Nuclear Fusion as an
Alternative Energy Source
By: Wil Schrader
PHYS 110 802
We
depend on electricity to power our homes and oil to power our vehicles, but the
ways in which we currently achieve these two things are neither clean nor
unlimited. Energy is quickly becoming a commodity we can not live without, and
as our population increases, and our needs grow, this becomes even truer.
Fortunately
there are many different alternative energy sources in the works. This paper
details one such source: Nuclear Fusion. While fusion is not a new idea, and in
fact has been in development since the 1950’s, we are inching ever closer at
being able to capture this virtually unlimited and clean source of energy that
can help power us in to the future on this planet and beyond.
The
common nuclear fission power plants, while clean initially, produce a lot of radioactive
weapons grade waste. The fuel, uranium, is also not as abundant as we would
like. It is estimated that if all the worlds energy needs were placed on
fission power, the worlds Uranium supply would not last longer than 70 years.
Fission
works on the premise that if you split apart a heavy element, such as Uranium,
in to two smaller elements that power is produced. Putting these two lighter
elements back together also creates energy. This reverse process is called
nuclear fusion.
Nuclear
fusion is when two atoms of light element, such as hydrogen, are fused together
either through pressure or heat. In this process massive amounts of energy are
expelled in the form of high-energy neutrons. This is the same process that is
done within hydrogen bombs and the Sun.
When
these two lighter elements fuse together, their masses are not equal as if you
had weighed them before and after fusion. E=MC^2 shows that this lost mass is
converted to energy. This process is only applicable, however, when the
elements used are lighter than that of iron. Once you begin to fuse heavy
elements, energy is absorbed in the fusion process. It is for this reason that
energy is released when splitting apart heavier elements in fission.
The
fuel that is used in fusion is an isotope of hydrogen; either deuterium or
tritium. Deuterium is a naturally occurring isotope found in sea water, about 1
part in 5000. This amounts to over 10^15 tons of deuterium available within our
oceans. A gallon of seawater could produce as much power as 300 gallons of
gasoline.
Tritium
is created from Lithium, which is also found abundantly in the earths crust,
but tritium is radioactive with a half life of 10 years. Tritium can be created
through the fusion process when the high-energy neutrons collide with lithium
blankets. These two hydrogen isotopes are used as not only are they easier to
fuse than hydrogen itself, but the energy output is also much more substantial.
Currently
the majority of the focus is on deuterium-tritium fusion as this combination is
the easiest to fuse. In the future, ideally fusion reactors would work on an
entirely deuterium fusion process as not only is more energy released in this
process but deuterium is both more abundant but also non-radioactive.
When
atoms fuse, their nuclei must come together. Hydrogen atoms repel each other as
they are both positively charged, so the first hurtle to overcome is breaking
the Coulomb barrier. This is the barrier due to electrostatic interaction that
the two nuclei need to overcome to they can get close enough to undergo nuclear
fusion. Once this barrier is broken, the Strong force takes over and fuses the
nuclei together.
Despite
the energy required to break the Coulomb barrier being extremely high, the
energy yielded from the fusion is enough to encourage continued research. There
are two distinct ways to overcome this barrier: heat and pressure.
Heat
in the order of 100 million Kelvin, which is about six times hotter than the
suns core, is what is required for atoms to fuse. At this temperature, hydrogen
takes on the form of plasma and not a gas, which is mostly a mix of ionized
atoms and electrons. The sun achieves this temperature via gravitational forces
compressing its large mass toward the center.
The
problem with this, however, is that this temperature is so great that we can
not contain it conventionally. There are several ways to contain plasma without
the structure containing it melting. First, if you have a large enough ball of
plasma, gravity will take over and contain the plasma which is how the Sun
functions.
The
other method of containing the plasma is through magnetism. Charged particles
placed within a magnetic field will gyrate in circles. If you arrange the
magnetic field correctly, the plasma will be contained by it. You then heat the
plasma through various methods, such as microwaves, particle beams, and
resistive heating by driving current through the plasma. Once a stable fusion
initiates, enough energy is released by the process that it becomes almost
entirely self-sustaining. This containment process is called Magnetic
Confinement.
The
second way to initiate a fusion reaction is through pressure. This is the same
principle behind the hydrogen bomb; implode a small pellet of fusion fuel which
in turn compresses the material resulting in pressure and heat large enough to
initiate fusion. The inertia of the implosion contains the fusion process, so
this method is aptly named Inertial Confinement.
Fusion
reactors will take two radically different looks depending on which process is
used to initiate a fusion process; however the process on which electricity is
generated will remain the same. The high-energy neutrons are collected from the
fusion process creating heat which is then converted to steam that in turn drives
an electricity generating turbine.
Magnetic
Confinement is used to contain the plasma in a magnetic field, flowing around
the inside of a donut shape enclosure called a toroid.
The magnetic field would keep the plasma of deuterium and tritium away from the
sides of the containing structure, allowing the plasma to flow around the
center without melting the structure.
