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Fusion power

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Fusion power is the technique of extracting net energy 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.

Some suggested advantages of commercial fusion reactors as power producers are:

  • An effectively inexhaustible supply of fuel -- at essentially zero cost on an energy production scale;
  • A fuel supply that is available from the oceans to all countries and therefore cannot be interrupted by other nations;
  • No possibility of nuclear runaway;
  • No chemical combustion products as effluents;
  • No afterheat cooling problem in case of an accidental loss of coolant;
  • No use of weapons grade nuclear materials; thus no possibility of diversion for purposes of blackmail or sabotage;
  • Low amount of radioactive by-products with significantly shorter half-life relative to fission reactors.

Some argue that fusion is the best option for a truly sustainable or long term energy source because the fuel is virtually inexhaustible and readily available throughout the world. Deuterium can be taken from water, and a thimble full of deuterium is equivalent to 20 tons of coal in energy production - a medium size lake contains enough deuterium to supply a nation with energy for centuries using fusion.

Current development centers on the more easily attained deuterium-tritium reaction (D+T) which has a fuel cycle which requires the relatively rare metal lithium to generate tritium. Claims for a truly inexhaustible fuel supply refer to the possibility of using D-D reactions in second generation fusion reactors.

The claim that fusion will have a smaller problem with radioactive waste than fission is also questionable for the D+T reaction. The fuel cycle depends on the use of the neutrons produced to irradiate lithium. As D+T produces only one neutron, any neutron losses at all will mean that not enough tritium is produced in the fusion reactor, and needs to be supplied form an external source, such as the fission reactors currently used to produce tritium for experiments.

The neutron flux expected in a commercial D+T fusion reactor is about 100 times that of current fission power reactors, posing enormous problems for material design, and potential waste problems. 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 cavity was sufficiently radioactive that remote handling needed to be used for the year following the tests.

It is also interesting to note that, unlike fission, fusion produces radioactivity that is in a sense permanent. The radioactive material produced by a fission plant has a shorter half-life than the fuel used, so after a long period of time (but far shorter than is often thought, ranging from tens to thousands of years), the waste is actually less radioactive than the fuel would have been had it been left in the ground. However, neither the lithium nor the deuterium used by the proposed fusion fuel cycle is naturally radioactive, so this crossover point is never reached with fusion waste.

Like fission, fusion can be environmentally sound without atmospheric pollutants or contribution to global warming (compared to fossil fuels where 64 lbs of CO2 is produced per American per day from fossil fuel usage).

In addition fusion power may be more attractive than existing fission systems as a nuclear power source. Much less radioactive waste results from fusion than from fission plants. During the D-T reaction, neutrons are released which cause the reactor vessel to become radioactive, but this radioactivity can be greatly reduced by using "low activation" materials. Such materials would have half-lives of tens of years, rather than the tens of thousands of years for radioactive waste produced from fission. Modern fission power plants with the exception of the RBMK have been designed to eliminate the possibility of nuclear runaway, and most current plants do not use weapons grade materials.

However, critics point out that it is far from clear that nuclear fusion will indeed be economically competitive with other forms of power. It is possible that fusion advocates are making some of the same mistakes in creating unrealistic economic projections that fission advocates have made in the past. When the cost of the plant is factored in, 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 the economic viability of fusion power, depending on what the input assumptions of the models are. Fusion advocates would counter that it is very dificult to predict these future costs, especially as they depend upon political climates which would set regulatory standards, and was a large source of the rising price of fission power, for instance. It has also been argued, although most economists would disagree, that it is dificult to weigh an increased economic cost with the environmental advantages of fusion.

Fusion does also have potential safety concerns. Although there intrinsically would be no danger of a runaway fusion reaction (a meltdown) and any malfunction would result in a rapid shutdown of the plant, there are possible scenarios which are safety concerns. In 1973 the American Association for the Advancement of Science (AAAS) pointed out several concerns for a fusion power plant, including the possibility of a tritium leak, lithium fire or the accidental release of magnetic energy. These concerns would need to be addressed as part of any reactor design.

Unfortunately, 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 far from clear that an economically viable fusion plant is even possible. It is an extremely difficult task to harness a 100 million degree plasma in an economically efficient way, so a working reactor is still many years down the road and is an active part of plasma physics research.

Table of contents

Power plant design

Humanity has been able to create artificial large-scale fusion reactions since 1949, when the United States detonated a hydrogen bomb as a test. However, an uncontrolled explosive reaction of that magnitude is not well-suited to power generation. Theoretically, 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 to ever be constructed for a variety of reasons.

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 energy needed to start the reaction is being given back off by the reaction itself. We have been capable of reaching this break-even point for over a decade. However there are other break-even points that are important, one is that the electricity out is the same as the electricity in, and perhaps most important, the point where the system is generating enough money to pay for itself. This last goal looks to be at least 50 years off at any given point in time.

Fusion systems are typically classified by the type of "confinement" system they used to handle the hot plasma that is the result of a fusion reaction. The vast majority of research has focussed 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 received the most attention since it was first introduced. Other systems include the magnetic pinch machines where a current running through the plasma generates its own magnetic field, inertial confinement systems that use lasers to explosively compress small pellets of fuel, and ions being "sprayed" into the reaction chamber as in the fusor.

The different forms of reactor all have advantages and disadvantages. Tokamaks are the most developed magnetic confinement scheme. Inertial confinement produces plasmas with impressive densities and temperatures, and might be best suited to research, X-ray generation, very small reactors, and rocketry. They rely on "perfect" fuel pellets in order to generate a symmetrical inward shock wave to produce the high-density plasma, and in practice these have proven extremely difficult to manufacture.

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 in the past few years.

Most controversially, some researchers have claimed to observe neutron production in electrochemical systems, the so-called cold fusion systems. Peers have not been able to reproduce this. Today cold fusion is regarded as pseudoscience, as part of a long list of scientific hoaxes and frauds.

Plasma Heating

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.

Ohmic Heating

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

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.

Magnetic Compression

A gas can be heated by sudden compression. In the same way, the temperature of a plasma is increased if it is compressed rapidly by increasing the confining magnetic field. In a tokamak system this compression is achieved simply by moving the plasma into a region of higher magnetic field (i.e., radially inward). Since plasma compression brings the ions closer together, the process has an additional benefit of facilitating attainment of the required density for a fusion reactor.

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.

Fuel Cycle First generation fusion reactors are expected to use deuterium and tritium as fuel. Several environmental drawbacks are, however, commonly attributed to DT 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, which limits the extent to which direct energy conversion techniques might be applied.
  3. The use of DT fusion power depends on lithium resources, which are less abundant than deuterium resources.

These drawbacks of DT 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 tritium and that release all of their energy in the form of charged particles. Although such alternatives to DT 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 breakeven (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 genera- tion 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.

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