Manhattan Project (1942–1945): The Race for the Atomic Bomb

The Manhattan Project was the secret programme that built the world’s first atomic bombs during the Second World War. Run by the United States Army from August 1942, with British and Canadian partners, it grew into a vast industrial and scientific enterprise spread across dozens of sites, culminating in the Trinity test in New Mexico on 16 July 1945. Its weapons were used against Hiroshima and Nagasaki weeks later, and its legacy shaped the nuclear age that followed.

Background: from fission to fear

The project grew out of the discovery of nuclear fission in Berlin in December 1938 and the finding in 1939 that each split uranium nucleus releases further neutrons, making a chain reaction possible. Many leading physicists who understood the implications were refugees from Nazi Germany and feared that Hitler’s scientists would build a bomb first.

In August 1939 the Hungarian-born physicist Leo Szilard drafted a letter, signed by Albert Einstein, warning President Franklin D. Roosevelt of the danger. Roosevelt set up a small Advisory Committee on Uranium in October 1939, but progress was slow. The decisive push came from Britain. In 1940 Otto Frisch and Rudolf Peierls in Birmingham calculated that a bomb would need kilograms, not tons, of the rare isotope uranium-235. The British MAUD Committee endorsed the idea in July 1941, and its report persuaded American leaders that a weapon was feasible within the war.

On 9 October 1941 Roosevelt approved an accelerated programme under the Office of Scientific Research and Development, headed by Vannevar Bush with James B. Conant. After the attack on Pearl Harbor brought the United States into the war in December 1941, the effort expanded rapidly.

Organisation: the Army takes charge

Because the work required huge construction projects, responsibility passed in 1942 to the Army Corps of Engineers. The Manhattan Engineer District was established in August 1942, named after the location of its first headquarters in New York. In September 1942 Colonel Leslie R. Groves, who had just overseen construction of the Pentagon, was put in charge and promoted to brigadier general.

Groves was forceful, demanding and highly effective. Within days he secured a top priority rating for materials, bought uranium ore stored on Staten Island and acquired land in Tennessee. In October 1942 he chose the theoretical physicist J. Robert Oppenheimer, of the University of California, Berkeley, to lead a new central laboratory for bomb design, a surprising choice since Oppenheimer had no administrative experience and no Nobel Prize.

Secrecy was intense. The project relied on compartmentalisation, meaning workers were told only what they needed for their own task. Most of the people who built the plants never knew they were making material for a bomb.

The three main sites

Oak Ridge, Tennessee: separating uranium

Natural uranium is more than 99 per cent uranium-238; only about 0.7 per cent is the fissile uranium-235. Separating the two isotopes, which are chemically identical, was one of the hardest problems of the project. At the Clinton Engineer Works, around the new town of Oak Ridge, the project built several competing plants:

• Y-12, using electromagnetic separators called calutrons, developed by Ernest Lawrence at Berkeley. Because copper was needed for the war, about 14,700 tons of silver were borrowed from the US Treasury for the magnet windings.
• K-25, a gaseous diffusion plant that, when completed, was among the largest buildings in the world.
• S-50, a smaller thermal diffusion plant added in 1944.
• X-10, an experimental graphite reactor that went critical in November 1943 and produced the first gram-scale samples of plutonium.

Chicago and Hanford, Washington: making plutonium

A second route to a bomb used plutonium, a new element made when uranium-238 absorbs a neutron inside a reactor. On 2 December 1942, under the stands of Stagg Field at the University of Chicago, Enrico Fermi’s team achieved the first controlled, self-sustaining nuclear chain reaction in Chicago Pile-1.

Full-scale production was entrusted to the DuPont company at the Hanford Engineer Works on the Columbia River in Washington State. The B Reactor started up in September 1944, briefly stalled because of an unexpected effect called xenon poisoning, and then ran with extra fuel channels that DuPont engineers had built in as a precaution. Plutonium from Hanford reached Los Alamos in early 1945.

Los Alamos, New Mexico: designing the bomb

The weapons laboratory, known as Project Y, opened in spring 1943 on a remote mesa that had been the site of the Los Alamos Ranch School. Oppenheimer gathered a remarkable group of scientists, including Hans Bethe, Edward Teller, Richard Feynman, Robert Serber, Emilio Segrè and later Enrico Fermi, together with engineers, technicians and soldiers.

Two designs emerged. A gun-type weapon fired one piece of uranium-235 into another to form a supercritical mass. It was so reliable that it was never tested before use. For plutonium, however, a crisis struck in 1944: reactor-made plutonium contained the isotope plutonium-240, which fissions spontaneously and would make a gun-type bomb fizzle. The laboratory was reorganised to develop implosion, in which carefully shaped explosive charges crush a sphere of plutonium inward. The work, led by figures such as Seth Neddermeyer, George Kistiakowsky and the mathematician John von Neumann, was so uncertain that a full test was judged essential.

