Nuclear Fusion Power

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Nuclear Fusion Power (Updated May 2010)  l l Fusion power offers the prospect of an almost inexhaustible source of energy for future generations, but it also presents so far insurmountable scientific and engineering challenges.  The main hope is centred on tokamak reactors which confine a deuterium-tritium plasma magnetically.  Today, many countries take part in fusion research to some extent, led by the European Union, the USA, Russia and Japan, with vigorous programs also underway in China,
  Nuclear Fusion Power (Updated May 2010)   l Fusion power offers the prospect of an almost inexhaustible source of energy for futuregenerations, but it also presents so far insurmountable scientific and engineeringchallenges.   l The main hope is centred on tokamak reactors which confine a deuterium-tritium plasmamagnetically.   Today, many countries take part in fusion research to some extent, led by the European Union, theUSA, Russia and Japan, with vigorous programs also underway in China, Brazil, Canada, andKorea. 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. Following abreakthrough at the Soviet tokamak, fusion research became 'big science' in the 1970s. But thecost and complexity of the devices involved increased to the point where international co-operationwas the only way forward.Fusion technologyFusion powers the Sun and stars as hydrogen atoms fuse together to form helium, and matter isconverted into energy. Hydrogen, heated to very high temperatures changes from a gas to aplasma in which the negatively charged electrons are separated from the positively charged atomicnuclei (ions). Normally, fusion is not possible because the strongly repulsive electrostatic forcesbetween the positively charged nuclei prevent them from getting close enough together for fusion tooccur. However, if the conditions are such that the nuclei can overcome the electrostatic forces tothe extent that they can come within a very close range of each other, then the attractive nuclearforce (which binds protons and neutrons together in atomic nuclei) between the nuclei will outweighthe repulsive (electrostatic) force, allowing the nuclei to fuse together. Such conditions can occurwhen the temperature increases, causing the ions to move faster and eventually reach speeds highenough to bring the ions close enough together. The nuclei can then fuse, causing a release ofenergy.In the Sun, massive gravitational forces create the right conditions for fusion, but on Earth they aremuch harder to achieve. Fusion fuel  – different isotopes of hydrogen  – must be heated to extremetemperatures of the order of 100 million degrees Celsius, and must be kept dense enough, andconfined for long enough, to allow the nuclei to fuse. The aim of the controlled fusion researchprogram is to achieve 'ignition', which occurs when enough fusion reactions take place for theprocess to become self-sustaining, with fresh fuel then being added to continue it.With current technology, the reaction most readily feasible is between the nuclei of the two heavyforms (isotopes) of hydrogen  – deuterium (D) and tritium (T). Each D-T fusion event releases 17.6MeV (2.8 x 10 -12 joule, compared with 200 MeV for a U-235 fission). a Deuterium occurs naturally inseawater (30 grams per cubic metre), which makes it very abundant relative to other energyresources. Tritium does not occur naturally and is radioactive, with a half-life of around 12 years. Itcan be made in a conventional nuclear reactor, or in the present context, bred in a fusion systemfrom lithium. b Lithium is found in large quantities (30 parts per million) in the Earth's crust and inweaker concentrations in the sea.  1 / 11   In a fusion reactor, the concept is that neutrons generated from the D-T fusion reaction will beabsorbed in a blanket containing lithium which surrounds the core. The lithium is then transformedinto tritium (which is used to fuel the reactor) and helium. The blanket must be thick enough (about 1metre) to slow down the neutrons. The kinetic energy of the neutrons is absorbed by the blanket,causing it to heat up. The heat energy is collected by the coolant (water, helium or Li-Pb eutectic)flowing through the blanket and, in a fusion power plant, this energy will be used to generateelectricity by conventional methods.The difficulty has been to develop a device that can heat the D-T fuel to a high enough temperatureand confine it long enough so that more energy is released through fusion reactions than is used toget the reaction going. While the D-T reaction is the main focus of attention, long-term hopes are fora D-D reaction, but this requires much higher temperatures.At present, two main experimental approaches are being studied: magnetic confinement andinertial