The	  Nuclear	
  Fusion	
  Distillate	
  
	
  
	
  
	
  
	
  An	
  Energy	  Technology	
  Distillate	
  from	  the	  Andlinger	
  Center	
  for	
  Energy	
  and	  the	
  Environment	
  at	
  Princeton	
  University	
  
	
  
This	
  Distillate	
  was	  researched,	
  synthesized,	
  and	
  written	
  by	
  10	
  Ph.D.	
  students	
  in	
  the	
  Princeton	
  Energy	
  and	
  Climate	
  Scholars	
  (PECS)	
  program	
  at	
  Princeton	
  University,	
  along	
  with	  faculty	
  mentor,	  Robert	
  Socolow.	
  
	
  
The	
  fusion	
  distillate	
  is	
  written	
  for	  the	
  reader	
  who	
  has	  an	
  appetite	
  for	
  technological	
  argument	
  but	
  has	  no	
  background	
  and	  no	
  comfort	
  even	
  with	  algebra.	
  It	
  treats	
  both	  technology	
  and	  policy	
  and	
  seeks	
  be	
  evenhanded.	
  	
  Professor	
  Socolow	
  would	
  welcome	
  your	  comments	
  and	
  feedback	
  regarding	
  this	  distillate.	
  	
  Please	  send	  your	  response	  to	
  Robert	
  H.	
  Socolow	
  socolow@princeton.edu.	
  	
  Fusion Energy

via Magnetic Confinement
An Energy Technology Distillate
from the Andlinger Center for Energy and the Environment at
Princeton University
Contributors

Cleo B. Chou, Janam Jhaveri, Jane W. Baldwin, Phillip M. Hannam, Kyle Keller,
Wei Peng, Sam Rabin, Arvind P. Ravikumar, Annette M. Trierweiler,
Xingchen T. Wang, Robert Socolow
Table of Contents

Prologue .........................................................................................................................page 2
Article 1: Overview.........................................................................................................page 3
Article 2: Key Concepts and Vocabulary.......................................................................page 5
Article 3: Technology......................................................................................................page 8
Article 4: Economics................................................................................................... page 16
Article 5: Fusion and Fission...................................................................................... page 22
Article 6: Politics and Progress................................................................................... page 29
The contributors would like to acknowledge help and useful feedback from Sharon Adarlo, Jan Beyea, William Brinkman, Emily
Carter, Adam Cohen, Steven Cowley, Jeanne Jackson DeVoe, Krista Dulon, Nathaniel Fisch, Alexander Glaser, Robert Goldston,
Gregory Hammett, Egemen Kolemen, Page Kyle, Eric Larson, Dale Meade, Stewart Prager, Robert Rosner, Dennis Whyte, andAndrew Zwicker.

1Prologue
The goal of this report is to inform the reader about
the challenges facing magnetic confinement nuclear
fusion technology, which could someday provide
the planet with a new and unique source of energy.
Like the other Andlinger Center Energy Technology“Distillates,” this Distillate aims to provide succinct yet
substantive information to policymakers, educators,
students, and other citizens – to fill a gap between
presentations in textbooks and in the popularliterature.
This Distillate was researched, synthesized, andwritten by 10 Ph.D. students in the Princeton Energy
and Climate Scholars (PECS) program at Princeton
University, along with our faculty mentor, Robert
Socolow. PECS is a platform that enables a groupof Ph.D. students working on disparate aspectsof energy and climate to have an interdisciplinary
exchange of ideas. The students involved in thisDistillate have a range of expertise: biogeochemistry,
climate modeling, ecology, electrical engineering,
psychology, and public policy. Notably, none of us isengaged in a dissertation related to nuclear fusion.
The lack of fusion scientists among the authors has
required significant effort on our part to learn andunderstand the intricacies of the fusion field. We wereaided by extensive consultations with experts. We

hope that the reader will benefit from our collective
learning process and our fresh perspective on
the fusion enterprise. An exposition emphasizing
objectivity may be particularly helpful for fusion
because the field is currently conducting expensive
experiments that compete for very limited government
funding in many nations, resulting in discourse that
may at times be hindered by the high stakes between
competing approaches and projects.
A special challenge to our objectivity arises from thefact that the Princeton Plasma Physics Laboratory(PPPL) is located at the University. Although we
consulted with several fusion experts at PPPL, thisreport was written independently of PPPL and does
not represent its views. Nonetheless, the reader may
well anticipate that any document from Princeton
will present fusion’s potential to become a futureenergy generation option in an excessively favorable
light. We have sought to write an impartial andrigorous assessment, the kind that we would mostwant to read ourselves. In the process, we hope wehave written a document that will be useful for every
reader with an appetite for introductory technologicalanalysis and a desire to understand nuclear fusion as
an energy source.

2

Article 1: Overview
In the current century the world faces a dual
challenge: providing the energy that enables
continued growth in desired human activity, while
limiting the severity of climate change by constraining
the emissions of carbon dioxide from fossil fuels.
Although this dual challenge is expected to be met
in part by efficient use of energy and extensive
deployment of already well-envisioned low-carbon
energy sources, additional low-carbon options could
help secure a robust path to sustainable global
development. Nuclear fusion energy may be such an
option, provided that significant progress in several
areas of science and technology is made in the next
few decades.
Fusion-generated energy would be uniquely valuable
because of a combination of characteristics notshared by any existing technology:
1) Fusion could dependably provide energy at a
sufficient scale to meet a significant fraction of
global demand.
2) Locations for fusion plants include sites whererenewable energy sources are unsuitable.
3) Fusion power is expected to have low impact on
public health and local environments.
4) Fusion power has no direct carbon dioxide
emissions. The indirect emissions that areassociated with plant construction and fuelproduction are modest.
5) The global supply of fusion fuels is essentiallyinexhaustible. Lithium and deuterium extracted
from seawater could provide enough fusion fuelto meet foreseeable energy demand for millionsof years.
Several significant challenges help explain why themultiple benefits of fusion energy have not yet led
to its presence in the global energy system, eventhough the theory of fusion reactions was developedin the 1920s and 1930s. The most direct challenges
are in science and technology. Nuclear fusion is
the method by which the Sun creates its energy,
and replicating such a process on Earth requires
recreating conditions comparable with those foundin the Sun’s core. The temperature of the fusion fuelmust be raised to approximately 200 million degrees
Celsius, versus 15 million degrees Celsius in the coreof the Sun. At such temperatures, the fuel is fully
ionized, that is, the fuel’s atoms have been strippedof all their electrons. This distinctive state of matteris called “plasma.” Magnetic confinement uses

magnetic fields to retain plasma heat and to control
plasma movement while energy is produced within
the plasma by nuclear reactions.
Creating, maintaining, and manipulating a hightemperature plasma are unique scientific challenges.
Basic questions in the science of plasma physics
and applied questions related to engineering andmaterials have been addressed in both small andlarge facilities, but many questions can be tested onlyin the largest facilities, which have become steadily
more expensive.
Early research focused on plasma control andlimited the number of nuclear fusion events in orderto reduce complications. Now the frontier includes“burning plasmas,” where enough fusion reactions
occur to maintain the high temperature of the plasma
with little or no external heating. The behavior of aburning plasma is new territory for science.
The scientific, technological, and economic challenges
of fusion energy research go hand in hand with thepolicy challenges of fusion energy. Because fusion
research requires large and long-term financing,funding has come primarily from governments rather
than private investors. However, in national politics,
it is difficult to sustain many decades of investmentson the scale of hundreds of millions of dollars
annually when the outcomes are uncertain and theend goal – a commercial reactor – will at best be
economically competitive decades in the future. The
fusion endeavor struggles to fund expensive researchfacilities and long-term horizons with bounded andfluctuating budgets.
One way that the fusion community is dealing withfinancial challenges is by forming international
collaborations to pool funds for its larger experiments.
The most ambitious collaboration in fusion is theInternational Thermonuclear Experimental Reactor(ITER), which is currently under construction in
France. It will be discussed at some length in thisDistillate. ITER is expected to allow for extensive
experimentation under burning plasma conditions.
Magnetic confinement fusion is one of the two
principal approaches to achieving nuclear fusionenergy currently being explored. The other is inertial
confinement fusion, which uses pulses from multiple
lasers or particle beams to squeeze tiny pellets of fueland trigger a rapid succession of fusion reactions.
Both magnetic and inertial confinement fusion are
based on the same nuclear fusion reactions, but due
to their vastly different configurations, the obstacles

3

in these two paths to commercial fusion energy are
entirely different. Here, we focus nearly exclusively onmagnetic confinement fusion, presenting the major
technological and economic issues associated with
its potential for commercial success. To further boundour Distillate, we do not discuss the relatively few,
small-scale, private-sector-funded fusion venturesthat are seeking alternatives to the mainstream
government-funded approaches.
A commercially competitive fusion power plant would
be a remarkable human achievement. As the science
unfolds, however, it is possible that the emergent
technological, economic, and political requirements
will not reveal a path forward. For now, achieving
competitive nuclear fusion energy is an open-ended
endeavor.

The remainder of this Distillate consists of fivearticles. Article 2, “Key Concepts and Vocabulary,”
provides background for the later articles. Article 3,“Technology,” presents some of the basic science
relevant to fusion energy and a few of the central
technical challenges being addressed in current
fusion research. Article 4, “Economics” discusses
issues likely to determine the prospects forcommercial fusion: the costs of reactor construction
and ongoing maintenance, the strength of climate
policy, and the success of fusion’s competitors. Article
5, “Fusion and Fission,” discusses how well nuclear
fusion power will address several of the vexing
problems that currently challenge nuclear fission
power. Article 6, “Politics and Progress,” reviews thecurrent global effort to develop nuclear fusion.

4

Article 2: Key Concepts andVocabulary
Power and energy
Energy is a fundamental physical quantity that comesin many different forms, and the quantity of energy
does not change as it is transformed from one form
to another. In a magnetic fusion reactor, there aremany forms of energy. Electric currents flowing
through coils of conducting wires create magnetic
energy in the form of magnetic fields that confine theregion where fusion occurs. During a fusion event,injected fuel reacts and releases nuclear energy thatis then transformed into kinetic energy of the fusion
products. These fusion products slow down throughcollisions with the surrounding material, transforming
their kinetic energy into thermal energy. Some of thethermal energy sustains the plasma temperature,and some is converted into electricity. Finally, someof the electricity is used within the power plant topower magnets and other auxiliary systems, but
most of the electricity is sent on transmission lines toprovide consumers with power for refrigerators, lights,
computers, and many other devices.
Although amounts of energy are expressed inmany different units, depending on the country
and the context, the scientific community uses aunified system of joules for macroscopic energy andelectron volts for energy at the scale of atoms andnuclei. Associated with the joule is a rate of energy
consumption, one joule per second, which is called
the watt. Two multiples of the watt are widely used:
the kilowatt (1,000 watts) and the megawatt (onemillion watts). Most nuclear fission power plants
today have the capacity to produce electricity at a
rate of about 1,000 megawatts, and fusion plants are
currently expected to have a similar capacity.
At the microscopic level, the reference unit energy,
the electron volt (eV), is approximately six quintillion(six billion billion, or six followed by 18 zeros) times
smaller than the joule. One million electron volts, acommon unit for nuclear science, is abbreviated MeV.

Atoms, nuclei, electrons,ions, plasma
Matter is made of nuclei and electrons. When matter
is not very hot, it is structured as atoms, each ofwhich has a tiny, positively charged, dense nucleus
at its center. The nucleus is made of protons (p,positively charged) and neutrons (n, electricallyneutral). Surrounding the nucleus are electrons, eachcarrying one negative electric charge. In an electricallyneutral atom, the positive charge of the nucleus isbalanced by the negative charges of electrons. When
there is not perfect balance, the atom is called an ion;
an ion can be positively or negatively charged.
At a very high temperature, such as that created ina fusion reactor, nearly all the electrons become
free from their nuclei, and the result is a state ofmatter, called “plasma,” where 1) nearly all matter
is ions and electrons, and 2) overall, there is no (orvery little) net electric charge. A plasma behavesfundamentally differently from a gas because in aplasma the particles attract or repel one anotherover a distance through electromagnetic forces, but
in a gas the particles interact by colliding with eachother. Plasmas are called the fourth state of matter,alongside solids, liquids, and gases. The field ofplasma physics examines the behavior of plasmas,
including the conditions under which they are stable
or unstable. Our Sun is a plasma whose core is at
a temperature of 15.6 million degrees Celsius. The
temperatures in fusion research reactors are evenhigher.

