Timeline of the Manhattan Project: Wikis


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The following is a timeline of the Manhattan Project, the effort by the United States, United Kingdom, and Canada to develop the first nuclear weapons for use during World War II. The following includes a number of events prior to the official formation of the Manhattan Project as the Manhattan Engineering District (MED) in August 1945 and a number of events after the atomic bombings of Hiroshima and Nagasaki, until the MED was formally replaced by the United States Atomic Energy Commission in 1947.





  • February 26: Conclusive discovery of plutonium by Glenn Seaborg and Arthur Wahl.
  • May 17: A report by Arthur Compton and the National Academy of Sciences is issued which finds favorable the prospects of developing nuclear power production for military use. Vannevar Bush creates the Office of Scientific Research and Development (OSRD)
  • 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
  • August: Mark Oliphant travels to USA to urge development of a bomb rather than power production [1]
  • October 3: Official copy of MAUD Report reaches Bush
  • October 9: Bush takes MAUD Report to Roosevelt who approves project to confirm MAUD's findings
  • December 6: Vannevar 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.
  • December 7: The Japanese attack Pearl Harbor. The United States issues a formal declaration of war against Japan the next day. Four days later, Nazi Germany declares war on the United States.
  • December 18: First meeting of the OSRD sponsored S-1 project, dedicated to developing fission weapons.


Gen. Leslie Groves and physicist Robert Oppenheimer became the military and scientific heads of the Manhattan Project.


Massive calutrons at Oak Ridge, Tennessee, worked around the clock to enrich uranium for a bomb.


The two types of fission weapon designs pursued during the Manhattan Project.
  • 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.
  • May: Fermi at Los Alamos tests the world's third reactor, LOPO, the first aqueous homogeneous reactor, and the first fueled by enriched uranium.
  • July 4: Oppenheimer reveals Segrè's final measurements to the Los Alamos staff, and the development of the gun-type plutonium weapon "Thin Man" is abandoned. Designing a workable "implosion" design becomes top priority of the laboratory.
  • July 20: The Los Alamos organizational structure is completely changed to reflect the new priority of "implosion".
  • July 25: First preliminary test of the RaLa Experiment series performed
  • September 2: chemists Peter N. Bragg, Jr. [2] and Douglas P. Meigs [3] are killed, and Arnold Kramish almost killed, while attempting to unclog a uranium enrichment device which is part of the pilot thermal diffusion plant at the Philadelphia Navy Yard. Two soldiers, George LeFevre and John Tompkins, also receive extensive injuries. An explosion of liquid uranium hexafluoride burst nearby steam pipes, and steam combined with the uranium hexafluoride to spray them with highly corrosive hydrofluoric acid. See also [4] [5] and [6]
  • September 22: First RaLa test with a radioactive source performed
  • December 9: 509th Composite Group of USAAF constituted to deliver the bomb
  • December 14: Definite evidence of achievable compression obtained in a RaLa test
  • Mid-December: Successful test of explosive lens for Fat Man.


The atomic bombings of Hiroshima and Nagasaki were the culmination of the wartime effort.


  • May 21: Louis Slotin, a physicist, received a fatal dose of radiation (2100 rems) when the screwdriver he was using to keep two beryllium hemispheres apart slipped; they were placed around the same plutonium core that had irradiated Daghilan. The upper hemisphere fell, causing a "prompt critical" reaction with a burst of hard radiation. Slotin lifted the upper hemisphere with his left hand and dropped it on the floor, so preventing a more serious accident. He was rushed to hospital, and died nine days later on May 30. Slotin had spent many hours with the dying Daghlian in 1945.



  • Richard Rhodes, The Making of the Atomic Bomb (Simon and Schuster: New York, 1986). ISBN 9780671441333, ISBN 9780684813783.

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