We saw in Chapter 4 that nuclei in the neighbourhood of 56Fe have the greatest binding energy per nucleon (Fig. 4.7). In principle therefore, nuclear potential energy can be released into kinetic energy and made available as heat by forming nuclei closer in mass to iron, either from heavy nuclei by fission or from light nuclei by fusion. This chapter is devoted to the physics of nuclear fission and its application in power reactors. There were, world-wide, some 430 nuclear power stations operating in 1997, and these generated about 17% of the global electricity supply. In the UK about 28% of all electricity generated came from nuclear fission.

Induced fission

The spontaneous fission of nuclei such as 236U was discussed in §6.3; the Coulomb barriers inhibiting spontaneous fission are in the range 5–6 MeV for nuclei with A ≈ 240. If a neutron of zero kinetic energy enters a nucleus to form a compound nucleus, the compound nucleus will have an excitation energy above its ground state equal to the neutron's binding energy in that ground state. For example, a zero-energy neutron entering 235U forms a state of 236U with an excitation energy of 6.46 MeV. This energy is above the fission barrier, and the compound nucleus quickly undergoes fission, with fission products similar to those found in the spontaneous fission of 236U. To induce fission in 238U, on the other hand, requires a neutron with a kinetic energy in excess of about 1.4 MeV.