Nuclear Chemistry
Chain Reaction (December 2, 1942)
Dr. Frank Settle

    By early 1942, it was known that the two naturally occurring isotopes of uranium reacted with neutrons as follows:

    235U92 +   1n0    -->   fission products + (2.5)1n0   +  200 MeV Energy

    238U92   1n0    -->  239U92   

    239U92       -->  239Np93   +    ß-1       t1/2=23.5 min. 

    239Np93     --->  239Pu94+   ß-1         t1/2=2.33 days

    Each U-235 that undergoes fission produces an average of 2.5 neutrons. In contrast, some U-238 nuclei capture neutrons, become U-239, and subsequently emit two beta particles to produce Pu-239. The times are half-lives for the successive beta emissions.

    The first generations of a nuclear chain reaction

    The answers to two questions were critical to the production of plutonium for atomic bombs. Is it possible, using natural uranium (99.3% U-238 and 0.7% U-235), to achieve a controlled chain reaction on a large scale? If so, some of the excess neutrons produced by the fission of U-235 would be absorbed by U-238 and produce fissionable Pu-239. The second question concerned how to separate (in a reasonable period of time) the relatively small quantities of Pu-239 from the unreacted uranium and the highly radioactive fission-product elements.

    Although fission had been observed on a small scale in many laboratories, noone had carried out a controlled chain reaction. Fermi thought that he could achieve a controlled chain reaction using natural uranium. He had started this work with Leo Szilard at Columbia University, but moved to the University of Chicago in early 1942. The nuclear reactor, called a pile, was composed of 80,590 lbs. of uranium oxide, 12,400 lbs. of uranium, and 771,000 lbs. of ultrapure graphite arranged in a manner to maximize neutron propagation. On December 2, 1942, the first controlled nuclear chain reaction occurred in a squash court under the football field at the University of Chicago. At around 2:20 p.m. the reactor went critical; that is, it produced one neutron for every neutron absorbed by the uranium nuclei. Fermi allowed the reaction to continue for the next 27 minutes before inserting neutron-absorbing cadmium control rods. In addition to excess neutrons and energy, the pile also produced a small amount of Pu-239, the other known fissionable material.

    The first controlled chain reaction, Stagg Field, Chicago, Dec. 2, 1942
    (Courtesy of the Argonne National Laboratory)

    The achievement of the first sustained nuclear reaction was the beginning of a new age in nuclear physics and the study of the atom. Humankind could now use the tremendous potential energy contained in the nucleus of the atom. However, while a controlled chain reaction was achieved with natural uranium, it would be necessary to separate U-235 from U-238 to build a uranium bomb. On December 28, 1942, upon reviewing a report from his advisors, President Roosevelt recommended building full-scale plants to produce both U-235 and Pu-239. This changed the effort to develop nuclear weapons from experimental work in academic laboratories administered by the U.S. Office of Scientific Research and Development to a huge effort by private industry. This work, supervised by the U.S. Army Corps of Engineers, was codenamed the Manhattan Project. It spread throughout the entire United States, with the facilities for uranium and plutonium production being located at Oak Ridge, Tennessee, and Hanford, Washington, respectively. Work on plutonium production continued at the University of Chicago, at what became known as the Metallurgical Laboratory or Met Lab. A new laboratory at Los Alamos, New Mexico, became the focal point for development of the uranium and plutonium bombs.

    Complete Bibliography on Nuclear Physics from the ALSOS Digital Library for Nuclear Issues


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