Isotopes and radioactivity
The chemical behavior of an atom is determined by
its electrons. For example, a neutral atom with 26
electrons (iron) will have a nucleus with 26 protons.
The chemical behavior of an atom does not dependon how many neutrons are in the nucleus, and thenuclei of an element can have varying numbers ofneutrons. Atoms with the same number of protons
but different numbers of neutrons in their nuclei arecalled isotopes of one another. A widely used notation
for isotopes provides the name of the atom and thetotal number of protons and neutrons. Iron-56, oneof the isotopes of iron, has a total of 56 neutronsand protons; since the iron atom has 26 electrons
balancing a total positive nuclear charge of 26, it has
26 protons and 30 neutrons. Iron-56 is also written as
56
Fe; Fe is the chemist’s abbreviation for iron.
Isotopes can be either stable or radioactive. All
stable isotopes are found on Earth, along withsome radioactive isotopes. The stable isotopes inthe Earth’s crust occur in specific proportions. For
example, 56Fe happens to be the most common
isotope of iron in the Earth’s crust, accounting for
92 percent of the iron in the crust; the next mostcommon is 54Fe, accounting for six percent. As for5radioactive isotopes of iron, these can be createdin nuclear reactions. For example, when the steel
in a nuclear fission or fusion reactor is exposed tobombardment by energetic neutrons, some of theresulting isotopes are the iron isotopes, 55Fe, 59Fe,
and 60Fe.
Every radioactive isotope has a half-life, meaning that
if a given amount of an isotope is present at a given
moment, then – as a result of radioactive decay – half
as much will be present after one half-life. The otherhalf will have been transformed into another isotope,
usually an isotope of another element. The half-life of
59
Fe, for example, is 45 days; after 45 days, half of it
has become 59Co, a stable (non-radioactive) isotopeof cobalt, and half is still 59Fe. After 90 days (two halflives) three quarters has become 59Co. The half-livesof 55Fe, 59Fe, and 60Fe are 2.7 years, 45 days, and 2.6
million years, respectively. They are the radioactive
isotopes of iron with the three longest half-lives.
Importantly for fusion, there are three isotopes of
the hydrogen nucleus: the ordinary hydrogen nucleus(called the proton, written as either 1H or p, consistingof one proton and no neutron); the deuterium nucleus(called the deuteron, written as either 2H or D,
consisting of one proton and one neutron); and thetritium nucleus (called the triton, written as either 3H
or T, and consisting of one proton and two neutrons).
1
H and 2H are the two stable isotopes of hydrogen,and ordinary hydrogen is far more prevalent on Earth.
Only one out of every 6,500 hydrogen atoms at the
Earth’s surface is deuterium. Tritium is radioactive,
with a half-life of 12.3 years. Due to its short half-life
almost no tritium exists naturally on Earth.

Nuclear reactions
Just as chemicals react to produce other chemicals
in a chemical reaction, nuclei can react to produce
other nuclei in a nuclear reaction. Many reactions
in both cases also produce energy. For example,
when gasoline burns, this is a chemical reaction that
produces carbon dioxide and water and releases
energy; energy is also released in the nuclear
reactions discussed below. Nuclear reactions typicallyrelease roughly a million times more energy thanchemical reactions.
For a nuclear fusion reaction to be possible, the total
number of protons must be the same in the reactants
and the products, and the equality must also holdtrue for the neutrons. We give two examples of energyproducing nuclear reactions important to fusion here:
Reaction 1: 2H + 3H → n + 4He + 17.6 MeV. The
reactants are a deuteron and a triton, with a total of
two protons and three neutrons. The products are aneutron (n) and a “helium-4” nucleus. The helium-4
nucleus has two protons and two neutrons. (It has a

special name, the “alpha” particle.) Thus, in all, thetwo products have two protons and three neutrons,
just as the reactants do. Also, 17.6 million electron
volts (MeV) are released, initially in the form of the
kinetic energy of the two products. Of all the nuclear
reactions, this “deuterium-tritium” or “D-T” reactionis the least difficult to produce in a hot plasma andaccordingly is the focus of most of the work seeking
to commercialize fusion energy. It will be discussed
extensively in this distillate.
Reaction 2: n + 6Li → 3H + 4He + 4.8 MeV. The
lithium atom has three electrons, and this particularlithium nucleus, the lithium-6 nucleus, has three
protons and three neutrons. Thus, there are a totalof three protons and four neutrons in the reactants.
There are also three protons and four neutrons in the
products, which are tritium and the helium-4 nucleus.
Reaction 2 plays a critical role in D-T fusion bycreating tritium from lithium. Lithium is an abundantresource, but tritium, as noted above, is essentiallyabsent from the Earth and yet is needed as a reactant
for D-T fusion. Reactions 1 and 2 can be combined sothat the neutron produced in Reaction 1 becomes areactant for Reaction 2. Then the net result is that two
4
He nuclei are produced, and a deuterium nucleus
and a lithium-6 nucleus are consumed. Tritium is
destroyed in the first reaction but replenished in thesecond. The two reactions produce a total of 22.4
MeV.

Magnetic confinement ofcharged particles
A magnetic confinement fusion reactor featureslarge and very strong magnets. The magnetic fields
created by these magnets can be shaped into specificconfigurations that force all of the charged particles
in the plasma to move within a specific region of thefusion reactor’s vacuum vessel. One then says thatthe plasma is “confined.”
At a fusion plant with conventional magnets, thepower requirements for the magnets can becomea significant fraction of the energy produced by
the power plant. However, when currents run
along wires that are made of a particular class ofmaterials called “superconductors,” no power isrequired to sustain the current and the magnetic
fields. Magnets produced by current flowing in wires
made from superconducting materials are calledsuperconducting magnets. The world’s largest
and strongest superconducting magnets are being
designed for use in fusion test facilities.
To date, no material has been found that issuperconducting at room temperature, butsome materials are superconducting at very low
temperatures, only a few degrees above absolute zero

6

(zero Kelvin degrees, which is equivalent to -273.15
degrees Celsius and -459.67 degrees Fahrenheit).
Thus, to enable superconducting magnets to be
used, a large amount of cooling equipment must be
installed at a fusion plant. However, even taking thecooling equipment into account, superconducting
magnets require much less power than ordinary
magnets.
In the most common configuration, a plasma is
confined in a space resembling a donut, also calleda torus. Figure 2.1 differentiates the two kinds ofloops on a torus: toroidal and poloidal. In fusion
research, one speaks of toroidal and poloidal
magnetic fields.
			

Toroidal loop
Poloidal loop
Figure 2.1: The two kinds of circular paths around adonut (torus) are identified.

7

Article 3: Technology
We consider only magnetic confinement fusion here.
The history of magnetic confinement fusion research
can be split roughly into two eras: From the 1950s
to the 1990s fusion research focused mostly onmagnetic confinement and the physical properties of
the core of the plasma. The hydrogen in the plasmas
was usually deuterium without tritium, because inan all-deuterium plasma the D-D nuclear reactions(reactions between pairs of deuterium nuclei) are rareat the temperatures of the laboratory plasmas and, asa result, little radioactivity builds up in the walls of thereactors. However, the small amount of radioactivity
was useful for diagnosis of the detailed performanceof the plasma; a plasma made of ordinary hydrogen
would produce too little radioactivity to serve thispurpose.
We are now in the second era, where the goal isto achieve a “burning plasma” – a plasma heated
predominantly by the energy from fusion reactions
occurring within the plasma, rather than by externalsources. The new era began cautiously in the 1990s
when deuterium-tritium (D-T) plasmas, which aremuch more likely to lead to nuclear reactions thanD-D plasmas, were created in some reactors. Also,
research attention shifted to the edges of the plasma
where heat is lost and materials are damaged. The
new era features the intertwining of two strands: thescience of plasmas and the science of nuclear fusion
reactions.
Here, we first introduce these strands separately.
Then we present some of the issues that arise when
they are combined, such as the burning plasma,neutron bombardment of structural materials, and theregeneration (“breeding”) of tritium.

The combination of toroidal and poloidal magnetic
fields confines the plasma, steering it away from solid
surfaces.
Fusion research is focusing on two toroidal
configurations: the tokamak and the stellarator.
Tokamaks were initially developed in the Soviet Union
at the same time as stellarators were being developedin the United States and elsewhere. Tokamaks
proved able to achieve better confinement and higher
temperatures and became the dominant design. ITER
is a tokamak. However, stellarators may be making
a comeback because they have advantages in twoareas relevant to commercial viability: 1) stellarators
have intrinsic advantages in sustaining a plasma
continuously, and 2) stellarators may be better at
avoiding the large-scale disruptive instabilities thatcan seriously damage plasma-facing components.
Figure 3.1 shows the complex array of magnets
that confines a plasma in a generic tokamak. The
tokamak, in addition to its toroidal and poloidal
magnets, has a central structure running through thedonut hole, called a solenoid. When the current in
the coils of the solenoid changes, it induces a voltagethat drives the plasma’s current. (A transformer on anelectric utility’s distribution network transfers powerby the same inductive process.) The ITER tokamak will
have a central solenoid 13 meters high that weighs
1,000 tons, as well as 18 D-shaped toroidal-field coils
and six ring-shaped poloidal-field coils. Constructionof these components in the ITER member countries
has begun (see Article 6).
Coils for toroidal
magnetic fields

Solenoid

Coil current

Coils for poloidal
magnetic fields

Occasionally, for specificity, we refer to the expected
performance of the International Thermonuclear
Experimental Reactor (ITER), the large international
magnetic confinement research project slated to
begin operating in 2026. We discuss the political
history of ITER in Article 6 of this Distillate.

Magnetic Confinement
Tokamaks and stellarators
The principal configurations for plasma confinement
being explored today have the shape of a donut,
formally called a torus. On the torus there are twodifferent directions, toroidal and poloidal (see Figure2.1). Toroidal field magnets produce magnetic fields
in the toroidal direction, and poloidal field magnets
produce magnetic fields in the poloidal direction [1].

VesselPlasmaPlasma current

Figure 3.1: Magnetic coils and fields in a tokamak
reactor. Note that the coils for the toroidal magnetic
fields follow a poloidal path and the coils for thepoloidal magnetic fields follow a toroidal path. Image
credit: Max Planck Institute for Plasma Physics.

8

Figure 3.2 shows configuration of the most recent
stellarator, Germany’s Wendelstein 7-X (W7-X); it
began running in the summer of 2015. Stellarators
do not have a central solenoid, but instead havea complex three-dimensional geometry that is
an engineering challenge to manufacture and iscurrently responsible for added costs. Stellarators
are benefiting from the arrival of supercomputers
powerful enough to design an optimal magnetic coilconfiguration for plasma confinement.

years of fusion research. The highest temperatures
reached at the end of the 1990s are approximately
as large as the temperature goal for ITER. Far more
improvement is required for the other variables thanfor temperature.
The plasma temperatures required for fusion reactors
generally cannot be reached without supplementing
the energy from the reactor’s electric fields withadditional energy sources. One strategy is to injecta beam of energetic neutral particles, like diatomic
deuterium (D2), that collide with the plasma’s
particles and raise their temperature. Radiofrequency
heating is also used.

In a plasma, the temperatures of the positively
charged nuclei (ions) and the negatively charged
electrons can be different when one or the otheris being heated or cooled selectively. The ion
temperature will exceed the electron temperaturewhen an external beam of neutral particles heating
the plasma preferentially heats the ions. The same
inequality in temperature occurs in plasmas whenelectrons cool themselves by emitting radiation;
radiation cooling can be one of a plasma’s important
Figure3.2:
Configurationofmagnetic
coils
in
the
energy loss
Figure 3.2: Configuration of magnetic coils in the Wendelstein 7-X stellarator[2]. mechanisms.
Wendelstein 7-X stellarator [2].

cubic meter) and the confinement time (in seconds).
Also shown are the parameters for two importantdevices at the frontier of current fusion science: ITER
and W7-X. Below, we elaborate on temperature,density, and confinement time.

