Nuclear fusion occurs when light nuclei (such as hydrogen isotopes) combine to form a heavier nucleus. This releases energy because the product nucleus is more tightly bound.
Fusion requires extremely high temperatures (millions of degrees) to overcome the electrostatic repulsion between positively charged nuclei. This is why fusion occurs naturally in stars.
Fusion releases more energy per kilogram than fission and produces no long-lived radioactive waste, but sustaining controlled fusion on Earth remains a major engineering challenge.
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