The workers, including a health physicist and technicians, were wearing protective gear including booties, gloves and respirators, he said.

"Their initial readings detected elevated amounts of radiation when they opened the piping. They took some swipes and surveyed the swipes and also had air monitors," Rohrer said.

"As they were doing all of the above, they recognized the rad levels were higher than anticipated," he said. "They issued a 'stop work' order and put the pipe in a safe configuration."

Tests of nasal swipes, fecal and urine samples showed that three of the eight workers had been exposed to plutonium.

"The exposures are relatively low, but they're high enough that it concerns us," Rohrer said.

He said the workers probably inhaled plutonium either while they were disconnecting the pipe's cap or while they were taking off their protection equipment.

The regulatory standard for radiation workers is 5,000 millirem per year. The test site maintains an administrative guideline of not more than 500 millirem per year.

A millirem is one-thousandth of a rem. A rem is the unit of dose for ionizing radiation.

One of the exposed, a man, had an estimated internal dose equivalent to 100 millirem that could be expected as the small amount of plutonium slowly decays over 50 years.

The other two, received a 400 millirem "effective dose" equivalent.

In all three cases, the amount of plutonium that officials think they inhaled and the estimated doses were far smaller than allowed under guidelines for one year.

The "effective dose," Rohrer said, means for those amounts that it would take 50 years for the two workers with the highest exposure to approach the test site's one-year, administrative threshold.

"We are completing an investigation" to see whether any corrective actions are warranted, Rohrer said.

The two women who registered the highest exposures have not been released to go back into radiation areas, and the other six have been allowed to perform normal duties, he said. For privacy reasons, their names were not released.

A 1995 study by scientists at Lawrence Livermore National Laboratory, in California, that focused on terrorists using plutonium to contaminate a city's air and drinking water concluded that "plutonium is hazardous, but it is not as immediately hazardous to health as many more common chemicals."

The study continued: "This is not to say that plutonium is not a dangerous, toxic material. Chronic exposure to even small amounts should be a matter of concern."

The report downplayed the potential danger from a terrorist threat involving plutonium but said, "The severity of the radiation dose, and the organs that are irradiated, depend primarily on the quantity of plutonium taken into the body and on the route by which it enters the body.

"In general, plutonium that is inhaled is far more hazardous than plutonium that is ingested, because it is more readily absorbed into the blood stream via the lungs than via the gastrointestinal tract," the Lawrence Livermore report said.

A spokeswoman for the contractor, Gillian Silver, said, "Obviously we're going to monitor the lessons learned from this experience to make sure any occupational health issues are addressed.

"We're continuing our own internal investigation. We don't have any reason to believe there were any security or safety considerations here," she said.

Rohrer said the incident represented the first significant exposure to workers that he is aware of in the pipe plugging project, which is part of the test site's environmental restoration program.