Manhattan Project

Shortly before Christmas 1938, Otto Hahn and Fritz Strassmann found in their Berlin laboratory that uranium nuclei bombarded with neutrons broke into two roughly equal pieces. Lise Meitner, a refugee in Sweden, and her nephew Otto Frisch calculated the energy released, and Frisch, borrowing a term from biology, named the process “fission.”⁠[1]

Niels Bohr arrived in New York City on January 16, 1939, and ten days later he and Enrico FermiEnrico FermiEnrico Fermi (1901–1954) was an Italian-American physicist who received the 1938 Nobel Prize in Physics and in 1942 led the team that achieved the first controlled nuclear chain reaction in Chicago.Open the full entry presented the discovery at a conference on theoretical physics in Washington, D.C. Bohr and John A. Wheeler then developed the theory of fission at Princeton UniversityPrinceton UniversityPrinceton University is a private university and Ivy League member in Princeton, New Jersey. Chartered on October 22, 1746 as the College of New Jersey, it moved to Princeton in 1756 and adopted its present name in 1896. Undergraduate education leads to A.B. and B.S.E. degrees and emphasizes small-group discussion, independent research and residential college life. Graduate education spans the humanities, social sciences, natural sciences and engineering. As of October 2026, Christopher L. Eisgruber was president. Its 2026–27 operating budget was $3.407 billion, including $342 million for undergraduate aid.Open the full entry, while Fermi and Leo Szilard collaborated with Walter H. Zinn and Herbert L. Anderson at Columbia UniversityColumbia UniversityColumbia University, formally Columbia University in the City of New York, is a private research university in New York City and a member of the Ivy League. Founded in 1754 by royal charter from King George II as King's College, it is the oldest institution of higher education in New York State and the fifth oldest in the United States. It was renamed Columbia College in 1784 and adopted the university name and moved to Manhattan's Morningside Heights in 1896. It has 17 schools and four affiliated colleges, with 35,978 students enrolled in fall 2025. Legal scholar Jennifer L. Mnookin became president on July 1, 2026.Open the full entry. By March 1940 John R. Dunning’s group at Columbia University, working with Alfred Nier of the University of Minnesota, had shown that it was uranium-235 that fissioned with slow neutrons, an isotope that makes up only about 0.7 percent of natural uranium.⁠[1]

The Department of Energy’s history states that Bohr confirmed the validity of the findings while sailing to New York City, and the department’s 2012 overview notes that the neutron was first identified in 1932 by James Chadwick. BritannicaEncyclopædia BritannicaEncyclopædia Britannica is a general English-language encyclopedia first published in Edinburgh, Scotland, in 1768; the name also refers to its publisher, Chicago-based Encyclopædia Britannica, Inc. (which presents itself as The Britannica Group). The print edition ended in 2012, and the company’s main businesses since then have been the online encyclopedia Britannica.com, digital education products for schools and libraries, and Merriam-Webster dictionaries.Open the full entry’s article records a conference between Enrico Fermi and the Navy Department in March 1939. The Atomic Heritage Foundation’s account says that the research at Columbia University took place in Pupin Hall and Schermerhorn Hall and that the cyclotron in the basement of Pupin Hall, built by John Dunning, an associate professor of physics, and George Pegram, remained in use until 1965.⁠[1][2][3][4]

On August 2, 1939, Albert EinsteinAlbert EinsteinAlbert Einstein (1879–1955) was a German-born physicist who formulated the special theory of relativity in 1905 and the general theory in 1915 and received the 1921 Nobel Prize in Physics for his discovery of the law of the photoelectric effect. From 1933 until his death he was a professor at the Institute for Advanced Study in Princeton, New Jersey, United States.Open the full entry, with the help of Leo Szilard, wrote to President Franklin D. RooseveltFranklin D. RooseveltFranklin D. Roosevelt (1882–1945) was an American politician who served as the 32nd president of the United States from 1933 to 1945, the only president elected four times; he led the country through the Great Depression and World War II and greatly expanded the federal government’s role through the New Deal.Open the full entry that chain reactions in uranium could probably generate vast amounts of power, that “extremely powerful bombs” were therefore conceivable, and that the German government was supporting such research. The economist Alexander Sachs met Roosevelt on October 11 and summarized the letter, and the two met again over breakfast the next morning; on October 19 Roosevelt wrote back to Einstein that he had set up a committee of civilian and military representatives to study uranium.⁠[1]

This Advisory Committee on Uranium was headed by Lyman J. Briggs, director of the National Bureau of Standards, with Colonel Keith F. Adamson and Commander Gilbert C. Hoover representing the Army and the Navy. It met for the first time on October 21, 1939, with Sachs, Szilard, Edward Teller and Eugene Wigner in attendance, and its first report, issued on November 1, recommended the purchase of four tons of graphite and fifty tons of uranium oxide. In February 1940 the government made its first outlay for the work, $6,000, which went to Fermi’s chain-reaction research at Columbia University.⁠[1][5]

The Department of Energy’s interactive history states that the Hungarian refugee physicists Leo Szilard, Edward Teller and Eugene Wigner regarded it as their responsibility to alert Americans that Germany might build an atomic bomb first; that Roosevelt, “preoccupied with events in Europe,” took over two months after receiving the letter to meet Sachs; and that Szilard and his colleagues read the delay as a sign that the president did not take the threat seriously. Both that site and the department’s history describe Sachs as a Wall Street economist and unofficial advisor to Roosevelt, and the history adds that he was a longtime friend; Roosevelt was at first noncommittal and worried about where the funds would come from, and only at the breakfast meeting the next morning became convinced of the value of exploring atomic energy. The history’s chronology has Roosevelt deciding to act on October 11–12, 1939, and appointing Lyman J. Briggs to head the committee.⁠[6][1]

In June 1940 Roosevelt approved the creation of the National Defense Research Committee (NDRC) under Vannevar Bush. The Uranium Committee was reorganized as a scientific body under the NDRC without military members; Bush barred foreign-born scientists from membership and blocked further publication of articles on uranium research.⁠[1]

Ernest LawrenceErnest LawrenceErnest Lawrence (in full Ernest Orlando Lawrence, 1901–1958) was an American physicist who invented the cyclotron and received the 1939 Nobel Prize in Physics for its invention and development and for the results obtained with it. He founded and directed the Radiation Laboratory at the University of California, Berkeley, and led the electromagnetic separation of uranium isotopes for the Manhattan Project; Lawrence Berkeley National Laboratory, Lawrence Livermore National Laboratory and element 103, lawrencium, are named after him.Open the full entry, director of the Radiation Laboratory at the University of California, BerkeleyUniversity of California, BerkeleyThe University of California, Berkeley (UC Berkeley) is a campus of the University of California system; the University of California was founded in 1868 through a merger of two institutions and moved to Berkeley, California, in 1873.Open the full entry, pressed for faster mobilization. In February 1941 Glenn T. Seaborg, working at that laboratory, identified element 94, which he later named plutonium, and by May he had shown that plutonium-239 was 1.7 times as likely as uranium-235 to fission.⁠[1][2]

Bush asked the National Academy of Sciences to review the program, and the review committee was headed by Arthur Compton of the University of Chicago. Its first report, submitted on May 17, 1941, held that a bomb of enormous power could be expected at an indeterminate point but certainly not before 1945; a second committee endorsed that report on July 11.⁠[1]

On June 28, 1941, the Office of Scientific Research and Development (OSRD) was established by executive order, with Bush as director reporting directly to the president. James B. Conant, president of Harvard UniversityHarvard UniversityHarvard University is a private research university in Massachusetts, established by the Massachusetts Bay Colony legislature in 1636 and named in 1639 for early benefactor John Harvard. Its principal campuses are in Cambridge and the Allston and Longwood neighborhoods of Boston. It includes Harvard College, graduate and professional schools, and the Radcliffe Institute for Advanced Study. Two governing boards share oversight. Alan M. Garber was president as of October 2026, and the endowment stood at $56.9 billion at the end of fiscal 2025.Open the full entry, took over the NDRC, which became an advisory body, and the Uranium Committee became the OSRD Section on Uranium, codenamed S-1.⁠[1]

The Department of Energy’s interactive history records that on June 15, 1940, Roosevelt directed that the Uranium Committee be placed under the NDRC, and that Bush dropped the Army and Navy representatives and added five scientists: Merle Tuve, George Pegram, Jesse Beams, Ross Gunn and Harold Urey. In December 1941 the committee became the OSRD S-1 Section, the word “uranium” being dropped from its name for security reasons. The department’s history describes Bush as “president of the Carnegie Foundation” and states that Szilard was among the most vocal of those advocating a program to develop bombs and that in 1940 Fermi and Szilard increasingly focused their attention on carbon in the form of graphite.⁠[5][1]

The Department of Energy’s history states that early in 1941 Lawrence proposed converting his smaller cyclotron into a spectrograph to produce uranium-235, that the device was converted from his thirty-seven-inch cyclotron, and that in 1942 he refined his new 184-inch magnet.⁠[1]

The British MAUD Committee was set up in the spring of 1940, in direct response to the memorandum “On the Construction of a ‘Super-Bomb’” that Otto Frisch and Rudolf Peierls had written at Birmingham University.⁠[1][7]

The MAUD report concluded on July 2, 1941, that a bomb was feasible, and Bush and Conant received it on July 14. It estimated that a critical mass of ten kilograms of pure uranium-235 would be enough to produce an enormous explosion and that a bomb could be ready in about two years, and it called for gaseous diffusion on a massive scale; it also recommended collaboration with the United States.⁠[1][7]

Britain then created the Directorate of Tube Alloys, and Tube Alloys became the wartime code name of the British bomb project.⁠[7]

MAUD is not an acronym but the result of a misunderstanding: a telegram from Niels Bohr ended with the words “Cockroft and Maud Ray Kent,” which the British scientists first took for a coded message; Maud Ray Kent had in fact looked after the Bohr children — “the former governess of Bohr’s children who subsequently moved to England” in the Department of Energy’s interactive history, “the former caretaker of the Bohrs’ children” in the Atomic Heritage Foundation’s article. The Atomic Heritage Foundation’s article also states that Mark Oliphant reorganized research at Birmingham University and brought Frisch and Peierls together; that the two were classed as “illegal alien[s]” and excluded from the committee; and that Hans Halban and Lew Kowarski, who had come to England after the fall of France with the world’s supply of heavy water, were excluded as well, and that the two later conducted vital experiments independent of the MAUD Committee.⁠[8][7]

The Department of Energy’s history states that the theoretical work of Peierls and Frisch on atomic bombs was done in 1940 and 1941 and that Americans had been in touch with the MAUD Committee since the fall of 1940; a draft of the report was forwarded from the NDRC liaison office in London, and the body of the history has Bush and Conant receiving it several days after July 11. The report held that such a bomb could be loaded on existing aircraft. The department’s interactive history states that Bush and Conant immediately went to work strengthening the Uranium Committee, adding Fermi as head of theoretical studies and Harold C. Urey as head of isotope separation and heavy water research, and that Bush asked a reconstituted National Academy of Sciences committee to evaluate the uranium program.⁠[1][8]

On October 9, 1941, Bush briefed Roosevelt and Vice President Henry A. Wallace on the British findings and on the estimates of cost and time. Roosevelt allowed him to explore construction needs with the Army but not to proceed to the production stage without further presidential authorization, and asked him to draft a letter so that the British government could be approached “at the top.”⁠[1][9]

Arthur Compton’s committee delivered its report on November 6, 1941, estimating that a critical mass of between two and 100 kilograms of uranium-235 would produce a powerful fission bomb and that isotope separation in sufficient quantities could be accomplished for $50–100 million; Bush forwarded it to Roosevelt on November 27. Japan attacked Pearl Harbor on December 7, and the United States entered the war. On January 19, 1942, Roosevelt returned the papers with a handwritten note: “V. B. OK -- returned -- I think you had best keep this in your own safe FDR.”⁠[9][1]

On Roosevelt’s instructions Bush removed all uranium work from the NDRC: the chemical engineer Eger V. Murphree oversaw engineering studies and pilot plants, Harold C. Urey led work on diffusion, the centrifuge and heavy water, Lawrence led electromagnetic and plutonium work, and Arthur Compton led work on the chain reaction and weapon theory. Broad policy rested with the Top Policy Group of Bush, Conant, Wallace, Secretary of War Henry L. Stimson and Army Chief of Staff George C. MarshallGeorge C. MarshallGeorge C. Marshall (1880–1959) was a five-star U.S. Army general and statesman. He was Army chief of staff during World War II and later served as secretary of state and secretary of defense, and he received the 1953 Nobel Peace Prize for proposing and supervising the plan for Europe’s economic recovery, the Marshall Plan.Open the full entry. A conference convened by Wallace on December 16, 1941, approved these arrangements, and two days later the S-1 Committee gave Lawrence $400,000 to continue his electromagnetic work.⁠[1][9]

The chronology of the Department of Energy’s history gives December 16, 1941, as the date on which the Top Policy Committee became primarily responsible for broad policy decisions on uranium research, and it summarizes Roosevelt’s note of January 19, 1942, as approving production of the atomic bomb.⁠[1]

In early 1942 Urey worked on gaseous diffusion and the centrifuge at Columbia University, Lawrence continued his investigations of electromagnetic separation, and Arthur Compton concentrated experiments on the reactor (then called a pile) at the Metallurgical Laboratory of the University of Chicago. On March 9 Bush sent Roosevelt a progress report in which he judged that, if matters were expedited, a bomb was possible in 1944.⁠[1][10]

