📖 In-Depth Theory
What Is Nuclear Fission?
NUCLEAR FISSION is the splitting of a large, unstable nucleus into two smaller nuclei when it absorbs a NEUTRON.
FISSILABLE MATERIALS:
Uranium-235 (²³⁵U): most commonly used.
Plutonium-239 (²³⁹Pu): used in some reactors and weapons.
WHAT HAPPENS:
1. A slow (thermal) NEUTRON is absorbed by a ²³⁵U nucleus.
2. The nucleus becomes highly unstable (²³⁶U) and splits.
3. Produces TWO SMALLER NUCLEI (fission fragments) + 2–3 NEUTRONS + ENERGY.
EXAMPLE:
²³⁵U + n → ²³⁶U → ¹⁴¹Ba + ⁹²Kr + 3n + energy
ENERGY RELEASED:
The total mass of products is LESS than the total mass of reactants.
The 'missing mass' is converted to energy: E = mc².
This tiny mass difference produces an enormous amount of energy.
One fission reaction releases ~200 MeV — vastly more than any chemical reaction (few eV).
Chain Reactions
CHAIN REACTION:
The 2–3 neutrons released by each fission can trigger further fission events.
Those produce more neutrons → trigger more fissions → chain reaction.
CONTROLLED chain reaction (nuclear reactor): carefully managed — produces steady thermal energy.
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 Withdrawn → more fission.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.