How is energy released by splitting and by joining nuclei?
Nuclear fission and fusion: the process of fission and the chain reaction, the process of fusion, and how each releases energy (separate physics only).
A focused answer to AQA GCSE Physics 4.4.3, covering nuclear fission as the splitting of a large unstable nucleus, the chain reaction in a reactor, nuclear fusion as the joining of light nuclei in stars, and how each process releases energy.
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What this dot point is asking
AQA wants you to describe nuclear fission as the splitting of a large unstable nucleus, explain the chain reaction in a reactor, describe nuclear fusion as the joining of light nuclei, and explain that each process releases energy. This is topic 4.4.3 of the AQA GCSE Physics (8463) specification and is separate physics only (it is not on the combined-science route).
Nuclear fission
The two smaller nuclei produced are called daughter nuclei, and they are usually of roughly similar (but not equal) size and are themselves radioactive. The energy released appears mainly as the kinetic energy of these fast-moving fragments. When they collide with surrounding atoms this kinetic energy is transferred to the thermal energy store of the fuel and coolant. In a power station this thermal energy heats water to make steam, which turns a turbine connected to a generator. The amount of energy released per fission is far greater than the energy released per atom in any chemical reaction such as burning coal, which is why a small mass of nuclear fuel can run a power station for a long time.
The chain reaction
The key idea is that, on average, exactly one neutron from each fission must go on to cause the next fission for the reaction to be steady. If more than one does, the rate grows uncontrollably (as in a bomb); if fewer than one does, the reaction dies out. Reactors manage this in two ways. Control rods made of a neutron-absorbing material such as boron are raised and lowered to soak up surplus neutrons, slowing or speeding the reaction. A moderator (often water or graphite) slows the fast neutrons released by fission to lower speeds, because uranium-235 absorbs slow neutrons far more readily than fast ones. Together these allow operators to hold the chain reaction at a steady, useful rate.
Nuclear fusion
Fusion needs extremely high temperatures and pressures because the two nuclei are both positively charged and repel each other, so they must be moving very fast to get close enough to join. In the core of the Sun, hydrogen nuclei fuse to form helium at temperatures of millions of degrees, and the energy released is what makes stars shine. Recreating these conditions on Earth is extremely difficult, which is why fusion power stations do not yet exist despite fusion being, in principle, a clean energy source with abundant fuel. The contrast with fission is important: fission is comparatively easy to start and control because the heavy nucleus is already on the edge of stability, while fusion is hard to start because the repulsion between light nuclei is a large barrier to overcome.
Try this
Q1. Describe what happens during nuclear fission. [2 marks]
- Cue. A large unstable nucleus (often after absorbing a neutron) splits into two smaller nuclei, releasing energy and two or three neutrons.
Q2. Explain why nuclear fusion requires very high temperatures. [2 marks]
- Cue. The two nuclei are positively charged and repel each other, so they need very high speeds (high temperature) to get close enough to fuse.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20196 marksDescribe the process of nuclear fission and explain how a chain reaction is set up and controlled in a nuclear reactor.Show worked answer →
This is a level-of-response question marked in bands. A top-band answer describes fission first: a large unstable nucleus such as uranium-235 absorbs a slow-moving neutron, becomes unstable, and splits into two smaller daughter nuclei. The process releases energy (as kinetic energy of the fragments, transferred to thermal energy) and two or three further neutrons, plus gamma radiation. It then explains the chain reaction: each released neutron can be absorbed by another uranium-235 nucleus and cause a further fission, so the number of fissions can grow rapidly. Finally it explains control: control rods (often boron) are lowered into the core to absorb surplus neutrons so that on average one neutron from each fission goes on to cause the next, keeping the rate steady, and a moderator slows the neutrons so they are more readily absorbed. Markers reward correct physics, logical sequence, and the link between neutron count and reaction rate.
AQA 20214 marksCompare nuclear fission and nuclear fusion. In your answer, refer to the type of nuclei involved, what happens to them, and the conditions each process needs.Show worked answer →
Fission splits a large, unstable nucleus (such as uranium-235) into two smaller nuclei (1 mark), and it can proceed at the relatively modest conditions inside a reactor because the heavy nucleus is already unstable and only needs to absorb a neutron (1 mark). Fusion joins two light nuclei (such as hydrogen isotopes) to form a heavier nucleus (1 mark), but it needs extremely high temperatures and pressures because the two positively charged nuclei repel each other and must move fast enough to get close enough to fuse (1 mark). Both release energy. Markers reward a clear point-by-point comparison rather than two separate descriptions, and penalise candidates who mix up which process splits and which joins.
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Sources & how we know this
- AQA GCSE Physics (8463) specification — AQA (2016)