Emilio Segrè Visual Archives

More than 30,000 photos of scientists and their work

University of Chicago H22

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University of Chicago H22

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Credit line: 
Argonne National Laboratory, courtesy of AIP Emilio Segrè Visual Archives

These exponential piles were two of at least twenty-nine similar uranium-graphite lattice structures built to furnish basic information necessary for the proper design of CP-1, the world's first nuclear reactor. Of primary interest was the determination of the "k"or reproduction factor for the final reactor and the effects on the reproduction factor of the varying grades of the available uranium and graphite. For a nuclear chain reaction to be self-sustaining the "k" factor must be greater than 1.0.  Variations in the purity, quantity, and arrangement of the uranium fuel and graphite moderator greatly influenced the value of "k." Data obtained from the small test piles (99 in. x 99 in. x 124 in. high) made it possible for the scientists to successfully design and construct the much larger CP-1.

The tops and sides of the piles were covered with cadmium sheets to eliminate errors by reducing the return of slow neutrons to the piles by scattering or reflection from the surroundings. Fast-neutron radium-beryllium sources were used to promote production of slow neutrons in these subcritical assemblies.

The identity of the man in the background is unknown.

The exponential piles were constructed in 1942 under the West Stands of Stagg Field at The University of Chicago. On December 2, 1942, a group of scientists, led by the late Enrico Fermi, successfully completed the historic experiment.

Photo taken May 16, 1942.

Photo date: 
May 16, 1942
Original format: 
1 photographic print (black and white; 8.5 x 11 inches)