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Nuclear Fusion: Holy Grail of Energy

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Chapter
Nuclear Fusion: Holy Grail of
Energy
Quamrul Haider
Abstract
The declining reserves of fossil fuels and their detrimental effects on the environ-
ment have thrust nuclear power based on fission reaction into the limelight as a
promising option to energy-starved economies around the world. However, the 1986
Chernobyl and 2011 Fukushima accidents have heightened our fears about nuclear
technologys ability to provide a safe way of generating clean power. There is another
kind of nuclear energy that has been powering the Sun and stars since their forma-
tion. It is nuclear fusiona process in which two lighter nuclei, typically isotopes of
hydrogen, combine together under conditions of extreme pressure and temperature
to form a heavier nucleus. In this chapter, harnessing the energy produced in nuclear
fusion reaction in a laboratory environment is discussed. Various research programs
dedicated to building fusion reactors are also discussed. Emphasis is given on over-
coming some of the technological challenges, such as surmounting the Coulomb
barrier, confining the plasma, and achieving the ignitiontemperature for fusion.
Keywords: nuclear fission and fusion, fission and fusion reactors, fusion in the Sun,
fusion on Earth, cold fusion, Coulomb barrier, fusion ignitiontemperature,
Lawson criterion, Debye length, plasma confinement, magnetic and inertial
confinements, tokamak, stellarators, fusion torch
1. Introduction
The 1930s were heady times for nuclear physics. A hit paradeof discoveries
gave us new insights into the properties of the nucleus. The means of unlocking
enormous amount of energy stored inside a nucleus seemed at hand. Finally, the
discovery of nuclear fission in 1938 ushered in a new era in the history of mankind
the nuclear age [1].
Nuclear energy is a technologically proven nonfossil energy source that made
significant contribution to the worlds energy supply in the past 6 decades. There are
two nuclear processes in which enormous amount of energy is released from
nuclear bonds between the particles within the nucleus. They are nuclear fission
and nuclear fusion.
2. Nuclear fission
The importance of nuclear fission for the production of energy is obvious. In
fission reactions, a heavy nucleus is split into two lighter fragments and two or three
1
neutrons. About 180 MeV of energy is produced in the fission of an actinide to one
of its most probable daughter pairs. This means that 1 kg of uranium (235 U) is
capable of producing enough energy to keep a 100-Watt light bulb running for
about 25,000 years [2].
2.1 Fission reactors
All nuclear power plants in operation today rely on controlled fission of the
isotopes of uranium and plutonium [3]. The reactor functions primarily as an exotic
heat source to turn water into pressurized steam. Only the source of heat energy
differsnuclear power plants use fissile radioactive nucleus, while nonnuclear
power plants use fossil fuel. The rest of the power train is the same. The steam turns
the turbine blades, the blades generate mechanical energy, the energy runs the
generator, and the generator produces electricity. The major improvement is the
elimination of the combustion products of fossil fuelsthe greenhouse gases, which
have destroyed our environment beyond repair.
Because of its abundance in nature, most nuclear reactors use uranium as fuel.
Natural uranium contains 0.7% of the fissile 235U; the rest is non-fissile 238U. When
235U is bombarded with a slow neutron, it captures the neutron to form 236 U, which
undergoes fission producing two lighter fragments and releases energy together
with two or three neutrons. The neutrons produced in the reaction cause more
fission resulting in a self-sustaining chain reaction. A reactor is considered safe
when a self-sustained chain reaction is maintained with exactly one neutron from
each fission inducing yet another fission reaction.
2.2 Problems and concerns with fission reactors
Although fission-based nuclear reactors generate huge amounts of electricity
with zero greenhouse gas emissions, and thus was hailed as a solution not only to
global warming but also to global energy needs, nuclear energy is now seen by
many, and with good reasons, as the misbegotten stepchild of nuclear weapons
programs. Besides, it is by no means certain that the safety systems designed to shut
down the reactor in the event of a runaway reaction are 100% foolproof and will
work as designed. Another area of great concern is the hazards associated with the
disposal of highly radioactive waste products.
What has raised our fear in regard to nuclear power more than anything else are
the accidents at Chernobyl in 1986 and Fukushima in 2011. They were a sobering
reminder of what we can expect from an accident due to catastrophic reactor failure
or human errors. The Fukushima disaster in particular has shattered the zero risk
myth of power reactors and heightened our concern about the invisibility of the
added lethal component, nuclear radiation. Consequently, they have spurred our
interest in the other source of nuclear energyfusion.
3. Nuclear fusion
Nuclear fusion is the process in which two lighter nuclei, typically isotopes of
hydrogen, combine together under conditions of extreme pressure and temperature
to form a heavier nucleus, resulting in the release of enormous amount of energy.
