At temperatures of millions of degrees, the nuclei of hydrogen atoms can overcome their natural tendency to repel one another and join or fuse to form helium nuclei. This releases enormous energy. Fusion is the opposite of fission, which is the release of energy by splitting heavy uranium or plutonium nuclei. These are complex and large machines that rely on high magnetic fields or powerful lasers to compress and heat fusion fuel, typically the isotopes of hydrogen, deuterium and tritium. The controlled hot fusion program has made enormous strides, but all agree that the earliest possible time when practical hot fusion devices might be available is about three decades away.
Hot fusion is a very tough engineering problem.
A commercial hot fusion power plant would not be on-line until at least 2040. Cold fusion releases enormous quantities of energy in the form of heat, not ionizing radiation, as in hot fusion. This heat energy is hundreds to thousands of times what ordinary chemical reactions could possibly yield. Cold fusion, in contrast to hot fusion, occurs in relatively simple apparatus. Cold fusion reactions are not at all like conventional hot fusion reactions.