Nuclear reactions as a source of energy

4 marks · Hard

Nuclear fusion is the process that powers stars, while nuclear fission is used in reactor power plants on Earth.

Compare and explain, in detail, why it is much more difficult to start and sustain nuclear fusion reactions on Earth than nuclear fission. Your answer should refer to the forces involved and the conditions needed.

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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Fusion requires two positively charged nuclei to come very close together to overcome electrostatic repulsion. At low temperatures and pressures, this repulsion prevents fusion. Very high temperatures and pressures (like in stars) are needed for nuclei to collide with enough energy for fusion. Fission, by contrast, requires only that a heavy nucleus absorb a neutron, which is not repelled, so it can happen at much lower temperatures and pressures.

Also accept: Fusion needs to overcome the Coulomb barrier. That needs much higher kinetic energies than possible on Earth without special technology.

Common mistakes: Incorrectly stating that fusion happens easily at room temperature; Not mentioning electrostatic forces or charge of nuclei

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Marking points

M1 · 1 mark · Recognises that nuclei in fusion are both positively charged and repel each other (Coulomb/barrier).

M2 · 1 mark · States that very high temperature and/or pressure is needed to overcome this repulsion in fusion.

M3 · 1 mark · Explains that fission occurs by absorption of a neutron, which is not repelled by the nucleus, so does not require such extreme conditions.

A1 · 1 mark · Clear comparison of fusion and fission in terms of forces involved and conditions required.

Worked solution

For fusion, two nuclei (e.g. hydrogen nuclei in the Sun) both carry a positive charge and approach each other. As they get close, they experience a strong electrostatic (Coulomb) repulsion. To overcome this force so they can combine (fuse), the nuclei must have extremely high kinetic energies — which happens at the very high temperatures and pressures in the core of a star.

On Earth, such temperatures and pressures are very difficult to create and sustain, so fusion is hard to initiate and maintain here. Fission, on the other hand, simply needs a neutron to be absorbed by a large nucleus like uranium-235. Neutrons are not repelled by the positively charged nucleus, so this process can occur at much lower temperatures and standard pressures.