What is nuclear fission?

Nuclear fission is defined as the process where a nucleus splits into smaller parts, typically resulting in the release of a considerable amount of energy. This reaction occurs when a heavy nucleus, such as that of uranium-235 or plutonium-239, absorbs a neutron and becomes unstable. As this unstable nucleus splits, it forms two or more smaller nuclei, known as fission products, along with additional neutrons and a significant release of energy. This energy release is what makes fission suitable for use in nuclear reactors and atomic bombs.

The other choices touch on different concepts in nuclear physics. The first option describes nuclear fusion, which involves combining light nuclei to form a heavier nucleus. The third option is related to gamma radiation, which is a type of high-energy radiation emitted by radioactive substances, but it is not a process itself like fission. The fourth choice refers to thermal decay, which is not a standard term in nuclear physics and does not accurately describe nuclear fission. Overall, the definition provided in the second option captures the essence of the fission process and its role in nuclear reactions.