In a nuclear reactor, the process that provides power is controlled fission. This involves the splitting of heavy atomic nuclei, such as uranium-235 or plutonium-239, into smaller nuclei. During this process, a significant amount of energy is released in the form of heat. This heat is then used to produce steam, which drives turbines to generate electricity. Controlled fission is essential because it allows for a steady release of energy, as opposed to uncontrolled fission, which can lead to catastrophic explosions. Understanding the difference between fission and fusion is crucial; while fission splits heavy nuclei, fusion combines light nuclei (like hydrogen) to form heavier ones, releasing energy in the process, but it is not the reaction used in current nuclear reactors.