  Ion temperature
The ion temperature at the core of the plasma isplotted in Figure 3.3. The temperature is quantified
in two ways: 1) as the absolute temperature, indegrees Kelvin (K), which at such high values istrivially different from the temperature in degrees
Celsius; and 2) as the energy equivalent of theabsolute temperature in thousands of electron volts(keV). 1 keV = 11.6 million degrees Kelvin, as can be
confirmed by comparing the two horizontal scales in
Figure 3.3. The core ion temperature has marched
upward in actual fusion devices by a factor of about
1,000 (from two hundredths keV to 20 keV, or,equivalently, from about 200 thousand degrees to
200 million degrees) over approximately the first 40

Product of core ion density and confinement time(Ions/ m3) • s

Steady progress has been made in plasma confinement since the 1960s. In Figure 3.3,
Steadymadeisintracked
plasmawith the help of two 1021
progress by decadefor progress
tokamakshas
and beenstellarators
since
thetemperature
1960s. In Figure3.3,(mostly,
progressparameters. Theconfinement
horizontal axis
is theof the ionshydrogen ions) inbyThedecadeforaxistokamaksand stellarators
is tracked
the plasma core.
verticalis the productof the densityof ions in the plasma
1020(measured in ions
per
cubic
the confinement
time (in seconds).
withthehelpmeter)of twoandparameters.
The horizontal
axis is Also shown
are the parameters
for two importantdevices:
1) the ITER,
under ions)construction
in France
the temperatureof theions (mostly,hydrogenin
19
and the world’s thelargesttokamak,
Wendelstein
stellarator
that 10has
plasmacore.andThe2)verticalaxis is7-X
the(W7-X),
producta ofthejust come online in Germany. Below, we elaborate on temperature, density, anddensity of ions in the plasma (measured in ions perconfinement time.

106

Core Ion Temperature (K)107108

Plasma Breakeven

109
Ignition

Tokamak (~1990)

ITER

Stellarator (~2000)
Tokamak (~1980)

W7-X

1018

1017

1016

Tokamak (~1970)
Tokamak (~1960)
0.1

1
10
Core Ion Temperature (keV)

100

Figure 3.3: Performance of the world’s tokamaks andstellarators over the past decades [3]. Many devices
are included within each oval. The current goals of ITER
(a tokamak) and Wendelstein 7-X (W7-X, a stellarator)
are also shown. The lower U-shaped band (dark brown)is the approximate region for “plasma breakeven,”
where as much energy is generated within a plasma as
is supplied to the plasma from external sources. The
upper U-shaped band (dark blue) shows the “ignition”
region, where fusion energy output is sustained without
external energy
input.
11  9

Ion density
Characteristic best values associated with the 1990s
tokamaks are a confinement time (see below) of
one second and an ion density of 1x1020 ions per
cubic meter, resulting in values of their product near
the top in Figure 3.3. The density of atoms in a gas
at atmospheric pressure and room temperature
is approximately 2.5x1025 atoms per cubic meter– about 250,000 times greater than this plasma
ion density. Doubling the ion density in a plasma
results in four times more fusion reactions (since thereactions are encounters of pairs of ions) and thus
four times more power is generated in that volume,other things being equal.
Confinement time
The confinement time is a measure of the ability ofthe plasma to stay hot in spite of thermal losses;
the more the energy in the plasma is insulated
against these losses, the longer the confinement
time. Quantitatively, the confinement time is theamount of energy in the plasma divided by the rate at
which energy is being lost from the plasma, which inequilibrium is the same as the rate at which heat isbeing provided to the plasma to sustain it. The heatcan be provided to the plasma either externally orfrom the energetic helium-4 nuclei produced in thefusion reactions in the plasma, or both ways. The
longest confinement times to date have been about
one second.
As seen in Figure 3.3, the product of confinement
time and ion density improved about 10,000 times
during the first era of fusion energy. Some of thelengthening of confinement time was the resultof experimental fusion reactors becoming larger.
Ions and electrons simply take longer to diffuse to
the walls from the center of the plasma in a bigger
reactor, other things being equal.
Plasma breakeven and ignition
Figure 3.3 shows, at the upper right, two parabolic
bands labeled “plasma breakeven” and “ignition.”
A plasma has achieved plasma breakeven when the
nuclear energy generated within the plasma is aslarge as the energy that sustains the plasma fromexternal sources. Ignition occurs when fusion energy
can be sustained with no external energy source atall: the energy deposited within the plasma arises
entirely from its nuclear reactions. The two bands are
U-shaped and the ignition band is displaced directly
upward by less than a factor of ten. At the bottom of
the U, the product of density and confinement time issmallest, and the core temperature, for both plasma
breakeven and ignition, is about 20 to 30 keV. Since
increasing the confinement time or the density isdifficult, research with the goals of plasma breakeven
and ignition has sought to achieve a plasma whose
core temperature is near this minimum.

Two tokamaks in the 1990s were fueled with
deuterium and tritium and for about one second
achieved conditions only slightly below plasma
breakeven. ITER’s goal is to generate 10 times asmuch fusion power as the external power required tosustain the plasma, thereby coming close to achieving
ignition. The temperature in the core of the plasma is
expected to reach about 20 keV.
Superconducting magnets and pulse duration
Fusion research reactors in the 1990s created fusion
power as high as 16 megawatts, but in short pulses– pulses lasting about one second. To achieve longer
pulses, superconducting magnets are required rather
than ordinary magnets. The distinctive characteristicof a superconducting magnet is that it does not
require energy to sustain a magnetic field because
the superconducting material exerts no resistanceto current flow. At a fusion plant, these savingsin magnetic energy would be far higher than theenergy for the refrigeration that lowers the magnet
temperature to where it is superconducting – close toabsolute zero. In addition, superconducting magnets
can create stronger magnetic fields for long pulses
than ordinary magnets. Several experimental fusion
reactors with superconducting magnets are now in
operation, including recently built tokamaks in China
and South Korea and Germany’s W7-X stellarator.
The world’s largest superconducting magnets areheading for ITER, where a fusion output power of 500
megawatts is expected to be sustained for at least
400 seconds.
For every superconductive material, there isa temperature below which the material issuperconducting (has zero resistance to current flow)
and above which it is no longer superconducting andhas finite resistance. The transition temperaturedepends on the magnetic field in the material: thestronger the magnetic field, the lower the transition
temperature. Above some critical magnetic field, thematerial is no longer superconducting, no matter
how low the temperature. For the superconducting
materials used today in fusion reactors, the magnets
are cooled by liquid helium, which enables thetemperature to come close to absolute zero. The twokinds of superconducting magnets being installedat ITER are based on niobium-titanium (NbTi) andniobium-tin (Nb3Sn) superconductors.
If a superconducting magnet suddenly transitions outof its superconducting state (e.g., by warming up),rapid heating ensues due to the large currents thatflow through the magnet. These quenching events
represent a potential explosion hazard because of
the large amounts of energy they can release [4].
However, with modern designs superconducting
magnets can be protected against such quenches.

10electric fields generated during a disruption in a
Instabilities
tokamak can generate such electrons. In this case,A plasma can have instabilities at a range of scales.
the damage was created deliberately at the JET
Small-scale instabilities cause turbulent transfer
research laboratory in the United Kingdom to studyof heat from the core to the edge and limit thethe phenomenon.
confinement time. They are nearly always present.
Large-scale instabilities can be triggered, particularly
in tokamaks, when the plasma’s pressure or current
density varies too strongly and in too many places
within the plasma. Severe large-scale instabilities can
A “bound” system is any system that requires energy
drive the hot plasma into a wall and damage the wall,
to separate it into its components. The nuclei inwhereupon the plasma becomes too cold to sustain
nature are bound combinations of neutrons andfusion reactions and the reactor shuts itself down.
protons, collectively called “nucleons,” and each
Learning to control and avoid large instabilities isnucleus has a specific binding energy. The amountone of the major science frontiers. Figure 3.4 showsof bindingof any nucleus, divided by its number oftheresultsofhigh-energy“runawayelectrons”
hitting
on an inner surface of the plasma containment chamber at the JET
fusion researchfacilityin theKingdom.
Thean
large
electric
fieldsduring a disruption
in a in Figure 3.5, for all nuclei.
is displayed
a portionof Uniteda beryllium
tile oninner
surfaceofgeneratednucleons,tokamakcan containment
generate suchchamberelectrons.at
In thethis JET
case,
the damage(Actually,
was createddeliberately
since
many nuclei have the same total
the plasma
fusionat the JET research laboratory in the United Kingdom to study the phenomenon.
number of nucleons, what is plotted is either the
research facility in the United Kingdom. The large
value for one of these
nuclei or some average.)
The number of nucleonsranges from one for both theneutron (n) and the proton(1H) to 238 for uranium-238(238U). Zero is at the top of
the vertical scale, and thefurther down from zero, thestronger the binding per
nucleon.

Fusion energy and nuclear
reactionsFigure 3.4: A plasma disruption leads to “runaway electrons” striking and partiallyplasma disruption“runaway electrons”
meltingFigure
one of3.4:
theAberyllium
tiles on aleads
beamtoprotecting
the innerstriking
wall of andthepartiallymelting oneof theon a[5].
beam
the inner
wallofconfinement
chamberat the
JET beryllium
research tiles
facility
Theprotecting
thin cut lines
on thetile
the confinement chamber at the JET research facility [5]. The thin cut lines on theare approximately
12 millimetersapart, andtheandtile theis approximately
50 millimeters
tile are approximately
12 millimetersapart,
tile is approximately
50
deep. millimeters deep. In this instance, the instability was created deliberately to study
the phenomenon.

n, 1H
Binding 0
energy energy and nuclear reactions
Nuclear Binding Energy
Fusion(MeV
per 1 2H
nucleon )A “bound”2 system Energy
is anyreleasesystem that requires energy to separate it into its components.
He

3

The nuclei
3 in3Hnature are bound combinations of neutrons and protons, collectively called“nucleons,” and each nucleus has a specific binding energy. The amount of binding of
4 divided by its number of nucleons, is displayed in Figure 3.5, for all nuclei.
any nucleus,
6
Li many nuclei have the same total number of nucleons, what is plotted is(Actually,5since7either the6valueLi for one of these nuclei or some average.) The number of nucleons
Energy release
ranges from 1 for both the neutron (n) and the proton (1H) to 238 for uranium-238 (238U).
7
235
U
Zero is at the
4 top of the vertical scale, and the further down from zero, the stronger theHe
238
U
8 nucleon.
binding per
16
56
O
Fe

9 has a U-shape, because the most bound nuclei in nature are near the iron
The curve
20 40
60
80 100 120 140 160 180 200 220 240
0nucleus that has a total of 56 neutrons and protons (56Fe). A nuclear reaction can beof nucleonsin nucleus
thought of as a ball rolling down towardNumber
iron fromeither endof the curve. Starting with
nucleithanCurve
iron (toofthe
left of Energy”
iron in the
figure),nuclear energy
can be thereleased
Figurelighter
3.5: “TheBinding[6]:
The horizontal
axis orders
nucleiwhen the protons and neutrons rearrange in new combinations closer to iron; suchin nature by their total number of nucleons (protons and neutrons). The vertical
rearrangements include fusion reactions. Similarly, fission reactions start with nuclei
axis is thebindingenergyperrelease
nucleon:the amountrequiredoftotakeheavierthaniron, andthey too
nuclearenergy ofby energy
rearrangement
theprotonsapart
a nucleus and create separated protons and neutrons, divided by the number
andneutrons.

The curve has a U-shape,
because the most boundnuclei in nature are near
the iron nucleus that has
a total of 56 neutrons andprotons (56Fe). A nuclear
reaction can be thoughtof as a ball rolling down
toward iron from either endof the curve. Starting with
nuclei lighter than iron (tothe left of iron in the figure),
nuclear energy can bereleased when the protons
and neutrons rearrange innew combinations closer toiron; such rearrangements
include fusion reactions.
Similarly, fission reactions
start with nuclei heavier
than iron, and they too
release nuclear energy
by rearrangement of the
protons and neutrons.

of nucleons. The five reactants or products involved in the energy-releasing
deuterium-tritium reaction and the tritium regeneration reaction (Reactions 1 and
15  2 in the text) are in red.

