to which we wanted to contribute most. Nevertheless, Oppenheimer… and many of the most prominent men in the laboratory continued to say that the job at Los Alamos would not be complete if we should remain in doubt whether or not a thermonuclear bomb was feasible.
To that end Oppenheimer in May discussed tritium production with Groves and Du Pont's Crawford Greenewalt. The chemical company had built a pilot-scale air-cooled pile at Oak Ridge that produced neutrons to spare; Greenewalt agreed to put some of them to use bombarding lithium. Teller departed the Theoretical Division. Rudolf Peierls took his place. Oppenheimer arranged then to meet with Teller weekly for an hour of freewheeling talk. That was a remarkable concession when the laboratory was working overtime six days a week to build a bomb before the end of the war. Oppenheimer may well have thought Teller's imaginative originality worth it. He also understood his extreme sensitivity to slight. Later that summer, when Cherwell visited Los Alamos, Oppenheimer gave a party and inadvertently failed to invite Peierls, who was deputy head of the British mission under James Chadwick. Oppenheimer sought out Peierls the next day and apologized, adding: “But there is an element of relief in this situation: it might have happened with Edward Teller.”
George Kistiakowsky adjusted himself to Seth Neddermeyer until he felt that not only he but also the project was suffering; then he reviewed his alternatives and, on June 3, wrote Oppenheimer a memorandum. He and Neddermeyer had established a certain
He proposed three possible solutions. He could resign, the solution he thought best and fairest to Neddermeyer. Or Neddermeyer could resign, but that would disturb his staff and slow the work; it would also be unfair to a good physicist. Or Neddermeyer could “take over more vigorous scientific and technical direction of the project but dissociate himself completely from all administrative and personnel matters.”
Oppenheimer had come to value Kistiakowsky too highly to choose any of these alternatives. He proposed a fourth. Kistiakowsky worked out the details and the men met painfully to present it to Neddermeyer on a Thursday evening in mid-June: Kistiakowsky would assume full responsibility for implosion work as an associate division leader under Parsons. Neddermeyer and Luis Alvarez, recently arrived from Chicago, would become senior technical advisers. Neddermeyer left the meeting early, as well he might. “I am asking you to accept the assignment,” Oppenheimer wrote him the same evening. “… In behalf of the success of the whole project, as well as the peace of mind and effectiveness of the workers in the H. E. program, I am making this request of you. I hope you will be able to accept it.” With enduring bitterness Neddermeyer did.
The air-cooled, pilot-scale reactor at Oak Ridge had gone critical at five o'clock in the morning on November 4, 1943; the loading crews, realizing during the night that they were nearing criticality sooner than expected, had enjoyed rousting Arthur Compton and Enrico Fermi out of bed at the Oak Ridge guest house to witness the event. The pile, which was designated X-10, was a graphite cube twenty-four feet on a side drilled with 1,248 channels that could be loaded with canned uranium-metal slugs and through which large fans blew cooling air. The channels extended for loading through the seven feet of high-density concrete that composed the pile face; at the back they opened onto a subterranean pool like the pools planned for Hanford into which irradiated slugs could be pushed to shield them until they lost their more intense short-term radioactivity. Chemists then processed the slugs in a remote-controlled pilot-scale separations plant using the chemical separation processes Glenn Seaborg and his colleagues had developed at ultramicrochemical scale in Chicago.
A few days before Compton moved to Oak Ridge to supervise the X-10 operation, at the end of November, workers discharged the first five tons of irradiated uranium from the pile. Chemical separations began the following month. By the summer of 1944 batches of plutonium nitrate containing gram quantities of plutonium had begun arriving at Los Alamos. The man-made element was quickly used and reused in extensive experiments to study its unfamiliar chemistry and metallurgy — more than two thousand separate experiments by the end of the summer.
