Excerpted from Section 2 of Nuclear Weapons Frequently Asked Qustions by Carey Sublette.
Fusion reactions, also called thermonuclear reactions, are reactions between the nuclei of certain isotopes of light elements. If the nuclei collide with sufficient energy (provided by heat in a star or a bomb, or by a particle accelerator in the laboratory) then there is a significant chance that they will merge to form one or more new nuclei with the release of energy. Different nuclei combinations have different inherent likelihoods of reacting in a collision at a particular temperature. The rates of all fusion reactions is affected by both temperature and density. The hotter and denser the fusion fuel, the faster the fusion "burn".
The fusion reactions that occur in stars are not the same as the ones that occur in thermonuclear weapons or (laboratory fusion reactors). The somewhat complex catalyzed fusion cycle in stars that converts light hydrogen (protium) into helium is extremely slow, which is why the lifetime of the Sun is measured in billions of years. The fusion reactions used in bombs and prospective powerplant designs are simple, and extremely fast - which is essential since the fuel must be fully consumed within microseconds. These reactions thus are based on the same general principles as stellar fusion, but are completely different in detail.
The most important fusion reactions for thermonuclear weapons are given below:1. D + T -> He-4 + n + 17.588 MeV 2. D + D -> He-3 + n + 3.268 MeV 3. D + D -> T + p + 4.03 MeV 4. He-3 + D -> He-4 + p + 18.34 MeV 5. Li-6 + n -> T + He-4 + 4.78 MeV 6. Li-7 + n -> T + He-4 + n - 2.47 MeV [D and T stand for deuteron or deuterium (H-2), and triton or tritium (H-3) respectively.]
At the temperatures found in fission bombs reaction 1 has a rate 100 times faster than the next fastest candidate (reactions 2 and 3 combined), which are in turn 10 times faster than reaction 4. The rates of reactions 1 - 4 all increase rapidly (exponentially) with temperature, but not in the same proportion. At the higher temperatures achievable by fusion, reaction 4 exceeds the combined rate of reactions 2 and 3. Other reactions also occur between the isotopes listed here, but the reactions rates are too low to be important.
Some additional important facts about these reactions: The neutron produced in reaction 1 is extremely energetic, it carries away 14.06 MeV of the reaction energy, the alpha particle (He-4 nucleus) only 3.52 MeV.
The neutron produced in reaction 2 has an energy of only 2.45 MeV (similar to the faster fission neutrons), with the He-3 carrying 0.82 MeV. The division of energy in reaction 3 is 1.01 MeV for the triton, and 3.03 MeV for the proton. The two D+D reactions are equally likely and each will occur half the time.
In reaction 4 the alpha particle carries off 3.67 MeV, the proton 14.67 MeV.
Reactions 5 and 6 are not thermonuclear reactions, strictly speaking. They are neutronic reactions, like fission, and do not require heat or pressure, just neutrons in the correct energy range. This distinction is usually ignored in the literature about nuclear weapons however. The Li-6 + n reaction requires neutrons with energies is the low MeV range or below. The Li-7 + n reaction is only significant when the energies are above 4 MeV.