The two key characteristics of nuclear fission important for the practical
release of nuclear energy are both evident in equation (2). First, the
energy per fission is very large. In practical units, the fission of 1 kg
(2.2 lb) of uranium-235 releases 18.7 million kilowatt-hours as heat.
Second, the fission process initiated by the absorption of one neutron in
uranium-235 releases about 2.5 neutrons, on the average, from the split
nuclei. The neutrons released in this manner quickly cause the fission of
two more atoms, thereby releasing four or more additional neutrons and
initiating a self-sustaining series of nuclear fissions, or a chain
reaction, which results in continuous release of nuclear energy.
Naturally occurring uranium contains only 0.71 percent uranium-235; the
remainder is the nonfissile isotope uranium-238. A mass of natural uranium
by itself, no matter how large, cannot sustain a chain reaction because only
the uranium-235 is easily fissionable. The probability that a fission
neutron with an initial energy of about 1 MeV will induce fission is rather
low, but the probability can be increased by a factor of hundreds when the
neutron is slowed down through a series of elastic collisions with light
nuclei such as hydrogen, deuterium, or carbon. This fact is the basis for
the design of practical energy-producing fission reactors.
In December 1942 at the University of Chicago, the Italian physicist Enrico
Fermi succeeded in producing the first nuclear chain reaction. This was done
with an arrangement of natural uranium lumps distributed within a large
stack of pure graphite, a form of carbon. In Fermi's “pile,” or nuclear
reactor, the graphite moderator served to slow the neutrons.
