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Fusion power is safe, sustainable power generated by nuclear fusion reactions. In this kind of reaction two light atomic nuclei fuse together to form a heavier nucleus and release energy. The largest current experiment, JET, has resulted in fusion power production somewhat larger than the power put into the plasma, maintained for a few seconds. In June 2005, the construction of the experimental reactor ITER, designed to produce several times more fusion power than the power into the plasma over many minutes, was announced. The production of net electrical power from fusion is planned for the next generation experiment after ITER.

Fuel cycle

For two nuclei to fuse, they must collide with enough energy to overcome the repulsive electrostatic force between them. Most fusion generation experiments therefore raise their fuel to very high temperatures. If two light nuclei come close enough to each other, they may fuse to form a single nucleus with a slightly smaller mass than the sum of their original masses. The difference in mass is released as energy according to Einstein's equation E = mc². (If the input nuclei are sufficiently massive, the resulting fusion product will be heavier than the reactants, and the reaction requires the addition of energy to convert into the additional mass; in this case the reverse process of nuclear fission will release energy, which can be used, for example, in nuclear reactors or bombs.)

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 (³He or ⁴He).

Perhaps the three most widely considered fuel cycles are based on the D-T, D-D, and p-¹¹B reactions. Other fuel cycles (D-³He and ³He-³He) would require a supply of ³He, either from other nuclear reactions or from extra-terrestrial sources, such as the surface of the moon or the atmospheres of the gas giant planets. The details of the calculations comparing these reactions can be found here.

The D-T fuel cycle

The easiest and most immediately promising nuclear reaction to be used for fusion power is:

D + T → ⁴He + n

Deuterium is a naturally occurring 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 + ⁶Li → T + ⁴He

n + ⁷Li → T + ⁴He + n

The reactant neutron is supplied by the D-T fusion reaction shown above, the one which also produces the useful energy. The reaction with ⁶Li is exothermic, providing a small energy gain for the reactor. The reaction with ⁷Li is endothermic but does not consume the neutron. At least some ⁷Li reactions are required to replace the neutrons lost by reactions with other elements. Most reactor designs use the naturally occurring 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 demand for hundreds of years.

Several drawbacks are commonly attributed to D-T fusion power.

1. It produces substantial amounts of neutrons that result in induced radioactivity within the reactor structure, and it requires the handling of the radioisotope tritium.
2. 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.
3. 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.

On the other hand, the volumetric deposition of neutron power can also be seen as an advantage. If all the power of a fusion reactor had to be transported by conduction through the surface enclosing the plasma, it would be very difficult to find materials and a construction that would survive, and it would probably entail a relatively poor efficiency.

The D-D fuel cycle

After the reaction of tritium with deuterium, it is easiest to achieve fusion through the reaction of deuterium with itself. This reaction has two branches that occur with nearly equal probability:

D + D	→ T	+ p
	→ 3He	+ n

The optimum temperature for this reaction is 15 keV, only slightly higher than the optimum for the D-T reaction. The first branch does not produce neutrons, but it does produce tritium, so that a D-D reactor will not be completely tritium-free, even though it does not require an input of tritium or lithium. Most of the tritium produced will be burned before leaving the reactor, which reduces the tritium handling required, but also means that more neutrons are produced and that some of these are very energetic. The neutron from the second branch has an energy of only 2.45 MeV, whereas the neutron from the D-T reaction has an energy of 14.1 MeV, resulting in a wider range of isotope production and material damage. Assuming complete tritium burn-up, the reduction in the fraction of fusion energy carried by neutrons is only about 18%, so that the primary advantage of the D-D fuel cycle is that tritium breeding is not required. Other advantages are independence from limitations of lithium resources and a somewhat softer neutron spectrum. The price to pay compared to D-T is that the energy confinement (at a given pressure) must be 30 times better and the power produced (at a given pressure and volume) is 68 times less.

