Manhattan Project

Manhattan Project

The Manhattan Project was a research and development project carried out during World War II that produced the first nuclear weapons, led by the United States with support from the United Kingdom and Canada. From 1942 to 1946, the project was under the direction of Major General Leslie Groves of the United States Army Corps of Engineers, while nuclear physicist Robert Oppenheimer was the director of Los Alamos National Laboratory, where the nuclear bombs themselves were designed.

The Manhattan Project
Active 1942-August 15, 1947
Countries United States
United Kingdom
Branch/s U.S. Army Corps of Engineers
Type Military project
Quartering Oak Ridge, Tennessee, United States
High command
James C. Marshall
Kenneth Nichols
Shoulder patch adopted in 1945

Shoulder patch adopted in 1945

Unofficial emblem Manhattan Project Unofficial emblem
Culture and history
Anniversaries August 13, 1942
Wars and battles
  • Allied invasion of Italy
  • Operation Overlord
  • Invasion of the Western Allies of Germany
  • Atomic bombings of Hiroshima and Nagasaki
  • Occupation of Japan

The military unit participating in the project received the designation Manhattan District, a name that gradually replaced the official code name, Substitute Material Development. In its course the project absorbed its previous British equivalent, the Tube Alloys project. The Manhattan Project began modestly, growing progressively to more than 130,000 employees and reaching a cost of nearly $2 billion. More than 90% of the budget was spent on factory construction and the production of fissile materials, with less than 10% going to weapons development and production. Research and production took place at more than 30 locations throughout the United States, United Kingdom and Canada.

Two types of atomic bombs were developed simultaneously during the war: a relatively simple ballistic-type fission weapon and a more complex implosion nuclear weapon. The fission design of the Thin Man bomb proved impractical for use with plutonium, so a simpler weapon called the Little Boy was developed that used uranium-235, an isotope that makes up only 0.7% of uranium in its natural state. Project workers had difficulty separating this isotope from uranium-238 because of their chemical and mass similarities. Three methods were used for uranium enrichment: the use of calutrons, gaseous diffusion and thermophoresis. Most of this work was carried out at the Clinton Engineer Works in Oak Ridge, Tennessee.

In parallel with research with uranium, the project continued work on plutonium production. After the feasibility of the world’s first artificial nuclear reactor in Chicago was demonstrated at the Metallurgical Laboratory, the X-10 graphite reactor at Oak Ridge and the production reactors at the Hanford Engineer Works facilities were designed, in which uranium was irradiated and transmuted into plutonium, and then chemically separated the plutonium from the uranium. The Fat Man implosion nuclear weapon was developed through concerted design and development at the Los Alamos laboratory.

The project also carried out counterintelligence work on the German nuclear weapons program. Through Operation Alsos, several members of the Manhattan Project served in Europe, sometimes behind enemy lines, seizing nuclear materials and documentation and transporting German scientists to Allied countries. On the other hand, despite the tight security of the project, several Soviet “atom spies” managed to infiltrate the program.

The first nuclear device detonated was an implosion bomb in the Trinity test, conducted at the Alamogordo firing and bombing range on July 16, 1945. Two more Little Boy and Fat Man bombs were used respectively a month later in the atomic bombings of Hiroshima and Nagasaki. In the years immediately following the war, the Manhattan Project conducted several weapons tests on Bikini Atoll as part of Operation Crossroads, developed new weapons, promoted the development of the national laboratory network, supported medical research on radiology, and laid the foundations of the nuclear navy. The project retained control over U.S. nuclear weapons research and production until the formation of the U.S. Atomic Energy Commission in January 1947.


The discovery of nuclear fission by German chemists Otto Hahn and Fritz Strassmann in 1938, along with their theoretical explanation by Lise Meitner and Otto Robert Frisch, made the development of an atomic bomb a theoretical possibility. It was feared that the Germans would be the first to develop one of these bombs through a project of their own, especially by scientists who had taken refuge from Nazi Germany and other fascist countries.

In August 1939, Hungarian-born physicists Leó Szilárd and Eugene Wigner wrote the Einstein-Szilárd letter, warning of the potential development of “extremely powerful bombs of a new class”. They urged the United States to take steps to acquire uranium ore reserves and accelerate research by Enrico Fermi and other scientists on nuclear chain reactions. Albert Einstein signed this letter, which was delivered to President Franklin D. Roosevelt. Roosevelt asked Lyman Briggs of the National Standards Institute to lead the Uranium Advisory Committee to investigate the problems outlined in this letter. Briggs organized a meeting on October 21, 1939, attended by Szilárd, Wigner, and Edward Teller. The committee informed Roosevelt in November that uranium “would supply a possible source of bombs with a capacity for destruction far greater than any known until then.”

The Uranium Advisory Committee became the National Defense Research Committee on June 27, 1940. Briggs proposed spending $167,000 for research on uranium, particularly the isotope uranium-235, and the newly discovered plutonium. On June 28, 1941, Roosevelt signed Executive Order 8807, creating the Office of Scientific Research and Development (OSRD) with Vannevar Bush as director. This office had the power to engage in large engineering projects in addition to research projects. The Uranium Committee became Section S-1 of the OSRD, removing the word “uranium” from the name for safety reasons.

By June 1939 in the United Kingdom, Frisch and Rudolf Peierls of the University of Birmingham had made significant advances in research into the critical mass of uranium-235. Their calculations indicated that such mass would be within an order of magnitude of 10 kg (22 lb), an amount small enough to carry loaded on a bomber. His Frisch-Peierls memorandum of March 1940 initiated the British atomic project and its MAUD Committee, which unanimously recommended pursuing the development of an atomic bomb. In July 1940 the United Kingdom offered the United States access to its scientific research and John Cockcroft was commissioned to brief American scientists on British developments, as part of the Tizard Mission. Cockcroft knew then that the American project was smaller than the British one and was not as advanced.

As part of the scientific exchange, the findings of the MAUD Committee were transmitted to the United States. One of its members, Australian physicist Mark Oliphant, flew to the United States in late August 1941 and learned that the data provided by the committee had not reached several leading American scientists. Oliphant sought to know why these discoveries were apparently being ignored. He met with the S-1 section of the OSRD and visited Berkeley, California, talking persuasively with Ernest Lawrence. Lawrence was impressed enough to begin his own research into uranium. Oliphant continued to meet with other researchers, including James B. Conant, Arthur Compton and George B. Pegram, thus making leading American physicists aware of the potential of atomic bombs.

On October 9, 1941, President Roosevelt approved the atomic program after a meeting with Vannevar Bush and Vice President Henry A. Wallace. To control the program he created a Senior Political Group composed of himself—although he never attended any of its meetings—Wallace, Bush, Conant, Secretary of War Henry L. Stimson, and Army Chief of Staff George C. Marshall. Roosevelt selected the Army to lead the project instead of the Navy as the Army had more experience in handling large-scale construction projects. He also agreed to coordinate the work with the British, sending a message to Prime Minister Winston Churchill on October 11, 1941, in which he suggested that they agreed on atomic matters.

Viability of the Manhattan Project


The S-1 Committee held a meeting on December 18, 1941 “imbued with an atmosphere of enthusiasm and urgency” following the attack on Pearl Harbor and the subsequent U.S. declaration of war on Japan and Germany. Research was ongoing on three isotope separation techniques to separate uranium-235 from the more abundant uranium-238. Lawrence and his team at the University of California investigated electromagnetic separation, while the team of Eger Murphree and Jesse Wakefield Beams investigated gaseous diffusion at Columbia University and Philip Abelson led research on thermal diffusion at the Carnegie Institution in Washington, D.C. and later at the Naval Research Laboratory. Murphree also led another unsuccessful separation research project, using a gas centrifuge.

At the same time, there were two lines of research in progress for nuclear reactor technology, with Harold Urey continuing heavy water research at Columbia, while Arthur Compton brought together scientists working under his supervision from Columbia, California and Princeton University on his team at the University of Chicago, where he organized the Metallurgical Laboratory in early 1942 to study plutonium and reactors using graphite. as a neutron moderator. Briggs, Compton, Lawrence, Murphree, and Urey met on May 23, 1942, to finalize the recommendations of the S-1 Committee, which urged the pursuit of the five technologies.

The proposed measures were approved by Bush, Conant, and Brigadier General Wilhelm D. Styer, chief of staff of the Supply Services to Maj. Gen. Brehon B. Somervell, who had been appointed the Army’s representative on nuclear affairs. Bush and Conant took the recommendations to the Higher Policy Group with a budget proposal of $54 million for construction by the U.S. Army Corps of Engineers, $31 million for research and development for OSRD, and $5 million for contingencies in fiscal year 1943. The Group sent this proposal to the President on June 17, 1942, and it was finally approved.

Pump Design Concepts

Compton asked theoretical physicist, Robert Oppenheimer of the University of California, California, to take over the research on fast neutron calculations—which were the key to calculations of critical mass and weapons detonation—from Gregory Breit, who had resigned on May 18, 1942, concerned about lax operational safety. John H. Manley, one of the physicists at the Metallurgical Laboratory, was commissioned to assist Oppenheimer by contacting and coordinating groups of experimental physicists spread across the country.

Oppenheimer and Robert Serber of the University of Illinois examined the problems of neutron diffusion—how neutrons move in a nuclear chain reaction—and hydrodynamics—how the explosion produced by the chain reaction would behave. To review this work and the general theory of fission reactions, Oppenheimer and Enrico Fermi held several meetings at the University of Chicago in June and at the University of California in July, along with theoretical physicists Hans Bethe, John Van Vleck, Edward Teller, Emil Konopinski, Robert Serber, Stan Frankel, and Eldred C. Nelson. the last three former students of Oppenheimer himself, and with experimental physicists Emilio Segrè, Felix Bloch, Franco Rasetti, John Henry Manley and Edwin McMillan. The group tentatively confirmed that a fission bomb was theoretically possible.

However, there were still many factors unknown to scientists. The properties of pure uranium-235 were relatively unknown, as were those of plutonium, which had been discovered in February 1941 by Glenn Seaborg and his team. Scientists at the Berkeley conference had a vision to create plutonium in nuclear reactors where uranium-238 atoms would absorb neutrons emitted from the fission of uranium-235 atoms. At that time no reactor had yet been built and only small amounts of plutonium obtained by means of a cyclotron were available.

By December 1943 only 2 mg of this element had been produced. The simplest way to bring the fissile material to a critical mass was to fire a “cylindrical plug” into a sphere of “active material” with a “lock,” a dense material that would focus the neutrons inward and hold the reactive mass together to increase its efficiency. They also explored spheroid designs, a primitive form of “implosion” suggested by Richard C. Tolman, as well as the possibility of carrying out autocatalytic methods that would increase the efficiency of the bomb during its explosion.

Considering that the fission bomb idea was theoretically settled, at least until more experimental data were obtained, the Berkeley conference veered in a new direction. Edward Teller promoted the debate about a more powerful bomb, the so-called “super”, later known as “hydrogen bomb”, which would use the explosive force of the detonation of a fission bomb to initiate a nuclear fusion reaction of deuterium and tritium. Teller proposed several schemes, all of which were rejected by Bethe, and the idea of fusion was set aside to concentrate on the production of fission bombs.

Teller also noted the speculative possibility that an atomic bomb could “ignite” the atmosphere because of a hypothetical fusion reaction of nitrogen nuclei. Bethe calculated that this could not happen and in a report co-authored by Teller they showed that “there is no chance of it initiating a self-propagation of nuclear chain reactions.” In Serber’s words, Oppenheimer mentioned this to Arthur Compton, who “did not have enough common sense to keep quiet on the subject… somehow it ended up in a document that was sent to Washington,” and “the matter was never over.”

