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Fission Or Fusion

Fission or Fusion


I think that right now, fission is the only way that we can get more
energy out of a nuclear reaction than we put in. 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 non-fissile 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 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.
Nuclear fusion was first achieved on earth in the early 1930s by
bombarding a target containing deuterium, the mass-2 isotope of hydrogen, with
high-energy deuterons in a cyclotron. To accelerate the deuteron beam a great
deal of energy is required, most of which appeared as heat in...