Site A/Plot M, Illinois

The Site A/Plot M Decommissioned Reactor Site is located in the Palos Forest Preserve in Cook County, Illinois, 20 miles southwest of Chicago. The site is underlain by 100 feet of glacial till overlying dolomite bedrock. About 8.9 million people live within 50 miles of the site; the population within a 5-mile radius is about 150,000.

The Site A/Plot M area is the former site of Argonne National Laboratory and its predecessor, the University of Chicago Metallurgical Laboratory, which was part of the World War II Manhattan Engineer District. In 1942, the U.S. Army Corps of Engineers leased 1,025 acres of land in the Palos Forest Preserve from the Forest Preserve District of Cook County. The newly constructed laboratory used two locations in the Palos Forest Preserve: Site A, a 19-acre area that contained experimental laboratories and nuclear reactor research facilities, and Plot M, a 150-foot by 140-foot area that was used for burial of radioactive waste. Plot M is located about 1,500 feet north of Site A.

Site A was the operational facility for two of the nation's first nuclear reactors, referred to as Chicago Pile-2 and Chicago Pile-3 (CP 2 and CP 3). Besides the two reactors, an estimated 35 support buildings were constructed at the site, including laboratories, dormitories, a cafeteria, dog kennels, and a lead foundry.