at the crippled Fukushima nuclear complex heightened alarm on Tuesday over Japan's lengthy battle to contain the world's worst atomic crisis in 25 years.
Plant operator Tokyo Electric Power Co (TEPCO) said the radioactive material, which is used in nuclear bombs, was traced in soil at five locations at the complex, hit by an earthquake and tsunami on March 11.
The drama at the six-reactor facility has compounded the Asian nation's agony after the natural disasters killed at least 11,000 people, left more than 17,000 missing, and made about a quarter of a million people homeless in the devastated north.
In a briefing around midnight in Tokyo, the vice-president of under-pressure TEPCO apologised for making people even more worried with the announcement -- but stressed the traces of plutonium 238, 239 and 240 were not dangerous.
"Plutonium found this time is at a similar level seen in soil in a regular environment and it's not at the level that's harmful to human health," Sakae Muto told reporters.
Muto said the readings were similar to those found in the past in other parts of Japan due to particles carried in the atmosphere after 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 as well as from the atmosphere, plutonium may have come from spent fuel rods at Fukushima or damage to reactor No. 3, the only one to use plutonium fuel.
The United Nations' nuclear watchdog, IAEA, said the find was to be expected. "It means that there is degradation of the fuel, which is not news," senior IAEA official Denis Flory said.
Japan's own nuclear safety agency was concerned at the samples which ranged from 0.18-0.54 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 Fukushima where engineers are resigned to a protracted struggle of weeks or possibly months to reestablish cooling systems vital to control the reactors.
On Monday, TEPCO said highly reactive water showing 1,000 millisieverts per hour had been found in concrete tunnels that extend beyond one reactor.
Fires, explosions and radiation leaks have forced engineers to suspend work at times, including at the weekend when radiation levels 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 for the high levels.
With Japan's towns and villages in the northeast coast reduced to apocalyptic landscapes of mud and debris, the cost of damage could top $300 billion, making it the world's costliest natural disaster.
Greenpeace said its experts had confirmed radiation levels of up to 10 microsieverts per hour in the village of Iitate, 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.
It urged the government to acknowledge the danger and "stop choosing politics over science".
More than 70,000 people have been evacuated from an area within 20 km (12 miles) of the plant and 130,000 people within a zone extending a further 10 km are advised to stay indoors. They have been encouraged to leave.
Beyond the evacuation zone, traces of radiation have been found in tap water in Tokyo and as far away as Iceland.
Japanese officials and international experts have generally said the levels away from the plant were not dangerous for people, who in any case face higher radiation doses on a daily basis from natural sources, X-rays or flying.
TEPCO, which has conceded it faces a long and uncertain operation to contain the crisis, sought outside help from French firms including Electricite de France SA and Areva SA, a French government minister said.
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.
He said the risk of a core meltdown had receded, and hopefully water pumps would be restarted in the next couple of weeks to get the reactors into a safe "cold shutdown" stage.
"From what I've seen so far, what has been released so far would not require the exclusion zone to become permanent. I don't think that's realistic. The eventual clean-up still looks like it should be easily manageable," he added.
(Additional reporting by Sylvia Westall in Vienna, David Sheppard in New York, Eileen O'Grady in Houston, Alister Doyle in Houston; Writing by Andrew Cawthorne)