LONDONDERRY TOWNSHIP, PENNSYLVANIA (BNO NEWS) — More than one-hundred employees working inside the shutdown Three Mile Island Unit 1 containment building were sent home on Saturday after a small amount of contamination was found inside, officials said on Sunday.

About 150 employees were inside when the airborne radiological contamination alarm went off around 4 p.m. EST on Saturday.

The unit of the nuclear power plant in central Pennsylvania has been shut down since October 26 for refueling, maintenance and a steam generator replacement.

“No contamination was found outside of the containment building,” said Beth Archer, a spokeswoman for Exelon Nuclear. “Radiological surveys showed that the contamination was confirmed to surfaces inside the containment building.”

Archer says there was never a threat to public health and safety, and said there was no leak.

The spokeswoman said that a monitor showed a slight increase in a reading when new steam generators were being moved inside. The reading returned to normal quickly while two other monitors did not show any abnormal readings.

“Workers were sent home because work couldn’t continue until we clean the area,” Exelon Vice President Bill Noll, the senior executive at Three Mile Island, said. “We hope to be able to resume activities tomorrow.”

Exelon technicians on Saturday night were checking employees who had been working in the building at the time for possible unusual radiation exposure. One worker was found to have received 16 millirem of exposure, and other workers received contamination of lower levels.

The annual occupational dose limit for nuclear workers at Exelon Nuclear plants is 2,000 millirem and did not exceed personal contamination limits imposed by the Nuclear Regulatory Commission.

Numerous work activities were going on in the building at the time the alarm sounded, and Exelon engineers are working to determine the cause the incident. The NRC, Pennsylvania Department of Environmental Protection and local elected officials were notified of the event.

Exelon Corp. is one of the nation’s largest electric utilities.

The incident is being investigated.