UNCONTROLLED chain reaction (nuclear weapon): exponential growth — massive energy release almost instantaneously.
CRITICAL MASS:
Minimum mass of fissile material needed to sustain a chain reaction.
Below critical mass: too many neutrons escape — chain reaction dies out.
Above critical mass: chain reaction sustained or grows.
MODERATOR:
Neutrons from fission are FAST — must be SLOWED DOWN to be captured by U-235 nuclei.
Moderator (water or graphite) slows neutrons by elastic collisions.
Slow (thermal) neutrons are much more likely to cause fission.
Nuclear Reactors
A NUCLEAR REACTOR generates thermal energy from controlled fission, which is used to produce electricity.
KEY COMPONENTS:
FUEL RODS: contain enriched uranium (mostly U-238 with ~3–5% U-235).
MODERATOR (water or graphite): slows neutrons to useful speed.
CONTROL RODS (boron or cadmium): absorb neutrons. Inserted deeper → fewer neutrons → less fission. Withdrawn → more fission. Used to control power output.
COOLANT (water or CO₂): transfers thermal energy from reactor core to steam generator.
STEAM GENERATOR: hot coolant boils water → steam → drives turbine → generator → electricity.
PRESSURE VESSEL AND SHIELDING: contains radiation, maintains pressure.
ADVANTAGES of nuclear power:
Low CO₂ emissions during operation.
Very energy-dense fuel — small amount produces huge energy.
Reliable base-load power.
DISADVANTAGES:
High cost of building and decommissioning.
Nuclear waste: some isotopes remain radioactive for thousands of years — difficult storage.
Small risk of catastrophic accident (Chernobyl, Fukushima).
⚠️ Common Mistake
In nuclear fission, the NEUTRON is ABSORBED first, making the nucleus unstable — then it splits. Fission is NOT the same as radioactive decay. The energy comes from the conversion of a tiny amount of MASS into energy (mass defect). Control rods ABSORB neutrons — inserting them SLOWS or stops the reaction.
📐 Key Equations
E = mc² (mass-energy equivalence)
📌 Key Note
Fission: large nucleus absorbs neutron → splits → two smaller nuclei + 2–3 neutrons + energy. Chain reaction: neutrons trigger further fissions. Critical mass: minimum for sustained chain reaction. Reactor: fuel rods + moderator (slows neutrons) + control rods (absorb neutrons) + coolant. Energy from mass defect (E = mc²).
🎯 Matching Activity — Nuclear Reactor Components
Match each reactor component to its function. — drag the symbols on the right to match the component names on the left.
Fuel rods
Drop here
Moderator
Drop here
Control rods
Drop here
Coolant
Drop here
Transfers thermal energy from core to steam generator — water or CO₂
Slows fast neutrons to thermal speed — water or graphite
Absorb neutrons — inserted deeper to slow reaction, withdrawn to increase power
Contain enriched uranium-235 — fission occurs here releasing thermal energy
⭐ Higher Tier Only
Explain the chain reaction quantitatively — how one fission produces 2-3 neutrons. Explain critical mass. Describe the difference between controlled (reactor) and uncontrolled (weapon) chain reactions. Explain how each reactor component contributes to safe, controlled power production.
🔬 Triple Science Only
Nuclear fission (physics only) — not in Combined Science.
🎯 Test Yourself
Question 1 of 2
1. What is the purpose of control rods in a nuclear reactor?
2. Why is energy released in a nuclear fission reaction?
⭐ How Well Do You Understand This Topic?
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