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Controlled Thermonuclear Technology
By:
Mike Pompura
Seminole Community College
September 15, 2004
OUTLINE
Thesis:
Even with the extremely difficult conditions necessary to initiate and
maintain a controlled nuclear fusion reaction, the opportunity of
having a viable energy source that will last for millions of years
continues to provide the main initiative for continuing research and
development in the field.
1: Thermonuclear Theory
A History and Background
B. Lawson Criteria
C. Fuel Supply
D. Advantages as a Power Source
2: Plasma containment
A. Inertial Confinement
B. Magnetic Confinement
a. Open Systems
b. Closed Systems
3: Future Applications
4: Because the fuel is available in almost unlimited supply, I
believe that fusion energy will become the major power source of the
future long after the petroleum sources have depleted and made the
internal combustion engine obsolete. Not only is this energy source
"clean" and environmental-friendly it also has the potential for a
higher efficiency in the fuel utilization; nothing is wasted in the
conversion process.
Nuclear fusion has been attained on the earth in the form of the
hydrogen bomb. The bomb is a form of uncontrolled fusion which has no
practical value except to make large holes in the ground quickly.
Controlled fusion presents unique problems which scientists have yet
to solve. Once the problems are solved, fusion power promises to be a
source of energy that could be used for a variety of purposes.
The primary fuel for the fusion process is Deuterium which is
abundant in seawater "There is one Deuterium atom in every 6,500
ordinary hydrogen atoms of seawater. The Deuterium in one gallon of
seawater has the fusion energy equivalent to 300 gallons of gasoline,
or the fusion energy available from a cubic mile of seawater has been
calculated to be the equivalent to the combustion of 5,700 billion
barrels of crude oil Ö the amount of 2.5 times the world's entire oil
reserves."1 At the current rate of fuel oil consumption the
Deuterium in the oceans could last for 500,000,000 years. The
possibility of a low cost fuel in abundant supply provides a strong
initiative for further research and development in the field of thermonuclear energy.
Fusion reactions were first discovered with a particle
accelerator when scientists directed a beam of high speed neutrons
into a target of frozen Deuterium. The energy released from these
experiments was far less than the energy required to initiate them,but
it did prove that the fusion process was actually possible. Project
Sherwood was the code name given to the experiments conducted into the
fusion research during the early 1950's.
A thermonuclear reaction takes place when two nuclei fuse
together to form a stable heavier one, thereby releasing elementary
particles and kinetic energy in the process. The nucleus consists of
protons and neutrons and it carries a positive electric charge which
tends to repel other nuclei. The greater number of protons in the
nucleus relates directly to a stronger repulsive force; therefore the
lightest nuclei are the easiest ones to fuse. To overcome this
repulsive force a nucleus must have enough kinetic energy to fuse with
another one. The kinetic energy required to fuse atoms amounts to
several thousand electron volts, but the energy liberated in the
fusion reaction totals in the million electron volt range. "One
electron volt is the energy that a singly charged particle gains in falling through a
potential difference of one volt."2
The usual way of accelerating atoms to sufficient kinetic
energies for fusion reactionsis to heat them, therefore the term
thermonuclear is applied. Amoung the many ways to heat atoms are:
1: Electrical Currents
2: Magnetic Fields
3: Laser Beams
When matter is superheated to extreme temperatures the atoms are
stripped of their electrons and form positive ions. This cloud of
ions and electrons is called a plasma. Two fields of science that
deal with plasmas are hydromagnetics and plasma physics. Plasma
physics deals with the physics of hot ionized gases and hydromagnetics
deals with the dynamics of electrically conducting fluids interacting with magnetic fields.
The usual way ofhandling these plasmas is to confine them in a magnetic field.
The Lawson Criterion was first proposed by the British scientist
J.D. Lawson in 1956, and it states that if a fusion reaction output is
to exceed its input, the value (nT) must exceed a critical number.
The value "n" is measured in particles per cubic centimeter, and the
value "T" is measured in fractions of a second. "The nuclear energy
released per unit time is proportional to the product of the ion
number density squared (n2), the nuclear reaction cross-section, and
the ion-ion collision velocity. The thermal energy supplied to this
volume is proportional to the product of the ion number density (n),
the mean thermal energy, and the reciprocal of the
containment time (T), which is the average time that a hot Deuterium
or Tritium nucleus spends in the reacting region."3 For a typical D-T
reaction the value is 1014 at a temperature of 200 million degrees
Kelvin. For a D-D reaction the value is 1016 at a temperature of
1,000 million degrees Kelvin. The basic requirements for achieving
useful power from a fusion reactor are to heat the fuel to a high temperature,
keep it free from impurities, squeeze it to an adequate density, and hold the
plasma together long enough.
