Section 1 of 9What fusion is
Nuclear fusion is the joining together of two light nuclei to form a single heavier nucleus.
Fusion is the process that makes the Sun and other stars shine. In the Sun’s core, hydrogen nuclei (protons) are fused in several steps to make helium, releasing energy that eventually leaves the Sun as light and heat.
Section 2 of 9Fusion reactions
The reaction considered most promising for power stations on Earth is between two heavy isotopes of hydrogen, deuterium (21H) and tritium (31H):
21H + 31H → 42He + 10n + energy
Check
- top 2 + 3 = 5 and 4 + 1 = 5;
- bottom 1 + 1 = 2 and 2 + 0 = 2.
- The neutron carries away most of the energy (about 80%), which is why fusion reactors would have to absorb the neutrons in a surrounding blanket where the energy can be collected as heat.
Within stars like the Sun, protons fuse via intermediate steps involving deuterium and helium-3 to make helium-4; the overall result is four hydrogen nuclei becoming one helium nucleus, two positrons and neutrinos, with energy released.
Section 3 of 9Fusion and fission compared
| Fission | Fusion | |
|---|---|---|
| Process | Large nucleus splits | Small nuclei join |
| Typical fuel | Uranium-235, plutonium-239 | Hydrogen isotopes (deuterium, tritium) |
| Energy per kg of fuel | Very large | Several times larger still |
| Conditions | Slow neutrons, critical mass | Extremely high temperature and pressure |
| Waste | Long-lived radioactive fragments | Mainly helium; some structural materials made radioactive but with shorter half-lives |
| Present use | Nuclear power stations | Stars; experimental reactors only |
Section 4 of 9Why fusion is attractive
- If the conditions fail the reaction simply stops.
The great challenge is that fusion needs extreme conditions, as discussed in the next lesson, and no experimental reactor has yet produced steady net electricity for the grid.
Section 5 of 9Where fusion happens in nature
Stars spend most of their lives fusing hydrogen into helium in their cores.
Section 6 of 9Energy from fusion in numbers
The deuterium-tritium reaction releases about 2.8 × 10−12 J per reaction.
- so the energy per kilogram is several times that of fission, roughly 3 × 1014 J.
- which shows why scientists continue to work on fusion despite its difficulties.
Remember also that fusion is the energy source linking all stars.
- which is why it has been shining for about 4.6 billion years and has fuel for billions more.
- and note that fusion in the Sun, like the fusion in a future reactor, depends on the nuclei overcoming their mutual repulsion.
Section 7 of 9Worked examples
Balance the fusion equation: 21H + 21H → 32He + xyn.
Top numbers: 2 + 2 = 4 on the left. On the right 3 + x = 4, so x = 1.
Bottom numbers: 1 + 1 = 2 on the left. On the right 2 + y = 2, so y = 0.
The particle is a neutron, 10n.
In a deuterium-tritium fusion reaction the total mass decreases by 3.1 × 10−29 kg. Calculate the energy released. (c = 3.0 × 108 m s−1)
E = mc2 = 3.1 × 10−29 × 9.0 × 1016
What the symbols mean: E = energy (J) · m or Δm = mass (change) (kg) · c = 3.00 × 108 m s−1= 2.8 × 10−12 J (about 17.6 MeV).
Give two advantages and one disadvantage of fusion as a power source compared with fission.
Advantages: fuel (deuterium) is plentiful in sea water; the main product is helium, so less long-lived radioactive waste; there is no runaway chain reaction.
Disadvantage: extremely high temperature and pressure are needed, and it has not yet been shown to produce sustained net electrical power.
Section 8 of 9Key terms
| Term | Meaning |
|---|---|
| Nuclear fusion | Joining of two light nuclei to make a heavier nucleus, releasing energy. |
| Deuterium | Hydrogen isotope with one proton and one neutron. |
| Tritium | Hydrogen isotope with one proton and two neutrons. |
| Main sequence | Long stable stage of a star’s life in which hydrogen is fused to helium. |
| Isotope | Atoms with the same proton number but different numbers of neutrons. |