Splitting heavy atomic nuclei to generate energy is not the only way to perform a nuclear reaction. It is also possible to take two very light nuclei, such as deuterium and tritium (isotopes of hydrogen), and push them close enough together to cause them to fuse into a heavier atomic nucleus.

Watch the following animation showing fusion of Deuterium and Tritium into Helium:

This process is known as nuclear fusion.

The amount of energy (E = mc² again) per reaction is a little bit smaller (2.82x10^-12 J), but since the atoms themselves are so much smaller and lighter than uranium, there are a lot more atoms in one gram, and the total energy yield is much larger. Because of this very large release of energy, and the fact that the waste products of nuclear fusion are relatively easy to dispose of, there has been a long-term effort in the physics community to develop a practical fusion reactor. A fusion reactor would be an environmentally clean electrical energy generator, whose fuel source is practically limitless because of the large amount of hydrogen on earth (hydrogen is a major constituent of water). At this point we have not yet created a successful fusion reactor for two reasons. First, with current technology, it takes more energy to create the fusion reaction than what the reaction gives off. Second, once the reaction is started, it is currently still too difficult to control.