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Uranium is the largest/heaviest atom that occurs naturally. It has the most protons. A transuranic element has more protons than uranium. In a nuclear reactor that produces "transuranic" elements, neutrons are collided with the uranium. These neutrons will sometimes stick to the uranium, making it a heavier version of uranium. Because the heavier uranium has too many neutrons, a neutron experiences "beta decay". It turns into a proton and an electron, making the atom transuranic. Plutonium is such an element.


Nuclear reactors produce lots of neutrons from fission reactions. Some of these neutrons are captured by heavy nuclei, such as uranium-238. This makes a new nucleus with the same atomic number but a higher atomic mass. Some of these new heavier nuclei can then convert to nuclei of other elements by beta decay, in which one neutron converts to a proton, an electron (beta particle), and an electron anti-neutrino (usually undetectable). So the mass of the nucleus stays about the same, but the number of neutrons goes down by one and the number of protons goes up by one. Plutonium-239, for instance, is produced by on electron capture by uranium-238 followed by two beta decays:

U-238 + n --> U-239 --> Np-239 + beta --> Pu-239 + beta

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois

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