Gaseous Diffusion: a Historical Account of Isotope Separation

The race to create the world’s first atomic bomb was a huge project that involved the work of tens of thousands of people and billions of dollars. Most of these resources, however, did not go into the theory of designing a bomb, as design was a relatively small part of the budget; rather, the money and manpower were needed to produce weapons-grade plutonium and uranium. Gaseous diffusion was, at the time, the most efficient way of separating the 235 U and 92
238 U isotopes.1 92
The question of why scientists entertained the idea of a uranium bomb at all is an intriguing one. Bohr had already demonstrated that uranium, when undergoing slow fission, gives off neutrons. When those neutrons strike impure 238 U, a new element is produced: 92
Neptunian. Neptunian, undergoing beta decay with a half-life of about 56 hours, produces plutonium—plutonium, like 235 U, is fissionable.2
The process by which plutonium is produced is scientifically straightforward— a modern reactor with an output of 100 MW/day can produce enough plutonium for a bomb in about two months. Plutonium, therefore, was the logical fissionable material considered for an atomic weapon. However, a problem soon arose, as scientists realized that a chain reaction could not be initiated with plutonium in the same way as with uranium. Rather, the plutonium would have to
1 http://www.childrenofthemanhattanproject.org/HISTORY/H-04b2.htm
2 Michael Frayn, Copenhagen, (New York, New York: Anchor Books, 1998) p. 37
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1 be imploded.3 Although the implosion model worked theoretically, many scientists, including Enrico Fermi, harbored serious doubts about the practical implementation of a plutonium implosion. These doubts from his top scientists forced Oppenheimer to suggest that, to guarantee the success of the entire Manhattan project, both a plutonium and uranium bomb should be produced.
Oak Ridge, the home of the current United States National Laboratory, was first created in 1940 to produce the