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Published in the proceedings of
the 18th World Energy Congress
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HAMACHER AND A
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BRADSHAW
Max-Planck-Institut für Plasmaphysik
Garching/Greifswald, Germany
1
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Such power plants
would be safe and environmentally friendly
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Fusion reactors would have almost limitless supplies of fuel and
could be sited anywhere in the world
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The aim of the present paper is to present the current status of fusion research and to describe the steps
ahead that will lead to power generation
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We then go on to describe the advances
made in fusion research in the last few years and note that the so-called break-even point has almost
been reached at the Joint European research facility JET in Culham, UK
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Finally, we consider the role which fusion might play in
various energy scenarios in the second half of the century
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Principles of fusion

2
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Their constituents are
positively charged nuclei surrounded by negatively charged electrons
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Figure I depicts the reaction of heavy hydrogen and super-heavy hydrogen, deuterium and tritium (known
as isotopes of hydrogen), to give helium (an α particle) and a sub-atomic particle, the neutron
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At the
same time, mass is lost: the combined mass of the products is lower than that of the reactants
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The advantage of deuterium and
tritium is their high reaction probability
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Under
normal conditions nuclei are separated at least by the so-called atomic radius which reflects the presence
of the surrounding electron cloud
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If the atoms are
heated, the motion of the electrons and the nuclei will increase until the electrons have separated
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Deuterium

Neutron
(14 MeV)

Tritium

Helium
(3,5 MeV)

Figure I: Schematic of the fusion reaction in which deuterium and tritium form a helium atom and a
neutron
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Even in a plasma, however, the nuclei do not come close enough to react because of mutually repulsive
forces
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As an
analogy, we can think of a fast ball rolling up a hill against the gravitational force
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The production of the plasma and its subsequent heating require of course energy
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The ratio of the power generated to that consumed (the fusion power amplification
factor) is called the Q value
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g
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With increasing temperature, however, the number of fusion reactions also increases and the
fusion reaction itself heats the plasma due to the production of the energetic helium atoms (actually ions,
or α particles)
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The energy is distributed to the fuel nuclei via
collisions, as in a game of billiards
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In practice, however, power plant
operation would probably correspond to a Q value of 20-40
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The latter value describes the
ability of the plasma to maintain its high temperature; in other words, it is a measure for the degree of
insulation of the plasma
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2
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Obviously no solid material
is able to confine a medium with such a high temperature
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(The
charged particles gyrate around the magnetic field lines
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This scheme is referred to as magnetic confinement
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The
most successful approach has been the tokamak, first realised in Russia [3]
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The magnetic field is the sum of the toroidal magnetic field produced by the coils shown and the magnetic
field produced by a current in the plasma
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The most important concept applied today is to place another magnetic coil in the
centre of the tokamak (see figure II: solenoid magnet) and to ramp the current in this coil up or down
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This voltage can only be sustained for a limited time - one or two hours at the very most
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The magnetic
field cage - necessary to confine the charged particles - is produced by the superposition of a toroidal
magnetic field and a poloidal magnetic field produced by a current in the plasma
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Many current
R&D activities are directed towards finding alternative ways of driving the current in the plasma (via
microwave heating or particle beam injection) or to concentrate on the stellarator, successfully pursued in
several countries, in particular Germany and Japan, in which no current is necessary
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3

Alternative path to fusion

Two alternatives to magnetic confinement are discussed briefly here: inertial confinement and muonic
fusion
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(Densities of twenty times the density
of lead and temperatures of 100 Mio ° C are envisaged
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The outer layer heats up and evaporates
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Inertial confinement is mainly
investigated in the US and France and to a lesser extent in Japan, Britain and other European countries
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Inertial fusion is considerably less developed
than magnetic confinement fusion with respect to the realisation of a power plant
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The idea is to produce muons, which are the heavy sisters of the electron
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There is a finite probability that the muon will be captured by
3

a tritium or deuterium atom and form a deuterium-tritium molecule
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Therefore the nuclei will be much closer to each other and there is a greater likelihood that they will
undergo a fusion reaction
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2
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A flow chart of the energy and material flows in a
fusion plant are depicted in figure III
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The neutrons leave the plasma and
are stopped in the so-called blankets which are modules surrounding the plasma
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The blankets also contain lithium in order to breed fresh
supplies of tritium via a nuclear reaction (see 4
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The "ash" of the fusion reaction – helium – is removed
via the divertor
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The outer magnetic field lines of the tokamak are especially shaped so that they intersect the
wall at special places, namely the divertor plates
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The tritium produced in the
blankets is extracted with a flushing gas - most likely helium - and delivered to the fuel cycle
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The heat produced in the blanket and the divertor is transported via water or helium to the steam
generator and used to produce electricity to feed to the grid
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Electrical power is required mainly for the cryo-system which
produces low temperature helium for the super-conducting magnets, the current in the magnets, the
current drive and the plasma heating systems
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The inner region is the plasma, surrounded
by first wall and blanket
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Outside the vacuum vessel are the
coils for the magnetic field
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4

3
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1

Plasma physics: “break-even” at JET

Progress on the path to ignition in magnetic confinement fusion research is best characterised by the
improvement in the triple product
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Figure IV depicts the increase of the triple
product by five orders of magnitude in the last three decades
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The first promising results were achieved in the Russian tokamak T3, following
which tokamaks were constructed in many countries at the beginning of the seventies
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It went into operation in 1983 and remains
the largest fusion device in the world
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The energy confinement time depends strongly on the plasma dimensions
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The confinement time is - as mentioned above - a measure of the heat
insulation of the plasma core and it is clear that a larger plasma insulates the core better than a smaller
plasma
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e
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Increased
understanding of the underlying physics and many experimental studies have led to the discovery of new
plasma modes, such as the so-called H-mode in 1982 [6]
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The temperature 1 keV is equivalent to 11 Mio
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5

