What is nuclear fusion?

Nuclear fusion is a process where nuclear reactions between light elements form heavier elements, releasing substantial energy, especially when the interacting nuclei are elements with low atomic numbers, like hydrogen or its isotopes. Fusion reactions are the fundamental energy source of stars, including the Sun, where hydrogen "burning" leads to the formation of helium. For practical energy generation, reactions between deuterium and tritium are most important because they have high cross sections, require moderate plasma temperatures, and yield high energy. Harnessing fusion energy involves magnetic and inertial confinement to contain high-temperature plasma.

How does nuclear fusion differ from nuclear fission?

Nuclear fission involves splitting a heavy atomic nucleus, like uranium or plutonium, into two smaller nuclei, releasing a large amount of energy. This process may occur spontaneously or be induced by particles or electromagnetic radiation. Nuclear fusion is the joining of the nuclei of atoms. Fusion involves nuclear reactions between light elements to form heavier elements, releasing substantial amounts of energy when the interacting nuclei belong to elements with low atomic numbers, such as hydrogen. Fission is dependent on neutrons to induce reactions in uranium or plutonium. Fusion occurs only when particles are sufficiently energetic to overcome the Coulomb repulsive force, requiring the heating of gaseous reactants to extreme temperatures to create plasma.

Where does nuclear fusion naturally occur?

Nuclear fusion naturally occurs in stars, including the Sun, where it serves as the primary energy source. This process involves the fusion of hydrogen nuclei to form helium, releasing vast amounts of energy. In many stars, the fusion of helium into heavier elements, up to iron, follows the production of helium. The evolution of stars can be viewed as a series of stages marked by thermonuclear reactions and changes in composition caused by nucleosynthesis over extended periods.

What conditions are needed for nuclear fusion to happen?

Nuclear fusion is when the nuclei of two atoms join to form a single heavier atom. Fusion happens when the kinetic energy of nuclei overcomes the long-range electrostatic repulsive force between them. For fusion to occur, the nuclei must have sufficient energy to overcome the Coulomb barrier, which is the electrostatic repulsion between positively charged nuclei. This requires extremely high temperatures, in the range of tens of millions of degrees. At these temperatures, the material is in a plasma state , where the atoms are ionized. Also, a high density of the fusion materials is needed.

Why is nuclear fusion considered a potential source of clean energy?

Nuclear fusion is considered a potential source of clean energy because it offers several key advantages. Fusion reactions, particularly those involving deuterium and tritium , release vast amounts of energy by combining light nuclei to form heavier ones. Unlike fossil fuels , fusion does not produce pollutants or greenhouse gases that contribute to global warming . Also, a fusion reactor cannot undergo a runaway chain reaction or meltdown because the fusion reaction requires a confined hot plasma, and any interruption of a plasma control system would extinguish the plasma and terminate fusion. The main product of fusion, helium, is not radioactive. With fuels like deuterium from seawater and lithium, a fusion reactor would have an effectively inexhaustible source of energy.

What are some challenges scientists face in building a fusion power plant?

Building a fusion power plant presents several challenges for scientists. Achieving sufficient plasma temperature and heat insulation are crucial, as hot gases tend to escape the enclosing magnetic structure. Maintaining plasma confinement for a sufficient time, balancing energy losses, and ensuring the system can produce electricity safely and cost-effectively with minimal radioactive waste are also major hurdles. Additionally, materials development is essential to withstand intense neutron bombardment on the reactor walls.

nuclear fusion

Introduction
The fusion reaction
- Two types of fusion reactions
- Energy released in fusion reactions
- Rate and yield of fusion reactions
- The plasma state
- Fusion reactions in stars
- Fusion reactions for controlled power generation
Methods of achieving fusion energy
- Magnetic confinement
- Inertial confinement fusion (ICF)
- Muon-catalyzed fusion
- Cold fusion and bubble fusion
- Conditions for practical fusion yield
History of fusion energy research

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What is the difference between nuclear fission and fusion?

Diagram showing the sequence of uranium nucleus fission after being struck by a neutron: the nucleus absorbs the neutron, deforms, splits into fragments, releases more neutrons and gamma rays, and emits beta particles as the fragments decay. Labels identify neutrons, protons, beta particles, and gamma rays, with accompanying explanatory text.