Fuel for the fusion reactor will most likely be one of four choices:
1. Deuterium
2. Tritium
3. Helium
4. Lithium
Deuterium and Tritium are isotopes of hydrogen. Deuterium has
one proton and neutron in its nucleus which is called a Deuteron; it
is a stable isotope quite abundant in nature. Tritium has one proton
and two neutrons in its nucleus which is called a Triton. This
isotope is radioactive and rarely found in nature, but it can be
easily produced by bombarding Lithium with neutrons. Lithium is a
metal which is quite abundant in nature. The stable isotope of
Helium, He3, is another possible fuel for fusion reactions. The most
logical choice would be a combination of Deuterium and Tritium because
of their availability and ease of fusion. Deuterium can easily be
separated from ordinary hydrogen by the electrolysis of water. Tritium
can easily be obtained from Lithium metal, alloys or salts. Lithium
could be used as blankets around the reactor core, liberating tritium
as the neutron flux penetrated it.
Among the many advantages of a fusion reactor is the fact that
only a very small amount of fuel would be present in the reactor
vessel at any given time, thereby eliminating the possibility of a
runaway explosion. The interior of the reactor vessel would be
radioactive, but the waste products would not. This would eliminate
the problem of handling highly radioactive waste disposal now common
to all operational fission reactors. The efficiency of the fusion
power plant could be raised to 90% in certain fuel cycles that
would permit direct conversion of the plasma into electricity. Also,
the fuel itself could not be used to make an explosive device; one
would first require a fissionable trigger to detonate the fuel.
Operating a fusion reactor would not require burning any oxygen or
hydrocarbons , and it would not release carbon dioxide or other
combustion products into the air. The only source of
problems would be from the Tritium. Tritium diffuses through most
metallic containers, and is difficult to contain. Routine release of
Tritium would be necessary for operation of the reactor,but it poses
little serious threat as compared to fission reactor byproducts.
There are two general approaches to plasma containment; inertial
and magnetic. Inertial confinement is actually a misnomer since
actual confinement does not occur. In theory, a dense plasma is
heated very rapidly by using lasers or particle beams. "Laser beams
would first heat the surface of a tiny Deuterium-Tritium pellet
causing the material on the surface to blow off; the inward
counterforce would implode the remaining material causing a fusion
reaction to occur."4
Development of this type of research is still quite new compared to
the applications of magnetic confinement. The main hinderance is the
power required for the laser beam. What is needed is a million joules
of energy delivered in less than a nano-second. Magnetic confinement
of plasmas can be divided into open and closed systems. Magnetic
systems have been studied as early as the 1950's. "In an open systems
device the magnetic lines depart from the plasma region rather than
close in on themselves to form a loop."5 The open systems operate
either on the mirror reflection principle, magnetic well, or theta
pinch theory. The mirror reflection device is usually an open tube
with a magnetic field which is weak in the middle and strong at the
ends, thus trapping the plasma in the center. The open systems tend
to leak plasma more readily than the closed systems although both
operate on the principle of magnetic confinement. The best conditions
that these machines have indicated to date are a plasma temperature
of 200 million degrees Kelvin contained for .0001 second with
a particle density of only 108 ions per cubic centimeter.
Reactors operating on the open systems principle are susceptible to
an inherent instability known as micro-instability, which renders them
marginal for use in practical power production units. A modification
was made to the mirror reflection device, and it was renamed the
magnetic well. With the well device, experiments have achieved ion
densities in the 1013 range with a containment time of .0003 second at
a temperature of 200 million degrees Kelvin.
"In most theta pinch devices, a single turn coil is at each end
of an open cylinder. A large capacitor storage bank is rapidly
discharged through the coil, thereby inducing an electric current in
the gas in a direction encircling the axis of the cylindrical volume.
This direction is the 0 direction in the cylindrical coordinates,
thereby giving rise to the name Theta Pinch."6 This electrical
discharge serves to provide a magnetic field, ionize and heat the plasma,
all in a micro-second. The Z pinch device is similar to the theta pinch,
but the difference lies in the direction of the applied magnetic field.
In a closed system device the magnetic field closes in on itself,
forming a circle. The usual configuration for closed systems is the
torus which looks like a donut. The closed systems can be classified
into three types:
1: Stellarators
2: Tokamaks
3: Internal Ring
The stellarator was first built in 1952 at Princeton University.
Coils are built around the torus, and are spaced at intervals. These
coils produce a magnetic field which twists around the central axis of
the toroid. An electrical current is discharged into the plasma to
heat it to high temperatures. The best results from these machines
has been a temperature of only 2 million degrees Kelvin, which is not
even close to the 200 million that is required; and an ion density of
10x13 particles per cubic centimeter with a containment time of 50-4
second.
