6.1.12Radiation
Fusion
5 min read 4 parts Lesson 12 of 14
Section 1 of 4The Fusion Process
Fusion is the process that releases energy in stars and is being researched as a future power source.
Definition
- Nuclear fusion is the joining of two small nuclei, such as hydrogen isotopes, to make a larger nucleus with the release of energy.
Example
- Deuterium and tritium, two isotopes of hydrogen, fuse to form helium and a neutron, releasing a large amount of energy.
Where it occurs
- Fusion powers the Sun and other stars, where hydrogen is gradually converted into helium in the core.
Energy source
- The energy released per kilogram of fuel is even greater than in fission.
Fuel supply
- Hydrogen isotopes can be obtained from water, so the fuel is abundant and widely available.
Section 2 of 4Conditions Needed
Conditions Needed
Repulsion
- Nuclei are positively charged and repel each other strongly, so they must be brought very close together to fuse.
High temperature
- Millions of degrees are needed to give nuclei enough kinetic energy to overcome the repulsion.
High pressure
- High pressure packs the nuclei closely so that collisions happen frequently enough to sustain the reaction.
Plasma
- At these temperatures atoms lose their electrons, forming a very hot gas of ions called plasma.
Containment
- No material can hold plasma at millions of degrees, so strong magnetic fields confine it in experimental reactors.
| Feature | Fission | Fusion |
|---|---|---|
| Process | Large nucleus splits | Small nuclei join |
| Fuel | Uranium, plutonium | Hydrogen isotopes |
| Conditions | Normal temperature, neutron needed | Extreme temperature and pressure |
| Waste | Long-lived radioactive waste | Little long-lived waste |
| Current use | Power stations | Stars, research reactors |
Section 3 of 4Fusion as an Energy Source
1
Research reactors heat a small amount of hydrogen fuel to over a hundred million degrees inside a magnetic cage.
2
At present these experiments have used more energy to heat the plasma than they usefully produce, so fusion is not yet a commercial power source.
3
If it succeeds, the fuel is almost unlimited and the reaction does not produce carbon dioxide or long-lived radioactive waste.
4
A fusion reaction stops at once if conditions are lost, so there is no runaway chain reaction like fission.
5
Engineering challenges include materials that survive the neutron bombardment and keeping the plasma stable for long periods.
Section 4 of 4Exam Checklist & Tips
6.1.12 – Key Points for the Exam
- Nuclear fusion The joining of two small nuclei to form a larger one, releasing energy.
- Deuterium An isotope of hydrogen with one proton and one neutron.
- Plasma A very hot gas of charged ions and free electrons.
- Magnetic confinement Using magnetic fields to hold plasma away from the walls.
- Isotope An atom of an element with a different number of neutrons.
Common Mistakes
- Mixing up fission and fusion, such as saying fusion splits large nuclei.
- Forgetting that nuclei repel each other, which is why high temperature is needed.
- Stating that fusion is already used in power stations.
- Claiming fusion produces no waste at all, when materials can become radioactive.