MIDDLETOWN, PA. - Ventilation fans at a building at Three Mile Island nuclear plant caused Saturday afternoon's release of small amounts of radioactive material, Exelon Corp. said today.

The fans caused a change in air pressure inside the containment building at Unit 1 and dislodged small irradiated particles in the reactor piping system. The material escaped through a hole cut in the pipe during renovation work. The fans have since been modified to prevent the situation from reoccurring, Exelon, which owns the plant, said in a news release today.

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About 150 workers stationed in the building were sent home late Saturday when monitors detected the airborne contamination. Tests over the weekend confirmed that no employee received radiation doses above what they might normally see doing work in the building, the company said.

Exelon began calling those employees back to work yesterday, and as of this morning all have returned to their jobs at the plant 10 miles south of Harrisburg.

Unit 1 was shut down on Oct. 26 for refueling of the reactor and to replace its two steam generators.

"Things are back to normal," site vice president Bill Noll said. "We are back performing outage activities as we had originally planned."

No contamination was found outside the containment building, and the event never posed a threat to the public, the company reiterated today.

A partial meltdown occurred in Three Mile Island's Unit 2 reactor in March 1979. It was the nation's first nuclear accident.

Contact staff writer Mario F. Cattabiani at 717-787-5990 or mcattabiani@phillynews.com.