Unusually high levels of radioactive particles were found at an underground nuclear waste site in New Mexico on Saturday in what a spokesman said looked like the first real alarm since the plant opened in 1999.
U.S. officials were testing for radiation in air samples at the site where radioactive waste, such as plutonium used in defense research and nuclear weapon making, is dumped half a mile below ground in an ancient salt formation.
"They (air monitors) have alarmed in the past as a false positive because of malfunctions, or because of fluctuations in levels of radon (a naturally occurring radioactive gas)," Department of Energy spokesman Roger Nelson said.
"But I believe it's safe to say we've never seen a level like we are seeing. We just don't know if it's a real event, but it looks like one," he said.
It was not yet clear what caused the air-monitoring system to indicate that radioactive particles were present at unsafe levels, Nelson said.