🌡️ High temperature
A very high temperature means the nuclei move very fast. Only fast-moving nuclei have enough kinetic energy to push against the repulsion and get close enough to fuse. At low temperatures they are too slow and just bounce apart.
Fission and Fusion » Nuclear Fusion
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 7.23, 7.24, 7.25, 7.26
- What nuclear fusion is and how it differs from fission
- Why joining small nuclei releases energy
- Why fusion powers the stars
- Why fusion needs very high temperatures and pressures
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Unlock This CourseJoining nuclei together
In the last two lessons you met fission, where one large nucleus splits. Fusion works the other way round. In nuclear fusion, two small nuclei join together to make one larger nucleus. This is the creation of a larger nucleus from smaller ones.
Fusion is another nuclear reaction that is a source of energy. The usual example is hydrogen nuclei fusing to make helium.
Key terms:
- Nuclear fusion: the joining of two small nuclei to make a larger nucleus, releasing energy.
Where does the energy come from?
When small nuclei fuse, the larger nucleus that forms has a little less mass than the small nuclei had before. This is a loss of mass. The missing mass has not vanished: it has been transferred as energy. That energy is released.
The loss of mass in one fusion is tiny. But a star contains an enormous number of nuclei, so the total energy released is huge.
Worked example
Stars mainly fuse hydrogen into helium. Here is one example of a fusion reaction, written as a nuclear equation (it is the type that researchers try to use on Earth). Hydrogen-2 and hydrogen-3 nuclei fuse:
hydrogen-2 + hydrogen-3 → helium-4 + neutron
Mass numbers: 2 + 3 = 5 on the left, and 4 + 1 = 5 on the right. Atomic numbers: 1 + 1 = 2 on the left, and 2 + 0 = 2 on the right. The equation balances, just like the ones in Balancing Nuclear Equations. The tiny difference in actual mass is what appears as released energy.
Fusion in the stars
Fusion is the energy source for stars. Our Sun is mainly hydrogen. In its core, hydrogen nuclei fuse to make helium nuclei, and the energy released is what makes the Sun shine. The light and heat that reach Earth began as fusion energy.
Stars are so massive that their own gravity squeezes the core to a very high pressure. The core is also at a very high temperature. These are exactly the conditions that fusion needs, as you will see below.
Fusion or fission?
Both reactions release energy from nuclei, but they do opposite things.
| Feature | Fission | Fusion |
|---|---|---|
| What happens | A large nucleus splits | Small nuclei join together |
| Nuclei before | One large nucleus, such as uranium-235 | Two small nuclei, such as hydrogen |
| Nuclei after | Two smaller nuclei and neutrons | One larger nucleus |
| Where it happens | Nuclear reactors | Stars |
| Conditions needed | A neutron to start it | Very high temperature and pressure |
Why fusion is so hard to start
Think about pushing two magnets together with the same poles facing. The closer they get, the harder they push back. Nuclei behave in a similar way, because of their electric charge.
Every nucleus is positively charged, because it contains protons. You know that like charges repel. So when two nuclei get close, there is an electrostatic repulsion of protons that pushes them apart. To fuse, the nuclei must get close enough to overcome this repulsion.
🗜️ High pressure
A very high pressure squeezes the nuclei very close together. Close nuclei are far more likely to meet and fuse. At low pressure they are too far apart.
So fusion does not happen at low temperatures and pressures. It only happens in places like the cores of stars.
This is why fusion power stations are so hard to build. On Earth we would need to heat hydrogen to a temperature of many millions of degrees and hold it at a high pressure for long enough. No material can touch something that hot, so building a working fusion power station is a very difficult problem. Fission reactors do not have this problem, which is why we have them but not fusion ones.
Quick summary of the energy idea
Fission: one large nucleus becomes two smaller ones, and energy is released. Fusion: two small nuclei become one larger one, and energy is released. Both are nuclear reactions that are a source of energy. In fusion, the larger nucleus has a little less mass than the small nuclei had, and the lost mass is transferred as energy.
Common mistakes
Common mistakes
- Mixing up the two words. Fission is splitting. Fusion is joining. Think of "fuse" meaning "join".
- Saying the mass is gained. In fusion the mass goes down slightly and energy is released.
- Saying fusion is hard because of "neutrons". The reason is the electrostatic repulsion between positively charged nuclei.
- Saying fusion only needs a high temperature. It needs a very high temperature and a very high pressure.
- Writing "high temperature" without saying why. Say that the nuclei move faster, so they can overcome the repulsion.
Exam-style question
(a) State what happens to the nuclei in nuclear fusion. [1 mark]
(b) State the energy source for stars. [1 mark]
(c) Explain why fusion does not happen at low temperatures and pressures. [3 marks]
Model answer
(a) Small nuclei join together to make a larger nucleus (1).
(b) Nuclear fusion (1).
(c) Nuclei are positively charged, so they repel each other (1). At a low temperature the nuclei move too slowly to get close enough to fuse (1). At a low pressure the nuclei are too far apart (1).
Exam tip
For a 3 mark "explain why" answer, give the cause first (the repulsion between positive nuclei), then link each condition to it: temperature to speed, pressure to distance.