Nuclear Fusion
Overview
Nuclear Fusion is the joining of two light nuclei to form a heavier nucleus. This process releases large amounts of energy and powers the stars, including the Sun.
Fusion is attractive as an energy source because it can provide:
- very high energy output
- abundant fuel sources
- lower long-lived radioactive waste than fission
This topic links closely with:
Core Ideas
- fusion joins light nuclei into a heavier nucleus
- energy is released because the products have higher binding energy per nucleon
- very high temperature is needed to overcome electrostatic repulsion
- fusion fuel becomes plasma at the required temperature
- controlled fusion requires confinement long enough for useful reactions to occur
What Is Fusion?
Fusion occurs when two light nuclei combine to form a heavier nucleus.
General features:
- nuclei must come very close together
- extremely high temperature is usually needed
- mass decreases after the reaction
- energy is released
Fusion differs from fission:
- fusion joins light nuclei
- fission splits heavy nuclei
Typical Example: Hydrogen Isotopes
A common fusion reaction is:
where:
- = deuterium
- = tritium
Products of Fusion
Fusion commonly produces:
- a heavier nucleus
- one or more particles, often neutrons
- gamma radiation in some reactions
- large kinetic energy
In the deuterium-tritium reaction, the products are:
- helium-4 nucleus
- neutron
- released energy
Why Energy Is Released
1. Mass Defect
The total mass after the reaction is smaller than before the reaction.
The missing mass becomes energy:
2. Higher Binding Energy per Nucleon
Light nuclei have lower binding energy per nucleon than medium-mass nuclei.
After fusion:
- the product nucleus is more tightly bound
- total binding energy increases
- energy is released
See Mass Defect and Binding Energy.
Why High Temperature Is Needed
Nuclei are positively charged.
They repel each other through electrostatic force.
To get close enough for the strong nuclear force to act, nuclei need:
- very high speeds
- very high kinetic energy
- therefore very high temperature
Typical fusion temperatures are many millions of kelvin.
Coulomb Repulsion Overview
Two positive nuclei repel each other before touching.
This repulsive barrier is often called the Coulomb barrier.
Fusion occurs only if nuclei approach closely enough for the strong nuclear force to dominate at short range.
Confinement Overview
At fusion temperatures, matter becomes ionised gas, that is plasma.
Hot plasma tends to expand and escape.
Therefore it must be confined long enough for significant fusion to occur.
Methods include:
- gravitational confinement in stars
- magnetic confinement
- inertial confinement
See Fusion Conditions and Confinement.
Fusion in Stars
Stars are powered by fusion in their cores.
Conditions there include:
- extremely high temperature
- enormous pressure
- high density
Gravity compresses matter, helping maintain fusion conditions.
The Sun mainly converts hydrogen into helium through a sequence of fusion reactions.
Thermonuclear Reactor Overview
A fusion reactor aims to reproduce stellar fusion on Earth in a controlled way.
General idea:
- heat fuel to plasma state
- confine the plasma
- allow fusion reactions to occur
- collect the released energy
- generate electricity
The main challenge is achieving reliable net energy gain.
Advantages of Nuclear Fusion
- very high energy output per mass
- fuel sources such as deuterium are abundant
- no carbon dioxide emitted during operation
- lower risk of runaway chain reaction than fission
- less long-lived high-level waste than many fission systems
Disadvantages / Challenges
- extremely high temperature required
- difficult plasma confinement
- expensive technology
- materials may be damaged by high-energy neutrons
- not yet widely commercialised
Safety and Waste Comparison with Fission
Compared with Fission
Fusion generally has:
- no self-sustaining neutron chain reaction of the fission type
- lower long-lived waste
- lower meltdown-style risk
However
Fusion can still involve:
- neutron radiation
- activated reactor materials
- engineering hazards
See Ionizing Radiation and Safety.
Short Worked Examples
Example 1: Why High Temperature Is Needed
Answer:
Nuclei need very high kinetic energy so that some collisions bring them close enough together for fusion to occur despite strong electrostatic repulsion.
Example 2: Why Energy Is Released
Answer:
The products have higher binding energy per nucleon, so mass decreases and energy is released.
Example 3: Why Stars Sustain Fusion
Answer:
Gravity provides strong compression, producing high temperature and pressure.
Exam Relevance
Students should be able to:
- describe a typical fusion reaction using hydrogen isotopes
- explain energy release using mass defect and binding energy
- explain why high temperature is needed
- compare fusion with fission
- describe confinement ideas qualitatively and explain why controlled fusion is difficult
Formula / Relationship Summary
Mass-Energy
Fusion Example
Stability Idea
Light nuclei move toward a higher binding-energy-per-nucleon region after fusion.
Common Exam Traps Overview
Students often confuse:
- fusion with fission
- energy released simply because nuclei join
- stars burning chemically
- fusion producing no radiation
- confinement being easy once fuel is hot
- fission reactor components applying directly to fusion
See Nuclear Fusion Common Exam Traps.
Quick Revision Summary
- fusion joins light nuclei into heavier nuclei
- high temperature is needed to overcome repulsion
- energy comes from mass defect and increased binding energy
- stars are powered by fusion
- controlled fusion requires plasma confinement
- fusion offers major potential but remains technically challenging