Allies, spies and the German programme

Britain merged its own programme, code-named Tube Alloys, into the American effort under the Quebec Agreement signed by Roosevelt and Winston Churchill in August 1943. A British Mission including James Chadwick, Rudolf Peierls and Otto Frisch worked at Los Alamos and elsewhere, and Canada contributed uranium and a heavy-water research programme at Montreal and Chalk River.

Despite tight security, the Soviet Union obtained detailed information through spies, including Klaus Fuchs of the British Mission, Theodore Hall and David Greenglass. Meanwhile the Alsos Mission followed Allied armies into Europe to find out how far Germany had progressed. It found that the German project had never come close to a working reactor, let alone a bomb.

Trinity and the bombing of Japan

The Trinity test

At 5:29 a.m. on 16 July 1945, the plutonium implosion device known as the Gadget was detonated on top of a 100-foot (about 30-metre) steel tower in the Jornada del Muerto desert, part of the Alamogordo Bombing Range in New Mexico. Oppenheimer had chosen the code name Trinity. The flash was seen more than 150 miles away, and the yield is now estimated at roughly 20–25 kilotons of TNT. Oppenheimer later recalled that a line from the Hindu scripture the Bhagavad Gita came to his mind: “Now I am become Death, the destroyer of worlds.”

The test proved that the implosion design worked. News reached President Harry S. Truman that evening in Potsdam, where he was attending the Allied conference that opened the next day.

Use against Japan and debate

Germany had surrendered in May 1945, removing the original reason for the project. Some scientists, especially at the Chicago Metallurgical Laboratory, urged restraint: the Franck Report of June 1945 proposed a demonstration of the bomb before using it on a city, and Szilard circulated a petition to the president in July. A government Interim Committee nevertheless recommended military use without prior warning.

A uranium gun-type bomb, Little Boy, was dropped on Hiroshima on 6 August 1945, and a plutonium implosion bomb, Fat Man, on Nagasaki on 9 August. Japan announced its surrender on 15 August. Whether the bombings were necessary to end the war remains debated by historians.

By the numbers

Legacy

After the war the Atomic Energy Act of 1946 transferred control of nuclear research from the Army to a civilian Atomic Energy Commission, which took over on 1 January 1947; the Manhattan District itself was abolished later that year. The laboratories at Los Alamos, Oak Ridge and Argonne, and the Hanford complex, became the core of the US national laboratory system and the post-war weapons programme.

The American nuclear monopoly lasted only four years, ending with the first Soviet atomic bomb test in August 1949. The arms race that followed defined the Cold War. The project also left environmental and human costs: radioactive waste at Hanford remains one of the largest clean-up challenges in the United States, and residents downwind of the Trinity site have long sought recognition for possible health effects.

Scientifically and organisationally, the Manhattan Project became the model for “big science”, large state-funded projects combining universities, industry and government. Parts of Los Alamos, Oak Ridge and Hanford became the Manhattan Project National Historical Park in 2015.

Myths and facts

• Myth: the project was based in Manhattan. Only its first headquarters were in New York; the main work took place in Tennessee, Washington State, New Mexico, Chicago and Berkeley, among more than 30 sites.
• Myth: Einstein worked on the bomb. Einstein signed the 1939 letter to Roosevelt but was not given security clearance and took no part in the project’s work.
• Myth: the Germans were close to a bomb. The Alsos Mission found that the German programme was far behind, without a working reactor in 1945.

Timeline

• 1939, 2 August: Einstein signs the letter to Roosevelt drafted by Szilard.
• 1941, July: The British MAUD Committee concludes a uranium bomb is feasible.
• 1942, August: The Manhattan Engineer District is established.
• 1942, September: Leslie Groves is appointed to command the project.
• 1942, 2 December: Chicago Pile-1 achieves the first self-sustaining chain reaction.
• 1943, spring: The Los Alamos laboratory opens under Oppenheimer.
• 1943, August: The Quebec Agreement brings Britain into the project.
• 1944, September: The Hanford B Reactor starts up.
• 1945, 16 July: The Trinity test in New Mexico.
• 1945, 6 and 9 August: Atomic bombs are dropped on Hiroshima and Nagasaki.

Why it matters

The Manhattan Project turned a laboratory discovery into a deployable weapon in under three years, changing warfare and international politics permanently. It introduced the world to nuclear deterrence, the arms race and the ethical dilemmas of scientists working for the state, and it created the institutions that dominated science in the second half of the twentieth century.