19.8: Nuclear Fusion

The process of converting very light nuclei into heavier nuclei is also accompanied by the conversion of mass into large amounts of energy, a process called fusion. The principal source of energy in the sun is a net fusion reaction in which four hydrogen nuclei fuse and ultimately produce one helium nucleus and two positrons.

A helium nucleus has a mass that is 0.7% less than that of four hydrogen nuclei; this lost mass is converted into energy during the fusion. This reaction produces about 1.7 × 10⁹ to 2.6 × 10⁹ kilojoules of energy per mole of helium-4 produced, depending on the fusion pathway. This is somewhat less than the energy produced by the nuclear fission of one mole of U-235 (1.8 × 10¹⁰ kJ). However, the fusion of one gram of helium-4 produces about 6.5 × 10⁸ kJ, which is greater than the energy produced by the fission of one gram of U-235 (8.5 × 10⁷ kJ). This is particularly notable because the reactants for helium fusion are less expensive and far more abundant than U-235 is.

It has been determined that the nuclei of the heavy isotopes of hydrogen, a deuteron and a triton, undergo thermonuclear fusion at extremely high temperatures to form a helium nucleus and a neutron. This change proceeds with a mass loss of 0.0188 amu, corresponding to the release of 1.69 × 10⁹ kilojoules per mole of helium-4 formed. The very high temperature is necessary to give the nuclei enough kinetic energy to overcome the very strong repulsive forces resulting from the positive charges on their nuclei so they can collide.

Useful fusion reactions require very high temperatures for their initiation—about 15,000,000 K or more. At these temperatures, all molecules dissociate into atoms, and the atoms ionize, forming plasma. These conditions occur in an extremely large number of locations throughout the universe—stars are powered by fusion.

It is a challenging task to create fusion reactors because no solid materials are stable at such high temperatures and mechanical devices cannot contain the plasma in which fusion reactions occur. Two techniques to contain plasma at the density and temperature necessary for a fusion reaction are currently the focus of intensive research efforts: containment by a magnetic field in a tokamak reactor and the use of focused laser beams. However, at present there are no self-sustaining fusion reactors operating in the world, although small-scale controlled fusion reactions have been run for very brief periods.

This text is adapted from the Openstax, Chemistry 2e, Section 21.4: Transmutation and Nuclear Energy.

The combination of small nuclei like hydrogen to produce larger ones such as helium is called nuclear fusion. Because nuclei have to overcome electrostatic repulsion, fusion reactions require temperatures of 40 million kelvins or more and so are known as thermonuclear reactions.

Nuclides with mass numbers between 40 and 100 have high binding energies per nucleon and are generally stable. Thus, lighter nuclei with low nuclear binding energies per nucleon tend to combine, yielding heavier nuclei with higher binding energies. 

The difference between the nuclear binding energies of product and reactant nuclides generates a huge amount of energy. Notably, the energy released during the formation of one gram of helium-4 is significantly larger than that of the fission of one gram of uranium-235.

So, is fusion used to produce electricity? Well, not yet! At the high temperatures required for fusion, all molecules dissociate into atoms, which ionize, forming a plasma. For such reactions, a strong, torus-shaped magnetic field serves as a reactor. However, its efficient use is still a technical challenge. 

Indeed, the fusion of hydrogen to helium is one of the major hydrogen-burning processes in main-sequence stars like the sun.

Once stars begin helium fusion, two helium nuclei combine into beryllium-8. Unlike helium-4, beryllium-8 is highly unstable, making this an endothermic, easily reversed fusion reaction. 

As helium fusion accelerates, beryllium-8 becomes more abundant and fuses with helium-4, producing excited-state carbon-12, which occasionally relaxes to stable carbon-12.

In massive stars, a chain of fusion reactions initiated by the combination of carbon-12 and helium-4 produces a sequence of elements up to magnesium-24.

As further fusion reactions create heavier nuclides, the decreasing difference in binding energies between reactants and products results in less energy being produced from these reactions. 

The sequence ends at nickel-56, which has one of the highest binding energies per nucleon. Heavier elements are instead produced by multiple neutron- or proton-capture events just before and during the unique explosions of stars, or supernovae.