4PH1
Fission & Fusion
Radioactivity & Particles · 3 question types
Exam Frequency Analysis
Past paper frequency (2018 to 2024)
This topic accounts for approximately 6% of your exam marks.
stable
Low
Stable6%
Chain reactions, conditions for fusion and energy release compared between fission and fusion.
| Nuclear fission | Nuclear fusion | |
|---|---|---|
| What happens | One large nucleus splits into two smaller ones | Two small nuclei join to form a larger one |
| Typical fuel | Uranium-235, plutonium-239 | Deuterium and tritium (isotopes of hydrogen) |
| Conditions needed | Slow (thermal) neutrons; critical mass of fuel | Extremely high temperature and pressure |
| Where it happens | Nuclear power stations and weapons on Earth | The cores of stars; experimental reactors on Earth |
| Products | Two smaller daughter nuclei (usually radioactive) + 2 or 3 neutrons + gamma rays | A larger nucleus (usually stable, not radioactive) + sometimes a neutron + gamma rays |
| Radioactive waste? | Yes, long-lived, dangerous, must be stored for thousands of years | No long-lived waste; the helium product is inert |
| Energy per kg of fuel | Very high (≈ 10⁷ × chemical reactions) | Even higher (≈ 4 × fission per kg) |
| Fuel availability | Uranium ore is rare and concentrated in few countries | Deuterium can be extracted from sea water; abundant everywhere |
| Practical on Earth? | Yes, has powered electricity for 70 years | Not yet, still experimental |
- Fusion is, in principle, a far better energy source than fission: more energy per kg, more abundant fuel, no long-lived radioactive waste, no risk of meltdown. The only problem (but it is an enormous one) is that no one has yet built a fusion reactor that produces more electrical energy than it consumes