Talk:Nuclear fusion/Pompura paper
Controlled
Thermonuclear
Technology
By:
Mike Pompura
Seminole Community College
September 15, 2004
ENC 1101
Maria Brandon
Research Paper 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.
Mike Pompura
Maria Brandon
ENC-1101
9/15/04
CONTROLLED THERMONUCLEAR TECHNOLOGY
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 –
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 of handling 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
1013 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 3013 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 1015 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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Corliss, William. Direct Conversion of Energy. Washington,D.C.: GPO. 1964.
Glasstone, Samuel. Controlled Nuclear Fusion. Wasington,D.C.: GPO, 1968.
Jacobs, D.J. Sources of Tritium and its Behavior Upon Release to the Enviroment.
Washington,D.C.: GPO. 1968.
Laquer, Henry, Cryogenics – The Uncommon Cold. Washiington,D.C.: GPO 1967.
Post, Richard. “Fusion Principles”. Encyclopedia Americana. Ed 1977.
Seaborg, Glenn. Peaceful Uses of Nuclear Energy. Washington,D.C.: GPO, 7/70.
Simon, Albert. “Controlled Nuclear Fusion”. Encyclopedia Americana. ed.1977.
U.S. Atomic Energy Commission. Fundamental Nuclear Energy Researcg-1970.
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U.S Atomic Energy Commission. Fundamental Nuclear Energy Research-1971.
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