  11Nuclear reactions for deuterium-tritium fusion
Expanding on the discussion in Article 2, the key
nuclear fusion reaction in today’s research is:2

H + H → n + He + 17.6 MeV
34(Reaction 1)

On the left hand side, 2H and 3H are isotopes ofhydrogen, also called deuterium (D) and tritium (T)respectively. The two products, a helium nucleus
and a neutron, emerge in opposite directions, theneutron carrying away 80 percent and the helium
nucleus carrying away 20 percent of the energy
released in the fusion reaction. Because the helium
nuclei have electric charge, they slow down in theplasma. They cool down to the thermal plasma
temperature, primarily by colliding with the plasma
electrons and to a lesser extent with the deuterium
and tritium nuclei. The energy they transfer to theparticles that slow them down heats the plasma.
Nearly all of the thermalized helium ash would thenbe guided out of the reactor, although a small fraction
would become embedded in structural materials,
eventually damaging them. The neutrons would not
be confined by the electromagnetic forces and would
travel beyond the plasma retaining their initial energy.
Except for the negligible fraction of the neutrons
which would decay (neutrons have a 10 minutehalf-life), every neutron would then be absorbed asa result of a nuclear reaction with some structural
material surrounding the core. A very large fraction of
the neutrons would be absorbed in the blanket, where
the neutron would react with lithium (see Reaction 2
below).
Inspecting Figure 3.5, the two reactants and two
products involved in Reaction 1 are found at the far
left: the two nuclei entering into the fusion reaction(2H and 3H) are only a little way down, 4He is quite far
down, and n at the top (in fact, not bound at all). The
exiting combination of n and 4He is a more deeply
bound system than the entering combination of 2H
and 3H, and the extra binding is what enables therelease of energy. Reaction 1 releases an enormous
amount of energy: twenty million times more energy
is released when a kilogram of deuterium reacts withtritium than when a kilogram of gasoline is burned in
air.
Because tritium is radioactive with a short half-life(12.3 years), it is present in only negligible quantities
on the Earth and is very expensive to produce.
Accordingly, the D-T fusion strategy for commercial
energy production presumes that tritium regeneration
will be integrated with tritium use and energy
production at a single facility. The regeneration is
expected to be accomplished by a nuclear reaction
between the neutrons produced in Reaction 1 andlithium embedded in the blanket surrounding theplasma. The products of this reaction are tritium andhelium. One such tritium-producing fusion reaction is:
n + 6Li → 3H + 4He + 4.8 MeV.

(Reaction 2)

Inspecting Figure 3.5, we see that Reaction 2 also
results in a net movement downward towards, on
average, a more deeply bound system, so that
Reaction 2 also produces energy.
Lithium in nature is a mixture of two isotopes oflithium, 6Li and 7Li. 6Li is less common (in the Earth’s
crust there are slightly more than twelve 7Li nuclei foreach 6LI nucleus), but 6Li is far better at absorbing aneutron and making tritium than 7Li. Accordingly, afuture fusion energy system might well include frontend lithium-enrichment facilities to create lithium thatis mostly 6Li for use in the reactor blanket.
Summing the energy release from Reactions 1 and
2, 22.4 MeV are released when a deuterium and a
Lithium-6 nucleus are consumed and two helium-4
nuclei are produced. Assuming that all of this energy
is available for use and that the neutron output
from Reaction 1 is the same as the neutron input toReaction 2, we can estimate the flows of reactants
and products for a 1,000-megawatt fusion reactor
converting fusion heat into electricity at 40 percent
efficiency and running 90 percent of the time. Eachyear Reaction 1 would consume approximately 80
kilograms of deuterium and 120 kilograms of tritium
fuel and would produce 160 kilograms of helium-4
and 40 kilograms of neutrons. Reaction 2 would
regenerate the 120 kilograms of tritium from the
40 kilograms of neutrons, while consuming 240
kilograms of lithium-6 and producing another 160
kilograms of helium-4.
Deuterium is not radioactive, and even though it is arare constituent of hydrogen, there is so much wateron the surface of the Earth that it can be consideredabundant. In the oceans, approximately one hydrogen
nucleus in 6,500 is deuterium, and all the rest are thecommon isotope (written either 1H or p, for proton).
Heavy water is the water molecule with both of its
hydrogen nuclei in the form of deuterium, written
D2O. (H2O is sometimes called “light water.”) Heavy
water is the principal industrial product containing
deuterium. To provide the heavy water for Canadian
nuclear fission reactors, the Bruce Heavy Water Plant
in Canada (the world’s largest heavy water production
plant) produced 700,000 kilograms of heavy
water per year from 1979 to 1997 [7], or 140,000
kilograms of deuterium per year. This production rate
is reassuringly large, in relation to the demand for
deuterium for fusion on a commercial scale. Imagine
that a central role for fusion in the future global
energy system entails the deployment of 1,000 of

12the 1,000-megawatt D-T facilities described above.
Using our estimate of 80 kilograms of deuterium
consumption per year for such a plant, 80,000
kilograms of deuterium would be consumed, which
would require only one deuterium production plant on
the scale of Bruce.
Today, the world has a modest tritium inventory, mostof it located in Canada at the Tritium Removal Facility
in Darlington, Ontario operated by Ontario Hydro, a
Canadian electric utility. This facility was built in the
1980s near one of the utility’s nuclear fission plants.
The utility at that time operated or was building
20 large CANDU nuclear fission power reactors. In
CANDU reactors, tritium is made in much greater
quantities than at any other kind of fission reactor,
because the coolant is heavy water. Tritium is madein these reactors when reactor neutrons collide with
coolant deuterium. On average, each of these CANDU
reactors (most of them 600 megawatt plants) was
expected to produce about 100 grams of tritium
per year, so the Tritium Removal Facility was sized
to process about two kilograms of tritium per year
and to remove for storage 97 percent of the tritium
processed. An estimate published in 2011 reported
that the inventory of tritium from CANDU reactors was
20 kilograms [8].
There is a second inventory of tritium in hydrogenbombs. The actual amount is classified, but it has
been declassified that each bomb generally has lessthan 20 grams. Assuming that four grams of tritium
are in each hydrogen bomb and that the world has
10,000 such weapons, the world inventory can be
estimated at 40 kilograms of tritium. About five
percent of the tritium decays each year, so that tokeep the tritium inventory constant would require
tritium production of two kilograms per year.
Compare both estimates of two kilograms per year
to the 120 kilograms of tritium per year consumed ina single one of our representative 1,000 megawatt
D-T facilities. The requirements for tritium in a global
energy system where D-T fusion power is widely
deployed dwarf all current flows of civilian and military
tritium.
With fusion power plants that combine burning
plasmas and lithium blankets, tritium is expected tobe generated at least as quickly as it is consumed.
But who will supply the tritium for the very first
D-T fusion reactors, if Ontario Hydro’s stock isinsufficient? Some portion of Ontario Hydro’s tritium
inventory will decay away – unused, the stock will
halve every 12 years. Another portion will be used at
ITER. Perhaps the very first fusion devices will be able
to be coupled to blankets that produce considerably
more tritium than the plasma consumes, so that thetritium inventory can be built up within the fusion

program. Perhaps some of the tritium required forthe first fusion reactors will be produced in CANDUs
or other nuclear fission reactors – at a rate that isdeliberately matched to the tritium needed for fusion
research and development.
Another potentially relevant nuclear reaction
The combination of Reactions 1 and 2 is not the only
path to fusion energy that scientists are investigating.
One particular alternative, the “proton-boron-eleven”(p-11B) reaction, is shown in Reaction 3.
p + 11B → 3 4He + 8.7 MeV.

(Reaction 3)

The reactants are a proton and the 11B nucleus, which
has five protons and six neutrons. Thus, the reactants,
all together, have six protons and six neutrons. The
products are three identical helium-4 nuclei, eachwith two protons and two neutrons.
Fusion power based on the p-11B reaction has
two advantages, relative to fusion based on the
D-T reaction with tritium regeneration: 1) the p-11B
reaction does not consume tritium, thereby avoiding
the need for a complex tritium-regeneration system,and 2) the p-11B reaction does not produce neutrons,
thereby avoiding the damage to structural materials
which neutron bombardment creates. However, thep-11B plasma can sustain the production of fusionenergy only at a temperature roughly 10 times higher
than the temperature required for the D-T plasma.
Sustaining such high temperatures exclusively byfusion generation may not be possible, because sucha hot plasma radiates energy away too quickly.
Moreover, no compelling strategy has been identified
for removing the charged helium-4 particles from thep-11B plasma while they still carry their initial kinetic
energy so that their kinetic energy can be converted tosome other form of energy outside the plasma rather
than within the plasma. Recall that in the D-T plasma
only the electrically neutral particles (the neutrons)
carry their kinetic energy beyond the plasma, while
the energetic charged helium-4 particles do not leave
the plasma and deposit their kinetic energy within theplasma.

Burning plasma
The next two decades of fusion research are expected
to reveal, for the first time, the dynamics of a burning
plasma – a plasma where the dominant energy
source is within the plasma itself. The detailed
behavior of a D-T plasma under these circumstances
cannot be completely predicted. New sources of
instability may appear, and they may or may not turnout to be straightforward to handle.
However, much else about the system in which theD-T plasma will be embedded can be anticipated.

13

Here we review challenges associated with recycling
tritium, recovering the thermal energy in the blanket,
and maintaining structural integrity for critical
reactor components in spite of incessant neutron
bombardment. These challenges are being addressedin many fusion research laboratories and are
expected to be a major focus at ITER.
Figure 3.6 shows the flows of fuels and products
at steady state in a deuterium-tritium power plant.
The figure is intended to represent the system being
developed today by the magnetic confinement fusion
research community, including for ITER. Within avacuum vessel are a deuterium-tritium fusion reactor
and a tritium-regenerating lithium blanket. The
plasma is confined within an inner region whose outerboundary is the first wall of the blanket. Not shown,the vessel has injection ports and is surrounded bymagnets.
The positively charged 4He nuclei produced in thefusion reaction lose their kinetic energy in theplasma, while the electrically neutral neutrons escape
the plasma but are stopped in the blanket, where
they generate tritium and heat. A heat exchanger
penetrating the blanket removes heat from theblanket to keep it at constant temperature andtransports the heat to a steam or gas turbine, which
generates electricity.
There are two critical tritium cycles.
1. The divertor cycle In today’s deuterium-tritium
reactors, only about 2 percent of the deuterium
and two percent of the tritium entering theplasma actually react before they are removed
from the plasma by a divertor. The “unburned”
98 percent of the deuterium and tritium isthen recycled and returned to the plasma. The
recycle time is expected to be approximately
one hour at ITER.
The divertor also pumps the helium out of theplasma; otherwise, the helium would dilute andcool the plasma. The divertor also removes
other impurities, such as materials ablated fromthe walls.
2. The blanket cycle Tritium is so scarce, tritium
production in the blanket must produce atleast as much tritium as is consumed in theplasma’s core: the blanket must accomplish
tritium “breeding.” One neutron is producedin the D-T reaction for each tritium nucleus
consumed (Reaction 1) and there is at least
some chance that this neutron will not enter
the blanket and react with lithium. This meansthat a fusion neutron that reaches the blanket
must produce somewhat more than one tritium
nucleus. Blankets are expected to embedneutron multipliers to make this possible. One
such multiplier is beryllium: when a neutronis absorbed by beryllium, two neutrons areproduced. Lead is another neutron multiplier.
The tritium produced in the lithium blanket must
be extracted from the blanket so that it can beinjected into the plasma. Tritium extraction isanother subject of current fusion research.
As for the lithium in the blanket, it is assumed
not to require replenishment any more rapidly
than the rest of the blanket.
Blanket T recycle

Vacuum vessel
BlanketFirst wall

T
D

D

6

n

Li

PlasmaConversion of heat to electricity

Divertor

Divertor D recycle

4

He

4

He

Divertor T recycle
4

He

Figure 3.6: Schematic of the materials flows ofdeuterium, tritium, and helium through a fusion
reactor system. Source: authors.

Heat extraction
To produce electricity at a fusion power plant, thermalenergy in the blanket (the result of Reactions 1 and
2) must be extracted and used to drive a generator.
The efficiency of the power plant depends criticallyon the temperature of the blanket and the efficiencyof extraction of heat from the blanket. Heat transferfrom the blanket can be accomplished by steam,by a gas such as helium, or by a liquid metal. Bothsteam and gas heat extraction methods were studied
in the European Power Plant Conceptual Study [9].
The water-cooled system is less technologicallydemanding; the maximum temperature in the blanket,
300 degrees Celsius, is similar to the temperature ina pressurized water fission reactor. The conversion
efficiency from blanket thermal energy to electricity
should be about 30 percent. Advanced blanket
systems might absorb the neutrons and regenerate
the tritium in a molten mixture of lithium and lead,which would allow blanket temperatures between 700
and 1100 degrees Celsius and efficiencies up to 60
percent.
Materials damage by radiation and plant
availability
A large commercial power reactor that operates nearlycontinuously throughout the year is called a baseload

14

power plant. Many nuclear and coal power plants arebaseload plants. Working as intended, they require
infrequent maintenance and at a time planned wellin advance. For a future fusion plant to be a baseload
plant, it will need to minimize unplanned shutdowns
and the time required for scheduled maintenance.
Accordingly, the durability of components subjected tohigh levels of neutron radiation is a major constraint
on fusion reactor design and materials choice. The
two major plasma-facing components of the fusion
reactor are especially vulnerable. These are the “firstwall” (the innermost surfaces of the blanket) andthe divertor. They must be replaced before they lose
structural integrity, and such replacements must not
be frequent.
The bombardment of structural materials by fusion
neutrons displaces atoms from their initial locationsin the material’s crystalline structure and also drives
nuclear reactions at these sites that create heliumgas. Both displacement and helium production
gradually reduce structural integrity and create
embrittlement. Structural damage is quantified by aparameter called “displacements per atom.” Steel ina fusion reactor may experience 15 displacements
per atom each year (on average, every atom has
moved 15 times per year!). These displacements may
include later ones that undo the effects of earlier
ones by sending an atom back to its original position[10].
There is little experience to draw on. Fusion neutrons
emerge from the fusion event (Reaction 1 above)with about seven times the average energy of fission
neutrons, so even the neutron damage in fission
reactors is only partially relevant. The joint EuropeanJapanese International Fusion Materials Irradiation
Facility (IFMIF) is expected to have the capability to
begin to fill this gap in knowledge.

