The Manhattan Project
Los Alamos, Trinity, and the Building of the Atomic Bomb
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Key technical challenges included uranium enrichment through electromagnetic separation, gaseous diffusion, and thermal diffusion methods. Reactors at Hanford produced plutonium while Los Alamos designed the actual weapons. The Trinity test in July 1945 proved the bomb worked. Later chapters detail the personnel involved, security measures, intelligence gathering, and the bombings of Hiroshima and Nagasaki.
This account covers the complex process of building the first atomic weapons, from scientific breakthroughs to wartime decisions. It examines the people behind the project, the enormous costs, and the lasting consequences. Readers interested in nuclear history, scientific development, or World War II technical achievements will find this detailed examination essential.
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In June 1942, Major General Eugene Reybold chose Colonel James C. Marshall to lead the Army’s role in the project. Marshall set up a liaison office in Washington, D.C., but his temporary headquarters were at 270 Broadway in New York. That location gave him access to administrative support from the Corps of Engineers' North Atlantic Division. It was near the Manhattan office of Stone & Webster, the main contractor, and close to Columbia University. He was allowed to pull staff from his old command, the Syracuse District, and he began with Lieutenant Colonel Kenneth Nichols, who became his deputy.
Marshall, whose work centered on construction, coordinated closely with Major General Thomas M. Robins, head of the Corps of Engineers Construction Division, and Robins's deputy, Colonel Leslie Groves. Reybold, Somervell, and Styer had chosen the name "Development of Substitute Materials" for the project, but Groves worried it would attract unwanted attention. Because engineer districts typically took their names from the cities where they were based, Marshall and Groves opted instead to call the Army's part the Manhattan District. Reybold formally established the district on August 13. People often referred to it informally as the Manhattan Engineer District, or MED. It differed from other districts in that it had no set geographic limits, and Marshall operated with the authority of a division engineer. Although "Development of Substitute Materials" stayed the official code name for the project overall, "Manhattan" gradually replaced it.
Marshall later admitted he hadn't known about atomic fission, but he did know that building even one plant for $90 million was unrealistic—especially when a single TNT facility Nichols had built in Pennsylvania had already cost $128 million. The estimates they were given weren't helpful at all; Groves said they were like telling a caterer to prepare for anywhere from ten to a thousand guests. A survey team from Stone & Webster had already looked at potential locations, and the War Production Board suggested areas near Knoxville, Tennessee, where the Tennessee Valley Authority could supply power and rivers could cool reactors. After reviewing several spots, the team chose one near Elza, Tennessee. Conant urged acquiring the land right away, Styer agreed, but Marshall held back, waiting for results from Conant's reactor tests. Of all the proposed methods, only Lawrence's electromagnetic separation process seemed ready enough to begin construction.
Marshall and Nichols moved quickly to gather what the project required, starting with securing the proper priority classification. The system used ratings from AA-1 through AA-4 for top-tier weapons and equipment, with AAA reserved only for emergencies. Colonel Lucius D. Clay, who held the position of deputy chief of staff at Services and Supply and was in charge of resources, felt he could not give the project anything higher than AA-3, though he said he would consider a AAA rating if absolutely needed. Nichols and Marshall were left feeling disappointed, since AA-3 was no better than the rating given to Nichols’ TNT facility in Pennsylvania.
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In the spring of 1942, Arthur Compton asked J. Robert Oppenheimer to take over research into fast neutron calculations from Gregory Breit, who had quit because of security concerns. John H. Manley was assigned to assist Oppenheimer by coordinating experimental physics groups around the country. Oppenheimer and Robert Serber looked at neutron diffusion and hydrodynamics, key to understanding how a nuclear chain reaction would behave and how an explosion might occur.
In June 1942, Oppenheimer and Fermi held a meeting at the University of Chicago with physicists Hans Bethe, John Van Vleck, Edward Teller, Emil Konopinski, Robert Serber, Stan Frankel, and Eldred C. (Carlyle) Nelson, plus experimental physicists Emilio Segrè, Felix Bloch, Franco Rasetti, Manley, and Edwin McMillan. They reviewed the theory of fission reactions and tentatively confirmed that a fission bomb was possible. The next month, in July, they met again at the University of California with the same group to continue their work.
At the July 1942 conference in Berkeley, scientists were grappling with the unknown properties of uranium-235 and plutonium, which had only just been isolated by Glenn Seaborg and his team in February 1941. They imagined creating plutonium in nuclear reactors where uranium-238 atoms would absorb neutrons emitted from fissioning uranium-235. But no reactor had been built yet, and only tiny amounts were available from cyclotrons—by December 1943, just two milligrams had been produced. The team considered many ways to bring the fissile material into a critical mass. One simple method involved shooting a cylindrical plug into a sphere of active material with a tamper to focus neutrons and contain the reaction. They also explored designs using spheroids, a basic form of implosion suggested by Richard C. Tolman, and the idea of autocatalytic methods to boost the bomb’s efficiency as it exploded.
As theoretical work on the fission bomb reached a tentative conclusion—though more experimental data was still needed—Edward Teller advocated for exploring a far more powerful option: what came to be known as the "super," later called the hydrogen bomb. This type of weapon would rely on the heat generated by a fission explosion to trigger nuclear fusion in deuterium and tritium. Such a device could theoretically deliver thousands of times the energy of a fission bomb. The physics involved were so complex, with many unknowns, that it remained unclear whether it would function at all. Since any "super" required a fission weapon as a first step, little effort or resources were devoted to studying it during the war. The very idea of its possibility was guarded even more closely than other aspects of the project.
At the Berkeley summer conference, discussions turned to fusion reactions, where Teller suggested that an atomic bomb might "ignite" the atmosphere through a hypothetical fusion of nitrogen nuclei. Bethe calculated this was "extremely unlikely," but the concern was serious enough that Oppenheimer reported it to Arthur Compton. Oppenheimer visited Compton in person, and Compton said that if the odds were greater than "three in a million," the project would have to halt. The theorists eventually concluded it had a "negligible" chance "for practical purposes." A postwar report co-authored by Teller stated that "whatever the temperature to which a section of the atmosphere may be heated, no self-propagating chain of nuclear reactions is likely to be started."
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Vannevar Bush grew frustrated with Colonel Marshall's slow progress on the project and believed it needed stronger leadership. He met with Harvey Bundy and Generals Marshall, Somervell, and Styer to voice his concerns. Bush suggested placing the project under a senior policy committee and recommended that a respected officer, ideally Styer, take charge as director.