Manhattan Project Organization

Manhattan District

In June 1942 the chief engineer, Major General Eugene Reybold, chose Colonel James C. Marshall to lead the Army portion of the project. Marshall established a liaison office in Washington, D.C., but established his headquarters at 18° 270 Broadway in New York City, from where he could receive administrative support from the North Atlantic Division of the Corps of Engineers. It was also close to the Manhattan office of Stone & Webster, the project’s prime contractor, and Columbia University. He also had permission to relocate personnel from his previous command, the Syracuse District, and began by recruiting Lieutenant Colonel Kenneth Nichols, who became his deputy.

Since much of the project involved construction work, Marshall worked in cooperation with the chief of the Corps of Engineers Construction Division, Major General Thomas M. Robbins, and his deputy, Colonel Leslie Groves. Eugene Reybold, Brehon B. Somervell, and Wilhelm D. Styer decided to call the project “Development of Substitute Materials,” but Groves felt that this name would attract too much attention. Since engineering districts were often named after the city where they were located, Marshall and Groves agreed to name the Army section of this project the Manhattan District.

This name became official on August 13 when Reybold issued an order for the creation of the new district. More informally it was also known as the Manhattan Engineering District. Unlike other districts it had no geographical boundaries and Marshall had the authority of a division engineer. The name Substitute Materials Development remained the official codename for the project as a whole, but was eventually replaced by “Manhattan”.

Marshall later stated, “I had never heard of atomic fission but I did know that you couldn’t build one plant, let alone four of them, for 90 million.” Nichols had been commissioned to build a TNT plant shortly before in Pennsylvania at a cost of $128 million. They were also unimpressed by estimates of the nearest order of magnitude, which Groves likened to telling a catering company to prepare for ten to a thousand guests. A Stone & Webster survey team had already explored a location for the production plants. The War Production Board recommended that they be locations near Knoxville, Tennessee, an isolated region where the Tennessee Valley Authority could provide high electrical capacity and where rivers could supply water for cooling reactors.

After examining several locations, the survey team selected one near Elza, Tennessee. Conant proposed that the land be acquired immediately and Styer agreed, but Marshall delayed the matter by waiting for the results of Conant’s reactor experiments before taking action. Of all the processes in perspective, only Lawrence’s electronic separation seemed sufficiently advanced in its definition for construction to begin.

Marshall and Nichols began to gather the resources they needed. The first step was to obtain a high-priority rating for the project. The highest ratings ranged from AA-1 to AA-4 in descending order, although there was also a special AAA rating reserved for emergencies. The AA-1 and AA-2 classifications were reserved for essential equipment and armament, so Colonel Lucius D. Clay, substitute chief of staff in the Department of Services and Supply, considered the highest rating he could give the project AA-3, although he was willing to give an AAA rating on request for critical materials if the need arose. Nichols and Marshall were disappointed with this, as AA-3 was the same priority that Nichols’ TNT plant in Pennsylvania had obtained.

Military Policy Committee

Oppenheimer and Groves in front of the remains of the Trinity test in September 1945
Oppenheimer and Groves in front of the remains of the Trinity test in September 1945, two months after the detonation and just after the end of World War II. White shoe covers prevented debris from sticking to the soles of the shoes

Vannevar Bush was dissatisfied with the slow progress of the project under Colonel Marshall and more specifically with the inability to acquire the Tennessee site, the low priority the army had assigned to the project, and the location of the headquarters in New York. He felt that the project needed more aggressive leadership and met with Harvey Bundy and Generals Marshall, Somervell and Styer to discuss their concerns. Bush wanted the project to be under a higher political committee, with a top official as general manager, preferably Styer.

Somervell and Styer chose Groves for this position, informing him of the decision and of his promotion to brigadier general on September 17, as they felt that the title of “general” would be of more influence to the academics working on the project. Groves thus came directly under Somervell’s command instead of Reybold and with Colonel Marshall under his command. Groves established his headquarters in Washington, D.C., on the fifth floor of the New War Department building, where Colonel Marshall had his liaison office. He assumed command of the Manhattan Project on September 23 and that same day attended a meeting convened by Stimson, at which a Military Policy Committee was established consisting of Bush (with Conant as a replacement), Styer, and Vice Admiral William R. Purnell. Tolman and Conant were later appointed scientific advisers to Groves.

On September 19, Groves met with Donald Nelson, chairman of the War Production Board, asking for greater authority to assign a AAA rating when necessary. Nelson initially objected, but agreed when Groves threatened to take the matter to the president of the United States. Groves promised that he would not use this classification unless it was strictly necessary. However, it turned out that the AAA rating was too high for the routine requirements of the project, while the AA-3 was too low. After a long campaign, Groves finally received the AA-1 classification on July 1, 1944. According to Groves: “There was an awareness in Washington of the importance of the highest priority. Almost everything proposed in the Roosevelt administration would have top priority. That would last about a week or two and then something else would have top priority.”

One of the first problems for Groves was finding a director for Project Y, the group that would design and build the pump. The most obvious choice for him was one of the heads of the three laboratories, Urey, Lawrence or Compton, but Groves could not afford to move them. Compton recommended Oppenheimer, who was already familiar with pump design concepts. However, Oppenheimer had very little administrative experience and unlike the other three laboratory chiefs, he had not won a Nobel Prize, something that many scientists felt the director of such an important laboratory should have.

There were also concerns about Oppenheimer’s security status, as many of his associates were communists, including his brother Frank Oppenheimer, his wife Kitty, and Robert’s girlfriend, Jean Tatlock. After a conversation during a train ride in October 1942, Groves and Nichols became convinced that Oppenheimer understood the challenges of establishing a laboratory in a remote area and should be appointed director. Groves personally waived the safety requirements and granted Oppenheimer clearance on July 20, 1943.

Collaboration with the United Kingdom for the Manhattan Project

The British and Americans exchanged nuclear information but at first did not join forces. The UK rejected attempts by Bush and Conant in 1941 to increase cooperation with their own project, codenamed Tube Alloys, as they were unwilling to share their technological leadership to help the US develop its own atomic bomb. Churchill did not respond to a personal letter from Roosevelt in which he offered to pay the costs of all the research and development of an Anglo-American project, so the United States decided in April 1942 that they would go ahead alone.

The British, who had made significant contributions in the early stages of the war, no longer possessed the resources to continue such a research program while fighting for their survival, so the Tube Alloys project lagged behind its American equivalent. On July 30, 1942, John Anderson, the minister responsible for the Tube Alloys project, told Churchill: “We must face the fact that … Our pioneering work… It is a dwindling asset and if we do not capitalize on it quickly, we will be at a disadvantage. We now have a real contribution to a “union”. In a short time we will have little or none.” That same month Churchill and Roosevelt reached an informal and unwritten agreement for collaboration on the atomic question.

However, the opportunity for equal collaboration no longer existed, and was demonstrated in August 1942 when the British unsuccessfully applied for substantial control over the project without covering any expenses. By 1943, the roles of the two countries had reversed compared to how they were at the end of 1941. In January Conant notified the British that they would no longer receive information about atomic research except in certain areas. The British were shocked by the abrogation of the previous agreement between Churchill and Roosevelt, but the head of Canada’s National Research Council C. J. Mackenzie was not so surprised, saying, “I can’t help but feel that the UK group overemphasized the importance of their contribution compared to the Americans.” As Conant and Bush told the British, the order came “from above.”

The negotiating position of the British had worsened. American scientists had decided that the United States no longer needed outside help and tried to prevent the United Kingdom from being able to take advantage of commercial applications for atomic energy after the war. The American committee agreed, with Roosevelt’s support, to restrict the flow of information to the United Kingdom during the war, especially regarding bomb design, even if doing so slowed down the American project. In early 1943, the British stopped sending research and scientists to the United States and, as a result, the Americans stopped sharing information.

The British considered cutting off the supply of uranium and heavy water from Canada to force the Americans to share information, but Canada needed American supplies to produce these elements. The British also investigated the possibility of carrying out an independent nuclear program but concluded that it would not be ready in time to affect the outcome of the war in Europe.

In March 1943 Conant decided that British aid would be beneficial in some areas of the project. James Chadwick and other British scientists were important enough that they were needed for the Los Alamos bomb design team, despite the risk of revealing secrets about weapons design. In August 1943 Churchill and Roosevelt negotiated the Quebec Agreement, resuming cooperation between scientists from the two countries. The United Kingdom accepted the information restrictions on the construction of large-scale production plants needed for the pump.

The subsequent Hyde Park Agreement in September 1944 extended this cooperation into the post-war period. The Quebec Agreement established the Combined Policy Committee to coordinate the efforts of the United States, the United Kingdom, and Canada. Stimson, Bush and Conant were the American members of this committee, Field Admiral John Dill and Colonel J. J. Llewellin were the British members and C. D. Howe the Canadian. Llewellin returned to the United Kingdom at the end of 1943 and was replaced on the committee by Ronald Ian Campbell, who in turn was subsequently replaced by the British ambassador to the United States Lord Halifax in early 1945. John Dill died in Washington, D.C. in November 1944 and was replaced by Field Admiral Henry Maitland Wilson.

Cooperation resumed after the Quebec Agreement and the British were surprised by the expenses and progress made by the Americans. The United States had spent more than 1 billion dollars, while the United Kingdom had invested 500 000 pounds. Chadwick lobbied for the British to get involved in the Manhattan Project altogether abandoning any hope of a British project during the war. Counting on Churchill’s support, he tried to ensure that all Groves’ pleas for help were met. The British mission that arrived in the United States in December 1943 included Niels Bohr, Otto Frisch, Klaus Fuchs, Rudolf Peierls and Ernest Titterton. More scientists arrived in early 1944.

While those assigned to gaseous diffusion left in the fall of 1944, the 35 who worked with Lawrence at Berkeley were assigned to existing laboratory groups and remained until the end of the war. The 19 that had been sent to Los Alamos also joined existing groups, mainly related to the assembly and implosion of the bomb, but not those related to plutonium. Part of the Quebec Agreement specified that nuclear weapons would not be used against any other country without mutual consent. In June 1945 Wilson agreed that the use of nuclear weapons against Japan would be recorded as a decision of the Combined Policy Committee.

The Combined Policy Committee created the Combined Development Fund in June 1944, with Groves as chairman, to procure uranium and thorium ore on international markets. The Belgian Congo and Canada possessed much of the world’s uranium outside Eastern Europe and the Belgian government-in-exile was then in London. The United Kingdom agreed to give the United States most of the Belgian ore, as they could not use it without research restricted by the Americans.

In 1944, the Fund purchased 1,560,000 kg of uranium oxide ore from companies operating mines in the Belgian Congo. In order to avoid informing US Treasury Secretary Henry Morgenthau Jr. about the project, they used a special bank account not subject to the usual audits and controls through which this type of funds had to pass. Between 1944 and the time he resigned from the Fund in 1947, Groves deposited a total of $37.5 million into the Fund’s account.

Groves appreciated the initial atomic research of the British and the contributions of British scientists to the project, but claimed that the United States would have achieved the same success without them. He also said that Churchill was “the best friend the atomic bomb project had, [for] he kept Roosevelt’s interest… I waved him all the time telling him how important I thought the project was.”

British involvement in the wartime project was crucial to the success of the UK’s independent nuclear weapons program after the war, when the McMahon Act of 1946 temporarily ended US nuclear cooperation.

