Nuclear Fusion

Some typical fusion reactions

Naturally occurring fusion

Thermonuclear reactions

Artificial fusion reactions

The hydrogen bomb

Peaceful applications of nuclear fusion

Inertial confinement

D–D and D–T reactions

Hope for the future

Cold fusion

Resources

Nuclear fusion is the process by which two light atomic nuclei combine to form a heavier atomic nucleus. As an example, a proton (the nucleus of a hydrogen atom) and a neutron will, under the proper circumstances, combine to form a deuteron (the nucleus of an atom of deuterium, an isotope of hydrogen). In general, the mass of the heavier nucleus thus produced is less than the total mass of the two lighter nuclei. This missing mass corresponds to energy released in the reaction.

When a proton and neutron combine, for example, the mass of the resulting deuteron is 0.00239 atomic mass unit less than the total mass of the proton and neutron combined. This “loss” of mass is expressed in the form of 2.23 MeV (million electron volts) of kinetic energy of the deuteron and other particles and as other forms of energy produced during the reaction. Nuclear fusion reactions are like nuclear fission reactions, therefore, in the respect that some quantity of mass is transformed into energy.

Some typical fusion reactions

The particles most commonly involved in nuclear fusion reactions include the proton, neutron, deuteron, a triton (a proton combined with two neutrons), a helium–3 nucleus (two protons combined with a neutron), and a helium–4 nucleus (two protons combined with two neutrons). Except for the neutron, all of these particles carry at least one positive electrical charge. That means that fusion reactions always require very large amounts of energy in order to overcome the force of repulsion between two like–charged particles. For example, in order to fuse two protons with each other, enough energy must be provided to overcome the force of repulsion between the two positively charged particles.

Naturally occurring fusion

As early as the 1930s, a number of physicists had considered the possibility that nuclear fusion reactions might be the mechanism by which energy is generated in stars. Certainly no familiar type of chemical reaction, such as oxidation, could possibly explain the vast amounts of energy released by even the smallest star over billions of years. In 1939, the German–American physicist Hans Bethe worked out the mathematics of fusion energy release, in which a proton first fuses with a carbon atom to form a nitrogen atom. The reaction then continues through a series of five more steps, the

net result of which is that four protons disappear and are replaced by one helium atom.

Bethe chose this sequence of reactions because it requires less energy than does the direct fusion of four protons and, thus, is more likely to take place in a star. Bethe was able to show that the total amount of energy released by this sequence of reactions was comparable to that which is actually observed in stars.

The Bethe carbon–cycle is not the only nuclear fusion reaction possible. Another approach, for example, would be one in which two protons fuse to form a deuteron. That deuteron could, then, fuse with a third proton to form a helium–3 nucleus. Finally, the helium–3 nucleus could fuse with a fourth proton to form a helium–4 nucleus. The net result of this sequence of reactions would be the combining of four protons (hydrogen nuclei) to form a single helium–4 nucleus. The only net difference between this reaction and Bethe’s carbon cycle is the amount of energy involved in the overall set of reactions.

Thermonuclear reactions

The reason that fusion reactions can occur in stars is that when they first form by gravitational compression of drifting gas, the temperatures in their centers are raised high enough to bring about fusion. Thereafter, gravity keeps a star together and fusion keeps it hot. Since the temperatures needed for fusion are in the millions of degrees, fusion reactions are also known as thermonuclear (“thermo” for heat) reactions.

Artificial fusion reactions

The understanding that fusion reactions are responsible for energy production in stars brought the accompanying realization that such reactions might be a source of energy for human needs. The practical problems with building a fusion power plant are great, however: it is extremely difficult to keep a fusion reaction going at all, much less drawing more energy out of the reaction than one puts into it (the “break–even point”). Scientists are still at least half a century from achieving affordable fusion power—by optimistic estimates. A simpler challenge, unfortunately, is to construct a fusion device that does not need to be controlled, that is, a fusion bomb.

The hydrogen bomb

Scientists who worked on the first fission (atomic) bomb during World War II were aware of the possibility of building an even more powerful bomb that operated on fusion. Here is how a basic fusion bomb works:

The core of the fusion bomb consists of a fission bomb, such as the one they were then developing. That core is then be surrounded by a casing filled with isotopes of hydrogen. Isotopes of hydrogen are forms of hydrogen that all have a single proton in their nucleus, but may have zero, one, or two neutrons. The nuclei of the hydrogen isotopes are, respectively, the proton, the deuteron, and the triton.

In the first fraction of a second of the detonation of such a device, the fission bomb would explode, releasing huge amounts of energy. In fact, the temperature at the heart of the fission bomb would reach several million degrees.

That temperature would not last very long, but in the microseconds that it did exist, it would provide the energy for fusion to begin to occur within the casing surrounding the fission bomb. Protons, deuterons, and tritons would begin fusing with each other, releasing more energy and triggering further fusion. The original explosion of the fission bomb would have ignited a small, starlike reaction in the casing surrounding it.

From a military standpoint, the fusion bomb had one powerful advantage over the fission bomb: there is no technical upper limit on the size of a fusion bomb. One simply makes the casing surrounding the fission bomb larger and larger, until (in theory) there is no longer a way to lift the bomb into the air so that it can be dropped on an enemy.