General Leslie Groves was selected for the role by Somervell and Styer, with General Marshall ensuring his promotion to brigadier general, believing the title "general" would carry more weight with the academic scientists involved. Groves assumed command of the Manhattan Project on 23 September 1942, establishing his headquarters in the New War Department Building in Washington, D.C., where Colonel Marshall had his liaison office. That same day, he attended a meeting convened by Stimson that formed a Military Policy Committee answerable to the Top Policy Group. The committee consisted of Bush, Conant as an alternate, Styer, and Rear Admiral William R. Purnell. Later, Tolman and Conant were appointed as Groves’ scientific advisers.
When Groves approached Donald Nelson, the chairman of the War Production Board, on 19 September, he asked for the power to assign a AAA rating whenever needed. Nelson resisted at first but gave in after Groves threatened to go straight to the president. Groves assured him he would only use the rating when truly necessary. Soon, it became clear that the AAA rating was excessive for regular project needs, while the AA-3 rating wasn’t enough. After persistent effort, Groves secured AA-1 authority on 1 July 1944. As Groves later said, “In Washington you became aware of the importance of top priority. Most everything proposed in the Roosevelt administration would have top priority. That would last for about a week or two and then something else would get top priority.”
One of Groves' early challenges was finding a director for Project Y, the team responsible for designing and building the bomb. The obvious candidates were Urey, Lawrence, or Arthur Compton, but they couldn’t be spared. Compton recommended Oppenheimer, who already understood the bomb design concepts. However, Oppenheimer had little administrative experience and no Nobel Prize, which many felt was necessary for leading such a vital lab. There were also concerns about his security clearance due to associates with communist ties. After a long conversation in October 1942, Groves and Nichols agreed Oppenheimer understood what it would take to run a remote laboratory and decided to appoint him. Groves personally cleared him despite the objections.
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The British and Americans shared nuclear information early on, but didn’t join forces at first. In 1940 and 1941, the British project known as Tube Alloys was bigger and further along. British leaders initially turned down an offer from Bush and Conant in August 1941 to combine their efforts. The British had made progress early in the war, but lacked the resources to keep going while dedicating most of their economy to fighting. Tube Alloys soon fell behind its American counterpart. By January 1943, Conant told the British they would no longer get atomic information unless it was in specific areas. The British considered going it alone, but decided they couldn’t build a weapon in time to affect the war in Europe.
By March 1943, Conant believed James Chadwick and a few other British scientists were important enough that the Los Alamos bomb design team should include them, even though it risked revealing sensitive weapon details. In August 1943, Churchill and Roosevelt agreed on the Quebec Agreement, which created the Combined Policy Committee to coordinate efforts between the US and UK. Canada wasn’t a signatory, but was given a representative on the committee due to its contributions. Later, in late September 1944, Roosevelt and Churchill signed the Hyde Park Aide-Mémoire, extending the Quebec Agreement into the postwar period. It suggested that “when a 'bomb' is finally available, it might perhaps, after mature consideration, be used against the Japanese, who should be warned that this bombardment will be repeated until they surrender.”
When cooperation restarted after the Quebec Agreement, the Americans' progress and spending surprised the British. Chadwick pushed for full British participation in the Manhattan Project and gave up on an independent effort during the war. With Churchill's support, he made sure every request from Groves for help was met. The British Mission arrived in the U.S. in December 1943, including Niels Bohr, Otto Frisch, Klaus Fuchs, Rudolf Peierls, and Ernest Titterton. More scientists joined in early 1944. Those working on gaseous diffusion left by fall 1944, but thirty-five under Oliphant with Lawrence at Berkeley were assigned to existing lab groups and stayed until the war ended. The nineteen sent to Los Alamos also joined existing teams, mainly focused on implosion and bomb assembly, not plutonium work. The Quebec Agreement said nuclear weapons wouldn't be used against another country without mutual consent from the U.S. and UK. In June 1945, Wilson agreed that the decision to bomb Japan would be recorded as a Combined Policy Committee action.
In June 1944, the Combined Policy Committee formed the Combined Development Trust, with General Groves as its chairman, to buy uranium and thorium ores from international sources. The Belgian Congo and Canada had most of the world’s uranium outside of Eastern Europe, and the Belgian Government in Exile was based in London. Britain agreed to share most of the Belgian ore with the United States, since it couldn’t use much of the supply without American research. That year, the Trust bought 3,440,000 pounds of uranium oxide ore from mines in the Belgian Congo. To avoid informing US Secretary of the Treasury Henry Morgenthau Jr., the Trust used a special account free from normal auditing and controls. Between 1944 and his resignation in 1947, Groves deposited $37.5 million into that account.
General Leslie Groves once described the British scientists' work on the Manhattan Project as "helpful but not vital," yet he also noted that without Britain—especially the push led by Winston Churchill—there almost certainly would have been no atomic bomb dropped on Hiroshima. Their involvement during the war proved essential for the development of their own nuclear weapons program after the McMahon Act of 1946 cut off American cooperation.
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The Manhattan Project needed uranium, which served as fuel for reactors, feedstock turned into plutonium, and the enriched form used in the atomic bomb itself. In 1940, there were four major uranium deposits: one each in Colorado, northern Canada, Joachimsthal in Czechoslovakia, and the Belgian Congo. All but Joachimsthal were under Allied control. A survey in 1942 confirmed enough uranium existed to meet project needs. Nichols worked with the State Department to place export controls on uranium oxide and arranged to buy 1,200 short tons of ore from the Belgian Congo, stored on Staten Island, and the rest from the Congo. He also negotiated with Eldorado Gold Mines for ore from its refinery in Port Hope, Ontario. The Canadian government later acquired a controlling interest in that company.
The ore from the Belgian Congo had the highest uranium content of all sources. American and British leaders saw controlling global uranium supplies as vital, so they moved to secure it. The Shinkolobwe mine was flooded and shut down, and Nichols tried but failed to convince Edgar Sengier, who ran the Union Minière du Haut-Katanga, to reopen it and sell all future output to the U.S. The issue was handled by the Combined Policy Committee. With British interests holding 30 percent of Union Minière's stock, the British led the talks. Sir John Anderson and Ambassador John Winant reached an agreement in May 1944 with Sengier and the Belgian government: the mine would reopen, and 1,720 short tons of ore would be bought at $1.45 per pound. To reduce reliance on the British and Canadians, Groves also arranged to buy the U.S. Vanadium Corporation's stockpile in Uravan, Colorado.
