UPDATE 1-Work resumes at Three Mile Island after leak

NEW YORK Nov 23 (Reuters) - About 150 workers in a containment building at the Three Mile Island nuclear plant in Pennsylvania have returned to work a day after being sent home when monitors detected a radiation leak, Exelon Corp (EXC.N) said on Monday.

The workers returned to work on Sunday in the containment building at the 786-megawatt Unit 1, Exelon said in a release. The workers had been sent home late on Saturday due to the contamination. The unit was shut about a month ago for planned refueling and steam generator replacement.

The affected unit was the sister of the unit at the Three Mile Island plant near the Pennsylvania state capital of Harrisburg that partially melted down in 1979 in the worst nuclear power accident in U.S. history.

In the latest incident, workers were cutting a pipe as part of the refueling and maintenance activities. There were some small irradiated particles inside the pipe, a spokeswoman said. Ventilation fans dislodged the irradiated particles and contaminated the indoor air, the company said.

Exelon said it modified the ventilation system to prevent similar incidents in the future.

Exelon said no contamination was found outside the containment building and the event never posed a threat to employees or the public.

The company said tests over the weekend confirmed that none of the containment workers received radiation doses above what they might normally receive during work in the building.

"Things are back to normal" at Unit 1, Site Vice President Bill Noll said in the release. "We are back performing outage activities as we had originally planned."

Throughout the weekend, more than 3,000 other plant and temporary workers continued outage work not associated with the containment building.

The unit that suffered the partial meltdown in 1979 was mothballed, and the accident made Three Mile Island synonymous with the dangers of nuclear power.

As of Sunday night, tests showed that 12 of the 150 workers received a detectable amount of radiation. The highest was 38.4 millirem, less than 1 percent of the federal occupational exposure limit of 5,000 millirem per year. Exelon said it has a more conservative standard of 2,000 per year.

Unit 1 was shut down on Oct. 26 for refueling of the reactor, which currently has no fuel, and replacement of the unit's two steam generators, Exelon said.

The unit, which last shut for refueling in 2007, is on a 24-month refueling cycle. (Reporting by Scott DiSavino; Editing by David Gregorio) ((scott.disavino@thomsonreuters.com; +1 646 223 6072; Reuters Messaging: scott.disavino.reuters.com@reuters.net)) ((For help: Click "Contact Us" in your desk top, click here [HELP] or call 1-800-738-8377 for Reuters Products and +1-888-463-3383 for Thomson products; For client training: training.americas@thomsonreuters.com; +1 646-223-5546))