Copyright © 1993 Grolier Electronic Publishing, Inc.
Fusion energy
A fusion reaction is one in which two atomic nuclei merge to form a heavier nucleus and, in most cases, an accompanying product such as a free nucleon. In almost all types of fusion reactions between light nuclei, a portion of their rest mass is converted into kinetic energy of the reaction products, or into gamma rays. Stars produce energy through a variety of fusion reactions. In main-sequence stars such as the Sun, the net effect of these reactions is to convert hydrogen nuclei (protons) into helium nuclei. The kinetic energy and gamma rays released in the process heat the stellar interior, maintaining it at the very high temperatures (greater than 10 million K) required to continue the fusion. Such conditions, where the thermal energy of the nuclei is sufficient to drive them together in spite of their electrostatic repulsion, are called thermonuclear.
This process, which has been driving the stars for billions of years, has clear potential as a power source on Earth, and scientists have worked for decades toward the goal of employing thermonuclear fusion reactions to produce useful power. The two fusion reactions that are by far the most promising both involve the heavier isotopes of hydrogen: deuterium (composed of one proton and one neutron), and tritium (composed of one proton and two neutrons). Deuterium occurs naturally as a minor constituent in all hydrogen-containing materials--such as water--in quantities sufficient to meet all the energy needs of societies for many billions of years. Tritium can be bred from lithium by a neutron-induced reaction in a blanket that could conceivably surround a fusion reactor. The western United States contains large lithium deposits in the salts of dry lake beds, and much larger quantities are dissolved in the sea. The reaction that occurs with the greatest probability and at the lowest temperatures involves the fusing of a deuterium nucleus with a tritium nucleus to form a helium (He4) nucleus and a neutron. The products contain 17.6 million electron volts (MeV) of released kinetic energy. The second promising reaction, involving the fusing of two deuterium nuclei, has two branches that occur with about equal probability. One leads to a He3 nucleus, a neutron, and 3.2 MeV of kinetic energy; the other produces a tritium nucleus, a proton, and 4.0 MeV. While the deuterium-deuterium reaction is the one that could furnish power beyond even the expected lifetime of the Sun, it is somewhat more easily produced deuterium-tritium reaction, which would itself be sufficient for many thousands of years, that will provide most of the energy in the next generation of research devices.
Fusion reactions can be induced easily by using a charged particle accelerator to bombard a solid or gaseous tritium target with energetic deuterium nuclei. This technique consumes power rather than producing it, however, because most of the accelerated nuclei lose their energy through elastic collisions with electrons and nuclei, without producing fusion reactions. A net energy gain is obtained only by mimicking the Sun and producing star-like thermonuclear conditions. Because a reactor must be much smaller than a star and must operate in a limited time frame, however, it must have a much higher power density and be several times hotter than the center of the Sun. The advantage of carrying out the reactions under thermonuclear conditions is that the energy transferred between particles in elastic collisions is not really wasted as long as it remains within the thermonuclear matter. That is, energy that is lost by one nucleus in an elastic collision is transferred to the particle it hits and is still available to eventually initiate a fusion reaction.
At thermonuclear temperatures, matter can exist only in the plasma state, consisting of electrons, positive ions, and very few neutral atoms. Fusion reactions that occur within a plasma serve to heat it further, because the portion of the reaction energy that remains with the electrically charged reaction products is transferred to the bulk of the plasma through collisions. In the deuterium-tritium reaction the positively charged helium nucleus carries 3.5 MeV. The neutron escapes the plasma with little interaction and, in a reactor, could deposit its 14.1 MeV in a surrounding lithium blanket. That would breed tritium and also heat an exchange medium (such as helium), which would then be used to produce steam to turn generator turbines. However, the plasma also loses thermal energy through a variety of processes: conduction, convection, and bremsstrahlung (electromagnetic radiation emitted when a charged particle decelerates). Energy also escapes through line radiation from electrons undergoing level transitions in heavier impurities, and through losses of hot nuclei that capture an electron and escape any confining fields. Ignition occurs when the energy deposited within the plasma by fusion reactions equals or exceeds the energy being lost.
In order to achieve ignition, a plasma must be confined and heated. Obviously, a plasma at millions of degrees is not compatible with an ordinary confining wall, but the effect of this incompatibility is not the destruction of the wall, as might be expected. Although the temperature of a thermonuclear plasma is very high and the power flowing through it may be large, the stored energy is relatively small and would quickly be radiated away by impurities if the plasma touched a wall and began to vaporize it. A thermonuclear plasma is thus self-limiting, because any significant contact with the vessel housing it causes its extinction within a few thousandths of a second.