The raw ore went through a series of chemical steps, first dissolving in nitric acid to form uranyl nitrate, then becoming uranium trioxide, and finally reduced into uranium dioxide. By July 1942, Mallinckrodt was making a ton of this pure oxide every day. But turning it into metal proved harder than expected. The process moved too slowly and the quality wasn’t good enough. So a team from the Metallurgical Laboratory set up shop at Iowa State College in Ames, Iowa, led by Frank Spedding. That group became known as the Ames Project, and their method, called the Ames process, started working in 1943.
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At the University of California Radiation Laboratory, scientists worked on electromagnetic isotope separation, using machines called calutrons. The name came from California, university, and cyclotron. In this process, magnetic fields separated charged particles based on their mass. It wasn’t scientifically neat or easy to build, and it needed more materials, workers, and money than other methods like gaseous diffusion or nuclear reactors. Still, it was chosen because the technology was already proven, which meant less risk. Plus, it could be built in parts and reached full industrial output quickly.
Marshall and Nichols determined that the electromagnetic isotope separation process required 5,000 short tons of copper, a resource in critically short supply. They realized silver could serve as a substitute, using an 11:10 ratio of copper to silver. Nichols approached Under Secretary of the Treasury Daniel W. Bell on August 3, 1942, requesting 6,000 tons of silver bullion from the West Point Bullion Depository. Ultimately, 14,700 short tons of silver were allocated for the project. Each silver bar weighed 1,000 troy ounces and was formed into cylindrical billets, then drawn into strips and wound onto magnetic coils.
In June 1942, Stone & Webster was given the job of designing and building the electromagnetic separation plant, later called Y-12. The plan included five Alpha racetracks for initial processing and two Beta units for final work, with construction starting in February 1943. By January 1944, the second Alpha I was running, and by March, the first Beta and first and third Alpha I’s were online. The fourth Alpha I came on line in April, and by October 1944, the four additional Alpha II racetracks were finished. Tennessee Eastman was hired to manage Y-12, and trained operators known as the Calutron Girls took over the calutrons.
The calutrons initially boosted the uranium-235 content to 13% or 15%, sending the first few hundred grams to Los Alamos in March 1944. Only about 1 part in 5,825 of the original uranium feed became usable product. Much was lost during processing, splattered across machinery. By January 1945, recovery efforts had improved output to 10% of the uranium-235 feed. In February, the Alpha racetracks started getting slightly enriched (1.4%) material from the new S-50 thermal diffusion plant. The next month they received even more enriched (5%) feed from the K-25 gaseous diffusion plant. By August, K-25 was producing enough enriched uranium to feed directly into the Beta tracks.
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At Columbia University, a team including Harold Urey, Karl P. Cohen, and John R. Dunning worked on research into gaseous diffusion, a method that stood out as both the most promising and the most difficult for separating isotopes. This technique relied on Graham’s law, which explains that the speed of a gas escaping through a membrane is linked to its molecular weight—lighter molecules escape faster. The plan was to arrange these selective chambers in a series, or cascade, where each step would gradually increase the concentration of the desired isotope.
In November 1942, the Military Policy Committee approved plans for a 600-stage gaseous diffusion plant, and on December 14 M. W. Kellogg accepted the contract to build it, codenamed K-25. A separate company, Kellex, was created for the work. The process required uranium hexafluoride, a highly corrosive gas with no viable substitute, and all equipment had to be sealed in vacuum and surrounded by inert gas. The main challenge was developing a barrier that was strong, porous, and resistant to corrosion. Edward Adler and Edward Norris designed a mesh barrier from electroplated nickel, but a six-stage pilot plant at Columbia revealed the prototype was too brittle. A competing version was later developed by Kellex, Bell Telephone Laboratories, and the Bakelite Corporation using powdered nickel. In January 1944, Groves ordered the Kellex design into production.
Kellex’s design for K-25 called for a massive U-shaped structure, four stories tall and half a mile long, containing 54 buildings split into nine sections with cells of six stages each. Construction began in May 1943 on the 500-acre site, and work on the main building started in October. By April 17, 1944, the six-stage pilot plant was ready. In 1945, General Groves canceled the upper stages and ordered Kellex to build a 540-stage side feed unit instead—K-27. That unit was turned over to Union Carbide and Carbon in September 1945. The total cost, including K-27, reached $480 million.
The production plant began operating in February 1945, and as each cascade came online, the output improved. By April, K-25 had reached 1.1% enrichment, and the S-50 thermal diffusion plant started feeding into the process. The following month, some of the product hit nearly 7% enrichment. In August, the final one of the 2,892 stages began operation. Later, in the early postwar years, K-25 and K-27 reached their full capacity, becoming models for the next generation of plants.
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The thermal diffusion process came from a theory by Sydney Chapman and David Enskog, explaining how mixed gases separate under temperature differences, with heavier atoms moving toward the cold end and lighter ones to the warm. U.S. Navy scientists developed it, but it wasn’t chosen early on for the Manhattan Project, partly because of doubts about whether it could work and due to rivalry between the Army and Navy. Research continued at the Naval Research Laboratory under Philip Abelson’s direction. It wasn’t connected to the Manhattan Project again until April 1944, when Captain William S. Parsons, in charge of ordnance at Los Alamos, told Oppenheimer about promising results. Oppenheimer passed the news to Groves, who approved building a thermal plant on June 24, 1944.
Groves hired the H. K. Ferguson Company of Cleveland to build the thermal diffusion plant known as S-50. The facility was designed with 2,142 columns standing 48 feet tall, organized into 21 racks. Each column had three pipes: an inner nickel tube carrying steam at 100 pounds per square inch and 545 degrees Fahrenheit, an outer iron pipe with water flowing upward at 155 degrees, and a middle copper pipe where uranium hexafluoride moved. Separation of isotopes happened between the nickel and copper pipes. Construction started on July 9, 1944, and S-50 began limited operations in September. Though leaks caused repeated shutdowns through the following months, by June 1945 the plant had produced over twelve thousand pounds of slightly enriched material.
By March 1945, all 21 production racks were running at full capacity. At first, the output from the S-50 thermal diffusion process went into Y-12, but starting that month, all three enrichment methods worked in series. The S-50 stage boosted uranium from 0.71% to 0.89% uranium-235. That material then moved to the K-25 plant, where the gaseous diffusion process raised it to about 23%. Next, Y-12 increased the concentration to roughly 89%, which was enough for nuclear weapons. By July 1945, about 50 kilograms of this highly enriched uranium reached Los Alamos. That entire amount, along with some 50%-enriched uranium averaging around 85% purity, was used in the first Little Boy bomb.
