Final answer:
Nuclear fission reactions occur when a uranium nucleus breaks up into fragments, releasing energy in a chain reaction.
Explanation:
Nuclear fission reactions occur when a uranium nucleus breaks up into fragments. This process involves splitting the nucleus of an atom and releasing energy, as seen in the fission chain reaction of Uranium-235. Fission reactions are induced by bombarding radioactive elements with neutrons, leading to the formation of lighter elements and additional neutrons. The chain reaction continues as the free neutrons produced bombard other fissionable elements, resulting in a rapid increase in nuclear reactions.
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