By Yoko Kubota and Chisa Fujioka
TOKYO (Reuters) - France and the United States will 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 humanitarian disaster, with 27,500 people dead or missing from a March 11 earthquake and tsunami.
New readings showed a spike in radioactive iodine in the sea off the plant to 3,355 times the legal limit, the state nuclear safety agency said on Wednesday, although it played down the impact, saying people had left the area and fishing had stopped.
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, a stance supported by the U.N. atomic agency.
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 pressure, criticized for safety lapses and a slow response. Its shares slid another 12.7 percent on Wednesday and are now down almost 75 percent since the quake, as the government considers nationalizing Asia's largest utility.
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 for his handling of the disaster and for not widening the exclusion zone beyond 20 km (12 miles) around Fukushima.
Kan said he was considering that step, which would force 130,000 people to move, in addition to 70,000 already displaced.
Authorities still have no idea when they will be able to stabilize the plant. "We are not in a situation where we can say we will have this under control by a certain period," Chief Cabinet Secretary Yukio Edano told a briefing.
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 and shipments of milk and some vegetables from areas nearby have been stopped.
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 U.S.-based Union of Concerned Scientists, a long-time nuclear watchdog group.
"They may have to make hard choices about releasing larger quantities of radiation to the environment ... There may not be any good choices."
Cabinet Secretary Edano said options being considered included transferring contaminated water to a ship and covering reactors to prevent radioactive particles escaping.
Nuclear plant operators might in future be forced to install backup power in case external power is knocked out, a nuclear safety agency official said.
(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)