Controlled thermonuclear reactors

Type of physical science: Nuclear physics

Field of study: Thermonuclear reactions

Light nuclei, such as isotopes of hydrogen, release energy when they combine to form heavier nuclei. In order to overcome the electrical repulsion between the light nuclei, which carry positive charges, and to bring them near enough for fusion to take place, they must be heated to temperatures of hundreds of millions of Kelvins or brought together by other means.

Overview

Nuclear fusion is the process of combining light nuclei to form heavier ones. Because nuclei are so tightly bound, fusion releases a measurable fraction of the mass of the fusing nuclei as pure energy. In particular, combining a tritium nucleus, which contains two neutrons and one proton, with a deuteron, which consists of one proton and one neutron, produces the nucleus of a helium atom with two protons and two neutrons, an extra neutron, and 17.6 million electronvolts of energy. In practical terms, the complete fusion of a kilogram of tritium and deuterium fuel would produce 364 million kilowatt hours of energy. For comparison, burning a kilogram of gasoline produces about 13.2 kilowatt hours. Most experimenters believe that the reaction between tritium and deuterium, rather than fusion reactions between deuterium and deuterium or other possible fusion reactions, provides the best chance of achieving a fusion reactor for practical electricity generation.

The nucleus of one of every sixty-seven hundred hydrogen atoms on Earth is a deuteron, and tritium can be formed when lithium absorbs neutrons from nuclear reactors or the extra neutrons from the fusion reaction itself. Thus, existing lithium and the deuterium in the oceans seem adequate to provide global energy needs into the distant future. Unlike nuclear fission reactions, the waste products of nuclear fusion reactions, helium and a neutron, are not inherently radioactive. The neutrons induce radioactivity in the reactor walls, however, and tritium itself β-decays with a half-life of twelve years and three months. Thus, nuclear fusion will produce some radioactivity that might be released into the environment, but the radionuclides have relatively short half-lives and pose considerably less of a long-term threat than the by-products of nuclear fission reactors.

In view of these advantages, nuclear fusion would seem to offer large energy supplies with minimum environmental damage. Unfortunately, nuclear fusion can take place only when tritium and deuterium nuclei approach one another closely enough that the nuclear force can overcome their mutual electrical repulsion. The range of the strong nuclear force is about 10 to the power of -15 meter, beyond which it drops abruptly to zero. The repulsive electrical force between the two positively charged nuclei extends to infinity, and its strength increases as the two nuclei move closer together. Most schemes for producing power from nuclear fusion impart enough kinetic energy to the nuclei that they can approach within 10^-15 meter of each other and let the strong force fuse them. This kinetic energy corresponds to temperatures of more than 100 million Kelvins, with temperature in this context taken as a measure of the kinetic energy of the nuclei.

At such temperatures, electrons are ripped off atoms and the detached nuclei and electrons form a new state of matter called a plasma. In order to sustain a fusion reaction, a plasma must be produced and held at a temperature of hundreds of millions of Kelvins at sufficient density and for a long enough time to generate useful power. Because materials vaporize long before they reach the required temperatures, the technology necessary to generate energy from nuclear fusion has not yet been developed. Nuclear fusion energy provides most of the explosive energy in thermonuclear weapons, in which the plasma is compressed and heated by the radiation from a nuclear fission warhead, but a more controlled release of nuclear fusion power has eluded researchers.

A fusion power scheme must reach breakeven; that is, the power put into the plasma to heat it must be less than the power produced in the plasma by fusion. Increasing the density of the plasma increases the numbers of collisions between nuclei and the output of power from fusion. The plasma cools down when it develops instabilities that let it escape from the magnetic fields that hold it. A measure of the time during which the plasma cools down by a certain fraction unless more heat is added is called the confinement time. A plasma is said to reach ignition when it produces enough energy from fusion reactions to compensate for energy losses from the plasma and to produce a self-sustaining fusion reaction. In order for a plasma to reach ignition, it must not only reach ignition temperatures above 100 million Kelvins, but the product of its density measured in particles per cubic centimeter and its confinement time measured in seconds must be greater than 10¹⁴ seconds per cubic centimeter as well. This is known as the Lawson criterion, and it is the goal toward which the designers of nuclear fusion reactors strive.

