Fusion is what makes the Sun shine
Definition
Nuclear fusion
The joining together of two light nuclei to form a heavier nucleus, releasing energy from the nuclear store.
- In the core of the Sun, hydrogen nuclei fuse in stages to form helium nuclei.
- The helium nucleus produced has slightly less mass than the particles that went into making it, and that missing mass appears as released energy.
- The energy released moves outwards and eventually leaves the Sun as electromagnetic radiation, mostly visible light and infrared, which is what warms the Earth.
- This is the process behind every star, and it is why the Sun has been able to shine steadily for billions of years on a fuel supply it has not yet exhausted.
- Fusion is the opposite of fission. Fission splits a heavy nucleus such as uranium into lighter ones; fusion joins light nuclei into a heavier one. Both release energy from the nuclear store.
Example
- In the Sun, four hydrogen nuclei are converted through a sequence of steps into one helium nucleus.
- The helium nucleus has a smaller mass than the four hydrogen nuclei combined, and the difference is released as energy.
Why fusion needs such extreme conditions
- Every nucleus carries a positive charge, so two nuclei approaching each other repel strongly.
- To fuse, they must be moving fast enough to overcome that repulsion and get close enough for the strong nuclear force to hold them together.
- Moving that fast means a temperature of millions of degrees, and getting enough collisions means an enormous pressure as well.
- In the Sun those conditions exist naturally, because the star's own gravity crushes and heats its core.
Turning fusion into a power station on Earth
- Fusion is an attractive target for electricity generation because its fuel comes from hydrogen, which is plentiful, and because it produces no long-lived radioactive waste of the kind fission reactors leave behind.
- The obstacle is reproducing the Sun's conditions without the Sun's gravity. Experimental reactors must heat a gas of hydrogen to well over 100100100 million degrees and hold it there.
- No material container can touch a gas that hot, so experimental machines confine it using strong magnetic fields, keeping the hot gas away from the walls.
- Research is being carried out worldwide to solve this. The difficulty is not producing fusion, which has been achieved, but sustaining it long enough to release more energy than is put in.
- No commercial fusion power station yet supplies electricity to any grid, so fusion is currently a research programme, not an energy resource in use.
Common Mistake
- Fusion is not fission. Existing nuclear power stations use fission of uranium. Fusion is still experimental.
- The Sun does not burn in the chemical sense. Burning is a reaction between chemicals; fusion changes the nuclei themselves.
- Fusion is not "free energy". Enormous amounts of energy must be supplied first to reach the required temperature.
- Do not say fusion produces no radioactivity at all. It produces far less, and shorter-lived, waste than fission.
Exam technique
- If asked what happens in the Sun, use the words hydrogen, fuse, helium and energy released. A vague answer about the Sun "burning fuel" scores nothing.
- When asked why fusion is difficult on Earth, name the two conditions: very high temperature and very high pressure, and explain that they are needed to overcome the repulsion between positively charged nuclei.
- Questions about fusion research usually want one benefit and one obstacle. Pair "plentiful fuel and little long-lived waste" with "the conditions are extremely hard to sustain".
Self review
- Define nuclear fusion.
- Name the two nuclei involved in the fusion process that powers the Sun.
- Explain why fusion requires extremely high temperatures.
- State one advantage of fusion over fission as a future energy resource.
- Explain why fusion is not yet used to generate electricity on a large scale.
