What is the nuclear reaction called where a massive nucleus splits into smaller nuclei with energy release?

The process described in the question is known as nuclear fission. This is a reaction in which a heavy nucleus, such as uranium-235 or plutonium-239, splits into two or more lighter nuclei, along with the release of a significant amount of energy. This energy release occurs because the binding energy per nucleon increases when heavier nuclei split into smaller, more stable products.

In nuclear fission, the nucleus may also release neutrons, which can initiate further fission reactions in a chain reaction, making it a crucial process in nuclear reactors and atomic bombs. The energy produced from fission is harnessed in various applications, particularly in electricity generation in nuclear power plants.

Understanding nuclear fission helps clarify that it is distinct from other processes like nuclear fusion, which involves combining light nuclei to form a heavier nucleus, or radioactive decay, where unstable nuclei release radiation without splitting. The concept of half-life also relates to radioactive decay and not to the splitting of nuclei.