US officials said they may have traced a power failure at a nuclear reactor in Illinois to a failed piece of equipment at a switchyard that delivers power to and from the plant.
After the shutdown yesterday morning at Exelon Nuclear's Byron Generating Station, operators began releasing steam to cool the reactor from the part of the plant where turbines produce 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.
The US nuclear regulatory commission spokeswoman declared the incident an "unusual event," the lowest of four levels of emergency. Commission officials also said the release of tritium was expected.
Spokeswoman Viktoria Mitlyng said officials can't yet calculate how much tritium was released. They know the amounts were small because monitors around the plant didn't show increased levels of radiation, she said.
Tritium is relatively short-lived and penetrates the body weakly through the air compared to other radioactive contaminants.
Exelon nuclear officials believe a failed piece of equipment at a switchyard caused the shutdown, but they were still investigating an exact cause.
The switchyard is similar to a large substation that delivers power to the plant from the electrical grid and from the plant to the electrical grid.
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."
Another reactor at the plant was operating normally.
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.