Nuclear Fusion | Research & Encyclopedia Articles

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.

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.

Scientists who worked on the first fission ("atomic") bomb during World War I 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 an 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. Since that date, both the Soviet Union (now Russia) and the United States have become proficient at manufacturing fusion bombs on an almost assembly-line schedule.

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 can't 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 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.

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 particle 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.

The two nuclear reactions now most commonly used for power production purposes are designated as 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 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 recent years, scientists have begun to explore approaches to fusion power that depart from the more traditional magnetic and inertial confinement techniques. One such approach is 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. Thus far, 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 astonished 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.

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