The fusion of four protons to form the helium nucleus 4He, two positrons, and two
neutrinos, for example, generates about 27 MeV of energy.
2
Nuclear Fusion
In the 1930s, scientists, particularly Hans Bethe, discovered that it is fusion that
has been powering the Sun and stars since their formation [4]. A fusion reactor
buried deep in the Suns interior produces in one heartbeat the energy of 100 billion
nuclear bombs. Beginning in the 1940s, researchers began to look for ways to initiate
and control fusion reactions to produce useful energy on Earth. We now have a very
good understanding of how and under what conditions two nuclei can fuse together.
3.1 Fusion in the Sun
The fusion of hydrogen into helium in the Sun and other stars occurs in three
stages. First, two ordinary hydrogen nuclei (1H), which are actually just single
protons, fuse to form an isotope of hydrogen called deuterium (2H), which contains
one proton and one neutron. A positron (eþ) and a neutrino (ν) are also produced.
The positron is very quickly annihilated in the collision with an electron, and the
neutrino travels right out of the Sun:
1Hþ1H!2Hþeþþν:(1)
Once created, the deuterium fuses with yet another hydrogen nucleus to
produce 3Hean isotope of 4He. At the same time, a high-energy photon, or γray,
is produced. The reaction is
2Hþ1H!3He þγ:(2)
The final step in the reaction chain, which is called the proton-proton cycle,
takes place when a second 3He nucleus, created in the same way as the first, collides
and fuses with another 3He, forming 4He and two protons. In symbols,
3He þ3He !4He þ21H:(3)
The net result of the proton-proton cycle is that four hydrogen nuclei combine to
create one helium nucleus. The mass of the end product is 0:0475 1027 kg less
than the combined mass of the 3He nuclei. This mass difference, known as mass
defect in the parlance of nuclear physics, is converted into 26.7 MeV of energy as
known from Einsteins equation E¼mc2.
The proton-proton cycle is particularly slowonly one collision in about 1026 for
the cycle tostart. As the cycle proceeds, the Suns temperature rises, and eventually
three 4He nuclei combine to produce 12C. Despite the slowness of the proton-proton
cycle, it is the main source of energy for the Sun and for stars less massive than the Sun.
The amount of energy released is enough to keep the Sun shiningfor billions of years.
Besides the proton-proton cycle, there is another important set of hydrogen-
burning reactions called the carbon-nitrogen-oxygen (CNO) cycle that occurs at
higher temperatures. Although CNO cycle contributes only a small amount to the
Suns luminosity, it dominates in stars that are more massive than a few times
the Suns mass. A star like Sirius with somewhat more than twice the mass of the
Sun derives almost all of its energy from the CNO cycle.
4. Coulomb barrier
An obstacle called the Coulomb barrier caused by the strongly repulsive electro-
static forces between the positively charged nuclei prevents them from fusing
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under normal circumstances. However, fusion can occur under conditions of
extreme pressure and temperature. That is why fusion reaction is often termed as
thermonuclear reaction.
Nuclei, which have positive charges, must collide at extremely high speeds to
overcome the Coulomb barrier. The speed of particles in a gas is governed by the
temperature. At the very center of the Sun and other stars, it is extremely hot and
density is very high. For the Sun, the temperature is around 15 million degrees
Celsius, and the central density is about 150 times that of water. Under such
extreme conditions, electrons in an atom become completely detached from the
atomic nucleus, thereby forming an ionized fluid called plasmaasoupof hot
gas, with bare, positively charged atomic nuclei and negatively charged electrons
whizzing about at extremely high speeds. The plasma as a mixture of positive ions
(nuclei) and negative electrons is overall electrically neutral.
Without the high pressure of the overlying layers, the hot plasma at the solar
core would simply explode into space, shutting off the nuclear reactions. The
pressure, which is about 250 billion atmospheres at the Suns core, squeezes the
nuclei so that they are within 1 fm (1015 m) of each other. At this distance, the
attractive strong nuclear force that binds protons and neutrons together in the
nucleus becomes dominant and pulls the incoming particles together, causing them
to fuse.
Additionally, massive gravitational force causes nuclei to be crowded together
very densely. This means collisions occur very frequently, another requirement if a
high fusion rate is to occur. A quick and crude calculation suggests that we need
about 1038 collisions per second to keep the Sun going, while within the core we get
about 1064 collisions per interactions per second, implying only one in 1026 colli-
sions needs to be a successful fusion event.
