Japan was Wednesday considering plans to drape shattered nuclear reactor buildings with special covers to limit radiation, and pump contaminated water into a tanker.  

The embattled nation, reeling from the triple calamity of a massive earthquake, tsunami and a crippled atomic power plant, was also inviting foreign experts to help stabilize the overheating Fukushima station.

In a file picture taken on March 23, 2011 Norio Tsuzumi (C-standing), vice president of Tokyo Electric Power Company,  and employees bow their heads to apologize to evacuees at a shelter in Fukushima prefecture

The United States has lent Japan robots of a model battle-tested in Iraq and Afghanistan that can crawl through, film and clear rubble in the blast-hit reactor buildings which humans can't enter because of very high radiation. 

And France, which relies on nuclear power for three-quarters of its domestic energy needs, was sending an expert team from Areva, its state-run reactor maker, to assist embattled operator Tokyo Electric Power Company (TEPCO). 

The strain of the crisis appeared to have taken a toll on TEPCO's president Masataka Shimizu, 66, who was hospitalized Tuesday evening with high blood pressure and dizziness, having not appeared in public for over two weeks. 

The company has seen its share price plunge by three-quarters and faced heavy criticism, most recently over news that it ignored expert warnings on the threat of a tsunami before a giant wave crashed into the plant on March 11.  

As Japan works to contain the world's worst nuclear disaster since Chernobyl in 1986, an official with the nuclear safety agency said Wednesday it was time to think outside the box. 

"We are in an unprecedented situation, so we need to think about different strategies, beyond what we normally think about," the official told AFP. 

Japan faces a dilemma in containing the nuclear crisis: it must pump water into reactors to stop them from overheating, even as highly radioactive runoff leaks out, halting crucial repair work and threatening the environment. 

Iodine-131 detected in Pacific Ocean water near the plant site surged to a new high of 3,355 times the legal limit, officials said - compared to the previous top level of 1,850 times the legal maximum taken days ago. 

"The figures are rising further," said nuclear safety agency spokesman Hidehiko Nishiyama. "We need to find out as quickly as possible the cause and stop them from rising any higher."

Radioactive steam has also wafted into the air, contaminating regional farm and dairy produce, and last week led to elevated iodine levels in drinking water in megacity Tokyo, 250 kilometres (155 milles) to the southwest. 

Japanese authorities have repeatedly stressed that none of the affected food products or water currently presents an immediate threat to human health, and that ocean currents will dilute radioactivity in the sea.

Among the options Japan's government is now considering is covering three badly damaged outer reactor buildings with special fabric caps and fitting air filters to limit radiation, the Asahi Shimbun reported.

Another plan is to anchor an empty tanker off reactor two, so that workers can pump several Olympic swimming pools worth of highly-radioactive runoff water into its hull, the daily said, citing unnamed government officials.

While the government did not explicitly confirm the report, Chief Cabinet Secretary Yukio Edano said Japan's leadership and nuclear experts were discussing "every possibility, including those mentioned in the press". 

With crucial control room functions still disabled, experts are not sure what exactly is happening inside the reactors - and some international experts have issued dire warnings that a meltdown may already be in progress. 

One of them is Richard Lahey, who was head of safety research for boiling-water reactors at General Electric when it installed the Fukushima units, and who was quoted by Britain's Guardian newspaper.

Available reactor and radiation data from the specially troubled unit two "suggest that the core has melted through the bottom of the pressure vessel" and onto the concrete floor, he was quoted as saying by the daily. "I hope I am wrong, but that is certainly what the evidence is pointing towards." 

In Washington, acting US assistant secretary in the Department of Energy's Office of Nuclear Energy, Peter Lyons, told senators Tuesday that "current information suggests that the plants are in a slow recovery".

"However, long-term cooling of the reactors and pools is essential during this period and has not been adequately restored to date, to the best of my knowledge. A massive cleanup operation obviously remains for the future."