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NEW TECHNOLOGIES RAU Scientific Reports. Computer Modeling & New Technologies, 1998, Volume 2, 50-56 Riga Aviation University, Lomonosov Str.1, Riga, LV-1019, Latvia NUCLEAR FUSION O. DUMBRAJS Department of Engineering Physics and Mathematics, Helsinki University of Technology, FIN-02150 Espoo, Finland, e-mail: [email protected] Nuclear fusion is the energy-production process that takes place continuously in the sun and stars. It is the reaction in which light nuclei combine together, or fuse, forming a heavier nucleus and releasing energy. Controlled thermonuclear fusion on earth is potentially a major vast new energy source suited to the industrialized word. A reactor based on nuclear fusion would be inherently safe and environmentally friendly. Furthermore, fuels are cheap, abundant and widely available. 1. Introduction At present about six billion people live on earth. Their energy consumption is about 13 TWa (1 TWa equals 1012 Watts per year), of which 3.8 TWa is the electric energy. The energy consumption on average per head is 2.2 kWa: in Europe 5.9 kWa, in the USA 10.9 kWa, in China 0.83 kWa, and in India 0.32 kWa. Annually 6% energy is produced in nuclear power stations and 7% in hydroelectric power stations. The rest comes from the burning of fossil fuels: 10% wood, 26% coal, 39% oil, and 22% gas. Annually 20% electrical energy is produced in nuclear power stations, 20% in hydroelectric power stations, and 60% by burning fossils. It is expected that by the year 2050 ten billion people will live on earth and will consume 30 TWa energy, of which 12 TWa as the electric energy. Two hundred years ago the production of CO2 was 280 PPM with zero growth rate. Now it is 350 PPM with the growth rate of 1.5 ppp/a, and in 50 years it may reach 430 PPM with the growth rate 2 PPM/a. The growth rate of CO2 emission now is 60 times higher than in all the previous 160 000 years together. The problem of the atmosphere pollution has become very acute. One TWa electric energy can be produced either by 3000 coal-fired power stations, or 2000 fission nuclear power stations, or l00 000 km2 solar panels, or 10 millions wind power stations, or by 1000 nuclear fusion power stations. It is rather obvious that nuclear energy is the only option for a truly sustainable or long term energy source. Unfortunately fission, in addition to safety problems (Chernobyl accident!), faces severe risks related to production of long-lived radioactive waste. Fusion will be environmentally sound without pollutants or contribution to global warming. The hazard potential would be limited since there would be no long-lived radioactive waste from the reaction products, including the fuel ”ash.” Radioactivity of the reactor structure, caused by the neutrons, decays rapidly and can be minimized by careful selection of low-activation materials. Fusion is also inherently safe since any malfunction results in a rapid shutdown of the reactor. It will be economically attractive and capable of producing the energy that future generations will require. 2. Fusion reactions The basic principle of nuclear fusion is the fusing together of light nuclei to form heavier ones and in this process a small quantity of mass is converted into a large amount of energy. For energy production on earth different fusion reactions can be used: D + T → 4He + n+ D + D → 3He + n+ D + D → T + p+ D + 4He → He + p+ 17.58 MeV 3.27 MeV 4.03 MeV 18.35 MeV 50 NEW TECHNOLOGIES For example, after fusion of the deuterium and tritium nuclei 17.58 MeV energy is released (1 MeV = 4.45x10-20 kWh). Just this reaction seems to be the most suitable reaction for future fusion power stations. There is no lack of fuel. Deuterium (heavy water) is abundant as it can be extracted from all forms of water. If all the world’s electricity were to be provided by fusion power stations, Deuterium supplies would last for millions of years. Tritium does not occur naturally but it can be manufacture from Lithium within the machine. Here the so-called Tritium breeding reactions are used: 7 6 Li + n → 4He + T + n Li + n → 4He + T Lithium, the lightest metal, is plentiful in the earth’s crust. Once the reaction is established, even though it occurs between Deuterium and Tritium, the consumable are Deuterium and Lithium. For example, 10 grams of Deuterium that can be extracted from 500 liters of water and 15 grams of Tritium produced from 30 grams of Lithium would produce enough fuel for the lifetime electricity needs of an average person in an industrialized country. 3. Conditions for a fusion reaction The problem of fusion reactions is the fact that initial particles are positively charged and to induce the reaction one has first to overcome the Coulomb barrier. This occurs in a plasma where the electrons have been separated from the atomic nuclei (usually called the ”ions”). 