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 ? 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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