The
fusion reaction, which takes about 70 megawatts to initiate, releases
high-energy neutrons that will heat up lithium blankets. These lithium blankets
collect the neutrons that are released creating both tritium and heat. The heat
is transferred through water to a heat exchanger that creates steam that will
in turn drive a turbine. This entire process will, at first, last only 300-500
seconds but will create 500 megawatts of power.
The
more promising of the two is that of Inertial Confinement reactors. Laser beams
are focused on a single point in chamber called a hohlraum.
At the focal point inside the target chamber would be a pea sized pellet of
deuterium and tritium in plastic cylinder. The lasers heat the chamber and this
will generate x-rays. The heat and radiation will convert the pellet into plasma
and compress it until fusion occurs.
This
process would be extremely short lived, with a total fusion reaction time of
one-millionth of a second, but would produce 50 to 100 times the amount of
energy than was needed to initiate the process. A reactor of this type would
have multiple targets that would be ignited in succession to generate a
sustained heat production. Each target fuel pellet could be made for as little
as twenty five cents, making this type of reactor cost effective.
In
addition to heat and pressure fusion methods, there has been talk of the so
called cold fusion. In 1989 researchers in the
Then
in 2005 scientists at UCLA initiated fusion using a pyroelectric
crystal in a container of helium. They warmed the crystal to produce an
electric field and inserted a wire to focus the charge. The focused electric
field forcefully repelled the positively charged hydrogen nuclei, and with this
repulsion the nuclei smashed into each other with enough force to fuse
together.
With
these new results, we may eventually have a third way of obtaining energy from
a much safer and easier fusion reaction, at room temperature, without all the
containment required for the “hot” fusion reactions. The understanding of this
reactive process and the possible uses are still in their infancy, however.
Currently,
there are no plans to build any power-producing fusion reactors. Current
reactors are operating only as a way to further study the fusion process, but
no commercial fusion reactors are expected before 2050. However, a recent paper
published in January of 2009 claims that small 50mW reactors are feasible.
In
March of 2009 the world’s largest, highest energy lasers were certified for
operation at the National Ignition Facility in
The
advantages of fusion over other alternative energies are many. There is
abundant energy – there is enough deuterium in the ocean to power fusion
reactors for thousands of years, and tritium can be created in the fusion
process. Since deuterium is available in all the worlds’ oceans, it is a widely
available source for all nations. Fusion also consumes less fuel per mass than
any other fuel-consuming energy source.
It
is safer than traditional fission reactors, as the amount of fuel used in
fusion reactions are comparably smaller than that used in fission. The amount
of radiation that is output in a fusion reaction is no more than the natural
background radiation we already live with in our daily lives.
As
with nuclear fission reactors, no combustion occurs in a fusion reactor so
there is no air pollution, greenhouse gases, or acid rain. On top of that, much
less nuclear waste will be created in the process of fusion, so storage will be
easier and what waste is created is of a non-nuclear weapons grade.
Fusion
creates more energy with less resource investment than any other source of
energy known to man. This means we would not need to devote acres of land to
wind and solar farms, or tear up the ground for coal and oil, nor dig holes in
the ground for geothermal energy.
However,
fusion is not without its own faults. The initial cost of building a fusion
reactor, about $500 million, and difficulty of maintaining and repairing the
fusion chamber has been questioned. Another pitfall is that the rare earth
metals and lithium blankets used to contain the neutrons will eventually decay
and become radioactive. However, as we get closer to a deuterium-deuterium
fusion process these radioactive issues will become less pertinent.
The
issue of storing what little waste that is created is also a problem, but not a
problem we are not already willing and capable of taking care of. The fact that
fusion reactors would create less waste than traditional fission reactors is
both a positive and a negative and an issue we would need to deal with.
We
have been developing fusion energy since the 1950’s and some estimate that
fusion power is still at least another 50 years off at best. We have made great
strides in the last 30 years, however, and who knows what breakthroughs will be
made that will help speed up the process. Still, there are some that question
whether or not fusion power will pan out in the long run, even when we continue
to inject billions of dollars in to the research and development of confinement
and reaction chambers.
Despite
these pitfalls, nuclear fusion may very well prove to be a very lucrative
source of energy, not only as a way of meeting the needs of those on this
planet, but as a way to power our space craft and even provide a source of
propulsion via nuclear pulse propulsion.
Nuclear
Fusion is just one of many sources of alternative energies that will no doubt
be used in the future to meet our ever increasing needs for electricity. Other
sources of energy will not be able to pull the load alone, but we should also
not put all of our eggs in one basket. Each has their own advantages and
disadvantages, and each can be used together to meet our needs now and in the
future.
References
http://hyperphysics.phy-astr.gsu.edu/HBASE/NucEne/fusion.html
http://science.howstuffworks.com/fusion-reactor.htm
http://en.wikipedia.org/wiki/Fusion_power
http://fusedweb.llnl.gov/faq.html
http://www.newscientist.com/article/dn8827-no-future-for-fusion-power-says-top-scientist.html
http://www.csmonitor.com/2005/0606/p25s01-stss.html