UPDATED Monday, January 30, 2012 --- 9:01 p.m.

BYRON, Ill. (AP) -- Officials at a northern Illinois nuclear power plant believe a failed piece of equipment at a switchyard caused it to shut down.

Exelon Nuclear officials say the switchyard is similar to a large substation that delivers power from the plant to the electrical grid. They said the investigation into the equipment failure is ongoing.

Unit 2 at Byron Generating Station shut down around 10:18 a.m. Monday after losing power. Generators began supplying power to the plant. Operators began releasing steam to reduce pressure, though not from within the nuclear reactor.

The Nuclear Regulatory Commission has declared it an "unusual event," the lowest of its four emergency classifications.

Officials say the steam contains low levels of tritium, a radioactive for of hydrogen.

The plant is about 95 miles northwest of Chicago

Copyright 2012 by The Associated Press. All Rights Reserved.

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Posted Monday, January 30, 2012 --- 4:40 p.m.

BYRON, Ill. (AP) -- Exelon Nuclear says a reactor at its Byron Generating Station has shut down after losing power, and steam is being vented to reduce pressure.

Plant operators declared an unusual event at 10:18 a.m. Monday when Unit 2 shut down after losing power from an off-site source.

They say the facility's generators are providing power to equipment, and the unit remains in safe condition.

Officials say Byron Station is designed to vent steam in the event of a power loss. They say the steam contains low levels of tritium, a radioactive isotope, but not at unsafe levels.

Officials say engineers are investigating why the unit lost offsite power. Unit 1 continues to operate normally.

Byron Generating Station is in Ogle County, about 95 miles southwest of Chicago.

Copyright 2012 by The Associated Press. All Rights Reserved.

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Press Release: Unit 2 automatically taken offline; diesel generators start as designed

BYRON, Ill. (January 30, 2012) – Operators at Byron Generating Station declared an Unusual Event at 10:18 a.m. CT, due to the loss of offsite power and Unit 2 coming offline.

The nuclear facility’s diesel generators activated as designed to provide power to the facility when there is a loss of offsite power to the facility. The facility remains in a safe condition. Station engineering experts are looking into the cause of the loss of offsite power.

Byron Station is designed to depressurize to reduce steam pressure as part of the many redundant safety systems built into the facility.

Steam from the unit is released through safety relief valves that are specifically designed for this purpose. The steam, which will evaporate quickly, contained expected levels of tritium. Local residents may see or hear the steam release in progress, which will continue throughout the day until the unit cools down. These types of station releases are regulated by the Nuclear Regulatory Commission.

There is no health or safety impact to workers or to the public from the release, and Exelon Nuclear has notified all appropriate local, state and federal officials of the Unusual Event.

An Unusual Event is the lowest of four emergency classifications established by the U.S. Nuclear Regulatory Commission.

Tritium is an isotope of hydrogen with a weak level of radioactivity. It is formed naturally in the upper atmosphere, is a component of rain and is found in virtually all of earth’s surface water. Tritium is produced in greater concentrations in commercial nuclear reactors and is routinely discharged into the environment under strict regulatory guidelines. Tritium eventually breaks down into helium.

Byron’s Unit 1 continues to supply clean electricity to Exelon customers.

Byron Generating Station is in Ogle County, Ill., about 25 miles southwest of Rockford.