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RAU Scientific Reports. Computer Modeling & New Technologies, 1998, Volume 2, 50-56
Riga Aviation University, Lomonosov Str.1, Riga, LV-1019, Latvia
NUCLEAR FUSION
O. DUMBRAJS
Department of Engineering Physics and Mathematics, Helsinki University of Technology, FIN-02150
Espoo, Finland, e-mail: [email protected]
Nuclear fusion is the energy-production process that takes place continuously in the sun and stars. It is the reaction in
which light nuclei combine together, or fuse, forming a heavier nucleus and releasing energy. Controlled thermonuclear fusion on
earth is potentially a major vast new energy source suited to the industrialized word. A reactor based on nuclear fusion would be
inherently safe and environmentally friendly. Furthermore, fuels are cheap, abundant and widely available.
1. Introduction
At present about six billion people live on earth. Their energy consumption is about 13 TWa (1
TWa equals 1012 Watts per year), of which 3.8 TWa is the electric energy. The energy consumption on
average per head is 2.2 kWa: in Europe 5.9 kWa, in the USA 10.9 kWa, in China 0.83 kWa, and in India
0.32 kWa. Annually 6% energy is produced in nuclear power stations and 7% in hydroelectric power
stations. The rest comes from the burning of fossil fuels: 10% wood, 26% coal, 39% oil, and 22% gas.
Annually 20% electrical energy is produced in nuclear power stations, 20% in hydroelectric power
stations, and 60% by burning fossils. It is expected that by the year 2050 ten billion people will live on
earth and will consume 30 TWa energy, of which 12 TWa as the electric energy.
Two hundred years ago the production of CO2 was 280 PPM with zero growth rate. Now it is
350 PPM with the growth rate of 1.5 ppp/a, and in 50 years it may reach 430 PPM with the growth rate 2
PPM/a. The growth rate of CO2 emission now is 60 times higher than in all the previous 160 000 years
together. The problem of the atmosphere pollution has become very acute.
One TWa electric energy can be produced either by 3000 coal-fired power stations, or 2000
fission nuclear power stations, or l00 000 km2 solar panels, or 10 millions wind power stations, or by 1000
nuclear fusion power stations.
It is rather obvious that nuclear energy is the only option for a truly sustainable or long term
energy source. Unfortunately fission, in addition to safety problems (Chernobyl accident!), faces severe
risks related to production of long-lived radioactive waste. Fusion will be environmentally sound without
pollutants or contribution to global warming. The hazard potential would be limited since there would be
no long-lived radioactive waste from the reaction products, including the fuel ”ash.” Radioactivity of the
reactor structure, caused by the neutrons, decays rapidly and can be minimized by careful selection of
low-activation materials. Fusion is also inherently safe since any malfunction results in a rapid shutdown
of the reactor. It will be economically attractive and capable of producing the energy that future
generations will require.
2. Fusion reactions
The basic principle of nuclear fusion is the fusing together of light nuclei to form heavier ones and in this
process a small quantity of mass is converted into a large amount of energy. For energy production on
earth different fusion reactions can be used:
D + T → 4He + n+
D + D → 3He + n+
D + D → T + p+
D + 4He → He + p+
17.58 MeV
3.27 MeV
4.03 MeV
18.35 MeV
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For example, after fusion of the deuterium and tritium nuclei 17.58 MeV energy is released (1
MeV = 4.45x10-20 kWh). Just this reaction seems to be the most suitable reaction for future fusion power
stations. There is no lack of fuel. Deuterium (heavy water) is abundant as it can be extracted from all
forms of water. If all the world’s electricity were to be provided by fusion power stations, Deuterium
supplies would last for millions of years. Tritium does not occur naturally but it can be manufacture from
Lithium within the machine. Here the so-called Tritium breeding reactions are used:
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Li + n → 4He + T + n
Li + n → 4He + T
Lithium, the lightest metal, is plentiful in the earth’s crust. Once the reaction is established, even though it
occurs between Deuterium and Tritium, the consumable are Deuterium and Lithium. For example, 10
grams of Deuterium that can be extracted from 500 liters of water and 15 grams of Tritium produced from
30 grams of Lithium would produce enough fuel for the lifetime electricity needs of an average person in
an industrialized country.
3. Conditions for a fusion reaction
The problem of fusion reactions is the fact that initial particles are positively charged and to
induce the reaction one has first to overcome the Coulomb barrier. This occurs in a plasma where the
electrons have been separated from the atomic nuclei (usually called the ”ions”).
