Like nuclear fission, nuclear fusion releases some of the energy normally contained in an atom's nucleus by converting some of the mass to energy in the form of fast-moving neutrons. While energy released through fission involves splitting nuclei, the opposite is true in fusion, where joining two nuclei produces energy.
Starting and sustaining the fusion process requires temperatures three times hotter than the sun's interior. The sun itself is a large nuclear-fusion reactor operating on a continuous basis, and the solar energy we receive is a byproduct of nuclear fusion.
The first controlled fusion reactions probably will use an isotope of hydrogen - known as tritium - bred from the element lithium.
Despite years of research, no one yet has been able to sustain a fusion reaction in the laboratory. But researchers believe they are progressing on several different approaches.
One is a magnetic confinement fusion device with torus. The deuterium-tritium plasma whirls around the torus, a doughnut-shaped vacuum chamber, where the magnetic field confines and compresses the plasma, causing it to heat up. A strong electric current from outside augments the heat. To achieve a fusion reaction, the device must keep this plasma at 180 million degrees Fahrenheit for about one second. The lithium blanket in the torus transfers heat to an exchanger to produce steam and drive a turbine-generator to produce electricity.
An alternative is the laser fusion device. Here laser beams strike the deuterium-tritium fuel pellets as they fall into the vessel. Each resulting fusion reaction is a small explosion that releases kinetic energy. When this kinetic energy becomes heat, it can power a conventional steam-turbine generator to make electricity.
Nuclear fusion has the potential to produce great quantities of electricity from cheap and virtually inexhaustible fuel, but development is still in its infancy. Even if scientists can solve fusion's problems, they do not expect to operate a demonstration plant before 2020.