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. Thermal energy is the energy a substance or system has related to its temperature, i.e., the energy of moving or vibrating molecules. Atoms and molecules, the smallest particles of any substance, are always in motion. The motion of thermal energy is usually not visible, but we can feel or see its effects. We use thermal energy to cook our food and heat our homes, and we use it to generate electricity.
Thermal energy is not the same as heat. Heat is energy transferred between substances or systems due to a temperature difference between them. So it is correct to say that a system contains thermal energy, but not that it "contains" heat, since heat means energy that is transferred from one thing to another.
The amount of heat transferred by a substance depends on the speed and number of atoms or molecules in motion. The faster the atoms or molecules move, the higher the temperature, and...