1.000.000
Zündung
Fusionprodukt = Dichte x Energieschlußzeit x Temperatur
(1017 Teilchen pro Kubikzentimeter x Sekunde x Grad)
JT 60-U
WENDELSTEIN 7-X
100.000
JET
TFTR
TRTF
(DT)
DIII-D
ALCATOR
10.000
JET
1.000
T10
Tore
ASDEX
Supra
WENDELSTEIN
7-AS
JET
(DT)
TFTR
JT 60
ASDEX
Pulsator
100
geplant
WENDELSTEIN
7-A
bis 1995
ISAR 1
ALCATOR: Boston, USA
ASDEX: Garching, D
D III-D: San-Diego, USA
ISAR 1: Garching, D
JET: Culham, GB
JT 60, JT 60-U: Naka, Japan
TFTR: Princeton, USA
Tore Supra: Cadarche, F
T3, T10: Moskau, Rußland
WENDELSTEIN: Garching, D
bis 1986
10
T3
bis 1977
T3
bis 1965
1
1
10
100
200
500
1.000
Temperatur (Millionen Grad)
Figure 1. Progress in nuclear fusion
Fusion reactions occur at a sufficient rate only at very high temperatures. The hot plasma must be
well isolated away from material surfaces in order to avoid cooling the plasma. The efficiency of
insulation is measured by a quantity called energy confinement time. This is the characteristic time-scale
for plasma cooling when the source of heat is removed. The density of fuel ions must be sufficiently large
for fusion reactions to take place at the required rate. The fusion power generated is reduced if the fuel is
diluted by impurity atoms released from surrounding material surfaces or by the accumulation of Helium
”ash” from the fusion reaction. As fuel ions are burnt in the fusion process they must be replaced by new
fuel and the Helium ash must be removed.
Numerical values for the Deuterium-Tritium reaction can be summarized as follows:
i) plasma temperature: T ≈ 100 – 200 million °C (10 - 20 keV), which is ten times greater than
the temperature in the center of the sun;
ii) energy confinement time: τ ≈ 1 – 2 seconds;
iii) plasma density: n ≈ 2-3·1014 particles cm-3 which corresponds to few thousandths of a gram per cubic
meter.
Overall the required conditions are measured by a product of these three values called the fusion
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product, n ⋅ τ ⋅ T , which is laid off as ordinate in the Figure 1 (see, [1] ).
It is encouraging to observe the tremendous progress achieved during the years l965 – 1995 in
approaching the plasma ignition (Zündung) region.
4. Plasma confinement
It is very difficult to provide the necessary confinement time. Here two principally different
methods are used. The first method is the inertial confinement by means of lasers in which extremely high
density of the plasma is achieved (n ∼ 1025 cm-3) which allows one significant1y to reduce the confinement
time ( τ ∼ 10-9 sec) by keeping constant the fusion product. This is the so-called inertial fusion that is also
of interest for developing nuclear weapons and which is not discussed further here. The second method is
the magnetic confinement of plasma by means of very strong magnetic fields. This is the so-called
magnetically confined fusion and constitutes the subject of the present discussion.
There are two different possibilities to magnetically confine the plasma: tokamaks and stellarators.
The tokamak is the most advanced concept for containing magnetically a hot dense plasma. It
originated in the USSR. A toroidal, axisymmetric plasma is confined by the combination of a large
toroidal magnetic field that is produced by coils surrounding the vacuum vessel, and a smaller poloidal
magnetic field created by a toroidal current through the plasma.
Tokamak
transformator coils
Figure 2. Principal scheme of TOKAMAK
vertical field coils
plasma current
plasma
In the stellarator there is only one
confining magnetic field that is
produced exclusively by a set of
complicated magnetic coils. No
plasma current is needed. This is a
great advantage in comparison with a
tokamak, because it makes a
continuous operation of the machine
possible in which no plasma
disruption can occur.
toroidal field coils
magnetic field line
Stellarator
Figure 3. Principal scheme
of stellarator
5. Plasma heating and current drive
To
ensure
the
high
temperature needed for occurrence of
fusion reactions, the plasma has to be
heated. In tokamaks also a plasma
current has to be generated (driven).
Three methods are used to accomplish
this.
i) Ohmic heating
The electric current is
induced in plasma which deposits
heating power. Here the plasma itself
plasma
magnetic coil
plays the role of the secondary
magnetic field line
winding. To ensure the same direction
of the plasma current all the time, only
positive or negative half-cycles of voltage can be used which means that the current pulsates. Moreover,
since the electric resistance of plasma decreases with increasing temperature, this method of heating can
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be effectively used only at low temperatures, that is, for preheating the plasma.
Figure 4. Ohmic heating of plasma
current
field lines
coil current
plasma current
Figure 5. Neutral beam heating
magnetic field
ion source and
accelerator
gas chamber
charge carrier
ii) Neutral beam heating
Beams of deuterium
or tritium ions, accelerated
by very high potential, are
injected into the plasma. In
order to penetrate the
confining magnetic field, the
accelerated
beams
are
neutralized. In the plasma,
the beams become ionized
and the fast ions give up
their energy to the plasma
ions and electrons by
colliding with them.
diverting
magnet
plasma
iii) Radio-frequency heating
The plasma ions
and electrons rotate around
in the magnetic field lines of
the tokamak or stellarator.
