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Nuclear fission splits heavy atomic nuclei; nuclear fusion joins light nuclei. Fission powers nuclear plants operating today. Fusion can release energy in experiments, but controlled fusion is still being developed as a way to generate commercial electricity—not a current replacement for the fission fleet.
How do fission and fusion work?
Fission splits a heavy nucleus
In fission, a neutron can strike a large nucleus—commonly uranium or plutonium—and cause it to split into smaller nuclei called fission products. The reaction releases energy and additional neutrons. Those neutrons can trigger further fissions, creating a chain reaction that must be controlled.
Fusion joins light nuclei
Fusion combines two light nuclei, commonly hydrogen isotopes, into a heavier nucleus and releases energy. On Earth, nuclei must be brought into conditions that allow them to fuse. Research systems use magnetic confinement, including tokamaks and stellarators, or inertial confinement, which uses lasers or particle beams.
Fission vs. fusion at a glance
| Comparison | Fission | Fusion |
|---|---|---|
| What happens to the nuclei? | A heavy nucleus splits into smaller nuclei. | Two light nuclei combine into a heavier nucleus. |
| Typical fuel | Uranium or plutonium. | Hydrogen isotopes. |
| Reaction conditions and confinement | A neutron interaction can initiate fission; reactor systems control the chain reaction. | Requires exceptionally high-temperature conditions and confinement; research approaches include magnetic and laser-driven inertial confinement. |
| Chain reaction? | Yes. Released neutrons can drive further fissions, so the reaction needs control. | No self-sustaining chain reaction. If operating conditions are not maintained, the fusion reaction stops. |
| How is electricity generated? | Reaction heat makes steam that turns a turbine-generator. | A future plant would need to capture fusion energy, manage heat and particles, and convert the energy into electricity; an integrated commercial system is still under development. |
| Deployment status | Used in nuclear power plants generating electricity today. | Controlled fusion electricity is not an established commercial power source. |
| Radioactive waste and safety | Produces radioactive waste, including long-lived waste, and requires safety controls. | Not expected to produce the same kind of long-lived waste as fission, but machines and materials can be radioactive and require safety oversight. |
| Unresolved engineering needs | Not stated in the cited comparison sources. | Materials that can withstand harsh conditions, tritium supply and fuel-cycle systems, heat and particle exhaust, maintenance, and electricity-conversion systems. |
Sources: U.S. Nuclear Regulatory Commission (NRC), “Fusion”; U.S. Department of Energy (DOE), “Fission and Fusion: What is the Difference?”; and DOE, “DOE Explains…Fusion Nuclear Science and Technology.”
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Why fission is used for power now, but fusion is not
Conventional fission plants already use the heat from controlled reactions to make steam and drive turbines. Fusion research has demonstrated ways to create fusion conditions, but producing a reaction is not the same milestone as operating a complete, reliable power plant that delivers electricity to the grid. A fusion plant would also need integrated systems for capturing energy, handling its fuel cycle, withstanding the reactor environment, and converting heat into useful electrical output.
The DOE identifies materials, tritium fuel supply, heat and particle exhaust, and electricity-conversion systems as continuing fusion science and engineering needs. The sources cited here do not establish an authoritative commercial deployment date, so a promised arrival year would be misleading.
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Do fission and fusion have different safety and waste profiles?
They do, but the distinction is not “dangerous” versus “risk-free.” Fission’s chain reaction requires control, and fission produces radioactive waste, including waste that remains radioactive for a long time. Fusion does not rely on a self-sustaining chain reaction, and it is not expected to produce the same kind of long-lived waste as fission. That is a relative comparison, not a claim that fusion has no radioactive material or safety concerns.
Fusion facilities can involve radioactive materials and components, while harsh operating conditions and tritium fuel-cycle systems pose engineering and safety challenges. In the United States, the NRC says the ADVANCE Act, enacted July 9, 2024, amended the Atomic Energy Act’s definition of byproduct material to include radioactive material produced by fusion machines. In some Agreement States, the state may oversee relevant material.
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For more detail, see the NRC’s fusion overview and the DOE’s account of fusion nuclear science and technology.
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What the difference means in practice
- Fission is the established nuclear-electricity technology: it splits heavy nuclei, sustains a controlled chain reaction, and supplies heat for steam turbines in operating plants.
- Fusion is a developing technology: it joins light nuclei under demanding conditions, without a self-sustaining chain reaction, but still needs an integrated system capable of producing commercial electricity.
- Neither label alone settles safety or environmental impact: waste, radioactive materials, fuel systems, facility design, and electricity production all matter.
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