References[1] ITER. Magnets. https://www.iter.org/mach/magnets [accessed March 2016].
[2] Milch, I. (2015). Magnet tests on Wendelstein 7-X successfully completed. http://www.ipp.mpg.
de/3895159/06_15 [accessed March 2016].
[3] Extensively modified from Yamada 2012, with the assistance of Dale Meade: Yamada, H. “Fusion Energy” in
Handbook of Climate Change Mitigation (eds. W.-Y. Chen et al., Springer, 2012).
[4] Duchateau, J. (2007). Superconducting magnets for fusion. Clefs CEA 56: 12-15.
[5] EUROFusion (2015). Generating runaway electrons in JET to benefit ITER. https://www.euro-fusion.
org/2015/06/generating-runaway-electrons-in-jet-to-benefit-iter/ [accessed March 2016].
[6] Redrawn from: European Nuclear Society. https://www.euronuclear.org/info/encyclopedia/n/nuclearfission.htm [accessed March 2016].
[7] Stuart, A. and M. Fairlie (1994). Integrated electrolysis: a utility springboard for sustainable development. In
Intersociety Energy Conversion Engineering Conference 4: 1647-55.
[8] Jason: Tritium (2011). Report JSR-11-345. The Mitre Corporation, McLean, Virginia.
[9] Maisonnier, D., Cook, I., Sardain, P., Andreani, R., Di Pace, L., Forrest, R., Giancarli, L., Hermsmeyer, S.,
Norajitra, P., Taylor, N., et al. (2005). A conceptual study of commercial fusion power plants: Final report of theEuropean Fusion Power Plant Conceptual Study (PPCS). European Fusion Development Agreement.
[10] Gilbert, M.R., Dudarev, S.L., Zheng, S., Packer, L.W. and J.-Ch. Sublet (2012). An integrated model formaterials in a fusion power plant: transmutation, gas production, and helium embrittlement under neutron
irradiation. Nuclear Fusion 52: 1-12.

15

Article 4: Economics
Understanding the economic potential of fusionenergy is a complex challenge since fusion is still
many years from being a market option. Uncertainties
abound at multiple scales, from the power plant to thefull global energy system.
This article has two parts. In the first part, we consider
the determinants of the cost of electricity from anindividual fusion power plant. In the second part,
we investigate how fusion will compete for energy
market share. We limit our discussion to magnetic
confinement fusion, and more particularly to its
tokamak configuration.

Estimates of the cost of
fusion electricity
The cost of fusion electricity is driven principally by its
capital cost and by how many hours the plant can run
each year. Quantitative discussion focuses on “thelevelized cost of electricity.” In essence, the levelized
cost is the total cost of building a plant and runningit over its lifetime, divided by the kilowatt hours ofenergy that the plant produces over its lifetime. Like
its fission counterpart, the total cost of a fusion power
plant is dominated by its initial capital costs. The
kilowatt hours produced over its lifetime are affectedprimarily by the size of the plant, the number of hours
that it is able to run each year, and the efficiency withwhich the thermal energy produced through fusion isconverted into electricity.
There are many estimates of the capital cost of afusion plant [1,2,3,4]. The estimates range from$2,700 to $9,700 per kilowatt of capacity. The
plants have a capacity between 1,000 and 1,500
megawatts. Assuming that the capital cost perkilowatt is roughly independent of the size across thissmall range of sizes, the estimated capital cost of afusion power plant with 1,000 megawatts of capacity
would range from 2.7 to 9.7 billion dollars. (Costs
throughout this article are in 2010 US dollars [5]).
The wide range of capital costs is partially explainedby varying assumptions about how many plants of thesame kind have been built prior to the plant whose
cost is being estimated. Fusion plants are expected
to become less expensive as more plants of a specificdesign are built. “Technological learning” captures
this issue: cost models often assume that costs will
fall at some well-defined rate as additional units ofthe same kind are installed.
The wide range of costs is also due to differencesin the assumed technological maturity of the plant.

Fusion plants are likely to become less expensive as
they incorporate successive advances in technology.
For example, with maturity may come greater
efficiency in converting the thermal energy of fusion
into electricity. Nearly a factor of two is at stake,with conversion efficiency ranging between 30 and
60 percent. This efficiency depends especially on
the temperature of the blanket; the blanket absorbs
the thermal energy released in the fusion reactions
and delivers most of that thermal energy to theturbine that produces electricity. The larger thedifference between the temperature of the blanket
and the temperature of the environment (ocean orriver water, for example), the higher the efficiency ofelectricity generation. Efficiencies of 30 percent are
representative when the blanket is water-cooled andmaintained at a temperature of 300 degrees Celsius,
so that steam enters the steam turbine at nearly 300
degrees Celsius. The higher 60 percent efficiency
might be realized if a blanket could be maintained atmuch higher temperatures as a result of being cooled
by a gas or a liquid metal.
Two kinds of costs are associated with any power
plant: the capital cost for building the plant (incurredin its first years) and the cost of running it (for manyfurther years). To combine these costs into a complete
cost per year that can be used to calculate cost ofelectricity requires “annualizing” the initial capitalcost, meaning transforming it into a cost per year
to match the units of the costs of running the plant.
Typically, this is done by multiplying the capital costby some factor (in units of percent per year). A typicalmultiplier is 15 percent per year, which not onlytransforms the capital cost into a cost per year, butalso incorporates the associated costs of borrowing
money, depreciation, insurance, and taxes. Using
this multiplier, if the capital cost for building a 1,000
megawatt plant is six billion dollars, the annualized
cost will be 900 million dollars per year, or 900
dollars per year per kilowatt of plant capacity. If theplant were to operate continuously over all the 8,760
hours in a year, the cost per kilowatt-hour associatedwith the capital cost alone would be approximately ten
cents.
However, plants do not run continuously for a whole
year; if the plant ran only half the year, the capitalcomponent of the cost per kilowatt-hour would betwenty cents. Accordingly, another source of variationin cost estimates is the assumption about the numberof hours that the plant runs each year. A capitalintensive technology like fusion generally requires
nearly full-time operation to be competitive. Unique16

in its significance for fusion
plants is “scheduled component
replacement,” which will affect its
availability. Critical components
degrade and need to be replaced
many times over the lifetime ofthe plant as a result of the wear
and tear that they sustain from
irradiation by fusion neutrons andcharged particles.

Availability 90(Percent)

12 Cost ofelectricity(cents perkilowatt hour)

80

10

70

8

60

0

5

10

15

20

25

6

Radiation absorbed by blanket before replacement
Accordingly, the fusion research(megawatt years per square meter)
community pays close attention
to the replacement of parts that
Figure 4.1: The dependence of fusion power plant availability and the costof fusion electricity on the durability of the fusion blanket [7]. The durability
lose their function after only a few
of the blanket is quantified as the maximum amount of absorbed radiation
years. The most important areas(in megawatt-years) that the blanket can tolerate per square meter ofof the plant, from the perspective
blanket surface area. The more radiation the blanket can absorb beforeof durability, are those in close
replacement, the fewer the periods during which the power plant is shutproximity to the fusion plasma
down for blanket replacement and the lower the cost of electricity.
itself, notably the first wall, theblanket, and the divertor. One
Figure 4.2 shows the allocation of the total cost of
current estimate indicates that divertor replacement
fusion power reported in the European Power Plant
could take four months and divertor replacement inConceptual Study [1]. A similar analysis is foundcombination with blanket replacement could takein the U.S. Advanced Research, Innovation, andsix months. Every replacement would also require a
Evaluation Study [2]. The European study investigates
one-month cooling period at the front end and a onefour variants of fusion plants, but only Model A, themonth conditioning period at the back end before thevariant described as requiring the least amount ofplant could produce power [6].
new technology, is shown in Figure 4.2. The costThus, there is an important relationship between
estimates are based on a “tenth of a kind” plant,
durability of components requiring replacement andmeaning that these are the costs for the tenth plant
the cost of fusion electricity. Figure 4.1 shows theof a similar kind – lower costs than the costs for the
benefit obtained when a blanket is able to withstandfirst plant because some technological learning has
a larger amount of radiation before replacement.
occurred. The capital cost is estimated to be $9,700
Consider the durability of the blanket for two values ofper kilowatt of capacity, the discount rate is 6 percent,the total tolerable neutron absorption before blanket
there is no taxation, the plant lifespan is 40 years, thereplacement is required: five and 20 megawatt-years
plant is expected to run 75 percent of the time, andof absorption of neutron irradiation per square meter
efficiency of conversion of fusion power to electricity isof surface. This four-fold increase in durability is seen
31 percent.
to produce an increase in power plant availability from
With these assumptions total costs can be grouped,
65 percent to 80 percent and a fall in the total costand shares for each group can be estimated: Capitalof a kilowatt-hour from ten cents to seven cents. The(73 percent), Divertor Replacement (12 percent),
cost curve has diminishing returns: at high durability,
Blanket/First Wall Replacement (4 percent), andfurther blanket durability has diminishing benefit, as
Operation and Maintenance (9 percent). Note that
other factors (such as divertor replacement) become
the combined cost for the replacement of the divertor
the more important causes of shutdowns.
and the blanket’s first wall is 16 percent of the
A priority for fusion, therefore, is the development
total cost. Other costs, mainly the costs of fuel andand demonstration of materials that will tolerate
decommissioning, are reported as being negligible.
the fusion environment for a long period. The
The cost of handling the regenerated tritium isdevelopment of analogous materials has enabled a
presumably placed in the Operation and Maintenancewell-operated fission plant to run roughly 90 percent
category, since it is a recurrent cost. The same studyof the time, with a single shutdown period roughly
reports that the largest contributions to the total
once a year. The most difficult component to replace
capital cost are 1) the combined cost of the magnets
in a fission plant, the pressure vessel within whichand cooling system, and 2) the combined cost of
the fissions occur, is expected to remain intact forbuildings and site preparation.
the lifetime of the plant. However, the neutrons fromfusion are more energetic than those from fission anddo more damage.

17

% total cost
100
90
80

Other
Operation and maintenanceBlanket/first wall
replacement
Divertor replacement

70

Capital
60
50
40
30
20
10
0
Figure 4.2: Components of the total cost of electricity
produced by a fusion power plant based on a tokamak
reactor, shown as a percent of total cost [1].

We draw five key messages from this section:
1. There is a wide range of estimates for the costof electricity from future fusion power plants.
2. Uncertainty regarding in what directions andhow rapidly technology will advance is a key
contributor to this range.
3. For all these estimates the primary
determinants of the cost of electricity arecapital costs and the proportion of time that theplant can operate.
4. Capital costs are dominated by the costs of themagnets and cooling system.
5. The proportion of time a plant can operate ismainly determined by how often the first wall,
blanket, and divertor need to be replaced.
While higher-level modeling of fusion energy’s future
economic competitiveness (such as the market
share study described immediately below) can
provide an orientation to fusion’s potential role in the
global energy system, the deep uncertainty aboutfusion’s likely future cost implies that all quantitative
economic estimates emerging from these models arehighly speculative at this stage.

