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Fusion PowerFusion power is the extraction of energy in some useful form, generally expected to be electricity, from a nuclear fusion reaction. Technically, most forms of power generation are indirectly fusion-powered, since the Sun is an extremely large natural fusion reactor and its radiation drives most energetic phenomena here on Earth, but the term is usually only used to refer to artificially sustained nuclear fusion. While experiments continue, no actual fusion power generators exist yet. Basic fusion Fusion reactions bring together two atomic nuclei and force them together to combine into one. It takes a large amount of energy to overcome the repuslive electrostatic force between the nuclei, but when they combine the resulting single nucleus has a mass slightly less than the two original ones. The difference in mass becomes energy, as described by E = mc. Hydrogen, the most abundant element in the universe, also has the smallest nuclear charge and therefore reacts at the lowest temperature. Helium has an extremely low mass per nucleon and therefore is energetically favored as a fusion product. As a consequence, most fusion reactions combine isotopes of hydrogen ("protium", deuterium, or tritium) to form isotopes of helium (He or He). The easiest reaction to utilize for fusion power is D + T → He + n. One disadvantage of this reaction is the large number of neutrons produced, so that aneutronic fuels are also of interest for fusion power. Fuel cycle The most probable fusion fuel cycle is based on deuterium and tritium. Deuterium is a naturally ocurring isotope of hydrogen and as such is universally available. The large mass ratio of the hydrogen isotopes makes the separation rather easy compared to the difficult uranium enrichment process. Tritium is also an isotope of hydrogen, but it occurs naturally in only negligible amounts due to its radioactive half-life of 12 years. Consequently, the deuterium-tritium fuel cycle requires the breeding of tritium from lithium using one of the following reactions: - n + Li → T + He
- n + Li → T + He + n
The reactant neutron is supplied by the D-T reaction which also produces the useful energy: - D + T → He + n
The reaction with Li is exothermic, providing a small energy gain for the reactor. The reaction with Li is exothermic but does not consume the neutron. At least some Li reactions are required to replace the neutrons lost by reactions with other elements. Most reactor designs use the naturally occuring mix of lithium isotopes. The supply of lithium is more limited than that of deuterium, but still large enough to supply the world's energy for hundreds of years. Other fuel cycles are possible but much more difficult to realize. Most of them would produce fewer neutrons and not require tritium breeding, both of which could have advantages in terms of fuel availability and safety. First generation fusion reactors are expected to use deuterium and tritium as fuel. Several environmental drawbacks are, however, commonly attributed to D-T fusion power. - It produces substantial amounts of neutrons that result in induced radioactivity within the reactor structure, and it requires the handling of the radioisotope tritium.
- Only about 20% of the fusion energy yield appears in the form of charged particles (the rest neutrons), which limits the extent to which direct energy conversion techniques might be applied.
- The use of D-T fusion power depends on lithium resources, which are less abundant than deuterium resources.
The neutron flux expected in a commercial D-T fusion reactor is about 100 times that of current fission power reactors, posing problems for material design. Design of suitable materials is underway but their actual use in a reactor is not proposed until the generation after ITER. After a single series of D-T tests at JET, the largest fusion reactor yet to use this fuel, the vacuum vessel was sufficiently radioactive that remote handling needed to be used for the year following the tests. These drawbacks of D-T fusion power have led to the proposal of alternatives for longer term application—for example, fusion power reactors based only on deuterium. Such systems are expected to (1) reduce the production of high energy neutrons and also the need to handle tritium; (2) produce more fusion power in the form of charged particles; and (3) be independent of lithium resources for tritium breeding. It has also been suggested that materials with slightly higher atomic numbers (like lithium, beryllium, and boron) be used as fusion fuels to provide power that is essentially free of neutrons and that release all of their energy in the form of charged particles. Although such alternatives to D-T fusion power are attractive, there is an important scientific caveat. To derive useful amounts of power from nuclear fusion, it will be necessary to confine a suitably dense plasma at fusion temperatures (108 K) for a specific length of time. This fundamental aspect of fusion power is expressible in terms of the product of the plasma density, n, and the energy confinement time, τ, required for fusion power break-even (i.e., the condition for which the fusion power release equals the power input necessary to heat and confine the plasma). The required product, nτ, depends on the fusion fuel and is primarily a function of the plasma temperature. Of all the-fusion fuels under current consideration, the deuterium-tritium fuel mixture requires the lowest value of nτ by at least an order of magnitude and the lowest fusion temperatures by at least a factor of 5. When the plasma requirements for significant power generation are compared with the anticipated plasma performance of current approaches to fusion power, it is apparent that fusion power must initially be based on a deuterium-tritium fuel economy. However, the eventual use of alternate fuel cycles remains an important ultimate goal and consequently attention will be given to identifying concepts which may permit their ultimate use. Fusion as a commercial power source Fusion is in several ways unique as a potential energy source. It is technologically difficult to achieve, but could potentially become abundant, safe, and clean compared to alternatives. Accident potential The accident potential of a fusion reactor is very much smaller than that of a fission reactor. The primary reason is that the fuel contained in the reaction chamber is only enough to sustain the reaction for about a minute, whereas a fission reactor contains about a year's supply of fuel. Effluents during normal operation The natural product of the fusion reaction is a small amount of helium, which is completely harmless to life and does not contribute to global warming. Of more concern is tritium, which, like other isotopes of hydrogen, is difficult to retain completely. During normal operation, some amount of tritium will be continually released. There would be no acute danger, but the cumulative effect on the world's population from a fusion economy could be a matter of concern. The 12 year half-life of tritium would at least prevent unlimited build-up and long-term contamination. Waste management The large flux of high-energy neutrons in a reactor will make the structural materials radioactive. The radioactive inventory at shut-down may be comparable to that of a fission reactor, but there are important differences. The half-life of the radioisotopes produced by fusion tend to be less than those from fission, so