The Manhattan Project was a research and development project that produced the first atomic bombs during World War II. It was led by the United States with the support of the United Kingdom and Canada. From 1942 to 1946, the project was under the direction of Major GeneralLeslie Groves of the US Army Corps of Engineers. The Army component of the project was designated the Manhattan District; "Manhattan" gradually became the codename for the entire project. Along the way, the project absorbed its earlier British counterpart, Tube Alloys. The Manhattan Project began modestly in 1939, but grew to employ more than 130,000 people and cost nearly US$2 billion (about $35.4 billion in 2023[1] dollars). Over 90% of the cost was for building factories and producing the fissionable materials, with less than 10% for development and production of the weapons.[2][3]
Two types of atomic bombs were developed during the war. A relatively simple gun-type fission weapon was made using uranium-235, an isotope that makes up only 0.7 percent of natural uranium. Since it is chemically identical to the most common isotope, uranium-238, and has almost the same mass, it proved difficult to separate. Three methods were employed for uranium enrichment: electromagnetic, gaseous and thermal. Most of this work was performed at Oak Ridge, Tennessee. In parallel with the work on uranium was an effort to produce plutonium. Reactors were constructed at Oak Ridge and Hanford, Washington, in which uranium was irradiated and transmuted into plutonium. The plutonium was then chemically separated from the uranium. The gun-type design proved impractical to use with plutonium so a more complex implosion-type nuclear weapon was developed in a concerted design and construction effort at the project's principal research and design laboratory in Los Alamos, New Mexico.
The following is a timeline of the Manhattan Project. It includes a number of events prior to the official formation of the Manhattan Project, and a number of events after the atomic bombings of Hiroshima and Nagasaki, until the Manhattan Project was formally replaced by the Atomic Energy Commission in 1947.
October 11: Economist Alexander Sachs meets with President Roosevelt and delivers the Einstein–Szilárd letter. Roosevelt authorizes the creation of the Advisory Committee on Uranium.[8]
September 6: Bush tells Briggs that the NDRC will provide $40,000 for the uranium project.[15]
September – Belgian mining engineer Edgar Sengier orders that half of the uranium stock available from the Shinkolobwe mine in the Belgian Congo—about 1,050 tons—be secretly dispatched to New York by African Metals Corp., a commercial division of Union Minière.[16][17]
July 2: The MAUD Committee chooses James Chadwick to write the second (and final) draft of its report on the design and costs of developing a bomb.[22]
July 15: The MAUD Committee issues final detailed technical report on design and costs to develop a bomb. Advance copy sent to Vannevar Bush who decides to wait for official version before taking any action.[23]
August: Mark Oliphant travels to USA to urge development of a bomb rather than power production.[24]
30 August 1941: Winston Churchill becomes the first national leader to approve a nuclear weapons programme: the project was named Tube Alloys
October 3: Official copy of MAUD Report (written by Chadwick) reaches Bush.[24]
October 9: Bush takes MAUD Report to Roosevelt, who approves Project to confirm MAUD's findings. Roosevelt asks Bush to draft a letter so that the British government could be approached "at the top."[26]
December 6: Bush holds a meeting to organize an accelerated research project, still managed by Arthur Compton. Harold Urey is assigned to develop research into gaseous diffusion as a uranium enrichment method, while Ernest O. Lawrence is assigned to investigate electromagnetic separation methods which resulted in the invention of Calutron.[27][28] Compton puts the case for plutonium before Bush and Conant.[29]
December 7: The Japanese attack Pearl Harbor. The United States and Great Britain issue a formal declaration of war against Japan the next day.[30]
December 11: The same day after Germany and Italy declare war on the United States, the United States declares war on Germany and Italy.[31]
December 18: First meeting of the OSRD sponsored S-1 Section, dedicated to developing nuclear weapons.[32]
September 23: Groves is promoted to brigadier general, and becomes director of the project. The Military Policy Committee, consisting of Bush (with Conant as his alternative), Styer and Rear AdmiralWilliam R. Purnell is created to oversee the project.[41]
