Courtesy of Argonne National Laboratory

December 2, 1942
Under the football stands of Stagg Field at the University of Chicago, Enrico Fermi leads a team of scientists in successfully creating the first controlled nuclear chain reaction.

Fermi began planning his full scale chain reacting pile in May 1942. CP-1 (Chicago pile one), as it was called, was to be built 20 miles southwest of Chicago in the Argonne forest. However in early November 1942 construction on the pile building came to a halt as construction workers went on strike. Faced with an indefinite delay, Fermi and Compton decided to build the pile in the doubles squash court under the football stands of Stagg Field on the University of Chicago campus. Construction began on November 16, 1942. Fermi's plan called for a spherical shape encased in a wood framework. However as the pile was assembled the upper layers ended up being unnecessary. The final shape resembled that of a doorknob. The pile was surrounded by a specially built balloon in case air had to be evacuated from the pile. This ended up being unnecessary. Slots were machined into the pile to contain wooden rods covered with cadmium foil. Cadmium has a huge neutron absorption cross section. With the cadmium rods in place, there weren't enough neutrons available for the chain reaction. With the rods pulled out, the chain reaction could proceed. The fact that delayed neutrons exist in the reaction allowed the operator to control the chain reaction using the cadmium rods. The pile contained 350,000 kg of graphite (in the form over over 40,000 blocks), 36,500 kg of uranium oxide and 5,600 kg of uranium metal (in the form of small pseudospheres). CP-1 cost about $1 million to produce and build. Fermi ran the pile for four and one half minutes, producing one half watt of power before shutting it down. It had a "k" value of 1.0006. The pile was torn down in February 1943 and reassembled as CP-2 at the Argonne forest location.

CP-1 Cutaway - Courtesy of Argonne National Laboratory

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