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Nuclear fusion: process, role in stars, and prospects for power

Nuclear fusion is the joining of light atomic nuclei into heavier ones, releasing energy. This article explains the physics, stellar role, confinement methods, earthly experiments, uses, history, and remaining challenges.

Author: Leandro Alegsa Creation date: November 13, 2022 Last updated: May 26, 2026

simple.wikipedia.org · Public domain

Overview

Nuclear fusion is the process by which two light atomic nuclei combine to form a heavier nucleus, a type of nuclear reaction. The fused nucleus weighs slightly less than the sum of its parts; the missing mass becomes energy according to E=mc², which accounts for the large amount of energy released. Fusion operates on subatomic constituents inside atoms and is distinct from fission, which splits heavy nuclei.

How fusion works

Two positively charged nuclei repel each other because of electrostatic force; overcoming this Coulomb barrier requires extremely high temperatures and often high pressures. At sufficient energy, quantum tunneling also allows nuclei to merge. Typical laboratory fuel choices focus on isotopes of hydrogen—deuterium and tritium—because lighter nuclei fuse more readily than heavy ones. The reaction produces a heavier element (for hydrogen fuels this is often helium) and energetic particles.

Confinement approaches include magnetic confinement (devices such as tokamaks and stellarators) and inertial confinement (intense lasers or particle beams).
In plasma form the fuel is ionized: electrons separated from nuclei so electromagnetic methods can control the charged particles.

Fusion in stars

Stellar cores are natural fusion reactors. In the Sun and similar stars, hydrogen nuclei (hydrogen) combine through chains of reactions to make helium, releasing the heat and light that power the star. More massive stars can fuse progressively heavier elements until they approach iron. Because iron (iron) has the highest binding energy per nucleon, further fusion is no longer energetically favorable, which is a key factor in the end stages of many stars.

Fusion on Earth

Reproducing stellar conditions on Earth is difficult: the fuel nuclei carry a net positive charge, so extremely high temperatures and confinement are needed to achieve the collision rates that permit fusion. Experimental facilities seek to produce more useful energy than they consume and to do so safely and stably. To date, uncontrolled fusion has been achieved in thermonuclear weapons, which use a fission device as a trigger, while controlled experiments employ magnetic or inertial techniques as part of research toward electricity generation and commercial fusion power.

Applications, benefits, and challenges

Interest in fusion stems from potential advantages: abundant fuel (deuterium from seawater, tritium bred from lithium), low greenhouse-gas emissions during operation, and limited long-lived radioactive waste compared with fission. However, challenges remain: sustaining a burning plasma long enough for net energy gain, managing neutron fluxes that activate materials, ensuring economic competitiveness, and solving engineering demands for heat extraction and materials endurance.

History and development

Ideas about fusion emerged from early 20th-century work on atomic structure and energy. Theoretical understanding of mass–energy equivalence and nuclear binding energy underpins the field. During the mid-20th century, both weapons and laboratory programs accelerated research into confinement schemes and diagnostic tools. Modern projects continue to iterate on device design, materials, and fuel cycles, aiming to transition from experimental demonstration to pilot electricity-producing plants.

Further notes and resources

Key distinctions include differences between fusion and fission reactions, the range of confinement technologies, and the varied fuel cycles that influence neutron production and waste. For introductory material and technical overviews see resources on atomic structure, general nuclear reactions, energy considerations (mass–energy), and the mass defect. General information about stellar fusion is available through popular summaries about stars and the Sun. For technical discussions of plasma conditions see materials linked on temperature, pressure, and charged-particle behavior (Coulomb forces). Consider also overviews addressing element formation, the role of iron in stellar evolution, and why some stars terminate in collapse or explosion. Earth-based research and policy material frequently discuss terrestrial experiments, prospects for power generation, and the broader ambition for commercial fusion energy. Additional context about materials (including metals) and the observable outputs of fusion, such as heat and light, can clarify practical engineering constraints and opportunities.

Fusion of deuterium and tritium into a helium nucleus

Binding energy per nucleon as a function of mass number

Research into nuclear fusion

Already the first observed nuclear reaction was an (endothermic) fusion reaction. It was discovered - long before nuclear fission - by Ernest Rutherford in 1917 during experiments with alpha particles. Protons of relatively high energy were found, which only occurred when the irradiated gas contained nitrogen. This nuclear reaction is called in today's notation ^14N(α,p)^17O or, written in detail:

${}^{14}\mathrm {N} +{}^{4}\mathrm {He} \,\rightarrow \,{}^{17}\mathrm {O} +{}^{1}\mathrm {H} -1,2\,\mathrm {MeV}$

This conversion of nitrogen into oxygen was, like the alpha decay itself, in contradiction to the classical theory, according to which the Coulomb barrier can only be overcome with sufficient energy. It was not until 1928 that George Gamow was able to explain such processes on the basis of the new quantum mechanics with the tunnel effect.

