How do fission and fusion release energy, and how do they differ?
Compare nuclear fission and fusion and explain the energy released using binding energy
A focused answer to the WACE Year 12 Physics Unit 4 content point on fission and fusion. How splitting heavy nuclei and joining light nuclei both move toward the binding-energy peak, chain reactions, the conditions fusion requires, and where each occurs.
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What this dot point is asking
WACE wants you to compare fission and fusion, explain the energy release through binding energy, and describe chain reactions and fusion conditions. Both are applications of the binding-energy curve.
Nuclear fission
In fission a heavy nucleus absorbs a neutron, becomes unstable and splits into two medium-sized daughter nuclei, releasing two or three neutrons and a large amount of energy. A typical example is
The fragments lie nearer the binding-energy peak than uranium, so they are more tightly bound and the mass defect emerges as energy.
Chain reactions
The neutrons released by one fission can be absorbed by other nuclei, causing further fissions. If on average more than one neutron from each fission triggers another, the reaction grows: this is a chain reaction. A reactor uses control rods to absorb surplus neutrons and a moderator to slow them, keeping the rate steady at one new fission per fission (criticality).
Nuclear fusion
In fusion, light nuclei combine to form a heavier nucleus, for example deuterium and tritium fusing to helium:
The helium nucleus is far more tightly bound per nucleon than the hydrogen isotopes, so fusion releases more energy per nucleon than fission. Fusion powers the Sun and stars.
Why fusion is hard on Earth
Fusing nuclei must approach closely enough for the short-range strong force to bind them, but both are positively charged and repel strongly. Overcoming this electrostatic barrier needs extremely high temperatures and pressures, conditions found in stellar cores but difficult to sustain in a controlled reactor. This is why fission is currently used in power stations while controlled fusion remains a research challenge.
Explaining the energy source
In any answer, anchor the energy release to the binding-energy curve: both processes move nuclei toward the iron peak, increasing binding energy per nucleon, so the products have less mass than the reactants and the difference is released as energy. Quote as the conversion.