(Corrects paragraph 26 to show Kan defended his actions rather than apologised for them)
By Chizu Nomiyama and Kazunori Takada
TOKYO (Reuters) - Plutonium found in soil at the Fukushima nuclear complex heightened alarm on Tuesday over Japan's battle to contain the world's worst atomic crisis in 25 years, while opposition MPs attacked the prime minister for his handling of the disaster.
Some opposition lawmakers lambasted Naoto Kan in parliament for not extending an evacuation zone around the plant. Kan said he was seeking advice on widening the area, which would force 130,000 people to move in addition to 70,000 already displaced.
Plant operator Tokyo Electric Power Co said plutonium was found at low-risk levels in five places at the facility, which was crippled by a massive earthquake and tsunami on March 11.
A by-product of atomic reactions and also used in nuclear bombs, plutonium is highly carcinogenic and one of the most dangerous substances on the planet, experts say.
They believe 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.
Japan's Nuclear and Industrial Safety Agency said while the plutonium levels were not harmful to human health, the discovery could mean a breach in the reactor's containment mechanism.
"Plutonium is a substance that's emitted when the temperature is high, and it's also heavy and so does not leak out easily," agency deputy director Hidehiko Nishiyama told a news conference.
"So if plutonium has emerged from the reactor, that tells us something about the damage to the fuel. And if it has breached the original containment system, it underlines the gravity and seriousness of this accident."
Sakae Muto, a Tokyo Electric vice-president, said the traces of plutonium-238, 239 and 240 were in keeping with levels found in Japan in the past due to particles in the atmosphere from nuclear testing abroad.
"I apologize for making people worried," Muto said.
Workers at Fukushima may have to struggle for weeks or months under extremely dangerous conditions to re-start cooling systems vital to control the reactors and avert total meltdown.
On Monday, highly contaminated water was found in concrete tunnels extending beyond one reactor, while at the weekend radiation hit 100,000 times over normal in water inside another.
That poses a major dilemma for Tokyo Electric, which wants to douse the reactors to cool them, but not worsen the radiation spread, Chief Cabinet Secretary Yukio Edano said on Tuesday.
"On the issue of pumping in water, we must avoid a situation in which the temperature (of the fuel rods) rises and the water boils off. So this cooling is a priority. On the other hand, on the standing water, under the circumstances work must proceed to remove it as quickly as possible," he said.
Japan says a partial meltdown of fuel rods inside reactor No. 2 has contributed to the radiation levels.
The crisis, the worst nuclear accident since Chernobyl in 1986, has contaminated vegetables and milk from the area, as well as the surrounding sea. U.S. experts said groundwater, reservoirs and the sea all faced "significant contamination".
Facing a long and uncertain operation, Tokyo Electric has sought help from firms including Electricite de France SA and Areva SA, the French government said.
Japan is also consulting Washington. The head of the U.S. Nuclear Regulatory Commission, Gregory Jaczko, met Japanese officials in Tokyo on Monday.
"The unprecedented challenge before us remains serious and our best experts remain fully engaged to help Japan address the situation," Jaczko said in a statement.
Experts have said the lack of information and some inconsistent data makes it hard to understand what is happening at Fukushima, which appears to have moved from a core-meltdown phase to one in which management of released radioactivity is paramount.
"There's a lot of radioactivity outside the containment barriers that poses a threat to workers and the public that needs to be addressed," said David Lochbaum, director of the nuclear safety project at the U.S.-based Union of Concerned Scientists, a long-time nuclear watchdog group.
"That's the top priority."
NATIONALISATION FOR NUCLEAR OPERATOR?
The crisis has put enormous pressure on Tokyo Electric, criticized for safety lapses and a slow disaster response. Its boss, Masataka Shimizu, has barely been seen.
The government might discuss nationalizing Tokyo Electric to deal with the crisis, National Strategy Minister Koichiro Gemba said. Its shares have tumbled 70 percent since the crisis.
Kan, leading Japan during its worst crisis since World War Two, was already deeply unpopular and under pressure to resign before the events of March 11.
He repeatedly defended his decision to fly over the stricken nuclear site the day after the quake, saying it had been important to see the situation with his own eyes. His top spokesman on Monday denied the visit had delayed operations to cool the reactors, as media reports had said.
He also assured lawmakers the government was disclosing all the information it had.
More than 70,000 people have been evacuated from a largely rural area within 20 km (12 miles) of the facility.
But opposition MP Yosuke Isozaki blasted Kan for not ordering people living in a band between 20 km and 30 km (12-19 miles) from the plant to also leave, asking "is there anything as irresponsible as this?".
The 130,000 people inside that zone have been encouraged -- but not ordered -- to leave.
Environmental group Greenpeace has urged an extension of the 20-km evacuation zone while the United States has recommended its citizens who live within 80 km (50 miles) of the plant to leave or shelter indoors.
Even though Japan's culture stresses group efficiency over individual charisma, many are unhappy and a weekend poll showed a majority feel Kan has not shown good leadership.
"The characters involved are too weak to take decisive actions," said Jesper Koll, analyst at JP Morgan Securities.
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 human beings, who in any case face higher radiation doses on a daily basis from natural sources, X-rays or flying.
The drama at the six-reactor facility has compounded Japan's agony after the double disaster left more than 28,000 people dead or missing in the devastated northeast.
With towns 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.
(Additional reporting by Linda Sieg, Mayumi Negishi, Yoko Nishikawa, leika Kihara and Phil Smith in Tokyo, 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 Bill Tarrant; Editing by Dean Yates)