Manhattan Project locations

Main locations of the Manhattan Project in the United States and Canada
Main locations of the Manhattan Project in the United States and Canada

Oak Ridge

The day after taking control of the project, Groves took a train to Tennessee with Colonel Marshall to inspect the proposed site there and was impressed upon arrival. On September 29, 1942, U.S. Deputy Secretary of War Robert P. Patterson authorized the Corps of Engineers to acquire 23,000 ha of land by expropriation at a cost of $3.5 million, with subsequent acquisition of another 1200 ha of land. Around a thousand families were affected by the expropriation order, which became effective on October 7. The various demonstrations, legal appeals and consultation in Congress in 1943 were unsuccessful.

In mid-November, sheriffs began affixing eviction signs to farm doors two weeks in advance and construction contractors began arriving. Some families received two weeks’ notice to leave their farms where they had lived for generations, while others had settled there after being evicted by the creation of Great Smoky Mountains National Park in the 1920s or the construction of Norris Dam in the 1930s. The final cost of acquiring land in the area, a process that was not completed until March 1945, was $2.6 million. When presented with Public Proclamation Number Two, which listed Oak Ridge as a total exclusion area that no one could enter without military permission, Tennessee Governor Prentice Cooper angrily tore it up.

Initially known as Kingston Demolition Range, the site was officially renamed Clinton Engineer Works (CEW) in early 1943. While Stone & Webster concentrated on production facilities, the architectural and engineering firm Skidmore, Owings & Merrill designed and built a residential community for 13,000 people, located in the hills of Black Oak Ridge, from which the new town of Oak Ridge took its name. The army’s presence in Oak Ridge increased in August 1943 when Nichols replaced Marshall as head of the Manhattan District. One of his first tasks was to move the district headquarters to Oak Ridge, although the name of the district was not changed.

In September 1943 the management of the communal facilities was subcontracted to Turner Construction through a subsidiary, Roane-Anderson Company (for Roane and Anderson counties, in which Oak Ridge was located). At these facilities, chemical engineers took part in the production of uranium-235 enriched between 10% and 12%, known as the code name “tuballoy tetroxide“, under high-security measures and rapid approvals of supply requests and materials. The population of Oak Ridge increased more than initially planned, reaching 75,000 residents in May 1945, with about 82,000 people working at Clinton Engineer Works and another 10,000 at Roane-Anderson.

Los Alamos

The idea of locating Project Y in Oak Ridge came to be considered, but in the end it was decided that this project should be done in a remote location. On the recommendation of Robert Oppenheimer, the search for a suitable location was limited to the surroundings of Albuquerque in New Mexico, where Oppenheimer owned a ranch. In October 1942, Officer John H. Dudley was sent to survey the area, recommending a location near Jemez Springs.

On November 16, Oppenheimer, Groves, Dudley and others visited the recommended area. Oppenheimer worried that the high boulders surrounding the site might make workers claustrophobic, while engineers were worried about the chances of flooding. The group then moved to nearby Los Alamos Ranch School. Oppenheimer was impressed and expressed a strong penchant for this place, citing its natural beauty and views of the Sierra de la Sangre de Cristo. Engineers were concerned about the poor access road to this area and whether the water supply would be ideal, but said the location was otherwise ideal.

Patterson approved the acquisition of the land on November 25, 1942, authorizing about $440,000 for the purchase of 22,000 ha of land, of which all but 3600 ha were already owned by the federal government. In addition, Secretary of Agriculture Claude R. Wickard ceded the use of another 18300 ha of land belonging to the United States Forest Service to the Department of War “as long as military necessity continues”. The need for land, a new road and later rights of way for a new 40 km power line, meant that the acquisition of land was finally about 18509 ha, although the expense was only 414 971 dollars.

Construction was assigned to the M. M. Sundt Company of Tucson, with Willard C. Kruger and Associates of Santa Fe as architects and engineers. Work began in December 1942. Groves initially allocated about $300,000 for construction, triple Oppenheimer’s estimate, with an estimated completion date of March 15, 1943. The scope of Project Y was greater than initially expected and by the time the works were completed on November 30, 1943, the cost had amounted to more than 7 million dollars.

Since it was secret, Los Alamos was called “Place Y” or “The Hill.” The birth certificates of those born in Los Alamos during the war indicated their place of birth in Santa Fe. Los Alamos was originally to be a military laboratory with Oppenheimer and other researchers commissioned into the military, but two of the project’s key physicists, Robert Bacher and Isidor Rabi, rejected this idea. Conant, Groves and Oppenheimer then defined a compromise whereby the laboratory would be operated by the University of California in a contract with the War Department.


During an Army and OSRD council on June 25, 1942, it was decided to build a pilot plant for the production of plutonium at Red Gate Woods, southeast of Chicago. In July Nichols agreed to a lease of 415 ha from the Cook County Forest Preservation District, and Captain James F. Grafton was appointed zone engineer in Chicago. Before long it became apparent that the scale of the planned operations was too great for that land, so it was finally decided to build the plant in Oak Ridge and maintain a research and testing facility in Chicago.

Delays in establishing the plant at Red Gate Woods prompted Compton to authorize the Metallurgical Laboratory to build the first nuclear reactor under the bleachers of Stagg Field football field at the University of Chicago. This reactor required a large number of graphite blocks and uranium balls. At that time the availability of pure uranium was limited. Frank Spedding of Iowa State University was able to produce only two short tons of pure uranium.

An additional three short tons of uranium metal were supplied by a Westinghouse Electric lamp factory in Bloomfield, New Jersey, produced quickly through an improvised process. Goodyear built a large square balloon to line the reactor. On December 2, 1942, a team led by Enrico Fermi initiated the first self-sustaining artificial nuclear chain reaction in an experimental reactor known as Chicago Pile-1. The point at which the reaction becomes self-sustaining was called the “tipping point.” Compton reported this success to Conant, who was in Washington, D.C., by means of a coded telephone call, saying, “The Italian navigator [Fermi] has just landed in the new world.”

In January 1943 Grafton’s successor, Arthur V. Peterson, ordered the dismantling of the Chicago Pile-1 reactor and its reassembly at Red Gate Woods, as he considered the operation of a reactor too dangerous to remain in a densely populated area. At the Argonne location, the Chicago Pile-3, the first heavy water reactor, reached the critical point on May 15, 1944. After the war, operations still continuing at Red Gate were moved to the new location of Argonne National Laboratory, about 9.7 km away.


In December 1942 concerns arose that even Oak Ridge was too close to a major population center (Knoxville) in the event of a serious nuclear accident. In November of that year, Groves had requested DuPont’s services as prime contractor for the construction of the plutonium production complex. The job offer for DuPont included a standard contract, but the company’s president, Walter S. Carpenter, Jr., did not wish to make a profit from it, so he requested that the contract be accommodated to explicitly exclude that the company could acquire any patent rights. For legal reasons, they had to agree on a fee of one dollar and after the war DuPont requested the termination of the contract before the date initially agreed and had to reimburse 33 cents.

DuPont recommended that the site be located far from the uranium production facilities already built in Oak Ridge. In December 1942 Groves sent Colonel Franklin Matthias and several DuPont engineers to probe potential locations. Matthias reported that Hanford Site near Richland, Washington, was “ideal in virtually every respect.” It was isolated and close to the Columbia River, which could supply enough water to cool the reactors that would produce the plutonium. Groves visited the site in January and the Hanford Engineer Works (HEW) was established, codenamed “Site W.”

Deputy Secretary Patterson gave his approval on February 9, 1943, allocating $5 million for the acquisition of 16,000 ha of land in the area. The federal government moved about 1500 residents of White Bluffs, Hanford and other localities in the area, in addition to the Wanapum and other natives present in the area. There were disputes with several farmers seeking compensation for the crops, which they had already planted before the government acquired the land, and the army allowed them to finish harvesting some of these crops in specific cases. The land acquisition process was time-consuming and not completed before the end of the Manhattan Project in December 1946.

Although progress in reactor design at the Metallurgical Laboratory and DuPont was not advanced enough to accurately predict the scope of the project, work on the facility began in April 1943 with an estimated 25,000 workers, with half of these living on site. As of July 1944, some 1200 buildings had been constructed and almost 51000 people lived in the construction camp. As an area engineer, Matthias exercised overall control of the site. At its peak, the construction site became the third most populous town in Washington state. Hanford operated a fleet of more than 900 buses, more than in the city of Chicago. Similar to Los Alamos and Oak Ridge, Richland was a gated community with restricted access, although it more closely resembled the fast-growing American populations of the day, as the military profile was lower and physical security features such as fencing and guard towers were less evident.


British Columbia

The Cominco company had been producing electrolytic hydrogen in Trail, British Columbia since 1930. In 1941 Urey suggested that it could also produce heavy water. Secondary electrolysis cells were added to the existing $10 million plant of 3215 cells with an electrical consumption of 75 MW to increase the concentration of deuterium in water from 2.3% to 99.8%.

For this process, Hugh Taylor of Princeton developed a platinum-on-carbon catalysis process for the first three stages, while Urey developed a nickel-chromium oxide process for the fourth phase tower. The final cost was $2.8 million and the Government of Canada did not become officially aware of this project until August 1942. Heavy water production at Trail began in January 1944 and continued until 1956. This heavy water was used in the Chicago Pile-3, the first nuclear reactor to use heavy water and natural uranium, which reached the critical point on May 15, 1944.


The Chalk River Laboratories in Ontario was established to house the Allied effort at the Montreal Laboratory away from urban areas. A new community was built in Deep River, Ontario to provide residences and facilities for team members. The site was chosen because of its proximity to the industrial zone of Ontario and Quebec and because of the proximity to a railway track adjacent to the military base of the Petawawa Garrison.

Located on one side of the Ottawa River, this site also had access to sufficient water. The first director of the new laboratory was Hans von Halban, succeeded in May 1944 by John Cockcroft and later by Bennett Lewis in September 1946. The first Canadian reactor was the pilot reactor known as the ZEEP (zero-energy experimental pile) reactor, and was also the first to be completed outside the United States when it reached its critical point in September 1945. The ZEEP reactor remained in use until 1970. In July 1947 a larger 10 MW reactor, the NRX, which was designed during the war, was completed and reached the critical point.

Northwest Territories

The Eldorado Mine at Port Radium in the Northwest Territories was a source of uranium ore.

Heavy water

Although DuPont’s preferred designs for nuclear reactors were helium-cooled and used graphite as a nuclear moderator, DuPont expressed interest in using heavy water as contingency support in case the graphite reactor design was unfeasible for any reason. For this purpose, it was estimated that about 3 tons of heavy water per month would be required. Project P-9 was the government’s code name for the heavy water production program.

As the Trail plant, which was still under construction, could produce half a ton per month, additional capacity was needed. Groves authorized DuPont to establish heavy water production facilities at Morgantown Ordnance Works near Morgantown, West Virginia; Wabash River Ordnance Works near Dana and Newport, Indiana; and finally at Alabama Ordnance Works, near Childersburg and Sylacuga, Alabama. Although known as the Ordnance Works and paid for by contracts on behalf of the Ordnance Department, they were built and operated by the Army Corps of Engineers. The U.S. plants used a different production process than Trail, in which heavy water was extracted by distillation, taking advantage of the slightly higher boiling point of heavy water.



A uranium metal "cookie" obtained after the reduction reaction
A uranium metal “cookie” obtained after the reduction reaction

The main material for the project was uranium, used as fuel for nuclear reactors, as a source for its transformation into plutonium and, in its enriched form, into the atomic bomb itself. In 1940 there were four known major uranium deposits: in Colorado, in northern Canada, in Joachimsthal (Czechoslovakia) and in the Belgian Congo, all except Joachimsthal in Allied hands. An investigation conducted in November 1942 determined that the quantities of uranium available were sufficient to meet the requirements of the project.

