Final answer:
Nuclear fission is the process of splitting a heavy atomic nucleus into two smaller nuclei, releasing energy and additional neutrons, which can cause a chain reaction. This process is used to generate electricity in nuclear power plants or can occur uncontrollably in nuclear weapons.
Step-by-step explanation:
Nuclear fission is a reaction in which the nucleus of an atom, typically a heavy nucleus such as uranium (U-235), is split into two smaller nuclei of nearly equal mass. This process releases a significant amount of energy due to the conversion of a small amount of mass into energy, according to Einstein's Mass-energy equivalence (E=mc^2). Furthermore, nuclear fission is accompanied by the release of additional neutrons, which can initiate more fission reactions, potentially creating a chain reaction if conditions allow. In a controlled environment, such as a nuclear power plant, this chain reaction is harnessed to generate electricity. However, uncontrolled nuclear fission is the principle behind nuclear weapons. It's important to note that a critical mass of the fissile material is required to sustain the reaction.