confinement. The first method uses strong magnetic fields to contain the hot plasma. Thesecond involves compressing a small pellet containing fusion fuel to extremely high densities usingstrong lasers or particle beams.Magnetic confinementIn magnetic confinement fusion, hundreds of cubic metres of D-T plasma at a density of less than amilligram per cubic metre are confined by a magnetic field at a few atmospheres pressure andheated to fusion temperature.Magnetic fields are ideal for confining a plasma because the electrical charges on the separatedions and electrons mean that they follow the magnetic field lines. The aim is to prevent the particlesfrom coming into contact with the reactor walls as this will dissipate their heat and slow them down.The most effective magnetic configuration is toroidal, shaped like a doughnut, in which themagnetic field is curved around to form a closed loop. For proper confinement, this toroidal fieldmust have superimposed upon it a perpendicular field component (a poloidal field). The result is amagnetic field with force lines following spiral (helical) paths that confine and control the plasma.There are several types of toroidal confinement system, the most important being tokamaks,stellarators and reversed field pinch (RFP) devices.In a tokamak, the toroidal field is created by a series of coils evenly spaced around the torus-shaped reactor, and the poloidal field is created by a system of horizontal coils outside the toroidalmagnet structure. A strong electric current is induced in the plasma using a central solenoid, andthis induced current also contributes to the poloidal field. In a stellarator, the helical lines of force areproduced by a series of coils which may themselves be helical in shape. Unlike tokamaks,stellarators do not require a toroidal current to be induced in the plasma. RFP devices have thesame toroidal and poloidal components as a tokamak, but the current flowing through the plasma ismuch stronger and the direction of the toroidal field within the plasma is reversed.In tokamaks and RFP devices, the current flowing through the plasma also serves to heat it to atemperature of about 10 million degrees Celsius. Beyond that, additional heating systems areneeded to achieve the temperatures necessary for fusion. In stellarators, these heating systemshave to supply all the energy needed.The tokamak ( toroidalnya kamera ee magnetnaya katushka     – torus-shaped magnetic chamber)  2 / 11 Nuclear_Fusion_Power   was designed in 1951 by Soviet physicists Andrei Sakharov and Igor Tamm. Tokamaks operatewithin limited parameters outside which sudden losses of energy confinement (disruptions) canoccur, causing major thermal and mechanical stresses to the structure and walls. Nevertheless, it isconsidered the most promising design, and research is continuing on various tokamaks around theworld.Research is also being carried out on several types of stellarator. Lyman Spitzer devised andbegan work on the first fusion device  – a stellarator  – at the Princeton Plasma Physics Laboratoryin 1951. Due to the difficulty in confining plasmas, stellarators fell out of favour until computermodelling techniques allowed accurate geometries to be calculated. Because stellarators have notoroidal plasma current, plasma stability is increased compared with tokamaks. Since the burningplasma can be more easily controlled and monitored, stellerators have an intrinsic potential forsteady-state, continuous operation. The disadvantage is that, due to their more complex shape,stellarators are much more complex than tokamaks to design and build.RFP devices differ from tokamaks mainly in the spatial distribution of the toroidal magnetic field,which changes sign at the edge of the plasma. The RFX machine in Padua, Italy is used to studythe physical problems arising from the spontaneous reorganisation of the magnetic field, which isan intrinsic feature of this configuration.Inertial confinementIn inertial confinement fusion, which is a newer line of research, laser or ion beams are focused veryprecisely onto the surface of a target, which is a pellet of D-T fuel, a few millimetres in diameter.This heats the outer layer of the material, which explodes outwards generating an inward-movingcompression front or implosion that compresses and heats the inner layers of material. The core ofthe fuel may be compressed to one thousand times its liquid density, resulting in conditions wherefusion can occur. The energy released then heats the surrounding fuel, which may also undergofusion leading to a chain reaction (known as ignition) as the reaction spre ads outwards through thefuel. The time required for these reactions to occur is limited by the inertia of the fuel (hence thename), but is less than a microsecond. So far, most inertial confinement work has involved