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 General Leslie 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 $ in 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.
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.
1939 August 2: Albert Einstein signs the letter (Einstein–Szilárd letter), authored by physicist Leó Szilárd and addressed to President Franklin D. Roosevelt, advising him to fund research into the possibility of using nuclear fission as a weapon as Nazi Germany may also be conducting such research. September 3: Great Britain and France declare war on Nazi Germany in response to its invasion of Poland, beginning World War II. 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. October 21: First meeting of the Advisory Committee on Uranium, headed by Lyman Briggs of the National Bureau of Standards; $6,000 is budgeted for neutron experiments.
1940 March 2: John R. Dunning's team at Columbia University verifies Niels Bohr's hypothesis that uranium 235 is responsible for fission by slow neutrons. March: University of Birmingham-based scientists Otto Frisch and Rudolf Peierls author the Frisch–Peierls memorandum, calculate that an atomic bomb might need as little as of enriched uranium to work. The memorandum is given to Mark Oliphant, who in turn hands it over to Sir Henry Tizard. April 10: MAUD Committee established by Tizard to investigate feasibility of an atomic bomb. May 21: George Kistiakowsky suggests using gaseous diffusion as a means of isotope separation. June 12: Roosevelt creates the National Defense Research Committee (NDRC) under Vannevar Bush, which absorbs the Uranium Committee. September 6: Bush tells Briggs that the NDRC will provide $40,000 for the uranium project. 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.
1941 February 25: Conclusive discovery of plutonium by Glenn Seaborg and Arthur Wahl at the University of California, Berkeley. 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. June 28: Roosevelt creates the Office of Scientific Research and Development (OSRD) under Vannevar Bush with the signing of Executive Order 8807. OSRD absorbs NDRC and the Uranium Committee. James B. Conant succeeds Bush as the head of NDRC. 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. 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. 30 August 1941: Winston Churchill becomes the first national leader to approve a nuclear weapons programme: the project was named Tube Alloys September 3: British Chiefs of Staff Committee approve Tube Alloys. October 3: Official copy of MAUD Report (written by Chadwick) reaches Bush. 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." 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. Compton puts the case for plutonium before Bush and Conant. December 7: The Japanese attack Pearl Harbor. The United States and Great Britain issue a formal declaration of war against Japan the next day. December 11: The same day after Germany and Italy declare war on the United States, the United States declares war on Germany and Italy. December 18: First meeting of the OSRD sponsored S-1 Section, dedicated to developing nuclear weapons.
1942 January 19: Roosevelt formally authorizes the atomic bomb project. January 24: Compton decides to centralize plutonium work at the University of Chicago. March: Selection of separation methods In the fall of 1942, the electromagnetic and gaseous diffusion processes were chosen for isotope separation of Uranium, as research into the centrifuge and liquid thermal processes had not progressed as rapidly as it has for the others. In 1944 a liquid thermal process plant (S-50 (Manhattan Project)|S-50) was installed at Clinton Engineer Works as the first stage, feeding into higher stages. The centrifrugal process was the usual process used postwar, and physicist Karl P. Cohen held that the process if used by the project could have produced the bomb and the defeat of Germany in 1944. June 19: S-1 Executive Committee is formed, consisting of Bush, Conant, Compton, Lawrence and Urey. June 25: S-1 Executive Committee selects Stone & Webster as primary contractor for construction at the Tennessee site. July–September: Physicist Robert Oppenheimer convenes a summer conference at the University of California, Berkeley to discuss the design of a fission bomb. Edward Teller brings up the possibility of a hydrogen bomb as a major point of discussion. July 30: Sir John Anderson urges Prime Minister Winston Churchill to pursue a joint project with the United States. August 13: The Manhattan Engineering District with James C. Marshall as District Engineer is established by the Chief of the United States Army Corps of Engineers, Major General Eugene Reybold, effective August 16. September 17: Major General Wilhelm D. Styer and Reybold order Colonel Leslie Groves to take over the project. 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 Admiral William R. Purnell is created to oversee the project. 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. September 26: The Manhattan Project is given permission to use the highest wartime priority rating by the War Production Board. September 29: Under Secretary of War Robert P. Patterson authorizes the Corps of Engineers to acquire in Tennessee for Site X, which will become the Oak Ridge, Tennessee, laboratory and production site. October - 100 tons of Sengier's uranium ore is sent to Canada for refining by Eldorado Mining and Refining in Port Hope, Ontario. October - A special detachment from United States Army Corps of Engineers arrives in the Belgian Congo to reopen the Shinkolobwe mine in the Belgian Congo. Work involves draining water from flooded workings, upgrading the plant machinery and constructing transportation facilities. October 15: after a meeting in Chicago on the Manhattan Project General Leslie Groves invited J. Robert Oppenheimer to join himself, James C. Marshall and Kenneth Nichols on their return trip to New York on the 20th Century Limited. After dinner on the train they discussed the project while squeezed into Nichol's one-person roomette (of about 40" by 80" or 1m by 2m). Shortly afterwards Oppenheimer was appointed to head the Los Alamos Laboratory (Site Y). October 19: Groves appoints Oppenheimer to coordinate the scientific research of the project at the Site Y laboratory. 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. November 16: Groves and Oppenheimer visit Los Alamos, New Mexico and designate it as the location for Site Y. December 2: Chicago Pile-1, the first nuclear reactor goes critical at the University of Chicago under the leadership and design of Enrico Fermi, achieving a self-sustaining reaction just one month after construction was started.
1943 January 16: Groves approves development of the Hanford Site. February 9: Patterson approves acquisition of at Hanford. February 18: Construction begins for Y-12, a massive electromagnetic separation plant for enriching uranium at Oak Ridge. April 1: Los Alamos laboratory is established. April 5–14: Robert Serber delivers introductory lectures at Los Alamos, later are compiled into The Los Alamos Primer. April 20: The University of California becomes the formal business manager of the Los Alamos laboratory. Mid-1943: The S-1 Committee was eliminated by mid-1943, as it had been superseded by the Military Policy Committee. June 2: Construction begins of K-25, the gaseous diffusion plant. July: The president proclaims Los Alamos, Clinton Engineer Works (CEW) and Hanford Engineer Works (HEW) as military districts. The Governor of Tennessee Prentice Cooper was officially handed the proclamation making Oak Ridge a military district not subject to state control by a junior officer (a lieutenant) he tore it up and refused to see the MED District Engineer Lt-Col James C. Marshall. The new District Engineer Kenneth Nichols had to placate him. July 10: First sample of plutonium arrives at Los Alamos.