The p-¹¹B fuel cycle

If aneutronic fusion is the goal, then the ideal reaction may be the proton-boron reaction:

p + ¹¹B → 3 ⁴He

Under reasonable assumptions, side reactions will result in about 0.1% of the fusion power being carried by neutrons. At 123 keV, the optimum temperature for this reaction is nearly ten times higher than that for the pure hydrogen reactions, the energy confinement must be 500 times better than that required for the D-T reaction, and the power density will be 2500 times lower than for D-T.

History of fusion energy

Fission as well as fusion were firstly used in the military field, in order to build very powerful bombs: A-bombs for fission and H-bombs for fusion.

Civilian applications, in which explosive energy production must be replaced by a controlled production, were developed later. Although it took less than ten years for going from military applications to civilian fission energy production^[1], it was very different in the fusion energy field, more than fifty years having already passed^[2] without any energy production plant being started up.

Registration of the first patent related to a fusion reactor^[3] by the United Kingdom Atomic Energy Authority, the inventors being Sir George Paget Thomson and Moses Blackman, dates back to 1946. Some basic principles used in ITER experiment are described in this patent: toroidal vacuum chamber, magnetic confinement, and radio frequency plasma heating.

In the magnetic confinement field, the theoretical works fulfilled in 1950-1951 by I.E. Tamm and A.D. Sakharov in Soviet Union, laid the foundations of the tokamak, subsequent research and developments inside the Kurchatov Institute of Moscow having materialized these ideas. Research equipments of this type were subsequently developed in numerous countries and, althouth stellarator competed with it for a while, the tokamak principle was selected for the ITER project.

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The Z-pinch phenomenon is known since the end of the 18^th century^[4]. Its use in the fusion field comes from research made on toroidal devices, initially in the Los Alamos National Laboratory right from 1952 (Perhapsatron), and in Great Britain from 1954 (ZETA), but its physical principles remained for a long time poorly understood and controlled. The appearance of the "wires array" concept in the 1980's allowed a more efficient use of this technique.

Although laser use in order to initiate fusions has been considered earlier, ICF experiments started in earnest in the mid-1970's, when lasers of the required power were first designed. The technique of "ablative" implosion of a microcapsule irradiated by laser beams, basis of the laser inertial confinement, was suggested in 1972 by the Lawrence Livermore National Laboratory.

Safety and environmental issues

Accident potential

The likelihood of a catastrophic accident in a fusion reactor in which injury or loss of life occurs is 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. Furthermore, fusion requires very extreme and precisely controlled conditions of temperature, pressure and magnetic field parameters. If the reactor were damaged, these would be disrupted and the reaction would be rapidly quenched(extinguished).

Although the plasma in a fusion power plant will have a volume of 1000 cubic meters or more, the total amount of fusion fuel in the vessel is very small: only two grams, enough for just a short time of operation. If the fuel supply is closed, the reaction stops without problems within seconds. Fusion is not a chain reaction and can therefore not run out of hand: in the normal situation, the fusion process runs at the fastest possible rate, and any deviation from this optimum leads to a decrease in energy production.

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 unique 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. This involves the design of new alloys with unusual chemical compositions, which is a complex process as the chemical composition also affects the materials' mechanical properties. However rather than the thousands of years for radioactive waste produced from fission, such low activation materials would have half-lives of tens of years and rapidly approach the radioactivity of coal ash. [1].

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 reactor 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 scientifically developed) magnetic confinement fusion.

Fusion power as a sustainable energy source

Fusion power is often described as a "clean", "renewable", or "sustainable" energy source. While it has the potential to be used indefinitely, like most forms of renewable energy, large scale reactors using neutronic fuels (e.g. ITER) and thermal power production (turbine based) are in fact most comparable from an engineering and economics viewpoint to fission power. Both fission and fusion power plants involve a relatively compact heat source powering a conventional steam turbine based power plant, while producing enough neutron radiation to make activation of the plant materials problematic. The main distinction is that fusion power produces no high-level radioactive waste (though activated plant materials still need to be disposed of). There are some power plant ideas which may significantly lower the cost or size of such plants, however research in these areas is nowhere near as advanced as in tokamaks.

