Edexcel A Level (IAL) Physics-5.12 Nuclear Fusion- Study Notes- New Syllabus

Mechanism of Nuclear Fusion and the Need for Very High Temperature and Density

Nuclear fusion is the process in which two light nuclei combine to form a heavier nucleus, releasing energy. Although fusion releases enormous energy, it is extremely difficult to initiate and sustain because of strong electrostatic repulsion between nuclei.

 The Basic Fusion Process

In a typical fusion reaction (such as in stars):

• Two light nuclei (e.g. hydrogen isotopes) approach each other.
• If they get close enough, the strong nuclear force binds them together.
• A new nucleus forms with a higher binding energy per nucleon.
• The mass defect is converted into energy using \( \Delta E = c^2 \Delta m \).

Key condition: Nuclei must come within about \( 1\times10^{-15}\ \mathrm{m} \) for the strong nuclear force to act.

 Coulomb Repulsion (Electrostatic Barrier)

All nuclei are positively charged, so they repel each other due to electrostatic force.

• This repulsion is called the Coulomb barrier.
• The force increases as nuclei get closer.
• Fusion cannot occur unless this barrier is overcome.

Conclusion: Very high kinetic energy is required for nuclei to get close enough to fuse.

 Why Very High Temperature Is Required

Temperature is a measure of the average kinetic energy of particles:

\( \text{average kinetic energy} = \tfrac{3}{2} kT \)

• High temperature → nuclei move very fast.
• High speed increases chance of overcoming Coulomb repulsion.
• Fusion typically requires temperatures of the order of \( 10^7 \) to \( 10^8\ \mathrm{K} \).

Example: The core of the Sun has a temperature of about \( 1.5\times10^7\ \mathrm{K} \).

 Why Very High Density Is Required

High density increases the probability of fusion reactions.

• More nuclei per unit volume.
• More frequent collisions between nuclei.
• Greater chance that collisions occur with sufficient energy.

Without high density:

• Collisions are too infrequent.
• Fusion rate is too low to sustain energy output.

Maintaining Nuclear Fusion

For fusion to be self-sustaining:

• Energy released must maintain high temperature.
• Particles must remain confined long enough to collide.
• Energy losses must be minimised.

This condition is known as plasma confinement.

Methods:

• Gravitational confinement: stars (e.g. the Sun)
• Magnetic confinement: tokamaks and fusion reactors
• Inertial confinement: high-powered lasers

Why Fusion Occurs Naturally in Stars

• Enormous gravitational pressure creates extremely high density.
• Gravitational energy raises temperature to fusion levels.
• Fusion energy balances gravitational collapse.

This balance keeps stars stable over billions of years.