Two nuclear reactions that release energy: fission splits a heavy nucleus into lighter ones, while fusion joins light nuclei into a heavier one; both convert a small loss of mass into energy.
- Fission: a heavy nucleus such as uranium-235 or plutonium-239 splits when struck by a neutron, releasing energy and more neutrons that sustain a chain reaction.
- Fission powers nuclear reactors (controlled chain reaction) and atom bombs (uncontrolled); reactors use control rods and moderators to manage the reaction.
- Fusion: light nuclei such as hydrogen isotopes (deuterium and tritium) fuse to form helium, releasing far more energy; this powers the Sun and stars.
- Fusion needs extremely high temperature and pressure and is not yet commercially viable; the international ITER project is researching controlled fusion.
- India's three-stage nuclear power programme, built around its thorium reserves, was conceived by Homi Bhabha; the Department of Atomic Energy oversees the civilian programme.
Fission versus fusion, the fuels involved, reactors versus bombs, and India's nuclear programme are recurring physics and science-technology facts with a strategic dimension.
Fission splits heavy nuclei (used in current reactors and bombs); fusion joins light nuclei (powers the Sun, not yet commercial). Fusion releases more energy per unit mass and produces less long-lived waste.
Fission splits heavy atoms (reactors, bombs); fusion joins light atoms (the Sun); both release energy from lost mass.