Manhattan Project

• 1. Introduction

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  The Manhattan Project built the first nuclear weapons in human history, yet it operated in such secrecy that most of its nearly 130,000 employees had no idea what they were working on. A sprawling research and development program led by the United States, with close collaboration from the United Kingdom and Canada, the project consumed nearly two billion dollars and stretched across dozens of sites from New Mexico to British Columbia. At its heart was a race against a terrifying possibility: that Nazi Germany might develop a nuclear bomb first. The questions worth sitting with are large. How did a theoretical discovery in a European laboratory translate, in just a few years, into weapons that could level a city? Who made the decisions, and who paid the price? And what did the project leave behind once the war was over?

• 2. A Letter and a Fear

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  Otto Hahn and Fritz Strassmann discovered nuclear fission in 1938, and within months the world of physics understood what it might mean. Lise Meitner and Otto Frisch worked out the theoretical explanation, and the implication was stark: a uranium bomb was no longer science fiction. In Britain, Rudolf Peierls and Otto Frisch at the University of Birmingham calculated in their March 1940 memorandum that the critical mass of uranium-235 was small enough to fit inside a contemporary bomber. That single calculation launched the British atomic bomb project and its MAUD Committee. Across the Atlantic, the American response was slower. Leo Szilard and Eugene Wigner drafted what became known as the Einstein-Szilard letter in August 1939, warning President Franklin D. Roosevelt of the potential for "extremely powerful bombs of a new type" and urging him to secure uranium stockpiles. Roosevelt responded by calling on Lyman Briggs to chair an Advisory Committee on Uranium, but critics inside the scientific community, including Karl T. Compton, complained that Briggs operated at a "peace-time government bureau tempo" that was dangerously out of step with the emergency at hand. The breakthrough came partly through a visit by the Australian physicist Mark Oliphant, who flew to the United States in late August 1941 and discovered that crucial MAUD Committee findings had not reached key American physicists. Oliphant spoke persuasively to Ernest O. Lawrence at Berkeley, who then spoke to James B. Conant, Arthur H. Compton, and George B. Pegram. On the 9th of October 1941, President Roosevelt approved accelerating the atomic program, forming a Top Policy Group that included Vice President Henry A. Wallace, Vannevar Bush, Secretary of War Henry L. Stimson, and General George C. Marshall.

• 3. Groves Takes Command

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  Colonel Leslie Groves was assigned to lead the Army's component of the project in June 1942, a posting he initially did not want. He assumed command on the 23rd of September 1942, and within a single day he had already attended a Stimson meeting that established the Military Policy Committee. General George C. Marshall ordered Groves promoted to brigadier general on the logic that a general's title would hold more sway with the academic scientists he would need to manage. Groves moved quickly to consolidate control. One early obstacle was the project's supply priority rating. The War Production Board's top civilian ratings were AA-1 through AA-4, and the project had been stuck at AA-3, the same level as a TNT plant in Pennsylvania. Groves went to the chairman of the War Production Board, Donald Nelson, and pressed for broad authority to issue an emergency AAA rating. Nelson initially refused. When Groves threatened to take the matter to the president, Nelson capitulated. Even so, AAA proved too powerful for routine needs and AA-3 too weak; Groves only secured the AA-1 authority he actually wanted on the 1st of July 1944. Finding a director for the weapons design laboratory, Project Y, was equally complicated. The obvious candidates were the laboratory heads Urey, Lawrence, and Arthur Compton, but none could be spared. Compton recommended J. Robert Oppenheimer, a theoretical physicist from the University of California who was already deeply familiar with bomb design. Oppenheimer had never won a Nobel Prize, which made some scientists skeptical, and his security file raised serious concerns: his wife Kitty, his girlfriend Jean Tatlock, and his brother Frank all had communist associations. A long conversation in October 1942 convinced Groves and Nichols that Oppenheimer was the right choice. Groves personally waived the security requirements and issued Oppenheimer's clearance on the 20th of July 1943.

