nuclear fusion/ kernfusie


In nuclear physics, nuclear chemistry and astrophysics nuclear fusion is the process by which two or more atomic nuclei join together, or "fuse", 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 must be brought close enough for the weak nuclear force to convert either of the identical protons into a neutron, thus 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— smaller nuclei are combined into larger nuclei.

Nuclear fusion occurs naturally in all active stars. 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 (see: fusion 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 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.(source:wikipedia)

Kernfusie is het samensmelten van de kernen van verschillende atomen, waarbij een andere, zwaardere kern wordt gevormd. Wanneer atomen van lichte elementen zoals waterstof samensmelten, wordt hierbij iets van de massa omgezet in energie, in het geval van waterstof ongeveer 0,67%. Het fuseren van zwaardere atomen kost daarentegen juist energie. De overgang tussen 'licht' en 'zwaar' ligt in deze context bij het element ijzer.

Voordat in 1938 de Duitse fysicus Hans Bethe het idee opperde dat de zon en de sterren hun energie opwekken door kernfusie, was het een raadsel waar al die energie vandaan kwam; alle in die tijd bekende chemische reacties leverden daarvoor veel te weinig op. De zon zet per seconde ongeveer 700 miljoen ton waterstof om in circa 695 miljoen ton helium. Het verschil in de massa, rond de 4,4 miljoen ton, is in energie omgezet, waarbij de beroemde formule van Albert Einstein, E = mc², geldt. Kernfusie is ook de energiebron van een waterstofbom, die vele malen krachtiger bleek dan de in de jaren veertig ontwikkelde atoomsplitsingsbom.

Kernfusie is geen kettingreactie; er komen geen deeltjes bij vrij die een nieuwe fusie kunnen veroorzaken. Het proces kan slechts aan de gang gehouden worden onder extreem hoge temperatuur en druk, zoals die rond het middelpunt van een ster heersen. Kernfusie laat, in tegenstelling tot kernsplijting, niet noodzakelijkerwijs radioactieve materialen achter als afval. Daarom proberen wetenschappers kernfusie op aarde te ontwikkelen als schone en veilige energiebron. Het vat waarbinnen de reactie plaatsvindt kan echter door bestraling wel radioactief worden.(bron:wikipedia)



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