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Nuclear Fusion

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 dreamscreating a fusion bombwas 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 ]

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Nuclear Fusion

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 bombone 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

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Nuclear Fusion

Nuclear Fusion


Nuclear fusion is a reaction whereby two smaller nuclei are combined to form a larger nucleus. It results in the release of energy for reactions that form nuclei of mass number below 60, with the largest energy release occurring with the lightest nuclides. This stands in contrast to the process of nuclear fission in which a heavy nucleus is split into two smaller nuclei with the release of energy. Since light nuclei have smaller repulsion energies (the energy required to bring two like charges together), fusion is much more likely to occur among these nuclei. Two deuterons, 2H, must have a total kinetic energy of 0.02 million electron volts (MeV) to be able to collide and react. Temperatures of greater than 200 million°C (360 million°F) are required for such kinetic energies. Atoms with kinetic energies of 0.02 MeV exist only as gases in which the atoms have lost their electrons. Such gases of ions and electrons are known as plasmas.

Temperatures required for fusion reactions exist in stars where fusion reactions are the principle components of energy release. In the Sun, approximately 90 percent of solar energy is a result of protonproton interactions in several steps to form helium of mass number 4. These steps all involve binuclear collisions since multinuclei collisions are very improbable events. Initially, two protons interact to form a deuterium nucleus (deuterium is an isotope of hydrogen with one proton and one neutron; the nucleus is a deuteron ) that collides with another proton to form a 3He (tritium) nucleus. This nucleus collides with a neutron or another 3He nucleus (with the emission of two protons) to form 4He. The net reaction can be represented as four protons fusing to form a 4He nucleus releasing 26.7 MeV. When a sufficient number of the 3He and 4He nuclei are formed in the star, they begin fusion reactions to form heavier nuclei such as 7Li and 7Be. The number of protonproton fusion reactions in the Sun amounts to 1.8 ×1038 s1. At present the Sun is 73 percent hydrogen, 25 percent helium, 2 percent carbon, nitrogen, oxygen, and all the other elements in the Periodic Table. Approximately 6 percent of the hydrogen originally in the Sun's stellar core has now been burnt.

Since the average mass of elements increases in stars, there is a transition from a proton cycle to a carbon cycle, as was proposed by American physicist Hans Bethe and German physicist Carl F. von Weizsacker in the 1930s. In such stars, the temperature and pressure reach higher values and the consumption of hydrogen accelerates. Since helium has a greater mass then hydrogen, it accumulates in the stellar core, while most of the hydrogen burning fusion of its nuclei moves to a layer outside that core. With an increased average mass in stars, reactions such as 8Be + 4He forming 12C begin to occur. The formation of 12C in sufficient quantities leads to reactions

with 4He to form oxygen, neon, and higher elements. Eventually, there is sufficient carbon present in some stars for the fusion of a pair of 12C nuclei to begin.

In stars with very heavy average masses, helium burning may last for only a few million years before it is replaced by carbon fusion. In time this leads to the production of elements such as calcium, titanium, chromium, iron, and nickel fusion partly by helium capture, partly by the direct fusion of heavy nuclides. For example, two 28Si can combine to form 56Ni that can decay to 56Co which then decays to stable 56Fe. These last steps of production may occur rather rapidly in a few thousand years. When the nuclear fuel for fusion is exhausted, the star collapses and a supernova results.

Nuclear fusion became important on Earth with the development of hydrogen bombs. A core of uranium or plutonium is used to initiate a fission reaction that raises the core's temperature to approximately 108 K, sufficient to cause fusion reactions between deuterium and tritium. In fusion bombs, LiD is used as 6Li reacts with fission neutrons to form tritium that then undergoes fusion with deuterium. It is estimated that approximately half the energy of a 50 megaton thermonuclear weapon comes from fusion and the other half from fission. Fusion reactions in these weapons also produce secondary fission since the high energy neutrons released in the fusion reactions make them very efficient in causing the fission of 238U.

The deuterium plus tritium and deuterium plus deuterium reactions are of interest in the development of controlled fusion devices for producing energy. A number of designs have been proposed for these fusion reactors, with most attention given to inertial confinement and magnetic confinement systems.

Inertial confinement is a pulsed system in which small pellets of D2 and T2 are irradiated by intense beams of photons or electrons. The surface of the pellet rapidly vaporizes, causing a temperature-pressure wave to move through the pellet, increasing its central temperature to greater than 108 K and causing fusion. If a fusion rate of approximately 100 pellets per second can be achieved, the result is a power output between 1 and 10 gigawatts.

At temperatures equal or greater than 107 K, hydrogen atoms are completely dissociated into protons and free electrons (the plasma state). Since construction material cannot withstand a plasma of this energy, the plasma is kept away from the walls by magnetic fields. The plasma density is limited by heat transfer and other considerations to approximately 1020 to 1021 particles m3. For a particle density of 3 × 1020 particles m3, confined for 0.1 to 1 second, the power density is estimated to be tens of megawatts per cubic meter. Several large machines based on magnetic confinement have been built, and confinement times of 2 seconds with particle densities of 5 × 1019 achieved. However, it seems unlikely that controlled thermonuclear reactors will be in operation for the purpose of power production before the year 2050 as significant technical problems remain to be solved. The availability of hydrogen and deuterium in the sea is so vast that nuclear fusion would outlast other nonrenewable energy sources. For example, a liter of seawater contains deuterium with an energy content equivalent to 300 liters (79.25 gallons) of gasoline.

see also Explosions; Nuclear Chemistry; Nuclear Fission.

Gregory R. Choppin

Bibliography

Choppin, Gregory R.; Liljenzin, Jan-Olov; and Rydberg, Jan (2001). Radiochemistry and Nuclear Chemistry, 3rd edition. Woburn, MA: Butterworth-Heinemann.

Friedlander, Gerhart; Kennedy, Joseph W.; Macias, Edward S.; and Miller, Julian (1981). Nuclear and Radiochemistry, 3rd edition. New York: Wiley-Interscience.

Internet Resources

FusEdWeb: Fusion Energy Educational Web Site. Available from <http://fusedweb.pppl.gov>.

General Atomics' Educational Web Site. Available from <http://fusioned.gat.com>.

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fusion, nuclear

fusion, nuclear Form of nuclear reaction in which nuclei of light atoms (such as hydrogen) combine to form one or more heavier nuclei with the release of large amounts of energy. The process takes place in the Sun and other stars, and has been reproduced on Earth in the hydrogen bomb. Efforts have since been concentrated on producing controlled fusion reaction in a thermonuclear reactor. Two main methods have been employed. In the first, a plasma of tritium and deuterium is raised to a temperature above 100 million°C in a tokamak reactor. The second method is laser fusion, in which a laser implodes a pellet of tritium and deuterium. At present, both methods fail to produce more energy from fusion than is put into the system. See also fission, nuclear; nuclear energy

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nuclear fusion

nu·cle·ar fu·sion • n. a nuclear reaction in which atomic nuclei of low atomic number fuse to form a heavier nucleus with the release of energy.

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nuclear fusion

nuclear fusion See fusion, nuclear

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