On May 23 the leaders of S-1 recommended pushing all three isotope-separation methods (centrifuge, gaseous diffusion and electromagnetic) and the pile as fast as possible to the pilot-plant stage; on June 17 Roosevelt approved the recommendation, with the Army Corps of Engineers to build the plants. Process development, materials procurement, engineering design and site selection passed to the Corps of Engineers, for which approximately sixty percent of the proposed 1943 budget, or $54 million, was earmarked; S-1 kept a budget of approximately $30 million and charge only of university research and pilot-plant studies.⁠[1][10]

On June 19 Bush authorized an S-1 Executive Committee, with Conant as chairman and Briggs, Arthur Compton, Lawrence, Murphree and Urey as members.⁠[5][1]

The Corps of Engineers first assigned Colonel James C. Marshall to the work; his organization was called the Laboratory for the Development of Substitute Metals (DSM), and he immediately moved from Syracuse to New York City. The Department of Energy’s 2010 history dates the establishment of the Manhattan Engineer District, by general order in New York City with James C. Marshall in command, to August 13, 1942; Britannica’s article has the Corps of Engineers’ Manhattan District initially assigned management of the construction work in June 1942.⁠[1][3]

The Atomic Heritage Foundation’s account states that the building at 270 Broadway in Manhattan became the project’s first headquarters in mid-June 1942 and that its 18th floor was then the headquarters of the Corps of Engineers’ North Atlantic Division. Britannica explains the name as follows: much of the early research had been done at Columbia University, in Manhattan, and Corps of Engineers districts are named after the city where they are located, so the project was called the Manhattan Engineer District; it eventually became known as the Manhattan Project.⁠[4][11]

Other sources give their own accounts of the name: the Atomic Heritage Foundation says that Groves decided to follow the custom of naming Corps of Engineers districts for the city in which they are located, that the name also masked the purpose of the project, and that the name first proposed was “Laboratory for the Development of Substitute Materials”; the National Park Service’s article says that the Manhattan Project took its name from an office in downtown Manhattan where its planning began; and the Hanford Site page says that it was named for a key Army Corps of Engineers official and that the name did not draw attention to its atomic bomb mission. The National Nuclear Security Administration’s article states that Groves kept the moniker “the Manhattan Project.” The Atomic Heritage Foundation adds that Stone & Webster, the Boston engineering firm tasked with building the Y-12 electromagnetic plant, also had offices at 270 Broadway, and that the district headquarters moved to Oak Ridge in August 1943.⁠[4][12][13][14]

On September 17, 1942, the Army appointed Colonel Leslie GrovesLeslie GrovesLeslie Groves (1896–1970) was a United States Army general who headed the Manhattan Project, the World War II effort to develop the atomic bomb.Open the full entry of the Corps of Engineers to head the project, and six days later he was promoted to brigadier general; his earlier work had included building the Pentagon. Within two days he moved to obtain the Tennessee site and secured a higher priority rating for project materials, and on September 19 he selected the site at Oak Ridge, Tennessee.⁠[1]

Groves moved the district headquarters from New York City to Washington, D.C. James C. Marshall remained District Engineer under him, and Marshall’s deputy, Colonel Kenneth D. Nichols, became the aide on whom Groves relied. On September 23 Stimson created the Military Policy Committee, with one representative each from the Army, the Navy and the OSRD.⁠[1]

The Department of Energy’s history describes Groves as “a practical-minded military man, brusque and goal-oriented,” with strong administrative abilities; Nichols called him “the biggest S.O.B. I have ever worked for” and described him as “most demanding,” “most critical,” “always a driver, never a praiser” and “abrasive and sarcastic,” and the history adds that Nichols characterized his heavyset boss as ruthless, egotistical and confident. It states that Groves held that even wrong decisions were better than no decisions, and that James C. Marshall’s cautious nature proved useful in later decision making. The Tennessee site near the Clinch River was then known as Site X.⁠[1]

On November 12, 1942, the Military Policy Committee decided that no centrifuge plant would be built and that gaseous diffusion, the pile and electromagnetic separation would skip the pilot-plant stage and go directly to full-scale plants; the S-1 Executive Committee endorsed these decisions on November 14.⁠[1]

On November 18 Groves appointed Warren K. Lewis of the Massachusetts Institute of Technology to head a final review committee made up of Lewis and three DuPont representatives. The committee elevated gaseous diffusion to first priority and had reservations about the electromagnetic program; on December 10 Conant and the committee reached a compromise on an electromagnetic plant of intermediate scale.⁠[1][15]

On October 19 Groves decided on a separate laboratory for bomb development. In mid-November he, J. Robert OppenheimerJ. Robert OppenheimerJ. Robert Oppenheimer (1904–1967) was an American theoretical physicist and science administrator who directed the Los Alamos laboratory that developed the atomic bomb during World War II.Open the full entry, Edwin M. McMillan and Lieutenant Colonel W. H. Dudley visited two sites in New Mexico, and on November 25 Groves selected Los Alamos, codenamed Project Y, with Oppenheimer to head the laboratory.⁠[1]

The Department of Energy’s history states that at Berkeley Oppenheimer headed a group of theoretical physicists he called the luminaries, among them Felix Bloch, Hans BetheHans Albrecht BetheHans Albrecht Bethe (1906–2005) was a German-born American theoretical physicist who joined Cornell University in 1935, headed the theoretical division at Los Alamos during the Second World War and received the 1967 Nobel Prize in Physics for his work on nuclear reactions and the energy production of stars.Open the full entry, Edward Teller and Robert Serber; their consensus was that approximately twice as much fissionable material would be required for a bomb, and Oppenheimer also advised that fusion explosions using deuterium (heavy hydrogen) might be possible. DuPont’s view at the time was that under the best of circumstances plutonium could be mass-produced by 1945, and it thought the chances of this happening were low. The S-1 Executive Committee discovered at one point that Arthur Compton was building the experimental pile at Stagg Field, a decision he had made without informing the committee or Groves; Groves and a committee of scientists took several days to confirm that the pile posed little danger.⁠[1]

It was Oppenheimer who suggested that the bomb laboratory operate secretly in an isolated area. The Department of Energy’s history describes him as “a philosophical man, attracted to Eastern mysticism” and states that, despite initial concerns about his administrative inexperience, his leftist political sympathies and his lack of a Nobel PrizeNobel PrizeThe Nobel Prize is an international award established under the will of Alfred Nobel, comprising five prizes in physics, chemistry, physiology or medicine, literature and peace, awarded since 1901; its funds are managed by the Nobel Foundation, whose Board is based in Stockholm. The Sveriges Riksbank Prize in Economic Sciences in Memory of Alfred Nobel, established in 1968, is not a Nobel Prize.Open the full entry, he proved to be an excellent director.⁠[1]

On the afternoon of December 2, 1942, in a converted squash court under the stands of the University of Chicago’s abandoned Stagg Field, the group led by Fermi brought Chicago Pile-1 (CP-1) to the first controlled, self-sustaining chain reaction, with 49 scientists present. The pile was built of 400 tons of graphite, six tons of uranium metal and fifty tons of uranium oxide laid in 57 layers; it ran at first at one-half watt, increased to 200 watts ten days later.⁠[1][16][17]

Arthur Compton passed the news to Conant in a coded phrase: “the Italian navigator has just landed in the new world.” CP-1 was later dismantled and moved to a forest preserve outside Chicago, and staff of the Metallurgical Laboratory later formed Argonne National Laboratory.⁠[1][17]

Sources differ on the moment of criticality: the Department of Energy’s history gives 3:20 p.m., the article of the department’s Office of Nuclear Energy 3:36 p.m., and the University of Chicago’s explainer 3:53 p.m., adding that the test had taken 28 minutes. The history places the pile in a racket court under the west grandstand at Stagg Field. The University of Chicago’s explainer also states that construction difficulties slowed progress and Arthur Compton decided to go ahead; that crews worked round the clock for two weeks to build a 20-foot-tall pile of graphite blocks studded with hundreds of smaller blocks of uranium, finishing on the evening of December 1, 1942; and that cadmium rods controlled the reaction. Compton said that he did not tell the university’s president, Robert Maynard Hutchins, and whether Hutchins knew is debated; the mayor of Chicago and other elected officials were not notified. After the test those present drank Chianti from paper cups and signed the straw wrapper of the bottle. Henry Moore’s sculpture Nuclear Energy was dedicated on December 2, 1967, the 25th anniversary of the reaction.⁠[1][16][17]

Bush submitted a report on December 16, 1942, that itemized about $0.5 billion in costs and held that bombs possibly could be produced during the first half of 1945. On December 28 Roosevelt approved the plans, authorizing full-scale gaseous diffusion and plutonium plants, the compromise electromagnetic plant and heavy-water production facilities.⁠[1][15]

In December 1942 the entire responsibility was transferred to the Manhattan Engineer District of the Army Corps of Engineers, along with the OSRD contracts; by May 1, 1943, the role of the OSRD in the program had for all practical purposes ended, and the S-1 Executive Committee held its last meeting on September 10–11, 1943.⁠[5]

Beginning on December 16, 1942, Colonel Franklin T. Matthias of Groves’s staff and two DuPont engineers viewed six locations in Washington, OregonOregonOregon (officially the State of Oregon) is a state on the Pacific coast of the western United States; its capital is Salem and its largest city is Portland. It was admitted to the Union on February 14, 1859, as the 33rd state. Its population in the 2020 census (April 1, 2020) was 4,237,256.Open the full entry and CaliforniaCaliforniaCalifornia is a U.S. state on the Pacific coast; its capital is Sacramento and its largest city is Los Angeles. California was admitted to the Union on September 9, 1850, as the 31st state. Its population at the 2020 Census (April 1, 2020) was 39,538,223, the largest of any state.Open the full entry in search of a site for the plutonium plants. On January 16, 1943, Groves selected Hanford, Washington, and authorized the Hanford Engineer Works, codenamed Site W. The site offered cooling water from the Columbia River and hydroelectric power from the Grand Coulee and Bonneville dams, and covered nearly one-half million acres.⁠[1]

DuPont built the plutonium production complex at Hanford for a fee of one dollar over actual costs. The three water-cooled piles, B, D and F, were placed about six miles apart on the south bank of the Columbia River, with the chemical separation plants nearly ten miles south of them. Ground was broken for the water-cooling plant of the 100-B area on August 27, 1943.⁠[1][2]

The construction workforce at Hanford reached 51,000, and in 30 months the workers built 554 buildings, 386 miles of road and 158 miles of railroad; B Reactor, the world’s first full-scale plutonium production reactor, was completed in 11 months.⁠[13]

The Department of Energy’s history also states that on October 3, 1942, DuPont agreed to design and build the chemical separation plant; that Crawford Greenewalt was given the responsibility of coordinating DuPont and Metallurgical Laboratory planning; that late in February 1943 Greenewalt reluctantly concluded that the Metallurgical Laboratory’s water-cooled model, while it had its problems, was superior to DuPont’s own helium-cooled design; and that for chemical separation Greenewalt chose bismuth phosphate in late May, although Seaborg admitted that little separated the two methods under consideration. The chronology words the entry for October 3, 1942, as DuPont agreeing to build the chemical separation plant at Oak Ridge. Robert S. Stone, in charge of the Metallurgical Laboratory’s health division from July 1942, established emission standards and conducted experiments on radiation hazards that informed planning for Oak Ridge and Hanford; the history states that safety was a major consideration because plutonium was highly radioactive and that precautions against radiation exposure influenced all aspects of plant design. Hanford’s population reached 50,000 by the summer of 1944; a photograph caption states that 880 hutment units were built, each housing ten workers; the caption gives their size as 16 by 40 feet and says that they housed men and women separately. Each chemical separation building contained forty process pools, and the planned fourth chemical separation plant was not built.⁠[1]

The Tennessee reservation, officially the Clinton Engineer Works, covered roughly ninety square miles (59,000 acres) in the ridges west of Knoxville. Land acquisition and construction of the town began in the fall of 1942, and in the summer of 1943 the district headquarters moved there from Washington, D.C., with Nichols in command.⁠[1][18]

Ground was broken for the Alpha plant of the Y-12 electromagnetic complex on February 18, 1943; Stone & Webster of Boston was the contractor and Tennessee Eastman the operator. The separation tanks were called calutrons in honor of the University of California. Because copper was scarce in wartime, the magnet windings were made of silver, of which the Army borrowed 14,700 tons from the United States Treasury.⁠[1][18]

The first Alpha racetrack, as each group of tanks was called, was plagued by failures in the fall of 1943, and Groves shut it down on December 15. By the end of February 1944 the second Alpha track had produced about 200 grams of twelve-percent uranium-235, and the first four Alpha tracks did not operate together until April 1944.⁠[1]

DuPont broke ground in February 1943 for the X-10 complex, with its graphite pile and pilot plutonium separation plant; the pile went critical in the early morning of November 4, 1943, and produced plutonium by the end of the month, and in the spring of 1944 Los Alamos began to receive plutonium samples from it.⁠[1][18]

The Department of Energy’s history states that the town of Oak Ridge was first planned for approximately 13,000 people and that the estimates were later revised upward to 40,000–45,000; that Stone & Webster took over design and construction of a 500-tank facility; and that almost 5,000 operating and maintenance personnel were hired and trained for Y-12. It gives the silver used for the magnet windings as “almost 15,000 tons,” slightly different from the interactive history’s 14,700 tons. Oppenheimer warned that three times more fissionable material would be required for a bomb, and the Alpha component of the Y-12 extension was designated Alpha II; Lawrence argued that Y-12 provided the only possible avenue to a bomb by 1945. The chronology records that in February 1944 Y-12 sent 200 grams of uranium-235 to Los Alamos and that the Beta building was completed in March 1944. The Oak Ridge page of the interactive history states that Y-12 eventually had 17 racetracks (nine Alpha and eight Beta), that X-10 was completed in November 1943, that the Tennessee Valley AuthorityTennessee Valley AuthorityThe Tennessee Valley Authority (TVA) is a United States corporate agency that is owned by the federal government but receives no tax funding. Created by Congress in 1933 to develop the resources of the Tennessee Valley, it operates the nation’s largest public power system and serves more than 10 million people across seven states.Open the full entry supplied the site’s power, and that Oak Ridge became the fifth largest urban area in Tennessee.⁠[1][18]

