Practise Nuclear fusion. 11 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
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Where does nuclear fusion happen naturally?
In stars, such as the Sun
The equation shows a fusion reaction that scientists hope to use in future power stations. \({}^{2}_{1}\mathrm{H} + {}^{3}_{1}\mathrm{H} \rightarrow {}^{4}_{2}\mathrm{He} + {}^{a}_{b}\mathrm{X}\) Determine the values of a and b, and identify particle X.
a = 1, b = 0: X is a neutron
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) What is nuclear fusion? Tick (✓) one box.[1]
The joining of two light nuclei to form a heavier nucleus
The splitting of a heavy nucleus into two lighter nuclei
The emission of radiation from an unstable nucleus
The absorption of a neutron by a nucleus
(b) Where does nuclear fusion happen naturally?[1]
(c) In a fusion reaction, some of the mass of the nuclei is converted into another form. What is it converted into?[1]
(d) Give one similarity between nuclear fission and nuclear fusion.[1]
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(a)Answer: The joining of two light nuclei to form a heavier nucleus
(b)Answer: In stars, such as the Sun
in stars / in the Sun
(c)Answer: Energy (radiation)
energy (of radiation)
(d)Answer: Both release energy.
both release energy
both are nuclear reactions / change the nuclei of atoms
Question 2Medium5 marks
(a) Give two differences between nuclear fission and nuclear fusion.[2]
(b) Explain why very high temperatures are needed for fusion to happen.[3]
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(a)Answer: Fission splits a heavy nucleus; fusion joins light nuclei. Fission is started by neutrons; fusion needs extremely high temperatures.
fission splits a large (heavy) nucleus, but fusion joins two small (light) nuclei
fission is usually started by a neutron, but fusion needs very high temperatures (and pressures)
fission is used in power stations now, but fusion power stations are still being developed
fission products are radioactive (long-lived waste), but fusion produces little long-lived radioactive waste
fusion is the energy source of stars, but fission is not
(b)Answer: The positive nuclei repel each other; only at extreme temperatures do they move fast enough to get close enough to fuse.
nuclei are positively charged, so they repel each other
at very high temperatures the nuclei move very fast / have a lot of kinetic energy
so they can get close enough (overcoming the repulsion) to join together
Question 3Hard6 marks
Scientists are trying to build power stations that use nuclear fusion. The fuel would be hydrogen-2, which can be extracted from sea water, and hydrogen-3, which can be made inside the reactor. The fuel must be heated to about 150 million °C.
(a) Suggest why the fuel cannot be held in an ordinary container.[2]
(b) Evaluate the use of fusion for generating electricity in the future, compared with fission.[4]
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(a)Answer: No material could withstand the temperature, and contact with walls would cool the fuel so fusion would stop.
any material would melt / vaporise at such a high temperature
the fuel would transfer energy to the walls and cool down, so fusion would stop
(b)Answer: Fusion has plentiful fuel, little long-lived waste and no runaway chain reaction, but it is not yet practical: current reactors use more energy than they release and are very hard to build. It could be better in the long term if these problems are solved.
advantage: the fuel is plentiful / cheap (hydrogen-2 from sea water)
advantage: much less long-lived radioactive waste (the helium produced is not radioactive)
advantage: no risk of a runaway chain reaction
advantage: no carbon dioxide is released (like fission)
disadvantage: no fusion reactor yet produces more energy than it uses / it is not yet available commercially
disadvantage: the very high temperatures make fusion reactors very difficult and expensive to build
a conclusion, e.g. fusion could be a better long-term energy resource if the technical problems are solved