Magnetic Confinement
Since the early 1950s, most fusion research has used magnetic fields to confine the charged particles that constitute a plasma. The density required in magnetic-confinement fusion is much lower than atmospheric density, so the plasma vessel is evacuated and then filled with the hydrogen-isotope fuel at 0.000001 times the density of the atmosphere. Magnetic-field configurations fall into two types: open and closed. In an open configuration the charged particles, which are spiraling along magnetic field lines maintained by a solenoid, are reflected at each end of a cell by stronger magnetic fields. In this simplest type of mirror machine, many particles that have most of their velocity parallel to the solenoidal magnetic field are not reflected and can escape. Present-day mirror machines retard this loss by using additional plasma cells to set up electrostatic potentials that help confine the hot ions within the central solenoidal field.
In closed configurations, the magnetic-field lines along which charged particles move are continuous within the plasma. This closure has most commonly taken the form of a torus, or doughnut shape, and the most common example is the tokamak. In a tokamak the primary confining field is toroidal and is produced by coils surrounding the vacuum vessel. Other coils cause current to flow through the plasma by induction. This toroidally flowing current engenders a poloidal magnetic field, at right angles, that wraps itself around the plasma. The poloidal field and the stronger toroidal field, acting together, yield magnetic-field lines that spiral around the torus. This spiraling ensures that a particle spends equal amounts of time above and below the toroidal midplane, thus canceling the effects of a vertical drift that occurs because the magnetic field is stronger on the inside of the torus than on the outside.
Plasma Heating
Tokamak plasmas can be heated to temperatures of 10-15 million K by the current flowing in the plasma. At higher temperatures the plasma resistance becomes too low for this method to be effective, and heating is accomplished by injecting beams of very energetic neutral particles into the plasma. These ionize, become trapped, and transfer their energy to the bulk plasma through collisions. Alternatively, radiofrequency waves are launched into the plasma at frequencies that resonate with various periodic particle motions. The waves give energy to these resonant particles, which then transfer it to the rest of the plasma through collisions.
Current Drive
Experiments are also under way in which radiofrequency waves are used to push electrons around the tokamak to maintain the plasma current. Such noninductive current drive allows the tokamak pulse to outlast the time limits imposed by the fact that, in a transformer-driven tokamak, the plasma current lasts only as long as the current in the secondary coils is changing. When the secondary coils reach their current limits, confinement is lost, and the plasma terminates until the transformer can be reset (a matter of at least seconds). Although the plasma in an inductively driven tokamak is pulsed, the electricity produced would not be, because the thermal inertia of the neutron-capturing blanket would sustain steam generation between pulses. By allowing longer pulse or steady-state plasma operation, however, radiofrequency current drive could lessen the thermal stresses in the fusion reaction.
Inertial Confinement
Another approach to fusion, pursued since about 1974, is termed inertial confinement. Its aim is to compress a solid pellet of frozen deuterium and tritium to very high temperatures and densities in a process analogous to what occurs in a thermonuclear (hydrogen) bomb. The compression is accomplished by bombarding the pellet from all sides, simultaneously, with an intense pulse of laser light, ions, or electrons. The outer pellet mass vaporizes and, by mechanical reaction, imparts inwardly directed momentum to the remaining pellet core. The inertia of the inwardly driven pellet material must be sufficient to localize the resulting fusion plasma for the approximately 10 to the power of -9 seconds required to get significant energy release. In 1988 it was learned that the U. S. government, which secretly had been using underground nuclear tests in Nevada to study inertial-confinement fusion, had achieved such fusion in 1986 by this means.
Progress toward Energy Production
The minimum confinement condition necessary to achieve energy gain in a deuterium-tritium plasma is that the product of density in ions per cubic cm and energy containment time in seconds must exceed 6 x 10 to the 13th power. This was attained for the first time in a hydrogen plasma at the Massachusetts Institute of Technology in 1983. The temperature required to ignite a fusion reactor is in the range of 100-250 million K, several times the temperature of the center of the Sun. Progress has been rapid over the last decade, but much research remains to be done before fusion power reactors can become a reality.
The goal of fusion--in effect, to make and hold a small star--is so daunting as to be widely considered the supreme technological challenge yet undertaken. That it is pursued nevertheless is an indication of the magnitude of the benefits that success could bring. In addition to an almost inexhaustible fuel supply, fusion has other attractive features: it is environmentally benign; the resulting ash is harmless helium and hydrogen; and the afterheat in the reactor structure would be much less than in a fission reactor and would be distributed through a greater thermal mass. In addition, because fusion is not a chain reaction, it cannot run out of control, and any perturbation would cause the plasma to extinguish itself. It would also be far more difficult to produce nuclear-weapons materials surreptitiously at a fusion plant than at a fission plant; because no fissionable material should ordinarily be present at a fusion plant, it would be a simple matter to detect characteristic gamma rays from such a source. Present levels of support for research are aimed at building the first demonstration fusion plant in the first quarter of the 21st century.
Larry R. Grisham
Bibliography: Heppenheimer, T. A., The Man-Made Sun (1984); O'Sullivan, Dermot A., "International Effort to Design Nuclear Fusion Reactor Launched," Chemical & Engineering News, May 23, 1988; Roth, J. Reece, Introduction to Fusion Energy (1986).
Posted By Colin Cashman.