that the inventory decreases more rapidly. Furthermore, there are fewer different species, and they tend to be non-volatile and biologically less active. As opposed to nuclear fission, where there is hardly any possibility to influence the spectrum of fission products, the problems can be further reduced by careful choice of the materials used. "Low activation" materials like vanadium, for example, would become much less radioactive than stainless steel. Such materials would have half-lives of tens of years, rather than the thousands of years for radioactive waste produced from fission. This involves the design of new alloys with unusual chemical compositions; a complex process as the chemical composition also affects the materials' mechanical properties. Nuclear proliferation Although fusion power uses nuclear technology, the overlap with nuclear weapons technology is small. Tritium is a component of the trigger of hydrogen bombs, but not a major problem in production. The copious neutrons from a fusion reator could be used to breed plutonium for an atomic bomb, but not without extensive redesign of the reactor, so that clandestine production would be easy to detect. The theoretical and computational tools needed for hydrogen bomb design are closely related to those needed for inertial confinement fusion, but have very little in common with (the more promising) magnetic confinement fusion. Achievability and economics Critics point out that it is far from clear that nuclear fusion will indeed be economically competitive with other forms of power. It is not clear that fusion will be cheaper than traditional forms of power, and although there are many economic estimates of the cost of fusion power, these estimates can give wildly different answers as to its economic viability. However it is also argued that fossil fuel is the recipient of many indirect government subsidies, such as the use of armed forces and military aid to provide security in places where fossil fuels are obtained. Including these factors makes an accurate cost comparison very difficult. Unfortunately, despite optimism dating back to the 1950's about the wide-scale harnessing of fusion power, there are still significant barriers standing between current scientific understanding and technological capabilities and the practical realization of fusion as an energy source, and it is unclear that an economically viable fusion plant is even possible. Research, while making steady progress, has continually thrown up new difficulties. Another barrier is materials to withstand the high neutron fluxes, which are speculated to be about 100 times those of existing PWRs. Materials design for a practical plant is still (as of 2004) in its infancy, with proper materials testing not possible in ITER and a proposed materials testing facility, IFMIF, still at the design stage in 2005. Power plant design Humans have known how to create large-scale fusion reactions since 1952, when the United States detonated the first hydrogen bomb, Ivy Mike, as a test. However, an uncontrolled explosive reaction of that magnitude is obviously not well-suited to power generation. It was once proposed that one could use existing large fusion bombs as a source of power by detonating them deep underground and then using the resulting heated cavern as a source of geothermal energy, but such a power plant is unlikely ever to be constructed, for a variety of reasons. See the PACER project for more details. Controlled nuclear fusion within a containment vessel has been possible for some time, but it remains quite difficult to make into a practical generation system. The fusion field refers to a break-even point where the amount of energy put into the reaction is equal to the amount of energy released from the reaction. There are other important energy balance points, however. One is that in a power plant the electricity produced should clearly be much more than the electricity used, a measure that factors in the efficiencies in extracting heat from the reactor and turning it into power. In a fusion reactor, a point usually referred to as "ignition" is defined as the point where the energy released from the fusion reaction itself is sufficient to sustain the reaction without additional energy input (for instance a D-T reaction would be mainly heated from the energy supplied by hot helium nuclei produced from the D-T fusion reactions). Perhaps most important is the balance point is where the system is generating enough money to pay for itself. This is a much more complex calculation, as in the past the price of electricity often fell when a new power plant was introduced, which ruined the economies of many nuclear plants in the 1970s. Fusion systems are typically classified by the type of "confinement" system they use to handle the hot plasma that is the result of a fusion reaction. The majority of research for potential use as an energy source has focused on magnetic confinement, where an arrangement of powerful magnets keeps the fuel in the center of a container. Of the variety of such systems, the Tokamak has produced the best results since it was first introduced. Other systems include the magnetic pinch fusion machines, where a current running through the plasma generates its own magnetic field; inertial confinement fusion systems that use lasers to explosively compress small pellets of fuel; and electrostatic confinement fusion systems, in which ions in the reaction chamber are confined and held at the center of the device by electrostatic forces, as in the Farnsworth-Hirsch Fusor. The different forms of reactor each have advantages and disadvantages. Tokamaks are arguably the most developed magnetic confinement scheme. Inertial confinement produces plasmas with impressive densities and temperatures, and appear to be best suited to weapons research, X-ray generation, very small reactors, and perhaps in the distant future, spaceflight. They rely on fuel pellets with a "perfect" shape in order to generate a symmetrical inward shock wave to produce the high-density plasma, and in practice these have proven difficult to produce. A recent development in the field of laser induced ICF is the use of ultrashort pulse multi-petawatt lasers to heat the plasma of an imploding pellet at exactly the moment of greatest density after it is imploded conventionally using terawatt scale lasers. This research will be carried out on the (currently being built)OMEGA EP petawatt and OMEGA lasers at the University of Rochester and at the GEKKO XII laser at the institute for laser engineering in Osaka Japan, which if fruitful, may have the effect of greatly reducing the cost of a laser fusion based power source. Competition between the various strands of fusion research for funding is fierce, with the large costs involved meaning that practical research has been concentrated mainly on Tokamaks and inertial confinement laser devices (eg. NIF) in the past few years. Most controversially, some researchers have claimed to observe neutron production in electrochemical systems, the so-called cold fusion systems. Other scientists have not been able to reproduce this, and today cold fusion is regarded as pseudoscience. Research into sonoluminescence induced fusion, sometimes known as bubble fusion, continues as well, although it is met with almost equal skepticism. Major controlled fusion experiments Laser driven Dismantled: Electrostatic confinement See also External links
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