The Pentagon was expected to announce that the aircraft carrier Ronald Reagan, which is sailing in the Pacific, passed through a radioactive cloud from stricken nuclear reactors in Japan, causing crew members on deck to receive a month’s worth of radiation in about an hour, government officials said Sunday.

The officials added that American helicopters flying missions about 60 miles north of the damaged reactors became coated with particulate radiation that had to be washed off.

There was no indication that any of the military personnel had experienced ill effects from the exposure. (Everyone is exposed to a small amount of natural background radiation.)

But the episodes showed that the prevailing winds were picking up radioactive material from crippled reactors in northeastern Japan. Ever since an earthquake struck Japan on Friday, the authorities worldwide have been laying plans to map where radioactive plumes might blow and determine what, if any, danger they could pose to people.