NUCLEAR FUSION

The nuclear fusion of light elements powers the sun and other stars. The fusion process in stars and supernovas creates from primordial hydrogen heavier elements, including those needed for chemistry and life. Nuclear fusion is analogous to the chemical reaction of burning, such as the joining (fusing) of hydrogen and oxygen to produce water vapor and release energy. Both processes produce something new while releasing energy.

Our planet contains large quantities of light elements that are theoretically suitable for providing energy from fusion reactions in central power plants. The potential energy resource from fusion is vast—so vast that the world's energy needs could be met for billions of years at current rates of consumption. Laboratory experiments, as well as nuclear explosions, have released fusion energy on the Earth, and it may be feasible to provide useful energy from fusion in the future in a controlled and safe manner. While the fusion process is a nuclear reaction, the results of the reaction do not include many of the problems with fission: First, there are no products like long-lived fission products that cause many of the problems associated with nuclear waste from fission. Second safeguarding uranium and plutonium is not an issue with fusion because those materials are not used. Fusion does not release compounds such as carbon dioxide that contribute to global warming through the greenhouse effect. Third, starting and maintaining a fusion reaction in a controlled manner appropriate for producing useful energy is difficult and always involves a limited amount of fuel at any given moment so that an accidental nuclear runaway reaction in a large quantity of reactants is not possible.

Fusion energy comes from rearranging electrostatic and nuclear forces inside the nucleus, and in this respect fusion is like fission. The rearrangements of both fission and fusion release energy in proportion to the decrease of mass during the reaction according to the famous formula from Albert Einstein's relativity, E= mc², where Eis the energy released, mis the mass lost, and cis the speed of light.

Chemical reactions, such as the burning of carbon in coal with oxygen, also get energy from rearranging electrostatic forces, but those forces are much smaller and consequently chemical energies released per atom are much smaller than nuclear energies released per nucleus.

Nuclei suitable for fusion must come near each other, where "near" means something like the nuclear radius of 10^-12 cm. For positively charged nuclei to make such a close approach it requires large head-on velocities, and therefore multimillion-degree Celsius temperature. In contrast, fission can occur at normal temperatures, either spontaneously or triggered by a particle, particularly an uncharged neutron, coming near a fissionable nucleus.

Above a temperature of 10,000 degrees or so, matter starts to become ionized, meaning that electrons separate from atoms. Ionized matter, containing ions and electrons, is called "plasma"—a term not to be confused with the "plasma" of physiology and medicine, which means a liquid in which cells are suspended.

Stars are giant spheres of plasma. Stars fuse hydrogen into helium for their primary source of energy. Omitting details of the catalytic role of carbon in stars, four hydrogens combine to yield one helium, two neutrinos, and two positively charged electrons (positrons) with a total kinetic energy of approximately twenty-eight MeV. The unit "MeV," or million electron volts, is convenient for discussions of nuclear energy, as is the "eV" or electron volt (or calorie and Btu) for treatments of chemical reactions. A million electron volts is 1.6 10^-13 joule, 3.8 10^-14 calorie, 1.5 10^-16 Btu, 4.5 10^-20 kWh (thermal), which makes the MeV sound like a very small unit until one considers that an MeV of released energy is typically associated with approximately one atomic mass unit (AMU), of which there are 6 10²³ in a gram.

The fusion reaction of four hydrogens to make a helium yields 185,000 kilowatt-hours (thermal) per gram of original hydrogen fuel—an enormous result for such a small mass. Fusion occurs mostly in the core of stars at a temperature near 15 million degrees and the energy released migrates to the much cooler 6,000 degree surface to produce the radiation emitted, some of which is visible light.

While stars shine from the fusion of plain hydrogen, that reaction is unsuitable for use on the earth. In substellar sizes the energy released leaves the reacting region as electromagnetic radiation much too quickly. Releasing fusion energy on a human scale requires fusion fuels that react with each other more rapidly than the slow-burning basic fuels of stars.

REACTIONS FOR USEFUL ENERGY

The most plausible fusion reaction for producing energy commercially involves two isotopes of hydrogen, deuterium (D) and tritium (T), or ¹H² and ¹H³. Deuterium contains one proton and one neutron for an atomic number of two. Tritium contains one proton and two neutrons for an atomic number of three. The reaction is

with the release of 3.5 and 14.1 MeV in the helium and neutron, respectively.

