Understanding Radiation: Scientific Basis of Nuclear Explosions - Thermonuclear Fusion Reactions
Energy production in the sun and stars is undoubtedly due to fusion reactions involving the nuclei of various light (low atomic weight) atoms. From experiments made in laboratories with charged-particle accelerators, it was concluded that the fusion of isotopes of hydrogen was possible. This element is known to exist in three isotopic forms, in which the nuclei have mass numbers of 1, 2, and 3, respectively.
These are generally referred to as hydrogen (1H), deuterium (2H or D), and tritium (3H or T). All the nuclei carry a single positive charge, i.e., they all contain one proton, but they differ in the number of neutrons. The lightest (1H) nuclei (or protons) contain no neutrons; deuterium (D) nuclei contain one neutron, and tritium (T) nuclei contain two neutrons.
Several different fusion reactions have been observed between the nuclei of the three hydrogen isotopes, involving either two similar or two different nuclei. In order to make these reactions occur to an appreciable extent, the nuclei must have high energies. One way in which this energy can be supplied is to raise the temperature to very high levels. In these circumstances the fusion processes are referred to as “thermonuclear reactions”.
Four thermonuclear fusion reactions appear to be of interest for the production of energy because they are expected to occur sufficiently rapidly at realizable temperatures; these are:
D + D = 3He + n + 3.2 MeV
D + D = T + 1H + 4.0 MeV
T + D = 4He + n + 17.6 MeV
T + T = 4He + 2n + 11.3 MeV,
where He is the symbol for helium and n (mass = 1) represents a neutron. The energy liberated in each case is given in million electron volt (MeV) units.
The first two of these reactions occur with almost equal probability at the temperatures associated with nuclear explosions (several tens of million degrees Kelvin), whereas the third reaction has a much higher probability and the fourth a much lower probability. Thus, a valid comparison of the energy released in fusion reactions with that produced in fission can be made by noting that, as a result of the first three reactions given above, five deuterium nuclei, with a total mass of 10 units, will liberate 24.8 MeV upon fusion. On the other hand, in the fission process, e.g., of uranium-235, a mass of 235 units will produce a total of about 200 MeV of energy. Weight for weight, therefore, the fusion of deuterium nuclei would produce nearly three times as much energy as the fission of uranium or plutonium.
Another reaction of thermonuclear weapons interest, with tritium as a product, is
6Li + n = 4He + 3T + 4.8 MeV,
where 6Li represents the lithium-6 isotope, which makes up about 7.4 percent of natural lithium. Other reactions can occur with lithium-6 or the more abundant isotope lithium-7 and various particles produced in the weapon. However, the reaction shown above is of most interest for two reasons:
(1) it has a high probability of occurrence, and
(2) if the lithium is placed in the weapon in the form of the compound lithium deuteride (LiD), the tritium formed in the reaction has a high probability of interacting with the deuterium.
Large amounts of energy are thus released by the third reaction and additional neutrons are produced to react with lithium-6.
In order to make the nuclear fusion reactions take place at the required rate, temperatures of the order of several tens of million degrees are necessary. The only practical way in which such temperatures can be obtained on earth is by means of a fission explosion. Consequently, by combining a quantity of deuterium or lithium deuteride (or a mixture of deuterium and tritium) with a fission device, it should be possible to initiate one or more of the thermonuclear fusion reactions given above.
If these reactions, accompanied by energy evolution, can be propagated rapidly through a volume of the hydrogen isotope (or isotopes) a thermonuclear explosion may be realized.
It will be observed that in three of the fusion reactions, neutrons are produced. Because of their small mass, these neutrons carry off most of the reaction energy; consequently, they have sufficient energy to cause fission of uranium-238 nuclei. As stated earlier, this process requires neutrons of high energy.
It is possible, therefore, to make use of the thermonuclear neutrons by surrounding the fusion weapon with a blanket of ordinary uranium. The high-energy neutrons are then captured by uranium-238 nuclei; the latter undergo fission, thereby contributing to the overall energy yield of the explosion, and also to the residual nuclear radiation arising from the fission products.
On the average, the energy released in the explosion of a thermonuclear weapon originates in roughly equal amounts from fission and fusion processes, although there may be variations in individual cases. In “boosted” fission weapons, thermonuclear neutrons serve to enhance the fission process; energy released in the thermonuclear reaction is then a small fraction of the total energy yield.