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.
Ordinary uranium as found in nature was not suitable for an atomic bomb because less than one percent of the atoms in it are fissionable isotope ²³⁵U.[3] It therefore became necessary to find some means for separating the rare ²³⁵U from the large quantity of ²³⁸U. Chemistry could not do it since the two isotopes are identical chemically.
Several methods of achieving large-scale separation were tried. The most successful and economical, known as “gaseous diffusion,” involves compressing normal uranium, in the form of uranium hexafluoride gas, against a porous barrier containing millions of holes, each smaller than two-millionths of an inch. Since the ²³⁵U molecules are slightly lighter than the ²³⁸U, they bounce against the barrier more frequently and have a greater chance of penetrating. Thus, although the gas at first contains only 0.7% ²³⁵U, the process of compression is repeated several thousand times, and the proportion gradually increases until the necessary concentration is reached.
For this operation an enormous plant containing a very large barrier area, miles of piping, and countless pumps was built at Oak Ridge, Tennessee.
At the same time that vast efforts were being made to produce a ²³⁵U bomb, another project of equal importance was being pursued to develop a different kind of fission bomb. Uncertainty as to whether it would be possible to separate usable amounts of ²³⁵U led to a decision to exploit a highly significant discovery about one of the transuranic elements.