lasers.Recent work at Osaka University's Institue of Laser Engineering in Japan suggests that ignition maybe achieved at lower temperature with a second very intense laser pulse guided through amillimetre-high gold cone into the compressed fuel, and timed to coincide with the peakcompression. This technique, known as 'fast ignition', means that fuel compression is separatedfrom hot spot generation with ignition, making the process more practical.A completely different concept, the 'Z-pinch' (or 'zeta pinch'), uses a strong electrical current in aplasma to generate X-rays, which compress a tiny D-T fuel cylinder.Hybrid fusionFusion can also be combined with fission in what is referred to as hybrid nuclear fusion where theblanket surrounding the core is a subcritical fission reactor. The fusion reaction acts as a source ofneutrons for the surrounding blanket, where these neutrons are captured, resulting in fissionreactions taking place. These fission reactions would also produce more neutrons, therebyassisting further fission reactions in the blanket.The concept of hybrid fusion can be compared with an accelerator-driven system (ADS), where anaccelerator is the source of neutrons for the blanket assembly, rather than nuclear fusion reactions  3 / 11 Nuclear_Fusion_Power   (see page onAccelerator-driven Nuclear Energy). The blanket of a hybrid fusion system cantherefore contain the same fuel as an ADS    – for example, the abundant element thorium or the long-lived heavy isotopes present in used nuclear fuel (from a conventional reactor) could be used asfuel.The blanket containing fission fuel in a hybrid fusion system would not require the development ofnew materials capable of withstanding constant neutron bombardment, whereas such materialswould be needed in the blanket of a 'conventional' fusion system. A further advantage of a hybridsystem is that the fusion part would not need to produce as many neutrons as a (non-hybrid) fusionreactor would in order to generate more power than is consumed  – so a commercial-scale fusionreactor in a hybrid system does not need to be as large as a fusion-only reactor.Fusion researchA long-standing joke about fusion points out that, since the 1970s, commercial deployment of fusionpower has always been about 40 years away. While there is some truth in this, many breakthroughshave been made, particularly in recent years, and there are a number of major projects underdevelopment that may bring research to the point where fusion power can be commercialised.Several tokamaks have been built, including the Joint European Torus (JET) in the UK and thetokamak fusion test reactor (TFTR) at Princeton in the USA. The ITER (International ThermonuclearExperimental Reactor) project currently under construction in Cadarache, France will be the largesttokomak.Much research has also been carried out on stellarators. The biggest of these, the Large HelicalDevice at Japan's National Institute of Fusion Research, began operating in 1998. It is being usedto study of the best magnetic configuration for plasma confinement. At the Garching site of the MaxPlanck Institute for Plasma Physics in Germany, research carried out at the Wendelstein 7-ASbetween 1988 and 2002 will be progressed at the Wendelstein 7-X, which is under constructionMax Planck Institute's Greifswald site. Due to be completed by 2015, Wendelstein 7-X, will be thelargest stellarator and it is planned to operate continuously for up to 30 minutes. Another stellarator,TJII, is in operation in Madrid, Spain. In the USA, at Princeton Plasma Physics Laboratory, wherethe first stellarators were built in 1951, construction on the NCSX stellerator was abandoned in2008 due to cost overruns and lack of funding 2 . There have also been significant developments in research into inertial confinement fusion.Construction of the National Ignition Facility at the Lawrence Livermore National Laboratory (LLNL)was completed in March 2009 and the first attempts at ignition are expected in 2010. The Laser Mégajoule in France’ s Bordeaux region is expected to be completed in 2010 with first experimentsthe following year.ITERIn 1985, the Soviet Union suggested building a next generation tokamak with Europe, Japan andthe USA. Collaboration was established under the auspices of the International Atomic EnergyAgency (IAEA). Between 1988 and 1990, the initial designs were drawn up for an InternationalThermonuclear Experimental Reactor (ITER, which also means 'a path' or 'journey' in Latin) with theaim of proving that fusion could produce useful energy. The four parties agreed in 1992 tocollaborate further on engineering design activities for ITER. Canada and Kazakhstan are alsoinvolved through Euratom and Russia, respectively.  4 / 11 Nuclear_Fusion_Power
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