Science
URANIUM CHEMISTRY AND METALLURGY
Science > Nuclear Physics
Uranium is the heaviest element to naturally exist in large quantities on the earth. Every atom of uranium contain 92 protons. There are two primary isotopes of uranium, with masses of 235 and 238. Uranium-235 contains 143 neutrons in addition to its 92 protons, while uranium-238 contains 146 neutrons. Because the two are isotopes of the same element, they behave the same way in chemical reactions. However, the slight difference in the nuclear structure of the two isotopes means that they do behave differently in terms of nuclear reactions. Uranium-235 will fission when it absorbs a neutron, while uranium-238 will absorb a neutron to become uranium-239 and will then undergo beta decay twice to become plutonium-239, another fissionable substance. The road to the atom bomb used both uranium-235 and plutonium-239 to create explosions. A third isotope, uranium-234, exists in trace quantities, and is useless for the purposes of building bombs. In order to create a bomb with uranium-235, this fissile isotope must be separated from the more common uranium-238. This heavier isotope comprises over 99% of naturally-occurring uranium. The process of separating these isotopes in order to get a uranium metal with a large concentration of uranium-235 is known as uranium enrichment. As a radioactive element, uranium decays naturally by emitting alpha particles. Uranium-235 has a half-life of 7.13x108 years, while uranium-238 has a half-life of 4.51x109 years. The discovery of uranium is attributed to the German chemist Martin Heinrich Klaproth in 1789. He named the element after the planet Uranus, discovered by William Herschel in 1781, and the first planet discovered in modern times. Antoine Becquerel discovered the phenomenon of radioactivity in 1896 while studying uranium. In 1934, Enrico Fermi generated beta rays by bombarding uranium with neutrons, leading to studies of fission by Otto Hahn, Fritz Strassman, Lise Meitner, and Otto Frisch. Uranium was needed in all its forms for the various parts of the Manhattan Project, a daunting task since its chemical properties were largely unknown. Its only stable gaseous form, uranium hexaflouride, it extremely corrosive to most metals, reacts violently with water, and is extremely dangerous to humans. Overcoming these difficulties and finding materials that could withstand this corrosive effect was central to the development of a gaseous diffusion process of uranium enrichment. Uranium metal was needed to fuel reactors such as those at the Hanford 300 Test Site. Finally, liquid uranium hexafluoride served as source material in the liquid thermal diffusion separation process.
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The text for this page is original to the Department of Energy's Office of History and Heritage Resources. A good source for the history of physics and chemistry is Helge Kragh, Quantum Generations: A History of Physics in the Twentieth Century (Princeton: Princeton University Press, 2002). The photograph of the blocks of uranium are courtesy LANL, reprinted in Rachel Fermi and Esther Samra, Picturing the Bomb: Photographs from the Secret World of the Manhattan Project (New York: Harry N. Abrams, Inc., Publishers, 1995), 109.