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 the target. As a
result, no net useful energy was produced. In the 1950s the first large-scale
but uncontrolled release of fusion energy was demonstrated in the tests of
thermonuclear weapons by the United States, the USSR, Great Britain, and France.
This was such a brief and uncontrolled release that it could not be used for the
production of electric power.
In the fission reactions I discussed earlier, the neutron, which has no
electric charge, can easily approach and react with a fissionable nucleus ,for
example, uranium-235. In the typical fusion reaction, however, the reacting
nuclei both have a positive electric charge, and the natural repulsion between
them, called Coulomb repulsion, must be overcome before they can join. This
occurs when the temperature of the reacting gas is sufficiently high, 50 to 100
million ° C (90 to 180 million ° F). In a gas of the heavy hydrogen isotopes
deuterium and tritium at such temperature, the fusion reaction occurs, releasing
about 17.6 MeV per fusion event. The energy appears first as kinetic energy of
the helium-4 nucleus and the neutron, but is soon transformed into heat in the
gas and surrounding materials.
If the density of the gas is sufficient—and at these temperatures the
density need be only 10-5 atm, or almost a vacuum—the energetic helium-4 nucleus
can transfer its energy to the surrounding hydrogen gas, thereby maintaining the
high temperature and allowing subsequent fusion reactions, or a fusion chain
reaction, to take place. Under these conditions, "nuclear ignition" is said to
have occurred. The basic problems in attaining useful nuclear fusion conditions
are to heat the gas to these very high temperatures, and to confine a
sufficient quantity of the reacting nuclei for a long enough time to permit the
release of more energy than is needed to heat and confine the gas. A subsequent
major problem is the capture of this energy and its conversion to electricity.
At temperatures of even 100,000° C (180,000° F), all the hydrogen atoms
are fully ionized. The gas consists of an electrically neutral assemblage of
positively charged nuclei and negatively charged free electrons. This state of
matter is called a plasma. A plasma hot enough for fusion cannot be contained by
ordinary materials. The plasma would cool very rapidly, and the vessel walls
would be destroyed by the temperatures present. However, since the plasma
consists of charged nuclei and electrons, which move in tight spirals around
strong magnetic field lines, the plasma can be contained in a properly shaped
magnetic field region without reacting with material walls.
In any useful fusion device, the energy output must exceed the energy
required to confine and heat the plasma. This condition can be met when the
product of