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Essay/Term paper: Fission or fusion

Essay, term paper, research paper:  Science Research Papers

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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 confinement time t and plasma density n exceeds about 1014. The
relationship t n ³ 1014 is called the Lawson criterion. Numerous schemes for the
magnetic confinement of plasma have been tried since 1950 in the United States,
the former USSR, Great Britain, Japan, and elsewhere. Thermonuclear reactions
have been observed, but the Lawson number rarely exceeded 1012. One device,
however, the tokamak, originally suggested in the USSR by Igor Tamm and Andrey
Sakharov, began to give encouraging results in the early 1960s. The confinement
chamber of a tokamak has the shape of a "torus", with a minor diameter of about
1 m (about 3.3 ft) and a major diameter of about 3 m (about 9.8 ft). A toroidal
magnetic field of about 50,000 gauss is established inside this chamber by large
electromagnets. A longitudinal current of several million amperes is induced in
the plasma by the transformer coils that link the torus. The resulting magnetic
field lines, spirals in the torus, stably confine the plasma.
Based on the successful operation of small tokamaks at several
laboratories, two large devices were built in the early 1980s, one at Princeton
University in the United States and one in the USSR. In the tokamak, high plasma
temperature naturally results from resistive heating by the very large toroidal
current, and additional heating by neutral beam injection in the new large
machines should result in ignition conditions.
Another possible route to fusion energy is that of inertial confinement.
In this concept, the fuel, tritium or deuterium ,is contained within a tiny
pellet that is then bombarded on several sides by a pulsed laser beam. This
causes an implosion of the pellet, setting off a thermonuclear reaction that
ignites the fuel. Several laboratories in the United States and elsewhere are
currently pursuing this possibility. Progress in fusion research has been
promising, but the development of practical systems for creating a stable fusion
reaction that produces more power than it consumes will probably take decades to
realize. The research is expensive, as well.
However, some progress has been made in the early 1990s. In 1991, for
the first time ever, a significant amount of energy, about 1.7 million watts,
was produced from controlled nuclear fusion at the Joint European Torus (JET)
Laboratory in England. In December 1993, researchers at Princeton University
used the Tokamak Fusion Test Reactor to produce a controlled fusion reaction
that output 5.6 million watts of power. However, both the JET and the Tokamak
Fusion Test Reactor consumed more energy than they produced during their
operation. If fusion energy does become practical, it offers the many advantages
includimg a limitless source of fuel, deuterium from the ocean, no possibility
of a reactor accident, as the amount of fuel in the system is very small, and
waste products much less radioactive and simpler to handle than those from
fission systems.
I conclude, that even though fusion is much better, cleaner, and safer,
than fission, we do not have the knowledge of how to create and contain the
energy realesed in a fusion reaction. So, until we do, fission is the only way
we can use the atom to create power.

 

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