TheManhattan Project was a research and development project that produced the firstatomic bombs duringWorld 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 ofMajor GeneralLeslie Groves of theUS 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 $36.3 billion in 2024[1] dollars). Over 90% of the cost was for building factories and producing thefissionable 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 simplegun-type fission weapon was made usinguranium-235, anisotope that makes up only 0.7 percent of naturaluranium. 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 foruranium enrichment:electromagnetic,gaseous andthermal. Most of this work was performed atOak Ridge, Tennessee. In parallel with the work on uranium was an effort to produceplutonium. Reactors were constructed at Oak Ridge andHanford, Washington, in which uranium was irradiated andtransmuted into plutonium. The plutonium was then chemically separated from the uranium. The gun-type design proved impractical to use with plutonium so a more compleximplosion-type nuclear weapon was developed in a concerted design and construction effort at the project's principal research and design laboratory inLos Alamos, New Mexico.
The following is atimeline 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 theatomic bombings of Hiroshima and Nagasaki, until the Manhattan Project was formally replaced by theAtomic Energy Commission in 1947.
October 11: EconomistAlexander 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 engineerEdgar Sengier orders that half of the uranium stock available from theShinkolobwe mine in the Belgian Congo—about 1,050 tons—be secretly dispatched to New York by African Metals Corp., a commercial division ofUnion Minière.[16][17]
July 2: The MAUD Committee choosesJames 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 namedTube 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 byArthur Compton.Harold Urey is assigned to develop research into gaseous diffusion as a uranium enrichment method, whileErnest O. Lawrence is assigned to investigate electromagnetic separation methods which resulted in the invention ofCalutron.[27][28] Compton puts the case for plutonium before Bush and Conant.[29]
December 7: The Japaneseattack 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 tobrigadier general, and becomes director of the project. The Military Policy Committee, consisting of Bush (with Conant as his alternative), Styer andRear AdmiralWilliam R. Purnell is created to oversee the project.[41]
September - Lieutenant ColonelKenneth Nichols meetsEdgar Sengier in the New York offices ofUnion 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 ofuranium 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 theWar Production Board.[43]
October 19: Groves appoints Oppenheimer to coordinate the scientific research of the project at theSite Y laboratory.[47]
November - The first uranium oxide shipment leaves the Congolese port ofLobito (it will later change toMatadi because of better security). Only two shipments will ever be lost at sea. Aerodromes atElizabethville andLeopoldville are expanded with US assistance. TheOSS is employed to prevent ore smuggling to Nazi Germany.[16]: 3, 6–7, 11 [17]: 45–49
November 16: Groves and Oppenheimer visitLos 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 signQuebec Agreement. Tube Alloys is merged with the Manhattan project.[63]
January 11: A special group of the Theoretical Division is created at Los Alamos underEdward Teller to studyimplosion.[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 agun-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, andArnold Kramish almost killed, after being sprayed with highly corrosivehydrofluoric acid while attempting to unclog a uranium enrichment device which is part of the pilot thermal diffusion plant at thePhiladelphia 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 (butLittle Boy was not abandoned).[91]
July 24: PresidentHarry S. Truman discloses to Soviet leaderJoseph Stalin at thePotsdam Conference 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-29Enola Gay dropsLittle Boy, a gun-type uranium-235 weapon, on the city of Hiroshima, the primary target.[95]
August 9: B-29Bockscar drops aFat 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: TheSmyth 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; asit 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 secondLittle Boy bomb would not be available until December.[99]
February: News of the Russian spy ring in Canada exposed by defectorIgor 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: PhysicistLouis Slotin receives a fatal dose of radiation (2100 rems) when the screwdriver he was using to keep twoberyllium hemispheres apart slips.[106]
August 1: Truman signs theAtomic 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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