Ulam remembers that Teller threatened to leave. Oppenheimer stepped in then to save him for the project. He encouraged Teller to give himself over to the Super—encouragement, Teller wrote in 1955, perhaps disingenuously, that he needed to move him on from the immediate task at hand:
Oppenheimer . . . continued to urge me with detailed and helpful advice to keep exploring what lay beyond the immediate aims of the laboratory. This was not easy advice to give, nor was it easy to take. It is easier to participate in the work of the scientific community, particularly when a goal of the highest interest and urgency has been clearly defined. Every one of us considered the present war and the completion of the A-bomb as the problems 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.2076
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.”2077
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.2078 He and Neddermeyer had established a certain modus vivendi, he wrote, but it was not what he had been asked to do, which was to administer implosion work while Neddermeyer did the science, and it was “not based on mutual confidence and a friendly give-and-take.”
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.”2079 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.2080
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 Segrè 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 Segrè 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.”2081
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.”2082 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.2083 “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.2084
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.2085
* * *
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.”2086 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 diesel engines, batteries and fuel.
Enrichment and separation are different goals, but the same technologies achieve them. Abelson began work by looking up the record of those technologies. Gaseous barrier diffusion was under study then at Columbia, electromagnetic separation at Berkeley, centrifuge separation at the University of Virginia. Abelson decided to try a process that had been pioneered in Germany before the war: liquid thermal diffusion (using glass tubes, Otto Frisch had experimented unsuccessfully with a similar process, gaseous thermal diffusion, at Birmingham). Thermal diffusion relied on the tendency of lighter isotopes to diffuse toward a hotter region while heavier isotopes diffused toward a colder region. The mechanism for driving such diffusion could be simple: a hot pipe inside a cold pipe with liquid uranium hexafluoride flowing between the two pipe walls. Depending on the difference in temperature and the spacing between the two pipes more or less diffusion would occur. At the same time the heating and cooling of the hex would start a convection current flowing up the hot pipe wall and down the cold. That would bring the U235-enriched fluid to the top of the column where it could be tapped off. To increase the enrichment a number of columns could be connected in series to make a cascade like the cascade of barrier tanks planned for K-25.
Abelson’s first technical contribution, in 1941, was inventing a relatively cheap way to make uranium hexafluoride. He processed the first hundred kilograms of hex produced in the United States. For the nominal sum of one dollar the Army contracted to borrow his patented process for Oak Ridge. He never saw the dollar.2087
The experimental thermal-diffusion columns Abelson built at the Naval Research Laboratory in 1941 and 1942 were 36 feet tall, each consisting of three pipes arranged one inside the other. The hot inner pipe, 1¼ inches in diameter, carried high-pressure steam at about 400° F. Surrounding that nickel pipe a copper pipe contained the liquid hex. The critical spacing between the two pipes where the hex flowed measured only about one-tenth of an inch. Surrounding both pipes a 4-inch pipe of galvanized iron carried water at about 130°, just above hex’s melting point, to cool the hex.2088
Pumps that circulated the water were the only moving parts. “The apparatus was run continuously with no shut down or break down what so ever,” Abelson reported to the Navy early in 1943. “Indeed, so constant were the various temperatures and operating characteristics that practically no attention was required to insure successful operation. Many days passed in which operating personnel did not touch any control device.”2089 To stop the flow of the hex out of a column Abelson simply dipped the bend of a U-shaped metal drain tube into a bucket of dry ice and alcohol, which froze the hex and plugged the tube. A flame to warm the tube started the flow again.
Abelson’s January 4, 1943, report, submitted jointly with his NRL colleague Ross Gunn, indicated that uranium could be enriched within a single thermal-diffusion column from its natural U235 content of 0.7 percent up to 1 percent or better. With several thousand columns connected in series Abelson thought he could produce 90 percent pure U235 at the rate of 1 kilogram per day at a total construction cost of no more than $26 million. Ninety percent purity was entirely sufficient to make a bomb. (That estimate proved optimistic, however, and equilibrium time for such a cascade appeared to be as long as 600 days.)
