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Fermi on Chicago Pile-1

Italian physicist Enrico Fermi directed the first experiment that proved an atomic chain-reaction could be self-sustaining in 1942 at the University of Chicago. Fermi describes the process of the experiment, which was named CP-1, or Chicago Pile-1, in the excerpt below.

Document Type:

From Fermi’s Own Story by Enrico Fermi

The year was 1939. A world war was about to start. The new possibilities appeared likely to be important, not only for peace, but also for war. A group of physicists in the United States—including Leo Szilard, Walter Zinn, now director of Argonne National Laboratory, Herbert Anderson, and myself—agreed privately to delay further publications of findings in this field.

We were afraid these findings might help the Nazis. Our action, of course, represented a break with scientific tradition and was not taken lightly. Subsequently, when the government became interested in the atom bomb project, secrecy became compulsory. Here it may be well to define what is meant by the “chain reaction,” which was to constitute our next objective in the search for a method of utilizing atomic energy.

An atomic chain reaction may be compared to the burning of a rubbish pile from spontaneous combustion. In such a fire, minute parts of the pile start to burn, and in turn ignite other tiny fragments. When sufficient numbers of these fractional parts are heated to the kindling points, the entire heap bursts into flames. A similar process takes place in an atomic pile, such as was constructed under the West Stands of Stagg Field at the University of Chicago in 1942.

The pile itself was constructed of uranium, a material that is embedded in a matrix of graphite. With sufficient uranium in the pile, the few neutrons emitted in a single fission that may accidentally occur strike neighboring atoms, which in turn undergo fission and produce more neutrons. These bombard other atoms and so on at an increasing rate until the atomic “fire” is going full blast. The atomic pile is controlled and prevented from burning itself to complete destruction by cadmium rods, which absorb neutrons and stop the bombardment process. The same effect might be achieved by running a pipe of cold water through a rubbish heap; by keeping the temperature low, the pipe would prevent the spontaneous burning.

The first atomic chain reaction experiment was designed to proceed at a slow rate. In this sense, it differed from the atomic bomb, which was designed to proceed at as fast a rate as was possible. Otherwise, the basic process is similar to that of the atomic bomb. The atomic chain reaction was the result of hard work by many hands and many heads. Arthur H. Compton, Walter Zinn, Herbert Anderson, Leo Szilard, Eugene Wigner, and many others worked directly on the problems at the University of Chicago. Very many experiments and calculations had to be performed. Finally, a plan was decided upon. 

Thirty “piles” of less than the size necessary to establish a chain reaction were built and tested. Then the plans were made for the final test of a full-sized pile. The scene of this test at the University of Chicago would have been confusing to an outsider—if he could have eluded the security guards and gained admittance. He would have seen only what appeared to be a crude pile of black bricks and wooden timbers. All but one side of the pile was obscured by a balloon cloth envelope.

As the pile grew toward its final shape during the days of preparation, the measurement performed many times a day indicated everything was going, if anything, a little bit better than predicted by calculations. Finally, the day came when we were ready to run the experiment. We gathered on a balcony about 10 feet above the floor of the large room in which the structure had been erected. Beneath us was a young scientist, George Weil, whose duty it was to handle the last control rod that was holding the reaction in check.

Every precaution had been taken against an accident. There were three sets of control rods in the pile. One set was automatic. Another consisted of a heavily weighted emergency safety held by a rope. Walter Zinn was holding the rope ready to release it at the least sign of trouble. The last rod left in the pile, which acted as starter, accelerator, and brake for the reaction, was the one handled by Weil. Since the experiment had never been tried before, a “liquid control squad” stood ready to flood the pile with cadmium salt solution in case the control rods failed. Before we began, we rehearsed the safety precautions carefully.

Finally, it was time to remove the control rods. Slowly, Weil started to withdraw the main control rod. On the balcony, we watched the indicators which measured the neutron count and told us how rapidly the disintegration of the uranium atoms under their neutron bombardment was proceeding. At 11:35 a.m., the counters were clicking rapidly. Then, with a loud clap, the automatic control rods slammed home. The safety point had been set too low.  

It seemed a good time to eat lunch. During lunch everyone was thinking about the experiment but nobody talked much about it. At 2:30, Weil pulled out the control rod in a series of measured adjustments. Shortly after, the intensity shown by the indicators began to rise at a slow but ever-increasing rate. At this moment we knew that the self-sustaining reaction was under way. The event was not spectacular, no fuses burned, no lights flashed. But to us it meant that release of atomic energy on a large scale would be only a matter of time. The further development of atomic energy during the next three years of the war was, of course, focused on the main objective of producing an effective weapon.

At the same time we all hoped that with the end of the war emphasis would be shifted decidedly from the weapon to the peaceful aspects of atomic energy. We hoped that perhaps the building of power plants, production of radioactive elements for science and medicine would become the paramount objectives. Unfortunately, the end of the war did not bring brotherly love among nations. The fabrication of weapons still is and must be the primary concern of the Atomic Energy Commission.

Secrecy that we thought was an unwelcome necessity of the war still appears to be an unwelcome necessity. The peaceful objectives must come second, although very considerable progress has been made also along those lines. The problems posed by this world situation are not for the scientist alone but for all people to resolve. Perhaps a time will come when all scientific and technical progress will be hailed for the advantages that it may bring to man, and never feared on account of its destructive possibilities.

More Historical Resources:

Enrico Fermi. Photo courtesy of Argonne National Laboratory.

Chicago Pile-1 scientists

One of the 24 John Cadel paintings recreating the CP-1 experiment. Image courtesy of Argonne National Laboratory.

One of the 24 John Cadel paintings recreating the CP-1 experiment. Image courtesy of Argonne National Laboratory.

One of the 24 John Cadel paintings recreating the CP-1 experiment. Image courtesy of Argonne National Laboratory.