Deuterium occurs naturally, mixed in with plain hydrogen in the tiny proportion of 0.015 percent; in other words, plain hydrogen is the more common isotope by a factor of 6,600. Tritium for fusion energy can be created from another nuclear process involving the interaction of the neutron (in the equation above) with lithium:

with the release of 4.8 MeV in the first reaction and the absorption of 2.5 MeV in the second reaction. The fuel resources for D-T fusion are, therefore, lithium and deuterium. Lithium occurs in brines, mines, and, at the high dilution of only 0.17 parts per million by weight, in sea salts. The resource of lithium in oceans dominates all other sources by a factor of approximately 10,000. And although lithium is plentiful, far fewer lithium atoms than deuterium atoms exist on the Earth making lithium the limiting the resource for D-T fusion.

An essential idea in fusion research is the following: The 3.5 MeV of energy carried by the ²He⁴, which is also called an ?-particle, stays in the fusing region to maintain the high temperature and keep the reaction going. The concept is called "ignition" by analogy to common chemical ignition of fuels in which a small initial spark starts a flame that continues on its own.

Other fusion reactions such as D plus ²He³ and D plus D (not to mention the H plus H of stars) require far more difficult physical conditions than D plus T, but offer potential advantages in reduced neutron production, and even larger reserves of potential energy in the case of D-D.

CONDITIONS FOR USEFUL ENERGY

D-T fusion in substellar sizes on a human scale requires ultrahigh temperatures, approximately 100 million degrees, for the power produced to exceed the fundamental and irreducible cooling effect of power lost to electromagnetic radiation caused by the particles colliding with each other (bremsstrahlung, or braking radiation). Approximations to data on D-T reactivity lead to a simple and often quoted figure of merit for progress on fusion research, ntT, where nis the fuel particle density, tis an average time that energy stays in the fusion region, and Tis the temperature. The best present experiments create ntTapproaching the requirements for producing net energy, or 10²¹ m^-3-second-keV. (A keV is a kilo electron volt, which is approximately 11 million degrees.) An equivalent statement is that in some present experiments the ratio of fusion energy produced to the energy needed to assemble the hot particles, usually called Q, is near unity. A Q of unity is often called "scientific break-even," to make clear the contrast with commercial feasibility which requires that fusion energy released be much more than the total energy to operate the machinery in the reactor power station.

The goal of research on nuclear fusion is to produce useful energy in the future as follows: A mixture of D and T is heated to ignition at 100 million degrees, and the fusion energy is released, largely carried by the 14 MeV neutrons produced, streams outwards to a sufficiently thick "blanket" containing lithium and a heat exchange medium, which could in fact be lithium or contain lithium. The neutrons interact with lithium to produce tritium that is retained at the reactor site for later use as fuel. The heat, which comes largely from absorbing the neutron energy in the blanket, is transferred to an external turbine engine to produce electricity with the standard efficiency of 35 to 40 percent. Low temperature heat—constituting of 60 to 65 percent—is not used for electricity. But it can be used for industrial processes, including space heating and refrigeration. The blanket absorbs most neutrons, and a shield outside the blanket absorbs any remaining neutrons and other escaping radiation. The region outside the blanket-shield is then largely free of radioactivity. The blanket, shield, and any material that must remain inside to operate the fusion reactor does become radioactive because of the interaction with the neutrons. The strength and nature of the radioactivity inside the outer surface of the shield has to do with the details of design and the materials used, but remote handling tools will be needed for maintenance. Equipment needed to create and maintain the reacting plasma is outside the shield: for example, electrical power supplies, lasers, ion beams, and superconducting magnets at a few degrees above absolute zero.

At the simplest level there is a remarkable feature of the fusion reactor: a 100 million degree reacting region is near the industrial heat source—the blanket, operating at several hundred degrees. Some kind of insulation must separate the reacting fusion region and the blanket. The two insulation options being explored are magnetic fields for "magnetic fusion" or space for "inertial fusion." The magnetic insulation is much like fiberglass in a walk or ceiling. Magnetic fields slow heat transfer from the motion of plasma particles, much as the fiberglass and trapped air slow the flow of heat through a wall. Space insulation is similar to that obtained in moving back from a blazing fire.

