Secret race won with Chicago's chain reaction
Sketch of the world's first Reactor. Because of wartime secrecy, there are no photographs of the completed reactor. The scientists who worked on the project were back row, from left, Norman Hilberry, Samuel Allison, Thomas Brill, Robert Nobles, Warren Nyer, and Marvin Wilkening. Middle row, Harold Agnew, William Sturm, Harold Lichtenberger, Leona Marshall and Leo Szilard,. Front row, Enrico Fermi, Walter Zinn, Albert Wattenber and Herbert Anderson.
Although Los Alamos had been selected as the site of the laboratory that would design nuclear weapons for the Manhattan Engineer District (MED), it was not until Dec. 2, 1942, that Enrico Fermi and his group at the metallurgical laboratory at the University of Chicago achieved an experimental demonstration of a chain reaction. This was a result of a program begun in 1939 in response to Albert Einstein's letter to President Franklin Delano Roosevelt warning of the German program to exploit fission for military purposes. Three years were required to achieve results that demonstrated the danger, although, unknown to American physicists, the Germans had not succeeded in achieving a chain reaction in their experiments.
The achievement of a chain reaction was important, not only because it would prove the principle of the atomic bomb, but also because a chain-reacting pile could be used to produce plutonium, one of the nuclear explosives American scientists hoped to use in their weapons. Indeed, MED Commander Leslie Groves had already asked the DuPont Corp. to build production reactors in Hanford, Wash., for that purpose.
Fermi was the champion of the American effort in the secret race to achieve a chain reaction in uranium. His antagonist was German physicist Werner Heisenberg one of the creators of quantum mechanics and among the most distinguished theoretical physicists in the world. Fermi had left Italy to escape fascist persecution in 1938, collecting his Nobel Prize in Stockholm, Sweden, en route to Columbia University, where he began his experiments to build a chain-reacting pile in 1939 after the discovery of fission. Heisenberg was recruited to work on a chain-reacting pile in September 1939 by Nazi physicist Kurt Diebner.
While Fermi chose graphite to slow down, or "moderate," the neutrons produced in the fission of uranium 235 so that they could cause further fissions in a chain reaction, Heisenberg chose heavy water, in part because experiments conducted by Walter Bothe at the University of Heidelberg indicated that pure graphite was inadequate as a moderator.
These results were based upon mistaken calculations and gave Fermi an advantage. Heavy water was also chosen because Heisenberg's early experiments with paraffin as a moderator failed to produce any chain reaction. Fermi's early experiments at Columbia, in contrast, used graphite Ń highly purified graphite, paid for by the federal government in response to Einstein's letter.
The price was $6,000 for 40 tons purified of boron and other neutron-absorbing elements that might inhibit a chain reaction. This was increased by $40,000 when the National Defense Research Committee was created in 1940. None of the piles built at Columbia with these funds succeeded in producing a neutron multiplication greater than one, which would indicate the presence of a chain reaction.
Heisenberg's first experiments with heavy water at the Kaiser-Wilhelm Institute in Berlin-Dahlem and in Leipzig, Germany, were encouraging enough for him to promote nuclear energy to the German government. He warned its representatives in the fall of 1941 that the Americans were pursuing a nuclear explosive (plutonium) that could be made in a chain-reacting pile. The warning resulted in receiving the highest priority for his work from Albert Speer, Hitler's minister of munitions. The Leipzig pile, however, had burned in a fire caused by a pyrophoric reaction of its powdered uranium with air, and Allied bombing of Berlin forced Heisenberg to move his materials there to Haigerloch in Wrttemberg, Germany.
After FDR's decision to commit the United States government to a full-scale atomic bomb program in October 1941, Fermi's pile had been moved to the University of Chicago, where it was rebuilt in a squash court under the west stands of Stagg Field, which were no longer in use. There, he used uranium metal, rather than the pyrophoric powder Heisenberg had used or the uranium oxide used in his earlier experiments at Columbia, to build a larger pile.
The Chicago pile included 400 tons of graphite and 22,000 "pseudospheres"of uranium stacked in 57 layers with cadmium sheets nailed to flat wooden strips as control and safety rods. on Dec. 2, Fermi supervised the final steps of his experiment. He calculated the number of fission neutrons to be expected as the control rod was pulled from the pile. In addition to safety rods controlled electronically so that they would be pushed into the pile if neutron detectors indicated a runaway reaction, another, weighted with lead, would fall into the pile if physicist Norman Hilberry cut a rope with a hatchet. If all else failed, a three-man "suicide squad" of physicists stood ready to drench the pile with cadmium sulfate.
That morning, Fermi ordered the control rod raised foot by foot from the pile until one of the safety rods, triggered at a preset point that he had forgotten, was released and slid into the pile, with weights crashing to the floor. He ordered it removed again, saw the counters indicate the previous flux and then broke the tension by ordering the crew to lunch. After lunch, the control rod was pulled out 13 feet again, then another six inches, then, after the safety rod was reinserted, another foot. The safety rod was withdrawn, and the counters rattled as the trace indicating neutron multiplication climbed straight up, indicating a self-sustaining chain reaction.
After operating for 28 minutes, producing about 200 watts of power, the pile was shut down. Eugene Wigner (a Hungarian physicist who helped to write the Einstein letter to FDR) handed Fermi a bottle of Bertolli Chianti and passed around paper cups to the crew. Leona Woods Marshall, the only woman present, broke the silence with the comment: "Let's hope that we are the first to succeed."
Arthur Compton called the head of the NDRC, James Conant, and told him cryptically, "the Italian navigator has just landed in the New World," the secret phrase agreed to, to signal success.
The success of the pile assured that Los Alamos would have plutonium, as well as uranium, as a potential nuclear explosive. Still to be built, however, were the giant piles at Hanford and the uranium isotope separation plants at Oak Ridge, Tenn., that would make enough material for these weapons.
- Bob Seidel
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