Nuclear fission and fusionAQA A-Level Physics: Subtopic test

1. 1
   Uranium-235 nuclei can be split by slow-moving neutrons. One fission reaction is ¹n + ²³⁵U → ¹⁴¹Ba + ⁹²Kr + 3 ¹n, and about 200 MeV of energy is released in each fission.

   (a)

   Why are slow (thermal) neutrons used to induce fission of uranium-235 in a reactor?

   [1 mark]

   • ASlow neutrons carry more kinetic energy, so they split the nucleus more violently.
   • BA slow neutron is more likely to be captured by a uranium-235 nucleus.
   • CSlow neutrons are attracted to the nucleus by electrostatic force.
   • DSlow neutrons are the only neutrons released by fission.

   (b)

   Which form of energy accounts for most of the energy released in a fission?

   [1 mark]

   • AGamma radiation emitted at the moment of fission
   • BThe kinetic energy of the neutrons released
   • CChemical energy released as the fuel oxidises
   • DThe kinetic energy of the fission fragments

   (c)

   Explain how a chain reaction can occur in a block of uranium-235.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A thermal nuclear reactor has fuel rods of uranium-235, a graphite moderator, boron control rods and a carbon dioxide coolant.

   (a)

   What is the main function of the control rods?

   [1 mark]

   • ATo absorb neutrons so that the rate of fission can be controlled
   • BTo slow fast neutrons down to thermal energies
   • CTo carry thermal energy away from the core to the boiler
   • DTo supply extra fissile material when the reaction slows

   (b)

   In a head-on elastic collision with a stationary nucleus, which nucleus takes the greatest fraction of a neutron's kinetic energy?

   [1 mark]

   • A²³⁸U
   • B¹²C
   • C¹H
   • D⁴He

   (c)

   Explain why a moderator is needed in a thermal reactor.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A thermal reactor operates in a steady state at a thermal power of 3.0 GW. Each fission of uranium-235 releases on average 200 MeV of energy and 2.4 neutrons. A uranium-235 atom has a mass of 235 u, where 1 u = 1.661 × 10⁻²⁷ kg; 1 MeV = 1.60 × 10⁻¹³ J.

   (a)

   Explain how the control rods keep the reactor operating in a steady state.

   [3 marks]

   (b)

   Calculate the mass of uranium-235 that undergoes fission in the reactor each day.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A government panel is advising on future electricity supply. It is considering building more thermal fission reactors, which use uranium-235 fuel, and also funding research into fusion. Fission of uranium-235 releases about 0.9 MeV per nucleon of the nucleus; deuterium–tritium fusion releases 17.6 MeV for the five nucleons involved.

   (a)

   Describe the function of the moderator, the control rods and the coolant in a thermal fission reactor, and explain the properties needed of the material chosen for each. Give an example of a suitable material for each.

   [6 marks]

   (b)

   Evaluate whether fusion is likely to be a better source of energy than fission, using the physics of the two processes.

   [6 marks]

   Total for question 4: 12 marks

End of questions