5. Nuclear fusion on Earth
One of the major challenges in initiating a fusion reaction in a laboratory envi-
ronment on Earth is to create conditions similar to that in the Sunextremely high
temperatures, perhaps more than 100 million degrees Celsius (equivalent to mean
particle kinetic energies of 10 keV) while simultaneously maintaining a high
enough density for a long enough time so that the rate of fusion reactions will be
large enough to generate the desired power.
5.1 Ignitiontemperature
We can estimate the minimum temperature required to initiate fusion by calcu-
lating the Coulomb barrier which opposes two protons approaching each other to
fuse. With e2¼1:44 MeV-fm, where eis the charge of a proton, and r¼1:0fm
(separation between two protons), the height of the Coulomb barrier is
U¼e2
r¼1:44 MeV:(4)
The kinetic energy of the nuclei moving with a speed vis related to the
temperature Tby
1
2mv2¼3
2kBT, (5)
4
Nuclear Fusion
where kB¼8:62 1011 MeV/K is the Boltzmann constant. By equating the average
thermal energy to the Coulomb barrier height and solving for Tgives a value for the
temperature of around 10 billion Kelvin (K).
The above back of the envelopcalculation, using classical physics, does not
take into consideration the quantum effect of tunneling, which predicts there will
be a small probability that the Coulomb barrier will be overcome by nuclei tunnel-
ing through it. The probability Pof such an event happening is
Pexp Z1Z2αc
v

,(6)
where Z1and Z2are the atomic numbers of the interacting particles, α¼1=137 is
the fine structure constant, and vis the relative velocity of the colliding nuclei.
Using the classical turning point r0and de Broglie wavelength λ¼h=p¼h=mv, the
above expression can be written as
Pexp r0
λ

:(7)
Thus, large values of v(high energies), or small λ, favor a fusion reaction.
Taking into account the tunneling probability, we can now estimate the
temperature for fusion to occur. In terms of de Broglie wavelength, the kinetic
energy is
K¼p2
2m¼h2
2mλ2:(8)
If we require that the nuclei must be closer than the de Broglie wavelength for
tunneling to take over and the nuclei to fuse, then the Coulomb barrier is given by
e2
λ¼h2
2mλ2!λ¼h2
2me2:(9)
If we use this wavelength as the distance of closest approach to calculate the
temperature, we obtain
3
2kBT¼e2
λ¼2me4
h2:(10)
Solving for the temperature, we get
T¼4me4
3h2kB
¼mc2α2
3π2kB
:(11)
For two hydrogen nuclei (mc2¼940 MeV), this gives a temperature of about 20
million Kelvin.
6. Fusion reactor
Since the 1950s, scientists have been working tirelessly to develop a reactor in
order to harness the nearly inexhaustible energy produced during fusion [5]. The
goals of fusion research at present include the following:
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1. To achieve the required temperature to ignite the fusion reaction.
2. To keep the plasma together at this temperature long enough to get useful
amounts of energy out of the thermonuclear fusion reactions.
3. To obtain more energy from the thermonuclear reactions than is used to heat
the plasma to the ignition temperature.
To date, much headway has been made toward achieving these goals.
6.1 Fuel
Just like the Sun, the fuel for a fusion reactor is hydrogen, the most abundant
element in the Universe. But without the benefit of gravitational force that is at
work in the Sun, achieving fusion on Earth requires a different approach. The
simplest reaction in which enormous amount of energy will be released is the fusion
of the hydrogen isotopes deuterium (2H) and tritium (3H) producing 4He and a
neutron. For the sake of brevity, we will use the notation d and t for deuterium and
tritium, respectively.
Deuterium is found aplenty in ocean water, enough to last for billions of years.
This makes it an attractive source of alternative energy relative to other sources of
energy. Naturally occurring tritium, on the other hand, is extremely rare. It is
radioactive with a half-life of around 12 years. Trace quantities of tritium can be
found in cosmic rays. Nevertheless, it can be produced inside a reactor by neutron
(n) activation of lithium (Li), the other raw material for fusion found in brines,
minerals, and clays. Because of the abundance of fusion fuel, the amount of energy
that can be released in controlled fusion reactions is virtually unlimited.
For d-t reaction, we must first create the tritium from either flavor of lithium:
6Li þn!4He þt,(12)
or
7Li þn!4He þ4H!4He þtþn:(13)
The next step in the reaction is
dþt!5He !4He þn:(14)
The neutrons generated from the d-t fusion can be used to bombard lithium to
produce helium and tritium, thereby starting a controlled, sustainable chain
reaction.
The mass of the resulting helium atom and neutron is not the exact sum of the
masses of deuterium and tritium. Once again, because of mass defect, each lithium
nucleus converted to tritium will end up yielding about 18 MeV of thermal energy.