On November 1, 1952, the United States the world’s first fusion bomb, annihilating Eniwetok Atoll in the Pacific and leaving an underwater crater over 6,000 feet across. This bomb was about 1,000 times more powerful than the fission bombs with which the U.S. had destroyed the Japanese cities of Hiroshima and Nagasaki in 1945. The Soviet Union exploded its first nuclear device in 1953. The two superpowers soon became proficient at manufacturing fusion bombs in an assembly–line manner, producing tens of thousands of the devices.

Peaceful applications of nuclear fusion

As research on fusion weapons was going on, attempts were also being made to develop peaceful uses for nuclear fusion. The most obvious challenge is simply to find a way to hold the nuclear fusion reaction in place as it occurs. One cannot build a machine made out of metal, plastic, glass, or any other common kind of material. At the temperatures at which fusion occurs, any one of these materials would vaporize instantly. Traditionally, two general approaches have been developed to solve this problem, namely, magnetic confinement and inertial confinement. To understand the first technique, imagine that a mixture of hydrogen isotopes has been heated to a very high temperature. At a sufficiently high temperature, the nature of the mixture begins to change. Atoms totally lose their electrons, and the mixture consists of a swirling mass of positively charged nuclei and electrons. Such a mixture is known as a plasma.

One way to control that plasma is with a magnetic field. One can design such a field so that a swirling hot mass of plasma within it can be held in any kind of a shape one chooses. The best–known example of this approach is a doughnut–shaped Russian machine known as a tokamak. In the tokamak, two powerful electromagnets create fields that are so powerful that they can hold a hot plasma in place as readily as a person can hold an orange in his hand.

The technique, then, is to heat the hydrogen isotopes to higher and higher temperatures while containing them within a confined space by means of the magnetic fields. At some critical temperatures, nuclear fusion will begin to occur. At that point, the tokamak is producing energy by means of fusion while the fuel is being held in suspension by the magnetic field.

Inertial confinement

A second method for creating controlled nuclear fusion makes use of a laser beam or a beam of electrons or atoms. In this approach, hydrogen isotopes are suspended at the middle of the machine in tiny hollow glass spheres known as microballoons. The microballoons are then bombarded by the laser, electron, or atomic beam and caused to implode. During implosion, enough energy is produced to initiate fusion among the hydrogen isotopes within the pellet. The plasma thus produced is then confined and controlled by means of the external beam.

The production of useful nuclear fusion energy by either of these methods depends on three factors: temperature, containment time, and energy release. That is, it is first necessary to raise the temperature of the fuel (the hydrogen isotopes) to a temperature of about 100 million degrees. Then, it is necessary to keep the fuel suspended at that temperature long enough for fusion to begin. Finally, some method must be found for tapping off the energy produced by fusion.

A measure of the success of a machine in producing useful fusion energy is known as the Lawson confinement parameter, the product of the density of particles in the plasma and the time the particles are confined. That is, in order for controlled fusion to occur, particles in the plasma must be brought close together and they must be kept together for some critical period of time. All of this must take place, of course, at a temperature at which fusion can occur.

D–D and D–T reactions

The two nuclear reactions now most commonly used for power research purposes are designated as DD and D–T reactions. The former stands for deuterium–deuterium and involves the combination of two deuterium nuclei to form a helium–3 nucleus and a free neutron. The second reaction stands for deuterium–tritium and involves the combination of a deuterium nucleus and a tritium nucleus to produce a helium–4 nucleus and a free neutron. The most common form of an inertial confinement machine, for example, uses a fuel that consists of equal parts of deuterium and tritium.

Hope for the future

Research on controlled fusion power has now been going on for a half century with disappointing results. Some experts believe that success is “just around the corner”—just a few billion dollars more!—but others argue that the problems of an economically feasible fusion power plant will never be solved.

In late twentieth century, scientists began to explore approaches to fusion power that departed from the more traditional magnetic and inertial confinement techniques. One such approach was called the PBFA process. In this machine, electric charge is allowed to accumulate in capacitors and then discharged in 40–nanosecond micropulses. Lithium ions are accelerated by means of these pulses and forced to collide with deuterium and tritium targets. Fusion among the lithium and hydrogen nuclei takes place, and energy is released. However, the PBFA approach to nuclear fusion has been no more successful than has that of more traditional methods.

KEY TERMS

Cold fusion —A form of fusion that some researchers believe can occur at or near room temperatures as the result of the combination of deuterons with palladium metal as a catalyst.

Deuteron —The nucleus of the deuterium atom, consisting of one proton combined with one neutron.

Isotopes —Two or more forms of the same element, whose atoms differ from each other in the number of neutrons contained in their nuclei.

Neutron —A subatomic particle with a mass of about one atomic mass unit and no electrical charge.

Nuclear fission —A nuclear reaction in which one large atomic nucleus breaks apart into at least two smaller particles.

Nucleus —The core of an atom consisting of one or more protons and, usually, one or more neutrons.

Plasma —A form of matter that consists of positively charged particles and electrons completely independent of each other.

Proton —A subatomic particle with a mass of about one atomic mass unit and a single positive charge.

Thermonuclear reaction —A nuclear reaction that takes place only at very high temperatures, usually of the order of a few million degrees.

