1. 1
   An engineer at a nuclear power station is explaining to new trainees how energy is released inside the reactor core when uranium fuel rods undergo a controlled chain reaction, and how this differs fundamentally from the process powering the Sun.

   (a)

   An engineer at a nuclear power station explains that a neutron collides with a U-235 nucleus, causing it to split apart. What is this process called?

   [1 mark]

   • ANuclear fission
   • BNuclear fusion
   • CRadioactive decay
   • DBeta emission

   (b)

   The engineer explains that when a U-235 nucleus splits, it produces two smaller radioactive daughter nuclei and releases a small number of neutrons. What happens to the kinetic energy released in this process?

   [1 mark]

   • AIt is entirely converted into gamma radiation with no thermal effect
   • BIt appears mainly as kinetic energy of the fission products, which is then transferred as thermal energy
   • CIt disappears completely as mass is destroyed
   • DIt is stored permanently within the daughter nuclei with no further transfer

   (c)

   Describe how a chain reaction is set up in the reactor once the first U-235 nucleus undergoes fission.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   During a routine safety inspection at the same nuclear power station, an engineer reviews the roles of the different components inside the reactor core, including the moderator, the control rods, and the surrounding shielding, and explains why each is essential for safe operation.

   (a)

   During a reactor safety inspection, an engineer checks the control rods, which can be raised or lowered into the reactor core. What is the main function of the control rods?

   [1 mark]

   • ATo shield workers outside the reactor from radiation
   • BTo slow down neutrons so they can be absorbed more easily by U-235
   • CTo absorb excess neutrons and control the rate of the chain reaction
   • DTo supply the initial neutrons needed to start the chain reaction

   (b)

   The inspection also covers the moderator material surrounding the fuel rods. What is the main function of the moderator?

   [1 mark]

   • ATo prevent any radiation from escaping the reactor core
   • BTo absorb neutrons so that the chain reaction stops completely
   • CTo convert kinetic energy of fission products directly into electricity
   • DTo slow down fast neutrons so they are more likely to cause further fission in U-235 nuclei

   (c)

   Explain the role of the shielding surrounding the reactor core, and why this is an essential safety feature in addition to the control rods and moderator.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A team of astrophysicists is studying how the Sun and other stars generate the enormous amounts of energy they radiate into space over billions of years, and comparing this process to the fission reactions used in nuclear power stations on Earth.

   (a)

   A team of astrophysicists studying the Sun explains that nuclear fusion, rather than fission, is responsible for the energy it produces. Describe the process of nuclear fusion, and explain how it differs fundamentally from nuclear fission.

   [3 marks]

   (b)

   Explain why nuclear fusion does not happen at the low temperatures and pressures found in a laboratory on Earth, and describe why extremely high temperatures and pressures, such as those found in the core of the Sun, are required instead.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A government energy panel is reviewing the future of nuclear power, comparing the practicalities of existing nuclear fission power stations with the long-term ambition of building commercially viable nuclear fusion power stations.

   (a)

   A government energy panel is comparing nuclear fission power stations with the long-term prospect of nuclear fusion power stations for large-scale electricity generation. Explain how a fission power station generates electricity from the energy released in the reactor core, and discuss why controlling a fission chain reaction safely requires the moderator, control rods and shielding described earlier.

   [6 marks]

   (b)

   Explain why nuclear fusion is often described as a potentially safer and cleaner long-term energy source than fission, but discuss why achieving controlled, sustained fusion for electricity generation on Earth remains extremely difficult.

   [6 marks]

   Total for question 4: 12 marks

End of questions