Lesson 3.8.1.7

3.8.1.7 Induced fission Quiz: AQA Physics, Unit 8

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 3.8.1.7, Induced fission: 20 multiple choice questions for the AQA Physics (7408), Unit 8: Nuclear physics (A-level only), written with Revision Ninja.

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The 20 questions

  1. How is fission induced in uranium-235 in a thermal reactor?

    • fast neutrons pass through uranium-235 without any effect
    • protons are absorbed to produce alpha particles
    • a slow (thermal) neutron is captured, and the nucleus splits into two smaller nuclei while releasing neutrons
    • gamma photons split the uranium nucleus into two equal halves

  2. What is required for a self-sustaining chain reaction?

    • on average, at least one neutron from each fission must go on to cause another fission
    • the fuel must be at least ten times heavier than the moderator
    • fission must release exactly two neutrons each time
    • every neutron from fission must be absorbed by a control rod

  3. What is the critical mass of a fissile material?

    • the mass of coolant needed to remove heat
    • the minimum mass that can sustain a chain reaction
    • the maximum mass of fuel allowed in a reactor
    • the mass of moderator needed to slow the neutrons

  4. What is the function of the moderator in a thermal reactor?

    • to absorb neutrons and stop the reaction
    • to slow fast neutrons by elastic collisions, so that they are more likely to cause fission
    • to cool the reactor core by carrying heat away
    • to hold the fuel rods apart and support them

  5. What is the function of control rods in a reactor?

    • to absorb neutrons, reducing the number available to cause fission
    • to carry heat away from the fuel to the turbines
    • to slow neutrons down so that they cause more fission
    • to release extra neutrons to speed up the reaction

  6. What is the function of the coolant in a thermal reactor?

    • to shield workers from radiation outside the core
    • to slow neutrons by elastic collisions
    • to carry thermal energy out of the core, for example to drive turbines
    • to absorb neutrons and stop the reaction

  7. Which material is a more effective moderator per collision?

    • a heavy metal, since heavy nuclei slow neutrons best
    • lead, since its large mass stops neutrons most quickly
    • uranium, since it absorbs the most neutron energy
    • a hydrogen-rich material such as water, since a light nucleus of similar mass to the neutron transfers the most energy

  8. A neutron collides head-on and elastically with a stationary nucleus of the same mass. What is its speed afterwards?

    • zero, since all its kinetic energy is transferred
    • half its original speed
    • unchanged
    • twice its original speed

  9. A neutron collides head-on and elastically with a carbon-12 nucleus at rest. What fraction of its kinetic energy is retained? Use ((A-1)/(A+1))^2.

    • about 0.14
    • about 0.50
    • about 0.72
    • about 0.28

  10. Why are control rods made from boron or cadmium?

    • they are good conductors of heat
    • they have a very low melting point
    • they have large neutron absorption cross-sections
    • they are the lightest materials available

  11. A reactor has a multiplication factor k = 1.00. What is happening?

    • the reaction is dying away, since neutrons are lost
    • the reaction has stopped, since all neutrons are absorbed
    • the reaction is growing exponentially, since each neutron causes more than one fission
    • the reaction is steady and self-sustaining at a constant rate

  12. A reactor has a multiplication factor k = 1.02. What happens?

    • the reaction stops immediately, since the moderator absorbs neutrons
    • the number of neutrons grows over successive generations, so the power increases
    • the reaction dies away, since fewer than all neutrons cause fission
    • the reaction stays steady, since 1.02 is the critical value

  13. Why must the fuel be enriched for a typical thermal reactor?

    • enrichment makes uranium more radioactive, which speeds the decay
    • enrichment removes the fast neutrons that would cause fission
    • natural uranium contains too little U-235 to sustain a thermal chain reaction, so enrichment raises its proportion
    • natural uranium is too light to split into two fragments

  14. What does the critical mass of a fissile sphere depend on?

    • its density, shape and purity, and whether it is surrounded by material that reflects neutrons back
    • only the number of electrons per atom
    • only the temperature of the room where it is kept
    • only its colour and surface finish

  15. Why does a thermal reactor depend on slowing neutrons with a moderator?

    • fast neutrons cause fission more readily, so the moderator must speed them up
    • slow neutrons are immediately absorbed by the control rods
    • thermal neutrons are much more likely to be captured by U-235 and cause fission than fast neutrons
    • the moderator slows the fission fragments and so cools them

  16. Why does a reactor need a coolant?

    • to reduce the mass of fuel required
    • to remove the heat released by fission, so that the core does not overheat
    • to increase the number of neutrons produced per fission
    • to make the fuel more radioactive

  17. A reactor has k = 0.99 and needs to reach k = 1.00. Which action is appropriate?

    • replace the moderator with a heavier material
    • reduce the flow of coolant through the core
    • insert the control rods further into the core
    • withdraw control rods slightly, reducing neutron absorption

  18. Why does U-235 fission readily with slow neutrons while U-238 does not readily fission?

    • U-235 has a high probability of fission after capturing a slow neutron, whereas U-238 mainly captures neutrons without fissioning
    • U-235 only fissions with fast neutrons, which it rarely captures
    • U-238 fissions more readily with slow neutrons because it is heavier
    • U-238 is not radioactive and so cannot undergo fission at all

  19. A reactor releases 200 MeV per fission. How many fissions per second correspond to a power of 1 W? Use 1 MeV = 1.6 x 10^-13 J.

    • 3.1 x 10^6 per second
    • 2.0 x 10^8 per second
    • 3.1 x 10^10 per second
    • 3.2 x 10^11 per second

  20. Why is an uncontrolled fission chain reaction explosive in a bomb but controlled in a reactor?

    • a bomb uses a moderator to speed neutrons, while reactors use none
    • a reactor uses fusion, which is slower than fission
    • in a reactor, control rods and moderation keep the reaction near critical, while uncontrolled devices let it grow rapidly
    • a reactor has a much larger fuel mass than a bomb, so it fissions more slowly

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