Fission (“splitting”) occurs when the nucleus of large, unstable atoms, like uranium and plutonium, break into smaller atoms, releasing energetic radiation and neutrons. Fission powers the “atomic” bomb that destroyed Hiroshima, and all nuclear power reactors.
Fusion (“joining”) occurs when light atoms, primarily isotopes of hydrogen, fuse into larger atoms, releasing fantastic quantities of energy. Fusion powers the sun and “hydrogen” bombs, which are called “thermonuclear” for the intense heat needed to overcome electrical repulsion between positively-charged hydrogen nuclei. Fusion, however, is extremely difficult to control; although billions have been spent to tame fusion for electricity, practical reactors are decades away.