Cold fusion

The scientific world was stunned in March, 1989 when two electrochemists, Stanley Pons and Martin Fleischmann, reported that they had obtained evidence for the occurrence of nuclear fusion at room temperatures. During the electrolysis of heavy water (deuterium oxide), it appeared that the fusion of deuterons was made possible by the presence of palladium electrodes used in the reaction. If such an observation could have been confirmed by other scientists, it would have been truly revolutionary. It would have meant that energy could be obtained from fusion reactions at moderate temperatures. The Pons–Fleischmann discovery was the subject of immediate and intense scrutiny by other scientists around the world.

It soon became apparent, however, that evidence for cold fusion could not consistently be obtained by other researchers. A number of alternative explanations were developed by scientists for the fusion results that Pons and Fleischmann believed they had obtained. Today, few scientists are still convinced that Pons and Fleischmann had made a breakthrough. In what has so far been a replay of the Pons–Fleischmann story, nuclear engineer Rusi Taleyarkhan of Purdue University announced the detection of desktop cold fusion in March, 2005. The fusion was supposedly triggered by the collapse of tiny bubbles in heavy water caused by ultrasonic waves. Taleyarkhan’s findings were published in a reputable peer–reviewed journal and, since the collapse of such bubbles is known to generate extraordinarily high temperatures, his work struck many scientists as more plausible than Pons and Fleischmann’s. However, as of late 2006 all attempts to duplicate Taleyarkhan’s results had failed, and most physicists believed that his apparent breakthrough was another case of honest measurement error.

Resources

BOOKS

Chen, Francis F. Introduction to Plasma Physics and Controlled Fusion. New York: Springer, 2006.

Peat, F. David. Cold Fusion: The Making of a Scientific Controversy. Chicago: Contemporary Books, 1989.

Proliferation: Threat and Response. Washington DC: Department of Defense, 2001. p.15.

PERIODICALS

Fuyuno, Ichiko. “China Set to Make Fusion History.” Nature. 442 (2006): 853.

Starr, Chauncey, et al. “Reexamining Fusion Power.” Science. 313 (2006): 170–171.

David E. Newton

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 α-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

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

Nuclear fusion

Nuclear fusion is the process by which two light atomic nuclei combine to form one heavier atomic nucleus. As an example, a proton (the nucleus of a hydrogen atom) and a neutron will, under the proper circumstances, combine to form a deuteron (the nucleus of an atom of "heavy hydrogen"). In general, the mass of the heavier product nucleus is less than the total mass of the two lighter nuclei.

When a proton and neutron combine, for example, the mass of the resulting deuteron is 0.00239 atomic mass unit less than the total mass of the proton and neutron combined. This "loss" of mass is expressed in the form of 2.23 MeV (million electron volts) of kinetic energy of the deuteron and other particles and as other forms of energy produced during the reaction. Nuclear fusion reactions are like nuclear fission reactions, therefore, in the respect that some quantity of mass is transformed into energy.

Some typical fusion reactions

The particles most commonly involved in nuclear fusion reactions include the proton, neutron, deuteron, a triton (a proton combined with two neutrons), a helium-3 nucleus (two protons combined with a neutron), and a helium-4 nucleus (two protons combined with two neutrons). Except for the neutron, all of these particles carry at least one positive electrical charge. That means that fusion reactions always require very large amounts of energy in order to overcome the force of repulsion between two like-charged particles. For example, in order to fuse two protons with each other, enough energy must be provided to overcome the force of repulsion between the two positively charged particles.

Naturally occurring fusion reactions

As early as the 1930s, a number of physicists had considered the possibility that nuclear fusion reactions might be the mechanism by which energy is generated in the stars. Certainly no familiar type of chemical reaction, such as oxidation, could possibly explain the vast amounts of energy released by even the smallest star . In 1939, the German-American physicist Hans Bethe worked out the mathematics of energy generation in which a proton first fuses with a carbon atom to form a nitrogen atom. The reaction then continues through a series of five more steps, the net result of which is that four protons disappear and are replaced by one helium atom.

Bethe chose this sequence of reactions because it requires less energy than does the direct fusion of four protons and, thus, is more likely to take place in a star. Bethe was able to show that the total amount of energy released by this sequence of reactions was comparable to that which is actually observed in stars.

The Bethe carbon-cycle is by no means the only nuclear fusion reaction that one might conceive. A more direct approach, for example, would be one in which two protons fuse to form a deuteron. That deuteron could, then, fuse with a third proton to form a helium-3 nucleus. Finally, the helium-3 nucleus could fuse with a fourth proton to form a helium-4 nucleus. The net result of this sequence of reactions would be the combining of four protons (hydrogen nuclei) to form a single helium-4 nucleus. The only net difference between this reaction and Bethe's carbon cycle is the amount of energy involved in the overall set of reactions.

Thermonuclear reactions

The term "less energy" used to describe Bethe's choice of nuclear reactions is relative, however, since huge amounts of energy must be provided in order to bring about any kind of fusion reaction. In fact, the reason that fusion reactions can occur in stars is that the temperatures in their interiors are great enough to provide the energy needed to bring about fusion. Since those temperatures generally amount to a few million degrees, fusion reactions are also known as thermonuclear (thermo = heat ) reactions.

