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 Withdrawn → more fission.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
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Coolant
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Absorb neutrons — inserted deeper to slow reaction, withdrawn to increase power
Transfers thermal energy from core to steam generator — water or CO₂
Slows fast neutrons to thermal speed — water or graphite
Contain enriched uranium-235 — fission occurs here releasing thermal energy
🔬 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?
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