The Manhattan Project - Statistics & Facts

Background & other nuclear programs

With the discovery of nuclear fission by Otto Hahn and his team in Berlin in 1938, nuclear weapons became a realistic prospect. Nuclear fission is where individual neutrons are fired at an atom’s nucleus, causing it to destabilize and split into two smaller atoms, while also releasing a large amount of energy. When it was realized that additional neutrons were released in this process, it was believed that a nuclear chain reaction could occur, releasing an even greater amount of energy. These findings highlighted the destructive potential of nuclear fission, and its military uses were then investigated.

The UK (with Canada) was the first major power to launch a nuclear weapons program, codenamed Tube Alloys. This was initially the largest and most advanced of its kind, but the demands of the war effort and proximity to the conflict then placed great strain on the resources and safety of the program – it was eventually absorbed by the Manhattan Project in August 1943. Germany was at the forefront of nuclear innovation in the 1930s, and a weapons program was quickly established following the discovery of fission. However, the reality was that Germany’s program (the “Uranium Club”) never threatened the Allies, as many top scientists had either fled or were conscripted, the production of nuclear weapons was not believed feasible until 1945, and Germany prioritized the development of conventional weapons as it sought to end the war quickly. Elsewhere, the Japanese nuclear program made even less progress than the Germans, while Soviet resources were so stretched that their nuclear program was primarily an espionage mission against the U.S.. Ultimately, the United States was the only major power in a position to develop nuclear weapons during the war.

Launch of the Manhattan Project

In 1939, almost three years before the attack on Pearl Harbor, members of the scientific community, including figures such as Albert Einstein, Enrico Fermi, and Leo Szilard who had fled anti-Jewish persecution in Axis countries, contacted the military to advocate the need for a nuclear weapons program. The Einstein-Szilard letter to Roosevelt was the most famous of these, convincing the President to approve nuclear research, which was then funded by the Navy. Einstein wrote about the importance of uranium in this letter, but believed the amount needed for a bomb would be so large it would have to be delivered by sea; in 1940, British scientists shared their findings that the amount needed was much lower than previously thought, meaning nuclear weapons could be dropped from the air. As knowledge increased and the threat of war grew larger, President Roosevelt approved the founding of an atomic weapons program in October 1941, transferred the project’s control from the Navy to the Army Corps of Engineers (USACE), and pushed for greater coordination with the British.

Following U.S. entry into the war, scientists from across the country convened to discuss the project's objectives, highlighting the need to produce high quantities of uranium and, potentially, plutonium and then weaponize these materials. The project was officially launched with the founding of the project’s USACE headquarters in Manhattan on August 13, 1942 with Brigadier General Leslie Groves placed in charge in September. The Army’s largest role was coordinating the construction of several sites across the U.S. to facilitate this research and produce the relevant materials – the construction and staffing of these was where the majority of the project’s funds were eventually allocated, with R&D and operational costs taking up a relatively small portion of the budget. Groves also oversaw intelligence and military operations related to the project, including a successful bombing raid on the head offices of Germany’s nuclear program.

Uranium

Almost two thirds of the project’s funding was directed toward the Oak Ridge facility in Tennessee: the site where uranium was enriched to weapons grade. This process is necessary as only the isotope uranium-235 is fissionable, but ²³⁵U makes up just 0.72 percent of all naturally occurring uranium and must be separated from the more abundant and stable ²³⁸U. Separation was achieved in three different ways: gaseous diffusion (carried out at the K-25 plant), thermal diffusion (at the Clinton Engineer Works), and electromagnetic separation (at the Y-12 plant). Most of the uranium used came from the Shinkolobwe Mine in the Belgian Congo (present-day DRC), which was the richest of the four uranium sources known at the time – over 4,000 tons was shipped to the U.S. for the beginning of the project, with monthly shipments of several hundred tons thereafter.

Oak Ridge itself was chosen for its low population, affordability, and transport links, as well as its inconspicuous setting. It transformed during the war: the population was between 3-4,000 people when construction began in 1942, and the influx of workers saw the population rise to around 75,000 by 1945. The plants built for uranium enrichment were also enormous, and the K-25 plant was the largest building in the world at the time. However, despite its scale, only a small number of personnel were actually aware of the site’s purpose - the majority of Oak Ridge staff did not learn of this until after the war.

Plutonium

Plutonium occurs only in trace amounts in nature, and therefore must be created synthetically from uranium. This had never been achieved before the Manhattan Project, but two of the project's most significant breakthroughs were in 1942, when scientists at Berkeley, led by Glenn T. Seaborg, were the first to successfully isolate and identify plutonium, before Enrico Fermi and his team at Chicago created the world’s first functioning nuclear reactor later that year. With these developments, it was now possible to place uranium-238 in a nuclear reactor and bombard it with neutrons to create uranium-239, which would eventually lose electrons and become plutonium-239.