Establishing the H-mode improves the energy confinement time by a factor of two
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Plasma stability is a matter of particular importance for the economics of fusion
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This ratio is very small in current
machines
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Limits also exist for the plasma density, although these are generally soft and considerable improvements
may be expected in future
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The particle and power exhaust seemed to a major problem for several years
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These plates are made from
special material, either carbon fibre composites (CFC) or tungsten
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Experimentally it has also been demonstrated that the residence time of helium in the plasma poses no
severe problem [10] and the helium "ash" can be transported efficiently to the divertor to be removed from
the system
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At JET, experiments with deuterium and tritium have led to considerable power production [11]
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1 MW
fusion power were produced for about a second and about 4 MW for a few seconds (see figure V)
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65
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3
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The next step in the international fusion programme, ITER (= International Thermonuclear Experimental
Reactor) will demonstrate the viability of fusion as an energy source
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Seven
tasks were set up in world-wide collaboration to design, construct and test these components
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With the exception of the toroidal magnet, where tests will start soon
(spring 2001) all the tasks have been successfully completed
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Remote handling proved to operate satisfactorily [13]
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Figure VI: Photograph of a the prototype sector of the vacuum vessel for ITER (Photo ITER)
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A number of materials have been identified as candidates for
future fusion power plants [15]
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4
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Via two major steps (ITER and
subsequently the demonstration reactor DEMO) the programme is intended to provide the scientific and
st
technological basis to build and operate economically viable fusion power plants by the middle of the 21
century
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ITER is a collaboration involving the European community, Japan and the Russian Federation
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The CDA phase began in April 1988 and was completed in December 1990
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In the current extension of the ITER-EDA the design is being modified to
produce a lower cost, lower performance version
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The design modifications do not change the major objectives of
ITER, namely the prove that fusion can deliver considerably more power than is required by the external
heating and that the complex technology can be mastered
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Further improvements of the magnetic confinement scheme are necessary
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Two lines of improvements are followed
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Many existing machines have already investigated such scenarios
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Two large stellarator
projects are now being pursued: the LHD stellarator in Japan went into operation in 1998; the
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WENDELSTEIN 7-X stellarator in Germany is expected to start operation in 2006
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The development of fusion materials requires the construction of an intense neutron source
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The conceptual design report was
produced at the end of 1996
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e
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5
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1

Fusion plant models

A number of detailed system studies have been performed in the last thirty years in order to study the
possible design of future fusion plants [17]
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The detailed design work on ITER adds useful complementary
material
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2

Fuel and material availability, energy requirements

One of the main motivations from the very beginning of fusion research has been that fusion can be
considered as a practically unlimited source of energy
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A 1 GWe fusion power
plant would require annually 110 kg deuterium and 380 kg lithium consumption
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In terrestrial hydrogen sources, such as sea water, deuterium makes up
one part in 6700
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The oceans have a total mass of 1
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6 * 10 kg of deuterium; moreover, there is already a mature technology for extracting the
deuterium
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Existing plants can produce up to 250 t/a of heavy water which means a production of 50
t/a of deuterium
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Obviously deuterium supply places no burden on the extensive use of fusion
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Lithium is found in nature in two different isotopes Li (7
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6
%)
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8 MeV

4

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Li + n -> T + He + n - 2
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In most blanket concepts the reaction with Li dominates, but in order to
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reach a breeding ratio exceeding unity the Li content might be essential
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015 % to 0
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; the concentration
varies between 0
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1 %
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173 mg/l (Li )
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Table I: Land reserves of Lithium
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At least a couple of hundred tons of lithium are necessary to build a blanket
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No detailed concept for recovering lithium has been developed so far
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Beside the land11
based resources there is a total amount of 2
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Techniques to extract lithium
from sea water have already been investigated [25]
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The ultimate lithium resources in sea water are thus practically unlimited
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1

Model 1 B
Model 2

0
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001
0
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Besides fuel numerous other materials will be necessary in order to construct and operate a fusion power
plant [21, 22]
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The material
required to build 1000 1 GWe fusion power plants are divided by the known reserves of these materials
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The energy necessary to produce, transport and manufacture all the materials to build a fusion plant add
up, in a conservative model, to 3
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The energy pay back time, the time necessary for the
plant to deliver the same amount of energy necessary for its construction, is roughly half a year and thus
comparable with conventional power plants
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3

Cost of electricity

Basis for the cost estimates of fusion power is a plant of 1 GWe capacity based on the tokamak concept
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The assumptions in the underlying physics and
technology seem well with reach based on current achievements
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Most of the components of the fusion power core are unique for fusion
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The ITER experience is of particular importance
because it combines system studies and real manufacturing experience
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The following discussion is based on [29,31,32]
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The rest splits up into numerous items
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The divertor will be replaced every
second year, the blanket every fifth year
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The pressure of the magnetic field has to balance the
pressure of the plasma
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Progress in plasma physics could reduce this
ratio in future and thus reduce the size and cost of the magnets
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Cost of electricity (COE) is the sum of the capital costs for the fusion core (39 %) and the rest of the plant
(23 %), the costs for the replacement of divertor and blanket during operation (30 %), fuel, operation,
maintenance and decommissioning (8 %)
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The investment costs for DEMO
are expected to be roughly 10000 Euro/kW (1995) [29] giv

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