Nichols defined with the State Department a series of export controls on uranium oxide and negotiated the purchase of 1200 tons of uranium ore from the Belgian Congo that was stored in a warehouse on Staten Island along with the remaining stocks of ore mined in the Congo. Negotiated with Eldorado Gold Mines the acquisition of ore from its refinery in Port Hope (Ontario) and its delivery in batches of 100 tons. The Canadian government subsequently bought shares in this company until gaining control of it.

Although these acquisitions ensured sufficient supply for wartime needs, American and British leaders concluded that it was in the interests of their respective countries to gain control of as many uranium deposits in the world as possible. The most abundant source of ore was the Shinkolobwe mine in the Belgian Congo, but it was flooded and closed. Nichols tried unsuccessfully to negotiate with Edgar Sengier, director of the Upper Katanga Mining Union, the company to which the mine belonged. The Combined Policy Committee then became involved in the matter.

As 30% of the shares of the Mining Union were controlled by British interests, they took the lead in the negotiations. John Anderson and Ambassador John Gilbert Winant reached an agreement with Sengier and the Belgian government in May 1944 to reopen the mine and acquire some 1750 tons of ore at a price of $1.45 per pound. To avoid dependence on the British and Canadians for the ore, Groves agreed to acquire the uranium reserves of the US Vanadium Corporation in Uravan, Colorado. Uranium mining in Colorado produced about 800 short tons (710 t) of ore.

Mallinckrodt Incorporated in St. Louis, Missouri, received the ore and dissolved it in nitric acid to produce uranium nitrate. He then added an ether in a liquid-liquid extraction process to separate the impurities from the nitrate. This was heated to form uranium trioxide, then reduced to high-purity uranium dioxide. By July 1942 Mallinckrodt was producing one ton of high-purity oxide per day, but at first, the process of converting the oxide into uranium metal proved more difficult for contractors Westinghouse Electric and Metal Hydrides. Production was too slow and quality too low. A special branch of the Metallurgical Laboratory was then established at Iowa State University in Ames under the leadership of Frank Spedding to investigate alternatives to this initial process. This became known as Project Ames and the new Ames process became available from 1943.

Isotope separation

Natural uranium is composed of 99.3% uranium-238 and 0.7% uranium-235, but only the latter is fissile. Both being chemically identical, uranium-235 had to be physically separated from the other, more abundant isotope. Several methods for uranium enrichment were considered during the project, most of them carried out at the Oak Ridge facility.

Centrifugation failed, but electromagnetic separation, gaseous diffusion, and thermal diffusion were successful and contributed to the project. In February 1943 Groves had the idea of using the production of some of the plants as the product to be used in others.


Until April 1942 the centrifugation process was considered the only promising method of separation. Jesse Beams had developed this process at the University of Virginia during the 1930s but had encountered technical difficulties. The process required high rotational speeds, but as it passed through certain speeds, harmonic vibrations were created that could break the machinery. Therefore, it was necessary to obtain rapid acceleration to overcome these speeds. In 1941 Beams began working with uranium hexafluoride, the only gaseous compound in uranium, and succeeded in separating uranium-235. At Columbia, Urey asked Karl Cohen to investigate the process and he produced a body of mathematical theory that made it possible to design a centrifugal separation unit, with Westinghouse in charge of its construction.

Scaling this process for a production plant was a major technical challenge. Urey and Cohen estimated that producing one kilo of uranium-235 per day would require up to 50,000 centrifuges with 1-meter rotors, or 10,000 centrifuges with 4-meter rotors, assuming it were possible to build the latter. The ability to keep so many rotors operating continuously at high speed was a challenge and when Beams started its experimental apparatus it only achieved 60% of the expected production. Beams, Urey and Cohen then began working on a series of improvements to increase the efficiency of the process. However, frequent failures at high speeds of the engines, shafts and supports delayed work at the pilot plant. In November 1942 the Military Policy Committee abandoned the centrifugation process following a recommendation by Conant, Nichols and August C. Klein of Stone & Webster.

Electromagnetic separation

Electromagnetic isotope separation was developed by Ernest Lawrence at the University of California Radiation Laboratory. This method used a device known as a calutron, a hybrid between the standard laboratory mass spectrometer and a cyclotron. The device’s name derives from the words “California,” “university,” and “cyclotron.” In the electromagnetic process, a magnetic field deflects charged particles according to mass. This process was not considered scientifically elegant or efficient on an industrial level. Compared to a gaseous diffusion plant or nuclear reactor, an electromagnetic separation plant consumed scarcer materials, required more labor to operate, and had a higher construction cost. However, the process was authorized as it was based on previously tested technology and therefore presented a lower risk. In addition, it could be built in phases and achieve industrial capacity quickly.

Marshall and Nichols concluded that this process of electromagnetic separation of isotopes would require about 4500 tons of copper, of which there was a significant lack of supply. On the other hand, they could use silver as a substitute, in a ratio of 11 to 10. On August 3, 1942, Nichols met with Deputy Secretary of the Treasury Daniel W. Bell and requested the transfer of 6,000 tons of silver bullion from the West Point Bullion Depot”.

Finally, they would end up using about 13300 tons of silver. The 31 kg silver bars were melted into cylindrical ingots and taken to the Phelps Dodge company in Elizabeth, New Jersey, where they were made into strips 15.9 mm thick, 76 mm wide and 12 m long. The Allis-Chalmers Company of Milwaukee, Wisconsin, was responsible for winding the strips into magnetic coils. After the war, all the machinery was dismantled and cleaned, extracting and burning the plates from the ground under it to recover as much silver as possible, of which only 1/3,600,000 would eventually be lost.

The S-1 committee assigned Stone & Webster responsibility for the design and construction of the electromagnetic separation plant, designated Y-12, in June 1942. The design called for five first-phase processing units, called Alpha circuits, and two final processing units, called Beta circuits. Construction began in February 1943, and in September 1943 Groves authorized the construction of four more circuits, designated Alpha II.

When the plant was commissioned for a scheduled test in October of that year, the 14-ton vacuum tanks were displaced from their alignment by the power of the magnets and had to be secured more firmly. A bigger problem subsequently arose when the magnetic coils began to short-circuit. In December of that year, Groves ordered one of the magnets dismantled for inspection, finding a large amount of rust inside it. Following this discovery, Groves ordered the circuits dismantled and the magnets sent back to the factory for cleaning.

An acid pickling facility was set up at the plant itself to clean pipes and other equipment. The second Alpha I circuit was not operational until late January 1944, the first Beta and the first and third Alpha I became available in March of that year, and the fourth Alpha I became operational in April. The four Alpha II circuits were completed between July and October 1944.

They hired Tennessee Eastman to manage the Y-12 plant under a usual fixed-cost plus fixed-rate contract, with a fee of $22500 per month plus $7500 per circuit for the first seven circuits and $4000 for each additional circuit. The calutron was initially operated by Berkeley scientists to eliminate faults and achieve a reasonable operational index. They were eventually replaced by operators trained by Tennessee Eastman who had only received secondary education. Nichols compared unit production data, telling Lawrence that hillbilly operators were doing better than their PhDs. The two agreed to pursue a “production career” that Lawrence lost, which meant a boost in morale for the workers and supervisors of Tennessee Eastman. According to Nichols himself, the young operators “were trained as soldiers not to reason why”, while “scientists could not avoid getting into long investigations into the cause of fluctuations in measuring instruments, even the smallest of them”.

Initially, the Y-12 plant enriched uranium-235 to 13-15%, sending the first few hundred grams of this product to Los Alamos in March 1944. Only 1 part in 5825 of consumed uranium product emerged as the final product. Much of the rest was splashed onto the equipment in the process. Several arduous recovery works helped raise production to 10% of the uranium-235 consumed in January 1945. In February of the same year, the Alfa circuits began using a slightly richer input product (1.4%) from the new S-50 thermal diffusion plant. The following month it received an improved product (5%) from the K-25 gaseous diffusion plant and by August this K-25 plant was producing uranium enriched enough to be used directly in the Beta circuits.

The Calutron Girls
The Calutron Girls were young women who monitored the control panels of the Calutron on Y-12. Gladys Owens, sitting in the foreground, did not know what she had been involved in until she saw this photo on a public tour of the facility 50 years later. Photograph by Ed Westcott.

Gaseous diffusion

The most promising, but also the most complicated, method of isotope separation was gaseous diffusion. Graham’s law states that the effusion rate of a gas is inversely proportional to the square root of its molecular mass, so in a container containing a semipermeable membrane and a mixture of two gases, lighter molecules will exit the container more quickly than heavier molecules. The gas leaving the container is enriched in the lighter molecules, while the residual gas is depleted. The idea proposed was that these containers could be arranged in the form of a cascade of pumps and membranes, with each successive stage containing a slightly more enriched mixture. Research into this process was conducted at Columbia University by a group that included Harold Urey, Karl P. Cohen, and John R. Dunning.

In November 1942 the Military Policy Committee approved the construction of a 600-stage gaseous diffusion plant. On December 14, M. W. Kellogg accepted an offer to build the plant, which was codenamed K-25. They negotiated a contract at cost plus a fixed rate, eventually getting a total of $2.5 million. A separate corporate entity called Kellex was created for this project, led by Percival C. Keith, one of M. W. Kellogg’s vice presidents.

The process had to face great technical difficulties. They had to use the highly corrosive uranium hexafluoride as a gas, as no substitute was found, and engines and pumps would have to be vacuum-sealed and surrounded by inert gas. The biggest problem was the design of the barrier, which had to be strong, porous and resistant to corrosion. The best choice for this purpose seemed to be nickel. Edward Adler and Edward Norris created a mesh barrier from electroplated nickel. A six-stage pilot plant was built at Columbia to test this process, but the Norris-Adler prototype proved too fragile. Kellex, Bell Telephone Laboratories and Bakelite Corporation developed a pulverized nickel-based barrier, and in January 1944 Groves ordered the production of this barrier.

Kellex’s design for the K-25 plant consisted of a long U-shaped structure 800 meters in length, containing 54 adjoining buildings. These buildings were divided into nine sections and within these were six-stage cells. The cells could be operated independently or consecutively within a section. Similarly, the sections could be operated separately or as part of a single cascade.

A survey group began construction marking the 2 km² location in May 1943. Work on the main building began in October of that same year and the six-stage pilot plant was ready for operation on April 17, 1944. In 1945 Groves canceled the plant’s upper stages, instructing Kellex to design a 540-stage supplementary feeding unit instead, which became known as K-27. Kellex transferred the last unit to the operation’s contractor, Union Carbide and Carbon, on September 11, 1945. The total cost, including the completion of the K-27 plant after the war, was $480 million.

The production plant began its operation in February 1945 and as the cascades began to be operational, the quality of the product was improving. By April 1945 the K-25 plant had achieved 1.1% enrichment and began using the output of the S-50 thermal diffusion plant as an input product. Part of the following month’s production achieved almost 7% enrichment. In August of that year, the last of the 2892 stages began operating. The K-25 and K-27 plants achieved their full potential in the early post-war years, eclipsing other production plants and becoming prototypes for a new generation of uranium enrichment plants.

Thermal diffusion

The thermal diffusion process was based on the theory of Sydney Chapman and David Enskog, which explains that when a mixed gas passes through a temperature gradient, the heavier tends to concentrate at the cold end and the lighter at the hot end. Since hot gases tend to rise and cold gases tend to descend, this can be used as a means of isotope separation. This process was first demonstrated by Klaus Clusius and Gerhard Dickel in Germany in 1938. The method was developed by U.S. Navy scientists, but it was not one of the uranium enrichment technologies initially selected for use in the Manhattan Project, as there were doubts about its technical feasibility, although internal rivalry between the Navy and Army services also played a role in this initial decision.