Energy is given to the
plasma at the precise
location
where
the
radiowaves resonate with the
particle rotation. Here the
two components of the
plasma: ions and electrons
have to be distinguished.
neutral particles
motion of
ions
Figure 6. Radio-frequency
heating
+
Ions
are
heated
by
electromagnetic waves at
frequencies between 10 and
direction of
l00 MHz. Such waves are
magnetic field
generated
by
klystrons.
direction of
Electrons are much lighter
electric field
than ions and are heated by
electromagnetic
waves
generated by gyrotrons at much higher frequencies between 60 and l70 GHz.
The helium nuclei (alpha-particles) produced when Deuterium and Tritium fuse remain within
the plasma’s magnetic trap. Their energy continues to heat the plasma to keep the fusion reaction going.
When the power from the alpha-particles is sufficient to maintain the plasma temperature, the reaction
becomes self-heated: a condition referred as ignition.
6. National programs of fusion research
Many countries have their own fusion research programs: European Union, USA, Russia, and
Japan, supported by vigorous programs in China, Brazil, Canada, and Korea. In the following Table 1 the
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most important current and future fusion experiments in the European Union are listed.
TABLE 1. Important current and future fusion experiments in the European Union
Experiment
JET
Type
tokamak
TEXTOR
TORE SUPRA
tokamak
tokamak
Laboratory
Joint European Torus, Culham,
UK
FA, Jülich. Germany
CEA, Cadarache, France
ASDEX Upgrade
tokamak
IPP, Garching, Germany
WENDELSTEIN
7-AS
stellarator
IPP, Garching, Germany
WENDELSTEIN
7-X
stellarator
IPP, Greifswald,
Germany
Task
Plasma physics studies in the
region close to ignition
Studies of plasma-wall interaction
Testing of super- conducting
coils, stationary operation
Plasma boundary studies in
divertor plasmas
Testing the principles of
”advanced stellarator”
Begin of operation
1983
Testing feasibility of ”advanced
stellarator” for power station
2004
1982
1988
1990
1988
It should be noted that there are fusion installations also in Belgium, Denmark, Portugal, Spain,
Sweden, and Switzerland. Two other EU countries, Finland and Greece, also have their own fusion
research programs, albeit without own fusion machines. In the European Union all the fusion research is
supervised and coordinated by EURATOM.
7. Finnish fusion research program
The following chart illustrates organization of fusion research in Finland:
Industry
Finnfusion
industrial R&D
prestudies
Academy of Science
TEKES, Sitra
Ministry of Trade and Industry
Steering Committee
Fusion Program Coordination
VTT - Energy
Material Research
- reactor materials
- materials testing
- ion beam studies
- fusion neutronics
Fusion Plasma
Research
Experiments
- theory/simulations
- rf-heating
- rf-current drive
- diagnostics
JET
Garching
Karlsruhe
Lausanne
Institutes: VTT Technical Research Center of Finland , Helsinki University of Technology , University of Helsinki , Technology
Center Pripoli (Finnfusion)
An important task of the plasma and fusion physics groups is also to sustain the basic knowledge
of fusion physics and reactor technology and to provide relevant education on these topics.
The total budget of the Finnish Fusion Research Program for the year 1997 was about four million US
dollars.
8. International thermonuclear experimental reactor (ITER)
The ITER project was initiated in l985. Its participants are European Union, Russia, Japan, and
USA. Each participant is represented by the so-called home team. The European home team resides in
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Garching, Germany. The headquarters coordinating the entire ITER relevant research are located in San
Diego, USA.
ITER may be considered as the first component in a worldwide collaborative program, to work
more efficiently and to achieve effective solutions to the scientific and technological challenges presented
by fusion research. The advantage of such a program must be to:
n reduce scientific and technological risks;
n allow flexibility to accommodate new concepts;
n provide a wider and more comprehensive database;
n offer flexibility in location and time scheduling;
n be more practical in industrial terms.
This program should minimize risks and overall costs and should ensure efficient use of world resources.
Figure 7. ITER tokamak
ITER is based on the tokamak concept. It will be able to operate for very long periods (over 1000 second
pulses) and will help to demonstrate the scientific and technological feasibility of fusion power. Most
importantly, it will be the first fusion device designed to achieve ignition and sustained burn – at which
point the reactor becomes self-heating and productive. ITER is planned to be operational by 2010. Its
location has yet to be determined. Characteristic ITER data are summarized in the following Table 2 .
TABLE 2. Characteristic ITER data
Radius
Height
Plasma radius
18 m
36 m
8.14 m
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Plasma height
Plasma width:
Plasma volume
Magnetic field:
Maximal plasma current
Heating and current drive
Wall losses induced by
neutrons
Fusion power:
Time of burning:
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9.4 m
5.6 m
2000 m3
5.7 Tesla
21 Megaamperes
100 Megawatts
∼1 Megawatt per m2
1500 Megawatts
> 1000 seconds
9. Conclusion
If all goes well, commercial application of nuclear fusion should he possible in the next century
providing humankind a safe, clean, and inexhaustible energy source for the future.
References
[1] Kernfusion im Forschungsverbund, Forschungszentrum Karlsruhe, Forschungszentrum Jülich,
Max-Planck-lnstitut für Plasmaphysik, Garching, Euratom Assoziationen, März l996.
Received on the 17th of January 1998
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