Section 1 of 5What Is Fusion?
Nuclear fusion is the joining of two light nuclei to form a heavier nucleus, releasing energy.
Reactants
- Light nuclei such as hydrogen isotopes, deuterium and tritium, are the fuels for fusion.
Product
- Two light nuclei combine to form a larger nucleus, such as helium, and a neutron may be released.
Energy released
- The new nucleus has slightly less mass than the starting nuclei, and the difference is released as energy.
Source of starlight
- Fusion in the cores of stars, such as the Sun, produces the energy they radiate.
Section 2 of 5Conditions for Fusion
Conditions for Fusion
Electrostatic repulsion
- Nuclei are positively charged, so they repel each other strongly when they are close together.
High temperature
- Millions of degrees are needed so that nuclei move fast enough to overcome the repulsion and collide.
High pressure
- Extreme pressure keeps the nuclei close together so collisions happen often enough to sustain the reaction.
Plasma
- At these temperatures atoms lose their electrons, forming a hot gas of charged particles called plasma.
Section 3 of 5Fusion Power on Earth
1
Scientists heat a small amount of deuterium and tritium to over a hundred million degrees in a doughnut-shaped reactor.
2
Strong magnetic fields hold the hot plasma away from the walls, because no material could contain it directly.
3
The nuclei fuse, and the neutrons released carry energy that could be used to produce steam.
4
So far, experimental reactors use more energy to run than they produce for any sustained period.
5
If fusion can be made to work, it would provide a large energy supply with little long-lived radioactive waste.
Section 4 of 5Fusion Compared with Fission
Fusion Compared with Fission
Fuel
- Fusion uses hydrogen isotopes, which are abundant in sea water, while fission uses rarer uranium or plutonium.
Waste
- Fusion produces far less long-lived radioactive waste than fission.
Safety
- Fusion cannot run away as a chain reaction, since the conditions are hard to maintain.
Development
- Fission power stations are in use today, but fusion remains experimental.
| Feature | Fission | Fusion |
|---|---|---|
| Process | Large nucleus splits | Small nuclei join |
| Conditions | Neutron needed | Extreme heat and pressure |
| Current use | Power stations | Experimental |
Section 5 of 5Exam Checklist & Tips
2.9.3 – Key Points for the Exam
- Nuclear fusion The joining of two light nuclei to form a heavier nucleus.
- Plasma A very hot gas in which the atoms have lost electrons.
- Deuterium An isotope of hydrogen with one proton and one neutron.
- Tritium An isotope of hydrogen with one proton and two neutrons.
Common Mistakes
- Mixing up fission and fusion; fission splits large nuclei and fusion joins small nuclei.
- Forgetting that nuclei repel each other, which is why fusion needs such high temperatures.
- Saying that fusion happens in a nuclear power station today; it is not yet a commercial energy source.
- Thinking the energy comes from the nuclei burning; it comes from a small loss of mass.