The Naval Research Laboratory continued this research under the direction of Philip Abelson, but contact with the Manhattan Project was minimal until April 1944 when Captain William S. Parsons, the naval officer in charge of artillery development at Los Alamos, informed Oppenheimer of promising progress in the Navy’s experiments on thermal diffusion. Oppenheimer wrote to Groves suggesting that production from a thermal diffusion plant could be used as an input product for the Y-12 plant. Groves created a committee consisting of Warren K. Lewis, Eger Murphree and Richard Tolman to investigate this idea, estimating that a thermal diffusion plant costing $3.5 million could enrich 50 kg of uranium per week to 0.9% uranium-235. Groves approved its construction on June 24, 1944.

Groves contracted with the H. K. Ferguson Company of Cleveland to build the thermal diffusion plant, designated S-50. Groves advisors Karl Cohen and W. I. Thompson of Standard Oil estimated that construction would take six months to complete, but Groves gave only four months. According to the plans they were to install 2142 diffusion columns 15 meters high arranged on 21 platforms and inside each column there would be three concentric tubes. Steam obtained from the nearby K-25 power plant, at a pressure of 6 900 kPa and a temperature of 285 °C, flowed downwards through the innermost 32 mm nickel tube, while water at 68 °C flowed upwards through the outermost iron tube. The separation of the isotopes took place in the uranium hexafluoride gas, between the nickel and copper tubes.

Construction work began on July 9, 1944, and the S-50 plant began partial operation in September of that same year. Ferguson operated the plant through its subsidiary Fercleve. In October the plant had produced 4.8 kg of 0.852% uranium-235%. Several leaks limited production and forced total stops during the following months, but in June 1945 it achieved a production of 5770 kg. By March 1945 all 21 production platforms were operating. At first, the output of the S-50 plant was used to feed the Y-12 plant, but from March 1945 the three enrichment processes were carried out in series. The S-50 became the first stage, enriching uranium from 0.71% to 0.89%. This material was used in the gaseous diffusion process at the K-25 plant, producing a product enriched up to 23% which in turn fed the Y-12 plant, reaching up to 89% there, enough for nuclear weapons.

Total production of uranium-235

By July 1945, about 50 kg of uranium enriched to 89% uranium-235 had been delivered to Los Alamos. This full 50 kg, along with additional 50% enriched uranium, gave a resulting average of 85% enriched uranium, which were used in the Little Boy bomb.


The second line of development pursued by the Manhattan Project used plutonium as a fissile element. Although there are small amounts of plutonium in its natural state, the best way to obtain large amounts of this element is in a nuclear reactor, bombarding the uranium with neutrons. Uranium-238 is transmuted into uranium-239, which rapidly decays first into neptunium-239 and then into plutonium-239. Only a small amount of uranium-238 is transformed, so the plutonium has to be chemically separated from the remaining uranium, initial impurities and nuclear fission products.

X-10 graphite reactor

In March 1943 DuPont began construction of a plutonium plant on a 0.5 km² site in Oak Ridge. Initially intended to serve as a pilot plant for Hanford’s larger production facility, it included the air-cooled X-10 graphite reactor, a chemical separation plant, and support facilities. Due to the subsequent decision to build water-cooled reactors at Hanford, only the chemical separation plant operated as a true pilot. The X-10 reactor was composed of a large block of graphite 7.3 m wide on each side, weighing about 1500 tons and surrounded by high-density cement 2.1 m thick as a radiation shield.

The main difficulty they encountered was related to the uranium casings produced by Mallinckrodt and Metal Hydrides. These had to be coated with aluminum to prevent corrosion and the escape of fission products into the cooling system. The Grasselli Chemical Company tried to develop a hot bath tinning process without success, while Alcoa tried a canning process. A new process for flux-free welding was then developed, and 97% of the housings passed a standard vacuum test, but high-temperature tests indicated a failure rate greater than 50%. Despite this, production began in June 1943. The Metallurgical Laboratory eventually developed an improved welding technique with the assistance of General Electric, which was incorporated into the production process in October 1943.

Overseen by Fermi and Compton, the X-10 graphite reactor reached the critical point on November 4, 1943, with about 30 tons of uranium. A week later the load was increased to 37 tons, increasing its power to 500 kW, and by the end of the month, the first 500 milligrams of plutonium were created. Modifications made over time increased the power to 4000 kW in July 1944. The X-10 reactor operated as a production plant until January 1945, when it was used for research activities.

Reactors in Hanford

Although an air-cooled design had been chosen for the Oak Ridge reactor to expedite construction, this would be unfeasible for larger production reactors. Initial designs from the Metallurgical Laboratory and DuPont used helium for cooling, before they determined that a water-cooled reactor would be simpler, cheaper, and faster to build. The new design was not available until October 4, 1943. Meanwhile, Matthias concentrated on improving the Hanford site by building lodgings, improving roads, building a railway interchange line, and improving electricity, water, and telephone lines.

As in Oak Ridge, the main difficulty encountered was related to the canning of uranium projectiles, a process that began in Hanford in March 1944. These were pickled to remove dirt and impurities, given molten baths of bronze, tin and an aluminum-silica alloy, canned using hydraulic dams, and subsequently sealed with arc welding under an argon atmosphere. Finally, they performed a series of tests to detect imperfect holes or welds. Most of the projectiles did not pass these tests, so in the beginning they obtained few units per day that served for the process. They made progressive advances until in June 1944 production was increased to the point that they would have enough shells to activate Reactor B as scheduled in August 1944.

Work on reactor B, the first of six planned 250 MW reactors, began on 10 October 1943. The reactor complexes were given designations with letters from A to F, with B, D and F being the first to be built, as this maximized the distance between reactors. These three were the only ones built during the Manhattan Project. The 37-meter-high building required about 400 tons of steel, 13300 m³ of concrete, 50000 concrete blocks and 71000 concrete bricks for its construction.

Construction of the reactor itself began in February 1944. Overseen by Compton, Matthias, Crawford Greenewalt of DuPont, Leona Woods and Fermi, who inserted the first bullet, the reactor was started on September 13, 1944. Over the next few days, 838 tubes were loaded and the reactor reached its critical point. Shortly after midnight on September 27, operators began removing the control rods to start production. At first everything seemed to be going well, but around 03:00 the power level began to drop and by 06:00 the reactor had stopped completely. They investigated the cooling water to try to determine if there was a leak or contamination. The next day they restarted the reactor, which stopped completely again in the same way.

Fermi contacted Chien-Shiung Wu, who identified the cause of the problem as neutron poisoning of xenon-135, which has a half-life of 9.2 hours. Fermi, Woods, Donald J. Hughes and John Archibald Wheeler then calculated the nuclear cross-section of xenon-135, which turned out to be 30,000 times that of uranium. However, DuPont engineer George Graves had deviated from the original Metallurgical Laboratory design, in which the reactor would have 1500 tubes arranged in a circle, adding another 504 additional tubes to fill the corners. Scientists had considered this to be engineering left over and a waste of time and money, but Fermi learned that if they loaded all of the 2004 tubes the reactor could achieve the required level of power and produce plutonium efficiently. The D reactor was started on December 17, 1944, and the F reactor on February 25, 1945.

Separation process

Chemists considered the problem of how to separate plutonium from uranium without knowing its chemical properties. Working with the minimal amounts of plutonium available at the Metallurgical Laboratory in 1942, a team led by Charles M. Cooper developed a lanthanum fluoride process to separate uranium and plutonium, chosen for the pilot separation plant. Glenn Seaborg and Stanly G. Thomson subsequently developed a second separation process, the bismuth phosphate process. The operation of this process was to switch the plutonium between its +4 and +6 oxidation states in bismuth phosphate solutions. In the first state, the plutonium was precipitated and in the last, it was kept in solution precipitating other products.

Greenewalt favored the bismuth phosphate process because of the corrosive nature of lanthanum fluoride and this process was chosen for the Hanford separation plants. As soon as the X-10 reactor began producing plutonium, the separation pilot plant began testing. The first batch was processed with an efficiency of 40%, which increased in the following months to 90%.

In Hanford, the installation in Area 300 was given top priority. It contained buildings for test materials, uranium preparation, and instrument assembly and calibration. One of the buildings contained canning equipment for uranium bullets, while another contained a small test reactor. Despite the high priority assigned, work on Area 300 was delayed according to the initial plan by the unique nature and complexity of the facilities, as well as the lack of workers and materials in wartime.

Initial plans called for the construction of two separation plants in each of the areas known as 200-West and 200-East. The plan was later reduced to just two, plants T and U, in the 200-West area, and one, plant B, in the 200-East area. Each separation plant was composed of four buildings: a processing cell or “cannon” building (known as 221), a concentration building (224), a purification building (231) and a loader warehouse (213). The guns had a length of 240 m and a width of 20 m each and were composed of 40 cells of 5.4 x 4 x 6.1 m.

Work on buildings 221-T and 221-U began in January 1944, with the first completed in September and the other in December 1944. Building 221-B was the next to be completed, in March 1945. Because of the high levels of radioactivity, all work in the separation plants had to be done by remote control using closed-circuit television. Maintenance was carried out with the help of a raised crane and tools designed specifically for this purpose.

The 224 buildings were smaller as they had to process less material and this was less radioactive. Buildings 224-T and 224-U were completed on October 8, 1944, and 224-B on February 10, 1945. The purification methods that would eventually be used on the 231-W were still unknown when construction began on April 8, 1944, but the plant was completed later that year with the methods already selected. On February 5, 1945, Matthias delivered the first shipment of 80 grams of 95% pure plutonium nitrate to a courier from Los Alamos in Los Angeles.

Weapon design

In 1943 design efforts were aimed at developing a ballistic-type fission weapon with plutonium called Thin Man. Initial research into the properties of plutonium was done using plutonium-239 generated in a cyclotron, extremely pure, but which could only be created in very small quantities. Los Alamos received the first sample of plutonium from Clinton’s X-10 reactor in April 1944 and three days later Emilio Segrè discovered a problem: the plutonium generated in the reactor had a higher concentration of plutonium-240, resulting in a spontaneous fission rate five times higher than the plutonium generated in the cyclotron. Seaborg had already correctly predicted in March 1943 that some of the plutonium-239 would absorb a neutron becoming plutonium-240.

This rendered reactor-generated plutonium useless for use in a ballistic-type weapon. Plutonium-240 would start the chain reaction too early, causing a pre-detonation that would release enough energy to disperse the critical mass with only a minimal amount of reacted plutonium (a long fire). Scientists suggested a faster weapon, but it turned out to be unfeasible. The possibility of isotope separation was also considered and rejected, as plutonium-240 is even more complicated to separate from plutonium-239 than uranium-235 from uranium-238.

Work on an alternative method of bomb design, known as implosion, had already begun earlier under the direction of physicist Seth Neddermeyer. The implosion used explosives to crush a subcritical sphere of fissile material into a smaller, denser shape. When fissile atoms are compressed together with each other, the neutron capture rate increases and the mass becomes critical. The metal needs to travel only a very short distance, so the critical mass is assembled in much less time than it would take with a ballistic method.

Early research into the implosion of Neddermeyer in 1943 and early 1944 was promising, but it also made it clear that the problem would be much more difficult from a theoretical and engineering perspective than for the design of the weapon itself. In September 1943 John von Neumann, who already had experience with hollow charges used in armor-piercing projectiles, argued that implosion would not only reduce the danger of pre-detonation and long fire, but also make more efficient use of fissile material and proposed using a spherical configuration instead of the cylindrical one Neddermeyer was working on.