Enhancement of the fusion reaction has been explored in attempts to avoid the extreme requirements for temperature, plus the stringent conditions on density and time. An "atom" of a D or T nucleus and the short lived muon, which is 207 times more massive than the electron, has a much smaller size than a normal D or T atom, so much so that it might be possible to bring the D and T nuclei close enough for fusion at modest conditions. Certain metals can absorb hydrogen so densely that some spontaneous fusioning of D and T might occur. The latter idea is behind the announcement in 1989 of "cold fusion" in an electrolytic cell having platinum and palladium electrodes—an announcement that numerous investigations subsequently proved to be in error. Catalysis, or enhancement of fusion rates has not been pursued as an energy source in recent years.

INERTIAL APPROACH

Inertial confinement fusion has long succeeded in the context of military explosions—the hydrogen bomb. In the military application a fission bomb produces xrays that drive an implosion of D-T fuel to enormous temperatures and densities such that fusion reactions occur during the short time that inertia keeps the fusing nuclei densely packed and hot.

For civilian purposes the inertial concept is to compress and heat a small sphere of D-T fuel with an external "driver" such as laser light, x-rays produced by lasers, or high energy ion beams. The resulting hot and extremely dense plasma burns the D-T mixture rapidly before the microexplosion expands appreciably. Time scales are nanoseconds (billionth of a second) for each microexplosion, repeated steadily to produce power. Density scales are thousands times solid density. The mission of inertial fusion in the United States is primarily to address the science of high energy densities for the Defense Department program in stewardship of nuclear weapons. Applications to civilian energy needs are a secondary mission.

Much of the driver energy goes into ablation, or blowing-off the surface of the sphere of fuel to force the compression (implosion) by the rocket effect. As a result, the ntTneeded for scientific break-even for inertial confinement is around twenty times higher than for magnetic confinement.

The largest inertial fusion program in the United States is the National Ignition Facility (NIF) now under construction at the Lawrence Livermore National Laboratory (LLNL) in Livermore, California (Figure 2). Completion of the stadium-sized construction project is planned for 2003 at an approximate total cost of $1 billion. The goal is to investigate the interaction between spherical targets and ultraviolet laser light producing x-rays in a "hohlraum"— cavity surrounding the target—at a power of 500 tera watts (trillion watts) for several nanoseconds for a total energy of 1.8 MJ (million Joules). Many other significant installations in the United States and around the world are investigating the potential of inertial fusion.

MAGNETIC APPROACH

Magnetic confinement has been pursued for civilian purposes since the 1950s. The idea is that a magnetic field, some of which might be generated by internal heating currents, slows the transfer of energy from the hot plasma core to the surroundings while extra heating power is applied from particle beams or radio frequency transmissions. Time scales are seconds for the energy replacement time, and indefinitely long for an energy-producing burn; density scales are ten thousands below atmospheric gaseous density.

In about 1968 the Soviet Union (now Russia) under the leadership of Academician Lev Artsimovitch took a large step forward in magnetic fusion with a concept called the "tokamak." The term is a Russian acronym for toroidal (inner-tube shaped) magnetic chamber. Since the mid 1970s the tokamak has dominated worldwide magnetic fusion research, culminating in highly successful D-T burning experiments in the United States with the Tokamak Fusion Test Reactor (TFTR) at Princeton University in New Jersey and the Joint European Torus (JET) in the United Kingdom. Numerous other tokamaks in many countries, including Japan (JT-60), France (Tore Supra) , Germany (ASDEX, TEXTOR), Italy (FTU), and the United States (DIII-D, Alcator-C), confirm and extend the progress while not using tritium.

Two essential ideas behind the tokamak are the externally supplied toroidal magnetic field that closes on itself repeatedly, and a slight twist to the field added by currents inside the reacting region. The closed magnetic field lines in the hot plasma do not strike a wall, so that energy is not instantly conducted to a wall. The twist is essential for good and stable confinement of energy. Prior to the emergence of the tokamak idea, the twist needed in closed, toroidal systems was provided often by external coils in the "stellarator" concept developed at Princeton University by astrophysicist Lyman Spitzer.