• 4. Cities Built in Secret

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  The Manhattan Project required industrial infrastructure on a scale that had never existed before, and it had to be constructed in near-total secrecy. The site in Tennessee that became Oak Ridge was selected partly because the Tennessee Valley Authority could supply ample electric power and the rivers could cool the reactors. On the 29th of September 1942, Under Secretary of War Robert P. Patterson authorized the acquisition of roughly 56,000 acres by eminent domain at a cost of about three and a half million dollars. Around 1,000 families were displaced; some received only two weeks' notice to leave farms their families had worked for generations. The population of Oak Ridge swelled far beyond initial projections, peaking at 75,000 in May 1945, by which time more than 82,000 people were employed at the Clinton Engineer Works alone. The weapons design laboratory at Los Alamos in New Mexico was chosen on Oppenheimer's recommendation, partly because he owned a ranch nearby and expressed a strong personal preference for the site, citing its natural beauty. Patterson approved the purchase on the 25th of November 1942. Groves initially set aside $300,000 for construction, three times Oppenheimer's own estimate, but by the time the contractor Sundt finished work on the 30th of November 1943, over seven million dollars had been spent. The Hanford Engineer Works in Washington state was established to house the plutonium production reactors, selected because it was isolated and sat beside the Columbia River. Colonel Franklin Matthias reported that the Hanford Site near Richland was "ideal in virtually all respects." At its peak, the construction camp at Hanford was the third most populous settlement in the state of Washington, operating a fleet of more than 900 buses, a larger fleet than the city of Chicago ran at the time.

• 5. Enriching the Fuel

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  Natural uranium is overwhelmingly composed of uranium-238, the non-fissile isotope, with uranium-235 making up only 0.7 percent. Separating the two was one of the project's central engineering challenges. Three technologies ultimately succeeded: electromagnetic separation, gaseous diffusion, and thermal diffusion, and in February 1943 Groves devised the scheme of feeding the output of one plant directly into the next as the input. The electromagnetic separation plant, designated Y-12, ran on a technology called the calutron, a name derived from California, university, and cyclotron. The process required an extraordinary amount of electrical copper, which was in desperately short supply. So the project borrowed silver instead, eventually using 14,700 short tons of bullion transferred from the West Point Bullion Depository. The operators who ran the calutrons were known as the Calutron Girls, trained Tennessee Eastman employees who had no idea what the machines actually did. Their calutrons initially enriched uranium-235 to between 13 and 15 percent. By January 1945, strenuous recovery efforts had raised the proportion of uranium feed that emerged as usable product to 10 percent. The gaseous diffusion plant, K-25, was a four-story structure half a mile long arranged in a U shape and containing 54 contiguous buildings. The total cost, including the later K-27 addition, reached $480 million. The thermal diffusion plant, S-50, built in just two months beginning in July 1944, became the first stage in the final cascade: it raised enrichment from 0.71 to 0.89 percent, the K-25 plant then brought it to roughly 23 percent, and Y-12 boosted it to approximately 89 percent. About 50 kilograms of uranium enriched to 89 percent were delivered to Los Alamos by July 1945.

• 6. Plutonium's Complications

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  Plutonium existed in nature only in trace amounts, and producing it in usable quantities meant building the world's first industrial-scale nuclear reactors. On the 2nd of December 1942, a team led by Enrico Fermi initiated the first artificial self-sustaining nuclear chain reaction beneath the bleachers of Stagg Field at the University of Chicago, in an experimental reactor called Chicago Pile-1. Fermi used a coded phone call to report the success to Conant in Washington, describing himself as "the Italian navigator" who had just landed in the new world. The far larger reactors at Hanford followed. Work began on Reactor B, the first of the planned 250-megawatt reactors, on the 10th of October 1943. When it was powered up beginning on the 13th of September 1944, disaster struck almost immediately. Shortly after midnight on the 27th of September, the power level began dropping and by 06:30 the reactor had shut down completely. The culprit, identified by physicist Chien-Shiung Wu, was neutron poisoning from xenon-135, whose nuclear cross section turned out to be 30,000 times that of uranium. The reactor could only recover because a DuPont engineer, George Graves, had added 504 tubes beyond the original design of 1,500, filling in the corners in a move the scientists had regarded as wasteful overengineering. By loading all 2,004 tubes, the reactor could overpower the xenon poisoning. Plutonium, once produced, presented a second problem at the weapon design stage. Los Alamos received its first sample of reactor-bred plutonium in April 1944, and within days Emilio Segrè discovered that it contained a much higher concentration of plutonium-240 than cyclotron plutonium, causing up to five times the spontaneous fission rate. Inserting this material into a gun-type weapon would cause a premature chain reaction, a failure mode the scientists called a fizzle. This discovery killed the gun-type plutonium design entirely and forced the project to bet everything on the more complex implosion approach.

