Section 1 of 6What Fusion Is
Fusion joins small nuclei into a larger one and releases energy.
Joining nuclei
- Two light nuclei, such as isotopes of hydrogen, merge to form a single heavier nucleus.
Energy release
- The new nucleus has slightly less mass than the starting nuclei, and the difference is released as energy.
In stars
- The Sun and other main sequence stars shine because hydrogen nuclei fuse to make helium nuclei in their cores.
Example reaction
- Deuterium and tritium fuse to give helium-4 and a neutron: ²₁H + ³₁H → ⁴₂He + ¹₀n, with energy released.
Section 2 of 6Conditions Needed
Fusion only happens when nuclei are forced extremely close together.
1
All nuclei carry positive charge, so any two nuclei repel each other through a strong electrostatic force.
2
To fuse, nuclei must approach so closely that the short range attraction between nucleons can take over.
3
This needs very high speeds, so the fuel must be heated to millions of degrees Celsius.
4
At such temperatures atoms lose their electrons and the fuel becomes a hot gas of charged particles called a plasma.
5
A very high pressure or density is also needed, so that collisions between nuclei happen often enough.
Section 3 of 6Fusion Compared with Fission
Fusion Compared with Fission
Fission
- A large unstable nucleus such as uranium-235 splits into two smaller nuclei after absorbing a neutron.
Fusion
- Small nuclei combine into one larger nucleus, and this process releases more energy per kilogram of fuel than fission.
Waste
- Fusion produces no long lived radioactive waste of the kind made in fission, though some reactor parts become radioactive.
Fuel
- Hydrogen isotopes can be obtained from water, so the fuel supply is very large.
| Feature | Fission | Fusion |
|---|---|---|
| Change to nuclei | One large nucleus splits | Two small nuclei join |
| Typical fuel | Uranium or plutonium | Hydrogen isotopes |
| Conditions | Controlled in a reactor at moderate temperature | Millions of degrees and high pressure |
| Used for power now? | Yes, in power stations | No, still experimental on Earth |
Section 4 of 6Fusion on Earth
Containment problem
- No solid container can survive plasma at millions of degrees, so strong magnetic fields hold the plasma away from the walls.
Energy balance
- So far experimental reactors have found it hard to produce more useful energy than they use to heat the plasma.
Cold fusion
- Claims of fusion at room temperature were never reliably repeated by other scientists, so they were not accepted.
Why keep trying
- Fusion could give a large, low carbon electricity supply with plentiful fuel, which is why research continues.
Section 5 of 6Worked Example
Question Show that the equation ²₁H + ³₁H → ⁴₂He + ¹₀n is balanced for nucleon number and proton number.
- Add the nucleon (mass) numbers on the left: 2 + 3 = 5.
- Add the nucleon numbers on the right: 4 + 1 = 5, so they match.
- Add the proton (atomic) numbers on the left: 1 + 1 = 2.
- Add the proton numbers on the right: 2 + 0 = 2, so they match.
Answer Both totals are equal on each side (5 and 2), so the equation is balanced.
Section 6 of 6Exam Checklist & Tips
6.2.5 – Key Points for the Exam
- Nuclear fusion The joining of two light nuclei to form a heavier nucleus, releasing energy.
- Plasma A very hot gas in which atoms have lost electrons, so it contains free nuclei and electrons.
- Electrostatic repulsion The force pushing positively charged nuclei apart, which must be overcome for fusion.
- Isotope Atoms of the same element with different numbers of neutrons, such as ²₁H and ³₁H.
- Nuclear fission The splitting of a large nucleus into smaller nuclei, releasing energy.
Common Mistakes
- Mixing up fission and fusionfission splits nuclei, fusion joins them.
- Saying fusion happens easily because it needs little energy; it needs extreme temperature and pressure.
- Forgetting that nuclei repel each other, which is the reason the conditions are so extreme.
- Writing equations whose nucleon numbers or proton numbers do not add up on both sides.