How does the splitting of a uranium nucleus by a neutron lead to a self-sustaining chain reaction?
Induced nuclear fission by thermal neutrons, the chain reaction, critical mass, and the factors controlling whether the reaction is sustained.
A focused answer to AQA A-Level Physics 3.8.1.8, covering induced fission by thermal neutrons, the products of fission, the chain reaction, critical mass and the conditions needed to sustain a controlled chain reaction.
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What this dot point is asking
AQA specification point 3.8.1.8 wants you to describe induced fission by thermal neutrons, explain the chain reaction and the role of the neutrons released, and define critical mass and the conditions for a sustained reaction.
The fission process
The two daughter nuclei are typically of unequal size (for example barium and krypton) and are themselves neutron-rich and radioactive, undergoing beta decay afterwards. Both nucleon number and proton number are conserved in the fission equation.
Thermal neutrons
The chain reaction
The average number of neutrons from one fission that go on to cause the next fission is the multiplication factor; keeping it at one is the central task of reactor control.
Critical mass
The critical mass depends on the material, its purity (enrichment) and its shape; a compact sphere has the smallest surface area for a given mass and so the lowest critical mass.
The energy released
The fragments and neutrons carry away kinetic energy, which appears as the energy of the reaction. The products have a higher binding energy per nucleon than the original heavy nucleus, so energy is released, as described by the binding energy curve. A single uranium-235 fission releases about , far more than any chemical reaction per atom.
Try this
Q1. Explain why a chain reaction requires a critical mass. [2 marks]
- Cue. Below the critical mass too many neutrons escape from the surface without causing further fission, so the reaction is not sustained.
Q2. State why fast neutrons are slowed down before causing further fission. [1 mark]
- Cue. Slow (thermal) neutrons are far more likely to be absorbed by uranium-235 and cause fission.
Q3. State approximately how much energy is released in a single uranium-235 fission. [1 mark]
- Cue. About .
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 20195 marksExplain what is meant by a chain reaction in nuclear fission, and describe the conditions needed for the reaction to be sustained at a steady rate.Show worked answer →
In induced fission, a uranium-235 nucleus absorbs a thermal neutron, becomes unstable and splits, releasing energy and typically two or three further neutrons. If these neutrons go on to cause further fissions, each producing more neutrons, the number of reactions can multiply: this is a chain reaction.
For a steady reaction, on average exactly one neutron from each fission must cause a further fission (a multiplication factor of one). This requires at least the critical mass of fissile material so that not too many neutrons escape from the surface, the neutrons to be slowed by a moderator so they are absorbed efficiently, and control rods to absorb the surplus neutrons.
Markers reward describing the multiplying neutrons, the condition of one neutron per fission continuing, and the roles of critical mass, moderator and control rods.
AQA 20213 marksA uranium-235 nucleus undergoes induced fission according to . Determine the nucleon number of the krypton nucleus, and state the conservation laws you used.Show worked answer →
Conserve nucleon number: the left side has nucleons. The right side has . Setting equal, , so .
Check proton number: left side ; right side , which balances.
The conservation laws used are conservation of nucleon (mass) number and conservation of proton (charge) number.
Markers reward balancing nucleon number to find and naming conservation of nucleon number and charge.
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Sources & how we know this
- AQA A-level Physics (7408) specification — AQA (2017)