As early as 1920, Arthur Eddington had suggested fusion reactions as a possible source of energy for stars, based on the precise measurements of isotopic masses by Francis William Aston (1919). Since it was known from spectroscopic observations that stars consist largely of hydrogen, its fusion into helium came into consideration here. In 1939 Hans Bethe published several mechanisms how this reaction could take place in stars.

The first targeted fusion reaction in the laboratory was the bombardment of deuterium with deuterium nuclei in 1934 by Mark Oliphant, Rutherford's assistant, and Paul Harteck. The fusion of this hydrogen isotope, which is rare in stars, branches into two product channels:

${}^{2}\mathrm {H} +{}^{2}\mathrm {H} \,\rightarrow \,{}^{3}\mathrm {He} +{}^{1}\mathrm {n} +3{,}3\,\mathrm {MeV}$

${}^{2}\mathrm {H} +{}^{2}\mathrm {H} \,\rightarrow \,{}^{3}\mathrm {H} +{}^{1}\mathrm {p} +4{,}0\,\mathrm {MeV}$

The technical use of thermonuclear fusion was first pursued with the aim of military weapons development. Therefore, this research took place in secret in the first decades after the Second World War. The USA had been in possession of the nuclear fission-based atomic bomb since 1945, the Soviet Union since 1949. Subsequently, Edward Teller and Stanislaw Ulam in the USA developed a concept for building a hydrogen bomb based on nuclear fusion, which promised a much higher explosive power. On November 1, 1952, the first hydrogen bomb, named Ivy Mike, was detonated at Eniwetok Atoll in the Pacific Ocean. This was proof that large amounts of energy could also be released on Earth through nuclear fusion.

Energy Balance

If the mass of the nuclei or particles created in the fusion is less than the sum of the masses of the initial nuclei, the mass difference Δ $\Delta m$ is released in the form of energy (as kinetic energy of the reaction products and possibly as electromagnetic radiation), as in every nuclear reaction according to the mass-energy equivalence formula $E=\Delta mc^{2}$ originating from Einstein. Exothermic, i.e. energy-releasing fusion reactions only occur in the fusion of light nuclei, since the binding energy per nucleon increases with increasing mass number only up to the element iron (isotope ^58Fe). However, it is very large in helium-4 producing reactions: The conversion of one gram of deuterium-tritium mixture in a nuclear fusion reactor would yield a thermal energy of about 100 megawatt-hours (MWh) or 12.3 tce.

The experiments to date on controlled thermonuclear fusion do not yet have a positive energy balance. The most successful so far was the British JET (Joint European Torus) facility, which was able to achieve a peak power of 16 MW for less than a second. In the process, 65 percent of the energy put into it could be recovered as fusion energy.

Questions and Answers

Q: What is nuclear fusion?

A: Nuclear fusion is the process of making a single heavy nucleus (part of an atom) from two lighter nuclei. This process is called a nuclear reaction and releases a large amount of energy.

Q: How does this process work?

A: The nucleus made by fusion is heavier than either of the starting nuclei, but not as heavy as the combination of their original mass. This lost mass is changed into lots of energy, which can be seen in Einstein's famous E=mc2 equation.

Q: Where does this process occur?

A: Fusion happens in the middle of stars, such as our Sun, where hydrogen atoms are fused together to make helium and release lots of energy that powers its heat and light.

Q: Are all elements able to be joined through fusion?

A: No, heavier elements are less easily joined than lighter ones and iron (a metal) cannot fuse with other atoms at all. This is what causes stars to die when they join all their atoms together to make heavier atoms until they start making iron which cannot be fused anymore.

Q: Is it easy to start nuclear fusion reactions on Earth?

A: No, it is very difficult because these reactions only happen at high temperature and pressure like in the Sun due to both nuclei having positive charges which repel each other so they must hit each other at very high speeds for them to fuse successfully.

Q: Has anyone been successful in controlling or containing these reactions for electricity generation?

A: Not yet - scientists and engineers have been trying for decades but still have many challenges before fusion power can be used as a clean source of energy.

Q: What has been successful so far with regards to nuclear fusion?

A: The only successful approach so far has been in nuclear weapons where the hydrogen bomb uses an atomic (fission) bomb to start the reaction.

Author

AlegsaOnline.com - Nuclear fusion - Leandro Alegsa

Creation date: November 13, 2022
Last updated: May 26, 2026

URL: https://en.alegsaonline.com/art/71360