Investigators were taking samples Sunday at a New Mexico underground nuclear waste site where airborne radiation was detected, though they stressed no contamination had been found.
Officials monitoring the possible radiation leak said there was no danger to people or the environment at the the Department of Energy's Waste Isolation Pilot Plant, the nation's first repository to seal away radioactive waste, mostly plutonium, used for defense research and the production of nuclear weapons.
The waste is dumped 2,150 feet underground in disposal rooms excavated in an ancient, stable salt formation.
No employees were underground when the continuous air monitor at WIPP near Carlsbad, N.M., detected underground radiation at 11:30 p.m. Friday, an agency statement said.
It said no staffer was found to be contaminated by the radiation.
Employees on the surface were told to shelter in place as a precautionary measure and were cleared to leave the site starting at 5 p.m. Saturday.
"We are continuing to monitor and we are emphasizing that there is no danger to human health and the environment," WIPP spokeswoman Deb Gill told AFP.
The agency stressed that "no contamination has been found on any equipment, personnel or facilities."
As soon as the airborne radiation was detected underground at the site where the US government dumps low-grade nuclear waste, WIPP's ventilation system automatically switched to filtration mode in order to prevent air exchange with the surface.
The site was shut down and not performing active operations at the time, according to Gill.
Earlier this month, an underground blaze prompted the evacuation of a different part of the site, after a truck hauling salt caught fire. Several workers suffered smoke inhalation.
But officials said the blaze was nowhere near radioactive material.