For both SL and HL: fusion and fission are nuclear processes that release energy because their products have a higher binding energy per nucleon than their reactants. Fusion combines light nuclei, whereas fission splits a heavy nucleus.
The Reasoning
In both processes, the total rest mass of the products is slightly less than that of the reactants. This mass defect is converted into released energy according to . Total mass-energy is conserved, although rest mass decreases.
The binding-energy-per-nucleon curve explains this energy release. Light nuclei release energy by combining and moving toward the curve's maximum; very heavy nuclei release energy by splitting into medium-mass nuclei that are more tightly bound.
| Feature | Nuclear fusion | Nuclear fission |
|---|---|---|
| Nuclear change | Light nuclei combine into a heavier nucleus | A heavy nucleus splits into smaller nuclei |
| Typical setting | Stars and experimental fusion reactors | Nuclear power reactors and some spontaneous decays |
| Conditions | Very high temperature and density help nuclei overcome electrostatic repulsion | Often initiated when a fissile nucleus absorbs a neutron |
| Reaction development | Difficult to sustain because extreme conditions must be maintained | Released neutrons can produce a chain reaction |
| Products | A heavier nucleus, often with particles such as neutrons | Smaller nuclei, neutrons and radiation |
| Waste | Usually less long-lived radioactive waste, although reactor materials may become radioactive | Radioactive fission fragments requiring careful management |
In stars, high temperature provides nuclei with sufficient kinetic energy for fusion, while high density increases collision frequency. Fusion supplies energy that helps support a star against gravitational collapse.
A common misconception is that energy is released merely because nuclei are joined or split. The correct explanation is that the products are more tightly bound, so decreased rest mass appears as released energy.
Exam Technique
For an Explain question, connect the mass defect to and increased binding energy per nucleon. When comparing, address the nuclear change, conditions, reaction behavior and products, not only that fusion joins while fission splits.