Following these successes, the USACE bought the Hanford Site in Washington and commissioned DuPont to design and build the reactors, processing plants, and other facilities necessary for the industrial scale production of plutonium. Despite the successful uranium enrichment projects at Oak Ridge, separation of ²³⁸U and ²³⁵U proved difficult and energy-intensive due to their structural similarities, whereas separating plutonium from uranium was much easier. After some interim plutonium production at Oak Ridge, the B Reactor in Hanford began large scale production of weapons-grade plutonium in March 1945.

Making the bombs

The next step of the project was the weaponization of this new technology. This took place at the Los Alamos Lab, New Mexico, under the leadership of J. Robert Oppenheimer. To accelerate the process, provisional calculations and weapons were made before sufficient amounts of uranium and plutonium were fabricated, and these were constantly adapted as new discoveries came to light. The remote location was chosen for the secrecy it afforded those working on the project, and the protocols used still form much of the basis of the classified information system used by the U.S. government today.

Two weapon types were designed: implosion and gun-type. Both were fission weapons that change the critical mass of radioactive material, forcing it to go supercritical and then explode. Implosion weapons achieve this by detonating conventional explosives within the bomb’s casing; the blast is then directed inward from all sides causing a small sphere of plutonium encased in the bomb’s center to compress and go supercritical. In contrast, gun-type weapons work by firing a small piece of material into a larger sphere, so that both subcritical parts will combine to go supercritical. Because of these functions, the implosion type weapon had to be powered by plutonium, as uranium cannot be compressed in the same way, whereas the gun-type weapon only worked with uranium as high levels of plutonium-240 cause a premature explosion with a smaller yield. Their functions also influenced the final design of the bombs, where the implosion bomb was more spherical, while the gun type weapon was longer – this is the source of the ”Fat Man” and “Thin Man” codenames, although Thin Man was later repurposed for use with uranium and renamed “Little Boy”.

In the desert of New Mexico, on July 16, 1945, the world’s first nuclear weapon was detonated. There was no test for a gun-type weapon as researchers were certain it would work, but concerns of a fizzle (partial explosion) and general uncertainty due to the implosion weapon’s complexity led to the “Trinity Test”. The test bomb, known as “The Gadget”, was almost identical to that dropped on Nagasaki, and the significance of the occasion drew the attendance of the project’s top figures. The test's success saw Oppenheimer secure his place in history as “the father of the atomic bomb”, and he later remarked that the explosion drew a mixture of celebration and cries, but mostly silence from observers. He said was reminded of a quote from Hindu scripture “now I am become death, destroyer of worlds” – a quote that has since become synonymous with both the man and the occasion.

Decision to drop the bomb

The Trinity Test was conducted on the earliest possible date, but this was already two months after Germany's surrender. Focus then shifted to the Pacific. As Allied forces pushed towards the Japanese mainland, resistance grew more extreme - civilian populations were used as shields, Japanese soldiers were ordered to fight to the death (famously in suicidal kamikaze and banzai attacks), and the Allies knew that an amphibious invasion of the mainland would see millions of deaths. Despite heavy bombing campaigns, including the firebombing of Tokyo in March 1945 that killed over 80,000 people in one night, Japan still did not surrender. Additionally, the bulk of the Soviet Army was en route to China to join the war against Japan, and the U.S. was keen to end things quickly to avoid ceding influence to the Soviets as it had done in Europe. In lieu of invasions, a blockade, or further bombings, a demonstration of a nuclear explosion in a neutral location was considered to try and convince Japan to surrender, but this idea was abandoned due to logistical complications, and fears that it was a dud or that it spurred the Japanese to fight harder. Ultimately, an atomic attack on a Japanese city was viewed as the most effective way to end the war.

The cities of Hiroshima and Kokura were chosen as the primary targets, as they were military production centers, relatively untouched by the war (making it easier to showcase the bomb's potential), and had little perceived cultural importance. Allied leadership met the day after the Trinity Test in Potsdam, Germany, to discuss the war's end and terms of postwar peace. Although allied, no information about nuclear weapons was shared with the Soviets, however, a number of spies embedded deep in the Manhattan Project meant that Stalin knew of their progress long before Truman told him of their "new weapon - in fact, Stalin knew of the bomb before Truman, who only found out about the project after Roosevelt's death in April 1945. On July 26, the leaders of the U.S., UK, and China (the USSR was yet to declare war on Japan) issued the Potsdam Declaration, outlining the terms for Japan's unconditional surrender, and threatening "prompt and utter destruction" if these were not accepted. While many in Japan's leadership found the terms agreeable, they chose to wait for a response from the Soviets, who they thought could mediate more favorable terms. What they did not know was that a Soviet invasion was imminent, and the U.S. took this delay as a rejection of the terms.

