Nuclear Fission and Fusion
This lesson explores the practical aspects of nuclear fission and fusion — how they work, how nuclear reactors are designed, the conditions required for fusion, and the role of nucleosynthesis in stars. This material is assessed in AQA sections 3.8.1 and 3.2.1.
Nuclear Fission
Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei (roughly equal in size), accompanied by the release of neutrons and a large amount of energy.
Induced Fission
Fission can be induced (triggered) by the absorption of a neutron. Uranium-235 is the most commonly used fissile material:
²³⁵₉₂U + ¹₀n → ⁹²₃₆Kr + ¹⁴¹₅₆Ba + 3¹₀n + energy (~200 MeV)
The incoming neutron must be a thermal (slow) neutron — one with kinetic energy of about 0.025 eV (corresponding to room temperature). Fast neutrons are much less likely to be captured by U-235.
Key features of fission:
- The products (fission fragments) vary — many different pairs of daughter nuclei are possible.
- Typically 2 or 3 neutrons are released per fission event.
- The energy is released mainly as kinetic energy of the fission fragments and neutrons.
- The fission fragments are usually radioactive (neutron-rich) and undergo further beta decays.
Chain Reactions
Each fission event releases 2–3 neutrons. If at least one of these neutrons goes on to cause another fission event, a chain reaction is sustained.
- If exactly 1 neutron per fission causes another fission, the reaction is critical — the rate is constant and controlled. This is the condition maintained in a nuclear power reactor.
- If more than 1 neutron per fission causes another fission, the reaction is supercritical — the rate increases exponentially. This is the condition in a nuclear weapon.
- If fewer than 1 neutron per fission causes another fission, the reaction is subcritical — the rate decreases and the reaction dies out.
Critical Mass
The critical mass is the minimum mass of fissile material needed to sustain a chain reaction. Below this mass, too many neutrons escape from the surface without causing further fissions. The critical mass depends on:
- The type of fissile material (U-235, Pu-239, etc.)
- The shape of the material (a sphere has the smallest surface-area-to-volume ratio)
- The purity/enrichment of the material
- Whether a neutron reflector is used
For pure U-235 (unreflected sphere), the critical mass is approximately 52 kg.
The Nuclear Fission Reactor
A nuclear fission reactor is designed to maintain a controlled, critical chain reaction. The key components are:
Fuel Rods
- Contain the fissile material, typically enriched uranium (3–5% U-235 in U-238) or sometimes plutonium-239.
- Natural uranium contains only 0.7% U-235 — enrichment increases this proportion.
- The fuel is formed into pellets and sealed in metal cladding (e.g., zirconium alloy) to contain the radioactive fission products.
Moderator
- Purpose: to slow down the fast neutrons released in fission to thermal energies, because U-235 has a much higher probability of capturing slow neutrons.
- Common moderators: graphite (carbon) or water (H₂O or heavy water D₂O).
- The moderator works by elastic collisions — neutrons bounce off moderator nuclei and transfer kinetic energy to them. The most effective moderation occurs when the moderator nuclei have a similar mass to the neutron (hence hydrogen/deuterium in water are very effective).
- The moderator must not absorb too many neutrons itself (otherwise the chain reaction cannot be sustained).
Control Rods
- Purpose: to absorb excess neutrons and control the rate of the chain reaction.
- Made from materials with a high neutron-absorption cross-section: boron or cadmium.
- Inserting the control rods further reduces the neutron population and slows the reaction.
- Withdrawing the control rods increases the neutron population and speeds up the reaction.
- In an emergency, the control rods are fully inserted (SCRAM) to shut down the reactor.
Coolant
- Purpose: to transfer thermal energy away from the reactor core to generate steam for electricity production.
- Common coolants: water, heavy water, CO₂ gas, or liquid sodium.
- In a pressurised water reactor (PWR), the same water acts as both moderator and coolant.
Exam Tip: The most common exam question on reactors asks you to explain the purpose of the moderator and control rods. Key points: the moderator slows neutrons (because U-235 is more likely to capture slow neutrons), and the control rods absorb neutrons to keep the reaction critical. Students often confuse these roles.
Nuclear Fusion
Nuclear fusion is the joining of two light nuclei to form a heavier nucleus, accompanied by a release of energy. Fusion is the energy source of stars.
The Fusion Process
²₁H + ³₁H → ⁴₂He + ¹₀n + 17.6 MeV