Los Alamos stands on a mesa about thirty miles northwest of Santa Fe. The National Park Service states that the Los Alamos Ranch School, which had operated there, was forced to shut down, that local homesteaders had to abandon their cabins, and that project workers repurposed some of the school’s 54 buildings; the Department of Energy’s 2010 history says that the boys’ school occupying the site was eager to sell. By the end of 1942 the Corps of Engineers’ district engineer in Albuquerque had orders to begin construction, and the University of California had contracted to provide supplies and personnel.⁠[1][19]

Oppenheimer spent the first three months of 1943 recruiting staff around the country and arrived at Los Alamos in mid-March; researchers began arriving in March, and bomb design work began in April. The first plan had been a military laboratory, and the scientists objected to serving as officers; a letter from Conant and Groves promised that the laboratory would remain civilian through 1943, and militarization never took place.⁠[1][20]

The laboratory was first organized into four divisions: theoretical (Hans Bethe), experimental physics (Robert F. Bacher), chemistry and metallurgy (Joseph W. Kennedy) and ordnance (William S. Parsons). Other scientists who worked there included Luis Alvarez, Norris Bradbury, Richard P. FeynmanRichard FeynmanRichard Feynman (Richard Phillips Feynman, 1918–1988) was an American theoretical physicist who rebuilt quantum electrodynamics with the path-integral formulation and Feynman diagrams, for which he shared the 1965 Nobel Prize in Physics with Julian Schwinger and Sin-Itiro Tomonaga. He took part in the Manhattan Project during World War II and then taught for decades at the California Institute of Technology, where his introductory physics course became The Feynman Lectures on Physics; in 1986, as a member of the presidential commission investigating the space shuttle Challenger accident, he used ice water to demonstrate that the O-ring lost its resilience at low temperature.Open the full entry, George Kistiakowsky, Seth Neddermeyer, John von Neumann, Emilio Segrè, Edward Teller, Victor Weisskopf and Robert Wilson.⁠[1][20]

The interactive history also names Eric Jette and Cyril Smith among the Los Alamos scientists; Isidor I. Rabi did not move to Los Alamos but served as a consultant. The letter from Conant and Groves was written at Oppenheimer’s request, Rabi and Robert F. Bacher having held that a military environment was not conducive to scientific research. The Department of Energy’s history states that a group of British scientists reinforced the Los Alamos staff at the beginning of 1944, Rudolf Peierls among them, with James Chadwick, the discoverer of the neutron, as head of the British contingent; Groves also called once again on Warren K. Lewis to head a committee to evaluate the Los Alamos program.⁠[20][1]

On August 19, 1943, Roosevelt and British Prime Minister Winston ChurchillWinston ChurchillSir Winston Churchill (1874–1965) was a British Conservative statesman and writer who served as Prime Minister in 1940–1945 and 1951–1955, led Britain through the Second World War and won the Nobel Prize in Literature in 1953.Open the full entry signed at Quebec an agreement on collaboration in the matter of Tube Alloys. The two governments agreed never to use the weapon against each other, not to use it against third parties or communicate information about it to third parties without each other’s consent, and, in view of the heavy burden of production falling on the United States, to settle postwar industrial and commercial advantages on terms specified by the President of the United States.⁠[21]

The agreement set up a Combined Policy Committee in Washington, D.C., to agree on the program of work, allocate scarce materials and settle questions about the interpretation or application of the agreement. Its members were the United States Secretary of War, Vannevar Bush and James B. Conant; Field Marshal Sir John Dill and Colonel J. J. Llewellin for the United Kingdom; and C. D. Howe for Canada.⁠[21]

In September 1944 Roosevelt and Churchill signed an aide-mémoire at Hyde Park, New York, pledging that full collaboration between the two countries would continue after the defeat of Japan “unless and until terminated by joint agreement.”⁠[7][1]

The text of the agreement is headed “The Citadel, Quebec”; among the functions of the Combined Policy Committee was also this: “To keep all sections of the project under constant review.”⁠[21]

The K-25 gaseous diffusion plant was built on the Clinch River eleven miles southwest of Oak Ridge. Site preparation for its power plant began in June 1943 and construction of the plant itself on September 27, 1943; the cascade was planned as fifty four-story buildings with 2,000,000 square feet of floor space, laid out in a U half a mile long.⁠[1]

Columbia University was the center of research and development for gaseous diffusion. No satisfactory barrier existed for a long time, and as 1944 began Groves ordered a crash barrier program; the Nash Garage Building at 3280 Broadway was converted into a pilot plant for barrier material, and Groves eventually decided on the powdered-nickel process developed by Clarence Johnson of the Kellex Corporation. Kellex was a separate entity created by the M. W. Kellogg Company for the sake of secrecy, and Union Carbide operated the plant; in the name K-25, K stood for Kellex and 25 for uranium-235.⁠[22][1][4]

Because of the difficulties at Y-12 and K-25, Groves decided in June 1944 to add the S-50 liquid thermal diffusion plant beside the K-25 powerhouse. The method had been developed by Philip Abelson with Navy support; the plant was built by the H. K. Ferguson Company of Cleveland and had 2,142 columns.⁠[1][18][22]

By the winter of 1944–1945 nine Alpha and three Beta racetracks were operating at Y-12. S-50 began operation in March 1945; for K-25, the Department of Energy’s 2010 history gives March 12, 1945, as the start of production, while the Oak Ridge page of the department’s interactive history says production began in stages in February 1945. K-25 and S-50 supplied feed for Y-12, whose Beta tracks carried out the final enrichment and delivered the product to Los Alamos.⁠[1][18]

For the start of work on the K-25 powerhouse, the Oak Ridge page of the interactive history gives May 1943, while the Department of Energy’s history has site preparation beginning in June. The history states that in late summer 1943 it was decided that K-25 would play a lesser role than originally intended, providing around fifty percent enrichment; that the plant covered nearly 43 acres; and that in 1942 nickel was the leading candidate for barrier material, though there was serious doubt whether a reliable nickel barrier could be ready in sufficient quantity by the end of the war. The Atomic Heritage Foundation’s account states that the corrosion-resistant nickel method was first developed by Edward Adler and Edward Norris and that, after the switch to the Kellex-Johnson process, the half-installed Norris-Adler equipment was removed from the plant at Decatur, IllinoisIllinoisIllinois is a state of the United States that borders Lake Michigan and the states of Wisconsin, Indiana, Kentucky, Missouri and Iowa; its capital is Springfield and its largest city is Chicago. It was admitted to the Union on December 3, 1818, as the 21st state, and its population in the 2020 census (April 1, 2020) was 12,812,508.Open the full entry.⁠[18][1][4]

On S-50, the Department of Energy’s history states that as the problems of Y-12 and K-25 reached crisis proportions in the spring and summer of 1944, Oppenheimer wrote to Groves late in April suggesting that Philip Abelson’s thermal diffusion experiments at the Philadelphia Naval Yard deserved a closer look; a quick analysis showed that a thermal diffusion plant could be in operation by early 1945, with steam available from the almost completed K-25 powerhouse. The body of the history gives the contractor’s deadline as ninety days from September 27, while its chronology dates Groves’s order to build the plant to June 21, 1944. The interactive history states that Abelson first experimented at the Carnegie Institution.⁠[1][22]

The Alsos Mission was the intelligence operation of the Manhattan Engineer District that sought to learn how far Germany had advanced toward an atomic bomb. Proposed by George C. Marshall in the fall of 1943, it was commanded by Lieutenant Colonel Boris T. Pash, and Samuel Goudsmit later became chief of its scientific staff. “Alsos” is Greek for “groves”; Groves disliked the code name but decided that changing it would only draw attention.⁠[23]

The mission to Italy was assembled in Algiers on December 14, 1943. On August 9, 1944, advance elements landed in France; in Paris, Frédéric Joliot-Curie gave the mission his view that the Germans had made little progress on atomic energy.⁠[23]

Alsos entered Germany on February 24, 1945. On April 24 it captured Hechingen, seized a physics laboratory and detained Otto Hahn, Carl von Weizsäcker and Max von Laue; uranium cubes were uncovered at Haigerloch, and Werner Heisenberg was then captured at Urfeld. A team under John Lansdale seized close to 1,100 tons of uranium ore at a salt mine near Stassfurt, and Groves wrote in a memorandum to George C. Marshall that the capture of this material, the bulk of the uranium supplies available in Europe, “would seem to remove definitely” any possibility of Germany using an atomic bomb in the war.⁠[23]

From July 3, 1945, to January 3, 1946, ten German scientists, among them Heisenberg and Hahn, were interned at Farm Hall, a bugged house in Godmanchester, England. Alsos had 114 personnel at the end of the war in Europe and was officially disbanded on October 15, 1945. The Department of Energy’s history judges that the Germans were little nearer to producing atomic weapons at the end of the war than they had been at its beginning.⁠[23][1]

The Atomic Heritage Foundation’s article also records the following. In December 1943 a liaison office was established in London, with a target list of up to fifty German nuclear scientists; the original Alsos detachment had thirteen military personnel and six scientists, and by the summer of 1944 it had seven operations officers and thirty-three scientists. The mission to Italy contacted Edoardo Amaldi and Gian Carlo Wick, both of whom said that they had not done any atomic research for the Germans or anyone else and estimated that a German bomb was at least a decade away. In Paris one of the mission’s primary objectives was Joliot-Curie’s laboratory at the College of France. On March 15, 1945, 612 Flying Fortresses of the 8th Air Force dropped close to 2,000 tons of high explosives on the Auergesellschaft Works at Oranienburg. The operation of April 1945 was called Operation Harborage, of which Groves wrote: “there could be no question but that American troops must be the first to arrive at this vital installation.” On April 27 the detained German scientists were transferred to Heidelberg for further questioning, where von Weizsäcker revealed the whereabouts of the German atomic research records, which the article says had been sealed in a metal drum and stored in a cesspool behind his house; Walther Gerlach was captured on May 1 and Kurt Diebner on May 3; Heisenberg, once captured, was likewise removed to Heidelberg with confiscated records. The salt mine near Stassfurt was known as the WIFO plant, and Groves’s memorandum to Marshall states that in 1940 the German army in Belgium confiscated and removed to Germany about 1,200 tons of uranium ore.⁠[23]

The detention of the ten German scientists was code-named Operation Epsilon, and the Atomic Heritage Foundation’s article also names Kurt Diebner, Carl von Weizsäcker and Max von Laue among those held. In the recorded conversations Diebner asked, “I wonder whether there are microphones installed here?” and Heisenberg replied, “Microphones installed? (Laughter) Oh, no, they’re not as cute as all that,” going on to say that he did not think they knew the real Gestapo methods and that they were a bit old-fashioned in that respect. The article states that several scientists, including Otto Hahn, expressed relief that Germany had not developed such a weapon for Hitler. On why Germany did not build a bomb, it says that much debate exists today: some, including Heisenberg himself, argued that the Germans did not develop the bomb for moral reasons; Groves held that the Germans had severely miscalculated the amount of fissionable material required; and some have made the case that the German program was farther along than most historians have believed, the article mentioning, as an allegation only, the claim that Diebner’s team tested a nuclear-related device at Ohrdruf in Thuringia on March 4, 1945.⁠[23]

In March 1944 the physicist Chien-Shiung WuChien-Shiung WuChien-Shiung Wu (1912–1997) was a Chinese-born American experimental physicist who spent most of her career at Columbia University and worked on the Manhattan Project. Her cobalt-60 beta-decay experiment of 1956–1957, carried out with collaborators, provided the first experimental proof that parity is not conserved in weak interactions.Open the full entry was invited to join the Manhattan Project at Columbia University as a senior scientist, working first on gaseous diffusion processes for separating uranium isotopes. At the Substitute Alloy Materials (SAM) Lab she also worked on radiation detectors and developed improved Geiger counters. The Atomic Heritage Foundation’s profile states that she is believed to have been the only Chinese person to have worked on the Manhattan Project.⁠[24][25][26]

The National Park Service’s biography states that Wu completed her PhD in 1940 at the University of California, Berkeley, and that the gaseous diffusion process she helped develop was the same process used at the K-25 plant in Oak Ridge.⁠[26]

The Atomic Heritage Foundation’s profile states that she married the physicist Luke Chia-Liu YuanLuke Chia-Liu YuanLuke Chia-Liu Yuan (1912–2003) was a Chinese-born American experimental physicist and the husband of the physicist Chien-Shiung Wu; he worked mainly at Brookhaven National Laboratory, where he was a senior physicist. He was elected an academician of Academia Sinica in 1959 and chaired the steering committee of Taiwan’s Synchrotron Radiation Research Center from 1983 to 2002.Open the full entry in 1942.⁠[25]

Los Alamos considered two methods of assembly: the gun method, a conventional artillery technique of firing one subcritical mass into another, intended for uranium-235, and implosion, in which symmetrical shock waves directed inward would compress a subcritical mass of plutonium. By March 1944 the laboratory had two bomb models in tests with B-29s: Thin Man, a plutonium gun named for Roosevelt, and Fat Man, an implosion prototype named after Churchill; the smaller uranium gun weapon became Little Boy.⁠[1]

