The Manhattan Project involved roughly 130,000 people at its peak, from Nobel Prize-winning physicists to tens of thousands of construction workers, engineers, and factory operators spread across secret sites in New Mexico, Tennessee, and Washington state. By the end of World War II, the project had spent $2.2 billion (about $37 billion today) to build the first atomic weapons. While most employees never knew what they were building, a core group of military leaders, scientists, and engineers drove the effort from concept to detonation in under three years.
The Two Men Who Ran the Project
General Leslie Groves and physicist J. Robert Oppenheimer formed the leadership pair at the heart of the Manhattan Project. Groves, an Army Corps of Engineers officer with a background in large-scale construction, oversaw the entire operation. He selected the sites, managed the budget, made staffing decisions, and coordinated with the U.S. administration on where and when to use the bombs. He also personally chose Oppenheimer to run the scientific side of the work, overriding security concerns about Oppenheimer’s past political associations. In a letter pushing for Oppenheimer’s clearance, Groves wrote that he was “absolutely essential to the project.”
Oppenheimer directed the Los Alamos laboratory in New Mexico, where the actual bomb design and assembly took place. Groves chose him not because he was the most accomplished physicist available, but because he had unusually broad knowledge across multiple areas of physics, exactly what was needed to coordinate dozens of specialized research teams. Oppenheimer managed the scientists, resolved technical disputes, and kept the work moving toward a deliverable weapon. He left Los Alamos shortly after the war ended, and Groves relinquished control of the project in 1947 when oversight transferred to the new civilian Atomic Energy Commission.
The Scientists Who Made It Possible
The scientific effort drew from a remarkable concentration of talent, including multiple Nobel Prize winners. Enrico Fermi led the team of 49 scientists who achieved the first sustained nuclear chain reaction on December 2, 1942, in a converted squash court beneath the University of Chicago’s abandoned Stagg Field. That experiment, called Chicago Pile-1, proved that a controlled chain reaction was possible and opened the path to both reactors and bombs. Eugene Wigner, a Hungarian-born physicist on the team, celebrated by opening a bottle of Chianti he had bought months earlier in anticipation.
Leo Szilard, another Hungarian émigré, was one of the people most responsible for getting the project started in the first place. Along with Edward Teller, Szilard convinced Albert Einstein to sign a 1939 letter to President Franklin Roosevelt warning that Germany might develop atomic weapons. That letter initiated the government-funded research program that eventually became the Manhattan Project. Einstein himself did not work on the project.
Ernest Lawrence, who had already won the Nobel Prize for inventing the cyclotron particle accelerator, contributed the electromagnetic separation technology used at Oak Ridge to enrich uranium. Arthur Compton, a Nobel laureate for his work on X-ray scattering, led the plutonium research effort in Chicago. Harold Urey, who won the Nobel Prize in Chemistry for discovering a heavy form of hydrogen, worked on isotope separation methods. Glenn Seaborg, whose team had first isolated plutonium, contributed to the chemistry needed to extract usable bomb material from reactor fuel. He later won the Nobel Prize in Chemistry in 1951.
A younger generation of American-born physicists also played key roles. Richard Feynman, then in his mid-twenties, worked on theoretical calculations at Los Alamos. Emilio Segrè, an Italian émigré, conducted research that influenced the bomb’s design. Maria Goeppert-Mayer, a German-born theoretical physicist, worked on uranium isotope separation at Columbia University’s Substitute Alloy Materials Laboratory and went on to become the first woman to win the Nobel Prize in Physics since Marie Curie.
The British Mission
The Manhattan Project was not purely an American effort. A group of 19 scientists known as the British Mission worked at Los Alamos, and despite their small numbers, their contributions were significant. The group included some of Europe’s best experimental and theoretical physicists along with specialists in electronics and explosives.
Fifteen members were British nationals, including William Penney, who later led Britain’s own nuclear weapons program. The rest were European refugees who had fled to Britain: Niels Bohr and his son Aage from Denmark, Otto Frisch from Austria, Rudolf Peierls and Klaus Fuchs from Germany, and Joseph Rotblat from Poland. Bohr and James Chadwick, who discovered the neutron, were both Nobel laureates. Frisch led the Critical Assemblies group at Los Alamos, testing how close materials could come to triggering a chain reaction. Fuchs, it was later revealed, had been passing nuclear secrets to the Soviet Union throughout his time on the project.
Three Secret Cities
The project’s work was split across three major sites, each with a distinct purpose and a workforce numbering in the tens of thousands.
Oak Ridge, Tennessee handled uranium enrichment. Three different industrial processes ran simultaneously there: electromagnetic separation (at the Y-12 plant), gaseous diffusion (at K-25), and thermal diffusion (at S-50). Each process partially enriched the uranium, and the output from one fed into the next until the material reached weapons-grade concentration. Kenneth Nichols, Groves’s deputy, coordinated the complicated feed schedule between the plants. Tennessee Eastman, a subsidiary of Eastman Kodak, operated the Y-12 facility. By early 1945, the system was producing enough enriched uranium for a bomb.
Hanford, Washington produced plutonium. Three nuclear reactors, designated B, D, and F, were built along the Columbia River. B Reactor went online in September 1944 but hit an unexpected problem: a byproduct of the nuclear reaction was poisoning the chain reaction and shutting it down. Engineers solved the issue, and by February 1945, the first plutonium from Hanford was on its way to Los Alamos. D Reactor went critical in December 1944, and F Reactor followed in February 1945.
Los Alamos, New Mexico was the weapons laboratory where the bomb itself was designed, built, and tested. Oppenheimer ran this site, which housed the scientific core of the project. The work there was useless without the enriched uranium from Oak Ridge and the plutonium from Hanford, and for much of 1944, it was unclear whether enough material would arrive in time.
The Workers Most People Forget
The vast majority of the 130,000 people who worked on the Manhattan Project were not scientists. They were construction workers, machine operators, secretaries, janitors, and technicians who built and ran the enormous industrial facilities. Most were never told what they were making.
More than 15,000 African Americans worked at Hanford, and approximately 7,000 worked at Oak Ridge. They came for higher wages and to support the war effort, but they faced the same Jim Crow segregation that existed throughout the country at the time. Most were assigned to lower-level positions as construction workers, laborers, janitors, or domestic workers. Housing, recreational facilities, and cafeterias were segregated. A limited number of African American men and women worked as scientists and technicians, but primarily at smaller project sites in New York and Chicago rather than at the major facilities.
Women filled critical roles across the project as well. Thousands worked as technicians and machine operators at Oak Ridge, where they monitored the calutron dials used in electromagnetic separation, often without knowing they were enriching uranium. On the scientific side, researchers like Goeppert-Mayer at Columbia and Chien-Shiung Wu, who worked on radiation detection, contributed directly to the technical success of the effort.
What Happened to Them After the War
When the Atomic Energy Commission took over from the military in 1947, it inherited the infrastructure and many of the people from the Manhattan Project. Robert Bacher, a physicist from Los Alamos, became the only scientist among the commission’s five founding members. The others included a New England businessman, an Iowa newspaper editor, and Lewis Strauss, a banker and reserve admiral who would later play a central role in revoking Oppenheimer’s security clearance.
Several Manhattan Project participants went on to win Nobel Prizes for work done before, during, or after the project. The list includes Fermi, Lawrence, Compton, Urey, Seaborg, Segrè, and Goeppert-Mayer. Others, like Feynman, built distinguished careers in physics and public life. Some, like Szilard and Rotblat, became vocal advocates for nuclear disarmament, troubled by what they had helped create. Rotblat won the Nobel Peace Prize in 1995 for his decades of work against nuclear weapons.