A scientist states: 'Nuclear fusion releases more energy per kilogram of fuel than nuclear fission, yet fusion is not currently used in power stations.' Evaluate this statement by explaining the advantages of fusion over fission and why fusion has not yet been used as a practical energy source.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (5 marks)
1. Fusion releases more energy per kilogram of fuel than fission – the energy released by fusing two light nuclei (e.g. D+T) is about 4 × 10^14 J kg^−1, roughly 10 times that of fission of uranium.
2. Fusion fuels such as deuterium and tritium are abundant – deuterium is found in seawater and tritium can be bred from lithium – whereas uranium is a rare element.
3. Fusion produces no long‑lived radioactive waste – the main products are helium and neutrons, unlike fission which creates long‑lived fission products and actinides.
4. Fusion requires extremely high temperatures (≈10^8 K) to overcome the Coulomb barrier between nuclei.
5. No material can contain plasma at such temperatures – magnetic or inertial confinement must be used, and it is difficult to sustain the reaction long enough to obtain a net positive energy output.
2. Fusion fuels such as deuterium and tritium are abundant – deuterium is found in seawater and tritium can be bred from lithium – whereas uranium is a rare element.
3. Fusion produces no long‑lived radioactive waste – the main products are helium and neutrons, unlike fission which creates long‑lived fission products and actinides.
4. Fusion requires extremely high temperatures (≈10^8 K) to overcome the Coulomb barrier between nuclei.
5. No material can contain plasma at such temperatures – magnetic or inertial confinement must be used, and it is difficult to sustain the reaction long enough to obtain a net positive energy output.
Examiner tips
- Use the exact terminology (e.g. ‘Coulomb barrier’, ‘net energy output’).
- Show the comparison of energy per kg to justify the first point.
- Explain both the abundance of fusion fuels and the lack of long‑lived waste.
- Mention the temperature requirement and containment difficulty.
Common mistakes
- Confusing the energy per kg of fusion with the total energy of a reactor.
- Claiming fusion produces no waste at all, ignoring neutron activation of reactor materials.
- Overlooking the need for sustained confinement to achieve net energy.
Mark scheme (5 marks)
- Nuclear fusion releases more energy per kilogram of fuel than fission (accept: fusion has a greater energy output per unit mass)
- Fusion fuels (such as hydrogen/deuterium/tritium) are more readily available / less rare than uranium used in fission
- Fusion does not produce long-lived radioactive waste (unlike fission)
- Fusion requires extremely high temperatures (millions of degrees) to overcome the repulsion between nuclei
- No material can contain plasma at such high temperatures / it is difficult to contain the reaction for long enough to produce a net energy output
Key terms in this question
nuclear fusion · nuclear fission
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