The binding energy per nucleon plot suggests that very heavy elements can gain stability and release energy if they fragment to yield middle-weight elements. This process is called fission. In contrast, very light elements can gain stability and release energy if they fuse together in a process called fusion.
Fission is more than simple radioactive decay. In a fission event, a nucleus breaks into fragments when a neutron strikes it.
Just as dropped plates will not break into the same fragments each time, many products are possible for a fission reaction. For example, more than 400 pathways have been identified for uranium-235, yielding nearly 800 different potential fission products.
The neutrons released by fission can induce more fission reactions. A chain reaction is a fission reaction that is sustained by the product neutrons. The mass of an isotope required to sustain fission is called the critical mass.
Fission is an exothermic process, meaning large amounts
of energy are released. Controlled fission can be used to generate electrical
power in a nuclear reactor. In the United States, about 22% of electricity
is produced by nuclear power plants. The disposal of depleted nuclear
fuel rods is a major obstacle to increased worldwide use of nuclear reactors
for the production of electricity.
Fusion reactions also release tremendous amounts of energy. Helium is produced in the sun and other stars by several fusion reactions.
The hydrogen isotopes required for fusion are cheap
and plentiful, and the products are nonradioactive and nonpolluting. However,
temperatures as high as 40 million kelvins are required to initiate fusion
reactions, so fusion methods are not yet practical as power sources.