As seen in Figure 2.2, the binding energy in each nucleon has an extremum as a function of the mass number. This means that energy can be obtained by the fission of heavy elements and by the fusion of light elements. (Fission is discussed in Section 6.2.1.)

The most important fusion reactions of the isotopes of hydrogen are exoergic:

The activation energy of the fusion processes, however, is very high. The igni­tion temperature is the lowest for the 2H—3H reaction (see Eq. (6.50) and Figure 6.7), it is about 107 K; the ignition temperature of the 2H—2H reaction (see Eqs. (6.47)—(6.49)) is in the range of 108 K. The H—H reaction requires an even higher temperature, about 1010 K. These reactions take place in stars. Natural fusion reactions will be discussed in Section 6.2.5.

Figure 6.7 The cross section of the D—T reaction as a function of temperature.

All products of the reactions ((6.44)—(6.51)) are inactive, so fusion energy pro­duction should be more desirable than fission energy production. However, there are many technical problems that have not been solved yet, as will be outlined in Section 7.4.

The thermonuclear reactions take place in the hydrogen bomb. The nuclear bomb will be discussed in Section 7.5.