In July 1944 Oppenheimer concluded that plutonium could not be used in a ballistic design and opted for the implosion design. The accelerated design effort for an implosion design, under the code name Fat Man, began in August 1944 when Oppenheimer put into operation a reorganization of the Los Alamos Laboratory. Two new groups were created in the laboratory, Division X (explosives) led by explosives expert George Kistiakowsky and Division G under the leadership of Robert Bacher. The new design that had been designed by von Neumann and the T Division (of theorist), mainly Rudolf Peierls, used explosive lenses to focus the explosion in a spherical shape using a combination of fast and slow explosion elements.

Designing lenses that detonated with the right shape and speeds proved to be slow, difficult and frustrating for scientists. They tested various explosives until they settled for composition B as a fast explosive and cheap as a slow explosive. The final design looked like a football, with 20 hexagonal and 12 pentagonal lenses, each weighing about 36 kg. Getting the detonation right required fast, reliable and electrically safe detonators, two for each lens for reliability. Therefore, they decided to use explosive bridge cable detonators, a new invention developed in Los Alamos by a group led by Luis Walter Álvarez. The Raytheon company was contracted to manufacture these devices.

To study the behavior of converging shock waves, Robert Serber devised the RaLa experiment, which used the short-lived radioisotope lanthanum-140, a potent source of gamma radiation. The gamma-ray source was located in the center of a metal sphere surrounded by explosive lenses, which in turn were inside an ionization chamber. This made it possible to film an X-ray film of the implosion. The explosive lenses were primarily designed using the results of this series of tests. In his history of the Los Alamos project, David Hawkins wrote, “RaLa became the most important experiment to affect the final design of the bomb.”

Inside the explosives were 110 mm thick aluminum reflectors that offered a regular transition from the lowest density explosive to the next layer, a 76 mm thick natural uranium lock. Its main function was to hold the critical mass together as long as possible, but it would also reflect neutrons back to the nucleus and some of it could fission as well. To prevent pre-detonation by external neutrons, it had a safety coated with a thin layer of boron.

A polonium-beryllium modulated neutron initiator, known as an “urchin” (sea urchin) because its shape resembled that of a sea urchin, was developed to start the chain reaction at just the right time. This work on the chemistry and metallurgy of radioactive polonium was led by Charles Allen Thomas of Monsanto and became known as the Dayton Project. The tests required up to 500 curies per month of polonium, which Monsanto could supply. The assembled assembly was clad in a bomb casing made of duralumin to protect it from bullets and anti-aircraft fire.

The most important challenge for metallurgists was figuring out how to shape plutonium into a sphere. The difficulties became apparent when attempts to measure the density of plutonium yielded inconsistent results. At first, scientists believed that these inconsistencies were due to contamination, but soon determined that there were multiple allotropes of plutonium. The unstable α phase that exists at room temperature molts to a plastic β phase at higher temperatures. The scientists focused on the most malleable δ phase in the temperature range between 300 °C and 450 °C. This was stable at room temperature in an alloy with aluminum, but aluminum emits neutrons when bombarded with alpha particles, something that would exacerbate the pre-detonation problem. The scientists obtained a plutonium-gallium alloy that stabilized this phase δ and could be hot-pressed into the desired spherical shape, which was given a nickel layer to prevent the plutonium from corroding.

These jobs were dangerous and by the end of the war half of the experienced chemists and metallurgists had to be discharged due to the appearance of high levels of plutonium in their urine. A small fire at Los Alamos in January 1945 gave rise to concerns that the plutonium laboratory might contaminate the entire locality, so Groves authorized the construction of a new facility for plutonium chemistry and metallurgy, which became known as the DP site. The hemispheres of the first plutonium core were produced and delivered on July 2, 1945, with three more hemispheres produced on July 23 and delivered three days later.

Try Trinity

Due to the complexity of an implosion weapon, it was decided that, despite the expenditure of fissile material, an initial test would be necessary. Groves passed this test, provided the active material could be recovered. Scientists considered provoking a controlled long fire, but Oppenheimer opted for a full nuclear test, which was codenamed “Trinity.”

In March 1944, Kenneth Bainbridge, professor of physics at Harvard University, was assigned to plan the test under Kistiakowsky’s direction. Bainbridge chose the bombing range near Alamogordo Army Airport as the test site. Bainbridge worked with Captain Samuel P. Davalos on the construction of Trinity Base Camp and its facilities, which included barracks, warehouses, workshops, a powder magazine, and a commissary.

Groves was not enthusiastic about explaining the loss of billions of dollars worth of plutonium to a Senate committee, so they built a containment vessel codenamed “Jumbo” to recover the active material in the event of test failure. At 7.6 m long and 3.7 m wide, this device was manufactured with 217 tons of iron and steel at Babcock & Wilcox’s Barberton, Ohio facility.

It was transported on a special railroad to a dead track in Pope (New Mexico) and from there it was carried the last 40 km to the test site pulled by two tractors. When he arrived at the site, confidence that the implosion method would work was high and the availability of plutonium sufficient, so Oppenheimer decided not to use it. Instead, they placed it on top of a steel tower 730m away from the weapon to gauge how powerful the blast would be. After the test the Jumbo survived, even though the tower it was in did not, which helped the belief that the Jumbo might have been enough to contain a long fire in the explosion.

On May 7, 1945, they conducted an initial test explosion to calibrate the instruments. They erected a wooden platform about 730 m from ground zero and piled up 100 tons of TNT by adding traces of nuclear fission products in the form of irradiated uranium from Hanford, which had been dissolved and poured into the explosive. Oppenheimer oversaw this explosion alongside Groves’ new deputy commander, Brigadier General Thomas Farrell. The data they obtained in this initial test turned out to be vital to the Trinity test.

For the test, they hoisted the weapon, codenamed “the gadget,” on top of a 30m steel turret, as detonation at that height would give a better indication of how the weapon would behave when launched from a bomber. Airborne detonation would maximize the energy applied directly to the target, and generate less radioactive waste. The instrument was mounted under the supervision of Norris Bradbury in the nearby McDonald Ranch House building on July 13 and was raised precariously with a winch the next day.

Among the observers of the test were Bush, Chadwick, Conant, Farrell, Fermi, Groves, Lawrence, Oppenheimer and Tolman. At 05:30 on July 16, 1945, the instrument exploded with an energy equivalent to about 20 kilotons of TNT, leaving a 76 m wide trinitite (radioactive crystal) crater in the desert. The shock wave was felt up to 160 km away and the mushroom cloud reached 12 km high. The detonation was heard in the city of El Paso (Texas), so Groves had to spread a story about an explosion in a powder keg in the Alamogordo field to cover up the test.

Oppenheimer later recalled that, while witnessing the explosion, he thought of a verse from the Hindu holy book, the Bhagavad-gītā (XI, 12):

कालोऽस्मि लोकक्षयकृत्प्रवृद्धो लोकान्समाहर्तुमिह प्रवृत्तः। ऋतेऽपि त्वां न भविष्यन्ति सर्वे येऽवस्थिताः प्रत्यनीकेषु योधाः॥ ११- ३२॥
If the radiance of a thousand suns burst out at the same time in the sky, it would be like the splendor of the mighty…

Years later he would explain that another verse had also gone through his head at that time:

We knew the world wouldn’t be the same. Some people laughed, some cried. Most people fell silent. I remembered the line of Hindu scriptures, the Bhagavad Gita; Vishnu is trying to persuade the Prince that he must do his duty and, to impress him, adopts his omniform and says, “Now I have become Death, the destroyer of worlds.” I guess we all think that, one way or another.
Oppenheimer read the original text in Sanskrit (XI,32)

Manhattan Project staff

In June 1944, the Manhattan Project had about 129,000 employed workers, of whom 84500 were construction workers, 40500 were plant operators, and 1800 were military personnel. As construction activity declined, the workforce dropped to 100,000 a year later, even though military personnel increased to 5600. Recruiting the required number of workers, especially those with high qualifications, in competition with other vital wartime programs, proved to be very difficult. In 1943 Groves obtained a special priority for the work of the War Personnel Commission and in March 1944 both this commission and the War Production Board gave the project the highest priority.

Tolman and Conant, in their role as scientific advisors to the project, drew up a list of candidate scientists who were evaluated by other scientists already working on the project. Groves then sent a personal letter to the presidents of their universities or heads of their companies asking them to release them to perform work essential to the war. At the University of Wisconsin-Madison, Stanislaw Ulam had to advance one of her exams to Joan Hinton so that she could go to work on the project. A few months later Ulam himself received a letter from Hans Bethe, inviting him to join the project, while Conant personally convinced Kistiakowsky to join as well.

One of the sources of qualified personnel was the army itself, in particular the army’s Specialized Training Program. In 1943 the Special Engineering Detachment was created, with an authorized force of 675 troops. Technicians and skilled workers recruited into the army were assigned to this detachment. Another source of personnel was the Army Women’s Corps. Initially arranged for office tasks in the handling of classified material, soon the personnel of this body was also assigned to scientific and technical tasks.

An associate professor of radiology at the University of Rochester, Stafford L. Warren, was appointed a colonel in the Army Medical Corps and appointed chief of the project’s medical section, as well as Groves’ medical adviser. His initial assignment was to direct medical personnel at the hospitals of Oak Ridge, Richland and Los Alamos.

The Medical Section was responsible for medical research and health and safety programs. This posed a major challenge, as workers handled a variety of toxic chemicals, used hazardous liquids and gases under high pressures, worked at high voltages, and conducted experiments with explosives, in addition to the unknown dangers of radioactivity and handling fissile materials. Nevertheless, in December 1945 the National Safety Council awarded the Manhattan Project the Distinguished Service in Safety Honor Award in recognition of its safety record. Between January 1943 and June 1945, there were 62 deaths and 3879 disabled wounded in the project, 62% below the rate of private industry.


A 1945 article in Life magazine estimated that before the atomic bombings of Hiroshima and Nagasaki “probably no more than a few dozen men throughout the country knew the full significance of the Manhattan Project and perhaps only a thousand others knew that it was work on atoms.” The magazine wrote that more than 100,000 employees on the project “worked like moles in the dark.” Warned that the dissemination of project secrets was punishable by 10 years in prison or a fine of 10,000 dollars (about 102,000 today), the workers saw huge quantities of raw material enter the factories without anything coming out of them and supervised “valves and switches while behind the thick concrete walls mysterious reactions took place” without knowing the purpose of their work.

Oak Ridge security personnel considered any private party with more than seven people suspicious, and residents — who believed government agents secretly infiltrated among them — repeatedly avoided inviting the same guests. Although the original residents of the area could be buried in existing cemeteries, all coffins had to be opened in front of a member of the security corps for inspection. All residents, including senior officers and their cars, had to go through screening when entering and leaving the project premises. An Oak Ridge worker publicly stated that “if you got inquisitive, you would be called by secret government agents in less than two hours.

Usually, those who called to give explanations accompanied us with their bags to the front door and ordered them to keep walking.” However, despite being told that their work would help end the war and perhaps all future wars, not seeing or understanding the results of their tedious tasks, the typical side effects of factory work and the end of the war in Europe without the use of their labor caused serious problems in the morale of the workers and that many rumors spread. One manager said that after the war:

“Well, it wasn’t that the work was hard… It was confusing. No one knew what was being done in Oak Ridge, not even me, and many people thought I was wasting my time here. It was up to me to explain to the dissatisfied workers that they were doing very important work. When they asked me what it was, I had to tell them it was a secret. But I almost went crazy trying to figure out what was going on”.

