Nov. 22 (Bloomberg) -- Exelon Corp. halted work yesterday on replacing equipment inside the Three Mile Island 1 reactor in Pennsylvania after radiological contamination was detected inside the containment building. The company plans to resume work “early” this week, a company spokeswoman said.

“We have stopped work inside the reactor building while we investigate the cause of the contamination,” Exelon spokeswoman Beth Archer said in a telephone interview. “There was no contamination outside the reactor building and there was no threat to public health and safety.”

A monitor near a temporary opening cut into the containment building created to move new steam generators inside “showed a slight increase in a reading and then returned to normal,” Exelon said in a statement yesterday. The 786-megawatt unit has been idled since Oct. 26 for refueling and maintenance during which workers will replace the steam generators.

About 150 workers inside the containment building were sent home at about 4 p.m. yesterday after an airborne radiological alarm sounded. One worker received 16 millirem of exposure, which is below the U.S. Nuclear Regulatory Commission’s limits. Exelon’s “annual occupational dose limit” for nuclear workers is 2,000 millirem.

Radiation Exposure

A millirem is a unit of measurement for a radiation dose. The average person is exposed to about 360 millirems a year from naturally occurring radiation, according to the American Nuclear Society.

The Nuclear Regulatory Commission has sent a radiation specialist and a regional manager to the power plant to “follow-up on yesterday’s incident” the commission said in an e-mailed statement today.

“The concern right now is making sure Exelon is checking the workers for contamination, making sure the source of the contamination is identified and addressed and to come up with a plan to deal with these issues if they want to resume work,” NRC spokesman Neil Sheehan said in a telephone interview today.

Exelon won’t be able to restart the replacement of the steam generators until the source of contamination is identified, Sheehan said.

“They’re narrowing it down but they still don’t have a definitive answer,” he said.

Other maintenance work outside of the reactor building continues as scheduled, according to Archer.

Unit 2 at Three Mile has been shut since 1979 following a partial core meltdown. It was the biggest nuclear accident in U.S. history.

To contact the reporter on this story: Aaron Clark in New York at aclark27@bloomberg.net; Jeran Wittenstein at jwittenstei1@bloomberg.net

Last Updated: November 22, 2009 19:01 EST