Explain, in terms of binding energy, why energy can be released when two nuclei undergo nuclear fusion. - AQA - A-Level Physics - Question 6 - 2021 - Paper 2

The energy released during nuclear fusion is primarily due to the increase in binding energy when two lighter nuclei fuse to form a heavier nucleus.

When two nuclei come together to form a single nucleus, the mass of the resulting nucleus is less than the sum of the original masses. This difference in mass (known as the mass defect) is converted into energy according to Einstein's equation, $E = riangle mc^2$, where $riangle m$ is the mass defect and $c$ is the speed of light.

Since the binding energy per nucleon is higher in the resulting nucleus compared to the individual ligther nuclei, this release of energy occurs as the system moves towards a more stable configuration.