Nuclear fusion is the combination of the nuclei of two light atoms to form a heavier one. The resulting atom has a smaller mass than the original ones; therefore, nuclear fusion is a method of transforming mass into energy. This reaction produces the energy of stars such as the sun. By weight, the fusion process yields 8 times more energy than the fission of uranium (see Nuclear Energy), and over a million times more than the burning of fossil fuels. Fusion is a very attractive energy source not only because of its high-energy yield but also because of the almost limitless abundance of its fuels and the fact that its principal by-product, helium, is inert, unlike the radioactive by-products of conventional fission reactors. Supplies of one major fuel, deuterium, a hydrogen isotope found in ordinary seawater, are virtually inexhaustible. The other major fuel, tritium, could be produced from lithium found in land deposits and seawater, which contain supplies for thousands of years.

The amount of fuel in the reactor at any time is very small, so there is no hazard of uncontrolled energy release or runaway reactions. Problems of radioactivity, fuel handling, contamination and waste disposal are small compared to those associated with fission reactors in nuclear power plants. The first man-made fusion reaction was the US thermonuclear hydrogen bomb tested in 1952. Unfortunately, the reaction has proved very difficult to contain and harness for peaceful purposes. Controlled experiments have barely reached the point where the energy released is greater than the energy put in, but if research and development proceed successfully, fusion could be an important commercial energy source early in the 21st century.

The important fusion reactions are those involving the isotopes of hydrogen: hydrogen (H), consisting of 1 proton and 1 electron; deuterium (D), 1 proton, 1 neutron and 1 electron; and tritium (T), 1 proton, 2 neutrons and 1 electron. The products of such reactions are helium (4He), also known as an alpha particle, and energetic neutrons (n) or protons (p). Fusion reactions are difficult to achieve because the interacting nuclei each have a positive electrical charge and, therefore, strongly repel one another. Fusion can occur only if the nuclei approach each other at very high velocities, sufficient to overcome their electrostatic repulsive forces.

To release energy at a practical level, using gaseous deuterium-tritium as a fuel, requires the heating of the mixture to a temperature of 100 million °C or more. Even at lower temperatures, the gas becomes ionized as the electrons become detached from the atoms. In this state, called a plasma, the separated negatively charged electrons and positively charged nuclei move freely, giving the mixture properties different from those of a normal gas. To release more energy than was supplied, it is necessary to confine the plasma to permit a sufficient number of fusion reactions to take place. In the sun the gravitational field heats and confines the hydrogen fuel, resulting in the formation of helium and other heavier elements. On Earth there are 2 classes of approach to containing and heating the plasma: magnetic confinement and inertial confinement.

Since a plasma is a very good conductor of electricity, it can be influenced by magnetic fields. In a magnetic field, the plasma particles are forced to follow spiral paths about the field line; hence, magnetic fields can confine the charged particles of the high-temperature plasma and prevent them from striking the walls of the containing vessel. Many magnetic containment schemes have been suggested and experimentally investigated. One very successful approach has been the tokamak, a closed magnetic-field device with a hollow, doughnut-shaped vessel through which magnetic fields twist to confine the plasma. The fields are produced by external magnetic-field coils and by electrical currents made to flow through the plasma.

Initial heating is often achieved by passing a current through the plasma or by rapidly changing the confining magnetic field, but the required temperatures cannot be reached by such methods. Hence, auxiliary heating techniques are used, eg, neutral beam injection, whereby high-energy neutral atoms are introduced into the hot plasma where they are immediately ionized and trapped by the magnetic field, and radio-frequency heating, which uses high-frequency electromagnetic waves generated by external oscillators and introduced into the plasma where the energy is transferred to the charged particles.

In the inertial-confinement approach to fusion, a small spherical pellet containing the fuel is compressed to extremely high density. This process heats the pellet to the required temperature and causes the fuel to ignite before the compressed mass can disassemble. The interaction occurs so rapidly that the compressed pellet remains together by its own inertia. High-power, short-pulsed lasers and ion-particle beams are the principal candidates for delivering the intense pulses of energy required to heat the outer layers of the fuel pellet rapidly. The ensuing blow off of vaporized material creates an implosion of the fuel. For ignition of D-T fuel to occur, compression of the order of 20 times the density of lead is necessary. The hydrogen bomb uses this approach.

Intensive research is being conducted on controlled fusion energy in many countries, particularly the US, the USSR, Japan and the European Economic Community.

In Varennes, Québec, Canada has a national research facility based on a tokamak machine. The $40-million facility was financed by NRC and Hydro-Québec. The research program is being conducted by a joint utilities/university/industry team composed of Institut de recherche de l'hydro-Québec, Intitut national de la recherche scientifique, Université de Montréal, MPB Technologies Inc and Canatom Inc under the management of both Atomic Energy of Canada Ltd and Hydro-Québec.