Fusion reactions on earth

The understanding that fusion reactions might be responsible for energy production in stars brought the accompanying realization that such reactions might be a very useful source of energy for human needs. Imagine that it would be possible to build and operate a small star on the outskirts of your community that operated on nuclear fusion. That power plant would be able to supply all of the community's energy needs as far into the future as anyone could see.

The practical problems of building a fusion power plant are incredible, however, and scientists are still a long way from achieving that goal. A much simpler challenge, however, is to construct a "fusion power plant" that does not need to be controlled, that is, a fusion bomb.

Uncontrolled fusion power: The hydrogen bomb

Scientists who worked on the first fission (atomic) bomb during World War II were aware of the potential for building an even more powerful bomb that operated on fusion principles. Here is how it would work.

The core of the fusion bomb would consist of a fission bomb, such as the one they were then developing. That core could then be surrounded by a casing filled with isotopes of hydrogen. Isotopes of hydrogen are various forms of hydrogen that all have a single proton in their nucleus, but may have zero , one, or two neutrons. The nuclei of the hydrogen isotopes are the proton, the deuteron, and the triton.

Imagine that a device such as the one described here could be exploded. In the first fraction of a second, the fission bomb would explode, releasing huge amounts of energy. In fact, the temperature at the heart of the fission bomb would reach a few millions degrees, the only way that humans know of for producing such high temperatures.

That temperature would not last very long, but in the microseconds that it did exist, it would provide the energy for fusion to begin to occur within the casing surrounding the fission bomb. Protons, deuterons, and tritons would begin fusing with each other, releasing more energy, and initiating other fusion reactions among other hydrogen isotopes. The original explosion of the fission bomb would have ignited a small star-like reaction in the casing surrounding it.

From a military standpoint, the fusion bomb had one powerful advantage over the fission bomb. For technical reasons, there is a limit to the size one can make a fission bomb. But there is no technical limit on the size of a fusion bomb. One simply makes the casing surrounding the fission bomb larger and larger, until there is no longer a way to lift the bomb into the air so that it can be dropped on an enemy.

On August 20, 1953, the Soviet Union announced the detonation of the world's first fusion bomb. It was about 1,000 times more powerful than was the fission bomb that had been dropped on Hiroshima less than a decade earlier. The Soviet Union (now Russia) and the United States became proficient at manufacturing fusion bombs on an almost assembly-line schedule.

Peaceful applications of nuclear fusion

As research on fusion weapons was going on, attempts were also being made to develop peaceful uses for nuclear fusion. The concept of a "star" power plant just outside the city was never out of sight for a number of nuclear scientists.

The problems to be solved in controlling the nuclear fusion reaction have, however, been enormous. The most obvious challenge is simply to find a way to hold the nuclear fusion reaction in place as it occurs. One cannot build a machine made out of metal , plastic, glass , or any other common kind of material. At the temperatures at which fusion occurs, any one of these materials would vaporize instantly. So how does one contain the nuclear fusion reaction?

Traditionally, two general approaches have been developed to solve this problem: magnetic and inertial containment. To understand the first technique, imagine that a mixture of hydrogen isotopes has been heated to a very high temperature. At a sufficiently high temperature, the nature of the mixture begins to change. Atoms totally lose their electrons, and the mixture consists of a swirling mass of positively charged nuclei and electrons. Such a mixture is known as a plasma.

One way to control that plasma is with a magnetic field. One can design such a field so that a swirling hot mass of plasma within it can be held in any kind of a shape one chooses. The best known example of this approach is a doughnut-shaped Russian machine known as a tokamak. In the tokamak, two powerful electromagnets create fields that are so powerful that they can hold a hot plasma in place as readily as a person can hold an orange in his hand.

The technique, then, is to heat the hydrogen isotopes to higher and higher temperatures while containing them within a confined space by means of the magnetic fields. At some critical temperatures, nuclear fusion will begin to occur. At that point, the tokamak is producing energy by means of fusion while the fuel is being held in suspension by the magnetic field.

Inertial confinement

A second method for creating controlled nuclear fusion makes use of a laser beam or a beam of electrons or atoms. In this approach, hydrogen isotopes are suspended at the middle of the machine in tiny hollow glass spheres known as microballoons. The microballoons are then bombarded by the laser, electron, or atomic beam and caused to implode. During implosion, enough energy is produced to initiate fusion among the hydrogen isotopes within the pellet. The plasma thus produced is then confined and controlled by means of the external beam.

The production of useful nuclear fusion energy by either of these methods depends on three factors: temperature, containment time, and energy release. That is, it is first necessary to raise the temperature of the fuel (the hydrogen isotopes) to a temperature of about 100 million degrees. Then, it is necessary to keep the fuel suspended at that temperature long enough for fusion to begin. Finally, some method must be found for tapping off the energy produced by fusion.

A measure of the success of a machine in producing useful fusion energy is known as the Lawson confinement parameter, the product of the density of particles in the plasma and the time the particles are confined. That is, in order for controlled fusion to occur, particles in the plasma must be brought close together and they must be kept together for some critical period of time. All of this must take place, of course, at a temperature at which fusion can occur.

