Section 1 of 4What Fusion Is
Fusion releases energy when light nuclei join together to form a heavier nucleus.
Definition
- Definition Nuclear fusion is the joining of two light nuclei to form a heavier nucleus, releasing energy in the process.
Mass change
- Mass change The product nucleus has slightly less mass than the original nuclei, and this lost mass is converted into energy.
Stars
- Stars Fusion of hydrogen nuclei in the core of a star produces helium and releases the energy that makes the star shine.
Equation
- Equation Deuterium and tritium fuse: ²₁H + ³₁H → ⁴₂He + ¹₀n, releasing a neutron and a large amount of energy.
Section 2 of 4Conditions for Fusion
Fusion is difficult to achieve because positively charged nuclei repel each other.
1
Nuclei are positively charged, so they repel each other strongly as they approach.
2
To overcome this repulsion, nuclei must collide at very high speeds, which requires very high temperatures.
3
In stars, the enormous pressure of the core pushes nuclei close enough together for fusion to occur.
4
On Earth, fusion reactors must contain plasma at temperatures of millions of degrees, which is technically very difficult.
Section 3 of 4Fusion Compared With Fission
Fusion and fission both release energy from nuclei, but they differ in several ways.
Fusion Compared With Fission
Process
- Process Fusion joins light nuclei together, while fission splits a heavy nucleus into smaller nuclei.
Fuel
- Fuel Fusion uses isotopes of hydrogen, such as deuterium, which are plentiful in sea water.
Waste
- Waste The fusion reaction itself produces no long-lived radioactive products, although reactor materials can become radioactive.
Current use
- Current use Fusion power stations are still experimental, so fusion does not yet contribute to the electricity supply.
| Feature | Fusion | Fission |
|---|---|---|
| Process | Light nuclei join | Heavy nucleus splits |
| Conditions | Very high temperature and pressure | Starts when a neutron is absorbed |
| Fuel | Hydrogen isotopes | Uranium-235 or plutonium-239 |
| Long-lived radioactive waste | Not produced by the reaction itself | Produced |
| Current use | Experimental | Used in nuclear power stations |
Section 4 of 4Exam Checklist & Tips
4.4.4 – Key Points for the Exam
- Nuclear fusion The joining of two light nuclei to form a heavier nucleus, releasing energy.
- Nuclear fission The splitting of a heavy nucleus into two smaller nuclei, releasing energy.
- Isotope Atoms of the same element with different numbers of neutrons.
- Plasma A very hot gas in which electrons have separated from the nuclei.
- Deuterium An isotope of hydrogen with one proton and one neutron in its nucleus.
Common Mistakes
- Saying fusion splits atoms; fusion joins light nuclei, while fission splits heavy nuclei.
- Stating that fusion is already used to generate electricity in power stations.
- Forgetting that fusion needs very high temperature and pressure, not just a supply of fuel.