Webster's New World defines fusion as "melting together." Therefore, nuclear fusion is the process by which more than one nuclei join together to form a heavier nucleus. Nuclear fusion is the process that powers our sun and the stars. Nuclear fission is the exact opposite process, in which the nucleus of an atom splits into two or more smaller nuclei.
What is the difference between nuclear fusion and nuclear fission?
As of today, fission is what produces the energy in nuclear power. Unlike fossil fuel energy sources like coal and gasoline, which produce tremendous air pollution that evidence suggests is depleting the earth's ozone layer, nuclear energy produces no air pollution. But the waste products of nuclear fission are highly radioactive and remain so for hundreds of years. At this time, a world-wide research program is underway to harness fusion energy to produce electricity, which would also create radioactive waste material but it would decay much more rapidly. The only by-product of nuclear fusion power would be Helium, a harmless gas.
Nuclear weapons use both fusion and fission, depending on the type of bomb. Fission bombs are known as atomic bombs, atom bombs, or A-bombs. In fission weapons, enriched uranium or plutonium is assembled into what's called a supercritical mass, which starts an exponentially growing nuclear chain reaction (the explosion). Fusion bombs (also called hydrogen bombs, H-bombs, thermonuclear bombs) work by detonating a fission bomb adjacent to a fusion fuel, which starts a fusion reaction. Fusion bombs are far more destructive than fission bombs. A one megaton fission bomb would level everything in its path up to about 1.7 miles away from the detonation site. A one megaton fusion bomb is about 80 times stronger than that.
Only six countries — United States, Russia, France, United Kingdom, China and India — are known to possess fusion bombs. Iran is currently doing research on nuclear fusion, claiming to have joined the race in developing fusion-based electricity. But the United Nations is concerned that Iran is actually developing nuclear weaponry.