Under the Grandstand

The first man-made chain reaction occurred under the grandstand of the University of Chicago football field on December 2, 1942, in what was known as an atomic "pile." It was so named because it was constructed of a "pile" of 45,000 high-purity graphite bricks (250 tons), with 19,000 drilled holes to contain the approximately 93,000 pounds of uranium metal and uranium oxide along with the cadmium control rods. When operating at its design point, it generated a half watt of power - enough to almost power a pencil sharpener. (Fortunately, it was not designed as a power reactor, but as an experiment to prove the "chain reaction" hypothesis.)

Why the "high-purity graphite bricks?" It has to do with the statement a few paragraphs back about "... if the energy of the neutron is within a certain range." When we want to make little rocks out of big rocks, we are accustomed to using a bigger hammer and swinging hard. Not so in the nuclear world. In order for a neutron to have a decent chance at fissioning a U235 nucleus, it must be slowed down by the action of a moderator. Carbon - as long as it is of high enough purity to avoid absorbing the neutrons - is a good moderator, although, as Chernobyl demonstrated, it has a few potential problems - which is why U.S. power reactors never use this material... or this type of "graphite reactor." It is typically used in military reactors for the production of plutonium - which reportedly was one of Chernobyl's functions, in addition to generating power. [Other uses would be in research reactors, as well as in reactors for use in creating medical radionuclides.]

Footnote to chapter: There is much evidence that a natural reactor "happened" in Western Africa in the Republic of Gabon at Oklo some 1.7 billion years ago when the ratio of U235 to U238 was considerably higher. It appears to have operated in accordance with the Nuclear Regulatory Commission rules of that time and was safely shut down after several hundred thousand years of operation. See Oklo Reactor, Scientific American, August 1976.