D-D and D-T reactions

The two nuclear reactions now most commonly used for power production purposes are designated as DD and D-T reactions. The former stands for deuterium-deuterium and involves the combination of two deuterium nuclei to form a helium-3 nucleus and a free neutron. The second reaction stands for deuterium-tritium and involves the combination of a deuterium nucleus and a tritium nucleus to produce a helium-4 nucleus and a free neutron. The most common form of an inertial confinement machine, for example, uses a fuel that consists of equal parts of deuterium and tritium.

Hope for the future

Research on controlled fusion power has now been going on for a half century with somewhat disappointing results. Some experts believe that success may be "just around the corner," but others argue that the problems of an economically feasible fusion power plant may never be solved.

In late twentieth century, scientists began to explore approaches to fusion power that departed from the more traditional magnetic and inertial confinement techniques. One such approach was called the PBFA process. In this machine, electric charge is allowed to accumulate in capacitors and then discharged in 40-nanosecond micropulses. Lithium ions are accelerated by means of these pulses and forced to collide with deuterium and tritium targets. Fusion among the lithium and hydrogen nuclei takes place, and energy is released. However, the PBFA approach to nuclear fusion has been no more successful than has that of more traditional methods.

Cold fusion

The scientific world was stunned in March of 1989 when two electrochemists, Stanley Pons and Martin Fleischmann, reported that they had obtained evidence for the occurrence of nuclear fusion at room temperatures. During the electrolysis of heavy water (deuterium oxide), it appeared that the fusion of deuterons was made possible by the presence of palladium electrodes used in the reaction. If such an observation could have been confirmed by other scientists, it would have been truly revolutionary. It would have meant that energy could be obtained from fusion reactions at moderate temperatures.

The Pons-Fleischmann discovery was the subject of immediate and intense scrutiny by other scientists around the world. It soon became apparent, however, that evidence for cold fusion could not consistently be obtained by other researchers. A number of alternative explanations were developed by scientists for the fusion results that Pons and Fleischmann believed they had obtained. Today, some scientists are still convinced that Pons and Fleischmann had made a real and important breakthrough in the area of fusion research. Most researchers, however, attribute the results they reported to other events that occurred during the electrolysis of the heavy water.

Resources

books

Carlisle, Rodney P. Encyclopedia of the Atomic Age. New York: Facts on File, 2001.

Morris, Robert. The Environmental Case for Nuclear Power. New York: Paragon Press, 2000.

Peat, F. David. Cold Fusion: The Making of a Scientific Controversy. Chicago: Contemporary Books, 1989.

Proliferation: Threat and Response. Washington DC: Department of Defense, 2001. p.15.

Richardson, Hazel, and Scoular Anderson. How to Split theAtom. Franklin Watts, 2001.

periodicals

Cordey, J. Geoffrey. "Progress toward a Tokamak Fusion Reactor." Physics Today January 1992, pp. 22-30.

Hogan, William J. "Energy from Inertial Fusion." PhysicsToday September 1992, pp. 42-50.

Jensen, Torkil H. "Fusion-A Potential Power Source." Journal of Chemical Education October 1994, pp. 820-823.

organizations

The Nuclear Energy Institute [cited March 2003] <http://www.nei.org/>.

other

U.S. Congress. Office of Technology Assessment. Starpower:The U.S. and the International Quest for Fusion Energy. Washington, DC: Office of Technology Assessment, 1987.

David E. Newton

KEY TERMS

Cold fusion

—A form of fusion that some researchers believe can occur at or near room temperatures as the result of the combination of deuterons with palladium metal as a catalyst.

Deuteron

—The nucleus of the deuterium atom, consisting of one proton combined with one neutron.

Isotopes

—Two or more forms of the same element, whose atoms differ from each other in the number of neutrons contained in their nuclei.

Neutron

—A subatomic particle with a mass of about one atomic mass unit and no electrical charge.

Nuclear fission

—A nuclear reaction in which one large atomic nucleus breaks apart into at least two smaller particles.

Nucleus

—The core of an atom consisting of one or more protons and, usually, one or more neutrons.

Plasma

—A form of matter that consists of positively charged particles and electrons completely independent of each other.

Proton

—A subatomic particle with a mass of about one atomic mass unit and a single positive charge.

Thermonuclear reaction

—A nuclear reaction that takes place only at very high temperatures, usually of the order of a few million degrees.

Nuclear fusion

Nuclear fusion is the process by which two light atomic nuclei combine to form one heavier atomic nucleus. As an example, a proton and a neutron can be made to combine with each other to form a single particle called a deuteron. In general, the mass of the heavier product nucleus (the deuteron, for example) is less than the total mass of the two lighter nuclei (the proton and the neutron).

The mass that "disappears" during fusion is actually converted into energy. The amount of energy (E) produced in such a reaction can be calculated using Einstein's formula for the equivalence of mass and energy: E = mc². This formula says even when the amount of mass (m) that disappears is very small, the amount of energy produced is very large. The reason is that the value of c² (the speed of light squared) is very large, approximately 900,000,000,000,000,000,000 meters per second.