Compared to fission, where each split of uranium releases about 200 MeV of
energy, it might appear that the energy released during fusion is rather small. The
discrepancy in the energies lies in the number of nucleons involved in the reactions
more than 200 for fission and 5 for fusion. On a per nucleon basis, fusion releases
18/5 = 3.6 MeV, while fission releases 200/236 = 0.85 MeV. So, fusion wins hands
down, by greater than a factor of 4.
The other fusion scheme for which the required fuel (4He ) will be produced is
dþd!4He. Another reaction, 2Hþ3He!4He þp, is an example of a fusion
6
Nuclear Fusion
reaction that releases its energy entirely in the form of charged particles, rather than
neutrons, thereby offering the possibility, at least in principle, of direct conversion
of fusion energy into electrical energy. However, the cross sections and reaction
rates for both the reactions are as much as a factor of 10 lower than the d-t reaction.
Moreover, because of the higher Coulomb barrier (2.88 MeV), the ignition tem-
peratures required for 2Hþ3He reaction are much higher than those of d-t fusion.
An interesting fusion reaction is a proton colliding with boron (B). The proton
fuses with 11B to form 12 C which immediately decays into three alpha (4H nucleus)
particles. A total energy of 8.7 MeV is released in the form of kinetic energy of the
alpha particles. Since it is relatively easy to control the energy of the proton with
todays accelerator technology, this fusion reaction can be easily initiated without
involving other interaction channels.
6.2 Conditions for fusion reaction
In order to attain the temperature for fusion to occur, the plasma has to meet
some conditions. They are Lawson criterion and Debye length.
6.2.1 Lawson criterion
In addition to providing a sufficiently high temperature to enable the particles to
overcome the Coulomb barrier, a critical density of the ions in the plasma must be
maintained to make the probability of fusion high enough to achieve a net yield of
energy from the reaction. The condition which must be met for a yield of more
energy than is required for the heating of the plasma is stated in terms of the
product of the plasma density (nd) and confinement time (τ). The product has to
satisfy the inequality:
ndτ31020 s=m3:(15)
This relation is called the Lawson criterion [6]. Researchers sometimes use the triple
product of nd,τ, and the plasma temperature T. Called the fusion product, the
condition for fusion to take place is
ndτT51021 s:keV=m3:(16)
To summarize, three main conditions are necessary for nuclear fusion:
1. The temperature must be hot enough to allow the ions to overcome the
Coulomb barrier and fuse together. This requires a temperature of at least 100
million degrees Celsius.
2. The ions have to be confined together in close proximity to allow them to fuse.
A suitable ion density is 2 31020 ions=m3.
3. The ions must be held together in close proximity at high temperature long
enough to avoid plasma cooling.
At higher densities, charged particles in the plasma moving at high speeds may
give rise to bremsstrahlungradiation given off by a charged particle (most often
an electron) due to its acceleration caused by an electric field of another charged
particle (most often a proton or an atomic nucleus). Bremsstrahlung could become
so dominant that all the energy in the plasma may radiate away. Other radiation
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losses, including synchrotron radiation from charged particles orbiting about mag-
netic fields would be negligible. A fusion reactor, therefore, has to be operated at a
temperature where the power gain from fusion would exceed the bremsstrahlung
losses.
6.2.2 Debye length
A parameter that determines the electrostatic properties of a plasma is called the
Debye length LD[7]:
LDffiffiffiffiffiffiffiffi
kBT
nd
s:(17)
It is a length scale over which electrons screen out electric fields in the plasma. In
other words, it is the distance over which significant charge separation can occur and
how far its electrostatic effect persists. For distances greater than the Debye length,
the energy of the particles in the plasma balances the electrostatic potential energy.
Using nd¼1028 particles/m3, the Debye length for a 10 keV plasma is of the
order of 10 nm, and the number of particles in a volume of the plasma of one Debye
length is about 104. For a more rarefied plasma, say nd¼1022 particles/m3,
LD¼10 μm, and the number of particles in a volume of dimension of one Debye
length is 107:In either of these two extreme cases, there are two basic properties:
the physical size of the plasma is far larger than the Debye length, and there are
many particles in a spherical volume of radius equal to one Debye length. They are
these two properties that describe the hot thermonuclear fuel.
7. Plasma confinement
Just like a conventional power plant, a fusion power plant will use the energy
released during fusion reaction to produce steam and then generate electricity by
way of turbines and generators. But as noted in the above discussions, it is hard to
harness the energy in a laboratory environment.
