In nuclear physics, nuclear chemistry, and astrophysics, nuclear fusion is the process in which two or more atomic nuclei join together to form a single heavier nucleus. This is usually accompanied by the release or absorption of large quantities of energy. Large scale thermonuclear fusion processes, involving many nuclei fusing at once, must occur in matter at very high densities and temperatures.
The fusion of two nuclei with lower masses than iron (which, along with nickel, has the largest binding energy per nucleon) generally releases energy while the fusion of nuclei heavier than iron absorbs energy. The opposite is true for the reverse process, nuclear fission.
In the simplest case of hydrogen fusion, two protons have to be brought close enough for the weak nuclear force to convert either of the identical protons into a neutron forming the hydrogen isotope deuterium. In more complex cases of heavy ion fusion involving two or more nucleons, the reaction mechanism is different, but the same result occurs - one of combining smaller nuclei into larger nuclei.
Nuclear fusion occurs naturally in all active stars. See astrophysics. Synthetic fusion as a result of human actions has also been achieved, although this has not yet been completely controlled as a source of nuclear power. In the laboratory, successful nuclear physics experiments have been carried out that involve the fusion of many different varieties of nuclei, but the energy output has been negligible in these studies. In fact, the amount of energy put into the process has always exceeded the energy output.
Uncontrolled nuclear fusion has been carried out many times in nuclear weapons testing, which always results in a deliberate explosion. These explosions have always used the heavy isotopes of hydrogen, deuterium (H-2) and tritium (H-3), and never the much more common isotope of hydrogen (H-1), sometimes called "protium".
Building upon the nuclear transmutation experiments by Ernest Rutherford, carried out several years earlier, the fusion of the light nuclei (hydrogen isotopes) was first accomplished by Mark Oliphant in 1932. Then, the steps of the main cycle of nuclear fusion in stars were first worked out by Hans Bethe throughout the remainder of that decade.
Research into fusion for military purposes began in the early 1940s as part of the Manhattan Project, but this was not accomplished until 1951 (see the Greenhouse Item nuclear test), and nuclear fusion on a large scale in an explosion was first carried out on November 1, 1952, in the Ivy Mike hydrogen bomb test. Research into developing controlled thermonuclear fusion for civil purposes also began in the 1950s, and it continues to this day.
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