Nuclear Fusion

Nuclear Fusion involves two or more small nuclei "fuse" together to form a larger nuclei. 

Fusion reactions release energy for the same reason as fission reactions - the binding energy per nucleon is greater after the reaction than before. 

The following example illustrates a fusion reaction which could typically occur in a fusion bomb (Hydrogen bomb) or in our Sun:

Figure 2.1 : Diagram showing the fusion of Deuterium and Tritium into Helium and one surplus neutron.

By examining the binding energy per nucleon graph it is possible to ascertain that Helium-4 has a far greater binding energy per nucleon than the isotopes of Hydrogen, Deuterium and Tritium. As such the fusion reaction shown in Figure 2.1 will release approximately 3.5MeV per nucleon. This is against the 0.9MeV released in the Uranium fission reaction and as such greater energy is released in a fusion reaction than a fission reaction.

The main obstacle to a fusion reaction is the repulsive Coulomb force which must be overcome as two nuclei approach. In order to fuse the two nuclei must enter the region where the strong nuclear force can bind them. This is typically of the order of around 10^-15m. 

Massive temperatures are therefore needed to provide the fusing nuclei with the kinetic energy needed to overcome the Coulomb repulsion. Temperatures needed are around the order of 40 million Kelvin. This is the main obstacle to using fusion reactions to provide the necessary energy for the human race.

In the case of the Hydrogen bomb, the necessary temperatures needed for fusion are provided by a fission reaction.