Nuclear fusion power
This EOE article is adapted from an information paper published by the World Nuclear Association (WNA). WNA information papers are frequently updated, so for greater detail or more up to date numbers, please see the latest version on WNA website (link at end of article).
Fusion powers the sun and stars as hydrogen atoms fuse together to form helium, and matter is converted into energy. 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). Normally, fusion is not possible because the positively charged nuclei naturally repel each other. But as the temperature increases the ions move faster, and they collide at speeds high enough to overcome the normal repulsion. The nuclei can then fuse, causing a release of energy.
4 1H + 2 e --> 4He + 2 neutrinos + 6 photons
Each time this reaction occurs, 26 million electronvolts (MeV) of energy are released.
In the sun, massive gravitational forces create the right conditions for this, but on Earth they are much harder to achieve. Fusion fuel—different isotopes of hydrogen—must be heated to extreme temperatures of some 100 million degrees Celsius, and must be kept dense enough, and confined for long enough (at least one second), to trigger the energy release. 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.
In principle, fusion has some extremely attractive features. The big advantage of fusion compared with fossil-fuel-based energy production is its relatively small fuel requirements. For the same amount of energy, fusion requires about six orders of magnitude (~106) less fuel compared with chemical energy sources (coal, oil, etc.). A convenient way to think about this is to consider that the hydrogen in an ordinary cup of tap water contains the energy equivalent of a full tank of motor gasoline in an automobile. That is, the approximately one drop of heavy water in that cup could, through fusion, provide as much energy as 20 gallons of motor gasoline.
Basic fusion technology
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). Each D-T fusion event releases 17.6 MeV (2.8 x 10-12 joule, compared with 200 MeV for a uranium-235 (235U) fission). Deuterium occurs naturally in seawater (30 grams per cubic meter), which makes it very abundant relative to other energy resources. Tritium does not occur naturally and is radioactive, with a half-life of around 12 years. It can be made in a conventional nuclear reactor, or in the present context, bred in a fusion system from lithium. Lithium is found in large quantities (30 parts per million) in the Earth's crust and in weaker concentrations in the sea. While the D-T reaction is the main focus of attention, long-term hopes are for a D-D reaction, but this requires much higher temperatures.
In a fusion reactor, the concept is that neutrons will be absorbed in a blanket containing lithium which surrounds the core. The lithium is then transformed into tritium and helium. The blanket must be thick enough (about 1 meter) to slow down the neutrons. This heats the blanket, and a coolant flowing through it then transfers the heat away to produce steam which can be used to generate electricity by conventional methods. The difficulty has been to develop a device that can heat the D-T fuel to a high enough temperature and confine it long enough so that more energy is released through fusion reactions than is used to get the reaction going.
At present, two different experimental approaches are being studied: fusion energy by magnetic confinement (MFE) and fusion by inertial confinement (ICF). The first method uses strong magnetic fields to trap the hot plasma. The second involves compressing a hydrogen pellet by smashing it with strong lasers or particle beams.
Magnetic confinement (MFE)
In magnetic confinement (MFE), hundreds of cubic meters of D-T plasma at a density of less than a milligram per cubic meter are confined by a magnetic field at a few atmospheres pressure and heated to fusion temperature.
Magnetic fields are ideal for confining a plasma because the electrical charges on the separated ions and electrons mean that they follow the magnetic field lines. The aim is to prevent the particles from 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 thin doughnut, in which the magnetic field is curved around to form a closed loop. For proper confinement, this toroidal field must have superimposed upon it a perpendicular field component (a poloidal field). The result is a magnetic field with force lines following spiral (helical) paths, along and around which the plasma particles are guided. There are several types of toroidal confinement systems, the most important being tokamaks, stellarators and reversed field pinch (RFP) devices.
The word tokamak means "toroidal chamber" in Russian. It is a magnetic fusion device that is in a shape of a torus (e.g., a doughnut). 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 strong electric current flowing through the plasma. In a stellarator, the helical lines of force are produced by a series of coils which may themselves be helical in shape. But no current is induced in the plasma. RFP devices have the same toroidal and poloidal components as a tokamak, but the current flowing through the plasma is much 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 a temperature of about 10 million degrees Celsius. Beyond that, additional heating systems are needed to achieve the temperatures necessary for fusion. In stellarators, these heating systems have to supply all the energy needed.
