The Atomic Bomb

Jessica Xu
February 20, 2016

Submitted as coursework for PH241, Stanford University, Winter 2016

The Manhattan Project

Before I delve into the atomic bomb, I want to give a brief background of its history in the United States. Specifically, I want to look at the Manhattan Project, a research and development project during World War II that ultimately produced the first nuclear weapons. Through this project, two types of atomic bombs were developed: a simple uranium gun-type fission weapon and a more complex plutonium implosion-type weapon. In my paper, I will go more in-depth with the uranium gun-type fission weapon and give a brief overview of the plutonium implosion-type weapon.

The Design and Impact of Little Boy

The first type of atomic bomb that was developed through the Manhattan Project was something called "Little Boy", the codename for the atomic bomb that was dropped in Hiroshima, Japan during World War II. When scientists first began developing this bomb, they analyzed U-235, a known fissionable isotope. As U-235 only makes up 1/140 of natural uranium, scientists had to separate the isotopes in order to collect the uranium necessary for the weapon. [1] In design, the Little Boy weighed about 4 tons or 8000 pounds. [2] Within the Little Boy were two important components: a hollow sub-critical mass of U-235 and a solid target cylinder. When the two pieces were explosively forced together, a nuclear chain reaction would be initiated. The interesting thing about the fission of uranium is that a large amount of energy is released and that when the neutron and U-235 fuses, fission fragments, 2 or 3 neutrons, and energy are all released. Therefore, in theory, a single neutron can start off a chain of nuclear fissions. For a nuclear explosion to take place, the weapon (Little Boy) must have a sufficient amount of fissionable uranium isotope (which we do have - U-235) for the critical mass to be exceeded. The total amount of energy emitted per fission doesn't vary greatly. [3] Breaking all this down, one can see that initiating a nuclear fission is not difficult while the energy released is tremendous. When the Little Boy was dropped on Hiroshima, Japan on August 6, 1945, 80,000 people were killed or missing. [4] Then, in 1953, Frederick Reines, an American physicist, calculated the yield of Little Man to be 13 kilotons. [5] All in all, Little Boy exploded with the energy equivalent of 15,000 tons of TNT. [6]

The Design and Impact of Fat Man

The 'Fat Man' was the codename for the second atomic bomb used in World War II, which was dropped on Nagasaki, Japan. Different from Little Boy, the Fat Man was an implosion-type nuclear weapon with a plutonium core (Pu-239), rather than a uranium core. Fat Man weighed 4.5 tons, or about 9000 pounds, which is 1000 pounds heavier than Little Boy. [2] What is interesting, however, is that the impact was not as strong as that of Little Boy because of the terrain of Nagasaki. When Fat Man exploded, 22 kilotons of TNT of energy was released. [6] About 35,000-40,000 were dead or missing and an equal number wounded. [4]

Radiation Impact

In measuring the radiation in Hiroshima and Nagasaki after the bombings, one finds that 1 meter above the ground at 1 km away from the hypocenter exists 4.5 Gy in Hiroshima and 8.7 Gy in Nagasaki. At 2 km away, the radiation levels were lower, with 0.08 Gy in Hiroshima and 0.14 Gy in Nagasaki. Comparatively, Fat Man increased the radiation level more than Little Boy did. During the period after the bombings from 1950-2000, 86,611 survivors were selected in a Life Span Study to estimate radiation dosage. The report finds that there have been 47,685 (55%) deaths of which 10,127 were from cancer and 296 were from leukemia. This goes to show that the atomic bomb at detrimental after effects. [7]


As can be seen, the impacts of both Little Boy and Fat Man were felt not just across the nation but also across the world as we learned the power of atomic bombs. Since World War II, no more atomic bombs have been used, but still they are being designed. While there have been no long-term effects of the radiation that was emitted during World War II, i.e. no hereditary issues related to radiation, the people who were in Japan at the time all had an increased risk to developing leukemia and other types of cancer.

© Jessica Xu. 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] V. C. Jones, Manhattan: The Army and the Atomic Bomb, Center of Military History, United States Army (U.S. Government Printing Office, 1985) [Reprinted by Createspace Independent Publishing, 2015].

[2] T. Imanaka, Casualties and Radiation Dosimetry of the Atomic Bombings on Hiroshima and Nagasaki," in Radiation Risk Estimates in Normal and Emergency Situations, ed. by A. A. Cigna and M. Durante (Springer, 2006).

[3] S. Glasstone and P. J. Dolan, Eds., The Effects of Nuclear Weapons, 3rd Ed., United States Department of Defense (U.S. Government Printing Office, 1977) [Reprinted by Literary Licensing LLC, 2013].

[4] F. D'Olier et al., "The Effects of the Atomic Bombings of Hiroshima and Nagasaki," U.S. Strategic Bombing Survey, June 1946, p. 37.

[5] L. Hoddeson et al., Critical Assembly: A Technical History of Los Alamos During the Oppenheimer Years, 1943-1945 (Cambridge U. Press, 2004), p. 398.

[6] W. Bach, "Nuclear War: The Effects of Smoke and Dust on Weather and Climate," Prog. Phys. Geog. 10, 315 (1986).

[7] Y. Shimizu et al., "Radiation Exposure and Circulatory Disease Risk: Hiroshima and Nagasaki Atomic Bomb Survivor Data, 1950-2003," BMJ 340, b5349 (2010).