About 150 employees working inside the shut-down Three Mile Island unit 1 containment building were sent home on Saturday 21 November after an airborne radiological contamination alarm sounded inside the reactor building. The levels of contamination were low and do not pose a health or safety concern. No radioactivity left the site as a result of the event and there was no threat to public health and safety.

A monitor at the temporary opening cut into the containment building wall to allow two new steam generators to be moved inside showed a slight increase in a reading and then returned to normal. Two other monitors displayed normal readings.

Radiological surveys showed that the contamination was confined to surfaces inside the containment building. It was later revealed that 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 the 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.

On Saturday night Exelon technicians checked employees who had been working in the building at the time for possible unusual radiation exposure. One worker was found to have received 16 millirem of exposure, and other workers have received contamination of lower level, according to an Exelon statement. The annual occupational dose limit for nuclear workers at Exelon Nuclear plants is 2000 millirem. The Nuclear Regulatory Commission has a higher limit of 5000 millirem.

NRC Resident Inspectors assigned on a full-time basis to the Middletown, Pennsylvania, plant went to the site on Saturday and Sunday to review Exelon’s response to the event, as did two radiation safety specialists from the agency’s Region I Office. With the assistance of these specialists, the resident inspectors are continuing to independently evaluate the company’s efforts to identify the source of the contamination, the adequacy of controls in place to prevent a recurrence and the development of a root cause evaluation of what occurred.

In a 23 November statement site vice president Bill Noll said that things are back to normal and that Exelon is back performing outage activities as originally planned.”

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