Theoretical Power plant designs

Confinement concepts

Confinement refers to all the conditions necessary to keep a plasma dense and hot long enough to undergo fusion:

• Equilibrium: There must be no net forces on any part of the plasma, otherwise it will rapidly disassemble. The exception, of course, is inertial confinement, where the relevant physics must occur faster than the disassembly time.
• Stability: The plasma must be so constructed that small deviations are restored to the initial state, otherwise some unavoidable disturbance will occur and grow exponentially until the plasma is destroyed.
• Transport: The loss of particles and heat in all channels must be sufficiently slow. The word "confinement" is often used in the restricted sense of "energy confinement".

The first human-made, large-scale production of fusion reactions was the test of the hydrogen bomb, Ivy Mike, in 1952. It was once proposed to use hydrogen bombs as a source of power by detonating them in underground caverns and then generating electricity from the heat produced, but such a power plant is unlikely ever to be constructed, for a variety of reasons. (See the PACER project for more details.) Controlled thermonuclear fusion (CTF) refers to the alternative of continuous power production, or at least the use of explosions that are so small that they do not destroy a significant portion of the machine that produces them.

To produce self-sustaining fusion, the energy released by the reaction (or at least a fraction of it) must be used to heat new reactant nuclei and keep them hot long enough that they also undergo fusion reactions. Retaining the heat is called energy confinement and may be accomplished in a number of ways.

The hydrogen bomb really has no confinement at all. The fuel is simply allowed to fly apart, but it takes a certain length of time to do this, and during this time fusion can occur. This approach is called inertial confinement. If more than milligram quantities of fuel are used (and efficiently fused), the explosion would destroy the machine, so theoretically, controlled thermonuclear fusion using inertial confinement would be done using tiny pellets of fuel which explode several times a second. To induce the explosion, the pellet must be compressed to about 30 times solid density with energetic beams. If the beams are focused directly on the pellet, it is called direct drive, which can in principle be very efficient, but in practice it is difficult to obtain the needed uniformity. An alternative approach is indirect drive, in which the beams heat a shell, and the shell radiates x-rays, which then implode the pellet. The beams are commonly laser beams, but heavy and light ion beams and electron beams have all been investigated.

Inertial confinement produces plasmas with impressively high densities and temperatures, and appears 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 close to 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.

At the temperatures required for fusion, the fuel is in the form of a plasma with very good electrical conductivity. This opens the possibility to confine the fuel and the energy with magnetic fields, an idea known as magnetic confinement. The Lorenz force works only perpendicular to the magnetic field, so that the first problem is how to prevent the plasma from leaking out the ends of the field lines. There are basically two solutions.

The first is to use the magnetic mirror effect. If particles following a field line encounter a region of higher field strength, then some of the particles will be stopped and reflected. Advantages of a magnetic mirror power plant would be simplified construction and maintenance due to a linear topology and the potential to apply direct conversion in a natural way, but the confinement achieved in the experiments was so poor that this approach has been essentially abandoned.

The second possibility to prevent end losses is to bend the field lines back on themselves, either in circles or more commonly in nested toroidal surfaces. The most highly developed system of this type is the tokamak, with the stellarator being a distant second, but still a serious contender. A third toroidal machine type is the Reversed-Field Pinch, which was never sufficiently able to realize its potential advantages. Compact toroids, especially the Field-Reversed Configuration and the spheromak, attempt to combine the advantages of toroidal magnetic surfaces with those of a simply connected (non-toroidal) machine. Compact toroids still have some enthusiastic supporters but are not backed as readily by the majority of the fusion community.

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A more subtle technique is to use more unusual particles to catalyse fusion. The best known of these is Muon-catalyzed fusion which uses muons, which behave somewhat like electrons and replace the electrons around the atoms. These muons allow atoms to get much closer and thus reduce the kinetic energy required to initiate fusion. Muons require more energy to produce than we can get back from muon-catalysed fusion, making this approach impractical for the generation of power.

Finally, there are also 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, but these are not believed capable of being developed into a practical power plant.