Plutonium made in a pile contained plutonium-240, which fissions spontaneously, so that in a gun the two pieces would predetonate before coming together. On July 17, 1944, the decision was made to cease work on the plutonium gun, and in the same month Oppenheimer reorganized the laboratory, with Bacher taking over G Division (for gadget) and Kistiakowsky X Division (for explosives), to concentrate on implosion.⁠[1]

On August 7, 1944, Groves presented a new timetable to George C. Marshall: implosion weapons in the second quarter of 1945 if experiments proved satisfactory, and a uranium gun bomb by August 1, 1945.⁠[1]

The Department of Energy’s history also states that Seth H. Neddermeyer performed his early implosion tests in relative obscurity, that John von Neumann suggested that high-speed assembly and high velocities would prevent predetonation, and that George Kistiakowsky was later assigned the task of perfecting implosion techniques; that two plutonium guns arrived in March and were field-tested successfully; that the two bomb models were developed by William S. Parsons’s group; and that Little Boy, the lighter and smaller uranium weapon, was called Thin Man’s brother. Groves’s timetable of August 7, 1944, also foresaw one or two more uranium gun bombs by the end of 1945; the chronology records the occasion as Bush briefing George C. Marshall. The department’s 2012 overview states that Los Alamos, working with the Army Air Forces, had developed the two bomb models by the spring of 1944.⁠[1][2]

In September 1944 Colonel Paul Tibbets began drilling the 393rd Bombardment Squadron of the 509th Composite Group at Wendover Field in western UtahUtahUtah is a U.S. state bordered by Idaho to the north, Wyoming to the northeast, Colorado to the east, Arizona to the south and Nevada to the west, with its capital at Salt Lake City. It was admitted to the Union on January 4, 1896, as the 45th state, and the 2020 census (April 1, 2020) counted 3,271,616 residents.Open the full entry, in test drops with 5,500-pound orange dummy bombs nicknamed “pumpkins.” In June 1945 Tibbets and his command moved to Tinian Island in the Marianas.⁠[1][14]

The design of the uranium gun bomb was frozen in February 1945, and the laboratory judged that it did not need a test before combat; the design of the implosion device was approved in March 1945. Kenneth T. Bainbridge was placed in charge of Project Trinity, the test, and Parsons headed Project Alberta, which was responsible for the combat weapons.⁠[1]

The Department of Energy’s history states that the test of the implosion device was scheduled for July 4 and that Project Alberta was also known as Project A. The interactive history calls the training base Wendover Army Air Field, an isolated air base in western Utah, and says that final training runs and bomb assembly were done at Tinian Island in the western Pacific.⁠[1][22]

On September 13, 1944, Fermi placed the first uranium slug in pile 100-B. The pile began running just after midnight on September 27; after about three hours its power level began to fall, and by the evening of the 28th it had stopped entirely. The cause was xenon-135, a fission product that absorbed neutrons faster than the pile produced them; because DuPont’s engineers had installed extra fuel tubes, the pile could be expanded to a power level sufficient to overwhelm the xenon poisoning. Chien-Shiung Wu helped identify the cause.⁠[1][25][26]

On December 25, 1944, the first irradiated slugs were discharged from 100-B. Of the chemical separation buildings, which the construction workers called Queen Marys, 221T and 221U were finished by December 1944; each was 800 feet long, 65 feet wide and 80 feet high, and operators worked by remote control behind thick shielding. Los Alamos received its first Hanford plutonium on February 2, 1945.⁠[1]

The National Academy of Sciences memoir describes Wu’s part as follows: Enrico Fermi had been asked to explain why the Hanford reactor was shutting down, and her very careful work on xenon-135 identified that isotope as the culprit in the malfunction. The Department of Energy’s history states that operators in the chemical separation buildings manipulated remote-control equipment by looking through television monitors and periscopes.⁠[24][1]

On April 12, 1945, Roosevelt died suddenly at Warm Springs, Georgia, and Harry S. TrumanHarry S. TrumanHarry S. Truman (1884–1972) was an American politician who served as a U.S. senator from Missouri and as vice president in 1945, succeeding Franklin D. Roosevelt on his death that year and serving as the 33rd president of the United States from 1945 to 1953, a period spanning the end of World War II and the early Cold War.Open the full entry became president. On April 25 Stimson briefed Truman on S-1 in a forty-five-minute meeting, with Groves present for part of it.⁠[1]

The Department of Energy’s history notes that Roosevelt died only weeks before Germany’s unconditional surrender on May 7 and that Truman, who succeeded him, was a veteran of the United States Senate. Its chronology records the Yalta conference of February 4–11, 1945, and the firebombing of Tokyo in March 1945, which resulted in 100,000 casualties.⁠[1]

The Interim Committee was an advisory group on atomic research whose members included Bush, Conant, Karl Compton, Under Secretary of the Navy Ralph A. Bard, William L. Clayton and James F. Byrnes; its Scientific Panel consisted of Oppenheimer, Fermi, Arthur Compton and Lawrence. Meeting on May 31 and June 1, 1945, the committee concluded that the bomb should be dropped without warning on a war plant surrounded by workers’ homes, and on June 6 Stimson informed Truman of its recommendations.⁠[1]

At the Metallurgical Laboratory, the Committee on the Social and Political Implications of the Atomic Bomb was chaired by James Franck and included Seaborg and Szilard. Its report of June 1945, the Franck Report, advocated international control of atomic power and proposed a demonstration in an uninhabited area. The Scientific Panel concluded that no technical test would convince Japan to surrender, and on June 21 the Interim Committee sided with the Scientific Panel.⁠[1]

A target selection group set up in late April 1945 was headed by Groves, who shared the responsibility with General Thomas Farrell; in late May the committee of scientists and Army Air Force officers listed Kokura Arsenal, Hiroshima, Niigata and Kyoto as the four best targets. Stimson vetoed Kyoto, and Nagasaki later took its place.⁠[1]

The interactive history’s page on the debate states that the Interim Committee was formed in early May 1945 by Stimson with Truman’s approval (the page prints one member’s name as Karl T. Compton); that at the meeting of May 31 Lawrence suggested that a demonstration might convince Japan to surrender, which was rejected partly for fear that the bomb “might be a dud”; and that it was conceded that the United States monopoly “could not be held long.” On the day after the meeting of May 31 the committee met four industrial leaders, Walter S. Carpenter of DuPont, James C. White of Tennessee Eastman, George H. Bucher of Westinghouse and James A. Rafferty of Union Carbide; the page says that this meeting confirmed that the United States had “a lead of three to ten years” on the Soviet Union in production facilities for bomb fabrication. On June 2 Arthur Compton briefed the scientists of the Metallurgical Laboratory on the committee’s conclusions. On June 6 Stimson informed Truman that the committee recommended keeping the bomb secret until Japan had been bombed and attacking “as soon as possible and without warning.” The page adds that Truman and Stimson agreed that the president would stall if the Soviet Union asked about atomic weapons at Potsdam, and that on June 21 the committee concluded that Truman should mention that the United States was preparing to use “a new kind of weapon” against Japan. The Department of Energy’s history states that William L. Clayton was Assistant Secretary of State and that James F. Byrnes was a future Secretary of State; that the Joint Chiefs of Staff continued to advocate the invasion of Kyushu, a plan identified as Operation Olympic; and that Stimson hoped an invasion could be avoided, either by redefining the surrender terms or by using the atomic bomb.⁠[27][1]

The test site lay in a desert area known as the Jornada del Muerto on the Alamogordo Bombing Range in New Mexico, 210 miles south of Los Alamos; the name Trinity was chosen by Oppenheimer. A test explosion with a small amount of fissionable material was conducted there on May 7, 1945. On July 12 the plutonium core was taken to the test area in an Army sedan, and by 5:00 p.m. on July 15 the device, which used about 13½ pounds of plutonium, had been assembled and hoisted to the top of a one-hundred-foot steel tower.⁠[1][2]

Rain delayed the test, originally set for 4:00 a.m. on July 16; at 5:30 a.m. the plutonium implosion device, known as the Gadget, was detonated. For its yield, the Department of Energy’s 2010 history gives approximately 18.6 kilotons of TNT, while the department’s 2012 overview and its interactive history give approximately 21 kilotons; Groves’s report to Stimson of July 21 estimated 15,000 to 20,000 tons.⁠[1][2][28][29]

The fireball was about 2,000 feet in diameter, the steel tower was vaporized, and the blast left a crater 10 feet deep and some 400 yards across; the nearest observers were in bunkers 10,000 yards away.⁠[2][1]

Stafford Warren, the district’s chief medical officer, afterward described a band of dust almost 30 miles wide extending almost 90 miles to the northeast; several ranch families received significant exposures and livestock were harmed, and he recommended that any future test site have a radius of at least 150 miles. The National Park Service states that people who lived near or downwind of the blast were exposed to dangerous levels of radiation, with long-lasting effects on their health.⁠[2][29]

The Trinity page of the interactive history states that the Gadget was assembled on July 13 in the McDonald ranch house; that Groves, Bush, Conant, Lawrence, Farrell, James Chadwick and others arrived on July 15, when it was pouring rain; that at 3:30 a.m. Groves and Oppenheimer pushed the test back to 5:30 and the rain stopped at 4:00; that Kistiakowsky’s team armed the device shortly after 5:00 a.m.; and that at zero Sam Allison cried “Now!” Of the rehearsal of May 7 the page says that a large amount of conventional explosives was detonated to test the instrumentation. Jumbo, a steel container weighing over 200 tons, was brought to the desert but not used in the test; standing half a mile from ground zero, it was nonetheless knocked ajar. The National Park Service gives the time of detonation as “5:30 am mountain war time,” and the Los Alamos National Laboratory page places the site 200 miles south of Los Alamos. Britannica’s article states that Oppenheimer’s name “Trinity” referred to one of John Donne’s Holy Sonnets; the Department of Energy’s history says that the name was inspired by the poems of John Donne.⁠[30][29][31][3][1]

The page also records the reactions of those present. Oppenheimer later recalled that a line came to his mind: “Now I am become Death, the destroyer of worlds.” Bainbridge, the test director, called the explosion a “foul and awesome display” and said to Oppenheimer, “Now we are all sons of bitches.” Farrell remarked that “the war is over,” and Groves replied, “Yes, after we drop two bombs on Japan.” About forty seconds after the blast Fermi, from the scraps of paper he had dropped, estimated the yield at 10,000 tons of TNT, less than half the 21,000 tons later calculated, a yield the page puts at four times what most at Los Alamos had predicted; Kistiakowsky, over five miles away, was knocked to the ground by the blast wave, got up, slapped Oppenheimer on the back and said, “Oppie, you owe me ten dollars”; the asphalt around the base of the tower was turned to green sand; and afterward Bush, Conant and Groves shook hands at the base camp. The Department of Energy’s history, in a photograph caption, quotes Farrell: “The whole country was lighted by a searing light with the intensity many times that of the midday sun.” It also states that Oppenheimer later said the test brought to his mind the legend of Prometheus, punished by Zeus for giving man fire.⁠[30][1]

On safety, the interactive history calls the test “the single most significant safety hazard of the entire Manhattan Project”: Army intelligence agents had located and mapped everyone within a forty-mile radius, and the nearest offsite habitation was about twenty miles away; test personnel were withdrawn almost six miles and sheltered behind barricades; an offsite monitoring system was set up and evacuation plans were prepared in case exposure levels became too high, and the Department of Energy’s history states that the Army stood ready to evacuate the people in surrounding areas. The cloud topped out at 25,000 feet and largely dispersed to the north-northeast within an hour; fallout on the test site was limited to about 1,200 yards from ground zero; the exposed ranch families showed little external injury, while livestock suffered skin burns, bleeding and loss of hair. Warren judged the site “too small for a repetition of a similar test of this magnitude” except under very special conditions. The National Park Service, for its part, gives the height of the mushroom cloud as 38,000 feet and says that the explosion was visible over 280 miles away.⁠[32][2][1][29]

The interactive history’s page on the evaluations of the test states that initial measurements suggested a yield “in excess of 5,000 tons of TNT.” George L. Harrison, Stimson’s special assistant on atomic issues in Washington, D.C., cabled him on July 16: “Operated on this morning. Diagnosis not yet complete but results seem satisfactory and already exceed expectations.” The same cable said, “Local press release necessary as interest extends great distance,” and “Dr. Groves pleased. He returns tomorrow. I will keep you posted.” A second cable, of July 18, read: “Doctor has just returned most enthusiastic and confident that the little boy is as husky as his big brother.” In the cable’s terms the flash could be seen as far away as 250 miles and the noise heard for fifty miles. Groves’s report of July 21 said that glass had shattered 125 miles away and that he “did not now consider the PentagonU.S. Department of DefenseThe U.S. Department of Defense is an executive department of the United States federal government, formed in 1949 from the National Military Establishment created in 1947 and headquartered at the Pentagon. The department’s website describes it as America’s oldest and largest government agency.Open the full entry safe from atomic attack.” Stimson recorded that Truman was “tremendously pepped up” by it and that it gave him an “entirely new feeling of confidence”; Churchill was “elated” and said that he now understood why Truman had been so forceful with Stalin the previous day.⁠[33][1]

The petition drafted by Szilard, “A Petition to the President of the United States,” is dated July 17, 1945; the Atomic Heritage Foundation’s note says that it was signed by seventy other scientists of the Met Lab (the Metallurgical Laboratory) in Chicago. It asked the president to rule that the United States “shall not resort to the use of atomic bombs” in the war unless the terms to be imposed on Japan had been made public in detail and Japan, knowing them, had refused to surrender. The same note states that neither the president nor the secretary of war saw the petition before the first bomb was dropped on Hiroshima.⁠[34]

