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By the end of this section, you will be able to:
  • Describe the process of nuclear fission in terms of its product and reactants
  • Calculate the energies of particles produced by a fission reaction
  • Explain the fission concept in the context of fission bombs and nuclear reactions

In 1934, Enrico Fermi bombarded chemical elements with neutrons in order to create isotopes of other elements. He assumed that bombarding uranium with neutrons would make it unstable and produce a new element. Unfortunately, Fermi could not determine the products of the reaction. Several years later, Otto Hahn and Fritz Strassman reproduced these experiments and discovered that the products of these reactions were smaller nuclei. From this, they concluded that the uranium nucleus had split into two smaller nuclei.

The splitting of a nucleus is called fission    . Interestingly, U-235 fission does not always produce the same fragments. Example fission reactions include:

0 1 n + 92 235 U 56 141 B a + 36 92 K r + 3 0 1 n + Q , 0 1 n + 92 235 U 54 140 X e + 38 94 S r + 2 0 1 n + Q , 0 1 n + 92 235 U 50 132 S n + 42 101 M o + 3 0 1 n + Q .

In each case, the sum of the masses of the product nuclei are less than the masses of the reactants, so the fission of uranium is an exothermic process ( Q > 0 ) . This is the idea behind the use of fission reactors as sources of energy ( [link] ). The energy carried away by the reaction takes the form of particles with kinetic energy. The percent yield of fragments from a U-235 fission is given in [link] .

An aerial photograph of The Phillipsburg Nuclear Power Plant.
The Phillipsburg Nuclear Power Plant in Germany uses a fission reactor to generate electricity.
A graph of percentage yield versus mass number A of fission fragment. The graph has two peaks at values A approximately equal to 95 and at A approximately equal to 137. There is a dip in the graph at A approximately equal to 118. The enclosed area under the graph is labeled 235 U Fission Fragments.
In this graph of fission fragments from U-235, the peaks in the graph indicate nuclei that are produced in the greatest abundance by the fission process.

Energy changes in a nuclear fission reaction can be understood in terms of the binding energy per nucleon curve ( [link] ). The BEN value for uranium ( A = 236 ) is slightly lower than its daughter nuclei, which lie closer to the iron (Fe) peak. This means that nucleons in the nuclear fragments are more tightly bound than those in the U-235 nucleus. Therefore, a fission reaction results in a drop in the average energy of a nucleon. This energy is carried away by high-energy neutrons.

Niels Bohr and John Wheeler developed the liquid drop model    to understand the fission process. According to this model, firing a neutron at a nucleus is analogous to disturbing a droplet of water ( [link] ). The analogy works because short-range forces between nucleons in a nucleus are similar to the attractive forces between water molecules in a water droplet. In particular, forces between nucleons at the surface of the nucleus result in a surface tension similar to that of a water droplet. A neutron fired into a uranium nucleus can set the nucleus into vibration. If this vibration is violent enough, the nucleus divides into smaller nuclei and also emits two or three individual neutrons.

The process of fission is shown in stages. A neutron strikes the circular nucleus of 235 U. The nucleus becomes oval shaped, labeled 236 U, unstable. Next, it develops the beginnings of a fissure in the middle. It then splits into two nuclei, each labeled fission fragment. This last stage also releases energy and neutrons.
In the liquid drop model of nuclear fission, the uranium nucleus is split into two lighter nuclei by a high-energy neutron.

U-235 fission can produce a chain reaction . In a compound consisting of many U-235 nuclei, neutrons in the decay of one U-235 nucleus can initiate the fission of additional U-235 nuclei ( [link] ). This chain reaction can proceed in a controlled manner, as in a nuclear reactor at a power plant, or proceed uncontrollably, as in an explosion.

Questions & Answers

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Source:  OpenStax, University physics volume 3. OpenStax CNX. Nov 04, 2016 Download for free at http://cnx.org/content/col12067/1.4
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