Each fusion reaction is characterized by a specific ignition temperature, which
must be surpassed before the reaction can occur. In stars, which are made of
plasma, fusion takes place because of immense gravitational forces and extreme
temperatures. Trying to create similar conditions here on Earth has required fun-
damental advances in a number of fields, from quantum physics to materials sci-
ence. Scientists and engineers have made enough progress over the past half
century, especially since the 1990s, so that a fusion reactor able to generate more
power than it takes to operate can be built. Supercomputing has helped enormously,
allowing researchers to precisely model the behavior of plasma under different
conditions.
One of the major requirements in the development of a fusion reactor is the
actual realization of the ignition temperature of d-t reaction, which is 100 million
degrees Celsius. Once all the conditions are realized, the challenge to contain and
control the staggering levels of heat in the plasma is formidable. That is because the
plasma must not only be heated to a temperature of at least 100 million degrees
Celsius, but the energy must also be confined within the plasma without being
carried to walls of the container for times long enough for the relatively infrequent
fusion events to occur. Otherwise, the plasma will exchange energy with the walls,
cool itself down, and melt the container.
8
Nuclear Fusion
Many techniques have been developed, but the two main experimental
approaches that seem capable of doing this task are magnetic confinement and
inertial confinement.
7.1 Magnetic confinement
This method uses strong magnetic fields to contain the hot plasma and prevent it
from coming into contact with the reactor walls. The magnetic fields keep the
plasma in perpetually looping paths because the electrical charges on the separated
ions and electrons mean that they follow the magnetic field lines. As a consequence,
the plasma does not touch the wall of the container.
There are several types of magnetic confinement system, but the approaches
that have been developed to the point of being used in a reactor are tokamak and
stellarator devices. Because of its versatility, tokamak is considered to be the most
developed magnetic confinement system. Hence, it is the workhorse of fusion.
7.1.1 Tokamak
The tokamak, acronym for the Russian phrase toroidálnaja kámera s magnitnymi
katúškami meaning toroidal chamber with magnetic coils, was designed in 1951 by
Soviet physicists Andrei Sakharov and Igor Tamm [8]. It is a doughnut-shaped
device in which the combination of two sets of magnetic coils, known as toroidal
and poloidal field coils, creates a field in both vertical and horizontal directions. The
magnetic fields hold and shape the charged particles of the plasma by forcing them
to follow the magnetic field lines. They essentially create a cage,a magnetic
bottle, inside which the plasma is confined. A strong electric current is induced in
the plasma using a central solenoid, and this induced current also contributes to the
poloidal field.
7.1.2 Stellarators
Unlike tokamaks, stellarators [9, 10] do not require a toroidal current to be
induced in the plasma. Instead, the plasma is confined and heated by means of
helical magnetic field lines. They are produced by a series of coils which may
themselves be helical in shape. As a result, plasma stability is increased compared
with tokamaks. Since heating the plasma can be more easily controlled and moni-
tored with stellarators, they have an intrinsic potential for steady-state, continuous
operation. The disadvantage is that, due to their more complex shape, stellarators
are much more complicated than tokamaks to design and build.
7.2 Inertial confinement
After the invention of laser in 1960 at Hughes Research Laboratory in California,
researchers sought to heat the fusion fuels with a laser so suddenly that the plasma
would not have time to escape before it was burned in the fusion reaction. It would
be trapped by its own inertia, hence the name inertial confinement,because it
relies on the inertia of the implosion to bring nuclei close together. This approach
to confinement was developed at Lawrence Livermore National Laboratory in
California [11].
Within the context of inertial confinement, laser beams with an intensity of the
order of 1014 1015 W=cm2are fired on a solid pellet filled with a low-density
mixture of deuterium and tritium. The energy of the laser vaporizes the pellet
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instantly producing a surrounding plasma environment for a short period of time.
During the process, the density and temperature of the fuel attains a high enough
value to ignite the fusion reaction.
The capability of present lasers does not allow the inertial confinement tech-
nique to obtain break-even conditions, simply because the efficiency for converting
electrical energy into radiation is very low, about 110%. Consequently, alternative
approaches are being explored to achieve the ignition temperature. One such
approach involves using beams of charged particles instead of lasers.
8. Cold fusion
In 1989, researchers at University of Utah (USA) and University of Southamp-
ton (UK) claimed to have achieved fusion at room temperature in a simple tabletop
experiment involving the electrolysis of heavy water (deuterium oxide) using pal-
ladium electrodes [12]. According to them, when electric current passed through
the water, palladium catalyzed fusion by allowing deuterium atoms to get close
enough for fusion to occur. Since other experimenters failed to replicate their claim,
most of the scientific community no longer considers it a real phenomenon.
But in 2005, cold fusion got a major boost. Scientists at UCLA initiated fusion
using a pyroelectric crystal [13]. They put the crystal into a small container filled
with hydrogen, warmed the crystal to produce an electric field, and inserted a metal
wire into the container to focus the charge. The focused electric field powerfully
repelled the positively charged hydrogen nuclei, and in the rush away from the
wire, the nuclei smashed into each other with enough force to fuse. The reaction
took place at room temperature.
