By Laura Zuckerman

(Reuters) - Underground sensors have detected excessive radiation levels inside a nuclear waste storage site deep below New Mexico's desert, but no workers have been exposed and there was no risk to public health, U.S. Department of Energy officials said on Sunday.

An air-monitoring alarm went off at 11:30 p.m. local time Friday indicating unsafe concentrations of radiation inside the Waste Isolation Pilot Plant in what DOE officials said appeared to be the first such mishap since the facility opened in 1999.

As of Sunday, the source of the high radiation readings had yet to be determined, and a plan to send inspection teams below ground to investigate was put on hold as a precaution.

"They will not go in today. It's a safety thing more than anything. We're waiting until we get other assessments done before we authorize re-entry," DOE spokesman Bill Mackie said.

The facility, located in southeastern New Mexico near Carlsbad, is designed as a repository for so-called transuranic waste, which includes discarded machinery, clothing and other materials contaminated with plutonium or other radioisotopes heavier than uranium.

The waste, shipped in from other DOE nuclear laboratories and weapons sites around the country, is buried in underground salt formations that gradually close in around the disposal casks and seal them from the outside world.

No workers were underground when the apparent radiation leak was detected in the vicinity of the plant's waste-disposal platform, and none of the 139 employees working above ground at the time was exposed, the Energy Department said.

The alarm automatically switched the underground ventilation system to filtration to keep any releases from reaching the surface, DOE officials said.

Subsequent testing of surface air in and around the facility showed the incident posed no danger to human health or the environment, Mackie said.

Air-monitor alarms at the facility have been tripped in the past by malfunctions or fluctuations in levels of radon, a naturally occurring radioactive gas. But officials said they believe this to be the first real alarm since the plant began operations.

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Just a few dozen essential personnel, including security officers, remained at the site over the weekend.

Inbound waste shipments had already been suspended at the plant since a truck caught fire there earlier this month in an accident that left several workers suffering smoke inhalation.

"We're in shutdown mode," Mackie said.

The facility in the Chihuahuan Desert normally receives up to 6,000 cubic meters of radioactive waste a year and employs more than 800 workers. The site is expected to continue to accept radiological materials until 2030, Mackie said.

(Reporting by Laura Zuckerman; Additional reporting by Kevin Murphy; Editing by Steve Gorman and Eric Walsh)