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The reactor at Oak Ridge went with an air-cooled design to speed up construction, but that wouldn’t work for the bigger production reactors. The Metallurgical Laboratory and DuPont first tried using helium for cooling, but then decided water cooling was simpler, cheaper, and faster to build. The water-cooled design didn’t show up until October 4, 1943. Until then, Matthias focused on improving the Hanford Site—putting up housing, fixing roads, building a railway switch line, and upgrading electricity, water, and phone systems.
At Hanford, the work of canning uranium slugs began in March 1944, just like it had at Oak Ridge. The process involved pickling the slugs to clean them, then dipping them in molten bronze, tin, and an aluminum-silicon alloy. They were then placed in hydraulic presses, capped with arc welding under argon gas, and tested for flaws. Initially, most of the canned slugs failed these tests, producing only a few good ones each day. But by June 1944, progress had been made, and there seemed to be enough合格的 slugs to begin assembling Reactor B on schedule in August 1944.
Work started on Reactor B on October 10, 1943, making it the first of six planned 250 MW reactors. The reactor sites were labeled A through F, with B, D, and F built first to maximize distance between them. Only those three were completed during the Manhattan Project. Reactor B rose 120 feet high, constructed from steel, concrete, blocks, and bricks. Some 390 short tons of steel went into its construction, along with 17,400 cubic yards of concrete, 50,000 concrete blocks, and 71,000 concrete bricks.
Construction of the reactor began in February 1944. Compton, Matthias, Crawford Greenewalt from DuPont, Leona Woods, and Fermi watched as the first slug was inserted. On 13 September 1944, the reactor was powered up; 838 tubes were loaded and it went critical. At midnight on 27 September, operators started withdrawing control rods to begin production. For a while, things seemed to go well, but by 3:00 a.m., power began dropping. By 6:30, the reactor had shut down completely. Cooling water was checked for leaks or contamination. The next day it restarted, only to shut down again.
Fermi contacted Chien-Shiung Wu to identify the issue, which she traced to neutron poisoning caused by xenon-135, an isotope with a half-life of 9.2 hours. Fermi, Woods, Donald J. Hughes, and John Archibald Wheeler then determined that the nuclear cross section of xenon-135 was 30,000 times greater than that of uranium. DuPont’s George Graves had changed the original design by adding 504 extra tubes to fill in the corners, deviating from the Metallurgical Laboratory's layout of 1,500 tubes in a circle. What once seemed like overengineering became key—Fermi realized that using all 2,004 tubes would allow the reactor to reach the necessary power level and produce plutonium efficiently. Reactor D began on December 17, 1944, and Reactor F started on February 25, 1945.
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While scientists worked to understand plutonium’s chemical nature, chemists at the Metallurgical Laboratory in 1942 began exploring ways to separate it from uranium. Under Charles M. Cooper, they developed a lanthanum fluoride process that was chosen for the pilot separation plant. A second method, the bismuth phosphate process, was later created by Seaborg and Stanly G. Thomson. Greenewalt preferred the bismuth phosphate process because of lanthanum fluoride’s corrosive effects, and it was selected for the Hanford plants. Once X-10 started producing plutonium, the pilot plant was tested. The first run achieved only 40% efficiency, but within a few months, that rose to 90%.
At Hanford, the 300 area received top priority for its installations, including buildings for testing materials, preparing uranium, and assembling instrumentation. One building held the canning equipment for uranium slugs, and another contained a small test reactor. Despite this high priority, work on the 300 area fell behind schedule because of the complex nature of the facilities and shortages of labor and materials during wartime.
The initial design called for two separation plants in both 200-West and 200-East. That plan was later revised to just two plants—T and U—in 200-West and one at 200-East, labeled B. Each plant had four buildings: a process cell building referred to as the “canyon,” a concentration structure, a purification unit, and a magazine storage area. The canyon buildings were each 800 feet long and 65 feet wide, made up of forty individual cells measuring 17.7 by 13 by 20 feet.
Work started on buildings 221-T and 221-U in January 1944, with the first finished by September and the second by December. The 221-B building came next, completed in March 1945. Due to high radioactivity, workers had to operate the plants remotely using closed-circuit TV, a new technology in 1943. Maintenance used overhead cranes and specially built tools. The smaller 224 buildings processed less material and were less radioactive. Buildings 224-T and 224-U were finished on October 8, 1944, with 224-B completed February 10, 1945. Methods for purifying plutonium in the 231-W plant weren’t known when construction began April 8, 1944, but the plant was done and the method chosen by year’s end. On February 5, 1945, Matthias personally delivered 80 grams of 95%-pure plutonium nitrate to a courier in Los Angeles for transport to Los Alamos.
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In 1943, scientists worked on a gun-type fission weapon using plutonium, named Thin Man. They first studied plutonium-239 made in cyclotrons, which was very pure but only available in tiny amounts. Then, in April 1944, Los Alamos got the first sample from the Clinton X-10 reactor. Within days, Emilio Segrè found a problem: the reactor-bred plutonium had more plutonium-240, causing up to five times more spontaneous fission than the cyclotron version.
The presence of plutonium-240 made plutonium-239 unfit for use in a gun-type weapon. Because plutonium-240 begins the chain reaction too early, it causes a premature explosion—what scientists called predetonation—that scatters the critical mass before much fission can occur, resulting in a weak, ineffective blast. A solution involving a higher-velocity gun was proposed but deemed unworkable. Another idea, separating the isotopes, was also dismissed. That approach was considered too difficult, even more so than isolating uranium-235 from uranium-238, and it was thought that attempting it would delay the development of the weapon indefinitely.
Work on a different kind of bomb design, called implosion, started earlier under the direction of physicist Seth Neddermeyer. This method used explosives to squeeze a subcritical sphere of fissile material into a smaller, denser form. The critical mass was assembled much faster than with the gun-type method. When atoms are packed closer together, neutron capture increases, making it more efficient. Neddermeyer’s investigations in 1943 and early 1944 showed promise but also revealed that an implosion weapon was more complex than the gun-type design. In September 1943, John von Neumann, who had experience with shaped charges, proposed using a spherical configuration instead of the cylindrical one Neddermeyer was working on.
In August 1944, Oppenheimer reorganized the Los Alamos laboratory to focus on the implosion design known as Fat Man. He created two new groups: X Division, led by explosives expert George Kistiakowsky, and G Division under Robert Bacher. The design used explosive lenses to compress the plutonium core into a sphere. Developing those lenses proved slow and difficult, with many explosives tested before settling on composition B and baratol. The final lens configuration looked like a soccer ball, made of 20 hexagonal and 12 pentagonal sections, each weighing about 80 pounds. Each lens needed two reliable detonators, which were supplied by exploding-bridgewire technology invented at Los Alamos by Luis Alvarez’s team.
