Fission & Fusion
Radioactivity & Particles
| 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 |
Common exam question
Weighing fission against fusion as an energy source
Question: Give disadvantages of using nuclear fission to generate electricity, or discuss why a fusion reactor would be safer than a fission one (2–3 marks).
Set in 2 of the 24 papers. For fission's drawbacks, any two of: it produces radioactive waste that needs long-term storage; uranium is itself radioactive; the fuel is non-renewable; there is a risk of a nuclear accident (naming one is fine); and the set-up and decommissioning costs are higher. An unqualified "it is expensive" is ignored.
For fusion's safety case, compare the two reactors: fusion's fuel is less hazardous and the products of fusion are not radioactive, unlike fission's daughter nuclei, so there is no cancer or cell damage from the products, no long-term waste storage, a lower risk of meltdown (or none) and no runaway chain reaction. Any three distinct points earn the three marks.
- 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