TOKYO (Reuters) - Japan called on the world not to impose "unjustifiable" import curbs on its goods as French President Nicolas Sarkozy was due to arrive on Thursday, the first leader to visit since an earthquake and tsunami damaged a nuclear plant, sparking the worst nuclear crisis since Chernobyl in 1986.
In a briefing to the World Trade Organization (WTO), Japan said it was monitoring radioactive contamination to prevent potential food safety risks and would provide the WTO with quick and precise information.
"In return, Japan asked members not to overreact," said a WTO official.
Several countries have banned milk and produce from the areas near the damaged Fukushima Daiichi nuclear plant, 240 km (150 miles) north of Tokyo, due to contamination fears. Japan has itself stopped exports of vegetables and milk from near the plant, which is leaking radiation.
As radiation fears linger, Singapore has told the U.N. nuclear watchdog that some cabbages imported from Japan had radiation levels up to nine times the levels recommended for international trade.
While food makes up only 1 percent of Japan's exports, the tsunami-crippled nuclear plant poses a serious risk to an economy burdened with huge public debt, an aging population and a big bill for rebuilding, possibly topping $300 billion.
Radioactive iodine in the sea off the damaged plant has hit record levels. The state nuclear safety agency said the amounts were 3,355 times the legal limit and highly toxic plutonium has been detected in the soil at the Fukushima Daiichi plant.
As operators struggle to regain control of the damaged reactors three weeks after the quake and tsunami, smoke was reported to be coming from a second damaged nuclear plant nearby on Wednesday, with authorities saying an electric distribution board powering a water pump was the problem.
The Daini plant several miles from the stricken Daiichi facility has been put into cold shutdown. Nuclear plant operator Tokyo Electric Power (TEPCO) said the incident would not cause any radiation effect externally.
The government has set up an evacuation area around the Daiichi plant with a 20-km (12-mile) radius and most of 70,000 people who live there are believed to have left. Another 130,000 people are within a zone extending a further 10 km in which residents are recommended to stay indoors.
But the International Atomic Energy Agency said radiation measured at a village 40 km from the nuclear complex exceeded a criterion for evacuation.
French President Nicolas Sarkozy, who chairs the G20 and G8 blocs of nations, will meet Prime Minister Naoto Khan to express support for Japan's efforts to end its nuclear crisis and rebuild after the quake and tsunami, which have left more than 27,500 people dead or missing.
France, the world's most nuclear-dependent country, has already flown in experts from its state-owned nuclear reactor maker Areva.
"The problem which worries TEPCO is water, so we are trying to see, because they are specialists in the treatment of radioactive waste, what they could advocate," said Areva Chief Executive Anne Lauvergeon, who arrived in Japan on Wednesday.
Japan has ordered an immediate safety upgrade at its 55 nuclear power plants, its first acknowledgement that standards were inadequate.
A Reuters investigation showed Japan and TEPCO repeatedly played down dangers at its nuclear plants and ignored warnings, including a 2007 tsunami study from the utility's senior safety engineer.
Nuclear plants will now be required by mid-April to deploy back-up mobile power generators and fire trucks able to pump water, while beefing up training programs and manuals.
Longer-term solutions such as higher sea walls at nuclear stations will be considered and Japan will review policy to encourage renewable energy.
Anger at Japan's nuclear crisis saw more than 100 people protest outside the Tokyo headquarters of TEPCO.
"We don't want to use electric power that can kill people," said Waseda University student Mina Umeda.
But the Japanese government says nuclear power will remain an integral supplier of power. Before the disaster, Japan's nuclear reactors provided about 30 percent of the country's electric power. That had been expected to rise to 50 percent by 2030, among the highest in the world.
TEPCO said it was inevitable it would have to scrap four of its six reactors at the Fukushima Daiichi plant.
But even just scrapping the damaged nuclear reactors may take decades , said Hidehiko Nishiyama, deputy director-general of the Nuclear and Industry Safety Agency (NISA).
"Even if they decide on scrapping the reactors, water spraying needs to be continued to prevent the fuels from overheating, and a sustainable cooling system needs to be established," he said.
"It would be 10 to 20 years before the scrapping process runs its course."
Jesper Koll, director of equity research at JPMorgan Securities in Tokyo, said a drawn-out battle to bring the plant under control and manage the radioactivity being released would perpetuate uncertainty and act as a drag on the economy.
"The worst-case scenario is that this drags on not one month or two months or six months, but for two years, or indefinitely," he said. "Japan will be bypassed. That is the real nightmare scenario."
Japan's Nikkei index has slid about 9 percent since the tsunami while TEPCO shares have fallen almost 80 percent.
TEPCO will test sprinkling synthetic resin in some areas of the Daiichi complex to prevent radioactive dust from flying into the air or being washed into the ocean by rain. The resin is water-soluble, but when the water evaporates, it becomes sticky and contains the dust.
"Radioactive dust from the hydrogen explosions in reactors No. 1 and 3 has drifted and is stuck on debris from the tsunami," said NISA's Nishiyama.
"We need to prevent that from spewing out into the sea along with the rain or from drifting away in the air."
Pollution of the ocean is a serious concern for a country where fish is central to the diet. Experts say the vastness of the ocean and a powerful current should dilute high levels of radiation, limiting the danger of marine contamination.
Hundreds of thousands of Japanese whose homes and livelihoods were wiped away by the tsunami that obliterated cities on the northeast coast have heard next to nothing from the government about whether it will help them rebuild.
About 173,600 were living in shelters on high ground above the vast plains of mud-covered debris, with temporary housing for only a few hundred currently under construction.