Truman learned of the success of the Trinity test while attending the Allied conference at Potsdam, Germany. On the evening of July 24, 1945, without an interpreter, he informed Joseph Stalin that the United States had a “new weapon of unusual destructive force”; Stalin replied only that he hoped the United States would make “good use of it against the Japanese.”⁠[35][28][1]

On July 25 a directive written by Groves, approved by Truman and issued by Stimson and George C. Marshall ordered the Army Air Forces’ 509th Composite Group to attack Hiroshima, Kokura, Niigata or Nagasaki as soon after August 3 as weather permitted; no further authorization was needed for subsequent atomic attacks.⁠[35][1]

The Potsdam Proclamation of July 26 called on Japan to surrender unconditionally or face “prompt and utter destruction”; it made no specific mention of the atomic bomb. The Department of Energy’s 2010 history states that it was issued in the name of the President of the United States, the president of China and the prime minister of Great Britain, and the Russians were not informed in advance. Japan rejected the proclamation on July 29.⁠[1][35]

Also on July 26 the USS Indianapolis reached Tinian with the gun assembly of Little Boy, and the remaining components arrived on C-54 transport planes. On July 29 the ship was sunk by a Japanese submarine, and of its crew of 1,199 only 316 survived.⁠[35][1]

The Department of Energy’s history states that the American delegation arrived in Berlin on July 15, and it words Stalin’s answer as a hope that the weapon would be used against Japan to good effect. The delivery of the Little Boy components to Tinian was called Operation Bronx. As to what the USS Indianapolis carried, the history says the gun and the uranium-235 projectile, while the interactive history says Little Boy minus its nuclear components; for the rest, the former has three C-54s and the latter five C-54 cargo planes. The interactive history also states that the four cities in the directive of July 25 were listed in order of preference and that half of the roughly 10,000 Japanese aircraft estimated by United States intelligence were kamikazes.⁠[1][35]

Early on August 6, 1945, the B-29 Enola Gay, piloted by Tibbets, the commander of the 509th Composite Group, took off from Tinian. At about 8:15 a.m. Hiroshima time it released Little Boy, a 9,700-pound uranium gun-type bomb, from about 31,000 feet; 43 seconds later the bomb exploded about 1,900 feet above the city, with a yield later estimated at 15 kilotons of TNT.⁠[36][1]

Hiroshima then held almost 300,000 civilians and about 43,000 soldiers. Nearly every structure within one mile of ground zero was destroyed and almost every building within three miles was damaged, and the fires merged into a firestorm that engulfed 4.4 square miles of the city.⁠[36]

Sixteen hours after the attack a statement by Truman was released, announcing that the weapon was an atomic bomb with more power than “20,000 tons of T.N.T.”; it warned that if Japan did not accept the terms it could expect “a rain of ruin from the air,” and said he would recommend that Congress consider establishing a commission to control the production and use of atomic power within the United States.⁠[37]

On August 8 the Soviet Union declared war on Japan and attacked Japanese forces in Manchuria.⁠[36][1]

The National Park Service’s timeline follows the mission in Tinian time, one hour ahead of Hiroshima and Nagasaki: at 2:45 the Enola Gay took off with a crew of twelve, accompanied by two aircraft, The Great Artiste and Necessary Evil; at 3:20 Little Boy was armed; at 7:30 Tibbets said to the crew, “We are carrying the world’s first atomic bomb.” At 8:09 air raid sirens sounded in Hiroshima, and at 9:12 the bombardier Thomas Ferebee took control, with the Aioi Bridge as the target point. The timeline gives the release altitude as 31,060 feet, the time of detonation as 8:16:02 a.m. Hiroshima time and the height of burst as 1,968 feet, about 550 feet from the Aioi Bridge; one second later the fireball was about 900 feet across, ground temperatures reached about 7,000 degrees Fahrenheit, the blast wave traveled at 984 miles per hour, and more than two-thirds of the city’s buildings were destroyed. At 10:55 the United States intercepted a Japanese message describing “a violent, large special-type bomb, giving the appearance of magnesium”; at 14:58 the Enola Gay landed at Tinian after a twelve-hour mission; and at 15:00 Tokyo news agencies carried word of an attack on Hiroshima with few details.⁠[38]

The Department of Energy’s history states that the Enola Gay first flew at low altitude on automatic pilot and climbed to 31,000 feet as it neared the target area; that the bomb exploded directly over a parade field where the Japanese Second Army was doing calisthenics and caused total devastation for five square miles; and that Tibbets recalled, “The city was hidden by that awful cloud boiling up, mushrooming, terrible and incredibly tall.” The interactive history states that Hiroshima was an important military center; that dry, combustible materials such as paper were instantly ignited as far as 6,400 feet from ground zero and the shadows of bodies were burned onto walls; that within minutes nine out of ten people half a mile or less from ground zero were dead; and that deaths from radiation sickness did not peak until three to four weeks after the attacks. For hours after the attack the Japanese government did not know for sure what had happened, and by August 9 American aircraft were showering leaflets all over Japan. The Hiroshima page states that radio stations began playing a prepared statement from Truman at 11:00 a.m. on August 6, Washington, D.C., time; the site’s page on informing the public says that the White House released the statement sixteen hours after the bombing, while Truman was aboard the USS Augusta on his way back from Potsdam, and quotes from it: “It is an atomic bomb.” The statement calls Hiroshima “an important Japanese Army base.”⁠[1][36][39][37]

At 3:47 a.m. on August 9 the B-29 Bock’s Car (also written Bockscar), commanded by Major Charles W. Sweeney, took off from Tinian for its primary target, Kokura Arsenal on the northern coast of Kyushu. Smoke and haze obscured Kokura, three passes failed to find the aiming point, and with fuel enough only to reach Okinawa the aircraft turned to the secondary target, Nagasaki.⁠[40][1][38]

At 11:02 a.m. Fat Man, a 10,000-pound plutonium implosion bomb, exploded 1,650 feet above Nagasaki, with a yield later estimated at 21 kilotons of TNT. It detonated roughly between the Mitsubishi Steel and Arms Works to the south and the Mitsubishi-Urakami Torpedo Works to the north; almost 200,000 people were still in the city. Of the city’s 52,000 homes, 14,000 were destroyed and 5,400 more were seriously damaged.⁠[40][1]

The National Park Service’s timeline (Tinian time) records that the support aircraft The Great Artiste and Big Stink took off at 3:51 and 3:53, the Enola Gay and Laggin’ Dragon being already airborne as weather planes; that at 4:00 Commander Fred Ashworth armed Fat Man; that Bockscar reached Kokura at 10:44; that at 11:32 Sweeney decided to turn to Nagasaki, 95 miles south of Kokura; that at 11:58 cloud cover left only one drop point, several miles from the intended target, and the bombardier Kermit Beahan released the bomb; and that all aircraft had returned to Tinian by 22:30. The timeline states that nearly half of Nagasaki was destroyed.⁠[38]

The Nagasaki page of the interactive history states that the next break in the weather was due three days after Hiroshima and the plutonium bomb was rushed into readiness for it; that Sweeney ordered the bomb armed ten minutes after takeoff; that a minor malfunction kept the aircraft from using its reserve fuel; and that it orbited Yaku-shima for almost an hour waiting for two escort B-29s, one of which never arrived. Over Kokura there were Japanese fighters and bursts of antiaircraft fire, which the crew member Jacob Beser is quoted as saying made things “a little hairy.” Nagasaki too was covered by cloud, Sweeney reluctantly approved a much less accurate radar approach, and the bombardier, Captain Kermit K. Beahan, released the bomb after glimpsing the stadium through a break in the clouds. The explosion affected about 43 square miles, of which about 8.5 were water and 33 only partially settled; the hills, the layout of the city and the detonation over an industrial area shielded parts of it. The official report termed the damage to the two Mitsubishi plants “spectacular”; a Nagasaki Prefectural report said that within one kilometer “men and animals died almost instantly”; and a United States Navy officer who visited in mid-September wrote that “a smell of death and corruption pervades the place.” The page adds that the yield was 40 percent greater than at Hiroshima. The Department of Energy’s history, for its part, says that Sweeney found unacceptable weather and flak over Kokura and that the bomb was dropped at 11:01 a.m.; that Nagasaki was home to the Mitsubishi plant that had made the torpedoes used at Pearl Harbor; that three square miles of the city were destroyed, less than at Hiroshima because of the steep hills around it; and that the aircraft then headed to Okinawa for an emergency landing.⁠[40][1]

At the Imperial Council on the night of August 9–10 the vote on surrender was tied three to three, and the emperor decided for surrender. On August 10 Japan offered to surrender on condition that the emperor keep his position. That day Groves advised that the next plutonium implosion bomb would be ready for delivery on the first suitable weather after “17 or 18 August”; Truman ordered a halt to atomic attacks while negotiations went on.⁠[41][1]

In its reply the United States accepted the surrender while making clear that the emperor could remain in a ceremonial capacity only, his authority subject to the Supreme Commander of the Allied Powers; the reply is dated August 12 in the Department of Energy’s interactive history and August 11 in the department’s 2010 history. On August 14 the emperor met his cabinet and ordered immediate acceptance, and on August 15 he announced the surrender by radio. The surrender documents were signed aboard the USS Missouri on September 2.⁠[41][1]

The Department of Energy’s history states that military extremists in Japan continued to advocate resistance to the end, that the United States answered on August 11 with British, Chinese and Russian concurrence, and that Japan surrendered on August 14, 1945. The interactive history words Japan’s condition as being that the emperor be allowed to remain the nominal head of state.⁠[1][41]

On August 12, 1945, the government released the Smyth Report, which Groves had the physicist Henry DeWolf Smyth prepare; it contained general technical information. Truman’s statement of August 6 had already disclosed that the project had plants and installations at Oak Ridge near Knoxville, Tennessee, at Richland near Pasco, Washington, and near Santa Fe, New Mexico.⁠[1][42][37]

The interactive history’s page on informing the public states that the press releases were the responsibility of Groves and his staff in Washington, D.C., and that a carefully designed public relations program began in early 1944; that Groves approached William Laurence, the science reporter of the New York TimesThe New York TimesThe New York Times is an American morning daily newspaper published in New York City. Founded in 1851, it is published by The New York Times Company.Open the full entry, who visited the major atomic facilities in the early months of 1945, witnessed the Trinity test and the bombing of Japan, and drafted most of the press releases. The Smyth Report’s full title is “Atomic Energy for Military Purposes: The Official Report on the Development of the Atomic Bomb under the Auspices of the United States Government, 1940-1945”; it ran to 182 pages, was released by the War Department on August 12, and was written by Henry D. Smyth, a Princeton University physicist and consultant to the Manhattan Project; Groves’s foreword said that “no requests for additional information should be made” and warned that disclosures were “subject to severe penalties under the Espionage Act”; and Princeton University Press reprinted the report as a “public service.” In one poll of August 1945 that asked whether people approved of the use of the atomic bomb, 85 percent of Americans replied “yes”; Paul Fussell, then a 21-year-old second lieutenant slated to be part of the invasion force going into Japan, wrote: “for all the fake manliness of our facades we cried with relief and joy,” and “We were going to live. We were going to grow up to adulthood after all.” The page says that the first reaction was overwhelmingly favorable, that later views ranged from dark pessimism to unbounded optimism, and that remorse slowly began to build among the public; an entire issue of The New Yorker in August 1946 was devoted to stories of the devastation of Hiroshima, later reprinted as John Hersey’s book Hiroshima. The Department of Energy’s history states that Americans were astounded to learn of a far-flung, government-run, top-secret operation, and that many workers did not know what they had been working on until they heard about the bombing of Hiroshima on the radio.⁠[39][1]

After the war Groves remained in command of the district. On September 9, 1945, the S-50 thermal diffusion plant was shut down; the Alpha tracks at Y-12 were put on standby in September, and K-25 now fed the Beta units directly. Hanford’s three piles continued to operate, while one of its two chemical separation areas was closed. Los Alamos was assigned to produce a stockpile of atomic weapons, and weapon assembly moved to Sandia Base at Albuquerque. Norris Bradbury succeeded Oppenheimer as director of the Los Alamos laboratory in 1945.⁠[1][31]

In July 1946, in Operation Crossroads at Bikini Atoll in the Marshall Islands, the district tested its third and fourth plutonium bombs: Able, dropped from a B-29 on July 1, and Baker, detonated ninety feet underwater on July 25, each with a yield of 21 kilotons of TNT. Before the tests the military removed the atoll’s 162 inhabitants; Baker produced a radioactive mist that deposited active products on the target fleet in amounts far greater than had been predicted, and a planned third shot was canceled. These were the last weapon tests of the Manhattan Project.⁠[1][2]

The Department of Energy’s history also states that in August 1944 the Military Policy Committee set up a Postwar Policy Committee, with Richard Tolman as chairman and Warren Lewis, Henry Smyth and Rear Admiral Earle W. Mills as members, and that on September 19, 1944, Bush and Conant contacted Stimson about postwar questions. During the sixteen months of Groves’s peacetime tenure the United States held a monopoly on atomic weapons; the Soviet secret atomic bomb program achieved success with the 1949 test of Joe I, which the Americans named after Joseph Stalin. A caption to a photograph of the Pajarito site at Los Alamos states that within a year of the Trinity test Louis Slotin perished when a plutonium core started an unintended fission chain reaction.⁠[1]

