Japan has ordered an immediate safety upgrade at its 55 nuclear power plants.
Nearly three weeks on from the 9.0 earthquake and tsunami that devastated north-east Japan, workers at the Fukushima Daiichi plant are battling to stabilise overheating reactors and control radiation spillage.
As well as radiation leakages around the nuclear complex, 240 km north of Tokyo, readings showed radioactive iodine in the sea off the plant at record levels as well as
The state nuclear safety agency said the amounts were 3,355 times the legal limit.
The new safety steps 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 this would be completed by the end of April.
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 said.
Before the disaster, Japan's 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.
"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 said.
The discovery of highly toxic plutonium in soil at the plant this week had already raised alarm over the disaster, while 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.
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.
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 disaster.
In comparison the 1995 Kobe quake cost $100 billion while Hurricane Katrina in 2005 caused $81 billion in damage.
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 the dilemma of needing to douse the reactors to prevent overheating, which 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 US-based Union of Concerned Scientists, a nuclear watchdog group.
"They may have to make hard choices about releasing larger quantities of radiation to the environment."