startup transformers. In this case the breaker feeding the transformers tripped automatically and the emergency diesel generators immediately started up to restore power to the safety buses. Additionally, all of the non-safety power generation buses transferred to the unit auxiliary transformers, which receive power from the plant's turbine generator, preventing a trip of the plant. After this second failed insulator event, Exelon took both units offline and replaced all of the insulators in the plant's switchyard. Surrounding population The Nuclear Regulatory Commission defines two emergency planning zones around nuclear power plants: a plume exposure pathway zone with a radius of , concerned primarily with exposure to, and inhalation of, airborne radioactive contamination, and an ingestion pathway zone of about , concerned primarily with ingestion of food and liquid contaminated by radioactivity. The 2010 U.S. population within of Byron was 25,679, an increase of 5.9 percent in a decade, according to an analysis of U.S. Census data for msnbc.com. The 2010 U.S. population within was 1,273,771, an increase of 14.5 percent since 2000. Cities within 50 miles include Rockford (17 miles to city center). Security Security at Byron was provided by The Wackenhut Corporation (now G4S Secure Solutions) until a story broke in late 2007 involving videotape that captured Wackenhut security guards asleep at the Peach Bottom Nuclear Generating Station in Pennsylvania. Security is now handled in-house by Exelon which owns and operates the power plant. After the attacks of September 11, 2001, a "security zone" was established by the United States Coast Guard around the Byron Nuclear Generating Station as part of a broader effort to secure nuclear facilities in the U.S. On December 10, 2004, the security zone at Byron was removed to allow for a security zone around the Hammond Intake Crib on Lake Michigan. Seismic risk The Nuclear Regulatory Commission's estimate of the risk each year of an earthquake intense enough to cause core damage to the reactor at Byron was 1 in 172,414, according to an NRC study published in August 2010. See also Nuclear safety in the U.S. Nuclear accidents in the United States List of largest power stations in the United States Largest nuclear power plants in the United States Notes External links Energy infrastructure completed in 1985 Byron, Illinois Buildings and structures in Ogle County, Illinois Exelon Nuclear power stations using pressurized water reactors Nuclear power plants in Illinois Towers in Illinois