Fission and fusion · Year 13
Huge amounts of energy are locked in atomic nuclei. In fission, a heavy nucleus like uranium-235 absorbs a neutron and splits, releasing energy and more neutrons that can start a chain reaction – the basis of nuclear power. In fusion, light nuclei join together, which is what powers the Sun. Both convert a little mass into energy, as E = mc² describes.
Key facts
- Fission: a heavy nucleus splits into two smaller nuclei plus 2–3 neutrons, releasing energy.
- A chain reaction happens when released neutrons cause further fissions; control rods absorb neutrons, the moderator slows them.
- Fusion: light nuclei (e.g. hydrogen) join to make heavier ones; it needs extreme temperature and pressure.
- E = mc² (c = 3.0 × 10⁸ m/s); 1 u of mass ≡ 931.5 MeV; 1 MeV = 1.6 × 10⁻¹³ J.
- Mass numbers and atomic numbers must balance on both sides of a nuclear equation.
- Binding energy per nucleon peaks around iron-56, which is why both fission of heavy and fusion of light nuclei release energy.