Not chemistry or metallurgy but physics nearly condemned the plutonium bomb to failure that summer. More than a year previously Glenn Seaborg had warned that the isotope Pu240 might form along with desirable Pu239 when uranium was irradiated to make plutonium. Pu240, an even-numbered isotope, was likely to exhibit a much higher rate of spontaneous fission than Pu239. The plutonium samples Emilio Segre had studied at his isolated log- cabin laboratory fissioned spontaneously at acceptable rates. They had been transmuted from uranium in one of the Berkeley cyclotrons. U238 needed one neutron to transmute to Pu239; for Pu240 it required two, and far more neutrons bombarded the uranium slugs cooking in the X-10 pile than a cyclotron could generate. When Segre measured the spontaneous fission rate of the X-10 plutonium he found it much higher than the Berkeley rate. The rate for Hanford plutonium, which would be exposed to an even heavier neutron flux, was likely to be higher still. That meant they would not need to cleanse the plutonium so thoroughly of light-element impurities. But it also signaled catastrophe. They could not use a gun to assemble a critical mass of such stuff: approaching each other even at 3,000 feet per second, the plutonium bullet and target would melt down and fizzle before the two parts had time to join.
Oppenheimer alerted Conant on July 11. The two men met with Compton, Groves, Nichols and Fermi in Chicago six days later and the next day Oppenheimer wrote Groves to confirm their conclusions. Pu240 was apparently long-lived, and since the two isotopes were elementally identical it could not be removed chemically. They had not considered separating Pu239 from Pu240 electromagnetically. Such an effort with isotopes that differed by only one mass unit and were highly toxic would dwarf the vast calutron operation at Oak Ridge and could not possibly be accomplished in time to influence the outcome of the war. “It appears reasonable,” Oppenheimer ended, “to discontinue the intensive effort to achieve higher purity for plutonium and to concentrate attention on methods of assembly which do not require a low neutron background for their success. At the present time the method to which an over-riding priority must be assigned is the method of implosion.”
That necessity was painful, as the Los Alamos technical history makes clear: “The implosion was the only real hope, and from current evidence not a very good one.” Oppenheimer agonized over the problem to the point that he considered resigning his directorship. Robert Bacher, the sturdy leader of the Experimental Physics Division, took long walks with him in those days to share his pain and eventually dissuaded him. There was no one else who could do the job, Bacher argued; without Oppenheimer there would be no bomb in time to shorten the war and save lives.
Action changed Oppenheimer's mood. “The Laboratory had at this time strong reserves of techniques, of trained manpower, and of morale,” says the technical history. “It was decided to attack the problems of the implosion with every means available, ‘to throw the book at it.’” Going over the prospects with Bacher and Kistiakowsky, Oppenheimer decided to carve two new divisions out of Parsons' Ordnance Division: G (for Gadget) under Bacher to master the physics of implosion and X (for eXplosives) under Kistiakowsky to perfect explosive lenses. The Navy captain howled, Kistiakowsky remembers:
[Oppenheimer] called a big meeting of all the group heads, and there he sprang on Parsons the fact that I had plans for completely re-designing the explosives establishment. Parsons was furious — he felt that I had by- passed him and that was outrageous. I can understand perfectly how he felt but I was a civilian, so was Oppie, and I didn't have to go through him… From then on Parsons and I were not on good terms. He was extremely suspicious of me.
Parsons had his hands full in any case designing the uranium gun, Little Boy, and arranging its eventual use. Oppenheimer prevailed: they would throw the book at implosion. In the months ahead the laboratory, which had swollen to 1,207 full-time employees by the previous May 1, would once again double and redouble in size.
Philip Abelson, the young Berkeley physicist to whom Luis Alvarez had run from his barber chair in January 1939 to announce the news of fission, had moved to the Naval Research Laboratory in 1941 to work on uranium enrichment for the Navy and had made valuable progress independently of the Manhattan Project in the intervening years. The Navy was interested in nuclear power as a motive force for submarines, to extend their range and to allow them to travel farther submerged. But a pile of the sort Fermi would build would be unwieldy; “it had become pretty obvious,” Abelson recalls, “that a reactor fueled with natural uranium would be big as a barn.” Increase the ratio of U235 to U238 in the reactor fuel — enrich the uranium — and the reactor could be correspondingly smaller; with enough enrichment, small enough to fit inside the hull of a submarine in the space previously reserved for
