Transcript Slide 1

Solar energy is any form of
energy radiated by the sun, including
light, radio waves, and X-rays.
Thermonuclear fusion is the energy
producing process which takes place
continuously in the sun and stars.
Nuclear fusion is a nuclear reaction in
which nuclei combine to form more
massive nuclei with the simultaneous
release of energy. The combining of the
different protons leads to the formation
of a new element. In the core of the sun
at temperatures of 10-15 million degrees
Celsius, Hydrogen is converted to
Helium providing more than enough
energy to sustain life on earth.
Regions of the sun include the
core, radiation zone, convection zone,
and photosphere. Gases in the core are
about 150 times as dense as water and
reach temperatures as high as 16
million degrees. Nuclear fusion of the
hydrogen atoms takes place in the core.
Fusion is what happens when two atomic nuclei are forced
together by high pressure ... high enough to overcome the strong
repulsive forces of the respective protons in the nuclei. When the
nuclei fuse, they form a new element, and release excess energy in
the form of a fast-moving neutrino and a positron. The energy is
'extra' because the mass of the newly formed nucleus is less than
the sum of the masses of the original two nuclei; the extra mass is
converted to energy according to Einstein's equation E=mc2 This
energy can be used to do useful work!
The main fusion reaction involved occurs between the nuclei of the
two hydrogen isotopes, Deuterium (D) and Tritium (T).
FUSION REACTION
In this drawing you can see a
diagram of a nuclear reaction
between two hydrogen isotopes as
they are fused together to produce
helium and a lone neutron. Nuclear
fusion actually combined the nuclei
(protons) of an element to produce a
different element.
Besides neutron it also ejects two
further particles: a positron (a
positively charged electron) and a
strange, almost mass-less, particle
called a neutrino.
The positron and neutrino go flying
off with kinetic energy supplied by
converting some of the mass of the
transmuted proton into kinetic energy,
in accordance with E = mc2.
10g of Deuterium (which can be extracted from 500
liters of water and 15g of Tritium produced from 30 g of
Lithium would produce enough fuel for the lifetime
electricity needs of an average person in an industrialized
country.
As you can see, an unbelievable amount of
energy can be produced through nuclear fusion.
Unfortunately, scientists have been unable to find practical
ways of accessing this energy. At the temperatures
required for the fusion of most hydrogen isotopes (over 100
million degrees Celsius!), the fuel has changed its state
from gas to plasma. Plasma is a fully ionized gas
containing approximately equal numbers of positive and
negative ions. It is an electric conductor and is affected by
magnetic fields. In a plasma, the electrons have been
separated from the atomic nuclei.
But there's a catch! In the sun, the energy to force nuclei together
comes from the sun's immense internal temperatures, approaching 16
million degrees or more at the center! In order to cause nuclei to fuse
here on earth they must either be heated to that temperature, or
caused to move fast enough to simulate a correspondingly high
temperature.
That has been done already, more than 50 years ago. The energy
to set off the fusion reaction was supplied by an atomic bomb, and the
fusion reaction that resulted was called a 'hydrogen bomb'! But the
energy release was all at once, and uncontrollable.
In order for fusion reactions to occur, the particles must be hot
enough (temperature), in sufficient number (density) and well
contained (confinement time). These simultaneous conditions are
represented by a fourth state of matter known as plasma. In a plasma,
electrons are stripped from their nuclei. A plasma, therefore, consists
of charged particles, ions and electrons.
Magnetic Confinement
Efforts to control fusion first relied on the
principle of magnetic confinement, in which
a powerful magnetic field traps a hot
deuterium-tritium plasma long enough for
fusion to begin.
In November 1997, researchers exploiting the magnetic
confinement approach created a fusion reaction that
produced 65 percent as much energy as was fed into it to
initiate the reaction. This milestone was achieved in England
at the Joint European Torus, a tokamak facility--a doughnutshaped vessel in which the plasma is magnetically confined.
A commercial fusion reactor would have to produce far more
energy than went into it to start or maintain the reaction.
At Princeton University's plasma physics laboratory in New Jersey, scientists
have produced a controlled fusion reaction at the Tokamak Fusion Test
Reactor there. During these reaction the temperature in the reactor
surpassed three times that of the core of the sun.
'Tokamak' is a Russian acronym for “toroidal magnetic chamber”. This device
was first developed by Russian scientists. A tokamak is a toroidal plasma
confinement device, resembling a doughnut in shape. The plasma is confined not by
the material walls but by magnetic fields. The reason for using magnetic confinement
is twofold. First, no known material can withstand the hundred-million degree
temperatures required for fusion. Second, keeping the plasma in a magnetic bottle
insulates it well, making it easier to heat up.
