Japan to scrap stricken nuclear reactors

Japan is to decommission four stricken reactors at the quake-hit Fukushima nuclear plant, the operator says.

Tokyo Electric Power (Tepco) made the announcement three weeks after failing to bring reactors 1 - 4 under control.

Locals would be consulted on reactors 5 and 6 which were shut down safely. Harmful levels of radioactivity have been detected in the area.

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

The embattled nation, reeling from the triple calamity of a huge earthquake, a massive tsunami and a crippled atomic power plant, has yet to stabilise the overheating Fukushima facility.

"Every possibility" was under discussion as Japan struggled to contain the world's worst nuclear disaster since Chernobyl a quarter of a century ago, Cabinet spokesman Yukio Edano said, reacting to reports of new measures.

These included covering three badly damaged outer reactor buildings with special fabric caps and fitting air filters to limit radiation, the Asahi Shimbun reported.

Another plan was 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.

"We are in an unprecedented situation, so we need to think about different strategies, beyond what we normally think about," an official with the nuclear safety agency told AFP, without detailing specific plans.

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

US President Barack Obama vowed continued help as he talked to Japanese Prime Minister Naoto Kan for the third time since the disaster, which has left almost 11,000 confirmed dead and 17,000 listed as missing.

"The president reiterated that the United States is determined to support the people of Japan in their efforts to deal with the devastating effects of this tragedy, both in the short and the long term," the White House said.

France, which relies on nuclear power for three-quarters of its 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 hospitalised 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.

TEPCO chairman Tsunehisa Katsumata said he saw little chance that the four stricken reactors in the six-reactor complex could ever resume operations.

"Looking at the reactors number 1 to 4 from an objective point of view, I think we will have to close them," he told a press conference.

For now 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 miles) to the southwest.

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

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 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."

Greenpeace radiation expert Jan van de Putte urged the Japanese government to take more aggressive measures to evacuate people near the plant, warning of long-term consequences for public health.

"Exposing a large number of people to this level of radiation creates a collective risk which is very significant over a long term, in terms of years. Our main concern is an increased incidence of cancer," he told reporters.