The source of energy in both nuclear fission and nuclear fusion is the change in nuclear binding energy when nuclei rearrange, which shows up as a tiny loss of mass that is converted to energy according to E=mc2E=mc^2E=mc2.

Key idea in simple words

  • Every atomic nucleus has a certain binding energy : the energy that holds protons and neutrons together.
  • When a nuclear reaction happens (either splitting a big nucleus or merging small ones), the total binding energy of the products is different from that of the reactants.
  • This difference in binding energy appears as released energy, which corresponds to a small “missing” mass: the mass defect.

In nuclear fission

  • A heavy nucleus (like uranium‑235) splits into two or more smaller nuclei plus some free neutrons.
  • The fission fragments (the smaller nuclei) have higher binding energy per nucleon than the original heavy nucleus.
  • Because they are more tightly bound, the system’s total mass decreases very slightly; this lost mass is converted into kinetic energy of the fragments and neutrons, plus gamma radiation.
  • That released energy is what heats water in a reactor or causes an atomic bomb’s blast.

In nuclear fusion

  • Light nuclei (like isotopes of hydrogen, deuterium and tritium) combine to form a heavier nucleus (like helium).
  • For light elements up to around iron, the fusion product has higher binding energy per nucleon than the original light nuclei.
  • Again, the total mass after the reaction is slightly less than before; that mass difference appears as enormous energy in the kinetic energy of the reaction products (fast neutrons, alpha particles, etc.).
  • This is the same mechanism that powers the Sun and stars: hydrogen nuclei fusing into helium and releasing energy from the increased binding energy.

One-sentence summary

The source of energy in both fusion and fission reactions is the mass defect —a small loss of mass due to nuclei moving to a more tightly bound configuration—converted into energy via E=mc2E=mc^2E=mc2.