9. Fusion research
The aim of the controlled fusion research program is to achieve ignition, which
occurs when enough fusion reactions take place for the process to become self-
sustaining, with fresh fuel then being added to continue it. Once ignition is
achieved, there is a net energy yieldabout four times as much as with nuclear
fission. As mentioned earlier, such conditions can occur when the temperature
increases, causing the ions in the plasma to move faster and eventually reach speeds
high enough to bring the ions close enough together. The nuclei can then fuse,
causing a release of energy.
The plasma temperature needed for ignition is produced by external heating.
Powerful methods were developed for this purpose. They are:
1. Heating by injection of neutral beams: In this method, neutralized particles
with high kinetic energy, produced in an ion source, are injected into the
plasma, whereby they transfer their energy to the plasma through collisions.
2. Heating by high-frequency radio or microwaves: When electromagnetic waves
of appropriate frequency are beamed into the plasma, the plasma particles
absorb energy from the field of the wave and transfer it to the other particles
through collisions.
3. Heating with current: When an electric current is passed through the plasma, it
generates heat in the plasma through its resistance. As the resistance decreases
with increasing temperature, this method is only suitable for initial heating.
10
Nuclear Fusion
These methods produce temperatures of 100 million degrees Celsius in present-day
fusion devices.
9.1 Research programs
Experiments with d-t fuel began in the early 1990s in the Tokamak Fusion Test
Reactor in Princeton (USA) [14] and the Joint European Torus (JET) in Culham
(UK) [15]. The worlds first controlled release of fusion power using a 5050 mix of
tritium and deuterium with a fusion output of 16 MW from an input of 24 MW
heating (Q-factor is 0.67) was achieved in 1991 by JET. The Q-factor is used to
represent the ratio of the power produced in the fusion reaction to the power
required to produce the fusion. It should not be confused with the Q-value of a
reaction, which is the amount of energy released by that reaction. Obviously,
Q-factor of 1 is breakeven. To achieve commercially viable fusion energy, the
Q-factor must be much greater than one.
The 35-nation International Thermonuclear Experimental Reactor (ITER, The
Wayin Latin) project currently under construction in Cadarache, France, is the
worlds largest tokamak fusion reactor [16]. The goals of ITER are:
1. To operate at 500 MW (for at least 400 s continuously) with less than 50 MW
of input power for a tenfold energy gain (Q-factor is 10).
2. Demonstrate the integrated operation of technologies for a fusion power
plant and test technologies for heating, control, diagnostics, cryogenics, and
remote maintenance.
3. Achieve a deuterium-tritium plasma in which the reaction is sustained
through internal heating and stays confined within the plasma efficiently
enough for the reaction to be sustained for a long duration.
4.Test tritium breeding because the world supply of tritium (used with
deuterium to fuel the fusion reaction) is not sufficient to cover the needs of
future power plants.
5. Demonstrate the safety characteristics of a fusion device, particularly the
control of the plasma and fusion reactions with negligible consequences to the
environment.
Launched in 2006, the project has been beset with technical delays, labyrinthine
decision-making, and cost estimates that have soared. The reactor is now expected
to be completed and become operational by 2030.
According to ITER Newsletter [16], When completed, the plasma circulating in
the core of the reactor will be 150 million degrees Celsius, or about 10 times hotter
than the Sun. The massive superconducting magnets surrounding the core will be
cooled to 270 degrees, as cold as the depths of space. So many of the technologies
involved are really at the cutting edge.
There is a considerable amount of research into many other fusion projects at
various stages of development, but ITER is the largest, with 10 times more plasma
capacity than any other reactor. Although China is a participating country in the
ITER project, the Chinese are nevertheless building a tokamak reactor by them-
selves. Known as the Experimental Advanced Superconducting Tokamak (EAST),
they managed to heat hydrogen gas to a temperature of about 50 million degrees
Celsius [17].
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Based on the information, technologies, and experience provided by ITER,
physicists and engineers at the Culham Laboratory in Oxfordshire (UK) are work-
ing to develop a Demonstration Power Station (DEMO) which, if successful in
terms of systems and performance, could be used as the commercial prototype,
creating a fast track to fusion power. In collaboration with the Princeton Plasma
Physics Laboratory, South Korea is also developing a tokamak fusion reactor named
Korean Demonstration Fusion Power Plant (K-DEMO) [18]. Both EAST and
K-DEMO are due for completion by year 2030.
