nuclear energy  nuclear energy  nuclear energy, the energy
stored in the nucleus of an atom and released through fission,
fusion, or radioactivity. In these processes a small amount of
mass is converted to energy according to the relationship E =
mc2, where E is energy, m is mass, and c is the speed of light
(see relativity). The most pressing problems concerning nuclear
energy are the possibility of an accident at a nuclear reactor
or fuel plant, such as those which occurred at Three Mile Island
(1979), Chernobyl (1986), and Takaimura, Japan (1999), and the
potential threat to the continued existence of the human race
posed by nuclear weapons (see disarmament, nuclear).  Nuclear
Reactions  The release of nuclear energy is associated with
changes from less stable to more stable nuclei and produces far
more energy for a given mass of fuel than any other source of
energy. In fission processes, a fissionable nucleus absorbs a
neutron, becomes unstable, and splits into two nearly equal
nuclei. In fusion processes