Remembering the Manhattan Project

Ethan Sperla
March 21, 2022

Submitted as coursework for PH241, Stanford University, Winter 2021


Fig. 1: The unofficial emblem of the Manhattan Project, circa 1946. (Source: Wikimedia Commons)

On July 16th, 1945, the United States ushered the world into the atomic age with the world's first nuclear explosion at a location nearly 210 miles south of Los Alamos, New Mexico. [1] The Trinity Test came to fruition because of approximately 130,000 individuals and over $2 billion dollars invested into the Manhattan Project. [1] The successful completion of that test would alter the course of mankind forever. But, it could have happened first in another country. After all, nuclear fission was discovered nearly six and a half years earlier in Nazi Germany. [1] Therefore, it is important to examine how exactly the Manhattan Project was accomplished by studying the origins of nuclear weapons and their development during World War II.

Surprising Beginnings

Nuclear fission was first discovered at the end of 1938 in Nazi Germany by two radiochemists, Otto Hahn, and Fritz Strassman, at the Kaiser Wilhelm Institute for Physical Chemistry in Dahlem. [1] The two scientists were bombarding a container of uranium nitrate with neutrons. Their goal was to find an element heavier than uranium; instead, they discovered barium, which is lighter than uranium. Stumped by their finding, they consulted with another physicist and confirmed that they had uncovered a new kind of nuclear radiation. The neutrons had managed to split the uranium nuclei, releasing energy equal to ten million times the amount initially used. Word of their finding would spread quickly across the globe and would gain its name, at least partially, from the process of binary fission from biology.

Leo Szilard began to question the implications of the process in early 1939 to produce large amounts of powers and supposedly mentioned the term atomic bombs. [1] Something nearly identical happened at the office of Robert Oppenheimer at Berkley after becoming aware of the finding. He began envisioning a bomb days after hearing the news. Physicists around the globe soon caught on and became enamored by the idea of creating one of these atomic bombs for deterrence and forwarding world peace. So, most industrial nations began developing these weapons. However, some countries were more committed than others, accepting the possibility of failure after having to invest billions of dollars. President Franklin D. Roosevelt trusted his scientists and authorized a full-scale nuclear weapons program on October 9th, 1941. [1]

Developing the Project

Robert Oppenheimer was tapped to lead the Anglo-American program that would become known as the Manhattan Project. Its unofficial emblem is showcased in Fig. 1. Oppenheimer began by preparing a functioning laboratory in New Mexico. [1] Then, he began recruiting scientists but did not go into details regarding the nature of their work. Oppenheimer was, however, notoriously known for saying that their work would end the war and save lives. [2] They first had to build an experimental reactor to prove that it was possible to achieve a controlled chain reaction in uranium. [1] Szilard and another colleague would develop a spherical assembly made up of graphite blocks drilled with holes, where uranium metal would be inserted. Their design required 800,000 pounds of highly purified graphite, 12,000 pounds of uranium metal, and 100,000 pounds of uranium oxide. [1]

Next, scientists had to find a site where plutonium could be produced for the bombs. Early requirements estimated that the manufacturing area had to be 12 by 16 miles, with no public expressway or railroad closer than 10 miles, no town with greater than 1,000 residents nearer than 20 miles, a sustainable water supply that could produce 25,000 gallons of water per minute, and finally, an electrical supply of at least 100,000 kilowatts. [3] Hanford, Washington, was ultimately chosen for the site, and the government was forced to relocate around 1,500 residents. [3] Construction soon would begin on three water-cooled production reactors; each referred to by a letter either B, D, or F. [3]

Getting Results

Fig. 2: A mockup of the Fat Man, the atomic bomb dropped on Nagasaki, Japan. (Source: Wikimedia Commons)

After completion of the three reactors at Hanford, production began right away. In late September 1944, the B Reactor was the first of three reactors to go critical. [3] By March of 1945, the remaining reactors had gone critical too. [2] Plutonium was cultivated and steadily delivered from Hanford to Los Alamos, usually by an army ambulance. [2] It was estimated that they would have a cumulated supply of 40 to 45 kilograms of plutonium by July 1st, 1945. [2]

There were two kinds of nuclear materials uranium and plutonium that could be used to create a weapon of mass destruction. Therefore, Oppenheimer would prepare two types of bombs. The first was named Little Boy, because of its relatively small design. It was a six-foot cannon with a cylinder of U-235 placed inside along with stacked rings of U235, which would be shot up the barrel. [2] When the rings slammed into the cylinder, they would form a supercritical mass and cause a chain reaction. Testing of this design was deemed unnecessary since prior experiments had proven it to be reliable. [4] The United States would drop the first version of the bomb on Hiroshima, Japan.