The tokamak (toroidalnya kamera ee magnetnaya katushka—torus-shaped magnetic chamber) was designed in 1951 by Soviet physicists Andrei Sakharov and Igor Tamm. Tokamaks operate within limited parameters outside which sudden losses of energy confinement (disruptions) can occur, causing major thermal and mechanical stresses to the structure and walls. Nevertheless, it is considered the most promising design, and research is continuing on various tokamaks around the world, the two largest being the Joint European Torus (JET) in the UK and the tokamak fusion test reactor (TFTR) at Princeton in the USA.
Research is also being carried out on several types of stellarators. The biggest of these, the Large Helical Device at Japan's National Institute of Fusion Research, began operating in 1998. It is being used to study of the best magnetic configuration for plasma confinement. At Garching in Germany, plasma is created and heated by electromagnetic waves, and this work will be progressed in the W7-X stellerator, to be built at the new German research center in Greifswald. Another stellarator, TJ-II, is under construction in Madrid, Spain. Because stellarators have no toroidal current, there are no disruptions and they can be operated continuously. The disadvantage is that, despite the stability, they do not confine the plasma so well.
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 study the physical problems arising from the spontaneous reorganization of the magnetic field, an intrinsic feature of this configuration.
Inertial confinement (ICF)
In inertial confinement (ICF), a newer line of research, laser or ion beams are focused very precisely onto the surface of a target—a sphere of D-T ice, a few millimeters in diameter. This evaporates or ionizes the outer layer of the material to form a plasma crown that expands, generating an inward-moving compression front or implosion that heats up the inner layers of material. The core or central hot spot of the fuel may be compressed to one thousand times its liquid density, and ignition occurs when the core temperature reaches about 100 million degrees Celsius. Thermonuclear combustion then spreads rapidly through the compressed fuel, producing several times more energy than was originally used to bombard the capsule. The time required for these reactions to occur is limited by the inertia of the fuel (hence the name), but is less than a microsecond. The aim is to produce repeated microexplosions.
Recent work at Osaka, Japan suggests that 'fast ignition' may be achieved at lower temperature with a second very intense laser pulse through a millimeter-high gold cone inside the compressed fuel, and timed to coincide with the peak compression. This technique means that fuel compression is separated from hot spot generation with ignition, making the process more practical.
So far, most inertial confinement work has involved lasers, although their low energy makes it unlikely that they would be used in an actual fusion reactor. The world's most powerful laser fusion facility is the NOVA at Lawrence Livermore Laboratory in the US, and declassified results show compressions to densities of up to 600 times that of the D-T liquid. Various light and heavy ion accelerator systems are also being studied, with a view to obtaining high particle densities.
In an operating fusion reactor, part of the energy generated will serve to maintain the plasma temperature as fresh deuterium and tritium are introduced. However, in the startup of a reactor, either initially or after a temporary shutdown, the plasma will have to be heated to 100 million degrees Celsius. In current tokamak (and other) magnetic fusion experiments, insufficient fusion energy is produced to maintain the plasma temperature. Consequently, the devices operate in short pulses and the plasma must be heated afresh in every pulse.
Since the plasma is an electrical conductor, it is possible to heat the plasma by passing a current through it; in fact, the current that generates the poloidal field also heats the plasma. This is called ohmic (or resistive) heating; it is the same kind of heating that occurs in an electric light bulb or in an electric heater.
The heat generated depends on the resistance of the plasma and the current. But as the temperature of heated plasma rises, the resistance decreases and the ohmic heating becomes less effective. It appears that the maximum plasma temperature attainable by ohmic heating in a tokamak is 20-30 million degrees Celsius. To obtain still higher temperatures, additional heating methods must be used.
Neutral-beam injection involves the introduction of high-energy (neutral) atoms into the ohmically – heated, magnetically – confined plasma. The atoms are immediately ionized and are trapped by the magnetic field. The high-energy ions then transfer part of their energy to the plasma particles in repeated collisions, thus increasing the plasma temperature.
