When large fissile atoms, usually uranium 235 or plutonium 239, absorbs a neutron, a fission of the atom often results. Fission splits the atom into two or more smaller nuclei and also releases gamma radiation and free neutrons. Some of the neutrons produced can be absorbed by other fissile atoms, which split and release more neutrons sustaining a chain reaction. The self-sustaining chain reaction can be controlled and used to heat water to steam and drive a turbine to produce electricity.

Nuclear fusion occurs in stars like our sun. The sun’s energy is produced from the fusion of two hydrogen nuclei to create a helium atom and energy. Nuclear fusion is the name for the general process where two or more atomic nuclei fuse together to make a heavier nucleus, a new element. When light nuclei fuse, energy is released until a nucleus reaches the size of iron. Beyond that, it requires energy to create larger atoms, which is why fission releases energy from the spliting of very large atoms. Controlled nuclear fusion has yet to be achieved by scientists for electricity production.