Unit 2 at Byron Generating Station, about 95 miles northwest of Chicago, shut down at 10:18 a.m., after losing power, Exelon officials said. Diesel generators began supplying power to the plant, and operators began releasing steam to cool the reactor from the part of the plant where turbines are producing electricity, not from within the nuclear reactor itself, officials said.
The steam contains low levels of tritium, a radioactive form of hydrogen, but federal and plant officials insisted the levels were safe for workers and the public.
"They're normal releases that we allow and this is an abnormal release, but it's still far below any limits that we have," U.S. Nuclear Regulatory Commission spokeswoman Viktoria Mitlyng told ABC7. "There is no threat to the public."
The U.S. Nuclear Regulatory Commission declared the incident an "unusual event," the lowest of four levels of emergency. Commission officials also said the release of tritium was expected.
Exelon Nuclear officials believe a failed piece of equipment at a switchyard caused the shutdown. The switchyard is similar to a large substation that delivers power to the plant from the electrical grid and that takes power from the plant to the electrical grid. Officials were still investigating the equipment failure.
Mitlyng said officials can't yet calculate how much tritium is being released. They know the amounts of tritium are small because monitors around the plant aren't showing increased levels of radiation, she said.
Tritium molecules are so small that tiny amounts are able to pass from radioactive steam from the reactor into the water used to cool the turbines and other equipment outside the reactor. The steam that was being released was coming from the turbine side.
The amount of releasing steam helps "take away some of that energy still being produced by nuclear reaction but that doesn't have anywhere to go now." Even though the turbine is not turning to produce electricity, she said, "you still need to cool the equipment."
"It's not unusual to lose off-site power. I don't know if we've had this situation before, but nuclear power plants have often lost power. So it's not uncommon to happen at plants in the country. We prepare for this with simulators, so it's something we train for," said Exelon spokesman Paul Dempsey.
Tritium is relatively short-lived and penetrates the body weakly through the air compared to other radioactive contaminants.
Candace Humphrey, Ogle County's emergency management coordinator, said county officials were notified of the incident as soon as it happened and that public safety was never in danger.
"It was standard procedure that they would notify county officials," she said. "There is always concern. But, it never crossed my mind that there was any danger to the people of Ogle County."
Unit 1 was operating normally while engineers investigate why Unit 2 lost power, which comes into the plant from the outside power grid, Mitlyng said. Smoke was seen from an onsite station transformer, she said, but no evidence of a fire was found when the plant's fire brigade responded.
Mitlyng said Nuclear Regulatory Commission inspectors were in the control room at Byron and in constant contact with the agency's incident response center in Lisle, Ill.
Residents are not worried about their safety.
"I hope everything is all right," said Matt Lesher. "I have lived here since I was four. This is the first time I've heard anything."
"My husband works there," said Kristin Sible. "I think we are fine."
In March 2008, federal officials said they were investigating a problem with electrical transformers at the plant after outside power to a unit was interrupted.
In an unrelated issue last April, the commission said it was conducting special inspections of backup water pumps at the Byron and Braidwood generating stations after the agency's inspectors raised concerns about whether the pumps would be able to cool the reactors if the normal system wasn't working. The plants' operator, Exelon Corp., initially said the pumps would work but later concluded they wouldn't.
The two nuclear generators in Byron went on line back in 1985 and 1987. According to the company's website, the plant can provide enough electricity to power nearly two million homes.
The Associated Press contributed to this report.