The Manhattan Project employed around 130,000 people at its peak, ranging from Nobel Prize-winning physicists to thousands of young women operating uranium enrichment machines they didn’t fully understand. The project spanned dozens of sites across the United States, Canada, and the United Kingdom, but three locations formed its core: Los Alamos, New Mexico; Oak Ridge, Tennessee; and Hanford, Washington. The people who built the world’s first nuclear weapons included some of the most brilliant scientists of the 20th century, tens of thousands of construction laborers, military personnel, engineers, and an international delegation of Allied researchers.
The Two Men Who Ran the Project
Brigadier General Leslie R. Groves led the Manhattan Project from its military side. Groves was an Army Corps of Engineers officer who had just overseen the construction of the Pentagon, and he brought that same blunt, relentless management style to atomic weapons development. He chose the project’s site locations, controlled its budget, and hand-picked the scientific leadership. His most consequential decision was selecting J. Robert Oppenheimer, a theoretical physicist at the University of California, Berkeley, to direct the weapons laboratory at Los Alamos.
Oppenheimer was an unusual choice. He was awkward, introverted, and had left-leaning political associations that made him a security risk on paper. But Groves saw in him someone who could manage a sprawling team of strong-willed scientists, and the bet paid off. Oppenheimer became the intellectual and organizational center of the bomb design effort, coordinating work across dozens of research groups and ultimately leading the project to the Trinity test on July 16, 1945.
The Nobel Laureates and Lead Scientists
The Manhattan Project concentrated an extraordinary density of scientific talent. Several participants had already won Nobel Prizes, and others would earn them for work done during or after the war.
Enrico Fermi, who won the Nobel Prize in Physics in 1938, led the team at the University of Chicago that built Chicago Pile-1, the world’s first nuclear reactor. That reactor demonstrated that a sustained, controlled chain reaction was possible, which was the foundational proof that the bomb could work. Fermi later moved to Los Alamos, where he headed the F Division and witnessed the Trinity test firsthand.
Arthur Compton, a 1927 Nobel laureate, oversaw the Metallurgical Laboratory in Chicago. This was the umbrella organization for Fermi’s reactor work and the plutonium research program. Compton also supervised the X-10 Graphite Reactor at Oak Ridge, a pilot-scale reactor that produced the first usable quantities of plutonium.
Ernest Lawrence, who won the Nobel Prize in 1939 for inventing the cyclotron particle accelerator, developed the electromagnetic method for separating uranium isotopes. His technique was scaled up into the massive Y-12 plant at Oak Ridge. Harold Urey, a 1934 Nobel laureate in Chemistry, ran a parallel effort at Columbia University developing gaseous diffusion, which became the primary uranium enrichment method used after the war.
Glenn Seaborg joined the Metallurgical Laboratory in 1942 and solved one of the project’s most critical chemistry problems: how to extract tiny amounts of plutonium from irradiated uranium. He developed the multi-stage chemical separation process that made plutonium production viable at industrial scale, work that later earned him the 1951 Nobel Prize in Chemistry. Emilio Segrè served as a group leader at Los Alamos from 1943 to 1946 and went on to win the Nobel Prize in 1959 for discovering the antiproton.
The British Mission
After the Quebec Agreement in 1943 formalized cooperation between the U.S. and Britain, a group of Allied scientists joined the Manhattan Project. This “British Mission” included Nobel laureates James Chadwick, who had discovered the neutron, and Niels Bohr, the Danish physicist who had pioneered quantum atomic theory. Future Nobel laureate Joseph Rotblat also participated.
The British scientists brought deep expertise in explosives and fluid dynamics. James Tuck helped develop the shaped explosive lenses needed for the implosion-type bomb. Rudolf Peierls contributed mathematical solutions to blast wave problems. Otto Frisch led the Critical Assemblies group, which did the dangerous work of determining exactly how much fissile material was needed for a bomb. William Penney calculated the optimal detonation altitude for the bombs dropped on Japan, working alongside American physicist Luis Alvarez to predict blast wave damage.
The British Mission also included Klaus Fuchs, a German-born physicist who was later confirmed to have passed nuclear secrets to the Soviet Union throughout his time on the project.
Women Scientists and Workers
Women contributed at every level of the Manhattan Project, though their roles were often overlooked at the time. Leona Woods Marshall Libby was 23 years old when she became the only woman on the team that built and operated Chicago Pile-1 alongside Enrico Fermi. She was present when the reactor first went critical in December 1942, and she was instrumental in building the Geiger counters used to monitor the experiment. She later moved to the Hanford site, where she helped solve the problem of xenon poisoning in the plutonium production reactors.
Maria Goeppert Mayer, a German-born theoretical physicist, worked at Columbia University’s Substitute Alloy Materials Laboratory on uranium isotope separation experiments. She later won the 1963 Nobel Prize in Physics for developing the nuclear shell model.
At the Y-12 plant in Oak Ridge, young women known as the “Calutron Girls” operated the electromagnetic separation machines that enriched uranium. Many were fresh out of high school. They monitored dials and adjusted controls on the calutrons without being told what the machines actually produced. In a direct comparison, these operators outperformed trained scientists at the task. The scientists kept trying to diagnose problems themselves, while the young women simply flagged issues to supervisors and maintained a steadier hand on the controls.
The Massive Workforce at Oak Ridge and Hanford
The three main sites each employed tens of thousands of people, most of whom had no idea they were building an atomic bomb. Oak Ridge, Tennessee, often called the “Secret City,” housed 75,000 people at its wartime peak. The city didn’t appear on any maps. Workers there enriched uranium using multiple competing methods, processed plutonium at pilot scale, and supported dozens of research programs.
Hanford, Washington, was the site of full-scale plutonium production reactors and chemical separation plants. Construction alone required a massive labor force. More than 15,000 African Americans arrived in the Tri-Cities area around Hanford during the project, and approximately 7,000 African Americans worked at Oak Ridge. Thousands of African American men and women contributed to the project across its sites, though most were assigned to construction, janitorial, or domestic work. A limited number worked as scientists and technicians at smaller Manhattan Project sites in New York and Chicago. Many of their individual stories remain undocumented.
Military Personnel and the Special Engineer Detachment
Beyond the civilian workforce, the Army embedded thousands of soldiers directly into the project’s technical operations. The Special Engineer Detachment, formed in May 1943, placed young men with engineering and science backgrounds at Manhattan Project sites to do work that looked nothing like typical military service. What started as a unit of 334 men grew to over 1,800 stationed at Los Alamos and more than 1,200 at Oak Ridge by September 1945. These soldiers contributed to the Trinity test preparations, helped manufacture the Little Boy and Fat Man bombs, and continued production work after the war ended.
Health and Safety Teams
The project also required people to handle a problem no one had faced before: protecting workers from radiation exposure at an industrial scale. Stafford Warren served as the Manhattan Project’s medical director and oversaw the broader health physics program. At the University of Chicago, cosmic ray physicist Ernest Wollan organized a team to study radiation hazards and protect coworkers. That team included Herbert Parker, who developed new radiation measurement units (including the rem, which remained in use for decades), and Karl Morgan, who went on to direct health physics at Oak Ridge from 1944 to 1972.
At Los Alamos, a young doctor named Louis Hempelmann was responsible for the health and safety of the entire site. His primary tool was regular blood sampling of workers, a crude but necessary method for tracking radiation exposure in an era when the biological effects of these materials were still poorly understood.