Officials investigating Byron reactor shutdown

Exelon Energy officials say they've traced a power failure at a nuclear reactor in northern Illinois to an electrical insulator in a switchyard.

Spokesman Paul Dempsey says the insulator failed and fell off Monday morning, causing one of the reactors at the Byron Generating Station to shut down automatically.

He says the bad insulator will be sent to a lab for analysis and officials hope to replace it by Tuesday evening. It's unclear how soon before the reactor could return to service.

Steam containing low levels of tritium, a radioactive form of hydrogen, is being vented to reduce pressure within the reactor. But federal and plant officials say the levels are safe for workers and the public.

The plant is 95 miles northwest of Chicago.

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. 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 on-site 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.

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 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.