Fusion’s market share in
the 21st century
The prospects for fusion energy as a source ofelectricity over the coming century will depend not
only on fusion’s own future costs but also on the
future global electricity market. The size of thatmarket depends on the rate of economic growth,
the amount of electrification of the energy system,and the efficiency with which electricity is used toprovide goods and services. Since fusion is a lowcarbon energy source, its future role in the market
also depends on the extent to which concern forclimate change is translated into carbon policies
that disadvantage high carbon energy sources (thefossil fuels: coal, oil, and gas) relative to low-carbon
sources. Also significant are the costs of other lowcarbon sources relative to the cost of fusion.
Little modeling of the impact of fusion onenergy markets has been done, largely because
cost estimates for fusion are so uncertain, as
demonstrated in the previous section of this article.
We report one of the few modeling studies where
fusion costs and related economic parameters(such as the time of fusion’s arrival) are assumed
and fusion’s impact on the full global energy system
is developed [8,9]. We provide quantitative resultsfrom this study that illustrate some of the factorsthat can influence fusion’s future market share. The
exact model results are unimportant relative to its
qualitative conclusions, which are consistent withother work [3,10,11,12].
The researchers used one of the best-known
integrated assessment models, known as GCAM,
developed and maintained at the Pacific Northwest
National Laboratory of the U.S. Department of Energy.
The model’s inputs include the performance of theeconomy, carbon-cycle science, climate policy, andthe costs of competing energy technologies. Its
outputs are representations of future energy markets
in 14 geopolitical regions every five years from 2015
to 2095. At each time step, the demand for electricity
is met by a broad array of energy technologies.
Although the model can describe the entire energy
system, here we focus on the electricity sector, where
the model calculates electricity prices, carbon prices(carbon taxes), and electricity demand. The study
we are reporting adds fusion to the list of electricitygenerating competitors that GCAM has modeled
previously. The key assumptions for fusion power are
the following:18

1. Timing The model’s base case assumes that thefirst fusion power plant becomes available in2035, at least ten plants are operating in 2050,
and at least 100 plants are on line in 2065.
This ambitious schedule is hard to reconcile
with the current schedule for ITER and follow-on
research projects (see Article 6).
2. Cost The median capital costs fall as fusion
deployment grows over time. The capital costof the initial plant is $6,000 per kilowatt ofcapacity, which is within the range of capital
cost estimates discussed above. The unit cost is
assumed to fall as additional units are built, as
may be appropriate for an immature technology.
The unit cost in 2065 is $3,800 per kilowatt,and costs continue to fall moderately after that.
3. Availability The plant is assumed to run 90
percent of the time. This would require that theplants achieve shorter shutdown periods for the
replacement of irradiated reactor components
than is currently expected (see the first part ofthis article).
The overall optimism in these assumptions suggeststhat these GCAM model runs probably overestimate
the market share of fusion at various future dates.
However, the qualitative results from these modelruns are instructive, especially those that reveal thedependence of fusion’s market share on alternative
policy environments. These results reveal theimportance of the assumed costs for two of fusion’s
main low-carbon competitors: carbon dioxide captureand storage (CCS) and nuclear fission. They also show
the strong dependence of fusion market share on theprice imposed on carbon dioxide emissions to theatmosphere.
In CCS power plants, a fossil fuel or biomass is burned
and the carbon dioxide emissions from combustion
are “captured” – prevented from escaping to theatmosphere. The carbon dioxide is then “stored,”typically, deep below ground in a porous saline
geological formation. The first CCS plants are now
running. GCAM allows alternative constraints to beplaced on the maximum amount of carbon dioxide
storage space available below ground in eachgeopolitical region.
As for fission, its median capital cost for the baseline
case in 2065 is $2,700 per kilowatt, about 30
percent less than the corresponding 2065 capital
cost for fusion. An alternative in the model is a “low”
fission case where in 2095 fission is more expensive(about 30 percent more expensive than the baselinecase), which closes the gap between the costs offission and fusion.

GCAM, for all cases, also restricts the contributionsof wind power and solar power to reflect theirintermittency.
Fusion competes with 14 other electricity production
technologies to meet electricity demand. Three
options are carbon-emissions-intensive: power fromcoal, natural gas, and oil without CCS. The other11 options are low-carbon – low-carbon rather
than zero-carbon because of the carbon dioxide
emissions associated with construction of the plantand other ancillary factors. Biomass-based electricity
without CCS is one of these. Another four are CCS
options, differing in their fuel: coal, natural gas, oil,and biomass. The remaining six are nuclear fission,
geothermal energy, hydropower, wind, ground-basedsolar, and rooftop solar. The market shares of all
15 production technologies from 1990 to 2095 areshown in Figure 4.3 for two representative GCAM
scenarios (the original study explores many morescenarios).
Panel I shows Scenario I, the base case and the pointof departure for the analysis. Circumstances areunfavorable for fusion because there is no climate
policy (no carbon price); all of fusion’s competitors,
including fission, have their baseline costs; and (notactually relevant, because without a carbon price CCS
cannot be viable) there is ample space below groundto store carbon dioxide. Efficiency in electricity use
has limited impact: global electricity demand, which in2010 was approximately 22 trillion kilowatt-hours (atrillion is a million million, or 1012), rises to 90 trillion
kilowatt-hours, more than four times the 2010 value,in 2095. Coal power dominates electricity supply
in 2095 with 29 percent market share, followed bynatural gas and nuclear fission, each with 18 percent.
The shares of wind and ground-based solar are sevenand five percent respectively. Fusion’s share is four
percent, or four trillion kilowatt-hours.
How many fusion power plants would be operating in
2095 in Scenario I if fusion actually were to produce
four trillion kilowatt-hours that year? Assuming a
representative plant that has a capacity of 1,000
megawatts (one million kilowatts) and runs 8,000
hours per year (approximately 90 percent of the time),
the plant would produce eight billion kilowatt-hours
each year. Thus, the answer is that approximately 500
fusion plants would be operating in 2095 in Scenario
I. This is approximately the scale of nuclear fission
power today.
Panel II presents Scenario II, a far more favorable
case for fusion, where there is significant climate
policy, a cost penalty for nuclear fission, and limits onavailable carbon dioxide storage space. Climate policy
is assumed to take the form of a concentration of19

Global Electricity Generation(trillion kilowatt-hours)
120
90

I. No carbon constraint;
baseline fission andCO2 capture and storage (CCS)

60

12%
7%
18%

30

59%

Hydropower
Geothermal

4%

Fission

0
2000

120

2025

2050

2075

II. 450 ppm cap;
low fission, low CO2
capture and storage (CCS)

35%

90

6%
60

26%

Roof-solar
Ground-solar
Wind

Biomass with CCS
Biomass
Oil with CCS
Oil
Gas with CCS
Gas
Coal with CCS
Coal
Fusion

1%

30

32%
0
2000

2025

2050

2075

Figure 4.3: Production of global electricity in the 21st century, by
source, when fusion is an option [8]. Panel I (above): Scenario I, acase unfavorable to fusion because there is no carbon policy andboth fission and carbon dioxide capture and storage (CCS) options are
fully available. Panel II (below): Scenario II, a case favorable to fusion
because there is a 450 parts per million (ppm) climate target and fission
and CCS options are constrained. The numbers at the right (in percent)are the shares of total electricity production in 2095 for five bracketed
power sources; from top to bottom, these are intermittent renewables,
non-intermittent renewables, nuclear fission, fossil sources with andwithout CCS, and nuclear fusion.

carbon dioxide in the atmosphere not to be exceededat the end of the century. Within GCAM there is
a carbon cycle model which links carbon dioxide
concentrations to carbon dioxide emissions andthereby limits emissions. Emissions reductions areachieved by a globally uniform price on carbon dioxide
emissions, which improves the competitiveness of
low-carbon energy relative to fossil fuels.
In Scenario II the global energy system is constrainedby the requirement that the maximum carbon dioxide
concentration in the atmosphere must never exceed
450 parts per million (ppm). (When the concentration
is 450 ppm, 450 carbon dioxide molecules arepresent in every million molecules of air.) This
constraint is one of the most common climate policy
objectives in the modeling literature [13]. A 450
ppm cap is estimated to have a 50 percent chanceof limiting the rise of the averagesurface temperature of the Earth totwo degrees Celsius relative to its
temperature 200 years ago, thereby
potentially avoiding some of thedangerous impacts of climate change.
The current concentration is alreadyabout 400 ppm. By comparison,
the carbon dioxide concentrationin 2095 for Scenario I is 810 ppm,
roughly double the concentration
today. Scenario I assumes that noclimate policy is enacted; specifically,
throughout the 21st century there is
no price on carbon dioxide emissions
to the atmosphere. By contrast, in
Scenario II a carbon price is imposedthat grows throughout the century andreaches more than $280 per ton ofcarbon dioxide in 2095.
Comparing Scenarios I and II reveals
that total electricity demand in
2095 is even higher in Scenario II
than in Scenario I. Global electricity
consumption in 2095 is 120 trillion
kilowatt hours for the 450 ppm climate
target, versus 90 trillion kilowatt
hours when there is no climate
target. The climate target pushes theenergy system toward electricity andaway from the direct use of fuel – forexample, toward electric vehicles andelectric space heating.

Fusion’s market share in 2095 is eighttimes larger in Scenario II (32 percent)than in Scenario I (four percent). In
Scenario II fusion is the dominant
supplier of electricity in 2095, providing
41 trillion kilowatt hours, or ten
times as much electricity as in Scenario I. Since weestimated that the fusion output in Scenario I would
require 500 representative fusion power plants, itfollows that in Scenario II 5,000 of these plants would
be operating.
But the differences between the scenarios extend
well beyond fusion. Electricity from fossil fuels andbiomass provides 59 percent of global electricity
when there is no carbon policy, but only one percent
when a 450 ppm target is met. In the presence of theclimate target, fusion, intermittent renewables, andfission all increase their market share to make up the
difference, gaining 28, 23, and 8 percentage pointsof market share respectively. The gain in fission’sshare happens in spite of its higher cost in Scenario
II than in Scenario I. Scenario I never uses CCS
technologies, because there is no price on carbon.

20

In Scenario II, CCS versions of biomass power plantsand (to a lesser extent) natural gas power plants playa significant role in the middle of the century but by
2095 they no longer contribute, presumably because
carbon dioxide storage costs have risen as theregional storage sites have filled up.
In summary, as with any similar study, the results
reported here are strongly dependent on theassumptions. The results are sensitive to the dateof the first commercial plant and its initial cost,
the rate of fall of unit cost through learning, andthe competitors’ costs and constraints. Notably,
the costs of fission and fusion are assumed to be
similar. Nonetheless two of the study’s results arebroadly relevant. First, without a carbon target andin the absence of any explicit penalties on fusion’scompetitors, fusion’s share of electricity at the endof the century is small, if it competes at all. Second,the combination of a carbon target and restraints
on fission and CCS increases fusion’s market share
dramatically.

References[1] Maisonnier, D., Cook, I., Sardain, P., Andreani, R., Di Pace, L., Forrest, R., Giancarli, L., Hermsmeyer, S.,
Norajitra, P., Taylor, N., et al. (2005). A conceptual study of commercial fusion power plants: Final report of theEuropean Fusion Power Plant Conceptual Study (PPCS). European Fusion Development Agreement.
[2] Najmabadi, F., The ARIES Team: Abdou, A., Bromberg, L., Brown, T., Chan, V.C., Chu. M.C., Dahlgren. F.,
El-Guebaly L., Heitzenroeder, P., Henderson, D., St. John, H.E., Kessel, C.E., Lao, L.L., Longhurst, G.R, Malang,S., Mau, T.K., Merrill, B.J., Miller, R.L., Mogahed, E., Moore, R.L, Petrie, T., Petti, D.A., Politzer, P., Raffray, A.R.,
Steiner, D., Sviatoslavsky, I., Synder, P., Syaebler, G.M., Turnbull, A.D., Tillack, M.S., Waganer, L.M., Wang, X.,
West, P. and P. Wilson (2006). The ARIES-AT advanced tokamak, advanced technology fusion power plant.
Fusion Engineering and Design 80(1-4): 3-23.
[3] Han, W.E. and D.J. Ward (2009). Revised assessments of the economics of fusion power. Fusion
Engineering and Design 84(2): 895-98.
[4] Bustreo, C., Casini, G., Zollino, G., Bolzonella, T. and R. Piovan (2013). FRESCO, a simplified code for cost
analysis of fusion power plants. Fusion Engineering and Design 88(12): 3141-51.
[5] When published costs were quoted in euros or dollars of a different year, we adjusted the cost using
http://x.rates.com and/or http://www.usinflationcalculator.com.
[6] Crofts, O. and J. Harman (2014). Maintenance duration estimate for a DEMO fusion power plant, based on
the EFDA WP12 pre-conceptual studies. Fusion Engineering and Design 89(9-10): 2383-87.
[7] Ward, D.J., Cook, I., Lechon, Y. and R. Saez (2005). The economic viability of fusion power. Fusion
Engineering and Design 75-79: 1221-1227.
[8] Turnbull, D. (2013). Identifying new saturation mechanisms hindering the development of plasma-basedlaser amplifiers utilizing stimulated raman backscattering. Ph.D. Dissertation. Princeton University, USA.
[9] Turnbull, D., Glaser, A. and R.J. Goldston (2015). Investigating the value of fusion energy using the Global
Change Assessment Model. Energy Economics 51: 346-53.
[10] Cook, I., Miller, R. and D. Ward (2002). Prospects for economic fusion electricity. Fusion Engineering and
Design 63-64: 25-33.
[11] Tokimatsu, K., Asaoka, Y., Konishi, S., Fujino, J., Ogawa, Y., Okano, K., Nishio, S., Yoshida, T., Hiwatari, R.
and K. Yamaji (2002). Studies of breakeven prices and electricity supply potentials of nuclear fusion by a longterm world energy and environment model. Nuclear Fusion 42(11): 1289-98.
[12] Gnansounou, E. and D. Bednyagin (2007). Multi-regional long-term electricity supply scenarios with fusion.
Fusion Science and Technology 52(3): 388-93.
[13] IPCC Climate Change 2014: Mitigation of Climate Change, Working Group III (eds. Edenhofer, O. et al.,Cambridge University Press, 2014).

