The nuclei of atoms are bound by the strong interactions. The most tightly bound nuclei are those of intermediate atomic mass. Because of this, energy is given off if very high mass nuclei break apart, or fission, into two intermediate mass products or when very low mass nuclei combine, or fuse, into heavier nuclei. Although very different in their underlying dynamics and their practical implementation, both nuclear fission and nuclear fusion are potentially sources of great amounts of energy for human use.
Nuclear fission is a high-energy-density, non-carbon energy source that is available with today's technology. In 2014, about 275GW, or 11% of the world's electricity, was generated by nuclear fission reactors . Nuclear fission energy could in principle supply a significant fraction (∼10–20%) of the world's total energy needs, which are roughly 15 TW, in the near-term future. Nuclear fission is not likely to be a long-term source of energy for human use, however, since the world's economically recoverable reserves of the basic fuel 235U, a rare isotope of the heavy metallic element uranium, are quite limited. Schemes to breed new fuel from the more common form of uranium (238U) or from thorium, another heavy metal element, are technically challenging and bedeviled by complex safety and security issues. If, however, it proves possible to convert uranium and/or thorium into fission fuels, and if the technical, social, and political problems that accompany nuclear power can be solved, then nuclear fission energy could in principle supply a large fraction of human energy needs for the foreseeable future.
Although nuclear fusion reactions power the Sun and other stars throughout the universe, fusion has proven extremely difficult to control on Earth. Efforts to control fusion processes continue to be the subject of both basic and applied research. If fusion power can be harnessed – and that is a big “if” – it too could supply human energy needs for a very long time.
Nuclear power presents some of the most difficult scientific, political, and ethical issues in the energy field. The environmental and human health hazards from radioactivity and nuclear waste, as well as the risks from nuclear weapons proliferation, are particularly challenging. On the other hand, the advantages of a large-scale, carbon-free energy source are also substantial, in view of climate issues discussed in §34.