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Nuclear fusion
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<p>Nuclear fusion is a <a href="/facts/Nuclear_reaction/202yClUr">reaction</a> in which two or more <a href="/facts/Atomic_nuclei/orpIm5Xl">atomic nuclei</a> combine to form a larger nuclei, nuclei/<a href="/facts/Neutrons/oRCM1geb">neutron</a> by-products. The difference in mass between the reactants and products is manifested as either the release or <a href="/facts/Absorption_(electromagnetic_radiation)/xBYKt7yS">absorption</a> of <a href="/facts/Energy/wd8PrQM7">energy</a>. This difference in mass arises as a result of the difference in <a href="/facts/Nuclear_binding_energy/PU8bREHW">nuclear binding energy</a> between the atomic nuclei before and after the fusion reaction. Nuclear fusion is the process that powers all active <a href="/facts/Stars/PFbd6ICx">stars</a>, via many <a href="/facts/Stellar_nucleosynthesis/qEQJdJe6">reaction pathways</a>. </p><p>Fusion processes require an extremely large <a href="/facts/Lawson_criterion/aPUrM8Tp">triple product</a> of temperature, density, and confinement time. These conditions occur only in <a href="/facts/Stellar_core/ExqHCYRh">stellar cores</a>, advanced <a href="/facts/Nuclear_weapon_design/gw0LEc74">nuclear weapons</a>, and are approached in <a href="/facts/List_of_fusion_experiments/yoVA6oc3">fusion power experiments</a>. </p><p>A nuclear fusion process that produces atomic nuclei lighter than <a href="/facts/Nickel-62/ePeXd1y1">nickel-62</a> is generally <a href="/facts/Exothermic/gCuSvpa4">exothermic</a>, due to the positive gradient of the <a href="/facts/Nuclear_binding_energy/PU8bREHW">nuclear binding energy curve</a>. The most fusible nuclei are among the lightest, especially <a href="/facts/Deuterium/Bv4wfCkw">deuterium</a>, <a href="/facts/Tritium/HWSa6EHy">tritium</a>, and <a href="/facts/Helium-3/SmPC5oT3">helium-3</a>. The opposite process, <a href="/facts/Nuclear_fission/LfkIylvm">nuclear fission</a>, is most energetic for very heavy nuclei, especially the <a href="/facts/Actinides/kHC4lxJV">actinides</a>. </p><p>Applications of fusion include <a href="/facts/Fusion_power/jyETSzm4">fusion power</a>, <a href="/facts/Thermonuclear_weapons/T9zFe3H0">thermonuclear weapons</a>, <a href="/facts/Boosted_fission_weapon/U11GQ2mf">boosted fission weapons</a>, <a href="/facts/Neutron_source/4BTQAu9l">neutron sources</a>, and <a href="/facts/Superheavy_element/9PjYvCc3">superheavy element</a> production. </p>