Event cause traced to airflow change inside containment building
- Edited by Randy Brich -
Outage workers at Three Mile Island Unit 1 who were sent home Saturday evening returned to work Sunday and today and normal outage work has resumed. About 150 workers stationed in the containment building of TMI Unit 1, which was shut down nearly a month ago for a planned refueling outage and steam generator replacement, were sent home late Saturday afternoon when monitors detected small amounts of airborne radiological contamination inside the containment building.
“Things are back to normal,” Site Vice President Bill Noll said. “We are back performing outage activities as we had originally planned.”
Outage work not associated with the containment building involving more than 3,000 other plant and temporary workers continued throughout the weekend. No contamination was found outside the containment building and the event never posed a threat to the health or safety of employees or the public.
Tests of the containment workers over the weekend confirmed that no employee received radiation doses above what they might normally see doing work in the containment building. 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 annual federal occupational exposure limit. A millirem is a measure of radiation exposure. The federal occupational limit is 5,000 millirem per year for workers; Exelon has a more conservative standard of 2,000 millirem per year.
The inside airborne contamination was caused by a change in air pressure inside the containment building that dislodged small irradiated particles in the reactor piping system. Some of the small particles became airborne inside the building and were detected by an array of monitors in place to detect such material. The air pressure change occurred when inside building ventilation fans were started to support outage activities. The site has modified the ventilation system to prevent future air pressure changes.
The shutdown of Unit 1 on Oct. 26, planning for which began more than four years ago, involves refueling of the reactor – which currently has no fuel in it – and replacement of the unit’s two steam generators. The plant will remain shut down until that work is completed.Beginning Saturday afternoon Exelon Nuclear took a series of steps to voluntarily inform the NRC, state officials, other key stakeholders and the media as soon as technical information could be confirmed.
Exelon Corporation is one of the nation’s largest electric utilities with approximately $19 billion in annual revenues. The company has one of the industry’s largest portfolios of electricity generation capacity, with a nationwide reach and strong positions in the Midwest and Mid-Atlantic. Exelon distributes electricity to approximately 5.4 million customers in northern Illinois and southeastern Pennsylvania and natural gas to approximately 485,000 customers in the Philadelphia area. Exelon is headquartered in Chicago.
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