what is the source of energy in fusion and fission reactions

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.