Under an Italian-Russian agreement, Italys National Agency for New Technolo-
gies, Energy and Sustainable Economic Development is developing a small tokamak
reactor by the name of Ignitor [19]. The reactor is based on the Alcator machine at
MIT [20] which pioneered the high magnetic field approach to plasma magnetic
confinement. The scientists of the project believe that unlike the larger ITER reac-
tor, Ignitor could be ready to begin operations within a few years.
By using magnetic fields that are twice as strong as those planned for ITER, two
spin-off companies, one in the USA and the other in the UK, hope to create a
sustainable fusion reaction in a machine as small as 1/70th the size of ITER. They
also believe, according to the August 2018 issue of Physics Today, that their reactor
will be able to produce more energy than they consume. It is expected to be
operational before ITER, possibly by the mid-2020s.
The Germans are working on a non-tokamak fusion reactor called Wendelstein
7-X [21]. In a test run, they produced helium plasma that lasted for one-quarter of a
second and achieved a temperature of 80 million degrees Celsius. The Germans
believe that their stellarator design, similar in principal to the tokamaks, will pro-
vide an inherently more stable environment for plasma and a more promising route
for nuclear fusion research in general.
Another stellarator, TJ-II, designed in collaboration with Oak Ridge National
Laboratory (USA), is in operation in Madrid, Spain [22]. This flagship project of the
National Fusion Laboratory of Spain is a flexible, medium-size stellaratorthe
second largest operational stellarator in Europe, after Wendelstein 7-X.
In 2014, scientists and engineers at the American aerospace conglomerate
Lockheed Martin claimed to have made a major technological breakthrough in the
development of a fusion reactor [23]. They are cautiously optimistic that an opera-
tional reactor with enough energy output to power a small city, yet small enough to
fit on the back of a truck, can be built before the end of this decade. However,
because of the absence of further details on how their reactor works, some scientists
are skeptical about the claim.
According to MIT Technology Review [24], while ongoing research centered on
large tokamaks may take decades before a commercially feasible fusion reactor is
built, several privately funded companies and small university-based research
groups pursuing novel fusion reactor designs have delivered promising results that
could shorten the timeline for producing a prototype machine from decades to
several years. On the other hand, scientists of the mega-projects believe that fusion
power could become a reality more quickly if the present international funding for
fusion research was increased.
There have also been significant developments in research into inertial confine-
ment fusion (ICF). Research on ICF in the USA is going on at the National Ignition
Facility [25] at the Lawrence Livermore National Laboratory in California and
Sandia Laboratories in New Mexico. At Sandia, an entirely different method of ICF
called the Z-pinch [26], which does not use laser at all, is being investigated.
Instead, it uses a strong electrical current in a plasma to generate X-rays, which
compresses a tiny d-t fuel cylinder. The other notable research activity on ICF is the
Laser Megajoule project in Bordeaux, France [27]. All three projects are designed to
12
Nuclear Fusion
deliver, in a few billionths of a second, nearly 2 million Joules of energy to targets
measuring a few millimeters in size. The main purpose of these projects is, however,
to support research for nuclear weapons programs.
Thus far, none of the ICF facilities have achieved scientific breakeven, which is a
gain of unity. However, for making fusion energy viable in commercial power
plants, the gain has to be much greater than breakeven. Since lasers are very
inefficient machines, gains of at least 100 are needed for a plant to produce net
power output. To that end, researchers at Lawrence Livermore National Laboratory
are exploring other approaches to developing ICF as a source for energy.
10. Advantages/disadvantages of fusion reactors
There are many advantages of fusion reactors:
1. They will produce at least five times more energy than the amount of energy it
will need to heat the fusing nuclei to the desired temperature. Furthermore, it
is estimated that to run a 1000 MW power plant for a year, a fusion reactor will
require about 3000 m3of water (source of deuterium) and 10 tonnes of
lithium ore, while the current fission reactors consume 2530 tonnes of
enriched uranium. Clearly, gram for gram, fusion reactor wins the energy race
by a wide margin.
2. Fusion fuels are widely available and nearly inexhaustible. Deuterium can be
distilled from all forms of water, while reserves of lithium, both terrestrial and
sea-based, which would be used to produce tritium, would fulfill needs of
fusion reactors for millions of years.
3. Unlike fission, fusion will have a low burden of radioactive waste. They will
not produce high-level nuclear wastes like their fission counterparts, so
disposal will be less of a problem. Fusions by-product is heliuman inert,
nontoxic, and nonradioactive gas used to inflate childrensballoons. Besides,
there will be no fissile material that could be diverted by terrorists to build
dirty bombs.Moreover, a fusion power station would not require the
transport of hazardous radioactive materials.
