In Chicago, Samuel K. Allison had found a suitable location 60 feet (18 m) long, 30 feet (9.1 m) wide and 26 feet (7.9 m) high, sunk slightly below ground level, in a space under the stands at Stagg Field originally built as a rackets court.
Stagg Field had been largely unused since the University of Chicago had given up playing American football in 1939, but the rackets courts under West Stands were still used for playing squash and handball.
Leona Woods and Anthony L. Turkevich played squash there in 1940.
Since it was intended for strenuous exercise, the area was unheated, and very cold in the winter.
The nearby North Stands had a pair of ice skating rinks on the ground floor, which although they were unrefrigerated, seldom melted in winter.
Allison used the rackets court area to construct a 7-foot (2.1 m) experimental pile before Fermi's group arrived in 1942.

The United States Army Corps of Engineers assumed control of the nuclear weapons program in June 1942, and Compton's Metallurgical Laboratory became part of what came to be called the Manhattan Project.
Brigadier General Leslie R. Groves, Jr. became director of the Manhattan Project on 23 September 1942.
He visited the Metallurgical Laboratory for the first time on 5 October.
Between 15 September and 15 November 1942, groups under Herbert Anderson and Walter Zinn constructed 16 experimental piles under the Stagg Field stands.

Fermi designed a new pile, which would be spherical to maximize k, which was predicted to be around 1.04, thereby achieving criticality.
Leona Woods was detailed to build boron trifluoride neutron detectors as soon as she completed her doctoral thesis.
She also helped Anderson locate the required large number of 4-by-6-inch (10 by 15 cm) timbers at lumber yards in Chicago's south side.
Shipments of high-purity graphite arrived, mainly from National Carbon, and high-purity uranium dioxide from Mallinckrodt in St Louis, which was now producing 30 short tons (27 t) a month.
Metallic uranium also began arriving in larger quantities, the product of newly developed techniques.

On 25 June, the Army and the Office of Scientific Research and Development (OSRD) had selected a site in the Argonne Forest near Chicago for a plutonium pilot plant.
This became known as Site A. 1,025 acres (415 ha) were leased from Cook County in August, but by September it was apparent that the proposed facilities would be too extensive for the site, and it was decided to build the pilot plant elsewhere.
The subcritical piles posed little danger, but Groves felt that it would be prudent to locate a critical pile—a fully functional nuclear reactor—at a more remote site.
A building at Argonne to house Fermi's experimental pile was commenced, with its completion scheduled for 20 October.
Due to industrial disputes, construction fell behind schedule, and it became clear the materials for Fermi's new pile would be on hand before the new structure was completed.
In early November, Fermi came to Compton with a proposal to build the experimental pile under the stands at Stagg Field.

The risk of building an operational reactor running at criticality in a populated area was a significant issue, as there was a danger of a catastrophic nuclear meltdown blanketing one of the United States' major urban areas in radioactive fission products.
But the physics of the system suggested that the pile could be safely shut down even in the event of a runaway reaction.
When a fuel atom undergoes fission, it releases neutrons that strike other fuel atoms in a chain reaction.
The time between absorbing the neutron and undergoing fission is measured in nanoseconds.
Szilard had noted that this reaction leaves behind fission products that may also release neutrons, but do so over much longer periods, from microseconds to as long as minutes.
In a slow reaction like the one in a pile where the fission products build up, these neutrons account for about three percent of the total neutron flux.

Fermi argued that by using the delayed neutrons, and by carefully controlling the reaction rates as the power
In Chicago, Samuel K. Allison had found a suitable location 60 feet (18 m) long, 30 feet (9.1 m) wide and 26 feet (7.9 m) high, sunk slightly below ground level, in a space under the stands at Stagg Field originally built as a rackets court. Stagg Field had been largely unused since the University of Chicago had given up playing American football in 1939, but the rackets courts under West Stands were still used for playing squash and handball. Leona Woods and Anthony L. Turkevich played squash there in 1940. Since it was intended for strenuous exercise, the area was unheated, and very cold in the winter. The nearby North Stands had a pair of ice skating rinks on the ground floor, which although they were unrefrigerated, seldom melted in winter. Allison used the rackets court area to construct a 7-foot (2.1 m) experimental pile before Fermi's group arrived in 1942.
The United States Army Corps of Engineers assumed control of the nuclear weapons program in June 1942, and Compton's Metallurgical Laboratory became part of what came to be called the Manhattan Project. Brigadier General Leslie R. Groves, Jr. became director of the Manhattan Project on 23 September 1942. He visited the Metallurgical Laboratory for the first time on 5 October. Between 15 September and 15 November 1942, groups under Herbert Anderson and Walter Zinn constructed 16 experimental piles under the Stagg Field stands.
Fermi designed a new pile, which would be spherical to maximize k, which was predicted to be around 1.04, thereby achieving criticality. Leona Woods was detailed to build boron trifluoride neutron detectors as soon as she completed her doctoral thesis. She also helped Anderson locate the required large number of 4-by-6-inch (10 by 15 cm) timbers at lumber yards in Chicago's south side. Shipments of high-purity graphite arrived, mainly from National Carbon, and high-purity uranium dioxide from Mallinckrodt in St Louis, which was now producing 30 short tons (27 t) a month. Metallic uranium also began arriving in larger quantities, the product of newly developed techniques.
On 25 June, the Army and the Office of Scientific Research and Development (OSRD) had selected a site in the Argonne Forest near Chicago for a plutonium pilot plant. This became known as Site A. 1,025 acres (415 ha) were leased from Cook County in August, but by September it was apparent that the proposed facilities would be too extensive for the site, and it was decided to build the pilot plant elsewhere. The subcritical piles posed little danger, but Groves felt that it would be prudent to locate a critical pile—a fully functional nuclear reactor—at a more remote site. A building at Argonne to house Fermi's experimental pile was commenced, with its completion scheduled for 20 October. Due to industrial disputes, construction fell behind schedule, and it became clear the materials for Fermi's new pile would be on hand before the new structure was completed. In early November, Fermi came to Compton with a proposal to build the experimental pile under the stands at Stagg Field.
The risk of building an operational reactor running at criticality in a populated area was a significant issue, as there was a danger of a catastrophic nuclear meltdown blanketing one of the United States' major urban areas in radioactive fission products. But the physics of the system suggested that the pile could be safely shut down even in the event of a runaway reaction. When a fuel atom undergoes fission, it releases neutrons that strike other fuel atoms in a chain reaction. The time between absorbing the neutron and undergoing fission is measured in nanoseconds. Szilard had noted that this reaction leaves behind fission products that may also release neutrons, but do so over much longer periods, from microseconds to as long as minutes. In a slow reaction like the one in a pile where the fission products build up, these neutrons account for about three percent of the total neutron flux.
Fermi argued that by using the delayed neutrons, and by carefully controlling the reaction rates as the power 
Chicago Pile-1