Fission and Fusion
4 marks · EasyThe diagram shows parts of a nuclear fission reactor and the process that occurs inside it. Nuclear fusion is also being researched as a future source of energy.
Describe the purpose of the moderator in the reactor.
State one reason why nuclear fusion is difficult to achieve in a reactor on Earth.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) The moderator slows down neutrons so they are more likely to cause further fission reactions. · must show slows neutrons, increases likelihood of fission
(b) Fusion is hard to achieve because very high temperatures and pressures are required to overcome the repulsion between nuclei. · must show requires high temperature and/or pressure, electrostatic repulsion
Must show: moderator slows neutrons, fusion requires high temperature/pressure
Show mark scheme and worked solution
Marking points
M1 (a) · 1 mark · States the moderator slows down neutrons.
A1 (a) · 1 mark · States that slow neutrons are more likely to cause fission reactions.
M2 (b) · 1 mark · States that high temperature or high pressure is needed for fusion.
A2 (b) · 1 mark · Mentions overcoming repulsion between nuclei (electrostatic repulsion/protons).
Worked solution
a) The moderator slows down fast neutrons in the reactor. Slow neutrons are more easily absorbed by uranium-235 nuclei, so this increases the likelihood of more fission reactions.
b) Fusion on Earth is difficult because the nuclei are both positively charged and repel each other. Very high temperatures and pressures are needed to give them enough energy to overcome this repulsion and collide.