About one in every 3,000 water molecules contains a deuterium atom. There is
enough deuterium in the oceans to provide for the world’s energy needs for
billions of years. One gram of fusion fuel can produce as much energy as 9,000
liters of oil. The amount of deuterium found naturally in one liter of water is
the energy equivalent of 300 liters of gasoline. Tritium is bred in the fusion
reactor. It is generated in the lithium blanket as a product of the reactor in
which neutrons are captured by the lithium nuclei. A fusion reactor would have
several attractive safety features. First, it is not subject to a runaway, or
meltdown, accident as is a fission reactor. The fusion reaction is not a chain
reaction; it requires a hot plasma. Accidental interruption of a plasma control
system would extinguish the plasma and terminate fusion. Second, the products of
a fusion reaction are not radioactive; hence, no long-term radioactive wastes
would be generated. Neutron bombardment would activate the walls of the
containment vessel, but such activated material is shorter-lived and less toxic
than the waste products of a fission reactor. Moreover, even this activation
problem may be eliminated, either by the development of advanced, low-activation
materials, such as vanadium-based materials, or by the employment of advanced
fusion-fuel cycles that do not produce neutrons, such as the fusion of deuterons
with helium-3 nuclei. Nearly neutron-free fusion systems, which require higher
temperatures than D-T fusion, might make up a second generation of fusion
reactors). Finally, a fusion reactor would not release the gaseous pollutants
that accompany the combustion of fossil fuels; hence, fusion would not produce a
greenhouse effect. The fusion process has been studied as part of nuclear
physics for much of the 20th century. In the late 1930s the German-born
physicist Hans A. Bethe first recognized that the fusion of hydrogen nuclei to
form deuterium is exoergic (there is release of energy) and, together with
subsequent reactions, accounts for the energy source in stars. Work proceeded
over the next two decades, motivated by the need to understand nuclear matter
and forces, to learn more about the nuclear physics of stellar objects, and to
develop thermonuclear weapons (the hydrogen bomb) and predict their performance.
During the late 1940s and early 1950s, research programs in the United States,
United Kingdom, and Soviet Union began to yield a better understanding of
nuclear fusion, and investigators embarked on ways of exploiting the process for
practical energy production.
This work focused on the use of magnetic fields and
electromagnetic forces to contain extremely hot gases called plasmas. A plasma
consists of unbound electrons and positive ions whose motion is dominated by
electromagnetic interactions. It is the only state of matter in which
thermonuclear reactions can occur in a self-sustaining manner. Astrophysics and
magnetic fusion research, among other fields, require extensive knowledge of how
gases behave in the plasma state. The inadequacy of the then-existent knowledge
became clearly apparent in the 1950s as the behavior of plasma in many of the
early magnetic confinement systems proved too complex to understand. Moreover,
researchers found that confining fusion plasma in a magnetic trap was far more
challenging than they had anticipated. Plasma must be heated to tens of millions
of degrees Kelvin or higher to induce and sustain the thermonuclear reaction
required to produce usable amounts of energy. At temperatures this high, the
nuclei in the plasma move rapidly enough to overcome their mutual repulsion and
fuse. It is exceedingly difficult to contain plasmas at such a temperature level
because the hot gases tend to expand and escape from the enclosing structure.
The work of the major American, British, and Soviet fusion programs was strictly
classified until 1958. That year, research objectives were made public, and many
of the topics being studied were found to be similar, as were the problems
encountered. Since that time, investigators have continued to study and measure
fusion reactions between the lighter elements and have arrived at more accurate
determinations of reaction rates. Also, the formulas developed by nuclear
physicists for predicting the rate of fusion-energy generation have been adopted
by astrophysicists to derive new information about the structure of the stellar
interior and about the evolution of stars. The late 1960s witnessed a major
advance in efforts to harness fusion reactions for practical energy production:
the Soviets announced the achievement of high plasma temperature (about
3,000,000 K), along with other physical parameters, in a tokamak, a toroidal
magnetic confinement system in which the plasma is kept generally stable both by
an externally generated, doughnut-shaped magnetic field and by electric currents
flowing within the plasma itself.
