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What is Fusion? |
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A fusion reactor design
Diagram courtesy of http://www.jet.edfa.org |
Fusion is the exact opposite of fission. While fission splits apart large atoms, fusion joins together small atoms. Fusion is very hard to produce due to the fact that it requires an incredible amount of heat and energy. In fact, the only two things that can currently produce fusion are stars and hydrogen bombs. Hydrogen bombs create fusion by using a fission bomb to start the fusion reaction. From there, the hydrogen atoms in the center of the bomb begin to fuse together and when they do, a tremendous amount of energy is released. Currently there are no fusion power plants, but several are being constructed. The sun produces fusion at temperatures of 15 million degrees Kelvin; there, hydrogen is fused into helium to support life on earth. For fusion to provide energy here on earth, a different fusion reaction is involved. The most suitable reaction occurs between the nuclei of the two heavy isotopes of hydrogen, Deuterium, and Tritium. Normal hydrogen has one proton and one electron; I will call this form of hydrogen “light hydrogen” for the sake of simplicity. Another form of hydrogen “heavy hydrogen” or deuterium, a rare form of hydrogen with a neutron, proton and an electron. Roughly 1 in 6,700 hydrogen atoms are in this form. The neutron chemically does nothing except add ballast to the atom. The deuterium, just like its “sister” hydrogen, combines with oxygen, to form “heavy water”. This heavy water is what several scientists used to try to start a cold fusion reaction. Heavy water is extracted from normal water, because the deuterium is one of the atoms used to form it. Deuterium as well as tritium, which I will explain shortly, is needed to start a fusion reaction. The reason both of these isotopes of hydrogen are needed is because normal hydrogen would not have an excess neutron to keep the reaction going. Tritium is the “super heavy” form of hydrogen and will also combine with oxygen to form “super heavy water”. Tritium is extremely rare; there is an average of one in every 1*1015 hydrogen atoms! Tritium is three times as heavy as normal hydrogen. It has one proton, two neutrons and one electron. The second neutron not only adds mass, it make tritium unstable, is radioactive and has a half-life of 12 ½ years. Although, tritium is found naturally in normal water, it is almost impossible to extract from water. However hard it is to extract from normal water, tritium forms in heavy water when neutrons bombard it. Because of this, a reactor in Canada, called CANDU, which extracts heavy water; also bombards the heavy water to form super heavy water. Bombarding lithium with neutrons can also produce tritium. In the United States the tritium used in hydrogen bombs was made by putting lithium in a fission reactor, so the neutrons could transmute it into tritium. Nuclear fusion of the tritium releases neutrons that act on the lithium to produce even more tritium.
Art Courtesy of http://www.plasmas.org |
