By Yoko Kubota and Chisa Fujioka
TOKYO (Reuters) - France and the United States are to help Japan in its battle to contain radiation from a crippled nuclear complex where plutonium finds have raised public alarm over the world's worst atomic crisis since Chernobyl in 1986.
The high-stakes operation at the Fukushima plant has added to Japan's unprecedented humanitarian disaster with 27,500 people dead or missing from a March 11 earthquake and tsunami.
French President Nicolas Sarkozy, who chairs the G20 and G8 blocs of nations, plans to visit Tokyo on Thursday. He will be the first foreign leader in Japan since the disaster.
In further support, France flew in two experts from its state-owned nuclear reactor maker Areva and its CEA nuclear research body to assist Japan's heavily criticized plant operator Tokyo Electric Power Co (TEPCO).
A global leader in the industry, France produces about 75 percent of its power from reactors so it has a strong interest in helping Japan get through the Fukushima disaster.
The United States is also weighing in to send some radiation-detecting robots to Japan to help explore the reactor cores and spent fuel pools, the Energy Department said.
With evidence mounting of radiation inside and beyond the plant, public fears rose a notch with Tuesday's announcement of plutonium traces in soil from five places within the facility.
A by-product of atomic reactions and a prime ingredient in nuclear bombs, plutonium is highly carcinogenic and one of the most dangerous substances on the planet, experts say.
Japan said, however, that only two of the plutonium traces had likely come from the plant, probably from overheating spent fuel rods or damage to reactor No. 3, with the others being particles in the atmosphere from past nuclear testing abroad.
The levels, of up to 0.54 becquerels per kg, were not considered harmful, Japanese officials said.
The U.N. atomic agency IAEA agreed. "Concentrations reported for both, plutonium-238 and plutonium-239/240, are similar to those deposited in Japan as a result of the testing of nuclear weapons," it said in its latest briefing.
First rattled by the earthquake and then engulfed by giant waves, the Fukushima plant resembles a bomb site, with steam and smoke occasionally rising from mangled pipes and twisted steel.
Plant operator TEPCO is under enormous pressure, criticized for safety lapses and a slow disaster response. Its shares are down almost 75 percent since the quake -- hitting a 47-year low on Tuesday -- and there is talk of a state takeover.
The government, too, is taking heat.
Already criticized for weak leadership during Japan's worst crisis since World War Two, Prime Minister Naoto Kan has been blasted by the opposition in parliament for his handling of the disaster and for not widening the exclusion zone beyond 20 km (12 miles) around Fukushima.
"Is there anything as irresponsible as this?" opposition legislator Yosuke Isozaki said on Tuesday.
Kan said he was considering that step, which would force 130,000 people to move, in addition to 70,000 already displaced.
There is rising despair among farmers and fishermen whose livelihoods have been turned upside down by the disaster.
One 64-year-old farmer hanged himself last week after saying "our vegetables are no good anymore", media said.
With entire towns on the northeast coast reduced to wastelands of mud and debris following the quake and tsunami, more than 175,000 people are living in shelters.
The event looks likely to be the world's costliest natural disaster, with estimates of damage topping $300 billion.
In a shock to high-tech Japanese whose economy is the world's third biggest, there has been electricity rationing after the disaster and 183,431 houses are still without power.
Workers at the Fukushima complex may have to struggle for weeks or months under extremely dangerous conditions to restart cooling systems vital to controlling the nuclear reactors.
More than a dozen workers have been injured at the plant, and they are said to be living in grim conditions, sleeping on the floor of a safe room when their shifts are over, and shoving packaged food down quickly to avoid contact with radiation.
At the site, highly tainted water has been found in some reactors and in concrete tunnels outside. Sea water has also showed radiation and shipments of milk and some vegetables from areas nearby have been stopped due to contamination.
Radiation has been found in tap water in Tokyo, 240 km (150 miles) to the south, and in tiny traces abroad.
Experts have said a lack of information and some inconsistent data made it hard to understand what was happening at Fukushima, which appears to have come back from the risk of a core meltdown -- the nightmare scenario -- to a situation where management of released radioactivity is paramount.
Engineers face a dilemma: they have to douse the reactors to prevent overheating, but that risks adding to the radiation problems by increasing water flows.
"If they need to increase cooling, it will increase runoff of highly contaminated water and they don't have any place to store it," said Edwin Lyman of the Union of Concerned Scientists.
"They may have to make hard choices about releasing larger quantities of radiation to the environment ... There may not be any good choices," he said.
While the government and many experts play down comparisons with Chernobyl, the radioactive substances being emitted are the same -- iodine-131, caesium-134 and caesium-137.
Some experts say contamination outside Fukushima remains at safe levels for humans, and point out the daily doses that people get unwittingly from X-rays, flights and natural means.
But anti-nuclear and environmental lobby groups accuse governments and academics, some with ties to the atomic industry, of glossing over the risks from Fukushima.
The crisis in Japan has sent ripples through the global economy, and disrupted supplies for the automobile and technology sectors. In the latest example, Toyota told North American dealers to curtail orders of replacement parts.
(Additional reporting by Elaine Lies, Chizu Nomiyama, Kazunori Takada, shinichi Saoshiro, Yoko Nishikawa in Tokyo, Roberta Rampton and Ayesha Rascoe in Washington, Eileen O'Grady in Houston; Writing by Andrew Cawthorne; Editing by Robert Birsel)