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…
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.
View the full transcript and gain access to JoVE Core videos
Q1: What is nuclear fusion and how does it differ from fission?
Nuclear fusion combines small nuclei like hydrogen into larger ones such as helium, releasing enormous energy. Unlike fission, which splits heavy nuclei, fusion merges light nuclei with low binding energies into heavier ones with higher binding energies. The energy difference between products and reactants generates the power. Fusion of one gram of helium-4 produces more energy than fission of one gram of uranium-235.
Q2: Why do fusion reactions require such extreme temperatures?
Fusion reactions require temperatures of 40 million kelvins or higher because nuclei must overcome strong electrostatic repulsion to collide and fuse. At these thermonuclear temperatures, all molecules dissociate into atoms, which ionize to form plasma. The high kinetic energy allows nuclei to approach each other closely enough for the strong nuclear force to bind them together.
Q3: How does the sun produce energy through fusion?
The sun powers itself through hydrogen-to-helium fusion, where four hydrogen nuclei fuse to produce one helium nucleus and two positrons. This net fusion reaction converts mass into approximately 1.7 × 10⁹ to 2.6 × 10⁹ kilojoules of energy per mole of helium-4 produced. This process, called the hydrogen-burning process, sustains main-sequence stars like our sun.
Q4: What happens during helium fusion in stars?
During helium fusion, two helium nuclei combine into beryllium-8, which is highly unstable and easily reverses. As the reaction accelerates, beryllium-8 becomes more abundant and fuses with helium-4, producing excited-state carbon-12 that relaxes to stable carbon-12. In massive stars, carbon-12 and helium-4 initiate a chain of fusion reactions producing elements up to magnesium-24.
Q5: Why haven't fusion reactors been successfully used for electricity generation?
Fusion reactors remain impractical because no solid materials withstand the extreme temperatures required, and mechanical devices cannot contain plasma. Scientists use magnetic fields in tokamak reactors or focused laser beams to contain plasma, but these techniques remain technically challenging. Currently, no self-sustaining fusion reactors operate worldwide, though small-scale controlled reactions have been briefly achieved.
Q6: How are elements heavier than nickel-56 formed in the universe?
Elements heavier than nickel-56 form through multiple neutron- or proton-capture events during supernovae explosions. As fusion reactions create heavier nuclides, the decreasing difference in binding energies between reactants and products yields less energy. Nickel-56, which has one of the highest binding energies per nucleon, marks the endpoint of stellar fusion processes.
Q7: Why is fusion more energy-efficient per gram than fission?
Fusion of one gram of helium-4 produces approximately 6.5 × 10⁸ kilojoules, exceeding the 8.5 × 10⁷ kilojoules from one gram of uranium-235 fission. Fusion reactants—heavy isotopes of hydrogen like deuterium and tritium—are far more abundant and less expensive than uranium-235. This combination of superior energy density and accessible fuel makes fusion theoretically attractive for future energy production.