Fission of a uranium nucleus by a neutron

reaction rate as a function of plasma temperature

Explore the physics behind nuclear fusion and the Sun

At a Glance

nuclear fusion summary

nuclear fusion

physics

Also known as: atomic fusion, fusion, thermonuclear fusion

Written by

Robert W. Conn

Fact-checked by

Britannica Editors

Last updated

Oct. 2, 2026 •History

Top Questions

What is nuclear fusion?
Nuclear fusion is a process where nuclear reactions between light elements form heavier elements, releasing substantial energy, especially when the interacting nuclei are elements with low atomic numbers, like hydrogen or its isotopes. Fusion reactions are the fundamental energy source of stars, including the Sun, where hydrogen "burning" leads to the formation of helium.

For practical energy generation, reactions between deuterium and tritium are most important because they have high cross sections, require moderate plasma temperatures, and yield high energy. Harnessing fusion energy involves magnetic and inertial confinement to contain high-temperature plasma.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
How does nuclear fusion differ from nuclear fission?
Nuclear fission involves splitting a heavy atomic nucleus, like uranium or plutonium, into two smaller nuclei, releasing a large amount of energy. This process may occur spontaneously or be induced by particles or electromagnetic radiation.

Nuclear fusion is the joining of the nuclei of atoms. Fusion involves nuclear reactions between light elements to form heavier elements, releasing substantial amounts of energy when the interacting nuclei belong to elements with low atomic numbers, such as hydrogen.

Fission is dependent on neutrons to induce reactions in uranium or plutonium. Fusion occurs only when particles are sufficiently energetic to overcome the Coulomb repulsive force, requiring the heating of gaseous reactants to extreme temperatures to create plasma.
5 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Where does nuclear fusion naturally occur?
Nuclear fusion naturally occurs in stars, including the Sun, where it serves as the primary energy source. This process involves the fusion of hydrogen nuclei to form helium, releasing vast amounts of energy. In many stars, the fusion of helium into heavier elements, up to iron, follows the production of helium. The evolution of stars can be viewed as a series of stages marked by thermonuclear reactions and changes in composition caused by nucleosynthesis over extended periods.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What conditions are needed for nuclear fusion to happen?
Nuclear fusion is when the nuclei of two atoms join to form a single heavier atom. Fusion happens when the kinetic energy of nuclei overcomes the long-range electrostatic repulsive force between them.

For fusion to occur, the nuclei must have sufficient energy to overcome the Coulomb barrier, which is the electrostatic repulsion between positively charged nuclei. This requires extremely high temperatures, in the range of tens of millions of degrees. At these temperatures, the material is in a plasma state, where the atoms are ionized. Also, a high density of the fusion materials is needed.
3 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
Why is nuclear fusion considered a potential source of clean energy?
Nuclear fusion is considered a potential source of clean energy because it offers several key advantages. Fusion reactions, particularly those involving deuterium and tritium, release vast amounts of energy by combining light nuclei to form heavier ones.

Unlike fossil fuels, fusion does not produce pollutants or greenhouse gases that contribute to global warming. Also, a fusion reactor cannot undergo a runaway chain reaction or meltdown because the fusion reaction requires a confined hot plasma, and any interruption of a plasma control system would extinguish the plasma and terminate fusion. The main product of fusion, helium, is not radioactive. With fuels like deuterium from seawater and lithium, a fusion reactor would have an effectively inexhaustible source of energy.
4 Britannica Sources
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.
What are some challenges scientists face in building a fusion power plant?
Building a fusion power plant presents several challenges for scientists. Achieving sufficient plasma temperature and heat insulation are crucial, as hot gases tend to escape the enclosing magnetic structure. Maintaining plasma confinement for a sufficient time, balancing energy losses, and ensuring the system can produce electricity safely and cost-effectively with minimal radioactive waste are also major hurdles. Additionally, materials development is essential to withstand intense neutron bombardment on the reactor walls.
2 Britannica Sources
1. nuclear fusion: History of fusion energy research
2. fusion reactor: General characteristics
Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