Naturally occurring fusion reactions

Scientists have long suspected that nuclear fusion reactions are common in the universe. The factual basis for such beliefs is that stars consist primarily of hydrogen gas. Over time, however, hydrogen gas is used up in stars, and helium gas is produced. One way to explain this phenomenon is to assume that hydrogen nuclei in the core of stars fuse with each other to form the nuclei of helium atoms. That is:

4 hydrogen nuclei → fuse → 1 helium nucleus

Words to Know

Cold fusion: A form of fusion that some researchers believe can occur at or near room temperatures as the result of the combination of deuterons during the electrolysis of water.

Deuteron: The nucleus of the deuterium atom, consisting of one proton combined with one neutron.

Electrolysis: The process by which an electrical current causes a chemical change, usually the breakdown of some substance.

Isotopes: Two or more forms of an element that have the same chemical properties but that differ in mass because of differences in the number of neutrons in their nuclei.

Neutron: A subatomic particle with a mass of about one atomic mass unit and no electrical charge.

Nuclear fission: A nuclear reaction in which one large atomic nucleus breaks apart into at least two smaller particles.

Nucleus: The core of an atom consisting of one or more protons and, usually, one or more neutrons.

Plasma: A form of matter that consists of positively charged particles and electrons completely independent of each other.

Proton: A subatomic particle with a mass of about one atomic mass unit and a single positive charge.

Subatomic particle: Basic unit of matter and energy (proton, neutron, electron, neutrino, and positron) smaller than an atom.

Thermonuclear reaction: A nuclear reaction that takes place only at very high temperatures, usually on the order of a few million degrees.

Over the past half century, a number of theories have been suggested as to how such fusion reactions might occur. One problem that must be resolved in such theories is the problem of electrostatic repulsion. Electrostatic repulsion is the force that tends to drive two particles with the same electric charge away from each other.

The nucleus of a hydrogen atom is a single proton, a positively charged particle. If fusion is to occur, two protons must combine with each other to form a single particle:

p⁺ + p⁺ → combined particle

But forcing two like-charged particles together requires a lot of energy. Where do stars get that energy?

Thermonuclear reactions

The answer to that question has many parts, but one part involves heat. If you raise the temperature of hydrogen gas, hydrogen atoms move faster and faster. They collide with each other with more and more energy. Eventually, they may collide in such a way that two protons will combine with (fuse with) each other. Reactions that require huge amounts of energy in order to occur are called thermonuclear reactions: thermo- means "heat" and -nuclear refers to the nuclei involved in such reactions.

The amount of heat needed to cause such reactions is truly astounding. It may require temperatures from a few millions to a few hundred millions of degrees Celsius. Such temperatures are usually unknown on Earth, although they are not uncommon at the center of stars.

Scientists now believe that fusion reactions are the means by which stars generate their energy. In these reactions, hydrogen is first converted to helium, with the release of large amounts of energy. At some point, no more hydrogen is available for fusion reactions, a star collapses, it heats up, and new fusion reactions begin. In the next stage of fusion reactions, helium nuclei may combine to form carbon nuclei. This stage of reactions requires higher temperatures but releases more energy. When no more helium remains for fusion reactions, yet another sequence of reactions begin. This time, carbon nuclei might be fused in the production of oxygen or neon nuclei. Again, more energy is required for such reactions, and more energy is released.

The end result of this sequence of fusion reactions is that stars heat up to temperatures they can no longer withstand. They explode as novas or supernovas, releasing to the universe the elements they have been creating in their cores.

Fusion reactions on Earth

Dreams of harnessing fusion power for human use developed alongside similar dreams for harnessing fission power. The first step in the realization of those dreams—creating a fusion bomb—was relatively simple, requiring a large batch of hydrogen (like the hydrogen in a star) and a source of heat that would raise the temperature of the hydrogen to a few million degrees Celsius.

Encapsulating the hydrogen was the easy part. A large container (the bomb casing) was built and filled with as much hydrogen as possible, probably in the form of liquid hydrogen. Obtaining the high temperature was more difficult. In general, there is no way to produce a temperature of 10,000,000°C on Earth. The only practical way to do so is to set off a fission (atomic) bomb. For a few moments after a fission bomb explodes, it produces temperatures in this range.

All that was needed to make a fusion bomb, then, was to pack a fission bomb at the center of the hydrogen-filled casing of the fusion bomb. When the fission bomb exploded, a temperature of a few million degrees Celsius would be produced, and fusion would begin within the hydrogen. As fusion proceeded, even greater amounts of energy would be produced, resulting in a bomb that was many times more powerful than the fission bomb itself.

For comparison, the fission bomb dropped on Hiroshima, Japan, in August of 1945 was given a power rating of about 20 kilotons. The measure 20 kilotons means that the bomb released as much energy as 20,000 tons of TNT, one of the most powerful chemical explosives known. In contrast, the first fusion (hydrogen) bomb tested had a power rating of 5 megatons, that is, the equivalent of 5 million tons of TNT.

Peaceful applications of nuclear fusion

As with nuclear fission, scientists were also very much committed to finding peaceful uses of nuclear fusion. The problems to be solved in controlling nuclear fusion reactions have, however, been enormous. The most obvious challenge is simply to find a way to "hold" the nuclear fusion reaction in place as it occurs. One cannot build a machine made out of metal, plastic, glass, or any other common kind of material. At the temperatures at which fusion occurs, any one of these materials would vaporize instantly. So how can the nuclear fusion reaction be contained?

