The Source of Nuclear Energy

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The source of nuclear energy was first discovered by Lise Meitner (assisted by Otto Frisch) and Otto Hahn (assisted by Fritz Stassmann).
Otto’s experiment was to bombard uranium with neutrons. Hahn, a chemist, was the first to chemically isolate the product barium for which he won the Nobel Prize for Chemistry in 1944.
Meitner, a physicist, realised that the product of Hahn’s experiment could only be explained if some mass had been converted into energy (calculated to be 200 MeV) and extra neutrons by fission (splitting) of the uranium atom. This could be a new source of energy for the world.
This concept was later turned into a reality by Enrico Fermi when he and others constructed and operated the world’s first nuclear reactor, ‘Chicago Pile number 1’, proving a sustainable controllable nuclear reaction was possible in December 1942.
Meitner and Frisch’s Nature paper (Meitner and Frisch, 1939) on fission states that the nucleus can be thought of as a ‘liquid drop’:
This liquid drop model of the nucleus had first been suggested by George Gamow, a nuclear physicist and cosmologist. It leads to the concept of the binding energy curve.
Binding energy


Binding energy is a result of the attractive strong nuclear force between all nucleons (protons and neutrons) that operates over distances the size of the atomic nucleus: 10-15 to 10-14 m.
Over these tiny distances, the strong nuclear force is much stronger than the electromagnetic Coulombic forces causing repulsion between the positively charged protons. However, it reduces exponentially with distance, whereas the Coulombic forces varies much less with distance. Therefore, the two opposing forces on the nucleus mean that big nuclei (about 6 nucleon diameters across, and mass, A, > 209) are unstable, as at these distances the Coulombic force starts to overcome the strong nuclear attractive force.
Other energy contributions that make up the semi-empirical ‘liquid drop’ model of the nucleus are:
  • surface energy term
  • asymmetry energy term
  • pairing energy term.
Surface Energy reduces binding of the nucleus, this means there is an effect of the surface area to volume ratio of a nucleus on its binding energy.
The asymmetry energy and the pairing energy are actually related to the fact the nucleus behaves according to quantum mechanics, as nucleons are quantum mechanical particles.

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