Final answer:
The process used in nuclear reactors that splits heavy nuclei into lighter ones to release energy is known as nuclear fission.
Step-by-step explanation:
The mechanism of energy generation used in nuclear reactors on Earth, in which heavy nuclei are split into lighter ones, releasing energy in the process is nuclear fission. In nuclear fission, large, complex nuclei such as Uranium-235 or Plutonium-239, are bombarded with neutrons, which causes them to split into smaller nuclei and other particles like more neutrons and gamma radiation. This splitting releases a significant amount of energy due to the conversion of some mass into energy, as per Einstein's theory of relativity (E=mc2). This process is different from nuclear fusion, which is the combining of light atomic nuclei to form heavier ones, and is the process that powers the sun and other stars. Fission can also occur spontaneously through natural radioactivity in unstable nuclei.