In this account, General Leslie R. Groves describes the challenges of planning amid great uncertainty. In meetings at the University of Chicago in 1942, Groves was taken aback when leading scientists hedged their estimate of how much plutonium was needed for an atomic bomb by a “factor of ten.”
From Now It Can Be Told
by Leslie R. Groves
It is essential for the reader to keep in mind the truly pioneering nature of the plutonium development as well as the short time available for research, to appreciate the gigantic steps taken by both scientists and engineers in moving as rapidly as they did from the idea stage to an operating plant of commercial size. It was a phenomenal achievement; an even greater venture into the unknown than the first voyage of Columbus.
The laboratory investigations had to be conducted in the face of incredible handicaps. At the laboratory in Chicago, we were seeking to split atoms, and in the process to transmute one element into another—that is, to change uranium into plutonium. The transmutation of an element involves the conversion of its atoms—the smallest known submicroscopic particles capable of existing alone which are not susceptible to further subdivision by chemical means—into atoms of another element possessing different chemical and physical properties. In effect, the scientists were reviving the classical, but always unsuccessful, search of the ancient alchemists for ways to convert base metals, such as lead, into gold; and the continuing, but theretofore unsuccessful, attempts of more modern chemists to change the character of elements. The precedents of history were surely all against us.
To carry out the transmutation process, even on a laboratory scale, and at an almost infinitesimal rate of production, a reactor, or as we often referred to it, a pile, of considerable size is necessary; for full scale production, obviously, a much bigger pile is needed. The laboratory unit, it was estimated, would require, among other items, some forty-five tons of uranium or uranium oxide. Such amounts were not available in sufficient purity until late in 1942. Even then, the laboratory unit would not be able to produce enough plutonium to permit normal laboratory research on its recovery—that is, on ways to separate it chemically from the basic uranium and the other radioactive materials that would also be produced.
In June, 1942, when the Corps of Engineers came into the picture, the necessary research on plutonium production and recovery had scarcely begun. There was no experimental proof that the hoped-for conversion would actually occur; it was predicated entirely on theoretical reasoning. Not until December 2, 1942, did we have any such proof, and this was weeks after we had decided to go ahead at full speed on the plutonium process, and many days after we had started to prepare the plans for a major plant. On October 5, 1942, I paid my first visit to the Metallurgical Laboratory at the University of Chicago, where Arthur Compton and I spent the morning inspecting the laboratory facilities and discussing with a number of scientists the work on which they were engaged.
That afternoon I had a meeting with Compton and about fifteen of his senior men. Among them were two other Nobel Prize winners, Enrico Fermi and James Franck, together with the brilliant Hungarian physicists Eugene Wigner and Leo Szilard, and Dr. Norman Hilberry, Compton’s assistant. The purpose of the meeting was to give me an idea of the extent of their knowledge about the plutonium process, and the anticipated explosive power of an atomic bomb, as well as of the amount of fissionable material that a single bomb would require. Of particular importance to me was an understanding of the gaps in knowledge that remained to be filled. I wanted to be sure also that everyone recognized the intermediate goals that had to be achieved before we would attain ultimate success, and that I, too, had a clear understanding of these goals. I was vitally interested in just how much plutonium or how much U-235 would be needed for a reasonably effective bomb. This was all-important, for it would determine the size of our production facilities, not only for plutonium, but also for Uranium-235.
Compton’s group discussed the problem with me thoroughly, backing up their postulations mathematically and eventually arriving at the answers I needed. In general, our discussion was quite matter-of-fact, although much of it was highly theoretical and based on completely unproven, but quite plausible, hypotheses on which all the other participants seemed to be in complete agreement.
As the meeting was drawing to a close, I asked the question that is always uppermost in the mind of an engineer: With respect to the amount of fissionable material needed for each bomb, how accurate did they think their estimate was? I expected a reply of “within twenty five or fifty per cent,” and would not have been greatly surprised at an even greater percentage, but I was horrified when they quite blandly replied that they thought it was correct within a factor of ten.
This meant, for example, that if they estimated that we would need one hundred pounds of plutonium for a bomb, the correct amount could be anywhere from ten to one thousand pounds. Most important of all, it completely destroyed any thought of reasonable planning for the production plants for fissionable materials. My position could well be compared with that of a caterer who is told he must be prepared to serve anywhere between ten and a thousand guests. But after extensive discussion of this point, I concluded that it simply was not possible then to arrive at a more precise answer.
While I had known that we were proceeding in the dark, this conversation brought it home to me with the impact of a pile driver. There was simply no ready solution to the problem that we faced, except to hope that the factor of error would prove to be not quite so fantastic. This uncertainty surrounding the amount of material needed for a bomb plagued us continuously until shortly before the explosion of the Alamogordo test bomb on July 16, 1945. Even after that we could not be sure that Uranium-235 (used in the Hiroshima bomb) would have the same characteristics as plutonium (used in the test and later against Nagasaki), although we knew of no reason why it should be greatly different.
- "Voices of the Manhattan Project," 1965 Interview with General Leslie R. Groves (11 Parts)
- Atomic Heritage Foundation, The Unlikely Pair