• 7. The Implosion Race

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  Physicist Seth Neddermeyer had been working on implosion since 1943 as an alternative weapon design: the idea was to use precisely shaped explosive charges to squeeze a subcritical sphere of fissile material into a smaller, denser configuration faster than a gun assembly could manage. In September 1943, John von Neumann, who had experience with shaped charges, proposed using a spherical configuration rather than the cylindrical one Neddermeyer had been pursuing. When reactor-bred plutonium ruled out the gun-type bomb in mid-1944, Oppenheimer reorganized the entire Los Alamos laboratory in August of that year to concentrate on implosion, creating two new divisions: X Division for explosives, led by George Kistiakowsky, and G Division for the gadget itself, led by Robert Bacher. The final design used 32 explosive lenses arranged to focus the detonation inward in a spherical shape, with 20 hexagonal and 12 pentagonal lenses, each weighing about 80 pounds, resembling a soccer ball. Getting the detonation timing precisely right required exploding-bridgewire detonators, a new invention developed at Los Alamos by a group led by Luis Alvarez, with two detonators per lens for reliability. To study converging shock waves, Robert Serber developed the RaLa Experiment, which placed a gamma ray source inside a metal sphere surrounded by explosive lenses and used an ionization chamber to capture what was effectively an X-ray movie of the implosion. David Hawkins later wrote in his history of the Los Alamos project that "RaLa became the most important single experiment affecting the final bomb design." Inside the assembly, a polonium-beryllium neutron initiator called an "urchin" was designed to start the chain reaction at precisely the right moment; the chemistry and metallurgy work on polonium was directed by Charles Allen Thomas of the Monsanto Company through what became known as the Dayton Project, which required up to 500 curies of polonium per month.

• 8. Secrets, Spies, and Trinity

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  Despite the project's intense security apparatus, Soviet atomic spies penetrated the program. The identities of those spies, and the extent of what they passed on, became some of the defining intelligence questions of the early Cold War. The British scientists who arrived at Los Alamos in December 1943, a group that included Niels Bohr, Otto Frisch, Klaus Fuchs, Rudolf Peierls, and Ernest Titterton, were among the project's most valuable contributors. Fuchs was later identified as one of the Soviet sources. Through Operation Alsos, Manhattan Project personnel also worked in Europe, sometimes behind enemy lines, gathering nuclear materials and documents and rounding up German scientists to assess how far Germany's own nuclear program had advanced. The first nuclear device ever detonated was an implosion bomb, tested at White Sands Proving Ground in New Mexico on the 16th of July 1945 in the Trinity test. The two weapon designs used in the war were Little Boy, a gun-type uranium bomb, and Fat Man, the implosion plutonium design. Fat Man had originally been a lower-priority fallback, but plutonium's incompatibility with gun-type assembly had made it the primary design. The Hyde Park Aide-Memoire, signed by Roosevelt and Churchill in late September 1944, had already suggested that once a bomb was "finally available" it might be used against Japan, who should be warned that bombardment would be repeated until surrender. In August 1945, Little Boy was dropped on Hiroshima and Fat Man on Nagasaki. The Manhattan Project maintained control over American atomic weapons research and production until the formation of the United States Atomic Energy Commission in January 1947, an institution whose existence it had made both necessary and inevitable.