Nuclear Fission
Overview
Nuclear Fission is the splitting of a heavy nucleus into two smaller nuclei, usually after absorbing a neutron.
This process releases:
- large amounts of energy
- additional neutrons
- ionising radiation
It is the basis of:
- nuclear reactors
- nuclear power generation
- nuclear weapons in the uncontrolled case
This topic links closely with:
Core Ideas
- a heavy nucleus may split after absorbing a neutron
- fission products are usually medium-mass nuclei plus neutrons and energy
- energy is released because the products have higher binding energy per nucleon
- the emitted neutrons may trigger a chain reaction
- reactors control the chain reaction using moderator, control rods, coolant, and shielding
What Is Fission?
A heavy unstable nucleus splits into two medium-mass nuclei.
General features:
- triggered by neutron absorption
- daughter nuclei are often radioactive
- several neutrons are emitted
- energy is released mainly as kinetic energy of the products
Typical Example: Uranium-235
A common fission reaction is:
Many different product combinations are possible.
Key Ideas
- a neutron is absorbed first
- an unstable uranium nucleus forms briefly
- the nucleus deforms and splits
- extra neutrons are emitted
Neutron-Induced Fission
Neutrons are useful because:
- they carry no electric charge
- they are not repelled by the positive nucleus
- they can enter the nucleus more easily than charged particles
Thermal, that is very slow, neutrons are especially effective for some fuels such as uranium-235.
Products of Fission
Fission usually produces:
- two medium-mass nuclei
- about 2 to 3 neutrons
- gamma radiation
- thermal energy
The daughter nuclei are often radioactive and may decay further.
Why Energy Is Released
1. Mass Defect
The total mass after the reaction is less than the total mass before the reaction.
The missing mass becomes energy:
2. Higher Binding Energy per Nucleon
Medium-mass nuclei are more tightly bound than very heavy nuclei.
So after splitting:
- total binding energy increases
- energy is released
See Mass Defect and Binding Energy.
Chain Reaction Overview
The neutrons released from one fission event may trigger more fission events.
This creates a chain reaction.
Example:
- 1 fission releases 3 neutrons
- some of those cause more fissions
- the reaction can multiply rapidly
Conditions for Sustaining Chain Reaction
A sustained chain reaction requires enough neutrons to continue causing new fissions.
Important factors include:
- sufficient fuel mass
- suitable geometry
- neutron losses not too large
- neutrons slowed where necessary
If too many neutrons escape or are lost, the reaction dies out.
Controlled vs Uncontrolled Fission
Controlled Fission
This occurs in nuclear reactors.
- the rate is kept steady
- energy is used gradually
- control rods regulate the neutron population
Uncontrolled Fission
This occurs in nuclear weapons.
- reactions multiply very rapidly
- enormous energy is released in a short time
Nuclear Reactor Overview
A reactor uses controlled chain reactions to generate heat, which is then used to produce electricity.
Main sequence:
- fission releases energy
- coolant carries heat away
- steam is produced directly or indirectly from the transferred heat
- steam drives a turbine
- the turbine drives a generator
Reactor Components and Roles
Fuel
Contains fissile material such as:
- uranium-235
- plutonium-239
It provides nuclei for fission.
Moderator
It slows fast neutrons so they are more likely to cause further fission.
Examples:
- water
- heavy water
- graphite
Control Rods
They absorb neutrons to regulate the reaction rate.
Materials may include:
- boron
- cadmium
Coolant
It transfers heat from the reactor core.
Examples:
- water
- gas
- liquid metal
Shielding
It protects the surroundings from radiation.
It is usually made using thick concrete and steel.
Advantages of Nuclear Fission
- very large energy output from small fuel mass
- low direct carbon dioxide emission during operation
- reliable base-load electricity
- small fuel transport volume
Disadvantages of Nuclear Fission
- radioactive waste disposal
- accident risk
- high construction cost
- security concerns
- possible thermal pollution
Safety and Waste Overview
Safety Systems
- shielding
- emergency shutdown systems
- cooling systems
- containment structures
Waste
Spent fuel remains radioactive for long periods and requires secure storage.
See Ionizing Radiation and Safety.
Short Worked Examples
Example 1: Why Neutrons Are Used
Question: Why use neutrons rather than protons?
Answer:
- neutrons are uncharged
- there is no electrostatic repulsion by the nucleus
Example 2: Why Energy Is Released
Question: Why does fission release energy?
Answer:
The products have higher binding energy per nucleon, so mass decreases and energy is released.
Example 3: Why Control Rods Are Needed
Answer:
They absorb excess neutrons and keep the reaction rate steady.
Exam Relevance
Students should be able to:
- describe a typical neutron-induced fission reaction
- explain energy release using binding energy and mass defect
- distinguish controlled from uncontrolled chain reactions
- state the qualitative functions of moderator, control rods, coolant, and shielding
- explain why not all emitted neutrons continue the chain reaction
Formula / Relationship Summary
Mass-Energy
Fission Logic
Heavy nucleus + neutron smaller nuclei + neutrons + energy
Stability Idea
Products are nearer the peak binding-energy-per-nucleon region.
Common Exam Traps Overview
Students often confuse:
- fission with radioactive decay
- moderator with control rods
- coolant with shielding
- energy released simply because a nucleus splits
- all neutrons always causing more fission
- controlled reactor operation with a bomb reaction
See Nuclear Fission Common Exam Traps.
Quick Revision Summary
- fission splits heavy nuclei into medium-mass nuclei
- it is usually initiated by neutron absorption
- energy is released through mass defect and increased binding energy
- extra neutrons can enable chain reactions
- reactors use moderator, control rods, coolant, and shielding
- controlled fission can generate electricity