( CNN ) -- Authorities at Pennsylvania 's Three Mile Island nuclear plant were investigating what caused a weekend radiation leak that resulted in 150 workers being sent home , officials said Sunday . An airborne radiological contamination alarm sounded about 4 p.m. Saturday in the Unit 1 containment building , according to a statement from Exelon Nuclear , which operates the Three Mile Island plant near Middletown , Pennsylvania . The unit had been shut down since October 26 for refueling , maintenance and steam generator replacement , the company said . `` A monitor at the temporary opening cut into the containment building wall to allow the new steam generators to be moved inside showed a slight increase in a reading and then returned to normal , '' the company said . `` Two other monitors displayed normal readings . '' Three Mile Island was the scene of the worst U.S. nuclear accident , a partial meltdown in 1979 that resulted in the plant 's second reactor being shut down permanently . Tests showed the contamination in Saturday 's incident was confined to the building itself , and none was found outside , Exelon said . There was no threat to public health and safety , but the workers were sent home because they could not continue until the area was cleaned , Bill Noll , Exelon vice president , said in the Saturday statement . One worker was found to have received 16 millirem of exposure , and others received lower levels of contamination . The annual occupational dose limit for workers at Exelon plants is 2,000 millirem , the statement said . Radiation exposure from a chest X-ray is about 6 millirem , according to the Nuclear Regulatory Commission . Although Noll said it was hoped work would resume at Unit 1 on Sunday , Exelon spokeswoman Beth Archer told CNN on Sunday it had not resumed , as the cause of the leak was still under investigation . Two radiation specialists from the NRC were scheduled to investigate Sunday . `` Numerous work activities were going on in the building at the time the alarm sounded , and Exelon engineers are working to determine the cause of the incident , '' the statement said . The March 1979 accident at Three Mile Island brought the nuclear industry to a standstill . The partial meltdown of Unit 2 's nuclear core resulted in no injuries to plant workers or nearby community members , but it triggered changes in nuclear power plant operations and emergency planning and led to tighter oversight of the industry by the NRC . The Unit 2 reactor is permanently shut down and defueled , the NRC said . In 2001 , FirstEnergy acquired it and contracted its monitoring to Exelon , which owns Unit 1 . The companies plan to keep Unit 2 in `` long-term , monitored storage '' until the Unit 1 operating license expires , at which time both plants will be decommissioned , the NRC said . A new generation of nuclear reactors is being considered in the United States as environmental concerns have intensified about coal-fired power plants . CNN 's Janet DiGiacomo contributed to this report .
Airborne radiological contamination alarm sounded at 4 p.m. Saturday . Radiation was confined to a single building and no one was in danger , company says . In 1979 , Three Mile Island was the scene of the worst U.S. nuclear accident .