The second design was stumbled upon accidentally in the spring of 1944 in the secret laboratory at Los Alamos. A mixture of plutonium 240 and higher isotopes from the plutonium created in Hanford was estimated to be so unstable that it was likely to cause an explosion at full yield when fired up a gun barrel, even at 3,000 feet per second. [2] Oppenheimer then ordered a pivot of current research and directed resources towards investigating this new process, nicknamed implosion. [2] They would use a lensed sphere of explosives to force a subcritical sphere of plutonium to critical density. However, implosion would require much more development before it would become ready for use.

The first full-scale test of an implosion-type device would take place on July 16th, 1945. [1] The explosion force was equivalent to 18 kilotons of TNT. The mushroom cloud from seconds after detonation can be seen in Fig. 3. It was described as a display that "no one.. could forget" by the director of the test, Ken Bainbridge. [2] This bomb would become known as Fat Man, because of its wide, round shape. A mockup of Fat Man can be seen in Fig. 2. On that same day, an infamous destroyer named the Indianapolis was leaving the San Francisco Bay carrying the Little Boy and its uranium bullet, bound for Tinian Island in the Marianas. [2]

Fig. 3: Mushroom cloud seconds after detonation of the Gadget, also known as the Trinity Test. (Source: Wikimedia Commons)

Changing the Tide of Conflict

There was a consensus among President Harry S. Truman and the armed services that dropping the atomic bombs had become necessary. So, the decision was made to use them to hasten Japan's surrender.

On August 6th, 1945, Little Boy was dropped on Hiroshima. [2] The bomb yielded 12.5 kilotons. [2] It is estimated that upwards of 140,000 people had died instantaneously or within five years. [2] There were approximately 76,000 buildings in the city before the bomb. [4] Afterward, 62.9 percent were completely destroyed or burned. [4] The destruction had delayed communication of what had happened in the city to Tokyo and the rest of Japan, meaning no surrender had been made in the days immediately following the dropping of the bomb. So, it was decided by President Truman that another atomic bomb was to be dropped on Japan.

On August 9th, 1945, Fat Man exploded over Nagasaki with a yield of 22 kilotons. [2] This bomb was estimated to have killed at least 70,000 people right away or within five years. [2] It also had similar effects on the city's buildings and infrastructure. Finally, on August 15th, the Emperor of Japan insisted that the government surrender, calling the new weapons a new and most cruel bomb. [2]


The close of the Second World War ultimately marked humanity's entrance into the dawn of the nuclear age. For the first time in history, humans had manufactured the means of their destruction. Death had become the most efficient it had ever been, with the capability to kill tens of thousands of people in an industrial fashion. Since then, the power of these weapons has only exponentially increased, and their detonation now poses a more significant threat to the world than ever before. Still, lessons can be drawn from the origins of nuclear weapons and the tremendous effort that was undertaken to develop them.

© Ethan Sperla. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.


[1] F.G. Gosling, "The Manhattan Project: Making the Atomic Bomb," U.S. Department of Energy, DOE/MA-0002, U.S. Department of Energy, January 2010.

[2] R. Rhodes, The Making of the Atomic Bomb (Simon and Schuster, 1995).

[3] D. Harvey, "History of the Hanford Site: 1943-1990," Pacific Northwest National Laboratory, (2000). (Courtesy of PNNL.)

[4] J. Borrie and T. Caughley, "An Illusion of Safety: Challenges of Nuclear Weapon Detonations for United Nations Humanitarian Coordination and Response," United Nations Institute for Disarmament Research, 2014.