Modern nuclear missiles, particularly strategic ones, are often multi-stage devices that can utilize both nuclear fission and nuclear fusion to achieve higher yields. Nuclear fission involves the splitting of heavy atomic nuclei (like uranium or plutonium) into lighter ones, releasing a large amount of energy. This is the principle behind the first stage of most nuclear weapons. Nuclear fusion involves the merging of light atomic nuclei (like isotopes of hydrogen) into heavier ones under extreme temperature and pressure, releasing even more energy than fission. This is often used to boost the yield of a fission bomb. Radioactive disintegration is a spontaneous process where unstable atomic nuclei lose energy by emitting radiation. While radioactive materials are involved in nuclear weapons (as fissile material), the primary energy release mechanism in a missile is not simply radioactive disintegration but controlled nuclear reactions.