nuclear fusion, process by which nuclear reactions between light elements form heavier elements (up to iron). In cases where the interacting nuclei belong to elements with low atomic numbers (e.g., hydrogen [atomic number 1] or its isotopes deuterium and tritium), substantial amounts of energy are released. The vast energy potential of nuclear fusion was first exploited in thermonuclear weapons, or hydrogen bombs, which were developed in the decade immediately following World War II. For a detailed history of this development, see nuclear weapon. Meanwhile, the potential peaceful applications of nuclear fusion, especially in view of the essentially limitless supply of fusion fuel on Earth, have encouraged an immense effort to harness this process for the production of power. For more detailed information on this effort, see fusion reactor.

This article focuses on the physics of the fusion reaction and on the principles of achieving sustained energy-producing fusion reactions.

The fusion reaction

Fusion reactions constitute the fundamental energy source of stars, including the Sun. The evolution of stars can be viewed as a passage through various stages as thermonuclear reactions and nucleosynthesis cause compositional changes over long time spans. Hydrogen (H) “burning” initiates the fusion energy source of stars and leads to the formation of helium (He). Generation of fusion energy for practical use also relies on fusion reactions between the lightest elements that burn to form helium. In fact, the heavy isotopes of hydrogen—deuterium (D) and tritium (T)—react more efficiently with each other, and, when they do undergo fusion, they yield more energy per reaction than do two hydrogen nuclei. (The hydrogen nucleus consists of a single proton. The deuterium nucleus has one proton and one neutron, while tritium has one proton and two neutrons.)

Fusion reactions between light elements, like fission reactions that split heavy elements, release energy because of a key feature of nuclear matter called the binding energy, which can be released through fusion or fission. The binding energy of the nucleus is a measure of the efficiency with which its constituent nucleons are bound together. Take, for example, an element with Z protons and N neutrons in its nucleus. The element’s atomic weight A is Z + N, and its atomic number is Z. The binding energy B is the energy associated with the mass difference between the Z protons and N neutrons considered separately and the nucleons bound together (Z + N) in a nucleus of mass M. The formula is B = (Zm_p + Nm_n − M)c², where m_p and m_n are the proton and neutron masses and c is the speed of light. It has been determined experimentally that the binding energy per nucleon is a maximum of about 1.4 10⁻¹² joule at an atomic mass number of approximately 60—that is, approximately the atomic mass number of iron. Accordingly, the fusion of elements lighter than iron or the splitting of heavier ones generally leads to a net release of energy.

Two types of fusion reactions

Fusion reactions are of two basic types: (1) those that preserve the number of protons and neutrons and (2) those that involve a conversion between protons and neutrons. Reactions of the first type are most important for practical fusion energy production, whereas those of the second type are crucial to the initiation of star burning. An arbitrary element is indicated by the notation ^A_ZX, where Z is the charge of the nucleus and A is the atomic weight. An important fusion reaction for practical energy generation is that between deuterium and tritium (the D-T fusion reaction). It produces helium (He) and a neutron (n) and is written D + T → He + n.

To the left of the arrow (before the reaction) there are two protons and three neutrons. The same is true on the right.

The other reaction, that which initiates star burning, involves the fusion of two hydrogen nuclei to form deuterium (the H-H fusion reaction): H + H → D + β⁺ + ν, where β⁺ represents a positron and ν stands for a neutrino. Before the reaction there are two hydrogen nuclei (that is, two protons). Afterward there are one proton and one neutron (bound together as the nucleus of deuterium) plus a positron and a neutrino (produced as a consequence of the conversion of one proton to a neutron).

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Both of these fusion reactions are exoergic and so yield energy. The German-born physicist Hans Bethe proposed in the 1930s that the H-H fusion reaction could occur with a net release of energy and provide, along with subsequent reactions, the fundamental energy source sustaining the stars. However, practical energy generation requires the D-T reaction for two reasons: first, the rate of reactions between deuterium and tritium is much higher than that between protons; second, the net energy release from the D-T reaction is 40 times greater than that from the H-H reaction.

nuclear fusion

The fusion reaction

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Two types of fusion reactions