Tokyo: Japan said it would upgrade its safety standards for nuclear power plants on Wednesday, its first acknowledgement that norms were insufficient when a tsunami wrecked one of its facilities, triggering the world's worst atomic disaster since Chernobyl in 1986.

The announcement came as the government conceded that there was no end in sight to the crisis and a spike in radioactive iodine levels in seawater added to evidence of reactor leakages around the complex and beyond.

The operator of a stricken nuclear power station in Japan said Wednesday that the plant's four troubled reactors would have to be decommissioned.  Tsunehisa Katsumata, chairman of Tokyo Electric Power Co (Tepco), said he was very sorry about causing concern and inconvenience to society due to the fires, explosions and radiation leaks from the plant. 

Plutonium finds in soil at the plant this week had already have raised public alarm over the accident, which has overshadowed the humanitarian calamity triggered by an earthquake and tsunami on March 11 that left 27,500 people dead or missing and hundreds of thousands homeless.

 "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 plant's operator, Tokyo Electric Power (Tepco) , said it would take a "fairly long time" to stabilise the overheating reactors. Meanwhile, the head of the company was in hospital due to high blood pressure, amid concerns his utility may collapse under the strain of paying for the disaster.

The Trade Ministry, which oversees nuclear safety, said in a statement comprehensive rules would be drawn up for power plant operators in light of the accident that ripped apart the facility in Fukushima, 240 km north of Tokyo.

Before the disaster, Japan's 55 nuclear reactors had provided about 30 per cent of the nation's electric power. The percentage had been expected to rise to 50 per cent by 2030, among the highest in the world.

A review of company and regulatory records by Reuters showed that Japan and its largest utility repeatedly downplayed dangers at its nuclear power plants and ignored warnings - including a 2007 tsunami study from Tokyo Electric's senior safety engineer.

The research paper concluded there was a roughly 10 per cent chance that a tsunami could test or overrun the defenses of the Fukushima Daiichi nuclear power plant within a 50-year span based on the most conservative assumptions. 
Iodine in sea 3,355 times legal limit

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, although it played down the impact, saying people had left the area and fishing had stopped.

Pollution of the ocean is a 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.

Prime Minister Naoto Kan, whose government faces mounting criticism for its handling of the disaster, 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. 
Plutonium find

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 per cent since the tsunami while Tepco shares have fallen almost 80 per cent. The government is considering a tax hike to pay for the damage it estimates at $300 billion in what could be the world's costliest natural disasters.

Public fears rose a notch on Tuesday with the announcement that plutonium traces had been found 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 the levels, of up to 0.54 becquerels per kg, were not considered harmful, a stance supported by the UN atomic agency.