TOKYO 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 at 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," said its latest briefing.
First rattled by the earthquake and then engulfed by a giant wave, 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 II, Prime Minister Naoto Kan was blasted by the opposition in parliament on Tuesday for his handling of the disaster and for not widening the exclusion zone beyond the current 20 km (12 miles) round Fukushima.
"Is there anything as irresponsible as this?" opposition legislator Yosuke Isozaki said.
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," local 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.
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.
Richard Wakeford, an expert at Britain's University of Manchester, said negligible plutonium levels were a "side issue" and the focus must remain on checking for iodine and caesium.
Former British government chief scientific adviser David King said there was an overreaction to events in Japan.
"As far as we know not one person has died from radiation" at Fukushima, he said. "Let's put this in context. In the same week 30 coal miners died...Is there a safer form of electricity production historically than nuclear power? The answer is no."
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 and Kazunori Takada in Tokyo, Roberta Rampton and Ayesha Rascoe in Washington, Kate Kelland and Daniel Fineren in London; Writing by Andrew Cawthorne)