A more efficient and most promising closed system is known as the
Tokamak, which was developed in Russia in 1968 by Lev Artsimovich.
The windings on a tokamak are quite simple compared to the
stellarator, and serve only to create an external magnetic field.
Because of this,tokamaks can be built to a higher aspect ratio which
tends to stabilize the plasma and permit a higher current discharge
into the plasma for denser confinement. The aspect ratio is the minor
radius compared to the major radius of the torus, meaning they can be
built to larger diameters.
Research at Princeton with a new type of tokamak known as the
Adiabatic Toroidal Compressor utilizing neutral particle injection,
have achieved ion densities of 30x13 at a temperature of 20 million
degrees Kelvin for .01 second. By using this device the plasma
density has increased to a large amount. Studies have concluded that
more optimum conditions can be available in building larger tokamaks.
Another tokamak called ORMAK located at Oak
Ridge laboratory has achieved favorable results. The difference in
ORMAK is the use of a super-cooled transformer. The torus has two
sets of coils around it which serve to center the plasma. The
transformer loops around the core of the torus and serves to heat the
plasma. All of this equipment sits in a large vacuum tank filled with
liquid nitrogen.
The internal ring devices utilize a ring inside the torus for the
purpose of achieving optimum magnetic confining fields with excellent
stability characteristics. These devices are only considered as
research tools and not possible fusion reactor prototypes. The
internal ring tends to conduct heat away from the plasma thus reducing
the probability of achieving the required
temperatures.
Fusion reactors operating on the magnetic confinement principle
will require a minimum temperature of 200 million degrees Kelvin with
an ion density of 10x15 particles per cubic centimeter for at least .1
second in order to undergo a successful fusion reaction process for
the production of useful energy. A typical fusion reactor will
probably use the D-T fuel cycle at first since it is the easiest to
undergo the fusion process. A lithium blanket would surround the
reactor core in order to absorb extraneous neutrons and release
Tritium for the fusion process to utilize. The lithium could also be
used as a heat transfer medium, absorbing the fusion core heat and
transferring it to a heat exchanger to make steam for driving a
turbogenerator. The efficiency would be rated at only about 60%. By
using other fuel cycles it would be possible to directly convert the
plasma stream into electrical current without the use of a
turbogenerator; thus increasing the efficiency closer to the 90%
mark.
The reactor could be a mirror machine where some of the plasma
could escape at one of the open ends and be made to pass though
electrostatic collectors which convert the ions and electrons into
direct current. Experiments at the Livermore laboratory have used the
kinetic energy of a 1,000 electron volt ion beam to directly convert
it into electricity. These studies prove that the theory will work
and could be utilized on a larger scale. The fusion plasma can also
be considered a high temperature heat source that could be used for a
variety of commercial purposes. The plasma can also be used as a
source of large amounts of ultraviolet radiation.
Among the many possible uses are:
1: Desalting of Water
2: Bulk Heating
3: Sterilization of Sewage/Waste
4: Ore/Mineral Processing for Aluminum/Steel
5: Reduction of toxic chemicals to their basic compounds
6: Direct Synthesis of Carbohydrates from carbon
dioxide/water
7: Production of Hydrogen
10: The neutrons could be used to shorten the half-lives of
radioactive wastes.
11: Production of fissionable reactor fuel from Thorium
Considerable progress has been made since the introduction of
fusion research in 1952. Plasma densities and temperatures have
increased significantly, and the confinement times have been shortened
and improved, but there are still problems that are required to be
resolved before a practical fusion reactor can be built for the
production of useable power. The reactor will probably be a
combination of machines now in development, using the advantages of
each one.
Nuclear fusion releases more energy per pound than the fission
process. There is 15 times more energy available in fusing a gram of
hydrogen than there is in fissioning a gram of uranium. When hydrogen
undergoes fusion it releases only .7% of its mass as energy. Further
possibilities of power production include the matter-antimatter
reactions, which would release 100% of their mass as energy and is
140 times more powerful than fusion reactions. Sadly, this reaction
only occurs in nuclear physics labs and is very remote in terms
of an energy source. The fusion process is being developed now,
and the fact that the fuel is almost inexhaustible provides the
strongest incentive for creating additional research and development in the field.
NOTES
1 U.S. Atomic Energy Commission, Atomic Energy Programs: 1972,
Washinton, D.C.: GPO, 1972, p.66
2 "Fusion Principles", Ecyclopedia Americana, 1977 ed. P.511.
3 Encyclopedia Americana, p.511
4 U.S. Atomic Energy Commision, Atomic Energy Programs: 1971.
Washington, D.C. : GPO, 1971, p.73.
5 "Controlled Fusion", Encyclopedia Americana, 1977 ed. P.513
6 Encyclopedia Americana, 1977, p.515
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