On Operation Crossroads, the Department of Energy’s 2012 overview states that the Joint Chiefs of Staff requested and received presidential approval for a test series in the summer of 1946; that the military moved the people of Bikini to another atoll; that the Able drop missed the anticipated ground zero by some distance and sank only three ships; that Baker spewed thousands of tons of water into the air and wreaked havoc on a seventy-four-vessel fleet of empty ships; that the Joint Chiefs’ evaluation board said the ships had become “radioactive stoves” that would have burned all living things aboard them; that Stafford Warren, head of the radiological safety section, warned of great risks of harm to personnel engaged in decontamination and survey work, that he prevailed and decontamination operations ceased; and that the planned third shot, to be detonated on the bottom of the lagoon, was canceled. The 2010 history states that the tests were watched by a large invited audience of journalists, scientists, military officers, congressmen and foreign observers and that shot Able failed to fulfill its pretest publicity buildup.⁠[2][1]

On October 3, 1945, Truman urged speedy passage of the May-Johnson bill; scientists at the Metallurgical Laboratory and Oak Ridge objected to the military control it provided. On December 20 Senator Brien McMahon of Connecticut introduced a substitute bill, which the Senate approved on June 1, 1946, and the House on July 20.⁠[1]

On August 1, 1946, Truman signed the McMahon Act, officially the Atomic Energy Act of 1946, which transferred atomic energy from the Army to an Atomic Energy Commission of five full-time civilian members. The Atomic Heritage Foundation’s article notes that United States nuclear research was classified under the act and that Truman did not honor the Hyde Park pledge.⁠[1][7]

On international control, Bernard Baruch presented the United States plan to the United NationsUnited NationsThe United Nations is an international organization founded in 1945 under the Charter of the United Nations. It has 193 Member States, and its headquarters are in New York City.Open the full entry Atomic Energy Commission on June 14, 1946, proposing an international atomic development authority with licensing and inspection powers; the Soviet Union, which insisted that the bomb be banned first, abstained in the vote of December 31, 1946, and by early 1947 the plan was a dead letter.⁠[1]

On domestic legislation, the Department of Energy’s history states that the terms of the debate were framed by draft legislation written by the Interim Committee in July 1945; that Groves, Bush and Conant testified at hearings in the House of Representatives; that Senator Arthur H. Vandenberg of Michigan held the May-Johnson bill up in the Senate through a parliamentary maneuver, while his attempt to create a joint committee failed; and that Brien McMahon succeeded in creating the Senate’s Special Committee on Atomic Energy and became its chair. Under the 1946 act the Atomic Energy Commission was assisted by a general advisory committee and a military liaison committee. On international control, the Acheson-Lilienthal report of March 1946 was fashioned primarily by Oppenheimer, and the Soviet Union was unwilling to surrender its veto power. The last entry of the chronology records that on December 31, 1947, the NDRC and the OSRD were abolished and their functions transferred to the Department of Defense. The book’s conclusion states: “Science had gone to war and contributed mightily to the outcome.”⁠[1]

The Department of Energy’s 2010 history has the Atomic Energy Commission succeeding the Manhattan Engineer District on January 1, 1947, with the district itself not abolished until August 15, 1947, and the Top Policy Group and the Military Policy Committee already disbanded; an article of the National Nuclear Security Administration dates the Army’s handover of primary responsibility to December 31, 1946. The National Park Service’s timeline gives the official span of the Manhattan Project as June 18, 1942, to August 25, 1947.⁠[1][14][43]

Under the Energy Reorganization Act of 1974 the Atomic Energy Commission was disbanded and its functions were divided between the Nuclear Regulatory Commission and the Energy Research and Development Administration, which was eliminated in 1977 when the Department of Energy was created. The Department of Energy describes itself as the direct descendant of the Manhattan Engineer District and continues to own and manage the federal properties at most of the major Manhattan Project sites, including Oak Ridge, Hanford and Los Alamos.⁠[3][44]

On August 29, 1949, the Soviet Union conducted its first nuclear test, of a device virtually identical in design to the one tested at Trinity four years earlier.⁠[45]

Klaus Fuchs, a British physicist who had worked in the Theoretical Division at Los Alamos, confessed in January 1950. His confession led the FBIFederal Bureau of InvestigationThe Federal Bureau of Investigation (FBI) is the federal investigative agency of the U.S. Department of Justice. It grew out of a force of special agents organized by Attorney General Charles J. Bonaparte in 1908, has used its present name since 1935 and is headquartered in Washington, D.C.Open the full entry to his handler, Harry Gold, and Gold led it to David Greenglass, an Army machinist at wartime Los Alamos. Greenglass confessed and implicated his wife Ruth and his brother-in-law Julius Rosenberg, and received 15 years; Julius and Ethel Rosenberg were sentenced to death, maintained their innocence, and were executed on June 19, 1953. Neither of the Rosenbergs had worked for the Manhattan Project.⁠[45]

On Theodore Hall, the Department of Energy’s interactive history states that the VENONA intercepts released in the mid-1990s revealed “an alleged second scientist-spy,” and that the FBI had learned of his espionage in the early 1950s, by which time his activities had apparently ended, so that the matter was quietly dropped.⁠[45]

In 2001 an expert panel convened by the Advisory Council on Historic Preservation at the Department of Energy’s request named the formal establishment of the department’s Signature Facilities as a collective unit as the ultimate goal. In 2004 Congress passed the Manhattan Project National Historical Park Study Act, directing the Secretary of the Interior, in consultation with the Secretary of Energy, to study the Oak Ridge, Los Alamos and Hanford sites; in July 2011 the Secretary of the Interior, with the Department of Energy’s concurrence, sent Congress a letter recommending a three-site park managed as a partnership of the National Park Service and the Department of Energy.⁠[46]

The provisions authorizing the park were included in the 2015 National Defense Authorization Act, which the House passed on December 4, 2014, and the Senate on December 12, and which President Barack ObamaBarack ObamaBarack Obama (born 1961) is an American politician and lawyer who served as the 44th president of the United States from 2009 to 2017, the first African American to hold the office. He was previously an Illinois state senator and a U.S. senator from Illinois, and he received the Nobel Peace Prize in 2009.Open the full entry signed on December 19. On November 10, 2015, Secretary of the Interior Sally Jewell and Secretary of Energy Ernest Moniz signed a memorandum of agreement, and the Manhattan Project National Historical Park was officially established.⁠[46][47][48]

The Department of Energy’s pages also record these steps: in the 1990s the department designated eight remaining major Manhattan Project properties as Signature Facilities; in March 2006 the National Park Service issued a notice of intent to prepare an environmental impact statement that would serve as the special resource study; in May 2007 the study team, with the department’s full participation, developed and assessed five alternatives; in early 2011 the two agencies agreed to recommend a three-unit park at Hanford, Oak Ridge and Los Alamos, with the department retaining ownership and control of access; and in 2012 and 2013 Congress held hearings in both houses but passed no authorizing bill. The 2014 law required the park to be established as a unit of the National Park Service no later than one year after enactment, after the Secretaries of the Interior and of Energy had entered into an agreement; the National Park Service’s management page states that the memorandum of agreement was to be executed by December 19, 2015. The department’s historical-resources page says that its public tours at Oak Ridge visit the X-10 Graphite Reactor and drive around the perimeter of the K-25 plant, and that tours will be expanded as the park is implemented.⁠[46][44][48][49]

The Radiation Exposure Compensation Act (RECA) was reauthorized under Pub. L. 119-21, enacted on July 4, 2025. The Department of Justice lists four groups of claimants: downwinders, onsite participants in nuclear tests, uranium workers and Manhattan Project waste applicants. The downwinder group includes people present in New Mexico for one year between September 24, 1944, and November 6, 1962, with compensation of a one-time lump sum of $100,000; the Manhattan Project waste group covers people who were physically present for at least two years after January 1, 1949, in designated zip codes in MissouriMissouriMissouri is a state of the United States bounded on the east by the Mississippi River; its capital is Jefferson City, its largest city is Kansas City, and its second-largest is St. Louis. It was admitted to the Union on August 10, 1821, as the 24th state, and its population in the 2020 census (April 1, 2020) was 6,154,913.Open the full entry, Tennessee, AlaskaAlaskaAlaska is the largest U.S. state by area and borders Canada to the east; its capital is Juneau and its largest city is Anchorage. Alaska was admitted to the Union on January 3, 1959, as the 49th state. Its population at the 2020 Census (April 1, 2020) was 733,391.Open the full entry and KentuckyKentuckyKentucky is a U.S. state bordered by Illinois, Indiana and Ohio to the north, West Virginia and Virginia to the east, Tennessee to the south and Missouri to the west; its capital is Frankfort and its largest city is Louisville. It entered the Union on June 1, 1792, as the 15th state and the first state west of the Appalachian Mountains, and had a population of 4,505,836 in the 2020 census (April 1, 2020).Open the full entry. All claims must be filed by December 31, 2027.⁠[50]

The Department of Justice page names the law under which the act was reauthorized as the One Big Beautiful Bill Act. The other conditions it lists are these: the downwinder group also includes people present in any affected area for one year between January 21, 1951, and November 6, 1962, or for the entire period from June 30 to July 31, 1962; the affected areas include the states of IdahoIdahoIdaho is a state in the northwestern United States, often classed as a Mountain state and also considered part of the Pacific Northwest; its capital and largest city is Boise. It was admitted to the Union on July 3, 1890, as the 43rd state, and its population in the 2020 census (April 1, 2020) was 1,839,106.Open the full entry, New Mexico and Utah, the counties of Coconino, Yavapai, Navajo, Apache, Gila and Mohave in ArizonaArizonaArizona is a state in the southwestern United States; its capital and largest city is Phoenix. Arizona was admitted to the Union on February 14, 1912, as the 48th state. Its population at the 2020 Census (April 1, 2020) was 7,151,502.Open the full entry, and, in NevadaNevadaNevada is a state in the western United States bordered by Oregon, Idaho, Utah, Arizona, and California; its capital is Carson City. It was admitted to the Union on October 31, 1864, as the 36th state, and its population in the 2020 census (April 1, 2020) was 3,104,614.Open the full entry, the counties of White Pine, Nye, Lander, Lincoln and Eureka and, within Clark County, townships 13 through 16 at ranges 63 through 71. Onsite participants are people who were present onsite during tests conducted by the United States before January 1, 1963; they receive a one-time lump sum of $100,000, offset by amounts paid by the Department of Veterans Affairs for the same illness. The uranium worker group covers people employed between January 1, 1942, and December 31, 1990, as uranium miners, millers or core drillers, in the transport of uranium or vanadium-uranium ore, or in remediation at uranium mines or mills in ColoradoColoradoColorado is a U.S. state classified as one of the Mountain states, with Denver as its capital. It was admitted to the Union on August 1, 1876, as the 38th state, and its population in the 2020 census (April 1, 2020) was 5,773,714.Open the full entry, New Mexico, Arizona, WyomingWyomingWyoming is a U.S. state, the ninth largest by area, with its capital at Cheyenne. Wyoming was admitted to the Union on July 10, 1890, as the 44th state, and the 2020 census (April 1, 2020) counted 576,851 residents.Open the full entry, South DakotaSouth DakotaSouth Dakota is a U.S. state bordered by North Dakota to the north, Minnesota and Iowa to the east, Nebraska to the south, and Wyoming and Montana to the west. Its capital is Pierre and its largest city is Sioux Falls. It was admitted to the Union on November 2, 1889, as the 40th state, and the 2020 census (April 1, 2020) counted 886,667 residents.Open the full entry, Washington StateWashington (state)Washington is a state on the Pacific coast of the northwestern United States; its capital is Olympia and its largest city is Seattle. Washington was admitted to the Union on November 11, 1889, as the 42nd state, and the 2020 census (April 1, 2020) counted 7,705,281 residents.Open the full entry, Utah, Idaho, North DakotaNorth DakotaNorth Dakota is a state in the north-central United States; its capital is Bismarck. It was admitted to the Union on November 2, 1889, as the 39th state, and its population in the 2020 census (April 1, 2020) was 779,094.Open the full entry, Oregon and TexasTexasTexas is a U.S. state in the south-central United States, with its capital at Austin and its largest city Houston. It was admitted to the Union on December 29, 1845, as the 28th state, and the 2020 census (April 1, 2020) counted 29,145,505 residents.Open the full entry, with a one-time lump sum of $100,000. Manhattan Project waste claimants living at the time of filing receive the greater of $50,000 or their documented out-of-pocket medical expenses; if the person has died, a surviving spouse receives $25,000, or, if there is none, the surviving children share that amount equally.⁠[50]

As of November 2026 the Manhattan Project National Historical Park is located in three states, New Mexico, Tennessee and Washington, and is co-managed by the National Park Service and the Department of Energy, with sites at Los Alamos, Oak Ridge and Hanford.⁠[51][52]

The park has visitor centers in Richland, Washington, in Oak Ridge and in Los Alamos. The Department of Energy runs guided tours of the B Reactor and of buildings that predate the Manhattan Project at Hanford, of Manhattan Project facilities in Oak Ridge, and of buildings behind the fence at Los Alamos; Los Alamos National Laboratory and the Department of Energy facilities at Oak Ridge are not accessible to the public. B Reactor was shut down on February 12, 1968, and became a National Historic Landmark in 2008.⁠[53][54][55][13]

The National Park Service’s Hanford visit page states that Hanford High School, which was converted into offices for the Manhattan Project, can be seen on a guided tour. The Hanford Site page lists the other designations of B Reactor: it was named a National Historic Mechanical Engineering Landmark by the American Society of Mechanical Engineers in 1976, listed in the National Register of Historic Places in 1992 and designated a National Historic Civil Engineering Landmark in 1994; at the ceremony conferring National Historic Landmark status, the acting Deputy Secretary of Energy announced that the reactor’s new path would be preservation and public access.⁠[53][13]

