Nuclear fusion is an atomic reaction in which multiple atom s combine to create a single, more massive atom. The resulting atom has a slightly smaller mass than the sum of the masses of the original atoms. The difference in mass is released in the form of energy during the reaction, according to the Einstein formula E = mc 2 , where E is the energy in joule s, m is the mass difference in kilogram s, and c is the speed of light (approximately 300,000,000 or 3 x 10 8 meters per second).
The most common nuclear fusion reaction in the universe, and the one of most interest to scientists, is the merging of hydrogen nuclei to form helium nuclei. This is the process that occurs in the interiors of stars including the sun. Hydrogen fusion is responsible for the enormous energy output that stars produce. The reaction involves three steps. First, two proton s combine to form a deuterium nucleus, which consists of one proton and one neutron . A positron (also called an anti-electron) and a neutrino (a particle with negligible mass but extreme penetrating power) are generated during this part of the process. Second, the deuterium nucleus combines with another proton, forming a nucleus of helium 3, which consists of two protons and one neutron. An energetic photon is produced during this part of the process, with a wavelength in the gamma-ray portion of the electromagnetic spectrum . Finally, two nuclei of helium 3 combine to form a nucleus of helium 4, which consists of two protons and two neutrons. In this part of the process, two protons (ordinary hydrogen nuclei) are released. These protons can eventually become involved in another fusion reaction.
Nuclear fusion requires extremely high temperatures, on the order of tens of millions of degrees Celsius . In addition, an intense attractive force, such as gravitation of the magnitude that occurs in the centers of stars, is necessary to overcome the electrostatic repulsion among positively charged nuclei. Scientists can generate the high temperatures and forces required to produce uncontrolled hydrogen fusion, the most notable example being the hydrogen bomb. However, sustaining these temperatures and forces indefinitely, in order to construct a hydrogen fusion reactor that can generate useful energy, has proven difficult. Research in this direction took a significant step forward in June 2005, with the announcement of an experimental fusion reactor to be built in the south of France.