Edexcel IGCSE Physics · Spec 7.5
Nuclear Fission & Fusion
The difference between nuclear fission and nuclear fusion.
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Nuclear Fission & Fusion
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Explained
Splitting nuclei and joining them
Both processes release energy from the nucleus, and they are opposites. Fission splits a large nucleus into smaller ones. Fusion joins small nuclei into a larger one.
Keeping the two words apart is the first thing, and the middle letters are the clue: fission for splitting, fusion for fusing together.
Fission
A slow moving neutron is absorbed by a large unstable nucleus, usually uranium 235 or plutonium 239.
The nucleus becomes even more unstable and splits into two smaller daughter nuclei, releasing energy and two or three more neutrons.
Those neutrons can be absorbed by other uranium nuclei, causing them to split too. That is a chain reaction, and it is what makes a nuclear reactor work.
Controlling the chain reaction
Control rods, made of boron or cadmium, absorb neutrons. Lowering them further into the reactor absorbs more neutrons, so fewer are left to cause further fissions and the reaction slows. Raising them speeds it up.
A moderator, usually water or graphite, slows the neutrons down. This is necessary because a slow neutron is much more likely to be absorbed than a fast one, so without a moderator the chain reaction would not be sustained.
Shielding, usually thick concrete, absorbs the radiation so that it does not escape and reach people.
Fusion
Two light nuclei, typically isotopes of hydrogen, join to form a heavier nucleus, releasing a very large amount of energy. This is the process powering the Sun and all other stars.
It is difficult to achieve on Earth because both nuclei are positively charged, so they repel each other electrostatically. Enormous temperatures are needed to give them enough kinetic energy to get close enough for fusion to occur, and enormous pressures to make collisions frequent enough.
Those conditions exist in the core of a star because of its vast mass and gravity. Reproducing them in a reactor is why fusion power remains experimental.
What the mark scheme accepts and rejects
An Edexcel International GCSE Physics mark scheme sets out a chain reaction as four steps: a neutron is absorbed by the uranium nucleus, the nucleus becomes unstable, it splits producing two smaller nuclei and some neutrons, and those neutrons are absorbed by other uranium nuclei. It condones undergoes fission in place of splits, and condones naming the daughter nuclei such as krypton and barium.
On the next part it asks for the difference between fusion and radioactive decay. Fusion is the joining of nuclei; radioactive decay is the spontaneous release of a particle from a nucleus. Its note then says to ignore any reference to splitting.
That instruction is worth pausing on. Neither of those two processes is splitting, so reaching for the word means fission has been brought into a question that was not about it.
The examiner report on that paper records a more specific confusion. Learners relied on chemical language such as successful collisions to explain the requirements for fusion. Collision theory belongs to chemistry; what fusion needs is enough kinetic energy to overcome the electrostatic repulsion between two positive nuclei, and saying that is what earns the marks.
Comparing them
Fission uses large unstable nuclei and works at ordinary temperatures once started, but produces radioactive waste that stays dangerous for thousands of years.
Fusion uses light nuclei that are abundant, releases more energy per kilogram of fuel, and produces very little radioactive waste. Its problem is entirely the conditions required.
If a question asks why fusion is not yet used commercially, the answer is the temperature and pressure, and the difficulty of containing a material that hot, rather than anything about the physics being unproven.
Spec 7.5
What you need to know
- Describe nuclear fission
- Explain a chain reaction
- Describe nuclear fusion
Active recall
Quick check
Answer each question before opening the answer.
What is nuclear fission?
A large unstable nucleus splits into smaller nuclei, releasing energy and neutrons.
What is nuclear fusion?
Two light nuclei join to form a heavier nucleus, releasing energy — as happens in stars.
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