BYRON, Ill. — A nuclear reactor at a northern Illinois plant shut down Monday after losing power, and steam was being vented to reduce pressure, according to officials from Exelon Nuclear and federal regulators.

Unit 2 at Byron Generating Station, about 95 miles (153 kilometres) 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, but 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.

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 from the plant to the electrical grid, they said. Officials were still investigating the equipment failure.

Nuclear Regulatory Commission spokeswoman Viktoria Mitlyng said 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."

Candace Humphrey, Ogle County's emergency management co-ordinator, 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 centre in Lisle, Illinois.