Most controversially, some researchers have claimed to have observed excess heat, neutrons, tritium, helium and other nuclear effects in so-called cold fusion systems. Most scientists consider cold fusion to be a pseudoscience. A peer review panel was commissioned by the US Department of Energy to study these claims [2] [3] and the majority of the panel did not consider the evidence for low energy nuclear reactions convincing, nor did any of the panel members advocate funding for cold fusion. However, some researchers, including some from Los Alamos, and the Naval Research Laboratory[4] continue to conduct research relating to cold fusion. Research into sonoluminescence induced fusion, sometimes known as "bubble fusion", also continues, although it is met with skepticism almost equal to cold fusion by most of the scientific community.

Subsystems

In fusion research, achieving a fusion energy gain factor Q = 1 is called breakeven and is considered a significant although somewhat artificial milestone. Ignition refers to an infinite Q, that is, a self-sustaining plasma where the losses are made up for by fusion power without any external input. In a practical fusion reactor, some external power will always be required for things like current drive, refueling, profile control, and burn control. A value on the order of Q = 20 will be required if the plant is to deliver much more energy than it uses internally.

There have been many design studies for fusion power plants. Despite many differences, there are several systems that are common to most. To begin with, a fusion power plant, like a fission power plant, is customarily divided into the nuclear island and the balance of plant. The balance of plant is the conventional part that converts high-temperature heat into electricity via steam turbines. It is much the same in a fusion power plant as in a fission or coal power plant. In a fusion power plant, the nuclear island has a plasma chamber with an associated vacuum system, surrounded by a plasma-facing components (first wall and divertor) maintaining the vacuum boundary and absorbing the thermal radiation coming from the plasma, surrounded in turn by a blanket where the neutrons are absorbed to breed tritium and heat a working fluid that transfers the power to the balance of plant. If magnetic confinement is used, a magnet system, using primarily cryogenic superconducting magnets, is needed, and usually systems for heating and refueling the plasma and for driving current. In inertial confinement, a driver (laser or accelerator) and a focusing system are needed, as well as a means for forming and positioning the pellets.

Although the standard solution for electricity production in fusion power plant designs is conventional steam turbines using the heat deposited by neutrons, there are also designs for direct conversion of the energy of the charged particles into electricity. These are of little value with a D-T fuel cycle, where 80% of the power is in the neutrons, but are indispensable with aneutronic fusion, where less than 1% is. Direct conversion has been most commonly proposed for open-ended magnetic configurations like magnetic mirrors or Field-Reversed Configurations, where charged particles are lost along the magnetic field lines, which are then expanded to convert a large fraction of the random energy of the fusion products into directed motion. The particles are then collected on electrodes at various large electrical potentials. Typically the claimed conversion efficiency is in the range of 80%, but the converter may approach the reactor itself in size and expense.

Materials

Developing materials for fusion reactors has long been recognized as a problem nearly as difficult and important as that of plasma confinement, but it has received only a fraction of the attention. The neutron flux in a fusion reactor is expected to be about 100 times that in existing pressurized water reactors (PWR). Each atom in the blanket of a fusion reactor is expected to be hit by a neutron and displaced about a hundred times before the material is replaced. Furthermore the high-energy neutrons will produce hydrogen and helium in various nuclear reactions that tends to form bubbles at grain boundaries and result in swelling, blistering or embrittlement. One also wishes to choose materials whose primary components and impurities do not result in long-lived radioactive wastes. Finally, the mechanical forces and temperatures are large, and there may be frequent cycling of both.

The problem is exacerbated because realistic material tests must expose samples to neutron fluxes of a similar level for a similar length of time as those expected in a fusion power plant. Such a neutron source is nearly as complicated and expensive as a fusion reactor itself would be. Proper materials testing will not be possible in ITER, and a proposed materials testing facility, IFMIF, is still at the design stage in 2005.

The material of the plasma facing components (PFC) is a special problem. The PFC do not have to withstand large mechanical loads, so neutron damage is much less of an issue. They do have to withstand extremely large thermal loads, up to 10 MW/m², which is a difficult but solvable problem. Regardless of the material chosen, the heat flux can only be accommodated without melting if the distance from the front surface to the coolant is not more than a centimeter or two. The primary issue is the interaction with the plasma. One can choose either a low-Z material, typified by graphite although for some purposes beryllium might be chosen, or a high-Z material, usually tungsten with molybdenum as a second choice.

