Final answer:
Nuclear fusion releases energy due to the mass-to-energy conversion of binding energy as light nuclei combine to form a heavier nucleus with higher binding energy, releasing heat, light, and radiation.
Step-by-step explanation:
Thermonuclear reactions, such as nuclear fusion, release energy because the resulting product or 'ash' from the reaction has less mass than the combined mass of the reactants or 'fuel'. This mass difference is due to the binding energy released during the reaction. According to the principle of mass-energy equivalence, as stated in Einstein's famous equation E=mc², this missing mass is converted into energy. In fusion reactions, when light nuclei combine to form a heavier nucleus, the binding energy per nucleon increases, which means less mass per nucleon. This mass defect results in the release of substantial energy, which manifests as heat, light, and radiation.
The energy released in fusion is more significant than in fission reactions and orders of magnitude greater than that in chemical reactions. During a fusion reaction, large amounts of energy are needed initially to overcome the Coulomb barrier - the repulsion between positively charged nuclei. Once overcome, the fusion of low-mass nuclei forms a larger nucleus with higher binding energy, releasing a tremendous amount of energy in the process, which can be harnessed as heat energy.