Nuclear fusion occurs when two light nuclei join together to form a heavier nucleus. It takes
great energy to force nuclei to fuse, even those of hydrogen (the lightest element).
When two nuclei collide with sufficient amounts of energy, it is possible that they will merge
to create one or more new nuclei. This process causes a release of energy. Temperature and
density affect the rate of nuclear fusion reactions. If the fusion fuel is hotter and more
dense, the fusion reactions become faster.
The fusion reactions in stars (e.g. the sun) are not identical to the fusion reactions that occur
in thermonuclear weapons. The fusion cycle in stars which converts hydrogen into helium is
complex and slow. The fusion reactions which occur in nuclear bombs are simple and very
fast. Within microseconds the fusion fuel is fully consumed.
In nuclear weapons, the most important fusion reaction is called the D-T reaction. These
hydrogen isotopes fuse together to form helium and free neutrons; heavy hydrogen-2
(deuterium, D) fuses with hydrogen-3 (tritium, T) to form helium-4 (He-4) plus one neutron
(n) and releases energy:
D + T > He-4 + n + 17.6 MeV
At the center of an exploding fission weapon the high pressure and temperature
environment is sufficient to compress and heat a mixture of tritium and deuterium gas. This
creates nuclear fusion.
The energy release from this fusion reaction is relatively small. However each neutron can
start a new fission chain reaction. As a result the rate of fission speeds up and more fissile
material is consumed. This greatly reduces the amount of material which is wasted when
expansion of the fissile material stops the fission chain reaction.