To understand how shock waves behave when they come together, Robert Serber created an experiment called the RaLa Experiment. It used a rare isotope, lanthanum-140, which gives off strong gamma rays. The radiation came from a source placed at the center of a metal sphere. Around that sphere were explosive lenses, and everything was inside an ionization chamber. This setup let scientists take X-ray movies of the implosion process. The design of the explosive lenses relied heavily on results from these tests. As David Hawkins wrote in his history of the Los Alamos project, "RaLa became the most important single experiment affecting the final bomb design."
Inside the explosives lay an aluminum pusher designed to ease transition from low-density material to the natural uranium tamper. That tamper had two key roles: keeping the critical mass intact as long as possible and bouncing neutrons back into the core, while some uranium would also undergo fission. To guard against premature detonation by external neutrons, a thin coating of boron—a neutron absorber—was applied. A polonium-beryllium initiator, nicknamed an "urchin," was created to spark the chain reaction at the exact moment needed. This work on the chemistry and metallurgy of radioactive polonium was led by Charles Allen Thomas of the Monsanto Company, who oversaw what came to be called the Dayton Project. Testing required as much as 500 curies of polonium per month, a supply Monsanto was able to provide. The entire assembly was then wrapped in a duralumin casing, meant to shield it from bullets and flak.
The metallurgists faced a major challenge in figuring out how to shape plutonium into a sphere. Initial attempts to measure its density produced unreliable results, leading to early suspicions of contamination. It was soon discovered that plutonium has several allotropes—different structural forms. The brittle α phase found at room temperature transforms into a plastic β phase at higher temperatures, and an even more malleable δ phase exists between 300 and 450 degrees Celsius. When alloyed with aluminum, the δ phase became stable at room temperature, but aluminum’s neutron emission posed a risk of pre-ignition. Eventually, they settled on a plutonium-gallium alloy that stabilized the δ phase and allowed for hot pressing into the right shape. Because plutonium corrodes easily, the final sphere was coated with nickel.
Working with plutonium was extremely hazardous. By 1945, half the chemists and metallurgists had to be removed from their duties because of dangerously high levels of the element found in their urine. A small fire at Los Alamos in January 1945 raised fears that a larger blaze in the plutonium lab could spread to the entire town. As a result, General Groves ordered the building of a new facility dedicated to plutonium chemistry and metallurgy. That site came to be called the DP-site. The work carried serious risks.
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Because the design of an implosion-style weapon was so complex, they concluded that a full-scale nuclear test was needed, even though it would waste fissile material. Oppenheimer named the test "Trinity." In March 1944, Kenneth Bainbridge was put in charge of planning, and he selected the Alamogordo Bombing Range for the site. A base camp was set up there, including living quarters, storage buildings, work areas, a place to store explosives, and a supply depot. On May 7, 1945, a calibration explosion took place to prepare the instruments. A wooden platform was constructed 800 yards from where the actual test would occur, and it was loaded with around 100 short tons of high explosives mixed with nuclear fission products.
Groves was determined to avoid any political fallout from a failed test, so he ordered the construction of a massive recovery container called “Jumbo” to catch plutonium if the experiment went wrong. Built from more than 200 tons of steel and iron, Jumbo was designed to capture the material in case of a fizzled explosion. But by the time it arrived, confidence in the implosion method had grown strong enough, and there was enough plutonium available that Oppenheimer decided not to use it. Instead, they positioned Jumbo on a tower 800 yards from the bomb site, using it as a rough gauge of the blast's power. The tower collapsed, but Jumbo remained intact—supporting the idea that it would have worked if needed.
For the test, the weapon known as “the gadget” was raised to the top of a 100-foot steel tower. This height was chosen so scientists could better understand how the weapon would perform when dropped from a bomber. Detonating the gadget in the air would direct more energy toward the target and reduce nuclear fallout. The gadget was put together under Norris Bradbury’s supervision at the McDonald Ranch House on 13 July, then carefully lifted up the tower the next day.
At 05:30 on 16 July 1945, the gadget detonated with the force of roughly 20 kilotons of TNT, carving a crater in the desert that measured 250 feet across. The blast created a mushroom cloud reaching seven and a half miles high, and the shock wave was felt over a hundred miles away. The explosion was audible even in El Paso, Texas. To cover up what had happened, military officials spread a story that an ammunition magazine at Alamogordo Field had exploded, involving gas shells.
Oppenheimer later said that as he watched the test explosion, a line from a sacred Indian text came to his mind. He remembered a verse from the Bhagavad Gita, specifically XI,12. That moment, at the Trinity test, left him with that particular quotation from the Hindu holy book.
The test had gone better than anyone expected. The news was sent right away to Secretary Stimson, who was then at the Potsdam Conference. Groves rushed to prepare a full report and sent it by courier. President Truman received the message and was deeply affected. Stimson wrote in his diary about sharing the news with Churchill. Churchill said, “Now I know what happened to Truman yesterday. I couldn't understand it. When he got to the meeting after having read this report, he was a changed man. He told the Russians just where they got on and off and generally bossed the whole meeting.”
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In June 1944, the Manhattan Project reached its peak workforce of roughly 129,000 people, with 84,500 in construction, 40,500 operating plants, and 1,800 military staff. By the following year, as building activity wound down, the total number of workers fell to 100,000, while the number of military personnel climbed to 5,600. Drawing together enough skilled laborers, especially under pressure from other wartime needs, proved extremely difficult. Over half a million individuals ultimately contributed to the project. African Americans were largely assigned to low-level roles, although some served as scientists or technicians. The project’s special demands also led to a higher representation of women in technical positions than later government initiatives.
In 1943, General Groves secured a special temporary priority for labor from the War Manpower Commission. By March 1944, both the War Production Board and the War Manpower Commission placed the project at the highest priority level. The Kansas commission director reported that from April to July 1944, every qualified applicant in the state who visited a U.S. Employment Service office was encouraged to work at the Hanford Site. No other job offer was made until the applicant explicitly turned down the opportunity. Scientific advisers Tolman and Conant compiled a list of candidate scientists and had them evaluated by those already working on the project. Groves then personally wrote to the heads of their universities or companies, requesting they release these individuals for essential war work.
The Army provided key personnel for the project, especially through the Army Specialized Training Program. In 1943, the Military Entrance Division created the Special Engineer Detachment, with 675 authorized personnel. Men drafted into the Army were assigned to this unit. The Women’s Army Corps also contributed, initially for clerical work with classified material, but later taking on technical and scientific roles. By February 1, 1945, all military personnel under the MED, including SED units, were reassigned to the 9812th Technical Service Unit—except at Los Alamos, where other military personnel, like WACs and Military Police, remained under the 4817th Service Command Unit.