If graphite is used, the gross erosion rates due to physical and chemical sputtering would be many meters per year, so one must rely on redeposition of the sputtered material. The location of the redeposition will not exactly coincide with the location of the sputtering, so one is still left with erosion rates that may be prohibitive. An even larger problem is the tritium co-deposited with the redeposited graphite. The tritium inventory in graphite layers and dust in a reactor could quickly build up to many kilograms, representing a waste of resources and a serious radiological hazard in case of an accident. The consensus of the fusion community seems to be that graphite, although a very attractive material for fusion experiments, cannot be the primary PFC material in a commercial reactor.

The sputtering rate of tungsten can be orders of magnitude smaller than that of carbon, and tritium is not so easily incorporated into redeposited tungsten, making this a more attractive choice. On the other hand, tungsten impurities in a plasma are much more damaging than carbon impurities, and self-sputtering of tungsten can be high, so it will be necessary to ensure that the plasma in contact with the tungsten is not too hot (a few tens of eV rather than hundreds of eV). Tungsten also has disadvantages in terms of eddy currents and melting in off-normal events, as well as some radiological issues.

Economics

It is far from clear whether or not nuclear fusion will be economically competitive with other forms of power. The many estimates that have been made of the cost of fusion power cover a wide range, and indirect costs of and subsidies for fusion power and its alternatives make any cost comparison difficult. The low estimates for fusion appear to be competitive with but not drastically lower than other alternatives. The high estimates are several times higher than alternatives.

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. Research, while making steady progress, has also continually thrown up new difficulties. Therefore it remains unclear that an economically viable fusion plant is even possible.

An important aspect of fusion energy in contrast to many other energy sources is that the cost of production is nearly perfectly elastic. With wind energy, for example, power from the best locations is cheap, but as more generators are installed, poorer sites must be used so the price goes up. With fusion energy, the production cost will not increase much, even if an extremely large number of plants are built. It has been suggested that perhaps even 100 times the current energy consumption of the world is within the realm of possibility. Some problems which are expected to be an issue in the next century such as fresh water shortages can actually be thought of merely as problems of energy supply. For example, in desalination plants, seawater can be converted into pure freshwater through a process of either distillation or reverse osmosis. However, these processes are energy intensive. Even if the first fusion plants are not competitive with alternative sources, fusion could still become competitive if large scale desalination requires more power than the alternatives are able to provide. Ultimately, there is a capacity limit to biofuel, hydro, terrestrial solar, and wind power, which is why some people want research of fusion power in spite of the large costs of such research and the relatively simple and inexpensive nature of other long-term energy solutions. Fusion power has many of the benefits of these long-term energy sources (such as a long-term continuous energy supply and no greenhouse gas emissions) but also some of the benefits of (relatively) short-term energy sources like hydrocarbons and nuclear fission (without reprocessing) , such as very high power-generation density (i.e. a coal-burning power plant takes up less room than an equivalent power solar power plant) and uninterrupted power delivery (i.e. coal plants work almost entirely independent of the day's weather, unlike wind and solar power).

External links

• U.S. Fusion Energy Science Program
• EURATOM/UKAEA Fusion Association
• ITER
• FIRE
• FUSION FAQ
• European Fusion Development Agreement
• Fusion Power Associates A Washington, DC area lobbying organiation; "a non-profit, tax-exempt research and educational foundation, providing timely information on the status of fusion development." Edits the Journal of Fusion Energy.
• Plasma/Fusion Glossary
• The Helimak Experiment, at the Fusion Research Center at UT Austin
• Investigations of the Formability, Weldability and Creep Resistance of Some Potential Low-activation Austenitic Stainless Steels for Fusion Reactor Applications (ISBN 0853111480):A.H. Bott, G.J. Butterworth, F. B. Pickering
• "Low Activation Material Candidates For Fusion Power Plants"; C.B.A. Forty and N.P. Taylor (PDF format)
• International Thermonuclear Experimental Reactor (Iter) fusion reactor work gets go-ahead (BBC news May 2006)

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