Another choice, more in keeping with the Navy’s interest in submarine propulsion, emphasized quantity enrichment rather than quality. Abelson proposed building a plant of 300 48-foot columns operating in parallel to make large amounts of slightly enriched uranium immediately. Chicago could use such slightly enriched uranium to advance its pile work, Abelson thought. He did not yet know that CP-1 had gone critical just one month before his report. “Information concerning the many experiments performed by [the Chicago] workers in the last six months has been denied to us,” he complained. “It is vitally necessary that there be an exchange of technical information if proper plans are to be made for future plants.”2090 The NRL had been the first research center Groves visited when he took charge of the Manhattan Project in September 1942. Months before then, Franklin Roosevelt had specifically instructed Vannevar Bush to exclude the Navy from atomic bomb development. Groves followed the NRL’s research and Bush encouraged its funding through the Military Policy Committee. But by 1943 the official flow of information on nuclear energy research ran from the Navy to the Army one-way only.
Unofficially, however, several of Groves’ compartments leaked. In November 1943 the Navy authorized Abelson to build his 300-column plant. He had searched for a sufficient source of steam—thermal diffusion used volcanic magnitudes of steam, one reason the Manhattan Project had chosen not to pursue it—and located the Naval Boiler and Turbine Laboratory at the Philadelphia Navy Yard. “They were testing good-sized boilers that would go into ships,” Abelson says. “They had the capability of making quantities of steam at a thousand pounds per square inch and they had Navy people standing twenty-four-hour watches to deliver the steam.”2091 The boiler laboratory’s waste steam would supply his 300-column plant, but before scaling up that far he planned to test his design by building and operating only the first 100 columns. Construction began in January 1944, with completion scheduled for July. By now Abelson knew more about the Manhattan Project.2092 He knew that the barriers which Houdaille-Hershey had been stripped and reequipped to manufacture were not yet passing inspection and that K-25, the gaseous-diffusion plant, was therefore woefully behind schedule. He knew Los Alamos had been founded with Robert Oppenheimer as its director. He knew Berkeley was struggling to make its calutrons work. He saw that his thermal-diffusion process might come to the bomb project’s rescue and he was generous enough and worried enough about the war to offer it despite the Army’s several previous rebuffs.
He chose not to work through the limited official channels that the Army and the OSRD had devised to constrict the flow of information. “I wanted to let Oppenheimer know what we were doing. Someone in the Bureau of Ships knew one of the people in the [Navy] Bureau of Ordnance who was going out to Los Alamos. I remember that I met the man at the old Warner Theater here in Washington, up in the balcony—real cloak and dagger stuff.”2093 Abelson briefed the BuOrd officer about t
he plant he was building. He said that he expected to be producing 5 grams a day of material enriched to 5 percent U235 by July. This vital information the BuOrd man carried to Los Alamos and passed along to Edward Teller.2094 Teller in turn briefed Oppenheimer. Oppenheimer apparently conspired then with Deke Parsons, the Hill’s ranking Navy man, to concoct a cover story: that Parsons had learned of the Abelson work on a visit to the Philadelphia Navy Yard. With the Navy thus protected, Oppenheimer on April 28 alerted Groves.
Oppenheimer had seen Abelson’s January 1943 report only a few months previously, a year after it was written. He was not impressed. Like his colleagues Oppenheimer had considered only those processes that enriched natural uranium all the way up to bomb grade, a requirement thermal diffusion could not efficiently meet. Now he realized that Abelson’s process offered a valuable alternative, the alternative Abelson had proposed in his report to help Chicago advance its pile work: slight enrichment of larger quantities. Feeding even slightly enriched material into the Oak Ridge calutrons would greatly increase their efficiency. A thermal-diffusion plant could therefore substitute at least temporarily for the stalled lower stages of the K-25 plant and supplement the output of the Alpha calutrons. Abelson’s 100-column plant with the columns operating in parallel, Oppenheimer calculated, should produce about 12 kilograms a day of uranium of 1 percent enrichment.
“Dr. Oppenheimer . . . suddenly told me that we had [made] a terrible scientific blunder,” Groves testified after the war.2095 “I think he was right. It is one of the things that I regret the most in the whole course of the operation. We had failed to consider [thermal diffusion] as a portion of the process as a whole.” From the beginning the leaders of the Manhattan Project had thought of the several enrichment and separation processes as competing horses in a race. That had blinded them to the possibility of harnessing the processes together. Groves had partly opened his eyes when barrier troubles delayed K-25; then he had decided to cancel the upper stages of the K-25 cascade and feed the lower-stage product to the Beta calutrons for final enrichment. So he was prepared to understand immediately Oppenheimer’s similar point about the value of a thermal-diffusion plant: “I at once decided that the idea was well worth investigating.”2096