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Why do fusion reactions between light nuclei release energy?

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laser-activated fusion — nuclear fusion Interior of the U.S. Department of Energy's National Ignition Facility (NIF), located at Lawrence Livermore National Laboratory, Livermore, California. The NIF target chamber uses a high-energy laser to heat fusion fuel to temperatures sufficient for thermonuclear ignition. The facility is used for basic science, fusion energy research, and nuclear weapons testing.

Fusion reactions between light nuclei release energy because the total mass of the resulting particles is less than the mass of the initial reactants. This difference in mass, known as the mass defect, is converted into energy according to Albert Einstein’s mass-energy equivalence equation, $E = m c^{2}$ .12

This process is fundamentally driven by binding energy, which measures the efficiency with which nucleons (protons and neutrons) are bound together within a nucleus. The binding energy per nucleon is relatively low for very light elements and reaches a maximum for nuclei with an atomic mass number near that of iron. When light nuclei fuse to form a heavier nucleus, the resulting product is more tightly bound than the original reactants. Consequently, the binding energy per nucleon increases, and the excess energy is liberated.32

While this process is exoergic, it requires overcoming the Coulomb barrier—the electrostatic repulsion between positively charged nuclei—which necessitates extremely high temperatures to achieve a significant reaction rate.4