Another worker testified how, working in a laundry, she passed a “special instrument” to the uniforms every day that emitted a “click”. It wasn’t until after the war that this worker learned she had been performing the task of searching for radiation with a Geiger counter. To improve morale among these workers, an extensive system of intramural sports leagues was created in Oak Ridge, including 10 baseballs, 81 softball and 26 football teams.


Voluntary censorship of atomic information began even before the Manhattan Project. After the beginning of the war in Europe in 1939, American scientists began to avoid the publication of research related to military topics and in 1940 scientific publications began to ask the National Academy of Sciences to give its approval for the publication of certain articles.

William L. Laurence of The New York Times wrote an article for The Saturday Evening Post on atomic fission in September 1940, later learning that in 1943 several government agents had petitioned libraries across the country to withdraw that issue. The Soviets learned of this silence and in April 1942 nuclear physicist Georgy Fliorov wrote to Joseph Stalin warning him of the absence of articles on nuclear fission in American publications. This triggered the Soviet Union to establish its own project for an atomic bomb.

The Manhattan Project operated under tight security to try to prevent its discovery from inducing the Axis powers, especially Germany, to accelerate their own nuclear projects or conduct covert operations against the project. By contrast, the government’s Bureau of Censorship relied on the press to abide by a voluntary code of conduct in its publications, and the project went unnoticed at first. In early 1943 newspapers began publishing reports of major construction in Tennessee and Washington based on public records, and the bureau began reviewing with project management how they could maintain secrecy.

In June, the Bureau of Censorship asked newspapers and broadcasters to avoid discussing “crushing atoms, atomic energy, atomic fission, atomic separation, or any of their equivalents,” as well as “the use for military purposes of radium or radioactive materials, heavy water, high-voltage discharge equipment, or cyclotrons.” The office also asked to avoid discussions on “polonium, uranium, ytterbium, hafnium, protactinium, radium, rhenium, thorium and deuterium”; Although only uranium was confidential, it was listed along with other elements to hide its importance.

Soviet espionage

The possibility of sabotage was present throughout the project, with suspicion sometimes arising when equipment failures occurred. Although it was confirmed that some problems were caused by dissatisfied or negligent employees, there were no confirmed cases of Axis-instigated sabotage. However, on March 10, 1945, a Japanese incendiary balloon hit a power line, causing a voltage surge that caused the temporary shutdown of three reactors at Hanford. With a very high number of people involved in the project, the security of the project was a complicated task.

A special detachment called the Counterintelligence Corps was formed to deal with the project’s security problems. In 1943 the Americans were certain that the Soviet Union was trying to infiltrate the project. Lt. Col. Boris T. Pash, head of the counterintelligence branch of the Western Defense Command, investigated a suspicion of Soviet espionage at the Radiation Laboratory in Berkeley. Oppenheimer confirmed to Pash that a fellow Berkeley professor, Haakon Chevalier, had asked him for some information to pass on to the Soviet Union.

The Soviet spy who had the greatest success was Klaus Fuchs, a member of the British mission with a prominent role in Los Alamos. The revelation in 1950 of his espionage activities damaged American nuclear cooperation with the United Kingdom and Canada. Other cases of espionage were subsequently uncovered, resulting in the arrest of Harry Gold, David Greenglass, and Ethel and Julius Rosenberg. Other spies such as George Koval and Theodore Hall were not discovered for several decades. The value of these espionage actions is difficult for historians to quantify, since the main constraint on the Soviet atomic bomb project was the shortage of uranium ore. The general consensus is that espionage saved the Soviets a year or two of research.

Foreign intelligence

In addition to the development of the atomic bomb, the Manhattan Project was tasked with gathering intelligence on the German nuclear power project. The Americans believed that Japan’s nuclear weapons program was not very advanced since Japan had access to very little uranium ore, but they did fear that Germany was very close to developing its own nuclear weapons. Instigated by the Manhattan Project, a bombing and sabotage campaign was carried out against heavy water plants in German-occupied Norway. A small mission was created with personnel from the Office of Naval Intelligence, the OSRD, the Manhattan Project itself, and the Army intelligence group G-2 to investigate the enemy’s scientific developments, not limited to those related to nuclear weapons. The army’s intelligence chief, Maj. Gen. George V. Strong, assigned Boris Pash command of the unit, which was given the code name “Alsos,” a Greek word meaning “grove.”

In Italy, the so-called Alsos mission interrogated the personnel of the physics laboratory of the University of Rome La Sapienza after the capture of the city in June 1944. Meanwhile, Pash formed a combined division of the British and American mission, in London, under the command of Captain Horace K. Calvert to participate in Operation Overlord. Groves felt that the risk that the Germans might try to stop the Normandy landings with radioactive poisons was enough to warn General Dwight D. Eisenhower of this by sending an officer to report to his chief of staff, Lieutenant General Walter Bedell Smith. Under the code name Operation Peppermint, special equipment was prepared and Chemical Warfare Service teams were trained to use it.

Following the first advances of the Allied armies in Europe, Pash and Calvert met with Frédéric Joliot-Curie to ask him about the activities of German scientists. They also spoke with officials from the Upper Katanga Mining Union about uranium shipments sent to Germany. They located 68 tons of ore in Belgium and another 30 tons in France. Interrogations of several German prisoners indicated that uranium and thorium were being processed at Oranienburg, about 32 km from Berlin, so Groves ordered their bombing on March 15, 1945.

A team from the Alsos mission traveled to Stassfurt in the Soviet occupation zone and recovered 11 tonnes of uranium ore from the facilities of the Wirtschaftliche Forschungsgesellschaft company. In April 1945 Pash, commanding a composite force known as the T-Force, carried out Operation Harborage, a sweep behind enemy lines of the cities of Hechingen, Bisingen and Haigerloch that formed the heart of the German nuclear effort. The T-Force captured nuclear laboratories, documentation, equipment and supplies, including heavy water and 1.5 tons of uranium metal.

Several Alsos mission teams were also tasked with capturing several German scientists, including Kurt Diebner, Otto Hahn, Walther Gerlach, Werner Heisenberg and Carl Friedrich von Weizsäcker, who were taken to England and interned in Farm Hall, a guarded residence in Godmanchester. Following the detonation of the bombs in Japan, the Germans were forced to face the fact that the Allies had done what they could not.

Allied soldiers dismantling the German experimental nuclear reactor at Haigerloch
Allied soldiers dismantling the German experimental nuclear reactor at Haigerloch

Atomic bombings of Hiroshima and Nagasaki


Beginning in November 1943, the Air Force Materiel Command at Wright Field, Ohio, began the Silverplate program, codenamed for the modification of Boeing B-29 Superfortress aircraft to carry bombs. They conducted bombing tests at Army Airfield at Muroc and at the Naval Armament Test Station at Inyokern, California. Groves met with the chief of the United States Army Air Forces (USAAF), General Henry H. Arnold, in March 1944 to discuss the delivery of the bombs upon completion. The only Allied aircraft capable of carrying the 5.2 m wide Thin Man bombs or the 150 cm wide Fat Man bombs was the British Avro Lancaster, but using a British aircraft would cause difficulties with its maintenance.

Groves hoped that the American B-29 Superfortress could be modified to carry a Thin Man bomb by joining its two bomb compartments together. Arnold promised that they would do everything possible to modify the B-29s and appointed Major General Oliver P. Echols as USAAF liaison officer for the Manhattan Project. Echols subsequently named Colonel Roscoe C. Wilson as his replacement and Wilson became the USAAF’s primary contact for the Manhattan Project. President Roosevelt instructed Groves that if the atomic bombs were ready before the end of the war with Germany, he should prepare to drop them on Germany.

The 509th Composite Group was activated on December 17, 1944, at Wendover Air Force Base in Utah, under the command of Colonel Paul W. Tibbets. This base, near the Nevada border, was codenamed “Kingman” or “W-47.” The trainings were held at Wendover and at San Antonio de los Baños Air Base in Cuba, where the 393rd Bomber Squadron practiced long-haul flights over the sea and dropped test pumpkin bombs. A special unit known as Project Alberta was formed at Los Alamos under the command of Navy Captain William S. Parsons of Project Y, as part of the Manhattan Project’s duties to assist in the preparations and delivery of the bombs.

Commander Frederick L. Ashworth of Alberta met with Fleet Admiral Chester W. Nimitz on Guam in February 1945 to brief him on the project. During his stay there Ashworth chose North Field on Pacific Island Tinian as a base for the 509th Composite Group and reserved site for the group and the necessary buildings, deploying it there in July 1945. Farrell arrived in Tinian on July 30 as a representative of the Manhattan Project.

Most of the components of the Little Boy bomb left San Francisco on the cruiser USS Indianapolis on 16 July and arrived in Tinian on July 26. Four days later the ship was sunk by the Japanese submarine I-58. The remaining components, including six uranium-235 rings, were delivered by three Douglas C-54 Skymasters of the 509th Group’s 320th Troop Transport Squadron. They carried two Fat Man assemblies to Tinian in specially modified B-29 aircraft belonging to Group 509 and the first plutonium core was carried in a special C-54.

A joint objectives committee was established between the Manhattan District and the USAAF to determine which cities in Japan should be targeted, recommending the cities of Kokura, Hiroshima, Niigata and Kyoto. It was then that Secretary of War Henry L. Stimson intervened, announcing that he would make the decision of the objectives and that he would not authorize the bombing of Kyoto because of its historical and religious importance. Groves then asked Arnold to remove Kyoto not only from the list of nuclear targets but also from the list of targets for conventional bombing. One of the cities chosen as a possible substitute target for Kyoto was Nagasaki.


In May 1945 the Interim Committee was set up to advise on the use of nuclear energy in wartime and post-war times. Its president was Stimson, with James F. Byrnes, former senator and later secretary of state, as personal representative of President Harry S. Truman; Ralph A. Bard, Deputy Secretary of the Navy; William L. Clayton, Assistant Secretary of State; Vannevar Bush; Karl T. Compton; James B. Conant and George L. Harrison, Stimson’s assistant and president of New York Life Insurance Company. This committee established a panel of scientists composed of Arthur Compton, Fermi, Lawrence and Oppenheimer to advise on scientific questions. In its presentation to the Interim Committee, the panel of scientists gave its opinion not only on the likely physical effects of an atomic bomb, but also on its likely military and political impact.

During the Potsdam conference in Germany, Truman received word that the Trinity test had been a success. There he told Stalin that the United States had a new superweapon, without giving him further details. This was the first official communication to the Soviet Union about the bomb, although Stalin already knew about it from his spies. With the authorization to use the bomb against Japan already granted, no alternative was considered following the Japanese rejection of the Potsdam declaration.

On August 6, 1945, a Boeing B-29 Superfortress named Enola Gay of the 393rd Bomber Squadron, piloted by Tibbets, took off from North Field with the Little Boy bomb in its cargo hold. Hiroshima was the primary objective of the mission as it was the headquarters of the 2nd General Army, the 5th Division and a port of embarkation, with Kokura and Nagasaki as alternatives. With Farrell’s permission, Parsons, the gunner in charge of the mission, completed the assembly of the bomb in the air to minimize risks during takeoff.

The bomb detonated at an altitude of 530 m with an explosion of an estimated equivalent to about 13 kilotons of TNT. An area of approximately 12 km² was destroyed. Japanese officials determined that 69% of Hiroshima’s buildings were destroyed and another 6–7% were damaged. Between 70,000 and 80,000 people, 20,000 of these Japanese soldiers and another 20,000 Korean slave laborers, 30% of the population of Hiroshima at the time, died immediately, with another 70,000 people injured.