September - Lieutenant Colonel Kenneth Nichols meets Edgar Sengier in the New York offices of Union Minière. Nichols has been ordered by General Groves to find uranium. Sengier's answer has become history: "You can have the ore now. It is in New York, a thousand tons of it. I was waiting for your visit." Nichols reaches an agreement with Sengier that an average of 400 tons of uranium oxide will begin shipping to the US from Shinkolobwe each month.[42]
September 26: The Manhattan Project is given permission to use the highest wartime priority rating by the War Production Board.[43]
October 19: Groves appoints Oppenheimer to coordinate the scientific research of the project at the Site Y laboratory.[47]
November - The first uranium oxide shipment leaves the Congolese port of Lobito (it will later change to Matadi because of better security). Only two shipments will ever be lost at sea. Aerodromes at Elizabethville and Leopoldville are expanded with US assistance. The OSS is employed to prevent ore smuggling to Nazi Germany.[16]: 3, 6–7, 11 [17]: 45–49
November 16: Groves and Oppenheimer visit Los Alamos, New Mexico and designate it as the location for Site Y.[48]
August 13: Kenneth Nichols replaces Marshall as head of the Manhattan Engineer District.[62] One of his first tasks as district engineer is to move the district headquarters to Oak Ridge, although its name did not change.[52]
August 19: Roosevelt and Churchill sign Quebec Agreement. Tube Alloys is merged with the Manhattan project.[63]
January 11: A special group of the Theoretical Division is created at Los Alamos under Edward Teller to study implosion.[68]
March 11: Beta calutrons commence operation at Oak Ridge.[69]
April 5: At Los Alamos, Emilio Segrè receives the first sample of reactor-bred plutonium from Oak Ridge, and within ten days discovers that the spontaneous fission rate is too high for use in a gun-type fission weapon (because of Pu-240 isotope present as an impurity in the Pu-239).[70]
July 4: Oppenheimer reveals Segrè's final measurements to the Los Alamos staff, and the development of the gun-type plutonium weapon[72]
July 17: "Thin Man" is abandoned. Designing a workable implosion design (Fat Man) becomes the top priority of the laboratory, and design of the uranium gun-type weapon (Little Boy) continued.[73]
July 20: The Los Alamos organizational structure is completely changed to reflect the new priority.[74]
September 2: Two chemists are killed, and Arnold Kramish almost killed, after being sprayed with highly corrosive hydrofluoric acid while attempting to unclog a uranium enrichment device which is part of the pilot thermal diffusion plant at the Philadelphia Navy Yard.[75]
September 22: First RaLa test with a radioactive source performed at Los Alamos.[76]
September 26: The largest nuclear reactor, the B reactor, goes critical at the Hanford Site.[77]
July 19: Oppenheimer recommends to Groves that gun-type design be abandoned and the uranium-235 used to make composite cores (but Little Boy was not abandoned).[91]
July 24: President Harry S. Truman discloses to Soviet leader Joseph Stalin that the United States has atomic weapons. Stalin feigns little surprise; he already knows this through espionage.[92]
July 25: GeneralCarl Spaatz is ordered to bomb one of the targets: Hiroshima, Kokura, Niigata or Nagasaki as soon as weather permitted, some time after August 3.[93]
July 26: Potsdam Declaration is issued, threatening Japan with "prompt and utter destruction".[94]
August 6: B-29 Enola Gay drops Little Boy, a gun-type uranium-235 weapon, on the city of Hiroshima, the primary target.[95]
August 9: B-29 Bockscar drops a Fat Man implosion-type plutonium weapon on the city of Nagasaki, the secondary target, as the primary, Kokura, is obscured by cloud and smoke.[96]
August 12: The Smyth Report is released to the public, giving the first technical history of the development of the first atomic bombs.[97]
August 13: Groves holds shipment of material for a third bomb, on his own authority as he could not reach Marshall or Stimson; as it would be a terrible mistake for us to send overseas the ingredients of another atomic bomb.[98]AFat Man bomb as enough U-235 for a second Little Boy bomb would not be available until December.[99]
February: News of the Russian spy ring in Canada exposed by defector Igor Gouzenko is made public, creating a mild "atomic spy" hysteria, pushing American Congressional discussions about postwar atomic regulation in a more conservative direction.[105]
May 21: Physicist Louis Slotin receives a fatal dose of radiation (2100 rems) when the screwdriver he was using to keep two beryllium hemispheres apart slips.[106]
August 1: Truman signs the Atomic Energy Act of 1946 into law, ending almost a year of uncertainty about the control of atomic research in the postwar United States.[108]
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