The fissionable material for a bomb could come from two sources. One was to separate the uranium-235 in natural uranium from uranium-238, isotopes that are chemically identical and differ in mass by less than one percent; the other was to produce plutonium in a uranium pile and separate it chemically.⁠[1][10]

Four methods of separating the uranium isotopes were investigated. The electromagnetic method, based on the mass spectrograph, was pioneered by Alfred Nier; gaseous diffusion relied on the principle that molecules of the lighter isotope pass more readily through a porous barrier; the centrifuge was pursued by Jesse W. Beams at the University of Virginia; and liquid thermal diffusion separated liquid uranium hexafluoride between concentric pipes. The MAUD report had dismissed plutonium production, thermal diffusion, the electromagnetic method and the centrifuge.⁠[1]

For the chemical separation of plutonium the project adopted the bismuth phosphate process found by Stanley G. Thompson’s team, in which the precipitate retained more than ninety-eight percent of the plutonium. Going from the laboratory to a separation plant meant a scale-up of roughly a billionfold.⁠[1]

Most of the uranium used by the project came from Canada and the Congo, with not insignificant amounts from the Colorado Plateau in the western United States. In 1940 Edgar Sengier, director of Union Minière du Haut Katanga, shipped about half of the uranium ore stored at the Shinkolobwe mine, some 1,200 tons, to the United States, where it sat for more than two years in a warehouse on Staten Island, New York.⁠[22][12][56]

In September 1942 the district bought the ore on Staten Island and the ore still in Africa: the National Nuclear Security Administration’s article has Groves acquiring 1,250 tons on September 18, and the Atomic Heritage Foundation’s profile gives a combined contract of 4,200 tons. The National Park Service’s article states that Union Minière supplied approximately two-thirds of the uranium the Manhattan Project acquired.⁠[14][56][12]

Processed uranium came from Eldorado Mining and Refining in Canada, which held large stocks of uranium left over from its radium refining; the Canadian government bought the company, and by 1946 more than 4,000 tons of ore concentrate, containing approximately 1,100 tons of uranium oxide, had been delivered to the district. During the Manhattan Project the Eldorado Mine at Port Radium, Northwest Territories, employed Dene people from the community of Déline to carry uranium. Between 1942 and 1945 the Ames Project, on the campus of Iowa State University, produced 2 million pounds of purified uranium.⁠[4][57][58]

The National Park Service’s article also states that Sengier was a Belgian mining engineer who rose through the ranks of Union Minière to become director; that he decided to ship the ore to the United States to keep the uranium out of German hands, to protect the company’s assets and potentially to sell to a higher bidder; that the ore was stockpiled in a three-story warehouse on Staten Island near the Bayonne Bridge while he conducted business in Manhattan, mere blocks from the office of the Manhattan Project; and that after Sengier applied to the State Department in the fall of 1942, Kenneth D. Nichols quickly arranged the purchase of that ore and of the stockpiles still in Africa. The Atomic Heritage Foundation’s profile says in its body text that the ore was stored in warehouses on Manhattan Island and in its timeline that it was shipped to Staten Island; it gives the ore sold to the Army from Shinkolobwe as 3,000 tons and states that in 1946 Sengier became the first non-American civilian to receive the Medal for Merit. The profile gives his dates as born on October 9, 1879, in Kortrijk, Belgium, and died on July 26, 1963, in Cannes, France.⁠[12][56]

The Atomic Heritage Foundation’s account also lists several related places in Manhattan: the Baker and Williams Warehouses on West 20th Street, which in the early 1940s were used for the short-term storage of tons of uranium concentrates that had been shipped in secret to the nearby Hudson River docks and were then distributed to government facilities working on the reactor and atomic weapons programs; the Madison Square Engineers Office, where approximately 300 people procured uranium, graphite and beryllium, run by Lieutenant Colonel John R. Ruhoff until September 1944 and then by Major Wilbur E. Kelly; the 18th floor of the Union Carbide building, which housed the Murray Hill Area Engineers Office and the Union Mines Development Corporation, a Union Carbide subsidiary that bought uranium ore from domestic sources; and the Woolworth Building, where Kellex had its offices. The account explains that the Canadian government bought Eldorado to accommodate American and British orders and to maintain secrecy. The National Park Service states that the building on the Iowa State University campus used by the Ames Project was nicknamed “Little Ankeny.”⁠[4][58]

The Department of Energy’s history states that the British made important early theoretical contributions but lacked the resources for a bomb program of their own, acceded to American leadership, and sent scientists to every Manhattan Project facility. Nineteen scientists from the British project worked at Los Alamos, among them James Chadwick, Rudolf Peierls, Klaus Fuchs and William Penney. The Atomic Heritage Foundation’s article states that Groves, who disapproved of the collaboration, put the British scientists in limited roles and that none of them had a complete picture of the Manhattan Project.⁠[1][7][59]

In 1942 C. D. Howe, Canada’s cabinet minister for wartime production, authorized the development of the Montreal Laboratory, a joint Canadian-British research effort, and in 1944 the work moved to new facilities at Chalk River, Ontario. On September 5, 1945, ZEEP, a reactor at Chalk River using uranium and heavy water, achieved a sustained and controlled nuclear reaction; it was the first nuclear reactor built and operated outside the United States.⁠[57]

Truman’s statement of August 6, 1945, said that Britain had pooled its scientific knowledge in this field with the United States from 1940, and that because Britain was exposed to constant air attack and still threatened with invasion, Churchill and Roosevelt had agreed to carry out the project in the United States.⁠[37]

The Atomic Heritage Foundation’s article on the British program also states that in August 1941 Roosevelt suggested collaboration and Churchill responded unenthusiastically; that in 1942 a mission led by Wallace Akers, director of Tube Alloys, visited the S-1 Committee and American universities, and Michael Perrin said the United States would “completely outstrip us in ideas, research and application of nuclear energy” and that “they will see no reason for our butting in”; that in the summer of 1942 Hans Halban’s Cambridge team moved to Canada; and that Churchill’s five-point agenda included a “free exchange of information” between the two nations, while Vannevar Bush and James B. Conant opposed collaboration. In the article’s summary the Quebec Agreement gave the Manhattan Project British uranium, set up a research center in Montreal and ensured that British scientists took part; the British team at Los Alamos numbered nineteen scientists; William Penney later witnessed the bombing of Nagasaki; and the Hyde Park memorandum, signed a few days after a summit at Quebec in September 1944, spoke of “Full collaboration between the United States and the British Government in developing Tube Alloys.” After the war Prime Minister Clement Attlee decided in 1947 to pursue a British bomb, and in May 1947 Penney was informed, “We’re going to make an atomic bomb. The Prime Minister has asked me to coordinate the work. They want you to lead it.” On October 3, 1952, Britain tested its first atomic bomb in Operation Hurricane at Monte Bello, Australia, and in 1958, after the United States amended the Atomic Energy Act of 1946, the two countries concluded their Mutual Defense Agreement.⁠[7]

The Canadian Encyclopedia’s article also states that George Laurence experimented with uranium fission in Ottawa in 1939–40 for the National Research Council, aiming at a uranium-graphite reactor, that his experiments continued until 1942 without a reactor being built, and that his uranium came from the Eldorado Mine at Port Radium; that the code name Tube Alloys dates from 1941 and the project was first based at the Cavendish Laboratory of the University of CambridgeUniversity of CambridgeThe University of Cambridge is a university in Cambridge, England, United Kingdom. It gives 1209 as its founding date, is made up of Schools, Faculties, Departments and autonomous Colleges, and is a member of the Russell Group.Open the full entry; that the Montreal Laboratory was based at McGill University and the Université de Montréal; and that Roosevelt, William Lyon Mackenzie King and Churchill met at Quebec in August 1943, where British and American nuclear weapons research was merged. It sums up Canada’s contribution under three heads: supplying and processing uranium (which it continued to supply to the United States for about two decades after the war), research on the extraction and production of plutonium, and the provision of researchers, scientists and facilities. ZEEP stands for Zero Energy Experimental Pile; the other experimental reactor at Chalk River, NRX, did not reach criticality until 1947, the Montreal Laboratory closed in 1946, and in 1952 the crown corporation Atomic Energy of Canada Ltd. took over research at Chalk River. On the Dene of Déline the article says that some Dene believe uranium from this mine became the fissile material in the bomb dropped on Hiroshima on August 6, 1945; that this is disputed by some historians, among them Robert Bothwell, who wrote a history of the Eldorado company in 1984 and suggests that the uranium the Dene carried was not pure enough to be used in the bomb; and that, whether or not it formed part of the weapon, at least some of this uranium was used in experiments that led to the development of nuclear arms and of Canada’s nuclear industry.⁠[57]

The Department of Energy’s history states that by the end of the war Groves and his staff had spent approximately $2.2 billion on production facilities and towns, and that expenditures reached $100 million a month by mid-1944. Truman’s statement of August 6, 1945, said that the United States had spent “two billion dollars,” put employment during peak construction at 125,000 and said that over 65,000 people were then engaged in operating the plants.⁠[1][37]

The Brookings Institution, drawing on figures in The New World by Richard G. Hewlett and Oscar E. Anderson Jr., lists cumulative costs through December 31, 1945: a total of $1,889,604,000, of which $1,188,352,000 was for Oak Ridge (K-25, $512,166,000; Y-12, $477,631,000), $390,124,000 for the Hanford Engineer Works and $74,055,000 for Los Alamos.⁠[60]

On numbers, the Department of Energy states that approximately 130,000 people were employed at the peak, a scale comparable to the American automobile industry of the time, while the National Park Service says that over 500,000 workers, scientists, engineers and political leaders contributed to the project. Oak Ridge grew in little more than two years into a city of 75,000, and the Clinton Engineer Works at one point consumed one-seventh of all the electric power produced in the country.⁠[1][61][62][18]

Most workers were White men, though the workforce also included African Americans, Native Americans, Hispanics and women, and most did not know the true purpose of their work. In May 1943 the Army created the Special Engineer Detachment, which let qualified enlisted men work on the project instead of going into combat; by 1945 it had more than 3,000 soldiers, 1,800 of them at Los Alamos.⁠[63][14]

The same Brookings table lists the remaining items in then-year dollars: Clinton Engineer Works headquarters and central utilities, $155,951,000; Clinton Laboratories, $26,932,000; the S-50 thermal diffusion plant, $15,672,000; special operating materials, $103,369,000; research and development, $69,681,000; government overhead, $37,255,000; and heavy water plants, $26,768,000. In constant 1996 dollars the total is $21,570,821,000, of which $13,565,662,000 was for Oak Ridge (K-25, $5,846,644,000; Y-12, $5,452,409,000; headquarters and central utilities, $1,780,263,000; Clinton Laboratories, $307,443,000; S-50, $178,904,000), $4,453,470,000 for the Hanford Engineer Works, $1,180,011,000 for special operating materials, $845,377,000 for Los Alamos, $795,445,000 for research and development, $425,285,000 for government overhead and $305,571,000 for heavy water plants. The page separately gives expenditures through August 1945 as $20 billion in constant 1996 dollars, including the 1940–42 costs of the NDRC and the OSRD and excluding $76 million spent by the Army Air Forces on Project SILVERPLATE from September 1943 to September 1945 (the modification of 46 B-29 bombers, the training of the 509th Composite Group and logistical support at Tinian Island); averaged over the three devices detonated (the Gadget, Little Boy and Fat Man), the cost per device or bomb was $5 billion, and a fourth bomb, “Bomb No. 4,” was not used. For comparison the page lists several United States Army expenditures of 1942–1945, taken from the War Department’s 1946 Statistical Review – World War II: A Summary of ASF Statistics: bombs, mines and grenades, $31.5 billion; small arms materiel excluding ammunition, $24 billion; all tanks, $64 billion; heavy field artillery, $4 billion; all other artillery, $33.6 billion; and it puts the total cost of the Second World War to the United States at roughly $3.3 trillion. The page explains that the figures were adjusted using 1944, the year of highest spending, as base year, that they include capital and operating costs from 1942 through 1945, and that actual costs per facility per year are apparently unknown.⁠[60]

Other sources put it as follows: Britannica’s article says that the original $6,000 authorized for the work had grown to $2 billion; the Atomic Heritage Foundation calls it a $2 billion government war project; the Department of Energy’s 2012 overview says that at its peak the project employed 130,000 workers and by the end of the war had spent $2.2 billion; the National Park Service’s timeline says that the Manhattan Project employed over 130,000 people throughout the United States, and its page on workers speaks of tens of thousands of workers; and the department’s interactive history describes those employed as ranging “from laborers to Nobel Prize winners.”⁠[3][42][2][43][63][61]

Every worker underwent an FBI background check and signed a pledge of silence; construction workers and most others were informed only on a “need-to-know” basis, and badges showed the holder’s picture, job and clearance level. Each site was fenced with barbed wire, and its checkpoints were guarded by military police around the clock. Congressional leaders agreed to secret budget processes for the project, without legislative oversight.⁠[42]

At Los Alamos the Technical Area was enclosed by a 9½-foot woven-wire fence; censorship of all correspondence except official mail began in December 1943, and all telegrams were reviewed. The laboratory held a weekly technical colloquium; Groves complained that the project as a whole was discussed there, and under a compromise attendance was limited to staff members.⁠[59]

The Department of Energy’s interactive history judges that security succeeded in keeping the bomb effort from Germany and Japan, neither of which had a spy inside the project, but was less successful with the Soviet Union, then an ally. In February 1943 counterintelligence learned of Soviet attempts to contact physicists at the Radiation Laboratory in Berkeley. Soviet intelligence had first learned of Anglo-American talk of an atomic bomb program in September 1941, and it gave the effort the code name ENORMOZ.⁠[45]