(Such reactors are inherently safe. If the plasma escapes, it immediately cools down,
and the reaction stops!)
Escaping neutrons and energy would heat a body of water; a steam turbine and
generator would produce electricity.
This magnetic confinement method for producing fusion is regarded by some
scientists as the most promising one for future commercial energy sites. This stems
from the way Magnetic Confinement fusion works, which allows for a sustained
reaction and thus continuous energy production. Many 'tokamaks' are in operation
currently, around the world, and more are planned for the future. But so far, none
have been able to sustain the reaction for more than a few seconds ... the plasma
leaks out. Improved magnet design and higher input power will perhaps allow these
reactors in the future to maintain a fusion reaction indefinitely, producing copious
amounts of power.
http://ippex.pppl.gov/temp/tokamak/tokamaknew.htm
NUCLEAR FUSION ADVANTAGES AND DISADVANTAGES
Nuclear fusion, if it can be developed, would have several advantages over conventional fossil fuel
and nuclear fission power plants. The fuels required for fusion reactors, deuterium and lithium, are
so abundant that the potential for fusion is virtually unlimited. Oil and gas fired power plants as well
as nuclear plants relying on uranium will eventually run into fuel shortages as these non-renewable
resources are consumed. Like conventional nuclear plants, fusion reactors have no emission of
carbon dioxide, the major contributor to global warming or sulphur dioxide, the main cause of acid
rain. Fossil fuel power plants burning coal, oil and natural gas are large contributors to global
warming and acid rain.
One of the barriers to the widespread use of conventional nuclear power plants has been public
concern over operational safety, and the disposal of radioactive waste. Major accidents, such as
Chernobyl, are virtually impossible with a fusion reactor because only a small amount of fuel is in
the reactor at any time. It is also so extremely difficult to sustain a fusion reaction, that should
anything go wrong, the reaction would invariably stop. Long lived highly radioactive wastes are
generated by conventional nuclear plants; these must be safely disposed of and represent a hazard
to living things for thousands of years. The radioactive wastes generated by a fusion reactor are
simply the walls of the containment vessel which have been exposed to neutrons. Although the
quantity of radioactive waste produced by a fusion reactor might be slightly greater than that from a
conventional nuclear plant, the wastes would have low levels of short lived radiation, decaying
almost completely within 100 years.
The major disadvantages of nuclear fusion are the vast amounts of time and money which will be
required before any electricity is generated by fusion. Fusion produces no greenhouse gases but
will not be able to contribute to reducing carbon dioxide emissions until close to the middle of the
next century. If the world does nothing and waits for fusion as the solution to the global warming
problem, it may well be too late. Similarly, every dollar spent on nuclear fusion would have a much
greater impact on reducing global warming if it was spent on reducing the demand for electricity.
There are dozens of ways to reduce electricity use through efficiency improvements and
conservation efforts, which are much less expensive than producing more electricity through
nuclear fusion. Development of other electricity supply technologies, such as photovoltaic cells
which convert sunlight directly into electricity, could also eliminate the need for fusion before it is
operational.
In short words...
Nuclear Fusion Pros:
Nuclear Fusion Cons:
. Vast new source of energy
. Fuels (mostly hydrogen) are plentiful
. Inherently safe since a malfunction
results in a rapid shutdown
. No atmospheric pollution leading to
acid rain or "greenhouse" effect
. Radioactivity of the reactor structure,
caused by neutrons, decays rapidly and
can be minimized by careful selection of
low-activation materials. Provisions for
geological time-span disposal is not
needed
. Usually high activation energy
. High temperatures are needed for
most reactions
. Hard to control in a given space and
even harder to maintain for any
significant amount of time
. Very expensive
Cold Fusion is a nuclear fusion reaction of deuterium at or relatively near
room temperature. It is still questionable as to whether or not this is
possible, although the ability to produce this magnitude of energy at such
low temperatures would definitely be desirable.
Nuclear fusion can produce energy when the nuclei of lighter elements
come together (fuse), creating larger nuclei. Energy is liberated when the
total mass of the end products is slightly less than the mass of the lighter
nuclei going into the process, with that difference in mass being
converted to energy via Einstein's famous E=mc2 relationship.
Because the protons in nuclei are all positively charged, and like charges
repel, nuclei need some convincing to get them to fuse. That convincing
ordinarily involves high temperature and pressure, such as exists at the
core of a star or under conditions created by a fission bomb.
Cold fusion is an attempt to get fusion to occur under less extreme
conditions, possibly as a result of chemical reactions. Despite the flurry of
publicity several years ago, cold fusion remains unrealized speculation for
now.