After the shut down Monday morning at Exelon Nuclear's Byron Generating Station, 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.

Diesel generators were supplying the reactor with electricity, though it hasn't been generating power during the investigation into what happened. One question is why smoke was seen from an onsite station transformer, though no evidence of a fire was found when the plant's fire brigade responded, U.S. Nuclear Regulatory Commission spokeswoman Viktoria Mitlyng said.

Exelon Nuclear officials believe a failed piece of equipment at a switchyard at the plant about 95 miles northwest of Chicago 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.

The commission declared the incident an "unusual event," the lowest of four levels of emergency. Commission officials also said the release of tritium was expected.

Mitlyng said officials can't yet calculate how much tritium was being released. They know the amounts were small because monitors around the plant didn't show increased levels of radiation, she said.

Tritium molecules are so microscopic that small amounts are able to pass from radioactive steam that originates in the reactor through tubing and into the water used to cool turbines and other equipment outside the reactor, Mitlyng said. The steam that was being released was coming from the turbine side.

Tritium is relatively short-lived and penetrates the body weakly through the air compared to other radioactive contaminants.

Releasing steam helps "take away some of that energy still being produced by nuclear reaction but that doesn't have anywhere to go now," Mitlyng said. Even though the turbine is not turning to produce electricity, she said, "you still need to cool the equipment."

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.

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.

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