Zion Nuclear Power Station was the third dual-reactor nuclear power plant in the Commonwealth Edison (ComEd) network and served Chicago and the northern quarter of Illinois. The plant was built in 1973, and the first unit started producing power in December, 1973. The second unit came online in September 1974. This power generating station is located on 257 acres (104 ha) of Lake Michigan shoreline, in the city of Zion, Lake County, Illinois.