Nuclear Fission and Fusion
Section: Nuclear Physics | Syllabus: Cambridge IGCSE Physics (0625)
Nuclear Fission and Fusion
Extended Only
Nuclear fission and nuclear fusion are processes involving the splitting or joining of nuclei, which release energy. In both processes, a small amount of mass is converted into a large amount of energy - numerical values of the mass and energy changes are not required.
Nuclear Fission
Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei.
- A heavy nucleus (e.g. uranium-235) absorbs a neutron, becomes unstable, and splits into two smaller nuclei
- Additional neutrons are released at the same time
- The total mass of the products is slightly less than the mass of the original nucleus and neutron - this "missing" mass is converted into the energy that is released
Nuclide equation for the fission of uranium-235:
Uranium-235 absorbs a neutron and splits into barium-141, krypton-92 and 3 neutrons, releasing energy.
Nuclear fission: a heavy nucleus splits into two lighter nuclei, releasing neutrons and energy
Nuclear Fusion
Nuclear fusion is the joining of two light nuclei to form a heavier nucleus.
- Two light nuclei (e.g. hydrogen isotopes) join together to form a heavier nucleus
- The mass of the product nucleus is slightly less than the total mass of the original nuclei - this "missing" mass is converted into the energy that is released
- This is the process that powers stars, including the Sun
Nuclide equation for the fusion of two hydrogen isotopes:
Deuterium and tritium fuse to form helium-4 and a neutron, releasing energy.
Nuclear fusion: two light nuclei join to form a heavier nucleus, releasing energy
Comparison: Fission vs Fusion
| Property | Fission | Fusion |
|---|---|---|
| Process | Splitting of a heavy nucleus | Joining of light nuclei |
| Nuclei involved | Heavy nuclei, e.g. uranium | Light nuclei, e.g. hydrogen isotopes |
| Mass change | Total mass decreases | Total mass decreases |
| Energy change | Energy is released | Energy is released |
| Where it occurs | Nuclear power stations | Stars, e.g. the Sun |
- Memory aid: Fission = Splitting (a heavy nucleus splits into smaller pieces); Fusion = Joining (light nuclei fuse together). In both processes, mass decreases and energy is released
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