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Chapter: 4 Atomic and nuclear physics
    Section: 4.7 Nuclear fission and fusion, and neutron interactions
        SubSection: 4.7.1 Nuclear fission



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4.7    Nuclear fission and fusion, and neutron interactions   

4.7.1   Nuclear fission   

When nuclei of the heavy elements capture neutrons, fission takes place; that is to say, the nuclei divide usually into two parts, though in a small fraction (< 1%) of cases a third light particle is emitted. Fission can also occur spontaneously but the half-life for this is usually long (see Ewbank et al., 1979). In the process of fission about 200 MeV of energy is released, largely in the form of kinetic energy of the two fission fragments. Fission is also accompanied by the emission of two or three neutrons and about ten gamma-rays, emitted from the fission fragments during their recoil. An approximate formula for the fission neutron spectrum is given in the section Neutron Cross-sections (Section 4.7.2) which also contains information on the total number of neutrons per fission for various fissile nuclei in three different neutron spectra.

The mass partition between the two fragments is in general asymmetrical, particularly for fission induced by low energy neutrons. The special cases of the fission of U-233, U-235 and Pu-239 by thermal and fast neutrons are illustrated in the diagram and it will be observed that despite its characteristic shape the fission yield curve is by no means a smooth function of mass. For a given neutron energy the high mass peak is very similar for all the fissioning nuclei while the low mass peak varies in position. As the neutron energy increases the valley between the peaks fills in and the wings of the peaks rise, while at very high energies and also for very heavy elements the mass yield becomes symmetrical and single humped.

The products of fission are themselves generally neutron rich and hence radioactive and the Table of Nuclides (section 4.6.1) should be consulted for their properties. When some fission fragments have undergone a β -decay they have sufficient excitation energy to emit a neutron. These neutrons, which are called delayed neutrons, are emitted with half lives of up to 55 s and are very important for the control of nuclear reactors. The βand gamma-ray decay energy of the fragments is emitted over a long period and the diagram shows for U-235 and Pu-239 the average β and gamma-ray energy emitted by fission products as a function of time after fission. In a reactor the actinides also make a significant contribution, particularly for decay times of ~105 s and greater than 108 s (see Tobias, 1980).

The average total energy released by slow-neutron induced fission in the fissile materials U-233, U-235 and Pu-239 is given below.





Instantaneously released energy




    Kinetic energy of fission fragments    .    .  .   .   .   .     .   .

168.2 MeV 

169.1 MeV 

175.8 MeV 

    Kinetic energy of prompt neutrons    .  .  .   .  .  .  .  .   .   .




    Energy carried by prompt γ-rays       .  .  .  .  .  .  .  .   .   .








Energy from decaying fission products




    Energy of β-particles    .   .   .   .   .   .   .   .   .   .   .   .   .




    Energy of anti-neutrinos   .   .  .   .   .   .   .  .  .  .   .   .   .  .




    Energy of delayed γ-rays  .   .  .   .   .   .   .   .   .   .   .   .  .
















In an operating reactor, all of this energy except that carried by the anti-neutrinos is converted into heat. There is, however, an additional source of energy arising from the binding energy released when those prompt neutrons which do not produce fission are finally captured. For thermal reactors this amounts to ~ 9.1 MeV for U-233, ~ 8.8 MeV for U-235 and ~ 11.5 MeV for Pu-239.

(click the Images to view Larger Images)
Fission yield (%)vs Mass number A

Beta and gamma-ray energy from fission product decay as a function of time after a thermal fission


Energy from Fission
       R. Sher (1981) BNL-NCS 51363, vol. II, p. 835.
       M. F. James (1969) J.N.E., 23, 517.
Decay Heat
       Proc. of a Specialists Meeting on Data for Decay Heat Predictions, OECD Report, NEACRP-302 ‘L’,
           NEANDC-245 ‘U’ (1987).
       A. Tobias (1980) Prog. in Nucl. En., 5(1), 3.

Fission Yields
       M. F. James, R. W. Mills and D. R. Weaver (1991) UKAEA Reports, AEA-TRS-1015, AEA-TRS-1018
           and AEA-TRS-1019.
       T. R. England and B. F. Rider (1992) OECD Report, NEA/NSC/DOC(92)9, p. 346.
Delayed Neutrons
       J. Blachot, M. C. Brady, A. Filip, R. W. Mills and D. R. Weaver (1990) OECD Report, NEACRP-L-323, NEANDC-299 ‘U’.
       G. R. Keepin (1965) Physics of Neutron Kinetics, Addison-Wesley.
Nuclear Fission
       A. Michaudon (ed.) (1981) Nuclear Fission and Neutron Induced Fission Cross-sections, Pergamon Press, Oxford.
Spontaneous Fission
       IAEA Technical Report Series No. 261 (1986).
       W. B. Ewbank, Y. A. Ellis and M. R. Schmorak (1979) Nuclear Data Sheets, 26, 1.



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