On the morning of 9 August 1945 the B-29 Bockscar, piloted by the commander of the 393rd Bomber Squadron, Major Charles Sweeney, took off with the Fat Man bomb in its cargo hold. This time Ashworth was the gunner and Kokura was the primary target. Sweeney took off with the bomb already assembled but with the electrical safety systems still activated. When they reached Kokura a cloud cover had darkened the city, preventing them from carrying out the visual approach required by the orders.

After three passes over the city and with less and less fuel, they headed towards the secondary objective, Nagasaki. Ashworth decided to use a radar approach in case the target was obscured, but clouds opened up over Nagasaki at the last moment, allowing them to make a visual approach following orders. The Fat Man bomb was dropped on the city’s industrial valley halfway between Mitsubishi’s steel and weapons facilities in the south and Mitsubishi-Urakami’s artillery installations in the north.

The resulting explosion had an equivalent of about 21 kilotons of TNT, about the same as the Trinity test, but was confined to the Urakami Valley and a large part of the city was protected by the intermediate mountains, resulting in the destruction of approximately 44% of the city. The bombing also greatly limited the city’s industrial production capacity and between 23,200 and 28,200 industrial workers were killed along with 150 Japanese soldiers. In total, between 35,000 and 40,000 people were killed and another 60,000 injured.

Groves hoped to have another atomic bomb ready for use by Aug. 19, along with three more in September and three more in October. Two more assemblies of Fat Man bombs scheduled to leave Kirtland Air Force Base for Tinian on August 11 and 14 were prepared. At Los Alamos, technicians worked 24 successive hours to mold another plutonium core that would still need pressing and coating, so it wouldn’t be ready until Aug. 16. However, on August 10, Truman requested that no more atomic bombs be dropped on Japan without her express authorization. Groves suspended the shipment of this third core using his own authority on August 13.

On August 11, Groves telephoned Warren to order him to organize a new survey team and investigate the damage and radioactivity in Hiroshima and Nagasaki. A group equipped with portable Geiger counters arrived in Hiroshima on September 8, led by Farrell and Warren, with Japanese Vice Admiral Masao Tsuzuki acting as translator. They remained in Hiroshima until September 14 and then surveyed Nagasaki from September 19 to October 8. This exploration, along with subsequent scientific missions in Japan, provided valuable historical and scientific data.

The necessity of the bombings of Hiroshima and Nagasaki became a controversial issue among historians. Some of them questioned whether “atomic diplomacy” would not have achieved the same goals and debated whether the bombings or the Soviet declaration of war against Japan were decisive. The Franck report of June 1945 was the main effort to prevent bombing, but was rejected by the scientific panel of the Interim Committee. The Szilárd petition, drafted in July 1945 and signed by dozens of scientists working on the Manhattan Project, was a belated attempt to warn President Truman of the liability required for the use of such weapons.

Post-war and dissolution

Seeing that the work done that they did not quite understand had produced the bombings of Hiroshima and Nagasaki, the workers of the Manhattan Project were as surprised as the rest of the world. The newspapers in Oak Ridge with the announcement of the bombing of Hiroshima came to sell for 1 dollar (about 11 today). Even though the existence of the bomb was already public, the secrecy in the project continued, many of the workers continued to ignore the purpose of their work, and many of the residents of Oak Ridge continued to avoid talking about “the stuff” in ordinary conversations.

In anticipation of the bombings, Groves ordered Henry DeWolf Smyth to prepare a story for the public. Atomic Energy for Military Purposes, better known as the “Smyth Report,” was published on August 12, 1945. Groves and Nichols awarded the Army Navy “E” Award to the major contractors involved in the project in secret up to that point. More than 20 presidential medals of merit were also awarded to contractors and scientists, including Bush and Oppenheimer. Military personnel received the Legion of Merit, including the commander of the Army Women’s Corps detachment, Captain Arlene G. Scheidenhelm.

At Hanford, plutonium production declined due to the depletion of reactors B, D and F, poisoned by fission products and inflammation of the graphite moderator, known as the Wigner effect. The inflammation damaged the cargo tubes where uranium was irradiated to produce the plutonium, rendering them useless. To maintain the supply of polonium for the “hedgehog” initiators, production was limited and the oldest unit, stack B, was closed so that at least one of the reactors would be available in the future. Research continued, with DuPont and the Metallurgical Laboratory developing a redox solvent extraction process as an alternative plutonium extraction technique to the bismuth-phosphate process, leaving the uranium unspent in a state from which it could not be easily recovered.

Pump engineering was continued by the Z Division, named for its director Jerrold R. Zacharias of Los Alamos. The Z Division was initially located in Wendover but moved to Oxnard Field, New Mexico, in September 1945 to be closer to Los Alamos. This marked the beginning of the Sandia Base. The air base near Kirtland was used as a base for B-29s for aircraft compatibility and launch tests. By October all Wendover facilities and personnel had been transferred to Sandia and the reserve officers who were demobilized were replaced by about 50 hand-selected regular officers.

Nichols recommended closing the S-50 plant and the Alpha circuits of the Y-12 plant, completing this in September. Although their performance was at an all-time high, the Alfa circuits could not compete with the K-25 and new K-27 plants, which had begun operations in January 1946. In December, the Y-12 plant was closed, reducing Tennessee Eastman’s daily wage costs from $8600 to $1500, saving about $2 million per month.

The main demobilization problem was in Los Alamos, where there was an exodus of talent even though more work was still required. They needed to make bombs like those used in Hiroshima and Nagasaki simpler, safer and more reliable. It was also necessary to develop implosion methods for uranium thus replacing the less efficient ballistic method and required nuclei composed of uranium-plutonium due to the lack of supplies of the latter due to problems with the reactors.

However, uncertainty about the lab’s future was a problem in getting workers to stay there. Oppenheimer returned to his job at the University of California and Groves named Norris Bradbury as an interim replacement, who would eventually remain in this position for the next 25 years. Groves attempted to combat dissatisfaction with the lack of services with a building program that included an improved water supply system, three hundred new residences, and recreational facilities.

In July 1946, two Fat Man bomb detonations were carried out on Bikini Atoll as part of Operation Crossroads to investigate the effect of nuclear weapons on warships. The “Able” bomb was detonated at an altitude of 158 m on 1 July 1946 and the “Baker” bomb was detonated at 27 m underwater on 25 July 1946.

After the bombings of Hiroshima and Nagasaki, several physicists from the Manhattan Project founded the Bulletin of the Atomic Scientists, initiated as an emergency action by scientists who saw an urgent need for an immediate educational program on atomic weapons. After seeing the destructive power of these new weapons and anticipating a nuclear arms race, several of the members of the project, including Bohr, Bush and Conant, expressed the view that it was necessary to reach an agreement on international control of nuclear research and nuclear weapons. The Baruch plan, revealed in a speech at the newly formed United Nations Atomic Energy Commission in June 1946, proposed the establishment of an international authority for atomic development, but the proposal was not adopted.

Following an internal debate over the permanent administration of the nuclear program, the United States Atomic Energy Commission was created by the Atomic Energy Act of 1946, with it responsible for the functions and assets of the Manhattan Project. This commission established civilian control over atomic development and separated the development, production and control of nuclear weapons from the army, while military matters were taken over by the Special Weapons Project for the Armed Forces. The Manhattan Project ceased to exist on December 31, 1946, while the Manhattan District remained until its dissolution on August 15, 1947.

Cost of the Manhattan Project

Costs of the Manhattan Project until December 31, 1945
Cost ($ in 1945) Cost ($ in 2016) % of total
Oak Ridge 1190 million 41800 million 62,9%
Hanford 390 million 13700 million 20,6%
Special Operations Materials 103 million 3640 million 5,5%
Los Alamos 74.1 million 2610 million 3,9%
Research and development 69.7 million 2450 million 3,7%
Government expenditures 37.3 million 1310 million 2,0%
Heavy water plants 26.8 million 942 million 1,4%
Total 1890 million 66500 million

Total project expenditure as of October 1, 1945, reached $1845 million, the equivalent of less than nine days of usual wartime spending, and reached $2191 million when the Atomic Energy Commission took over on January 1, 1947. The total budget was 2400 million dollars. More than 90% of the cost was due to the construction of the plants and the production of fissile materials, with less than 10% for the development and production of the weapons.

By the end of 1945, a total of four bombs had been produced (the Trinity test “instrument”, the Little Boy bomb, the Fat Man bomb and a fourth unused bomb), bringing the average cost of a bomb to $500 million in 1945. By comparison, the total cost of the project at the end of 1945 amounted to 90% of the total spent on small arms production (excluding ammunition) by the United States and 34% of the total spent on US tanks during the same period. Overall, it was the second most expensive armament project undertaken by the United States in World War II, behind only the design and production of the Boeing B-29 Superfortress.


The cultural and political impact of the development of nuclear weapons is considered profound and far-reaching. William L. Laurence of The New York Times, the first person to use the term “atomic age,” became the official correspondent of the Manhattan Project in the spring of 1945. In 1943 and 1944 he had tried unsuccessfully to persuade the Bureau of Censorship to allow him to write about the explosive potential of uranium, so government officials felt he had won the right to report on the war’s greatest secret.

Laurence witnessed both the Trinity test and the bombing of Nagasaki and wrote the official press releases for both events. He subsequently wrote a series of articles extolling the virtues of the new weapon. His articles before and after the bombings helped to raise public awareness of the potential of nuclear technology and were one of the motivations for its development in the United States and the Soviet Union.

The Manhattan Project left a legacy in the form of a network of national laboratories: Lawrence Berkeley National Laboratory, Los Alamos National Laboratory, Oak Ridge National Laboratory, Argonne National Laboratory, and Ames Laboratory. Groves established two more shortly after the war, Brookhaven National Laboratory in Upton, New York, and Sandia National Laboratories in Albuquerque. Groves allocated $72 million for research activities in the 1946–1947 fiscal year. This network of laboratories was at the forefront of large-scale research known as “Big Science,” a term coined by Alvin Weinberg, director of Oak Ridge National Laboratory.

The Naval Research Laboratory had long been interested in the possibility of using nuclear energy for the propulsion of warships, so it sought to create its own nuclear project. In May 1946 Chester Nimitz, then Chief of Naval Operations, decided that the Navy should work in conjunction with the Manhattan Project. He assigned a group of naval officers to Oak Ridge, the highest-ranking being Captain Hyman G. Rickover, who became assistant director there. These officers focused on the study of nuclear energy, laying the foundations of a nuclear navy. A similar group of Air Force personnel arrived at Oak Ridge in September 1946 with the intention of developing nuclear aircraft. Its Nuclear Power for Aircraft Propulsion (NEPA) project faced major technical difficulties and would eventually be canceled.

The ability of new reactors to create radioactive isotopes in previously impossible quantities initiated a revolution in nuclear medicine in the years immediately following the war. From mid-1946, Oak Ridge began distributing radioisotopes to hospitals and universities. Most of the orders were for iodine-131 and phosphorus-32, used in the diagnosis and treatment of cancer. In addition to medicine, these types of isotopes were used in biological, industrial and agricultural research.

In ceding control of nuclear weapons to the Atomic Energy Commission, Groves gave a farewell speech to the staff who had worked on the Manhattan Project:

“Five years ago, the idea of atomic energy was just a dream. You made this dream a reality. You got hold of some of the most confusing ideas and translated them into realities. You built cities where none was known before. You built industrial plants of a magnitude and precision previously considered impossible. You built the weapon that ended the war and thereby saved countless American lives. With regard to peacetime applications, you raised the curtain on the vision of a new world”.


In 2014, the United States Congress passed a law for the creation of a national park dedicated to the history of the Manhattan Project, finally created with the name of Manhattan Project National Historical Park on November 10, 2015.

References (sources)