In radio-frequency heating, high-frequency waves are generated by oscillators outside the torus. If the waves have a particular frequency (or wavelength), their energy can be transferred to the charged particles in the plasma, which in turn collide with other plasma particles, thus increasing the temperature of the bulk plasma.
Fusion power plants
In the most likely scenario for a fusion power plant, a deuterium-tritium (D-T) mixture is admitted to the evacuated reactor chamber and there ionized and heated to thermonuclear temperatures. The fuel is held away from the chamber walls by magnetic forces long enough for a useful number of reactions to take place. The charged helium nuclei which are formed give up energy of motion by colliding with newly injected cold fuel atoms which are then ionized and heated, thus sustaining the fusion reaction. The neutrons, having no charge, move in straight lines through the thin walls of the vacuum chamber with little loss of energy.
The neutrons and their 14 MeV of energy are absorbed in a "blanket" containing lithium which surrounds the fusion chamber. The neutrons' energy of motion is given up through many collisions with lithium nuclei, thus creating heat that is removed by a heat exchanger which conveys it to a conventional steam electric plant. The neutrons themselves ultimately enter into nuclear reactions with lithium to generate tritium which is separated and fed back into the reactor as a fuel.
The successful operation of a fusion power plant will require the use of materials resistant to energetic neutron bombardment, thermal stress, and magnetic forces. Additional work also needs to be done on the design of systems for the removal of spent gas.
In 1989, spectacular claims were made for another approach, when two researchers, in USA and UK, claimed to have achieved fusion in a simple tabletop apparatus working at room temperature. Other experimenters failed to replicate this "cold fusion", however, and most of the scientific community no longer considers it a real phenomenon. Nevertheless, research continues. Cold fusion involves the electrolysis of heavy water using palladium electrodes on which deuterium nuclei are said to concentrate at very high densities.
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 under way in China, Brazil, Canada, and Korea. 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 a breakthrough with the Soviet tokamak design, fusion research became big science in the 1970s. But the cost and complexity of the devices involved increased to the point where international co-operation was the only way forward.
In 1978, the European Community (with Sweden and Switzerland) launched the JET project in the UK. JET produced its first plasma in 1983, and saw successful experiments using a D-T fuel mix in 1991. In the USA, the PLT tokamak at Princeton produced a plasma temperature of more than 60 million degrees in 1978 and D-T experiments began on the Tokamak Fusion Test Reactor (TFTR) there in 1993. In Japan, experiments have been carried out since 1988 on the JT-60 Tokamak.
Joint European Torus (JET)
The Joint European Torus (JET) is the largest tokamak operating in the world today. Up to 16 MW of fusion power for one second has been achieved in D-T plasmas using the device and many experiments are conducted to study different heating schemes and other techniques. JET has been very successful in operating remote handling techniques in a radioactive environment to modify the interior of the device, and has shown that the remote handling maintenance of fusion devices is realistic.
In 2001, the U.S. Department of Energy (DOE) and the European Union agreed to conduct joint research in fusion energy extending an umbrella fusion agreement signed in 1986 between Europe and the DOE. Areas of cooperation include tokamaks, alternatives to tokamaks, magnetic fusion energy technology, plasma theory, and applied plasma physics.
International Thermonuclear Experimental Reactor (ITER)
In 1985, the Soviet Union suggested building a next generation tokamak with Europe, Japan and the USA. Collaboration was established under the auspices of the International Atomic Energy Agency (IAEA). Between 1988 and 1990, the initial designs were drawn up for an International Thermonuclear Experimental Reactor (ITER) with the aim of proving that fusion could produce useful energy. The four parties agreed in 1992 to collaborate further on Engineering Design Activities for ITER (ITER is both an acronym, and means 'a path' or 'journey' in Latin). Canada and Kazakhstan are also involved through Euratom and Russia respectively.