Colonel Stafford L. Warren, who had been an associate professor of Radiology at the University of Rochester School of Medicine, was commissioned into the United States Army Medical Corps and appointed chief of the Manhattan Engineer District’s Medical Section as well as Groves’ medical advisor. His responsibilities included staffing hospitals at Oak Ridge, Richland, and Los Alamos, while the Medical Section managed both research and health and safety programs. Workers dealt with toxic chemicals, high-pressure gases and liquids, dangerous voltages, explosives, and the unknown risks of radioactivity and fissile materials. Despite these hazards, the National Safety Council honored the Manhattan Project in December 1945 with its Award of Honor for Distinguished Service to Safety. From January 1943 through June 1945, there were 62 fatalities and 3,879 disabling injuries—roughly 62 percent fewer than the rate in private industry.
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The Manhattan Project was ordered to operate under "absolute secrecy" by Roosevelt, which meant no one could know about the project's existence. Keeping that secret was incredibly difficult because people already understood nuclear fission, many individuals were involved, and the facilities were enormous. General Groves implemented a strict need-to-know policy as part of an extreme system of compartmentalization.
The way I saw it, keeping knowledge locked away in compartments was the core of how you kept things secure. My rule was clear and simple—every person should know only what they needed to do their job, nothing more. Following that rule not only kept things safe but also made everyone more efficient, since they stayed focused on their own part of the work. It also made it obvious that this project wasn’t about letting people satisfy their curiosity or expand their science knowledge. It was about reaching a single, specific goal.
The need for secrecy clashed with the norms of many scientists, who argued that science couldn’t function under such strict requirements. Officials in charge of the Manhattan Project also struggled with journalists, Congressmen, federal workers not in the loop, local residents, judges handling land disputes, and anyone else who might spread rumors or leak information. There were real fears about espionage and sabotage. General Groves turned to the FBI and his own G-2 intelligence unit to look into possible security breaches. Over 1,500 cases involving “loose talk” were investigated during the war.
Byron Price, who led the Office of Censorship, later called the Manhattan Project "the best-kept secret of the war." In 1945, Life magazine estimated that fewer than a few dozen people in the country understood what the project truly meant, and perhaps only about a thousand others knew that atomic work was involved. More than 100,000 people worked on it, but they were told nothing of the purpose behind their efforts, working like moles in the dark. Those who learned even a little faced punishment of up to ten years in prison or a fine of $10,000—equivalent to about $179,000 today. They monitored dials and switches while behind thick concrete walls, watching mysterious reactions take place without knowing why.
In December 1945, the US Army released a secret report evaluating the security around the Manhattan Project. The report said that the project was “more drastically guarded than any other highly secret war development.” To keep things secure, the team checked 400,000 potential employees and 600 companies for risks during the early days of the project in 1943.
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While the Manhattan Project focused on building the atomic bomb, it also had to track the German nuclear program. At first, there was concern that Germany was close to developing its own weapons, unlike Japan, which lacked access to uranium ore. To counter this, the Project organized a bombing and sabotage mission against heavy water facilities in Nazi-occupied Norway. A special unit was formed, jointly staffed by the Office of Naval Intelligence, OSRD, the Manhattan Project, and Army Intelligence (G-2). Major General George V. Strong, who led Army Intelligence, appointed Boris Pash to command this effort, which was code-named "Alsos," meaning "grove" in Greek. Samuel Goudsmit served as the scientific director of the Alsos mission.
After the Allies captured Rome in June 1944, the Alsos Mission questioned staff at the University of Rome’s physics labs. In London, Pash helped form a joint British and American Alsos team led by Captain Horace K. Calvert, preparing for Operation Overlord. General Groves worried that the Germans might try to sabotage the Normandy landings using radioactive poisons, so he warned General Eisenhower and sent an officer to brief Lieutenant General Walter Bedell Smith. Under the code name Operation Peppermint, special equipment was readied and Chemical Warfare Service teams were trained in its use.
Following the advance of Allied forces, the Alsos team moved into liberated areas of France and Germany, questioning scientists and inspecting facilities to uncover what they knew about the German nuclear program. By November 1944, Goudsmit had concluded that the Germans had never progressed beyond laboratory work. As he later explained: "The evidence at hand proved definitely that Germany had no atom bomb and was not likely to have one in any reasonable time."
Interrogation of German prisoners revealed that uranium and thorium were being processed in Oranienburg, so General Groves ordered it bombed on 15 March 1945 to prevent Soviet capture. An Alsos team later went to Stassfurt in the Soviet Occupation Zone and recovered 11 tons of ore from WIFO. In April 1945, Pash, who was in command of a force called T-Force, led Operation Harborage—a mission behind enemy lines in Hechingen, Bisingen, and Haigerloch, areas central to the German nuclear program. T-Force seized nuclear laboratories, documents, equipment, supplies, heavy water, and 1.5 tons of metallic uranium.
The Alsos teams captured several German scientists, including Kurt Diebner and Otto Hahn, along with Werner Heisenberg, Walther Gerlach, and Carl Friedrich von Weizsäcker. These men were taken to England and held at a house called Farm Hall, where they were secretly watched and monitored.
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The only Allied plane that could carry the large atomic bombs was the British Avro Lancaster, but using it would have caused maintenance problems. General Groves wanted to modify the American B-29 Superfortress to carry the Thin Man bomb by combining its two bomb bays. That plan became unnecessary when the Thin Man design was dropped, since the Little Boy bomb fit into a single B-29 bay, though modifications were still needed. In November 1943, the Army Air Forces Materiel Command at Wright Field, Ohio, began a project called Silverplate to make those changes. Test drops were done at Muroc Army Air Field and the Naval Ordnance Test Station in California using pumpkin bombs shaped like the Thin Man and Fat Man to check their flight behavior, fuzing, and stability.
The 509th Composite Group was formed on December 17, 1944, at Wendover Army Air Field in Utah, with Colonel Paul W. Tibbets in command. Its 393rd Bombardment Squadron trained using Silverplate B-29s, practicing long flights and dropping pumpkin bombs. At Los Alamos, a unit called Project Alberta was created under Parsons to help prepare the bombs. The group moved to North Field on Tinian in July 1945. Most of the components for Little Boy left San Francisco aboard the USS Indianapolis on July 16 and reached Tinian on July 26. The rest, including six enriched uranium rings, were flown in three C-54 Skymasters from the 320th Troop Carrier Squadron. Two Fat Man assemblies arrived in specially modified B-29s, and the first plutonium core went aboard a special C-54.
