Nuclear fusion is a process by which the nuclei of lighter elements come together to form heavier elements. If the interacting nuclei are of elements with low atomic numbers then large quantities of energy are released. More energy is released per nuclear fusion reaction than for nuclear fission reactions.

There are two types of nuclear fusion reactions: a) those that preserve the number of neutrons and protons and, b) those that involve a conversion between neutrons and protons.

Fusion reactions that preserve the number of neutrons and protons are more important when considering nuclear fusion for practical energy production. An example of this type of reaction occurs when the hydrogen isotopes deuterium and tritium are fused:

Deuterium + Tritium => Helium + Neutron

Deuterium is a hydrogen isotope with one proton and one neutron in its nucleus; Tritium is a hydrogen isotope with one proton and two neutrons in its nucleus; Helium has two protons and two neutrons in its nucleus.
Therefore, we see the number of neutrons and protons is conserved. This reaction is sometimes called the deuterium-tritium (or D-T) reaction.

Fusion reactions that convert neutrons and protons are fundamentally important to the energy production mechanisms in stars. An example occurs to initiate the ‘burning’ of hydrogen in the core of stars when two hydrogen nuclei are fused to form deuterium:

Hydrogen + Hydrogen => Deuterium + Positron + Neutrino

Hydrogen has one proton in its nucleus; Deuterium has one proton and one neutron in its nucleus; Before the reaction, there were two protons (in the hydrogen nuclei); after there is a proton and a neutron bound together as a Deuterium nucleus. The conversion of one proton to a neutron yields a positron and a neutrino.

Nuclear fusion in the sun and other stars

In the late 1930s, Hans Bethe recognized that the fusion of hydrogen nuclei to form deuterium (a hydrogen isotope with one proton and one neutron in its nucleus) is an exoergic process (that is, there is a net release of energy.) By subsequent reactions, it leads to the synthesis of helium. The nucleosynthesis of helium (often called the proton-proton cycle) is the primary source of energy of normal stars, such as the Sun.

The proton-proton cycle starts out with the fusion of two hydrogen nuclei to form deuterium:

Hydrogen + Hydrogen => Deuterium + Positron + Neutrino

The deuterium then captures a further hydrogen nucleus to form helium-3:

Deuterium + Hydrogen => Helium-3 + Gamma ray

The reaction may take one of several paths at this point. At lower temperatures, the most likely continuation of the cycle couple two helium-3 nuclei to produce a helium-4 nucleus plus two protons,

Helium-3 + Helium-3 => Helium-4 + Hydrogen + Hydrogen

Whereas, at higher temperatures, large nuclei are formed, and more energetic neutrinos emitted,

Helium-3 + Helium-4 => Beryllium-7 + Gamma ray

Beryllium-7 + Deuterium => Boron-8 + Gamma ray

Boron-8 => Beryllium-8 + Positron + Neutrino

Beryllium-8 => Helium-4 + Helium-4

No matter which path the cycle takes, the result is Helium-4.

Power generation using nuclear fusion

Harnessing nuclear fusion for peaceful, controlled power generation is highly desirable but has proven more difficult to achieve than with nuclear fission. Nuclear reactions between deuterium and tritium are the most important to consider when pursuing controlled energy creation since such reactions occur frequently and yield high quantities of energy. Simulating conditions found in the core of stars is a difficult undertaking due to the high temperatures and pressures involved, and controlled energy production using nuclear fusion has largely settled in to two means of confirming a high-temperature plasma of elements undergoing nuclear fusion: magnetic confinement and inertial confinement.
In magnetic confinement, the plasma is held in place using magnetic fields. The plasma is held in pressure equilibrium by creating a magnetic well. Significant developments by Russian scientists in the 1960s led to the use of a toroidal magnetic confinement system known as a Tokomak. The Tokomak has been the primary focus of research efforts to harness nuclear fusion for controlled energy production; however, additional techniques such as the compact torus and stellarator have offered additional insight.

In order to achieve fusion, magnetically confined plasmas must be heated to temperatures at which nuclear fusion reactions occur rapidly. This is typically 75 000 000 K and above. This is achieved by magnetically compressing the plasma or by injecting beams of highly energised neutral atoms that ionize and heat the plasma.
Tokomak configurations have been use din the United States, Europe and Japan to create conditions under which practical nuclear fusion will occur. In the United States and Europe, experimental reactors have produced more than 10 MW of power via nuclear fusion.

Inertial confinement processes

In inertial confinement, fuel masses are compressed rapidly to densities up to 10 000 times greater than normal atmospheric pressure for very short periods. At the point of maximum compression, the fuel, in a cool plasma state, is heated by converging shock waves to temperatures high enough to facilitate nuclear fusion. This chain of events employed in this method is very similar to that used in both thermonuclear weapons and in star formation. The fuel mass is collapsed, heated under compression and then fusion is initiated. In order to collapse the fuel, high-power lasers are usually employed, though the same can be achieved using high-energy ions from particle accelerators. Due to the difficulty achieving the extreme conditions required to coerce nuclei to undergo nuclear fusion, for several decades now, researchers have been investigating potential ways in which nuclei can me made to approach each other very closely at much lower temperatures.

Muon-catalyzed fusion is one such technique. In muon-catalyzed fusion, muons are substituted for electrons that usually surround the nucleus of a fuel element. Muons are negatively charged, like electrons, but are 200-times heavier and are very unstable. The essence of this technique is to force the capture of a muon by a deuterium atom and a tritium nucleus, creating a muonic molecule.

Nuclear fusion holds great promise for civilian energy production, but unlike nuclear fission where civilian use was mastered within ten years of its military use, more than fifty years has now passed since nuclear fusion was used for military means and no controlled commercial energy production plant is in operation.

•