In addition to schemes that rely on the kinetic energy of nuclei to overcome the electrical repulsion between their positive charges, a fundamentally different technique for triggering nuclear fusion relies on other mechanisms for bringing nuclei close enough together that the nuclear force can bind them to one another with a resulting release of energy. A negative particle called a muon behaves like an electron with 207 times the electron mass. The muon can enter orbit around a deuterium or tritium nucleus and bind it to a second nucleus in a molecule just as electrons bind ordinary hydrogen molecules. Because of the greater mass of the muon, the nuclei in the muonic molecule are bound closely enough together that the nuclei have a relatively high probability of fusing. Muon-catalyzed fusion has been demonstrated in the laboratory, where it is a subject of great interest to students of both nuclear and molecular structure. As yet, however, practical fusion reactor designs have not employed it. Reports by two groups in Utah of using absorption by a platinum matrix to force deuterium nuclei close enough to start fusion reactions were controversial, and the effect was not demonstrated to the satisfaction of the majority of the scientific community. The fusion techniques that do not involve a hot plasma are known collectively as cold fusion.

Applications

Attempts to construct fusion reactors fall into two basic categories: magnetic confinement and inertial confinement. Magnetic confinement schemes use the forces of magnetic fields on moving charged particles to hold these particles of the plasma while they are heated.

The magnetic fields generated by electrical currents of the charged particles in the plasma itself play an important role in magnetic confinement. In principle, the hot plasma eventually reaches ignition temperatures and the energy from fusion reactions is released steadily. Inertial confinement schemes use a laser beam or a beam of charged particles to condense and heat a plasma simultaneously, producing a burst of fusion reactions. In effect, inertial confinement schemes generate a series of tiny thermonuclear explosions.

The first efforts at magnetic confinement relied on the pinch effect, which takes advantage of the fact that moving charged particles constitute electric currents. Electric currents flowing in the same direction attract one another and tend to compress the plasma into a narrow tube. Compressing the plasma heats it because plasmas are effectively gases. Pinch confinement is limited by the interactions of the plasma with the ends of the tube that contains it and by instabilities in the plasma tube, which cause the hot plasma to bend into contact with the walls of the tube or break into a series of lumps rather than a continuous path. The theory of hot plasmas relies both on the behavior of charged particles in electromagnetic fields and on the dynamics of the plasma considered as a fluid. Although modern computers have helped to model plasma behavior, the detailed theory of hot plasmas remains an unsolved problem in theoretical physics.

Plasma machines contain very little matter and thus very little heat, although they operate at extremely high temperatures. Any contact with the walls of the tube quickly cools the plasma. All proposed designs for nuclear fusion reactors contain a limited amount of fuel at any one time. In case of an accident, fusion reactors would automatically shut themselves down as the limited supply of plasma would quickly cool in contact with construction materials. This automatic shutdown is one of the major advantages of nuclear fusion over nuclear fission reactors.

The interaction with the ends of the tube can be eliminated by bending the plasma current into a circular path. In order to reduce instabilities within the plasma, a magnetic field is applied along the line of the circular current loop. The most successful magnetic confinement scheme has been the tokamak, in which external magnets provide a carefully adjusted magnetic field in the direction of the plasma currents which is combined with the magnetic fields generated by the plasma currents to produce a corkscrew magnetic field containing the plasma.

This device is particularly effective in preventing the loss of energetic electrons, which carry off much of the energy contained in the plasma. Modern modifications of the original tokamak design are among the experiments approaching an effective breakeven process.

Other methods for exploiting magnetic confinement use magnetic mirrors (shaped magnetic fields) to prevent contact between the plasma and the ends of a linear tube. Reactors using magnetic mirrors are popularly known as magnetic bottles. One modification of this scheme, which is believed to show promise for providing a sustained fusion reaction, is the dense z-pinch. In this scheme, a solid fiber of deuterium-tritium fuel suddenly carries a huge pulse of current. The fiber vaporizes and is compressed by the magnetic field of the current, which heats the resulting plasma. The development of solid fibers and better power supplies has increased experimental interest in the dense z-pinch.