4.Fusion reactors are inherently incapable of a runaway reaction that could result
in a core meltdown, the most serious calamity possible in a fission reactor. This
is because there is no critical mass required for fusion. Besides, fusion reactors
work like a gas burner; once the fuel supply is shut off, the reaction stops.
There will, therefore, be no off-site radiation-related deaths, even from a
severe accident.
5. Despite being technically nonrenewable, fusion has many of the benefits of
renewable energy sources, such as being a long-term source of energy
emitting no greenhouse gases. Besides, because it is not dependent on weather,
fusion could provide uninterrupted power delivery, unlike solar and wind
power.
Although fusion does not generate long-lived radioactive products and the
unburned gases can be treated on site, there are nevertheless few concerns related
to the radioactivity induced by the high-energy neutrons (14 MeV) that are
produced during the d-t reaction. They are:
13
Nuclear Fusion: Holy Grail of Energy
DOI: http://dx.doi.org/10.5772/intechopen.82335
1. Some radioactive wastes will be produced due to neutron activation of lithium
to produce tritium inside the reactor, but their inventory will be much less
than those from fission, and they will be short-lived. Nonetheless, if
accidentally released in the air or water, tritium will remain radioactive for a
period equal to at least 10 half-lives or 120 years.
2. The neutrons will irradiate the surrounding structures giving rise to radioactive
nuclides, which ultimately have to be disposed of in some waste facility. But
their stock will be considerably lower than that from actinides used in fission-
based reactors.
3. Since most of the energy in the d-t reaction is carried away by the neutrons,
this could lead to neutron leakage that could be significantly higher than
uranium reactors. More neutron leakage means more shielding and improved
protection for workers at the power plant.
11. Fusion torch
A fascinating application for the abundant energy that fusion may provide is the
fusion torch, a star-hot flame or high-temperature plasma into which all waste
materialswhether liquid sewage or solid industrial refusecould be dumped [28].
In the high-temperature environment, the materials would be reduced to their
constituent atoms and separated by a mass-spectrograph-type device into various
bins ranging from hydrogen to uranium. Thus, a single fusion plant could, in
principle, not only dispose of thousands of tons of solid wastes per day but also
convert them into a few reusable and saleable elements, thereby closing the cycle
from use to reuse.
12. Conclusion
Projection for the demand of energy depends on the growth of population,
because the more people there is, the more energy will be used. The current world
population of 7 billion is expected to reach 11 billion in 2100. This means if we want
to maintain a better or at least the same standard of living, global consumption of
energy could double, or perhaps triple, by the end of this century.
With the incorporation of improved safety features and new generation of
reactors, nuclear fission will probably continue to make a major contribution to
electricity generation. However, its growth could be curtailed by issues of public
and political acceptability. Supplies from some renewable sources of energy, such as
solar or wind, are not guaranteed either, because they are reliant on weather
conditions. Technological challenges for other sources, ocean thermal energy and
hydrokinetic energy from rivers, for example, have not yet been fully developed.
So, for future energy security, the answer is nuclear fusion.
Advocates acknowledge that fusion technology is likely many decades away. The
reason, these systems are intrinsically large, so large that we cannot test the physics
and technology of fusion on a lab bench and then mass-produce fusion reactors.
Consequently, these large, first-of-a-kind facilities take time to construct.
Despite the enormity of the projects, we have succeeded in creating a short-lived
artificial Sun on Earth via experimental fusion reactors. Once commercial fusion
reactors become a reality, there will be a paradigm-shifting development in the
global energy mix. In particular, our dependence on the rapidly depleting supply of
14
Nuclear Fusion
fossil fuels and uranium will be drastically reduced. More importantly, fusion
power can easily secure our planets future, given the abundance of fuels and near-
limitless energy produced from fusion reactions. Additionally, with no risk for
proliferation and minimum radioactive waste generated, nuclear fusion would offer
a clean, relatively safe, zero greenhouse gas-emitting, and long-term source of
energy, with the potential to produce at least 2025% of the worlds electricity by
2100.
To conclude, nuclear fusion energy may not have the magic wand that would
solve our energy problem. Nevertheless, it has the potential to be an attractive
energy source that can be deployed as major pressures rise on existing energy
supply options. Also, it would go a long way in slowing down, if not mitigating, the
unrelenting climb of the temperature of our planet.
Author details
Quamrul Haider
Department of Physics and Engineering Physics, Fordham University, Bronx, New
York, USA
*Address all correspondence to: haider@fordham.edu
© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms
of the Creative Commons Attribution License (http://creativecommons.org/licenses/
by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,
provided the original work is properly cited.
15
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DOI: http://dx.doi.org/10.5772/intechopen.82335
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Nuclear Fusion: Holy Grail of Energy
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