Company spokesman Ralph DeSantis said the airborne contamination was caused by a change in air pressure in the containment building that dislodged small irradiated particles in the reactor piping system.
Some of the small particles became airborne inside the building. An array of monitors detected them. The air pressure change occurred when inside building ventilation fans were started. The company has since modified the ventilation system to prevent that from happening, according to a company news release.
York County President Commissioner Steve Chronister said this morning that he's willing to accept Three Mile Island's assurance that the incident over the weekend was not an emergency.
Some Dauphin County officials have complained about the fact that they did not receive notification of the situation for hours.
"I would hope that in an emergency situation, it would be minutes instead of hours," Chronister said.
County Commissioner Doug Hoke said he believes York County officials should have been notified sooner.
"I would hope they would notify us immediately, because we have to make decisions if there is an emergency situation," Hoke said.
DeSantis said that company officials are aware of concerns about the time it took to notify county officials. After any such event, he said, the company reviews procedures to see how things were done, and if they can be done any better.
Exelon engineers were still working Sunday to determine the cause of a contamination alarm that sounded Saturday afternoon at the shut-down Three Mile Island Unit 1 containment building, prompting about 150 workers to be sent home.
Twenty workers received exposure to contaminants, but the exposure did not exceed U.S. Nuclear Regulatory Commission limits, said John White, a branch chief at the commission's Region 1 office, who visited the Londonderry Township, Dauphin County site Sunday to follow-up on the incident.
One worker received 16 millirem of exposure, according to Exelon, and other workers received lower levels of contamination. The annual limit for nuclear workers at Exelon plants is 2,000 millirem, according to Exelon, which owns and operates the plant.
"None of the levels were harmful," said Ralph DeSantis, a TMI spokesman.
The engineers determined two possible causes for the alarm, DeSantis said, declining to name the possible causes.
"Until (a cause) is determined, it'd be just speculating," DeSantis said.
Employees were working inside the building about 4 p.m. when the alarm went off. The unit has been shut down since Oct. 26 for refueling, maintenance and steam generator replacement.
No contamination was found outside the building, and the event posed no threat to public health and safety, according to Exelon.
The York County 911 Center received a notification more than six hours later, about 10:45 p.m. Saturday, that the incident had occurred, according to Kay Carman, director of the York County Office of Emergency Management.
That came in the form of an Event of Potential Public Interest, or EPPI, which is a courtesy notification, Carman wrote in an e-mail Sunday. EPPIs - used for incidents such as traffic accidents near the plant, an injured worker, or a fire truck or ambulance responding to the site - are different from the classifications used by TMI for accidents at the plant.
While Carman wrote that more than six hours was "a little excessive" to be notified, she added, "you must remember that we are not the host county for TMI."
DeSantis said that if the event was a declared emergency, even a low-level one, emergency service providers would have been notified within 15 minutes.
"Last night's event wasn't even close to the level of the lowest-level emergency," DeSantis said. "There was never even a potential threat to the public health and safety."
Daily Record/Sunday News reporter Tom Joyce contributed to this report.
Three Mile Island, located in Londonderry Township, Dauphin County, generates power for more than 800,000 homes. The U.S. Nuclear Regulatory Commission last month renewed TMI Unit 1's operating license until April 19, 2034.
TMI Unit 2 suffered a partial meltdown in 1979.
One TMI worker received 16 millirem of radiation exposure, according to Exelon, which sets an annual limit of 2,000 millirem for its nuclear workers.
According to the Nuclear Regulator Commission, here are some of the doses of radiation, in millirems, in everyday life:
300: Amount of natural background radiation - from the air, cosmic rays and the earth itself - the average American is exposed to annually.
3: Dose of radiation from a dental X-ray
40: Dose of radiation from an abdominal X-ray
130: Dose of radiation from a full-body CT scan
30: Dose of radiation the average person receives from food per year
Also of interest· Three Mile Island emergency indelibly written into memories.