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, 2 H, 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 proton–proton 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 3 He (tritium) nucleus. This nucleus collides with a neutron or another 3 He nucleus (with the emission of two protons) to form 4 He. The net reaction can be represented as four protons fusing to form a 4 He nucleus releasing 26.7 MeV. When a sufficient number of the 3 He and 4 He nuclei are formed in the star, they begin fusion reactions to form heavier nuclei such as 7 Li and 7 Be. The number of proton–proton fusion reactions in the Sun amounts to 1.8 ×10 38 s −1 . 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 8 Be + 4 He forming 12 C begin to occur. The formation of 12 C in sufficient quantities leads to reactions
with 4 He to form oxygen, neon, and higher elements. Eventually, there is sufficient carbon present in some stars for the fusion of a pair of 12 C 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 28 Si can combine to form 56 Ni that can decay to 56 Co which then decays to stable 56 Fe. 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 10 8 K, sufficient to cause fusion reactions between deuterium and tritium. In fusion bombs, LiD is used as 6 Li 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 238 U.
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 D 2 and T 2 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 10 8 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 10 7 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 10 20 to 10 21 particles m −3 . For a particle density of 3 × 10 20 particles m −3 , 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 × 10 19 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.
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.
FusEdWeb: Fusion Energy Educational Web Site. Available from http://fusedweb.pppl.gov .
General Atomics' Educational Web Site. Available from http://fusioned.gat.com .