The water pumps were ferried to Yokota Air Force Base for use at the crippled Fukushima plant, with four additional pumps delivered from Sasebo, in Japan's southwest, the US Pacific Fleet said in a statement.

The Pentagon, which provided two fire trucks earlier to help Japanese teams combat a fire at a reactor, has not been asked by Tokyo authorities so far to directly join the effort to contain damage at Fukushima, a spokesman said.

"We would certainly provide assistance if asked," Colonel Dave Lapan told reporters.

US commanders meanwhile set up an "evacuation zone" of 80 kilometers (50 miles) around the Fukushima plant, with American air crews barred from venturing into the potentially radioactive area without specific orders, Lapan said.

Japanese authorities have set up a smaller zone of 20 kilometers and it was unclear why US authorities had declared a larger perimeter.

The country's devastating 9.0-magnitude earthquake Friday was part one of an unprecedented triple disaster, including a killer tsunami that pulverized Japan's northeastern coast and damaged atomic reactors which have spewed radiation into the air and sparked fears of a nuclear meltdown.

Cooling down the reactor fuel rods is vital to containing the disaster, and the US high-pressure pumps would likely be used for such a purpose.

The US military also decided to give out anti-radiation pills as a precaution to some air crews who were scheduled to fly within the "vicinity" of the crippled plant, Lapan said.

Testing earlier showed 17 US helicopter crew members from the USS Ronald Reagan aircraft carrier had been contaminated with low levels of radiation. Out of those 17 crew members, two were given potassium iodide pills, he said.

The carrier, off the east coast of Japan, deployed further north a few days ago to stay clear of radiation carried by winds.

The US Navy's 7th Fleet said 15 US ships had arrived in or were steaming toward Japanese waters for a massive aid operation, including the fleet's flagship USS Blue Ridge as well as an amphibious ready group carrying a Marine expeditionary unit.

The USS Essex was leading the three-ship amphibious group with a fleet of helicopters and about 2,200 Marines, who were ordered to the Sea of Japan to the west to avoid debris and radiation off the east coast.

"In the coming days they will take position off the coast of Sakata on the western coast of Honshu to begin conducting disaster response operations," the fleet said, noting that the west coast "affords greater access to undamaged ports and roads, fewer navigational hazards, and prevailing winds that are upwind of the Fukushima power plant."

The Marine expeditionary unit was due to help clean up debris at the military airport in Sendai, officials said.

As of Wednesday, US forces have delivered more than 129,000 pounds of water and 4,200 pounds of food, and carried out 125 flights with military planes -- including unmanned Global Hawk surveillance aircraft -- and 113 helicopter flights for relief efforts.

earlier related report
Fukushima: Possible contamination scenarios
Paris (AFP) March 16, 2011 - Following is a snapshot of scenarios for radioactive contamination from the crippled Fukushima plant in Japan.

AREA AFFECTED

Short term:

The Japanese government on Wednesday said radiation posed no immediate health threat outside a 20-kilometre (12-mile) exclusion zone established around the plant.

In Tokyo, the US embassy Wednesday warned American citizens living within 50 miles of Fukushima to evacuate or seek shelter.

Jacques Repussard, head of France's Institute for Radiological Protection and Nuclear Safety (IRSN), said there was a radioactive plume in a zone of "several dozen kilometres" around the plant.

It will extend over a zone of several hundred kilometres "in the coming days" but have no consequences for health in Tokyo, an agglomeration of 30 million people lying 250 kilometres to the southeast, he said.

Long-term:

Scenarios are coloured by many unknowns concerning the rate and type of radioactive release.

Repussard, who was speaking at a French parliamentary committee meeting, said there could eventually be a "strongly contaminated zone" extending up to 60 kilometres around Fukushima, beyond which "there will be measurable impacts but not dramatic impacts."

Didier Champion, who overseas environment and intervention issues at the IRSN, said Fukushima would probably generate "more local impacts than were seen for Chernobyl."

"The downside is that contamination is likely to be more concentrated inside a 10- to 20- kilometre zone. The upside is less contamination over a larger area," he said.

MAIN SOURCES OF CONTAMINATION

Longevity of highly toxic nuclear waste is a major problem.

The likely contaminants from Fukushima are iodine-131 and caesium-137, with half-lives of eight days and 30 years, respectively.

The half-life is the number of years required for any amount of a radioactive element to decompose by half. Typically, such elements remain hazardous for a period 10 times their half-lives.

Radioactive iodine and caesium are carcinogenic and pose a threat to health, whether directly in the polluted air or water or indirectly through the food chain.

Radioactive iodine is highly volatile and disperses easily into the air in the form of vapour plumes such as those generated by the disabled Fukushima reactors.

It becomes a hazard when it settles on water sources or crops, such as leafy vegetables, that are later ingested by livestock or humans.

Since Iodine-131 has a half-life of eight days, it decays completely within a matter of months.

Everyone is exposed to limited, and apparently harmless, levels of caesium-137 as a result of atmospheric testing of nuclear weapons during the 1950s and 1960s, although much of that contamination has since decayed.

The Chernobyl reactor blast also generated caesium pollution.

Tainted soil can become airborne as dust, resulting in exposure to internal organs when breathed in, according to the US Environmental Protection Agency. Drinking the element in contaminated water also exposes living tissue.

In plants, the element is first absorbed by leaves, then roots. It also concentrates easily in mushrooms and wild birds.

CLEAN-UP

Cleaning up radioactive sites is massively costly, time-consuming and dangerous.

Contaminated soil can be removed and buried in lined landfills, but doing so does not eliminate the waste. Incineration in special furnaces renders toxic elements harmless, but is expensive and technically challenging.

Soil can also be "washed" by mixing it with solvents that transform the contaminants into a liquid form that must then be disposed of.

Water can be filtered using synthesised cloth that traps solid particles, but the technique works well only with low levels of contamination.

Demineralisation through electrodialysis -- to remove radioactive ions -- has also proved effective, as has boiling water to separate out contaminants as solids.

There are several methods for sealing off failed reactors which continue to generate radiation long after they have been shut down.

A concrete and steel sarcophagus was built to entomb the Chernobyl reactor, but the makeshift structure now has cracks and is being replaced.

SOURCES: US Environmental Protection Agency, UN Food and Agriculture Organisation, French Institute for Radiological Protection and Nuclear Safety

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