The significant point about the fission of a uranium (or plutonium) nucleus by means of a neutron, in addition to the release of a large quantity of energy, is that the process is accompanied by the instantaneous emission of two or more neutrons; thus,
Neutron + uranium-235 = fission fragments +
(or uraniurn-233) 2 or 3 neutrons + energy.
The neutrons liberated in this manner are able to induce fission of additional uranium (or plutonium) nuclei, each such process resulting in the emission of more neutrons which can produce further fission, and so on.
Thus, in principle, a single neutron could start off a chain of nuclear fissions, the number of nuclei suffering fission, and the energy liberated, increasing at a tremendous rate. as will be seen shortly.
There are many different ways in which the nuclei of a given fissionable species can split tip into two fission fragments (initial fission products), but the total amount of energy liberated per fission does not vary greatly. A satisfactory average value of this energy is 200 million electron volts.
The million electron volt (or 1 MeV) unit has been found convenient for expressing the energy released in nuclear reactions; it is equivalent to 1.6 x 10-6 erg or 1.6 x 10-13 joule. The manner in which this energy is distributed among the fission fragments and the various radiations associated with fission is shown in Table 3.
Table 6-2: Distribution of Fission Energy
Kinetic energy of fission fragments 165 ± 5
Instantaneous gamma-ray energy 7 ± 1
Kinetic energy of fission neutrons 5 ± 0.5
Beta particles front fission products 7 ± 1
Gamma rays from fission products 6 ± 1
Neutrinos from fission products 10____
Total energy per fission 200 ± 6
The results in Table 6-2 may he taken as being approximately applicable to either uranium-233, uranium-235, or plutonium-239. These are the only three known substances, which are reasonably stable so that they can be stored without appreciable decay, that are capable of undergoing fission by neutrons of all energies. Hence, they are the only materials that can be used to sustain a fission chain.
Uranium-238, the most abundant isotope in natural uranium, and thorium-232 will suffer fission by neutrons of high energy only, but not by those of lower energy. For this reason these substances cannot sustain a chain reaction. However, when fission does occur in these elements, the energy distribution is quite similar to that shown in the table.
Only part of the fission energy is immediately available in a nuclear explosion; this includes the kinetic energy of the fission fragments, most of the energy of the instantaneous gamma rays, which is converted into other forms of energy within the exploding weapon, and also most of the neutron kinetic energy, but only a small fraction of the decay energy of the fission products.
There is some compensation from energy released in reactions in which neutrons are captured by the weapon debris, and so it is usually accepted that about 180 MeV of energy are immediately available per fission. There are 6.02 x 1023 nuclei in 235 grams of uranium-235 (or 239 grams of plutonium-239), and by making use of familiar conversion factors the results quoted in Table 1.45 may be obtained for the energy (and other) equivalents of 1 kiloton of TNT. The calculations are based on an accepted, although somewhat arbitrary, figure of 1012 calories as the energy released in the explosion of this amount of TNT3.
Table 1.45 Equivalents of 1 Kiloton of TNT
Complete fission of 0.057 kg (57 grams or 2 ounces) fissionable material
Fission of 1.45 x 1025 nuclei
2.6 x 1025 million electron volts
4.48 x 1019 ergs (4.18 x 1012 joules)
1.16 x 106 kilowatt-hours
3.97 x 109 British thermal units