Six years later, the ITER Council approved the first comprehensive design of a fusion reactor based on well-established physics and technology with a price tag of US$6 billion. The USA then decided pull out of the project, forcing a 50% reduction in costs and the need for a redesign. The result was the ITER-Fusion Energy Advanced Tokomak (ITER-FEAT), expected to cost $3 billion but still achieve the targets of a self-sustaining reaction and a net energy gain. The energy gain is unlikely to be enough for a power plant, but it will demonstrate feasibility.
In 2003, the USA rejoined the project and China also announced that it would do so. After deadlocked discussion, the six partners agreed in mid-2005 to site ITER at Cadarache, in southern France. The deal involved major concessions to Japan, which had put forward Rokkasho as a preferred site. The EU and France will contribute half of the EUR 10 billion total cost, with the other partners—Japan, China, South Korea, USA and Russia—contributing 10% each. Japan will provide much of the high-tech components, will host an EUR 1 billion materials testing facility, and will have the right to host a subsequent demonstration fusion reactor. The total cost of the 500 MWt ITER comprises about half for the ten-year construction and half for 20 years of operation.
Assessing fusion power
Fusion power plants has the potential to substantially reduce the environmental impacts of increasing world electricity demands since, like nuclear fission power, they would make negligible contributions acid rain or the greenhouse effect compared to fossil fuels. Fusion power could easily satisfy the energy needs associated with continued economic growth, given the ready availability of fuels. There would be no danger of a runaway fusion reaction as this is intrinsically impossible and any malfunction would result in a rapid shutdown of the plant.
However, although fusion generates no radioactive fission products or transuranic elements, and the unburned gases can be treated on site, there would a short-term radioactive waste problem due to activation products. Some component materials will become radioactive during the lifetime of a nuclear reactor, due to bombardment with high-energy neutrons, and will eventually become radioactive waste. The volume of such waste would be similar to that due to activation products from a fission reactor. The radiotoxicity of these wastes would be relatively short-lived compared with the actinides (long-lived alpha-emitting transuranic isotopes) from a fission reactor.
There are also other concerns, such as those first raised in 1973 by the American Association for the Advancement of Science (AAAS). These include the hazard arising from an accident to the magnetic system. The total energy stored in the magnetic field would be similar to that of an average lightning bolt (100 billion joules, equivalent to about 45 tonnes of TNT). Attention was also drawn to the possibility of a lithium fire. In contact with air or water, lithium burns spontaneously and could release many times that amount of energy. Safety of nuclear fusion is a major issue.
But the AAAS was most concerned about the release of tritium into the environment. It is radioactive and very difficult to contain since it can penetrate concrete, rubber and some grades of steel. As an isotope of hydrogen it is easily incorporated into water, making the water itself weakly radioactive. With a half-life of 12.4 years, tritium remains a threat to health for over one hundred years after it is created, as a gas or in water. It can be inhaled, absorbed through the skin or ingested. Inhaled tritium spreads throughout the soft tissues and tritiated water mixes quickly with all the water in the body. The AAAS estimated that each fusion reactor could release up to 2x1012 Bequerels of tritium a day during operation through routine leaks, assuming the best containment systems—much more in a year than the Three Mile Island accident released altogether. Moreover, an accident would release even more. This is one reason why long-term hopes are for the deuterium-deuterium fusion process, dispensing with tritium.
Materials research and development will play a major role in determining fusion's future viability due to the very high energetic neutron bombardment, thermal stress, and magnetic forces.
At this point in time the economics of fusion power are largely unknown. The capital costs are likely to be large, given that a fusion power plant would be much larger in physical size and more complex than a conventional fission power plant. The levelized cost cost of electricity from current fission reactors is greater than that from fossil fuels and wind, so fusion must make significant progress on this front to compete in future electricity markets.
Thus, while the scientific community has made enormous progress in our scientific understanding of fusion, as of yet there is no clearly identified route to an attractive commercial fusion power plant that will sell in the energy marketplace of the 21st century and beyond. While fusion power clearly has much to offer if and when the technology is eventually developed, the problems associated with it also need to be addressed if is to become a widely used future energy source.
- WNA paper on Nuclear fusion power
- Fusion Basics (Princeton Plasma Physics Laboratory (PPPL))
- International Thermonuclear Experimental Reactor (ITER) Website
- Joint European Torus (JET) Website
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