At end of December 1944, Roosevelt told Groves and Stimson that if bombs were ready before Germany surrendered, they should prepare to drop them on Germany, though Japan was still seen as more likely. In late April 1945, targeting committee picked Kokura, Hiroshima, Niigata, and Kyoto as possible cities. Stimson stepped in and said he would make final call, ruling out Kyoto because of its historical value. Nagasaki replaced it. Then in May 1945, Interim Committee formed to guide nuclear use decisions. It created scientific group including Compton, Fermi, Lawrence, and Oppenheimer, who assessed bomb's physical power plus military and political effects. At meeting on 1 June, committee decided bomb should be used against Japan as soon as possible, on war plant near worker housing, and without warning.
At the Potsdam Conference in Germany, President Harry S. Truman informed Stalin about a new weapon of unusual destructive force, offering no specifics. He believed Stalin didn’t grasp the significance since he showed no particular interest. What Truman didn’t realize was that Soviet spies had already kept Stalin in the loop on both the project and the upcoming test.
On 25 July, a directive from General Thomas T. Handy to General Carl Spaatz received approval from Marshall and Stimson, stating that the “first special bomb” would be used “after about 3 August 1945,” with additional bombs to follow as soon as they were ready. The initial plan called for the first bomb on 2 August, the second on 10 August, and a third around 24 August. However, because of weather conditions and the requirement for visual bombing, the first mission was delayed until 6 August, and the second was moved up to 9 August.
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On August 6, 1945, a Boeing B-29 Superfortress named Enola Gay, flown by Colonel Tibbets, departed North Field loaded with a Little Boy atomic bomb. The mission's main target was Hiroshima, a key military center and port, though Kokura and Nagasaki were also options. A weaponeer named Parsons oversaw the bomb assembly while the plane was in flight to prevent an explosion during a possible crash. The device detonated 1,750 feet above the ground, with energy equal to 13 kilotons of TNT, flattening around 4.7 square miles. Japanese records showed that 69% of the city’s buildings were destroyed and another 6–7% damaged. Early reports listed 66,000 dead and 69,000 injured, but later tallies, which included Korean laborers and soldiers, estimated up to 140,000 fatalities by December 1945.
On the morning of 9 August 1945, the Bockscar lifted off from Tinian with a Fat Man bomb on board, piloted by Major Charles W. Sweeney of the 393d Bombardment Squadron. Weaponeer Ashworth was aboard, and Kokura was the intended target. Cloud cover obscured the city, so after three failed runs and with fuel low, they headed for Nagasaki as the secondary target. A radar approach was planned if visibility was poor, but a break in the clouds allowed a visual drop. The bomb was released over Nagasaki’s industrial valley, between the Mitsubishi Steel and Arms Works and the Mitsubishi-Urakami Ordnance Works. The explosion yielded 21 kilotons of TNT, similar to Trinity, but the blast was contained in the Urakami Valley, sparing much of the city center. About 44% of Nagasaki was destroyed, with casualties estimated between 40,000 and 80,000 killed and at least 60,000 injured.
By the time August 1945 rolled around, General Groves had planned for another atomic bomb to be ready for deployment on the 19th, with three additional ones scheduled for September and three more for October. Two more Fat Man assemblies were readied for shipment from Kirtland Field to Tinian, set to leave on August 11 and 14. Meanwhile, at Los Alamos, technicians pushed through the night to cast a new plutonium core. Even though the casting was finished, the core still needed pressing and coating, which wouldn’t be done until August 16.
On 10 August, Truman was told another bomb was ready. He said no more atomic bombs could be used without his personal approval. According to Henry A. Wallace, Truman told his cabinet the idea of killing 100,000 more people was too terrible. He didn’t want to kill “all those kids,” as he put it. Later, on 13 August, Groves stopped the shipment of the third core.
On 11 August, Groves called Warren with instructions to put together a team to assess the damage and radioactivity in Hiroshima and Nagasaki once the war was over. A group equipped with Geiger counters reached Hiroshima on 8 September, led by Farrell and Warren, accompanied by Japanese Rear Admiral Masao Tsuzuki, who served as translator. They stayed in Hiroshima until 14 September, then moved on to Nagasaki, where they began surveying on 19 September and continued until 8 October. These scientific missions provided key information about the atomic bomb’s effects and led to the creation of the Atomic Bomb Casualty Commission.
In the wake of the bombings, General Groves had asked physicist Henry DeWolf Smyth to create a simplified account of the project for the public. The plan to share this information openly was debated, but President Truman made the final call. The report was titled the "Smyth Report," and it went public on August 12, 1945.
Japan surrendered on August 15, sparking debate among historians about whether the bombings were necessary. Some argued that "atomic diplomacy" might have achieved the same result without using the bombs. The role of the Soviet Union's entry into the war also came into question, as did how much each factor influenced Japan’s decision to give up. The Franck Report proposed a public demonstration instead, but the Interim Committee’s scientific group rejected it. In July 1945, the Szilárd petition was signed by several scientists from the Manhattan Project, attempting to alert President Truman to the weight of his decision in deploying such weapons.
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After the war, the Manhattan Project became widely known, credited with ending the conflict, and its contractors were finally recognized. General Groves and Lieutenant Colonel Nichols awarded the Army–Navy "E" Awards, and more than twenty Presidential Medals for Merit went to key figures like Bush and Oppenheimer. Military personnel received the Legion of Merit. The project continued until December 31, 1946, and the Manhattan District lasted until August 15, 1947, though it faced ongoing issues from technical setbacks, rapid demobilization, and unclear long-term goals.
At Hanford, plutonium production dropped as Reactors B, D, and F reached the end of their life, damaged by fission products and the Wigner effect in the graphite moderator. The swelling ruined the charging tubes used to irradiate uranium, making them unusable. Reactor B pile was shut down to keep at least one unit running. Meanwhile, DuPont and the Metallurgical Laboratory worked on a new plutonium extraction method using redox solvent extraction, as an alternative to the bismuth phosphate process that left uranium difficult to recover. Bomb engineering moved from Wendover Field to Oxnard Field, New Mexico, in September 1945, starting what would become Sandia Base. Kirtland Field nearby served as a B-29 base for testing aircraft compatibility and drop trials. As reservist officers were released, about fifty regular officers were chosen to replace them.