21

Article 5: Fusion and Fission
Nuclear fission power has been under serious scrutiny
throughout the decades-long history of civilian fission
reactors. A complex regulatory system has emerged,
accompanied by sizeable public distrust in fourareas: nuclear weapons proliferation, the disposalof radioactive waste, reactor accidents, and terrorist
or military attack. Fusion power will probably needto deal with the same challenges and fears. But towhat extent are the risks from fusion and fission
power similar? In order to answer this question, we
explore the qualitative and quantitative differences
between fusion and fission technology for these four
issues. For the most part, we restrict the discussion tomagnetic confinement fusion.

Nuclear weapons
proliferation
There are two types of nuclear weapons: Fission
bombs (“atomic bombs”) harness the energy released
in fission reactions; the Hiroshima and Nagasaki
bombs were fission weapons. A few years after fission
bombs were developed, the energy from fusion
reactions was harnessed for weapons by using theenergy from a fission bomb to set off secondary
fusion reactions. These “hydrogen bombs” were evenmore destructive than fission-only weapons. At the
moment, nine countries possess atomic bombs: the
U.S., Russia, China, the United Kingdom, France,
Israel, India, Pakistan, and North Korea. The first five
– and probably Israel and India as well – also havehydrogen bombs, while Pakistan and North Korea
probably do not.
Nuclear proliferation refers to the developmentof nuclear weapons capabilities in new countries.
It requires access to special nuclear materials,
which the expansion of nuclear power plants could
potentially provide. It also requires specialized
training in nuclear technology, some of which would
be gained through experience with nuclear power.
Civilian fission power is tightly linked to fission
weapons through two special materials: plutonium
and highly enriched uranium. Either can sustain
an explosive chain reaction. The uranium found innature cannot be used for weapons without expensive
manipulation, called isotope enrichment. Isotope
enrichment, in which the ratio of uranium-235 touranium-238 is increased relative to the ratio foundin nature, is part of the preparation of uranium formost kinds of nuclear power plants. “Weaponsgrade” uranium is produced by further enrichment.
Plutonium is found in nature only in trace amounts so
it needs to be made in nuclear facilities. Plutonium isa by-product of the reactions in fission power plants,
but it is unavailable for weapons use unless steps aretaken to separate it from the highly radioactive “spentfuel” in which it resides.
Thus, fission power represents a step toward thenuclear materials for an atomic bomb, but the next
steps are not inevitable. There is technological spacefor civilian fission power without nuclear weapons,
and many countries have chosen to be in that space.
Because of the requirements for nuclear power, they
have nuclear scientists and nuclear equipment, butthey do not have and do not seek to have the required
separated plutonium or enriched uranium theseweapons require. Without these materials no bomb
can be built.
The relationship between civilian fusion power andhydrogen bombs is more complicated. First, a country
that chooses not to develop fission weapons cannot
develop fusion weapons because hydrogen bombs
require fission as well as fusion. Second, however,both uranium and plutonium for weapons can beproduced at a fusion plant using the abundant
neutrons generated in the reactor. Plutonium canbe produced at a fusion plant by placing naturaluranium just outside the reactor core and bombarding
it with neutrons from the fusion reactions. In thesame way, uranium-233 of high isotopic purity, aweapons material, can be generated from neutrons
bombarding thorium. No uranium enrichment plant
would be necessary. Diverting some of its neutrons,
a 1,000 megawatt fusion plant could conceivably
create enough uranium-233 or plutonium for one
fission bomb within a week, not including the timeneeded to put the necessary hardware in place, which
could add a few additional weeks to this proliferation
scenario [1].
It would be difficult to disguise the generationof plutonium or uranium-233 if an internationalsafeguards regime with inspections were operating,because a country which operates only fusion plants
would have no valid reason to have uranium or
thorium on hand. This has been called “the major
nonproliferation advantage” of fusion power plants[2]. This advantage would not apply to any “fusionfission hybrid” – a power plant designed to combine
elements of fusion and fission reactors.
Having a fusion reactor could conceivably abet a
country’s transition from fission weapons to follow-on
fusion weapons because the fusion reactor could
produce tritium for such weapons. Tritium is a heavy
isotope of hydrogen that scarcely exists in nature
because its half-life is 12 years. Fusion power plants22both consume and produce tritium, and if tritium
were to become a more common article of commerce
thanks to the fusion power industry, this might slightly
lower the barriers to fusion weapons.
A larger barrier than tritium availability is the highly
guarded secrets related to hydrogen bomb design.
But the prospect of fusion power raises concerns
here too, at least with regard to inertial confinement
fusion, where there is a significant overlap between
the technology for pellet compression and ignitionand hydrogen bomb design. It is telling that the
primary objective of the National Ignition Facility, themain focus of U.S. research in inertial confinement
fusion, is to better understand the physics behindthermonuclear weapons without testing them. A
particular concern is that scientific data from inertial
confinement fusion research could be used in thedevelopment of nuclear weapons [3], although thecivilian fusion power plants themselves are unlikely
to be useful in this regard [2]. Magnetic confinement
fusion, by contrast, has been the poster child ofinternational collaboration in scientific research,dating back to Soviet collaboration with Westerncountries in the 1950s – in part because there is no
credible pathway from magnetic confinement fusion
to nuclear weapons.

Waste disposal

the most challenging aspects of radioactive waste
management for fission power.
Current national regulations for nuclear power requirethat human beings living thousands of years fromnow should experience only minimal damage fromany radiation created by nuclear energy today. The
United States, for example, has targeted 10,000
years or even longer as the minimum duration forgeological storage of what is called “high-level”
nuclear radioactive waste at the proposed storage
facility at Yucca Mountain, Nevada. Such regulations
raise deep questions. The regulations governing the
Yucca Mountain facility note that “except for a few
archaeologic and natural analogs, there is a limited
experience base for the performance of complex,
engineered structures over periods longer than a few
hundred years” (10 C.F.R. § 63.102(h) [4]). To date,despite a number of test projects, no permanent
facility for civilian high-level waste has become
operational. All civilian high-level fission waste is
being held in temporary storage, awaiting a long-term
solution. A waste facility for military waste is operatingin the United States in New Mexico – the Waste
Isolation Pilot Plant. Advanced reactors are being
considered which may be able to destroy most of thelong-lived transuranic isotopes – but, in exchange,
radioactive isotopes with shorter half-lives would be
generated in abundance and would present their own
management issues.

In nuclear fission reactions the uranium nucleus splits
into two other nuclei. These nuclei include radioactive
isotopes with half-lives ranging from fractions of asecond to millions of years. One important radioactive
byproduct from the perspective of public health isiodine-131 (half-life, eight days). The human bodyconcentrates any iodine intake in the thyroid gland,
and when the iodine is radioactive (which it normallyis not) the result, especially in children, can bethyroid cancer. Other damaging isotopes have halflives of years to decades (cesium-134, cesium-137,
and strontium-90); these isotopes dominate theenvironment months to decades after any fissionreactor accident. The accident at Fukushima Daiichi
in Japan in March 2011 led to land contamination byradioactive cesium and the relocation of more than
100,000 people, some of them permanently.

Irradiation of the structural materials of a nuclear
fission reactor adds to the radioactivity it produces.
Most of this waste is “low-level” radioactive waste,
whose storage management is less daunting. UnitedStates nuclear regulations allow low-level waste to beburied in relatively shallow trenches. Required storage
times for low-level waste can still be as long as 100
years. As a result, even wastes classified as “lowlevel” present substantial institutional challenges.

Fission plants also inevitably produce radioactive“transuranic” elements (elements heavier thanuranium). Transuranic elements are found in nature
only in trace amounts, but they are produced inreactors by a succession of nuclear reactions
initiated by the absorption of a neutron by uranium.
(When a neutron strikes a nucleus of uranium, theresult can be either fission or absorption.) Severaltransuranic isotopes have half-lives of centuries
to millions of years, and these represent one of

The physical half-life of tritium is 12.3 years, meaningthat after 12.3 years half of any tritium initiallypresent has transformed itself into innocuous, nonradioactive helium-3. This twelve-year half-life is shortenough that tritium management does not require
costly and politically complex long-term storage; 99
percent is gone in 82 years. Moreover, the energy
released when a tritium nucleus decays is small
relative to most radioactive nuclei. Nonetheless,tritium is subject to intense regulation, because

A fusion power plant is a radioactive environment
for two principal reasons: 1) tritium is inserted into
the plasma, regenerated in the blanket, and heldin storage before being reinserted into the plasma,
inevitably with some losses along the way that end
up in waste streams; and 2) materials surrounding
the plasma are made radioactive by bombardment by
fusion neutrons.

23

being a heavy isotope of hydrogen, it acts chemicallylike ordinary hydrogen. In particular, it is readily
incorporated into water and can thus be ingested byorganisms. Tritium’s biological half-life – the time ittakes for half the tritium to be eliminated from thehuman body after ingestion – is about ten days, thesame as for water, because essentially all of thetritium is in the form of tritiated water, HTO. Thus,
it would take just over two months for a person toeliminate 99 percent of the tritium ingested. (Thebiological half-life, to be sure, varies from person toperson; it can be shortened by drinking large amountsof water.) Releases to the environment and worker
exposure are highly regulated; for example, themaximum concentration of tritium in drinking watersupplies allowed by the U.S. Environmental Protection
Agency (EPA) is just 2 trillionths of a gram per liter ofwater. Accordingly, the management of on-site tritiumis already a preoccupation of operators of fusion
research facilities. It will be a central concern at any
commercial fusion reactor.
The world’s largest civilian facility for tritium
processing is the Ontario Hydro’s Tritium Removal
Facility in Darlington, Ontario, Canada (see Section
3). The facility is on the shore of Lake Ontario andabout one hour from Toronto. It processes tritiatedwater (HTO) whose initial concentration of tritium
was 750 million times larger than EPA’s allowed
concentration in drinking water supplies and whose
final concentration after treatment (97 percent tritium
removal) was 20 million times larger [5].
As for the generation of radioactivity by neutron
bombardment of structural materials, this may well
be a larger issue for a fusion plant than for a fission
plant. About five times as many neutrons are createdin a deuterium-tritium fusion reactor as in a fission
reactor producing the same amount of energy, andthe neutrons made in the deuterium-tritium reaction
carry about seven times as much energy as theneutrons (on average) created in a fission event.
The search for better structural materials for fusion
reactors, although principally concerned with finding
materials that retain their strength under neutron
bombardment, also seeks materials whose demands
on waste disposal are reduced [6].
It may be possible, at some additional cost, to lessen
waste disposal costs by avoiding steels that containniobium and molybdenum. Neutron bombardment of
niobium will create niobium-94 (94Nb), a radioactive
isotope with a half-life of about 20,300 years. This
half-life is approximately the same as the half-life of
plutonium-239 (24,100 years), which is the isotope
whose half-life set the scale for the storage ofhigh-level waste from nuclear fission reactors half acentury ago. On the other hand, niobium is one of theelements in superconducting magnets (see Article3), and this raises the question of the extent to which
neutrons can make their way beyond the vacuum
vessel to the surrounding magnets to generate 94Nb insignificant quantities.
As for molybdenum, neutron bombardment
creates radioactive molybdenum-93 (93Mo)
from molybdenum-92 (92Mo), a stable isotopeof molybdenum constituting 15 percent of themolybdenum in the Earth’s crust. The half-life of93
Mo is 3,500 years, not a desirable half-life fromthe perspective of waste disposal. Molybdenum
provides resistance to wear and extra strength athigh temperatures, so metallurgists would rather not
remove molybdenum from steel. Under considerationis the use of an isotope-separation process toprovide specialty molybdenum for fusion reactorsthat contains negligible amounts of 92Mo, thereby
essentially eliminating 93Mo from the reactor’s
structural materials at time of disposal while enjoying
the improvements in performance that molybdenum
brings to steel [7].
The volume of activated material generated ata fusion power plant would be very large. In theEuropean Union’s Power Plant Conceptual Study, it
was estimated that over 70,000 metric tons of waste
would be generated during the 25-year lifetime of a
1,500 megawatt plant, with another 50,000 metrictons coming at decommissioning [8]. If all this waste
were considered low-level waste, it would need to
be stored for about 100 years [8] – a substantial
advantage when contrasted with the much longer
period required for high-level fission waste. Estimatesof the cost for storing low-level waste range from$100 to $10,000 per cubic meter and depend
strongly on the radioactivity level. Moreover, thedirect payment for storage is estimated to represent
only about 15 percent of the total disposal cost;
other expenses include evaluation, packaging, andtransportation [9].
An alternative to storage as low-level waste, called“clearance and recycling,” is being explored[10,11,12]. This system would begin with theseparation of tritium from irradiated structuralcomponents removed from a fusion plant either atthe end of their useful lives or when the plant itself isdecommissioned. The waste would then be stored fora “decay period,” after which materials would havelost enough of their radioactivity through decay to
be fit for release and sale into the general economy.
For example, concrete from the structure could be
crushed and used in road construction. Materials
whose radioactivity remains above legal limits
would be refabricated into new parts for use within
the nuclear industry. Only 10 percent of the wastefrom the plant would be ineligible for clearance andrecycling [8]. With further work to identify materials

