Japan is 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.

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).

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

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 kilometers (155 miles) to the southwest.