Chicago Pile 1 was encased within a balloon so that the air inside could be replaced by carbon dioxide.
Anderson had a dark gray balloon manufactured by Goodyear Tire and Rubber Company.
A 25-foot (7.6 m) cube-shaped balloon was somewhat unusual, but the Manhattan Project's AAA priority rating ensured prompt delivery with no questions asked.
A block and tackle was used to haul it into place, with the top secured to the ceiling and three sides to the walls.
The remaining side, the one facing the balcony from which Fermi directed the operation, was furled like an awning.
A circle was drawn on the floor, and the stacking of graphite blocks began on the morning of 16 November 1942.
The first layer placed was made up entirely of graphite blocks, with no uranium.
Layers without uranium were alternated with two layers containing uranium, so the uranium was enclosed in graphite.
Unlike later reactors, it had no radiation shielding or cooling system, as it was only intended to be operated at very low power.

The work was carried out in twelve-hour shifts, with a day shift under Zinn and a night shift under Anderson.
For a work force they hired thirty high school dropouts that were eager to earn a bit of money before being drafted into the Army.
They machined 45,000 graphite blocks enclosing 19,000 pieces of uranium metal and uranium oxide.
The graphite arrived from the manufacturers in 4.25-by-4.25-inch (10.8 by 10.8 cm) bars of various lengths.
They were cut into standard lengths of 16.5 inches (42 cm), each weighing 19 pounds (8.6 kg).
A lathe was used to drill 3.25-inch (8.3 cm) holes in the blocks for the control rods and the uranium.
A hydraulic press was used to shape the uranium oxide into "pseudospheres", cylinders with rounded ends.
Drill bits had to be sharpened after each 60 holes, which worked out to be about once an hour.
Graphite dust soon filled the air and made the floor slippery.

Another group, under Volney C. Wilson, was responsible for instrumentation.
They also fabricated the control rods, which were cadmium sheets nailed to flat wooden strips, cadmium being a potent neutron absorber, and the scram line, a manila rope that when cut would drop a control rod into the pile and stop the reaction.
Richard Fox, who made the control-rod mechanism for the pile, remarked that the manual speed control that the operator had over the rods was simply a variable resistor, controlling an electric motor that would spool the clothesline wire over a pulley that also had two lead weights attached to ensure it would fail-safe and return to its zero position when released.

About two layers were laid per shift.
Woods' boron trifluoride neutron counter was inserted at the 15th layer.
Thereafter, readings were taken at the end of each shift.
Fermi divided the square of the radius of the pile by the intensity of the radioactivity to obtain a metric that counted down to one as the pile approached criticality.
At the 15th layer, it was 390; at the 19th it was 320; at the 25th it was 270 and by the 36th it was only 149.
The original design was for a spherical pile, but as work proceeded, it became clear that this would not be necessary.
The new graphite was purer, and 6 short tons (5.4 t) of very pure metallic uranium began to arrive from the Ames Project at Iowa State University, where a team under Frank Spedding had developed a new process to produce uranium metal.
Westinghouse Lamp Plant supplied 3 short tons (2.7 t), which it produced in a rush with a makeshift process.

The 2.25-inch (5.7 cm) metallic uranium cylinders, known as "Spedding's eggs", were dropped in the holes in the graphite in lieu of the uranium oxide pseudospheres.
The process of filling the balloon with carbon dioxide would not be necessary, and twenty layers could be dispensed with.
According to Fermi's new calculations, the countdown would reach 1 between the 56th and 57th layers.
The resulting pile was therefore flatter on the top than on the bottom.
Anderson called a halt after the 57th layer was placed.
When completed, the wooden frame supported an elliptical-sh
Chicago Pile 1 was encased within a balloon so that the air inside could be replaced by carbon dioxide. Anderson had a dark gray balloon manufactured by Goodyear Tire and Rubber Company. A 25-foot (7.6 m) cube-shaped balloon was somewhat unusual, but the Manhattan Project's AAA priority rating ensured prompt delivery with no questions asked. A block and tackle was used to haul it into place, with the top secured to the ceiling and three sides to the walls. The remaining side, the one facing the balcony from which Fermi directed the operation, was furled like an awning. A circle was drawn on the floor, and the stacking of graphite blocks began on the morning of 16 November 1942. The first layer placed was made up entirely of graphite blocks, with no uranium. Layers without uranium were alternated with two layers containing uranium, so the uranium was enclosed in graphite. Unlike later reactors, it had no radiation shielding or cooling system, as it was only intended to be operated at very low power.
The work was carried out in twelve-hour shifts, with a day shift under Zinn and a night shift under Anderson. For a work force they hired thirty high school dropouts that were eager to earn a bit of money before being drafted into the Army. They machined 45,000 graphite blocks enclosing 19,000 pieces of uranium metal and uranium oxide. The graphite arrived from the manufacturers in 4.25-by-4.25-inch (10.8 by 10.8 cm) bars of various lengths. They were cut into standard lengths of 16.5 inches (42 cm), each weighing 19 pounds (8.6 kg). A lathe was used to drill 3.25-inch (8.3 cm) holes in the blocks for the control rods and the uranium. A hydraulic press was used to shape the uranium oxide into "pseudospheres", cylinders with rounded ends. Drill bits had to be sharpened after each 60 holes, which worked out to be about once an hour. Graphite dust soon filled the air and made the floor slippery.
Another group, under Volney C. Wilson, was responsible for instrumentation. They also fabricated the control rods, which were cadmium sheets nailed to flat wooden strips, cadmium being a potent neutron absorber, and the scram line, a manila rope that when cut would drop a control rod into the pile and stop the reaction. Richard Fox, who made the control-rod mechanism for the pile, remarked that the manual speed control that the operator had over the rods was simply a variable resistor, controlling an electric motor that would spool the clothesline wire over a pulley that also had two lead weights attached to ensure it would fail-safe and return to its zero position when released.
About two layers were laid per shift. Woods' boron trifluoride neutron counter was inserted at the 15th layer. Thereafter, readings were taken at the end of each shift. Fermi divided the square of the radius of the pile by the intensity of the radioactivity to obtain a metric that counted down to one as the pile approached criticality. At the 15th layer, it was 390; at the 19th it was 320; at the 25th it was 270 and by the 36th it was only 149. The original design was for a spherical pile, but as work proceeded, it became clear that this would not be necessary. The new graphite was purer, and 6 short tons (5.4 t) of very pure metallic uranium began to arrive from the Ames Project at Iowa State University, where a team under Frank Spedding had developed a new process to produce uranium metal. Westinghouse Lamp Plant supplied 3 short tons (2.7 t), which it produced in a rush with a makeshift process.
The 2.25-inch (5.7 cm) metallic uranium cylinders, known as "Spedding's eggs", were dropped in the holes in the graphite in lieu of the uranium oxide pseudospheres. The process of filling the balloon with carbon dioxide would not be necessary, and twenty layers could be dispensed with. According to Fermi's new calculations, the countdown would reach 1 between the 56th and 57th layers. The resulting pile was therefore flatter on the top than on the bottom. Anderson called a halt after the 57th layer was placed. When completed, the wooden frame supported an elliptical-sh
Chicago Pile-1