After the war, work at Y-12 plant wound down as Nichols recommended shutting down the S-50 and Alpha tracks in September. Though the Alpha tracks had been performing well, they couldn't compete with K-25 and the newly operational K-27, which began in January 1946. By December, the entire Y-12 plant closed, reducing the Tennessee Eastman payroll from 8,600 to 1,500 and saving two million dollars a month. At Los Alamos, the situation was more difficult, with many talented people leaving. There was still much work to do on the bombs used in Hiroshima and Nagasaki—making them simpler, safer, and more reliable. Implosion methods needed development for uranium, replacing the gun method, and new composite cores were required now that plutonium was scarce. Uncertainty about the lab's future made it hard to keep people. Oppenheimer returned to UC Berkeley, while Groves appointed Norris Bradbury as interim leader; Bradbury stayed in the role for twenty-five years. To ease dissatisfaction from lacking amenities, Groves launched a construction program including a new water supply, three hundred houses, and recreation facilities.
After the war, Manhattan Project staff took part in Operation Crossroads, a pair of nuclear tests at Bikini Atoll in July 1946. Two bombs similar to the Fat Man design were exploded—one as an airburst, one underwater—to study how nuclear weapons affected naval ships. The tests drew international attention, with press and observers from around the world in attendance. Back home, there was debate over how to manage the atomic program after the war. That led to the Atomic Energy Act of 1946, which formed the United States Atomic Energy Commission to oversee civilian control of atomic development. The military retained responsibility through the Armed Forces Special Weapons Project, or AFSWP.
After the bombs were dropped on Hiroshima and Nagasaki, some of the Manhattan Project physicists came together to form the Bulletin of the Atomic Scientists in 1945 and the Emergency Committee of Atomic Scientists in 1946. These groups started as urgent efforts to educate people about atomic weapons. Several project members, including Bohr, Bush, and Conant, believed it was necessary to reach an international agreement on controlling nuclear research and atomic weapons. In June 1946, the Baruch Plan was presented to the newly formed United Nations Atomic Energy Commission (UNAEC). It proposed creating an international atomic development authority, but the plan was not adopted.
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The Manhattan Project cost $1.845 billion by October 1, 1945, a fraction of wartime spending, and rose to $2.191 billion when the Atomic Energy Commission took over on January 1, 1947. The total budget was $2.4 billion. By the end of 1945, most of that money had gone toward building facilities at Oak Ridge and Hanford. At both sites, the biggest expenses were construction—74% at Oak Ridge and 87% at Hanford—with the rest covering operations.
The Manhattan Project was initially funded through the OSRD’s general budget, but as plans developed to transfer the work to the Corps of Engineers, Bush told Roosevelt in late 1942 that “it would be ruinous to the essential secrecy to have to defend before an appropriations committee any request for funds for this project.” As a result, early financing came from discretionary sources that Roosevelt controlled directly. Congress remained unaware of the project due to concerns that lawmakers might leak information and because it was feared the project would seem like a waste of money. Funds were quietly included in other legislation, but as expenses increased and the massive facilities appeared to yield nothing, oversight grew. The Truman Committee attempted to audit the project multiple times, yet each effort was turned down.
In the spring of 1944, Bush, Groves, and Stimson agreed that a small group of high-ranking Congressmen should be told about the project. By March 1945, seven had been officially informed. The funds were buried in appropriation requests with unremarkable titles such as “Engineer Service Army” and “Expediting Production.” Then, in late May 1945, to secure Albert J. Engel’s cooperation—after he threatened to reveal the project if he wasn’t told more—five more Congressmen were allowed to visit Oak Ridge. There they inspected the living accommodations and observed the size of the installations, with some of the project's complexities demonstrated to them.
The Manhattan Project cost about $500 million per bomb on average, though it actually made three used bombs—Trinity, Little Boy, and Fat Man—as well as one extra Fat Man that wasn’t needed. By the end of 1945, the project’s total spending reached about 90% of what the U.S. spent on small arms production during the war, and 34% of the money spent on tanks. It was the second most expensive weapons effort the U.S. undertook during World War II, only behind the Boeing B-29 Superfortress.
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William Laurence of The New York Times, who first used the phrase "Atomic Age," was named the official correspondent for the Manhattan Project in spring 1945. He saw both the Trinity test and the bombing of Nagasaki, writing the official press releases for each event. Afterward, he published a series of articles that praised the new weapon. His reporting brought widespread attention to the possibilities of nuclear technology and played a role in pushing its development forward in both the United States and the Soviet Union.
The Manhattan Project created a lasting network of national laboratories, including Lawrence Berkeley, Los Alamos, Oak Ridge, Argonne, and Ames. Two more were later established by Groves: Brookhaven in New York and Sandia in New Mexico. These labs would lead the way in what Alvin Weinberg, director of Oak Ridge, called Big Science. The project also shaped computational science, with Los Alamos operating one of the most advanced tabulating machine facilities in the world.
In May 1946, Chief of Naval Operations Nimitz decided the Navy would work with the Manhattan Project rather than pursue its own nuclear initiative. A group of naval officers, including Captain Hyman G. Rickover, was sent to Oak Ridge, where Rickover became assistant director and helped establish the groundwork for a nuclear-powered navy. At around the same time, in September 1946, another team from the Air Force arrived at Oak Ridge with plans to develop nuclear aircraft. That project encountered serious technical challenges and was eventually canceled.
Starting in mid-1946, Oak Ridge began sending radioisotopes to hospitals and universities, using iodine-131 and phosphorus-32 primarily for cancer diagnosis and treatment. These radioactive materials sparked a revolution in nuclear medicine, and isotopes also found use in biological, industrial, and agricultural research.
The Manhattan Project’s production sites left behind a lasting legacy of environmental harm. At Hanford, radioactive and corrosive wastes were stored in underground tanks that were meant to be temporary. These single-shell, steel-lined tanks were never properly maintained and eventually leaked. As a result, Hanford became one of the most contaminated nuclear waste sites in North America. It required major cleanup efforts once it was deactivated.
When handing over control to the Atomic Energy Commission, General Leslie Groves said farewell to the men and women who had worked on the Manhattan Project. He thanked them for their dedication and sacrifice during the effort to build the atomic bomb. The project had brought together scientists, engineers, and countless others at sites like Los Alamos and Trinity. Their work would change the world forever, and Groves recognized the weight of what they had accomplished.
Five years ago, the idea of Atomic Power was only a dream. You have made that dream a reality. You have seized upon the most nebulous of ideas and translated them into actualities. You have built cities where none were known before. You have constructed industrial plants of a magnitude and to a precision heretofore deemed impossible. You built the weapon which ended the War and thereby saved countless American lives. With regard to peacetime applications, you have raised the curtain on vistas of a new world.
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