1.000.000 Zündung Fusionprodukt = Dichte x Energieschlußzeit x Temperatur (1017 Teilchen pro Kubikzentimeter x Sekunde x Grad) JT 60-U WENDELSTEIN 7-X 100.000 JET TFTR TRTF (DT) DIII-D ALCATOR 10.000 JET 1.000 T10 Tore ASDEX Supra WENDELSTEIN 7-AS JET (DT) TFTR JT 60 ASDEX Pulsator 100 geplant WENDELSTEIN 7-A bis 1995 ISAR 1 ALCATOR: Boston, USA ASDEX: Garching, D D III-D: San-Diego, USA ISAR 1: Garching, D JET: Culham, GB JT 60, JT 60-U: Naka, Japan TFTR: Princeton, USA Tore Supra: Cadarche, F T3, T10: Moskau, Rußland WENDELSTEIN: Garching, D bis 1986 10 T3 bis 1977 T3 bis 1965 1 1 10 100 200 500 1.000 Temperatur (Millionen Grad) Figure 1. Progress in nuclear fusion Fusion reactions occur at a sufficient rate only at very high temperatures. The hot plasma must be well isolated away from material surfaces in order to avoid cooling the plasma. The efficiency of insulation is measured by a quantity called energy confinement time. This is the characteristic time-scale for plasma cooling when the source of heat is removed. The density of fuel ions must be sufficiently large for fusion reactions to take place at the required rate. The fusion power generated is reduced if the fuel is diluted by impurity atoms released from surrounding material surfaces or by the accumulation of Helium ”ash” from the fusion reaction. As fuel ions are burnt in the fusion process they must be replaced by new fuel and the Helium ash must be removed. Numerical values for the Deuterium-Tritium reaction can be summarized as follows: i) plasma temperature: T ≈ 100 – 200 million °C (10 - 20 keV), which is ten times greater than the temperature in the center of the sun; ii) energy confinement time: τ ≈ 1 – 2 seconds; iii) plasma density: n ≈ 2-3·1014 particles cm-3 which corresponds to few thousandths of a gram per cubic meter. Overall the required conditions are measured by a product of these three values called the fusion 51 NEW TECHNOLOGIES product, n ⋅ τ ⋅ T , which is laid off as ordinate in the Figure 1 (see, [1] ). It is encouraging to observe the tremendous progress achieved during the years l965 – 1995 in approaching the plasma ignition (Zündung) region. 4. Plasma confinement It is very difficult to provide the necessary confinement time. Here two principally different methods are used. The first method is the inertial confinement by means of lasers in which extremely high density of the plasma is achieved (n ∼ 1025 cm-3) which allows one significant1y to reduce the confinement time ( τ ∼ 10-9 sec) by keeping constant the fusion product. This is the so-called inertial fusion that is also of interest for developing nuclear weapons and which is not discussed further here. The second method is the magnetic confinement of plasma by means of very strong magnetic fields. This is the so-called magnetically confined fusion and constitutes the subject of the present discussion. There are two different possibilities to magnetically confine the plasma: tokamaks and stellarators. The tokamak is the most advanced concept for containing magnetically a hot dense plasma. It originated in the USSR. A toroidal, axisymmetric plasma is confined by the combination of a large toroidal magnetic field that is produced by coils surrounding the vacuum vessel, and a smaller poloidal magnetic field created by a toroidal current through the plasma. Tokamak transformator coils Figure 2. Principal scheme of TOKAMAK vertical field coils plasma current plasma In the stellarator there is only one confining magnetic field that is produced exclusively by a set of complicated magnetic coils. No plasma current is needed. This is a great advantage in comparison with a tokamak, because it makes a continuous operation of the machine possible in which no plasma disruption can occur. toroidal field coils magnetic field line Stellarator Figure 3. Principal scheme of stellarator 5. Plasma heating and current drive To ensure the high temperature needed for occurrence of fusion reactions, the plasma has to be heated. In tokamaks also a plasma current has to be generated (driven). Three methods are used to accomplish this. i) Ohmic heating The electric current is induced in plasma which deposits heating power. Here the plasma itself plasma magnetic coil plays the role of the secondary magnetic field line winding. To ensure the same direction of the plasma current all the time, only positive or negative half-cycles of voltage can be used which means that the current pulsates. Moreover, since the electric resistance of plasma decreases with increasing temperature, this method of heating can 52 NEW TECHNOLOGIES be effectively used only at low temperatures, that is, for preheating the plasma. Figure 4. Ohmic heating of plasma current field lines coil current plasma current Figure 5. Neutral beam heating magnetic field ion source and accelerator gas chamber charge carrier ii) Neutral beam heating Beams of deuterium or tritium ions, accelerated by very high potential, are injected into the plasma. In order to penetrate the confining magnetic field, the accelerated beams are neutralized. In the plasma, the beams become ionized and the fast ions give up their energy to the plasma ions and electrons by colliding with them. diverting magnet plasma iii) Radio-frequency heating The plasma ions and electrons rotate around in the magnetic field lines of the tokamak or stellarator. Energy is given to the plasma at the precise location where the radiowaves resonate with the particle rotation. Here the two components of the plasma: ions and electrons have to be distinguished. neutral particles motion of ions Figure 6. Radio-frequency heating + Ions are heated by electromagnetic waves at frequencies between 10 and direction of l00 MHz. Such waves are magnetic field generated by klystrons. direction of Electrons are much lighter electric field than ions and are heated by electromagnetic waves generated by gyrotrons at much higher frequencies between 60 and l70 GHz. The helium nuclei (alpha-particles) produced when Deuterium and Tritium fuse remain within the plasma’s magnetic trap. Their energy continues to heat the plasma to keep the fusion reaction going. When the power from the alpha-particles is sufficient to maintain the plasma temperature, the reaction becomes self-heated: a condition referred as ignition. 