The U.S. nuclear regulatory authorities have sent a group of experts to investigate a small radiation leak at the infamous Three Mile Island nuclear power plant (NPP), the CNN reported.

Exelon Nuclear, the operator of the Three Mile Island NPP near Middletown, Pennsylvania, earlier said in a statement that an airborne radiological contamination alarm sounded on Saturday in the Unit 1 containment building.

The unit had been shut down since October 26 for refueling, maintenance and steam generator replacement.

"No contamination was found outside of the containment building. Radiological surveys showed that the contamination was confined to surfaces inside the containment building," the company said.

According to Exelon Nuclear, the incident posed no threat to public health and safety although 150 workers were immediately sent home and "checked for possible unusual radiation exposure."

The Three Mile Island NPP became the scene of the worst U.S. nuclear accident in history when the plant's Unit 2 experienced a partial meltdown in 1979 and was shut down permanently even though it led to no deaths or injuries to plant workers or members of the nearby community.

The accident brought about sweeping changes involving emergency response planning, reactor operator training, human factors engineering, radiation protection, and many other areas of nuclear power plant operations.

It also caused the U.S. Nuclear Regulatory Commission to tighten and heighten its regulatory oversight. Resultant changes in the nuclear power industry and at the NRC had the effect of enhancing safety.