Problems with magnetic confinement schemes arise when the plasma is heated to a higher temperature. The theory of hot plasmas is imperfectly understood, and unexpected instabilities tend to plague reactors as the temperatures of the plasma are increased.

Experimenters attempt to remove these instabilities and increase the temperature of the plasma by modifications in the detailed design of the magnetic fields confining the plasma. The experiments divide themselves into those oriented toward attempts to understand the basic physics of the hot plasma's behavior and those oriented toward reaching breakeven and ignition.

All the experiments involve constructions of large, complex devices and thus are extremely expensive. Efforts to promote international cooperation in the study of fusion have been encouraging because of both the flow of scientific knowledge and the sharing of the expense of the experiments among nations. The detailed design and evaluation of a reactor from which useful heat can be extracted and used to generate electric power await the choice of a scheme for producing the self-sustaining fusion reactions in the plasma.

Inertial confinement schemes inject a pellet of deuterium-tritium fuel into the point at which several laser or charged-particle beams meet. The beams rapidly compress and heat the pellet. As it is compressed, the pellet vaporizes, forming a hot plasma in which, it is hoped, fusion reactions will occur. Energy is released as the pellet explodes. The debris--consisting of deuterium, tritium, helium nuclei, and neutrons--then strikes and heats a surrounding medium from which useful power can be extracted.

In one scheme, the medium is liquid lithium, which is swirled to provide a cavity into which the fuel pellet is dropped. The neutrons from the fusion breed new supplies of tritium in their interaction with the lithium, which is drawn off the bottom of the container to be chemically cleaned for salvaging tritium, deuterium, and helium. Heat deposited in the lithium by the fusion interactions can be used to heat water for the generation of power before the lithium reenters the fusion reactor. The force of the small fusion explosion could possibly be absorbed by bubbles in the lithium.

There are several problems plaguing inertial confinement schemes. Such technologies demand very powerful laser or particle beams that must be sharply focused on the pellet so that the beams strike it from several directions at precisely the same time. If the timing is off, then the pellet will not be uniformly compressed and heated. These requirements challenge state-of-the-art technology for both generating and handling the beams. Design of the fuel pellets themselves involves new technologies. In some experiments, the pellets surround layers of tritium and deuterium with thin layers of gold and other metals. The entire multilayer pellet must be a perfect sphere, and hundreds of them can fit on the head of a pin.

Finally, inertial confinement schemes work by generating a series of small fusion explosions. These explosions can be used to mimic the effects of nuclear weapons on materials and electronic circuits. Much of the research on inertial confinement is used by the military and is classified. While there has been less discussion of inertial confinement in electric power generation, research programs have been proceeding vigorously in the classified world.

Context

Experimental designs for nuclear fusion reactors have concentrated on reaching first breakeven and then ignition. They are approaching these conditions, but achieving ignition will prove only that nuclear fusion can provide a self-sustaining energy source in a laboratory environment.

Such experiments provide new basic information on the structure of nuclei and the behavior of plasmas. Since nuclear fusion reactions are believed to provide the power of stars, these results also interest the astrophysics community. In addition, fusion reactor designs use such new technologies as superconducting magnets and provide laboratories for developing them. The need to provide high currents in the plasma challenges the technologies used in power supplies and has spurred developments in this area.

Before nuclear fusion can provide a useful energy source, experimenters must demonstrate engineering feasibility by incorporating the laboratory device in a pilot plant that can generate electric power. Developers must examine problems that may arise in the design of larger plasma machines, as well as such issues as the response of reactor materials to the high flux of high-energy neutrons produced during nuclear fusion. Systems must be developed to breed tritium for the reactor fuel and to prevent tritium leaks to the environment. Other health and safety issues will certainly include mechanisms for shutting down the reactor without damaging any of its components.

In inertial confinement schemes, for example, there will be concern over the interface between the lithium, which carries heat from the fusion reactions, and the steam used to drive electric generators. Lithium reacts strongly with water, and mechanisms must be developed to guard against explosions. These problems have little to do with the nuclear fusion process itself, but they are critical to the operation of a practical system for generating electrical power. Other issues will include exploring the degree to which fusion reactors will produce radioactive wastes from the materials of which the reactor is constructed. Finally, the high-energy neutrons produced in the fusion reaction will require very heavy shielding to protect the operating crew of the power plant.

