Deuterium occurs naturally in seawater (30 grams per cubic metre), which makes it very abundant relative to other energy resources. a On a mass basis, the D-T fusion reaction releases over four times as much energy as uranium fission. Each D-T fusion event releases 17.6 MeV (2.8 x 10 -12 joule, compared with 200 MeV for a U-235 fission and 3-4 MeV for D-D fusion). With current technology, the reaction most readily feasible is between the nuclei of the two heavy forms (isotopes) of hydrogen – deuterium (D) and tritium (T). According to the Massachusetts Institute of Technology (MIT), the amount of power produced increases with the square of the pressure, so doubling the pressure leads to a fourfold increase in energy production. The aim of the controlled fusion research program is to achieve 'ignition', which occurs when enough fusion reactions take place for the process to become self-sustaining, with fresh fuel then being added to continue it. Once ignition is achieved, there is net energy yield – about four times as much as with nuclear fission. Fusion fuel – different isotopes of hydrogen – must be heated to extreme temperatures of the order of 50 million degrees Celsius, and must be kept stable under intense pressure, hence dense enough and confined for long enough to allow the nuclei to fuse. In the Sun, massive gravitational forces create the right conditions for fusion, but on Earth they are much harder to achieve. The nuclei can then fuse, causing a release of energy. Such conditions can occur when the temperature increases, causing the ions to move faster and eventually reach speeds high enough to bring the ions close enough together. However, if the conditions are such that the nuclei can overcome the electrostatic forces to the extent that they can come within a very close range of each other, then the attractive nuclear force (which binds protons and neutrons together in atomic nuclei) between the nuclei will outweigh the repulsive (electrostatic) force, allowing the nuclei to fuse together. Normally, fusion is not possible because the strongly repulsive electrostatic forces between the positively charged nuclei prevent them from getting close enough together to collide and for fusion to occur. Hydrogen, heated to very high temperatures changes from a gas to a plasma in which the negatively-charged electrons are separated from the positively-charged atomic nuclei (ions). But the cost and complexity of the devices involved increased to the point where international co-operation was the only way forward.įusion powers the Sun and stars as hydrogen atoms fuse together to form helium, and matter is converted into energy. Following a breakthrough at the Soviet tokamak, fusion research became 'big science' in the 1970s. Initially, fusion research in the USA and USSR was linked to atomic weapons development, and it remained classified until the 1958 Atoms for Peace conference in Geneva. Today, many countries take part in fusion research to some extent, led by the European Union, the USA, Russia and Japan, with vigorous programmes also under way in China, Brazil, Canada, and Korea. The main hope is centred on tokamak reactors and stellarators which confine a deuterium-tritium plasma magnetically.The fundamental challenge is to achieve a rate of heat emitted by a fusion plasma that exceeds the rate of energy injected into the plasma.Fusion power offers the prospect of an almost inexhaustible source of energy for future generations, but it also presents so far unresolved engineering challenges.
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