By Yoko Kubota and Chisa Fujioka
TOKYO (Reuters) - Japan ordered an immediate safety upgrade at its 55 nuclear power plants on Wednesday in its first acknowledgement that standards were inadequate when an earthquake and tsunami wrecked one of the facilities nearly three weeks ago.
Adding to the evidence of radiation leakages around the crippled nuclear complex, 240 km (150 miles) north of Tokyo, readings showed radioactive iodine in the sea off the plant at record levels. The state nuclear safety agency said the amounts were 3,355 times the legal limit.
A Reuters investigation showed Japan and plant operator Tokyo Electric Power (TEPCO) repeatedly played down dangers at its nuclear plants and ignored warnings -- including a 2007 tsunami study from the utility's senior safety engineer.
The research paper concluded there was a roughly 10 percent chance that a tsunami could test or overrun the defences of the Fukushima Daiichi nuclear power plant within a 50-year span based on the most conservative assumptions.
The new safety steps, to be completed by the end of April, include preparing back-up power in case of loss of power supply, and having fire trucks with hoses ready at all times to intervene and ensure cooling systems for both reactors and pools of used fuel are maintained, the Trade Ministry said.
Other measures such as building higher protective sea walls would be studied after a full assessment of the Fukushima disaster, officials said.
The immediate measures do not necessarily require nuclear plant operations to be halted, Minister of Economy, Trade and Industry Banri Kaieda told a news conference.
"These are the minimum steps we can think of right now that should be done immediately," said Kaieda.
"We shouldn't wait until a so-called overhaul or a comprehensive revision -- something major that would take a long time -- is prepared. We should do whatever we can if and when there is something (which safety authorities agree is) viable and necessary," he said.
Before the disaster, Japan's nuclear reactors had provided about 30 percent of the nation's electric power. The percentage had been expected to rise to 50 percent by 2030, among the highest in the world.
The government and TEPCO conceded there was no end in sight to the world's worst atomic disaster since Chernobyl in 1986.
"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 news briefing.
The discovery of highly toxic plutonium in soil at the plant this week had already raised alarm over the disaster, which has overshadowed the humanitarian calamity triggered by the earthquake and tsunami, which left 27,500 people dead or missing.
Pollution of the ocean is also 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 contamination to fish and other marine life.
However, just how radiation is spilling into the ocean is unclear and controlling leakage from the plant could take weeks or months, making precise risk assessments difficult.
Tokyo Electric said it would take a "fairly long time" to stabilise overheating reactors, adding four of the six reactors would need to be decommissioned.
Meanwhile, the head of the company was in hospital due to high blood pressure, another sign of the disarray at Asia's largest utility.
Prime Minister Naoto Kan, whose government faces mounting criticism for its handling of the crisis, won assurances of American support in a telephone conversation on Wednesday with President Barack Obama.
The United States has already agreed to send some radiation-detecting robots to Japan to help explore the reactor cores and spent fuel pools at the stricken nuclear plant.
French President Nicolas Sarkozy, who chairs the G20 and G8 blocs of nations, is due 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 TEPCO.
DRAG ON ECONOMY
Hundreds of engineers have been toiling for nearly three weeks to cool the plant's reactors and avert a catastrophic meltdown of fuel rods, although the situation appears to have moved back from that nightmare scenario.
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 the 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 main stock index has fallen about 9 percent since the tsunami while TEPCO shares have fallen almost 80 percent. The government is considering a tax hike to pay for the damage it estimates at $300 billion (187 billion pounds) in what could be the world's costliest natural disaster.
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 also been found in tap water in Tokyo and in tiny traces abroad.
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 run-off 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."
RISING ANGER
Already criticised for weak leadership during Japan's worst crisis since World War Two, 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.
Hundreds of thousands 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 to rebuild.
About 175,000 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.
(Additional reporting by Elaine Lies, Mayumi Negishi, Leika Kihara and Yoko Nishikawa in Tokyo, Roberta Rampton , Ayesha Rascoe and Deborah Zabarenko in Washington, Eileen O'Grady in Houston; Writing by Jon Herskovitz; Editing by John Chalmers)
