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Nuclear Fission/Fusion

  Nuclei are composed of positively charged protons and neutral neutrons. When a nucleus is surrounded by orbiting electrons, we have an atom. Atoms are neutral due to cancellation of the proton charge by the negatively charged electrons.

Early this century, it became obvious that there was something wrong with theories about nuclei. The masses of the free proton and neutron were both accurately known, as were the masses of many nuclei. However, when protons and neutrons were bound together to form a nucleus, the resulting nucleus always had slightly less mass than the total mass of its constituent particles.

This bizarre ‘mass defect’ was explained by Einstein\'s famous formula E = mc2. The protons in a nucleus repel each other very strongly as they are positively charged. This repulsion is overcome by the strong force which only operates at short range. However, the energy required to overcome the strong force is enormous, and can only come from the mass of the particles. In effect, some of the mass of the nucleus has been converted into energy in order to keep the protons from flying apart.

The stability of nuclei to fission or fusion is determined by this mass defect. We define the binding energy of a neutron or proton as the mass defect per particle. The more binding energy a particle has, the more stable it will be.

Nuclei fuse or fission to increase the binding energy per particle. The most stable nucleus is iron. Nuclei lighter than iron can gain stability by fusion and heavier nuclei by fission. Thus, it is only the lightest nuclei (hydrogen and helium) that are used in fusion experiments, while very heavy nuclei like uranium and plutonium will fission easily.

The tremendous energy releases that occur in fusion and fission reactions are due to excess binding energy being given off. It is possible to achieve a chain reaction in fission processes, as each fission gives out excess neutrons, which in turn go on to cause more fissions. There is no similar process for fusion, which goes some way towards explaining whay a fusion reactor is far more demanding to build than a fission one. Fusion would produce more energy more efficiently and cleanly than fission; the snag is that to persuade atoms to fuse, they must be subjected to enormously high temperatures and pressures. Designing a reactor with these requirements is proving to be a difficult task. Sustained fusion reactions have only been achieved by humans in the uncontrolled release of energy from a hydrogen bomb. In space, the story is very different. It is nuclear fusion that powers the Sun and all other stars. JJ

See also weak force/strong force.



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