Nuclear fusion (AQA GCSE Physics): Revision Notes
Nuclear fusion
What is nuclear fusion?
Nuclear fusion is when small, light nuclei combine together to create larger, heavier nuclei. This process releases lots of energy.
During fusion, some of the original mass gets converted into energy. This fundamental principle is what makes fusion such a powerful energy source. The combined mass of the new products is slightly less than the original nuclei - this "missing" mass becomes energy.
Key Example: Deuterium-Tritium Fusion
Deuterium and tritium (both types of hydrogen) can fuse together to make helium and release a neutron plus energy:
Deuterium + Tritium → Helium + Neutron + Energy
This is the most studied fusion reaction because it occurs at relatively lower temperatures than other fusion processes.
How nuclear fusion works
The process involves very light nuclei coming together at incredibly high speeds. When they collide and stick together, they form a new, heavier nucleus.
Simple Steps of Nuclear Fusion:
- Small nuclei move towards each other very fast
- They overcome the forces trying to push them apart
- They combine to make a larger nucleus
- Energy is released as heat and light
Understanding these basic steps helps explain why fusion requires such extreme conditions to occur naturally or in controlled environments.
Fusion in stars
Stars are giant fusion reactors! During a star's stable lifetime, huge amounts of hydrogen nuclei get converted into helium nuclei through fusion reactions.
This fusion process is what gives stars their energy. The Sun has been doing this for billions of years and will continue for billions more.
The Sun's Incredible Longevity
Our Sun converts approximately 4 million tonnes of matter into energy every second through nuclear fusion. Despite this enormous rate, the Sun has enough hydrogen fuel to continue this process for another 5 billion years!
What happens in stars:
- Hydrogen nuclei fuse to make helium
- This releases enormous amounts of energy
- The energy travels through space as heat and light
- Eventually, heavier elements form as the star ages
This stellar nucleosynthesis is responsible for creating most of the elements heavier than hydrogen and helium that we find in the universe today.
Why fusion is difficult to achieve
Nuclear fusion is much harder to achieve than nuclear fission. Understanding these challenges explains why fusion power remains elusive despite decades of research.
Electrostatic Repulsion Challenge
All nuclei have positive charges, so they naturally push each other away. Getting them close enough to fuse requires overcoming this strong electrostatic repulsion. This is the fundamental barrier that makes fusion so difficult.
Extreme Conditions Required
Fusion only happens under very high temperatures and pressures. The nuclei must be moving extremely fast to get close enough to fuse together. We're talking about temperatures of millions of degrees Celsius!
Energy Requirements
Creating these extreme conditions requires more energy than current fusion reactors have been able to produce efficiently. This is known as the energy breakeven problem - getting more energy out than you put in.
This is why we don't have fusion power stations yet, even though scientists have been working on them for decades.
Fusion vs fission
Understanding the differences between these two nuclear processes helps clarify why fusion is more challenging to harness.
Similarities:
- Both nuclear processes release large amounts of energy
- Both involve changes to atomic nuclei
Key differences:
- Fusion joins light nuclei together; fission splits heavy nuclei apart
- Fusion needs extremely high temperatures and pressures; fission needs a slow-moving neutron to be absorbed
- Fusion uses hydrogen isotopes; fission typically uses uranium-235
Key Points to Remember:
- Nuclear fusion combines small nuclei to make larger ones and releases energy
- Stars get their energy from hydrogen fusing into helium
- Fusion requires extremely high temperatures and pressures to overcome electrostatic repulsion
- The "missing" mass in fusion reactions gets converted into energy
- Fusion is much harder to achieve than fission, which is why we don't have fusion power stations yet