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 of nucelosynthesis.
The practical problems of building a fusion power plant are formidable, and the technology to construct a suitable containment vessel or field in which controlled fusion reactions could take place does not yet exist. Currently the only fusion reactions that take place on Earth are uncontrolled fusion reaction in nuclear weapons (e.g., H-bombs).
In April, 2003, Sandia scientists reported that they had achieved controlled thermonuclear fusion in a pulsed power source. If ultimately reproduced and verified, the process, and other competing approaches to controlled fusion, holds the promise of nearly unlimited clean power generation. Unlike fission reactions, fusion based energy technology would not produce long-lived radioactive waste.
Instead of using magnetic containment to compress hydrogen and thereby achieve temperatures hot enough for fusion to occur, Sandia scientists used pulsed releases of current to achieve a rapid series of limited micro fusion reactions. Using an improved and more powerful Z accelerator, high current is induced in a tungsten wire cage surrounding a 2 mm plastic capsule containing deuterium (an heavier isotope of hydrogen). The tungsten cage is vaporized, but the short-lived current impulse generated in the wires creates a powerful magnetic pulse and shock-wave of superheated tungsten that creates an intense x-ray source that, along with the shockwave compresses and heats the hydrogen to more than 20 million degrees Fahrenheit (more than 11 million degrees Celsius) to induce fusion.
The Sandia reaction process contrasts with another promising approach undertaken at the Lawrence Livermore National Laboratory (LLNL) that seeks to initiate fusion reactions by shining high energy lasers on hydrogen globules. The LLNL approach will be further explored at the National Ignition Facility.
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 fusion sequence. 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, 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. Because those temperatures generally amount to a few million degrees, fusion reactions are also known as thermonuclear (thermo = heat) reactions. The heat to drive a thermonuclear reaction is created during the conversion of mass to energy during other thermonuclear reaction.
Fusion bombs. 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 was 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.
Possible peaceful uses for fusion. As research on fusion weapons continued, 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 hold promise of possibly solving this problem: magnetic and inertial containment.
One way to control hot 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 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 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.
In January 2003, the United States rejoined the International Fusion Program, an international effort to construct an experimental fusion reactor. Recent progress in controlling plasmas and developing technologies for burning plasma reactors may eventually provide a workable containment system.
█ FURTHER READING:
Boyd, T. J. M. and J. J. Anderson The Physics of Plasma. Cambridge, UK: Cambridge University Press, 2003.
United Kingdom Atomic Energy Authority. "Focus on Fusion." < http://www.fusion.org.uk/focus/index.htm > (March 29, 2003).
United States Department of Energy, Office of Fusion Energy Sciences. "Welcome to the U.S. Fusion Energy Sciences Program." < http://wwwofe.er.doe.gov/ > (March 30, 2003).