6. National programs of fusion research Many countries have their own fusion research programs: European Union, USA, Russia, and Japan, supported by vigorous programs in China, Brazil, Canada, and Korea. In the following Table 1 the 53 NEW TECHNOLOGIES most important current and future fusion experiments in the European Union are listed. TABLE 1. Important current and future fusion experiments in the European Union Experiment JET Type tokamak TEXTOR TORE SUPRA tokamak tokamak Laboratory Joint European Torus, Culham, UK FA, Jülich. Germany CEA, Cadarache, France ASDEX Upgrade tokamak IPP, Garching, Germany WENDELSTEIN 7-AS stellarator IPP, Garching, Germany WENDELSTEIN 7-X stellarator IPP, Greifswald, Germany Task Plasma physics studies in the region close to ignition Studies of plasma-wall interaction Testing of super- conducting coils, stationary operation Plasma boundary studies in divertor plasmas Testing the principles of ”advanced stellarator” Begin of operation 1983 Testing feasibility of ”advanced stellarator” for power station 2004 1982 1988 1990 1988 It should be noted that there are fusion installations also in Belgium, Denmark, Portugal, Spain, Sweden, and Switzerland. Two other EU countries, Finland and Greece, also have their own fusion research programs, albeit without own fusion machines. In the European Union all the fusion research is supervised and coordinated by EURATOM. 7. Finnish fusion research program The following chart illustrates organization of fusion research in Finland: Industry Finnfusion industrial R&D prestudies Academy of Science TEKES, Sitra Ministry of Trade and Industry Steering Committee Fusion Program Coordination VTT - Energy Material Research - reactor materials - materials testing - ion beam studies - fusion neutronics Fusion Plasma Research Experiments - theory/simulations - rf-heating - rf-current drive - diagnostics JET Garching Karlsruhe Lausanne Institutes: VTT Technical Research Center of Finland , Helsinki University of Technology , University of Helsinki , Technology Center Pripoli (Finnfusion) An important task of the plasma and fusion physics groups is also to sustain the basic knowledge of fusion physics and reactor technology and to provide relevant education on these topics. The total budget of the Finnish Fusion Research Program for the year 1997 was about four million US dollars. 8. International thermonuclear experimental reactor (ITER) The ITER project was initiated in l985. Its participants are European Union, Russia, Japan, and USA. Each participant is represented by the so-called home team. The European home team resides in 54 NEW TECHNOLOGIES Garching, Germany. The headquarters coordinating the entire ITER relevant research are located in San Diego, USA. ITER may be considered as the first component in a worldwide collaborative program, to work more efficiently and to achieve effective solutions to the scientific and technological challenges presented by fusion research. The advantage of such a program must be to: n reduce scientific and technological risks; n allow flexibility to accommodate new concepts; n provide a wider and more comprehensive database; n offer flexibility in location and time scheduling; n be more practical in industrial terms. This program should minimize risks and overall costs and should ensure efficient use of world resources. Figure 7. ITER tokamak ITER is based on the tokamak concept. It will be able to operate for very long periods (over 1000 second pulses) and will help to demonstrate the scientific and technological feasibility of fusion power. Most importantly, it will be the first fusion device designed to achieve ignition and sustained burn – at which point the reactor becomes self-heating and productive. ITER is planned to be operational by 2010. Its location has yet to be determined. Characteristic ITER data are summarized in the following Table 2 . TABLE 2. Characteristic ITER data Radius Height Plasma radius 18 m 36 m 8.14 m 55 NEW Plasma height Plasma width: Plasma volume Magnetic field: Maximal plasma current Heating and current drive Wall losses induced by neutrons Fusion power: Time of burning: TECHNOLOGIES 9.4 m 5.6 m 2000 m3 5.7 Tesla 21 Megaamperes 100 Megawatts ∼1 Megawatt per m2 1500 Megawatts > 1000 seconds 9. Conclusion If all goes well, commercial application of nuclear fusion should he possible in the next century providing humankind a safe, clean, and inexhaustible energy source for the future. References [1] Kernfusion im Forschungsverbund, Forschungszentrum Karlsruhe, Forschungszentrum Jülich, Max-Planck-lnstitut für Plasmaphysik, Garching, Euratom Assoziationen, März l996. Received on the 17th of January 1998 56