One of the methods that has been tried is called magnetic confinement. To understand this technique, imagine that a mixture of hydrogen isotopes has been heated to a very high temperature. At a sufficiently high temperature, the nature of the mixture begins to change. Atoms totally lose their electrons, and the mixture consists of a swirling mass of positively charged nuclei and negatively charged electrons. Such a mixture is known as a plasma.

One way to control that plasma is with a magnetic field, which can be designed so that the swirling hot mass of plasma within the field is held in any kind of shape. The best known example of this approach is a doughnut-shaped Russian machine known as a tokamak. In the tokamak, two powerful electromagnets create fields that are so strong they can hold a hot plasma in place as readily as a person can hold an orange in his or her hand.

The technique, then, is to heat the hydrogen isotopes to higher and higher temperatures while containing them within a confined space by means of the magnetic fields. At some critical temperatures, nuclear fusion will begin to occur. At that point, the tokamak is producing energy by means of fusion while the fuel is being held in suspension by the magnetic field.

Hope for the future

Research on controlled fusion power has now been going on for a half century with somewhat disappointing results. Some experts argue that no method will ever be found for making fusion power by a method that humans can afford. The amount of energy produced by fusion, they say, will always be less than the amount of energy put into the process in the first place. Other scientists disagree. They believe that success may be soon in coming, and it is just a matter of finding solutions to the many technical problems surrounding the production of fusion power.

Cold fusion

The scientific world was astonished in March of 1989 when two electrochemists, Stanley Pons and Martin Fleischmann, reported that they had obtained evidence of the occurrence of nuclear fusion at room temperatures. Pons and Fleischmann passed an electric current through a form of water known as heavy water, or deuterium oxide. In the process, they reported fusion of deuterons had occurred. A deuteron is a particle consisting of a proton combined with a neutron. If such an observation could have been confirmed by other scientists, it would have been truly revolutionary: it would have meant that energy could be obtained from fusion reactions at moderate temperatures rather than at temperatures of millions of degrees.

The Pons-Fleischmann discovery was the subject of immediate and intense study by other scientists around the world. It soon became apparent, however, that evidence for cold fusion could not be obtained by other researchers with any degree of consistency. A number of alternative explanations were developed by scientists for the fusion results that Pons and Fleischmann believed they had obtained. Today, some scientists are still convinced that Pons and Fleischmann made a real and important breakthrough in the area of fusion research. Most researchers, however, attribute the results they reported to other events that occurred during the electrolysis of the heavy water.

[See also Nuclear fission; Nuclear power; Nuclear weapons ]

Nuclear fusion

Nuclear fusion is the process by which two light atomic nuclei combine to form one heavier atomic nucleus. As an example, a proton (the nucleus of a hydrogen atom ) and a neutron will, under the proper circumstances, combine to form a deuteron (the nucleus of an atom of "heavy" hydrogen). In general, the mass of the heavier product nucleus is less than the total mass of the two lighter nuclei. Nuclear fusion is the initial driving process for the process of nucelosynthesis.

When a proton and neutron combine, the mass of the resulting deuteron is 0.00239 atomic mass units (amu) less than the total mass of the proton and neutron combined. This "loss" of mass is expressed in the form of 2.23 MeV (million electron volts) of kinetic energy of the deuteron and other particles and as other forms of energy produced during the reaction. Nuclear fusion reactions are like nuclear fission reactions, therefore, in that some quantity of mass is transformed into energy. This is the reason stars "shine" (i.e., radiate tremendous amounts of electromagnetic energy into space ).

The particles most commonly involved in nuclear fusion reactions include the proton, neutron, deuteron, a triton (a proton combined with two neutrons), a helium-3 nucleus (two protons combined with a neutron), and a helium-4 nucleus (two protons combined with two neutrons). Except for the neutron, all of these particles carry at least one positive electrical charge. That means that fusion reactions always require very large amounts of energy in order to overcome the force of repulsion between two like-charged particles. For example, in order to fuse two protons with each other, enough energy must be provided to overcome the force of repulsion between the two positively charged particles.

As early as the 1930s, a number of physicists considered the possibility that nuclear fusion reactions might be the mechanism by which energy is generated in the stars. No familiar type of chemical reaction, such as combustion or oxidation, could possibly explain the vast amounts of energy released by even the smallest star. In 1939, the German-American physicist Hans Bethe worked out the mathematics of energy generation in which a proton first fuses with a carbon atom to form a nitrogen atom. The reaction then continues through a series of five more steps, the net result of which is that four protons are consumed in the generation of one helium atom.

Bethe chose this sequence of reactions because it requires less energy than does the direct fusion of four protons and, thus, is more likely to take place in a star. Bethe was able to show that the total amount of energy released by this sequence of reactions was comparable to that which is actually observed in stars.

The Bethe carbon-cycle is by no means the only nuclear fusion reaction. A more direct approach, for example, would be one in which two protons fuse to form a deuteron. That deuteron could then fuse with a third proton to form a helium-3 nucleus. Finally, the helium-3 nucleus could fuse with a fourth proton to form a helium-4 nucleus. The net result of this sequence of reactions would be the combining of four protons (hydrogen nuclei) to form a single helium-4 nucleus. The only net difference between this reaction and Bethe's carbon cycle is the amount of energy involved in the overall set of reactions.

