Fission and Fusion
7 marks · HardNuclear power stations rely on an understanding of both fission and fusion.
Outline how a chain reaction is started and controlled in a fission reactor, and explain why this is necessary for safe operation.
Explain why fusion is the main source of energy in stars but not in current nuclear power stations.
Give one reason why some scientists prefer the idea of fusion over fission as a future energy source 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) A chain reaction starts when neutrons from fission trigger further fission events; it is controlled by absorbing some neutrons with control rods to keep the reaction steady, preventing dangerous overheating or explosions. · must show Neutrons from fission start chain, Control rods absorb neutrons, Safety: prevent explosion/overheating
(b) Fusion powers stars because their temperature and pressure are high enough for nuclei to overcome repulsion and fuse; on Earth, those conditions are extremely hard to achieve, so we use fission. · must show Stars: high enough temperature/pressure for fusion, Earth: we use fission because of difficulty
(c) Fusion does not produce long-lived radioactive waste (or is safer/cleaner). · must show Fusion creates less radioactive waste
Common mistakes: Assuming fusion works at normal temperatures; Forgetting to mention control/safety in fission
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Marking points
B1 (a) · 1 mark · Correctly explains how neutrons from fission start the chain reaction.
B2 (a) · 1 mark · Explains that control rods absorb neutrons.
B3 (a) · 1 mark · Explains why controlling the reaction is necessary for safety.
B1 (b) · 1 mark · States fusion powers stars.
B2 (b) · 1 mark · Gives reason: high temperature and pressure in stars.
B3 (b) · 1 mark · States these conditions are difficult to reach on Earth, so we use fission.
B1 (c) · 1 mark · States one reason for preferring fusion (e.g., no radioactive waste).
Worked solution
a) In a fission reactor, a chain reaction is started when a neutron strikes a U-235 nucleus, causing it to split and release more neutrons. These neutrons can then cause other nuclei to split, sustaining the reaction. Control rods are inserted to absorb some of the neutrons, regulating the rate of reaction. Controlling the chain reaction is essential to avoid unsafe increases in temperature that could cause an explosion or meltdown.
b) Fusion is the main energy source in stars because their immense temperature and pressure allow hydrogen nuclei to fuse together. On Earth, achieving those conditions is technologically challenging; current nuclear power stations rely on fission, which is easier to control and sustain with present technology.
c) Fusion does not produce long-lived radioactive waste, so it is cleaner and potentially safer than fission.