A release of nuclear energy occurs when the nuclei of atoms are changed. Hydrogen and uranium are two kinds of matter used to produce nuclear energy. In a nuclear reaction, the tremendous binding energy inside a hydrogen or uranium nucleus is released.
Nuclear energy is released during atomic fission, when uranium nuclei are split. It is also released during fusion, when hydrogen nuclei combine to form a helium nucleus. In fission and fusion, nuclear energy produces thermal energy, which is given off as heat. Fission's heat is used to generate electric power in hundreds of locations worldwide. The sun and other stars use fusion to generate radiant and thermal energy. As stars give off energy, they lose mass. Someday humans may be able to harness nuclear fusion as well.
Nuclear energy also has other uses. In medicine, it is used in radiation therapy to treat cancer. The U.S. Navy uses nuclear energy to power some submarines and large ships. They can stay at sea for long periods without stopping to refuel, because their nuclear fuel takes up little space.