Nuclear fusion is a reaction in which two atomic nuclei combine to form a heavier nucleus, sometimes accompanied by the emission of other particles. Fusion of suitable light nuclei releases energy because the products have less total rest mass than the reactants. It powers the Sun and other stars and is investigated as a potential energy source on Earth. Unlike nuclear fission, which splits nuclei, fusion builds larger nuclear structures from smaller ones. (energy.gov)
Physical principles
The energy released depends on nuclear binding energy, the energy required to separate a nucleus into its constituent particles. When fusion products are more tightly bound than the reactants, the reduction in total rest mass appears as kinetic energy and radiation. This follows mass–energy equivalence, expressed as . Binding energy per nucleon generally increases toward the iron–nickel region, explaining why many light-nucleus fusion reactions release energy, whereas building substantially heavier nuclei generally requires energy. (energy.gov)
Positively charged nuclei repel one another electrically. To fuse, they must approach closely enough for the short-range nuclear attraction associated with the strong interaction to become effective. Quantum tunneling allows fusion below the energy needed to cross the repulsive barrier classically. Consequently, fusion rates depend on both collision energies and quantum-mechanical penetration probabilities, rather than on a single sharp temperature threshold. (arxiv.org)
Fusion in stars
In the Sun, hydrogen is converted into helium mainly through the proton–proton chain. In hotter, more massive hydrogen-burning stars, the carbon–nitrogen–oxygen cycle becomes more important. These are sequences of reactions, not a single collision in which four hydrogen nuclei merge simultaneously. Both ultimately convert hydrogen into helium and release energy; their reaction rates are central to models of stellar structure and evolution. (arxiv.org)
Fusion also contributes to nucleosynthesis, the formation of atomic nuclei. In later stellar stages, helium and heavier nuclei participate in reactions that produce carbon, oxygen, and other elements. Massive stars can undergo successive burning stages producing nuclei near the iron group. Formation of many heavier elements instead involves neutron-capture processes in environments such as stellar explosions and neutron-star mergers. (energy.gov)
Fuels and reaction products
Many fusion-energy experiments emphasize deuterium and tritium, two hydrogen isotopes. Deuterium has one proton and one neutron; tritium has one proton and two neutrons. Their reaction produces a helium-4 nucleus and a neutron:
Approximately 3.5 MeV goes to the helium nucleus and 14.1 MeV to the neutron. This reaction is attractive experimentally because useful reaction rates occur at lower temperatures than for many alternative fusion fuels. (iter.org)
In a magnetically confined reactor, charged helium nuclei can help heat the fuel, while uncharged neutrons escape the magnetic confinement and deposit energy in surrounding materials. Their energy can be recovered as heat, but neutron exposure also damages reactor components. Deuterium–tritium fusion therefore couples plasma physics to demanding materials and heat-extraction problems. (iter.org)
Conditions and confinement
Thermonuclear fusion experiments heat fuel into a plasma, containing nuclei and free electrons. Useful terrestrial fusion rates typically require temperatures exceeding 100 million degrees Celsius. High temperature alone is insufficient: enough fuel must remain together long enough for fusion heating to compete with energy losses. The Lawson criterion describes this balance through fuel density, temperature, and energy-confinement time, often combined into a “triple product.” (energy.gov)
Magnetic confinement fusion uses magnetic fields to restrict charged-particle motion and reduce contact between hot plasma and vessel walls. A tokamak uses a toroidal configuration with an important plasma-current contribution. A stellarator obtains its confining configuration primarily from externally generated, three-dimensional magnetic fields, offering a route toward steady-state operation without requiring a large confining plasma current. (nucleus.iaea.org)
Inertial confinement fusion instead rapidly compresses a small fuel capsule. Intense laser pulses can heat and ablate its outer layers, driving an inward implosion. The compressed fuel burns during a brief interval before it expands. Lasers may illuminate the capsule directly or generate X-rays inside a surrounding enclosure, which then drive compression. (nucleus.iaea.org)
Ignition and energy gain
Ignition means fusion-product heating is sufficient to sustain the burning fuel without continued external heating during the relevant confinement interval. In magnetic-fusion research, the gain factor usually compares fusion power with externally supplied plasma-heating power. It does not include every electrical demand of the facility or measure net electricity delivered to a grid. (iter.org)
On December 5, 2022, the National Ignition Facility at Lawrence Livermore National Laboratory achieved laboratory fusion ignition, producing 3.15 megajoules of fusion energy from 2.05 megajoules of laser energy delivered to the target. This demonstrated target-level energy gain, not net electrical generation: the comparison excluded the energy required to operate the laser system and the wider facility. (llnl.gov)
Power-plant engineering
A practical plant must integrate confinement, fuel supply, heat recovery, electricity generation, and maintainable components. Tritium is radioactive, scarce, and has a half-life of approximately 12.3 years. Proposed deuterium–tritium plants would produce replacement fuel in lithium-containing breeding blankets, using fusion neutrons to generate tritium while recovering heat. Efficient breeding, extraction, recycling, and containment remain essential development tasks. (iter.org)
Fusion does not rely on a self-sustaining neutron-driven fission chain reaction, but its engineering hazards are not absent. Tritium containment, neutron-induced radioactivity, intense heat loads, material degradation, and component replacement require dedicated systems. Commercial operation additionally demands reliable energy conversion and plant availability, beyond demonstrating fusion reactions or experimental energy gain. (nucleus.iaea.org)