The same history gives this account of the individuals: Klaus Fuchs offered his services to Soviet intelligence in late 1941 and, once at Los Alamos, passed on detailed information on weapon design; Theodore Hall made contact with Soviet intelligence in November 1944 and supplied data on implosion; David Greenglass funneled information through his wife Ruth to Julius Rosenberg, who turned it over to Soviet intelligence; and Allan Nunn May, a British physicist assigned to the Canadian effort to build a heavy-water reactor, passed information to Soviet intelligence in February 1945. It estimates that espionage probably hastened the Soviet acquisition of an atomic bomb by at least 12 to 18 months.⁠[45]

The interactive history states that the fence around the Technical Area at Los Alamos was a woven-wire fence with two barbed-wire top strands, an automatic alarm system attached to it and 159 mounted 1,500-watt floodlights, that prominent scientists used code names while traveling, Oppenheimer’s being “James Oberhelm,” and that long-distance telephone calls were subject to limited monitoring under the supervision of the security office. The Atomic Heritage Foundation’s article states that workers with lower clearances wore red or blue badges and those with the highest clearance white ones, and that generally only top-level staff knew that the overall goal was an atomic bomb; that billboards urged secrecy, one of them reading, “What you see here, what you do here, what you hear here, when you leave here, let it stay here!”; that mail in and out of Los Alamos was meticulously censored and Richard P. Feynman, who liked to send his wife coded puzzles in the mail, was questioned by security officials about the puzzles and asked to stop using coded words in his letters home; and that Groves created separate organizations for intelligence, counterintelligence and surveillance, operating outside regular military channels. The foundation’s article on Alsos quotes Groves: “Code names for our projects were deliberately innocuous.”⁠[59][42][23]

The interactive history further states that rumors of the Manhattan Project reached Japan as well but that, as with Germany, no Japanese spies penetrated the project; that Fuchs was caught largely through the VENONA intercepts; that Ruth Greenglass was never formally charged; that the Rosenbergs refused to cooperate; and that Allan Nunn May had come over with James Chadwick in 1943. The site’s page on security at Los Alamos calls Theodore Hall the youngest scientist at Los Alamos, a Harvard graduate at age 18, and states that Fuchs, one of the nineteen-member British mission, passed detailed information regarding atomic weapons design to his Soviet handlers. The Atomic Heritage Foundation’s article on the British program states that the British project was infiltrated by the Soviet spies Klaus Fuchs, Donald Maclean and Guy Burgess.⁠[45][59][7]

The Oak Ridge reservation had been home to several small rural communities; about 1,000 families were removed, a process largely completed by mid-February 1943.⁠[18][64]

At Hanford the federal government in 1943 took roughly 600 square miles along the Columbia River by eminent domain, and the residents of the towns of White Bluffs and Hanford were given 30 days to leave. The area was the traditional land of several tribal nations: the Wanapum Band, which had signed no treaty, lived there year-round, and members of the Confederated Tribes of the Umatilla Indian Reservation, the Confederated Tribes and Bands of the Yakama Nation and the Nez Perce Tribe wintered there; from 1943 the tribes were excluded by barbed-wire fences and security gates.⁠[65][13][66]

At Los Alamos the government in late 1942 took U.S. Forest Service land and private property on the Pajarito Plateau, and fences and checkpoints thereafter barred American Indians and former landowners from returning. The land taken included Tsirege, an ancestral pueblo of the Tewa speakers of the Pueblo de San Ildefonso.⁠[66]

The Hanford Site page states that when the government in early 1943 gave residents 30 days to evacuate their homes and abandon their farms, it offered a small amount of money to do so. The National Park Service’s page on tribal nations also states that oral tradition and archaeology show an American Indian presence in the Hanford area for more than 10,000 years; that the 1855 treaties of the Walla Walla Council moved most local tribes to reservations elsewhere while the tribes reserved access to usual and accustomed places for salmon, game and medicinal plants; and that the Department of Energy consults with three federally recognized tribes that it found to be affected under the Nuclear Waste Policy Act of 1982, the Yakama Nation, the Umatilla and the Nez Perce, while the Wanapum people at Priest Rapids, who are not federally recognized, have consulted with the department since the 1940s. At Los Alamos, use of the Pajarito Plateau dates to as early as 10,000 BC and large pueblo villages were built there between AD 1150 and 1600; Tsirege was occupied during the Classic period (AD 1325 to 1600); and the page lists as associated tribes the Jicarilla Apache Nation and the Pueblos of Cochiti, Jemez, Nambe, Ohkay Owingeh, Picuris, Pojoaque, Santa Clara, San Ildefonso and Taos. At Oak Ridge the reservation includes evidence of human settlement dating back at least 14,000 years; treaties with the Cherokee Nation in the 1790s brought a wave of white settlers, and the Indian Removal Act of 1830 led to the forced removal of tens of thousands of American Indians, along with thousands of enslaved African Americans, to Indian Territory, a removal known as the Trail of Tears during which, the page says, thousands died; some Cherokee people hid in the mountains, and the Eastern Band of Cherokee Indians bought 57,000 acres in North CarolinaNorth CarolinaNorth Carolina is a state in the South Atlantic region of the United States; its capital is Raleigh and its largest city is Charlotte. One of the original 13 states, it ratified the U.S. Constitution on November 21, 1789, becoming the 12th state, and its population in the 2020 census was 10,439,388.Open the full entry in the 1800s, the Qualla Boundary.⁠[13][66]

The Department of Energy’s 2010 history gives these figures: at Hiroshima 70,000 killed at once (including about twenty American airmen held as prisoners) and another 70,000 injured, a death toll of 140,000 by the end of 1945 and 200,000 five years later; at Nagasaki 40,000 killed and 60,000 injured, 70,000 dead by January 1946 and eventually 140,000.⁠[1]

The Department of Energy’s interactive history is more guarded: at Hiroshima the death toll by the end of 1945 was probably over 100,000 and the five-year total may have reached or even exceeded 200,000; at Nagasaki deaths probably approached 70,000 by January 1946, with perhaps about twice that number within five years. Britannica’s article has some 70,000 killed instantly at Hiroshima and more than 100,000 by the end of the year, and an estimated 40,000 killed instantly at Nagasaki with at least 30,000 more deaths afterward.⁠[36][40][3]

The Radiation Effects Research Foundation, a Japan–United States research institute, estimates acute deaths within two to four months of the bombings at 90,000–166,000 in Hiroshima (population then about 340,000–350,000) and 60,000–80,000 in Nagasaki (population about 250,000–270,000). It explains that the total is not known precisely because military personnel records in both cities were destroyed, entire families died leaving no one to register the deaths, and unknown numbers of forced laborers were present.⁠[67]

The National Park Service states that more than 200,000 people had died by the end of 1945 as a direct result of the two bombings. Survivors are known in Japanese as hibakusha; more than 525,000 deceased hibakusha are named in memorials, over 330,000 of them from Hiroshima and over 190,000 from Nagasaki.⁠[68][69]

The foundation’s Life Span Study follows about 120,000 people, about 94,000 of them survivors. Its summary states that the excess risk of leukemia, highest in the first ten years, has virtually disappeared, whereas the excess risk of solid cancers has stayed constant and is likely to persist throughout the survivors’ lifetimes.⁠[67]

Other sources give these figures: the National Park Service’s timeline of the bombings says that in Hiroshima eighty thousand people were instantly killed or grievously wounded and over 100,000 more died from the bomb’s effects in the following months, and that in Nagasaki between 40,000 and 75,000 people died instantly; the University of Chicago’s explainer says that more than 200,000 people died in the two blasts. The National Park Service’s page on survivors states that the word hibakusha translates as “bomb-affected-people” and that more than 160 people who were present at both Hiroshima and Nagasaki are known as niju hibakusha, double survivors. The Life Span Study of the Radiation Effects Research Foundation also includes 27,000 unexposed individuals.⁠[38][17][68][67]

The Department of Energy’s interactive history summarizes two main positions among historians: many argue that the bombing was necessary to end the war and saved lives by avoiding an invasion of Japan, while others argue that Japan would have surrendered even without the atomic bomb and that Truman and his advisors used it only in an effort to intimidate the Soviet Union. The site’s own judgment is that the prospect of avoiding another year of bloody warfare “may well have figured most importantly” in the decision.⁠[35]

As background the site lists the following: the Okinawa campaign lasted more than ten weeks and killed over 12,000 Americans, about 100,000 Japanese and perhaps another 100,000 Okinawans; United States intelligence estimated that Japan retained two million soldiers and about 10,000 aircraft for the defense of the homeland; intercepted messages showed that Japan was seeking a conditional surrender; and the alternatives considered included a demonstration, waiting for the Soviet declaration of war, and a blockade combined with conventional bombing.⁠[35]

Groves wrote in his farewell message to the soldiers of the district: “You built the weapon which ended the War and thereby saved countless American lives.” The National Park Service describes the loss of life in Japan as “one of the most tragic legacies” of the project.⁠[14][69]

The “Legacy” section of the Department of Energy’s 2012 overview states that the advent of nuclear weapons not only helped bring an end to the Second World War but ushered in the atomic age and determined how the next war, the Cold War, would be fought. The interactive history states that Truman hoped that the atomic bomb might give the United States an edge in postwar diplomacy.⁠[2][35]

The National Park Service states that the production and testing of the first atomic weapons released large quantities of radioactive byproducts into the environment and that residents near Hanford, Los Alamos and Oak Ridge were unknowingly exposed; they later called themselves Downwinders, and only in the 1980s did journalists, concerned citizens and activists obtain Department of Energy documents confirming the harm.⁠[70]

Plutonium production at Hanford continued until 1987, and nine production reactors stand along the river. The work left about 56 million gallons of radioactive and chemical waste stored in 177 large underground tanks. On May 15, 1989, the Department of Energy, the Environmental Protection Agency and the Washington State Department of Ecology signed a legally binding cleanup agreement, the Tri-Party Agreement; the Hanford Site page, last updated on June 17, 2025, says that about 10,000 Hanford workers are involved in the cleanup today.⁠[13]

The laboratory at Los Alamos continues as Los Alamos National Laboratory, which describes its basic mission as maintaining the safety, security and reliability of the nation’s nuclear deterrent without the need to return to underground testing. The University of Chicago calls Argonne National Laboratory the nation’s first national laboratory. A group of Chicago scientists who had helped build the first atomic bomb founded the Bulletin of the Atomic Scientists in December 1945.⁠[31][17][71]

Terrence R. Fehner and F. G. Gosling, historians of the Department of Energy, judge that the Manhattan Project became the organizational model for American “big science” in the second half of the twentieth century. President John F. KennedyJohn F. KennedyJohn F. Kennedy (1917–1963) was an American politician who represented Massachusetts in the U.S. House of Representatives and the U.S. Senate and served as the 35th president of the United States from 1961 to 1963. The youngest person elected president and the first Roman Catholic to hold the office, he was assassinated in Dallas on November 22, 1963.Open the full entry invoked the spirit of the project when he set the goal of putting a man on the moon by the end of the 1960s.⁠[2][61]

The Los Alamos National Laboratory page states that the laboratory began in 1943 for a single purpose: to design and build an atomic bomb. The first page of its list of directors gives, in order, Oppenheimer (1943–1945), Norris Bradbury (1945–1970), Harold M. Agnew (1970–1979), Donald M. Kerr (1979–1985), Siegfried S. Hecker (1985–1997), John C. Browne (1997–2003), G. Peter Nanos (2003–2005), Robert Kuckuck (2005–2006), Michael R. Anastasio (2006–2011) and Charles McMillan (2011–2017).⁠[31]

The University of Chicago’s explainer gives several later figures: at the peak in 1986 there were about 70,000 atomic weapons in six nations; about 500 bombs were detonated above ground before a 1963 treaty moved tests underground; about 12,000 weapons remained in the world as of 2025; nuclear reactors supply about 20 percent of the electricity of the United States and 10 percent worldwide; and the world’s first usable amount of electricity from nuclear energy was produced on December 20, 1951, lighting a string of four light bulbs. The Bulletin of the Atomic Scientists says of its founders that they realized they could no longer “remain aloof to the consequences of their work”; the University of Chicago’s explainer says that they objected to the use of the bomb and that the Bulletin’s Doomsday Clock has since grown to incorporate climate change and biological threats.⁠[17][71]

The multi-volume classified Manhattan District History was commissioned by Groves: the Department of Energy’s historical-resources page says that it was commissioned at the end of the war and that all 36 volumes are being made available in full text online, some with redactions, while the department’s background page describes a 35-volume history commissioned in late 1944. The New World, 1939–1946, published in 1962, was the first major history of the Manhattan Project and is the first volume of the official history of the Atomic Energy Commission; the U.S. Army Center of Military History released Manhattan: The Army and the Atomic Bomb in 1985.⁠[49][44]

The core administrative records of the Manhattan Engineer District, which came out of Oak Ridge, were transferred to the Southeast Region of the National Archives in Atlanta, Georgia, and the classified records were sent to National Archives headquarters (Archives II in College Park). On July 16, 2013, the 68th anniversary of the Trinity test, the Department of Energy launched the website The Manhattan Project: Resources.⁠[49][44]

Manhattan Project National Historical Park (National Park Service): Official website: https://www.nps.gov/mapr/ (opens in a new window).⁠[52]

U.S. Department of Energy (Manhattan Project Historical Resources page): Official website: https://www.energy.gov/lm/manhattan-project-historical-resources (opens in a new window).⁠[49]

Los Alamos National Laboratory (history page): Official website: https://www.lanl.gov/about/history-innovation (opens in a new window).⁠[31]

Hanford Site (history page): Official website: https://www.hanford.gov/page.cfm/understandPAST (opens in a new window).⁠[13]