Dropping the bombs

The first atomic bomb was dropped on Hiroshima on the morning of August 6, 1945. The B-29 Superfortress, named Enola Gay, departed from the Northern Mariana Islands in the early hours, carrying "Little Boy". Enola Gay arrived at its destination and dropped the bomb at 8:15am (local time), which detonated 43 seconds later at 1,500 feet above the city (atmospheric detonations have a wider blast radius, but send less radioactive fallout into the atmosphere than ground level explosions). Little Boy's explosion had a yield of 16KT, ground temperatures reached almost 4,000° Celsius, and almost everything within a one mile radius was vaporized or destroyed. People closest to the epicenter were killed instantly by both the blast force and the extreme temperatures, while thousands more were killed or injured further out due to the destruction or inflammation of their surroundings. Even more died in the aftermath of the attack due to burns, injuries, and radiation sickness. As the bomb exploded over Hiroshima's center, most of the city's medical professionals and many government officials were killed on the day, which greatly hindered the emergency response to the attack.

It took several hours for Japan's leadership to realize what had happened; they knew the attack was different and not part of a larger raid, but there was still confusion until Truman reiterated the Potsdam Declaration's demands later that night. Yet, Japan did not surrender - its scientists (correctly) estimated that the U.S. likely had only one or two more nuclear weapons ready, and they were willing to endure further attacks and then continue fighting. U.S. intelligence learned of Japan's intentions, and the second bomb was prepared.

The second attack was scheduled against the city of Kokura on August 11, with an almost identical plan of attack to Hiroshima, but the mission did not go as smoothly. A poor weather forecast brought the attack forward to August 9, faulty fuel pumps jeopardized the return journey, and a delayed in-air rendezvous of the accompanying planes allowed clouds and smoke to obscure the target. These obstacles prevented an attack on Kokura, and the bombing party headed south to the secondary target: Nagasaki. At 11:02, the plutonium-fueled Fat Man was dropped over Nagasaki's industrial valley, detonating 47 seconds later approximately 1,650 feet above the ground. The bomb had a yield of 21KT, bigger than that of Hiroshima, but the surrounding topography and location meant that casualties were lower than if the bomb had been dropped on the city center. Exact death tolls for both explosions remain unclear due to the destruction caused, poor wartime records, and the long-term effects of the injuries and radiation. Nonetheless, conservative estimates place Hiroshima's death toll at around 70,000, and Nagasaki's at around 40,000, although some sources cite a combined figure as high as 210,000. The vast majority of those killed were civilians.

With the Soviet invasion of Manchuria earlier that morning, compounded by food shortages and dropping morale, Japan's leadership accepted that the war was lost. Following a day of deliberation, Japan's Foreign Ministry sent telegrams to the Allies informing them of Japan's intention to surrender, bringing an end to hostilities. Emperor Hirohito publicly announced Japan's surrender on August 15, which was then formalized on September 2 with the Allied occupation of Japan.

Legacy

The nuclear attacks remain a topic of debate in both scholarly and public circles. Proponents argue that ending the war early prevented millions of deaths by conventional bombings or invasion, and avoided the starvation of millions by allowing aid into Japan. Opponents criticize the use of the bomb on ethical grounds, going as far as calling it a war crime - some argue that the Soviet invasion of Manchuria would have been enough to convince Japan to surrender, and accuse the U.S. of sacrificing Hiroshima and Nagasaki to intimidate the USSR. Based on a 2020 survey, 39 percent of Americans believe the U.S. was right to drop the bombs on Japan, while 33 percent disagree.

The Manhattan Project continued until 1946, and was then disbanded and split between the U.S. Atomic Energy Commission (USAEC) and Armed Forces Special Weapons Project (AFSWP). Both the U.S. and USSR forcibly repatriated thousands of German scientists as "intellectual reparations" to help them in their respective nuclear weapons programs. Then began the nuclear arms race that would come to overshadow the Cold War, with U.S. and Soviet arsenals exceeding 30,000 and 40,000 warheads at their respective peaks. The most powerful weapons ever built attained yields roughly 3,000 times more powerful than Fat Man or Little Boy.

The project's military leader, Leslie Groves, was made head of the AFSWP in 1947, but resigned from the military the following year after receiving a severe dressing down from General Eisenhower. Many of the scientists in the project sought to distance themselves from the further development of nuclear weapons after the war - even before the Trinity Test, many notable figures signed the Franck Report, warning against ever using the weapons in fear that it would lead to a nuclear arms race. Some of those involved in the project then founded Bulletin of Atomic Scientists in an attempt to educate the public about this new technology and its dangers. Public interest in the attacks also saw Oppenheimer move into the public eye as the face of the Manhattan Project. He then took a prominent role in the USACE and was a leading opponent within the government to the expansion of the U.S. nuclear arsenal, the development of the hydrogen bomb, and nuclear proliferation. Political maneuvering then saw him ousted from the government during a high profile security hearing in 1954, due to his former association with leftist organizations - the ruling was nullified and his reputation was formally restored in 2022.