Nuclear Fusion

What is Nuclear Fusion?

Nuclear fusion is the process where two light atomic nuclei combine to form a heavier nucleus. This process releases a large amount of energy because the mass of the resulting nucleus is slightly less than the sum of the original masses. The lost mass is converted into energy according to Einstein6s equation $E = mc^2$. Fusion releases energy because the binding energy per nucleon increases, making the resulting nucleus more stable.

Fusion requires extremely high temperatures and pressures to occur, allowing the nuclei to overcome their natural electrostatic repulsion (both nuclei are positively charged). Fusion is the process that powers stars, including our Sun.

For example, in the Sun, hydrogen nuclei fuse to form helium, releasing energy that sustains the star6s brightness and heat.

Conditions for Fusion

Fusion only happens under very extreme conditions because positively charged nuclei repel each other strongly. To achieve fusion, the following conditions are necessary:

• Extremely high temperature: Temperatures of millions of degrees Celsius are needed to give nuclei enough kinetic energy to collide and fuse.
• High pressure: High pressure increases the chance of collisions between nuclei by forcing them close together.
• Overcoming electrostatic repulsion: The positively charged nuclei repel each other due to electrostatic forces. High temperature and pressure help overcome this repulsion.
• Plasma state: At these high temperatures, matter exists as plasma1 hot, ionised gas of nuclei and electrons free to move.

These conditions are naturally found in the cores of stars, where fusion occurs continuously.

Energy from Fusion

When fusion occurs, some of the mass of the original nuclei is converted into energy. This energy release is much greater than the energy input required to start the fusion reaction, making fusion a potentially powerful energy source.

The energy released by fusion is the source of the Sun6s heat and light. Scientists are researching how to harness fusion on Earth as a clean and almost limitless energy source, since fusion produces no greenhouse gases and very little radioactive waste compared to nuclear fission.

For example, the fusion of two isotopes of hydrogen, deuterium and tritium, produces helium, a neutron, and a large amount of energy:

$$\mathrm{^2_1H} + \mathrm{^3_1H} \rightarrow \mathrm{^4_2He} + \mathrm{^1_0n} + \text{energy}$$

This reaction is the basis for experimental fusion reactors like tokamaks.

Fusion in Stars

Stars generate energy by fusing hydrogen nuclei (protons) into helium nuclei in their cores. This fusion process releases enormous amounts of energy, which balances the gravitational force trying to collapse the star.

The main fusion process in stars like the Sun is called the proton-proton chain, where four hydrogen nuclei combine through several steps to form one helium nucleus, releasing energy in the form of gamma rays, neutrinos, and kinetic energy of particles. (A simplified diagram or description can help understand this process.)

This energy radiates outwards, providing the heat and light that stars emit and sustaining life on planets like Earth.

The balance between the outward pressure from fusion energy and the inward pull of gravity keeps the star stable for millions to billions of years.

Learning Example

Calculate the energy released when 0.001 kg of mass is converted into energy during fusion.

Using Einstein6s equation $E = mc^2$, where:

• $m = 0.001\, \text{kg}$
• $c = 3.0 \times 10^8\, \text{m/s}$ (speed of light)

Calculate:

$$E = 0.001 \times (3.0 \times 10^8)^2 = 0.001 \times 9.0 \times 10^{16} = 9.0 \times 10^{13} \text{ joules}$$

This shows a tiny amount of mass can release an enormous amount of energy.