[Illustration: _Scale model of the CP-1 (Chicago Pile No. 1) used by Enrico Fermi and his associates on December 2, 1942, to achieve the first self-sustaining nuclear reaction. Alternate layers of graphite, containing uranium metal and/or uranium oxide, were separated by layers of solid graphite blocks. Graphite was used to slow down neutrons to increase the likelihood of fissions._] The excitement among physicists became even greater when it was realized that this newly discovered process of fission was accompanied by the release of several free neutrons from the splitting nucleus. Each new neutron could, if properly slowed down by a moderating material, cause another nucleus to split and release more energy and still more neutrons, and so on, as illustrated in Figure 5. (A moderator is necessary because fast, newly released neutrons are too readily absorbed by uranium-238 nuclei, which rarely split.) Apparently all that was needed to achieve this spectacular kind of a chain reaction was to assemble enough uranium in one place so that the released neutrons would have a good chance of finding another ²³⁵U nucleus before escaping from the pile. The amount of fissionable material required to sustain a chain reaction is termed the “critical mass.” A team of scientists led by Fermi achieved the first self-sustaining nuclear reaction on December 2, 1942, under the grandstand at the University of Chicago’s athletic field. This date is often referred to as the beginning of the Nuclear Age. [Illustration: Figure 5 _This diagram shows what happens in a chain reaction resulting from fission of uranium-235 atoms._] STRAY NEUTRON ²³⁵U ORIGINAL FISSION FISSION FRAGMENTS One to three neutrons from fission process A NEUTRON SOMETIMES LOST ²³⁸U CHANGES TO PLUTONIUM ²³⁵U ONE NEW FISSION FISSION FRAGMENT One to three neutrons again ²³⁵U ²³⁵U TWO NEW FISSIONS FISSION FRAGMENTS The Fission Bomb Is Exploded The American scientists present on that historic December day were part of the tremendous super-secret scientific and industrial complex that bore the unrevealing title Manhattan District. The United States had been at war almost a year. An uncontrolled fission reaction gave promise of producing an explosion of untold proportions. This promise, coupled with the possibility that enemy scientists might be nearing such a goal, had launched a vast Allied effort. The Manhattan Project, as it was commonly known, included a variety of “hush-hush” facilities. Each of these installations, in New York, Illinois, Tennessee, New Mexico, California, and Washington, had its own experts working night and day to solve the baffling problems surrounding development of a fission weapon.