How does nuclear fusion differ from nuclear fission?

In nuclear engineering, nuclear fission and nuclear fusion are distinct processes for releasing energy from atomic nuclei, representing opposite physical mechanisms.12

Nuclear Fission

Nuclear fission involves the splitting of a heavy atomic nucleus (such as uranium or plutonium) into two lighter, roughly equal fragments. This process releases a large amount of energy and emits [neutrons]. Because these neutrons can induce further fission in nearby nuclei, fission can sustain a chain reaction, which is the principle utilized in nuclear reactors for power generation.13

Nuclear Fusion

Nuclear fusion is the process of combining two light nuclei—typically isotopes of hydrogen—to form a heavier nucleus. This reaction releases significant energy because the resulting nucleus has less mass than the original components. Unlike fission, fusion requires overcoming the Coulomb force, an electrostatic repulsion between positively charged nuclei. This necessitates heating the fuel to extreme temperatures to create a plasma. While fusion is the energy source of stars, practical fusion reactors remain a subject of ongoing research and development.1245