Many other magnetic fusion concepts have been pursued over the years: for example, the "magnetic mirror" with open field lines that always strike a wall somewhere but with good stability properties; linear and toroidal "pinches" featuring rapid compression and heating. Today the stellarator is enjoying a significant revival with large programs in Japan (LHD) and in Germany (W-7-AS), plus smaller experiments elsewhere. At a basic research level, there is interest in a broad array of magnetic approaches.

Still, the tokamak is getting most of the attention, particularly regarding the matter of a large project to follow JET and TFTR and burn D-T so successfully that the plasma ignites—continues to burn on its own—thereby demonstrating in a convincing manner that with further work, the tokamak can be developed into a commercially viable source of energy. The working name for the ignition project is the International Thermonuclear Experimental Reactor (ITER). In 1998, they completed an integrated engineering design, supported by research and development on crucial components, and a reliable budget and schedule. The term "thermonuclear" refers to the multi-million degree temperatures needed to produce energy from nuclear fusion. The four international parties to ITER—Japan, the European Union, the United States, and the Confederation of Independent States (Russia)—had intended to proceed with cooperatively funded construction at a site to be chosen jointly. The cost estimate of approximately 10 billion and political factors have caused ITER to reexamine the design goals, particularly toward a less costly device, but one retaining the most basic and interesting scientific and technical goals. ITER management hopes to go ahead with construction after the year 2000, and operate ignited plasmas in a decade or so.

For both inertial or magnetic fusion energy the long range, but nonspecific, plan is the following: develop concepts to achieve ignition of D-T, whether in ITER, NIF, or other facilities, and proceed to a demonstration reactor (often called DEMO) and then to commercial implementation. DEMO and commercialization are decades away. Even though the ignition DEMO path is being followed by most people working on fusion, researchers generally agree on the need to continue developing the science of fusion and alternative approaches because uncertainties continue about what will eventually prove to be a successful approach.

SUMMARY

Creating useful energy from fusion on the earth has proved difficult: The forty year effort looks like it still has decades to go. The following inherent potential advantages tend to encourage continued effort: abundant fuel in lithium and virtually inexhaustible fuel in deuterium; absence of the greenhouse emissions of coal, oil, natural gas; no issues regarding the control of a nuclear chain reaction; radioactivity induced by fusion neutrons depends on the design of the device, not the fundamentals of nuclear physics.

If and when projects producing energy on an industrial scale begin to appear in the middle of the twenty-first century, the theoretical advantages will be subject to confirmation. Fusion, magnetic or inertial, might then join the mix of energy resources.

D. W. Ignat

BIBLIOGRAPHY

Bromberg, J. L. (1982). Fusion: Science, Politics and the Invention of a New Energy Source.Cambridge, MA: MIT Press.

Fowler, T. K. (1997). The Fusion Quest. Baltimore: Johns Hopkins University Press.

Häfele, W.; Holdren, J. P.; Kessler, G.; and Kulcinski, G. L. (1977). Fusion and Fast Breeder Reactors. Laxenburg, Austria: International Institute for Applied Systems Analysis.

Heppenheimer, T. A. (1984). The Man Made Sun: The Quest for Fusion Power. Boston: Little, Brown and Company.

Herman, R. (1990). Fusion: The Search for Endless Energy. New York: Cambridge University Press.

Huizenga, J. R. (1992). Cold Fusion: The Scientific Fiasco of the Century.Rochester: University of Rochester Press.

Lindl, J. D. (1998). Inertial Confinement Fusion: The Quest for Ignition and Energy Gain Using Indirect Drive. New York: Springer-Verlag.

McPhee, J. (1974). The Curve of Binding Energy.New York: Farrar, Straus and Giroux.

Niu, K. (1989). Nuclear Fusion. New York: Cambridge University Press.

Rhodes, R. (1995). Dark Sun: The Making of the Hydrogen Bomb.New York: Simon & Schuster.

Serber, R. (1992). The Los Alamos Primer: The First Lectures on How to Build an Atomic Bomb.Berkeley: University of California Press.

Sheffield, J. (1994). "The Physics of Magnetic Fusion Reactors." Reviews of Modern Physics 66:1015–1103.