sentences:  It has also become clear that the ultimate source of most terrestrial energy is nuclear, either through radiation from the Sun caused by stellar thermonuclear reactions or by radioactive decay of uranium within the Earth, the principal source of geothermal energy.In natural nuclear radiation, the byproducts are very small compared to the nuclei from which they originate.Nuclear fission is the process of splitting a nucleus into roughly equal parts, and releasing energy and neutrons in the process.