Nuclear fission and fusion, explained
Fission splits large nuclei apart; fusion joins small nuclei together. Both release enormous amounts of energy from tiny amounts of fuel. Fission powers nuclear reactors today, and fusion powers the Sun.
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Why nuclei release energy
The particles in a nucleus are held together by the strong nuclear force. Some arrangements of protons and neutrons are more tightly bound than others. Very heavy nuclei, such as uranium, and very light nuclei, such as hydrogen, are less tightly bound than medium-sized nuclei such as iron. So energy is released when heavy nuclei split into medium-sized ones (fission), and when light nuclei join into heavier ones (fusion). In both cases, the products have slightly less mass than the starting particles, and that missing mass becomes energy. At A level, this is described by Einstein's E = mc².
Nuclear fission
Fission is the splitting of a large, unstable nucleus into two smaller nuclei. In a reactor, the fuel is usually uranium-235 or plutonium-239.
- A slow-moving neutron is absorbed by a uranium-235 nucleus.
- The nucleus becomes very unstable and splits into two smaller nuclei, called daughter nuclei, of roughly similar size.
- The split also releases two or three neutrons, plus energy, mostly as the kinetic energy of the fragments, together with gamma radiation.
Spontaneous fission is rare; nearly all fission in a reactor happens because a nucleus has absorbed a neutron.
Chain reactions
The neutrons released by one fission can be absorbed by other uranium-235 nuclei, causing them to split and release more neutrons. This is a chain reaction. If each fission causes, on average, exactly one more, the reaction runs at a steady rate, as in a power station. If each fission causes more than one, the number of fissions grows extremely quickly, releasing a huge amount of energy almost at once, which is what happens in a nuclear weapon. A power station keeps the reaction carefully controlled.
Inside a nuclear reactor
- Fuel rods contain the uranium.
- The moderator, often water or graphite, slows down the fast neutrons released by fission, because slow neutrons are much more likely to be absorbed by uranium-235 and cause another fission.
- Control rods, made of a material such as boron that absorbs neutrons, are raised or lowered into the core. Lowering them absorbs more neutrons and slows the reaction; raising them speeds it up. In an emergency they are dropped fully in to stop the chain reaction.
- The coolant carries the thermal energy away to a heat exchanger, where it boils water. The steam drives turbines, which turn generators to produce electricity.
- Thick concrete shielding absorbs radiation and protects workers.
Nuclear fusion
Fusion is the joining of two light nuclei to form a heavier nucleus. In the Sun and other stars, hydrogen nuclei fuse, through several steps, to form helium, releasing the energy that makes stars shine. In experimental reactors on Earth, the fuel is usually two isotopes of hydrogen, deuterium and tritium, which fuse to make helium and a neutron.
Why fusion is so hard to achieve
Nuclei are positively charged, so they repel each other strongly. To fuse, they must collide at enormous speeds, which needs temperatures of around 100 million degrees Celsius on Earth, hotter than the centre of the Sun, and high pressures or densities. No material container can hold matter that hot, so experimental reactors use powerful magnetic fields to hold the hot plasma away from the walls. Scientists have achieved fusion, but producing more useful energy than is put in, reliably and economically, remains a major engineering challenge.
Fission and fusion compared
- What happens: fission splits heavy nuclei; fusion joins light nuclei.
- Fuel: fission uses uranium or plutonium; fusion uses isotopes of hydrogen, which are abundant in sea water.
- Waste: fission produces long-lived radioactive waste; fusion produces far less, much of it short-lived.
- Status: fission power stations operate worldwide; fusion power is still experimental.
Worked examples
Example 1: explaining a chain reaction
Explain how one fission can lead to a chain reaction.
- A uranium-235 nucleus absorbs a neutron and splits, releasing energy and two or three neutrons.
- These neutrons can be absorbed by other uranium-235 nuclei, which also split and release more neutrons. The process repeats, so each fission triggers further fissions.
Example 2: the role of control rods
A reactor is producing too much power. What should the operators do, and why does it work?
- Lower the control rods further into the core.
- The rods absorb more neutrons, so fewer neutrons cause further fissions, and the rate of the chain reaction falls.
Example 3: comparing processes
Give one similarity and one difference between fission and fusion.
- Similarity: both release energy from changes to nuclei, and in both the products have less mass than the reactants.
- Difference: fission splits a large nucleus, while fusion joins two small nuclei.
Nuclear power: benefits and risks
Nuclear power stations produce no carbon dioxide while running and provide reliable electricity regardless of the weather, from very small amounts of fuel. However, they produce radioactive waste that stays dangerous for thousands of years and must be stored securely, they are expensive to build and decommission, and accidents, though rare, can release radioactive material over a wide area. Exam questions often ask you to weigh these points, so give balanced arguments. A strong answer names at least one advantage and one disadvantage, explains each, and finishes with a reasoned judgement, such as whether nuclear power should be part of a low-carbon energy mix alongside renewables.
Common mistakes
- Mixing up fission (splitting) and fusion (joining).
- Saying control rods slow neutrons down; that is the moderator's job. Control rods absorb neutrons.
- Forgetting that fission releases extra neutrons, which is what makes a chain reaction possible.
- Saying fusion is easy because stars do it; stars have enormous gravity, temperature and pressure.
Practice questions
Try each one before opening the answer.
Q1. What particle must be absorbed to start fission of uranium-235?
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Q2. What is the job of the moderator in a reactor?
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Q3. What fuses in the core of the Sun?
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Q4. Why must fusion happen at very high temperatures?
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Frequently asked questions
Is fission and fusion in Combined Science?
Nuclear fission and fusion are in GCSE separate Physics on AQA. Combined Science covers atomic structure and radioactivity but not fission and fusion in detail. Check your own board's specification.
Is E = mc² needed for GCSE?
No. GCSE describes energy release in words. A level Physics uses E = mc² and binding energy to calculate the energy released.
Key takeaways
- Fission splits heavy nuclei after they absorb a neutron, releasing energy and more neutrons.
- Released neutrons can cause a chain reaction.
- Moderators slow neutrons; control rods absorb them.
- Fusion joins light nuclei and powers the stars.
- Fusion needs extreme temperatures because nuclei repel each other.