6.4NuclearStretch

Nuclear Fission and Fusion

Splitting a heavy nucleus apart and joining two light nuclei together are opposite processes, yet both release enormous energy — because both move the resulting nuclei closer to the binding-energy-per-nucleon peak near iron.

45 min Video by Science Shorts Module 6.4 Nuclear and particle physics

Fission & Fusion — GCSE & A-level PhysicsWatch the full walkthrough before the notes below.

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What you'll be able to do

Explain nuclear fission in terms of the binding energy per nucleon curve
Describe induced fission and a chain reaction
Explain the role of moderator, control rods and critical mass
Explain nuclear fusion and why it needs extreme temperatures

Nuclear fission

A heavy nucleus splits into two smaller daughter nuclei, releasing energy since the products have higher binding energy per nucleon. Induced fission of uranium-235 by a slow neutron releases 2–3 further neutrons, enabling a chain reaction.

Controlling a chain reaction

A moderator (e.g. graphite, water) slows fast neutrons to increase fission probability; control rods (e.g. boron) absorb excess neutrons to regulate the reaction rate. Critical mass is the minimum fissile mass needed to sustain the chain reaction.

Tip — Moderator SLOWS neutrons down; control rods TAKE neutrons OUT — different jobs, easy to confuse.

Nuclear fusion

Fusion joins light nuclei into a heavier one, moving toward the stability peak from the other side, releasing more energy per unit mass than fission. It requires extreme temperatures to overcome electrostatic (Coulomb) repulsion before the short-range strong force can bind the nuclei together.

Tip — The Coulomb barrier, not the strong force itself, is what makes fusion difficult — the strong force is what makes it release energy once nuclei are close enough.

Common mistakes to avoid

Thinking fission and fusion release energy for unrelated reasons.

Both release energy because products have higher binding energy per nucleon, moving toward the stability peak.

Confusing the roles of a moderator and control rods.

A moderator slows neutrons; control rods absorb them to regulate rate.

Key takeaways

Fission and fusion both release energy by moving nuclei toward the binding-energy-per-nucleon peak near iron.
Induced fission releases 2–3 neutrons per event, enabling a chain reaction if critical mass is reached.
A moderator slows neutrons; control rods absorb neutrons to regulate reaction rate.
Fusion needs extreme temperatures to overcome the Coulomb barrier between nuclei.

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