* No US forces in Japan show signs of radiation poisoning
* Potassium iodide tablets given to some flight crews
* No-go zone far larger than one set by Japan (Adds drone flights, paragraph 10)
By Phil Stewart
WASHINGTON, March 16 The U.S. military on Wednesday ordered troops to stay at least 50 miles (80 km) away from a crippled Japanese nuclear power plant and started prescribing medication ahead of higher-risk missions amid growing concerns about radiation.
The Pentagon said Japan's escalating nuclear crisis would not stop its massive relief operation, which has seen 14 U.S. warships take position offshore to ferry food and water to survivors of last week's devastating earthquake and tsunami.
Still, there is anxiety at U.S. bases in Japan about exposure to radiation. The U.S. Navy has advised families on two U.S. bases to limit outdoor activities and shut off external ventilation after detecting higher-than-normal -- but still low -- doses of radiation.
On a Facebook page for U.S. Naval Forces Japan, some Americans voiced concern. One living in Atsugi, Japan, where radiation was detected at a naval base, asked about a potential evacuation.
"Having a toddler and being pregnant, I need to know if they can get us going," wrote 21-year-old Chelsea Origer.
Another woman, identifying herself as Melanie Cobos Lopez, responded: "You know they will wait (until) the last (minute). Just book a flight and keep them babies safe."
"Who knows what (they're) not telling us," she wrote.
Japan's nuclear crisis appeared to be spinning out of control on Wednesday after workers withdrew briefly from the stricken power plant because of surging radiation levels and a helicopter failed to drop water on the most troubled reactor.
In a sign of desperation, police will try to cool spent nuclear fuel at one of the facility's reactors with water cannon, normally used to quell riots.
The U.S. military gave Japanese forces firetrucks and water pumps, but stressed Americans will not operate them. It said unmanned "Global Hawk" drone aircraft would gather surveillance data of "industrial sites," presumably including the plant.
Pentagon spokesman Colonel Dave Lapan said U.S. military personnel and their families would not be allowed within 50 miles of the plant, an area much larger than the evacuation zone of 12 miles (20 km) set by Japan's government.
Japan has asked people living between 12 miles and 18 miles (30 km) to stay indoors to guard against dangerous doses of radiation.
Still, Lapan said, the larger no-go zone for the U.S. military was not set in stone. Exceptions could be made if necessary to carry out a mission.
Asked whether the U.S. military might need to seek volunteers to go into the danger zone -- instead of just ordering them in normally -- given the potential radiological hazards, Lapan said: "No. But again, we're talking about the United States military."
"We train and equip all of our people to operate in all kinds of environments. So we know how to measure (radiation), we know how to test. We know how to respond. We know how to take precautions," Lapan said.
The Pentagon said some U.S. air crews started preventatively taking potassium iodide tablets on missions that were within 70 miles (110 km) of the power plant as a way to guard against effects of radiation.
Potassium iodide can saturate the thyroid gland and prevent the uptake of radioactive iodine. When given before or shortly after exposure, it can reduce risk of cancer in the long term.
No U.S. forces have shown signs of radiation poisoning, however, the Pentagon said.
Still, the U.S. Navy said more than 20 U.S. flight crews have been exposed to higher-than-normal levels of radiation requiring that they discard old clothing, scrub with soap and water or even take potassium iodide.
When flying relief missions, they are told to keep the helicopter windows shut and the sleeves of their flight suits rolled down and wear gloves and boots to minimize exposure.
"All of our crews that are out there (closer to the plant) are going to be exposed to some level of radiation. These are very low, manageable levels," said Lieutenant Anthony Falvo at the U.S. Navy's Seventh Fleet. (Editing by Mohammad Zargham)