Other fusion reactions include D-D and D-T reactions. The former stands for deuterium-deuterium and involves the combination of two deuterium nuclei to form a helium-3 nucleus and a free neutron. The second reaction stands for deuterium-tritium and involves the combination of a deuterium nucleus and a tritium nucleus to produce a helium-4 nucleus and a free neutron.

The term "less energy" used to describe Bethe's choice of nuclear reactions is relative, however, since huge amounts of energy must be provided in order to bring about any kind of fusion reaction. In fact, the reason that fusion reactions can occur in stars is that the temperatures in their interiors are great enough to provide the energy needed to bring about fusion. Since those temperatures generally amount to a few million degrees, fusion reactions are also known as thermonu-clear (thermo = heat) reactions. The heat to drive a thermonu-clear reaction is created during the conversion of mass to energy during other thermonuclear reactions.

The understanding that fusion reactions might be responsible for energy production in stars brought the accompanying realization that such reactions might be a very useful source of energy for human needs. The practical problems of building a fusion power plant are incredible, however, and scientists are still a long way from achieving a containment vessel or field in which controlled fusion reactions could take place. A much simpler challenge, however, is to construct a "fusion power plant" that does not need to be controlled, that is, a fusion bomb.

Scientists who worked on the first fission (atomic) bomb during World War II were aware of the potential for building an even more powerful bomb that operated on fusion principles. A fusion bomb uses a fission bomb as a trigger (a source of heat and pressure to create a fusion chain reaction. In the microseconds following a fission explosion, fusion begins to occur within the casing surrounding the fission bomb. Protons, deuterons, and tritons begin fusing with each other, releasing more energy, and initiating other fusion reactions among other hydrogen isotopes. The original explosion of the fission bomb would have ignited a small star-like reaction in the casing surrounding it.

From a military standpoint, the fusion bomb had one powerful advantage over the fission bomb. For technical reasons, there is a limit to the size one can make a fission bomb. However, there is no technical limit on the size of a fusion bomb—one simply makes the casing surrounding the fission bomb larger. On August 20, 1953, the Soviet Union announced the detonation of the world's first fusion bomb. It was about 1,000 times more powerful than was the fission bomb that had been dropped on Hiroshima less than a decade earlier. Since that date, both the Soviet Union (now Russia) and the United States have stockpiled thousands of fusion bombs and fusion missile warheads. The manufacture, maintenance, and destruction of these weapons remain a source of scientific and geopolitical debate.

With research on fusion weapons ongoing, attempts were also being made to develop peaceful uses for nuclear fusion. The containment vessel problems remain daunting because at the temperatures at which fusion occurs, known materials vaporize instantly. Traditionally, two general approaches have been developed to solve this problem: magnetic and inertial containment.

One way to control that plasma is with a magnetic field . One can design such a field so that a swirling hot mass of plasma within it can be held in a specified shape. Other proposed methods of control include the use of suspended microballoons that are then bombarded by the laser, electron, or atomic beam to cause implosion. During implosion, enough energy is produced to initiate fusion.

The production of useful nuclear fusion energy depends on three factors: temperature , containment time, and energy release. That is, it is first necessary to raise the temperature of the fuel (the hydrogen isotopes) to a temperature of about 100 million degrees. Then, it is necessary to keep the fuel suspended at that temperature long enough for fusion to begin. Finally, some method must be found for tapping off the energy produced by fusion.

In the late twentieth century, scientists began to explore approaches to fusion power that departed from magnetic and inertial confinement concepts. One such approach was called the PBFA process. In this machine, electric charge is allowed to accumulate in capacitors and then discharged in 40-nanosecond micropulses. Lithium ions are accelerated by means of these pulses and forced to collide with deuterium and tritium targets. Fusion among the lithium and hydrogen nuclei takes place, and energy is released. However, the PBFA approach to nuclear fusion has been no more successful than has that of more traditional methods.

In March of 1989, two University of Utah electro-chemists, Stanley Pons and Martin Fleischmann, reported that they had obtained evidence for the occurrence of nuclear fusion at room temperatures (i.e., cold fusion). During the electrolysis of heavy water (deuterium oxide), it appeared that the fusion of deuterons was made possible by the presence of palladium electrodes used in the reaction. If such an observation could have been confirmed by other scientists, it would have been truly revolutionary. It would have meant that energy could be obtained from fusion reactions at moderate temperatures. The Pons-Fleischmann discovery was the subject of immediate and intense scrutiny by other scientists around the world. It soon became apparent, however, that evidence for cold fusion could not consistently be obtained by other researchers. A number of alternative explanations were developed by scientists for the apparent fusion results that Pons and Fleischmann believed they had obtained and most researchers now assert that Pons and Fleischmann's report of "cold fusion" was an error and that the results reported were due to other chemical reactions that take place during the electrolysis of the heavy water.

See also Atom; Atomic mass and weight; Atomic number; Atomic theory; Big Bang theory; Chemical elements; Chemistry; Electricity and magnetism; Energy transformations; Radioactive waste storage (geological considerations); Radioactivity