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When heavier nuclei split apart into lighter ones, the process is known as nuclear fission. Spontaneous fission is rare in naturally occurring isotopes, the most common case being uranium-235. The binding energy of the protons and neutrons (nucleons) in a nucleus decreases with increasing size for elements heavier than iron, so the fission reaction for heavy elements like uranium releases a significant quantity of energy that can be used for power purposes.
Fission-based nuclear power reactors supply about 16% of the world's electrical energy at present. Almost all of this uses a mixture of uranium isotopes; in recent years the price of uranium has increased sharply due to temporary limits in supply and continued high demand. Among the byproducts of fission power are highly radioactive plutonium, other heavy elements, and the lighter fission breakdown products. Reactor waste is viewed as a danger to public safety, although the total volume is limited thanks to the large quantity of energy supplied from relatively small quantities of uranium. Plutonium and enriched uranium in reactors also hold dangers for production of nuclear weapons. For both these safety-related reasons, public opposition to nuclear power has become strong in some countries, limiting further growth of the technology.