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How is nuclear fusion different from nuclear fission?

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laser-activated fusion
nuclear fusion Interior of the U.S. Department of Energy's National Ignition Facility (NIF), located at Lawrence Livermore National Laboratory, Livermore, California. The NIF target chamber uses a high-energy laser to heat fusion fuel to temperatures sufficient for thermonuclear ignition. The facility is used for basic science, fusion energy research, and nuclear weapons testing.

The fundamental difference between nuclear fusion and nuclear fission lies in how they manipulate atomic nuclei to release energy.1

  • Nuclear Fission: This process involves the splitting of a heavy nucleus (such as uranium or plutonium) into two lighter nuclei of roughly equal mass. Because neutrons have no electric charge, they can easily enter a nucleus to induce this reaction, even at room temperature. This process can trigger a chain reaction where emitted neutrons induce further fission in nearby nuclei.12

  • Nuclear Fusion: This process involves the combining of light elements (such as hydrogen isotopes like deuterium and tritium) to form heavier elements. Unlike fission, fusion requires overcoming the Coulomb barrier—the electrostatic repulsion between positively charged nuclei. To achieve this, reactants must be heated to extreme temperatures (often millions of degrees) to reach a plasma state, providing the particles with enough energy to fuse.132