Once engineering feasibility has been established, nuclear fusion reactors must next demonstrate commercial feasibility. The costs of an electric generating plant based on nuclear fusion cannot yet be estimated. Completed design of a pilot plant will permit assessment of costs per kilowatt of electric power in the terms of both operating expenses and capital costs. Pilot plants will define the environmental effects of the plants and permit authorities to establish licensing procedures for them. Finally, the costs for constructing a fusion plant must be comparable to the costs of constructing equivalent nuclear fission or fossil fuel plants and must demonstrate superior environmental effects.

Principal terms

BREAKEVEN: the condition in which a plasma generates as much energy through fusion reactions as is put into the plasma to heat it; at breakeven, the plasma only replaces the heat that it loses naturally and does not produce net energy

DEUTERIUM: an isotope of hydrogen whose nucleus contains one proton and one neutron

FUSION: the process of combining light nuclei to form heavier ones, with a resulting release of energy

IGNITION: the point at which a plasma generates enough energy to sustain fusion reactions without external heating

INERTIAL CONFINEMENT: the family of techniques for holding a plasma while heating it; these methods use a laser beam or a beam of charged particles to compress a fuel pellet

LAWSON CRITERION: the product of the density of nuclei in a plasma and the confinement time for the plasma that will allow a positive output of energy

MAGNETIC CONFINEMENT: the family of techniques for holding a plasma by using the forces that a magnetic field exerts on the moving charged particles that compose the plasma

PLASMA: a state of matter composed of free (detached) electrons and nuclei without electrons

TRITIUM: a radioactive isotope of hydrogen whose nucleus contains one proton and two neutrons

Bibliography

Bromberg, Joan Lisa. FUSION: SCIENCE, POLITICS, AND THE INVENTION OF A NEW ENERGY SOURCE. Cambridge, Mass.: MIT Press, 1982. This volume describes the history and politics of the development of controlled fusion reactors in the United States as a serious effort in the history of science. The discussion is useful in explaining why some technical decisions were made at the time and the reason that only a few of the various approaches to producing fusion energy have been vigorously pursued.

Epstein, Gerald L. "Fusion Technology for Energy." In THE ENERGY SOURCEBOOK: A GUIDE TO TECHNOLOGY, RESOURCES, AND POLICY, edited by Ruth Howes and Anthony Fainberg. New York: American Institute of Physics, 1991. This chapter provides a summary of developments in fusion technology, including a discussion of muon catalysis and other less prominent techniques. It is written for a general audience and is accessible to the educated lay reader.

Hunt, S. E. FISSION, FUSION, AND THE ENERGY CRISIS. 2d ed. Elmsford, N.Y.: Pergamon Press, 1980. This discussion of the British approach to fusion power provides a good analysis of the criteria that fusion must meet in order to provide a useful energy source. The discussion of some of the problems with various magnetic confinement schemes is particularly readable and interesting.

U.S. Congress. Office of Technology Assessment. STARPOWER: THE U.S. AND THE INTERNATIONAL QUEST FOR FUSION ENERGY. OTA-E-338. Washington, D.C.: Government Printing Office, 1987. This volume provides a clear and readable discussion of the technologies and economics of nuclear fusion power. It is written initially for the lay reader, but it provides a summary of technology for the reader who is more interested in the details of the technology.

York, Herbert F., ed. ARMS CONTROL. San Francisco: W. H. Freeman, 1973. Contains three articles on fusion. The articles are: Richard F. Post, "Fusion Power" (originally published in December, 1957); Moshe H. Lubin and Arthur P. Fraas, "Fusion by Laser" (June, 1961); and William C. Gough and Bernard J. Eastlund, "The Prospects of Fusion Power" (February, 1971). These articles provide an excellent general introduction to nuclear fusion power. The article by Post presents a very elementary discussion of the fusion process and the problems it faces. The article by Lubin and Fraas is an equally readable introduction to inertial confinement problems and potentials. Finally, the article by Gough and Eastlund introduces more detail on various types of reactors.

Boiling-water reactor (BWR)

The Structure of the Atomic Nucleus

Forces on Charges and Currents

Thermonuclear Reactions in Stars