24that do not become highly activated or that quickly
shed their radioactivity, even further reduction in theburden of fusion waste management might result.
There are already a few examples of clearance andrecycling for activated waste from fission reactors. In
one instance, 100 tons of lead casks from the Idaho
National Engineering & Environmental Laboratory
were recycled into bricks for a radioactivity-shielding
wall at Idaho State University. The laboratory gave thebricks to the university for free and realized savingsof $0.70 per pound by recycling instead of disposingof the waste; moreover, the university avoided buying
lead bricks at a cost of about $1.75 per pound [13].
However, as of 2008, one review found there had
been only five instances of clearance/recycling fromfission reactors [14].
The implementation of a clearance and recycling
system for fusion power plants would confront serious
obstacles. For example, the waste would need to be
remotely dismantled, handled, and remanufactured;some of the needed techniques could be borrowed
from the fission power industry, but new methods
would need to be developed. There would alsoneed to be a market for cleared materials. As one
analysis observes: “the American scrap metalindustry is highly concerned about radioactivity intheir products as consumers may refuse to purchaseproducts they believe are tainted” [9]. Even if clearedmaterials are not avoided out of fear, they will not be
accepted if they are more expensive than alternative
products that already exist. Finally, the possibility
that regulations about radioactivity management
could become stricter over time introduces further
uncertainty into the clearance and recycling process[9].
In summary, fusion’s principal advantage overfission from the perspective of radioactive waste
management is the absence of the long-lived
radioactive isotopes that are inherent in the fission
process. Tritium is the most important radioactive
isotope intrinsic to deuterium-tritium fusion, and its
in-plant management and releases to air and water
are subject to demanding regulations. Tritium has
a half-life of only 12 years, so it is not subject tothe rules for millennial-scale storage of high-level
radioactive waste that currently hobbles fission
power. Moreover, there may be opportunities todevelop fusion reactor materials that reduce thewaste management challenges associated withactivated products, thereby compensating forthe greater energy of fusion neutrons relative tofission neutrons. (More energetic neutrons create
greater transformation of the nuclei in structural
materials into radioactive forms.) Nonetheless, themanagement of radioactive fusion wastes will not bea trivial matter by any means.

Reactor accidents
A “meltdown” occurs at a fission reactor when thereactor’s cooling system fails and the core’s highly
radioactive contents provide, after several hours,
sufficient heat to melt the core’s containment
vessel, whereupon the radioactive contents canmove to the floor of the reactor and beyond. The
Fukushima Daiichi accident in Japan, following a large
earthquake and tsunami, produced a meltdown atseveral of the reactors. Seconds after the earthquake,the systems designed to shut down the reactors
worked as intended and halted fission reactions.
However, the reactor’s backup cooling systems –needed to control the residual decay heat from thefission products in each reactor’s core – failed whena tsunami a few minutes later sent seawater over theprotective sea wall at the coast. Through containment
breaches and fires, these fission products weredispersed over hundreds of kilometers. As noted
above, more than 100,000 people were evacuatedand relocated to escape from exposure to radioactive
fission fragments. The social disruption andpsychological distress has been enormous.
A meltdown had occurred earlier, in 1979, at
the Three Mile Island nuclear power plant in
Pennsylvania, U.S. Although the reactor wasdestroyed, very little of the radioactivity released
within the facility left the reactor site.
A “criticality accident” can also disperse radioactivity
from a fission plant. A criticality accident occurred at
Ukraine’s Chernobyl plant in 1986. The chain reaction
at a nuclear reactor is controlled so that fission eventsoccur at a steady rate, but operators at Chernobyl
managed to put the reactor into a “runaway” condition
where this control was lost. A brief nuclear explosion
ensued that breached the reactor containment,
resulting in widespread contamination of food, soils,
and buildings by the radioactive fission products that
had been in the reactor core. Contamination forced
the abandonment of large amounts of land andinfrastructure and the displacement of even more
people than at Fukushima.
At fusion reactors, runaway chain reactions cannot
happen. When a fusion power plant experiences amalfunction, the conditions in the reaction chamberchange, and fusion inevitably stops. The energy
present in a fusion reactor is sufficient to melt
individual internal components, but not to breachthe containment vessel – where, in any case, far lessradioactivity is present than in a fission reactor’score. However, accidents that release radioactivity
are still possible. For example, a dramatic failure ofa superconducting magnet could cause the vacuum
vessel to rupture. This could release tritium andradioactive dust from the vacuum vessel walls,

25

potentially dispersing radioactivity beyond thewalls of the facility. Given appropriate plant design,
such accidents might not require evacuation of thesurrounding area. In one “worst-case” simulation,
the “most exposed individual” at a distance of
one kilometer from a fusion plant received a doseof radiation less than that experienced during a
mammogram [15,16,17]. Only in the event of an
unprecedented earthquake or similarly energetic
event could all safety measures be breached andresult in the mobilization of enough radioactive
material to require evacuation of the surrounding
population [18].

Terrorism and war
The fission power industry is a potential target of
attacks by terrorist organizations or military forces. In
addition to the loss of electricity production capacity
and the proliferation concerns discussed above, thefear is that an attack on a reactor or waste storagesite could lead to widespread radiation poisoning. The
most commonly feared possibilities are inducementof a meltdown or the deliberate dispersal ofradioactive material.
For bad actors to induce a meltdown intentionallyat a fission reactor, they would need to disable
numerous redundant safety systems. Spent fuelstorage buildings, by contrast, are today much lesswell-protected and therefore more vulnerable to thedeliberate disabling of their primary and backup
cooling systems [19,20]. Fortunately, the radioactivity
at spent fuel storage facilities produces heat far more
slowly than in reactor cores right after shutdown. As
a result, deliberately creating a leak to drain the poolof water in which the high-level waste is stored is lesscertain to induce a meltdown.
However, a highly energetic external event could
mobilize dangerous levels of radioactivity. Detailed
studies have not been performed, but it is possiblethat a bomb or airplane crash could qualify. The
amount of radioactivity that could be dispersed isvastly larger for fission plants than fusion plants.
In particular, used nuclear fuel from many years ofoperation of a nuclear fission power plant, full ofradioactive fission fragments, is often retained at a
fission reactor site, either relatively immobile in dry
casks or easily dispersed from pools of water.
Creating and detonating a “dirty bomb” –conventional explosives wrapped in radioactive
material – might be less difficult than inducing ameltdown. Attackers could steal or be provided withradioactive material from either fission plants orspent-fuel processing facilities [20]. Or they could

steal similar material from research reactors, medical
therapy machines, food irradiators, and other devices
where large amounts of radioactivity are found[20]. The U.S. National Research Council (2002)
considered the eventual use of a dirty bomb or otherradioactivity-dispersing device to be highly probable,with “materials and means… readily available” and“few preventative measures in place” [19].
The tritium and activated materials on the reactor
walls at a fusion power plant could be used in a dirty
bomb. One study found that the accidental release of
150 grams of tritium or 6 kilograms of tungsten dust
from one conceptual reactor would require evacuationof the facility and surrounding areas [21]. It follows
that such quantities incorporated into a dirty bomb
could cause significant contamination. However, it
seems unlikely that tritium would be used in a dirty
bomb, given that it currently costs about $30,000
per gram; a less expensive, more easily attainableradionuclide would probably be preferred.
The impacts on human health resulting from radiation
released by an induced reactor accident or dirty bomb
would probably develop slowly. Few people would die
from acute radiation poisoning, and the exposure of
the surrounding population to a radioactive plumemight result in only a small increase in cancer
occurrence [20]. However, in the event of such an
incident, this information might still create panic andlead to serious economic consequences [19,20].
The radioactive contamination from an induced
meltdown could force the abandonment of large
amounts of land and infrastructure, displacing
populations for years and creating a level of social
distress comparable to what happened at Fukushima.
Conceivably, by contrast, a dirty bomb might
result in less dislocation and distress, if the bombcontaminated only a few square kilometers and thedispersed radioactivity was promptly cleaned up[19,22].
Overall, the fusion power system presents far smallerrisks than the fission power system from the pointof view of becoming associated with the malevolent
dispersal of radioactivity. Neither the risk of an
attack on fusion power infrastructure nor the risk ofusing fusion waste in a dirty bomb would seem to besignificant, though the risk is present. By contrast,
despite the numerous safety measures in place, thecomparable risks from the fission power system are
far higher, both because of the possibility of meltdown
and because of the much greater quantities of highly
radioactive materials involved.
Table 5.1 summarizes this article through six
comparisons of fission and fusion energy systems.

		26

Fission

FusionPrimary reaction

Uranium fission

Deuterium-tritium fusion

Radioactive materials ofconcern: Short-lived

131

I, 137Cs, 90Sr

Tritium

Activation products,
transuranics

Activation productsRadioactive materialsof concern: Long-lived

Risk: Proliferation
Inherent in fuel cycle
Not inherent, but requires
		safeguards
Possible

Impossible

Focus of current concern

MinimalRisk: Meltdown
Risk: Terrorist attack

Table 5.1: Summary of the fission-fusion comparisons in this article.

References[1] Glaser, A. and R.J. Goldston (2012). Proliferation risks of magnetic fusion energy: clandestine production,
covert production and breakout. Nuclear Fusion 52(4): 043004.
[2] US National Research Council. Assessment of Inertial Confinement Fusion Targets. (The National Academies
Press, Washington, D.C, 2013).
[3] Goldston, R.J. and A. Glaser (2011). Inertial confinement fusion energy R&D and nuclear proliferation: The
need for direct and transparent review. Bulletin of the Atomic Scientists 67(3): 59-66.
[4] Disposal Of High-Level Radioactive Wastes In A Geologic Repository At Yucca Mountain, Nevada, 10 C.F.R. §63 (2001, as amended in 2009).
[5] Sood, S.K., Quelch, J. and R.B. Davidson (1990). Fusion technology experience at Ontario-Hydro Darlington
Tritium Removal Facility and heavy-water upgraders. Fusion Engineering and Design 12(3): 365-371.
[6] El-Guebaly, L., Huhn, T., Rowcliffe, A., Malang, S. and the ARIES-ACT Team (2013). Design challenges andactivation concerns for ARIES vacuum vessel. Fusion Science and Technology 64(3): 449-454.
[7] Dennis Whyte, private communication[8] Massaut, V., Bestwick, R., Bróden, K., Di Pace, L., Ooms, L. and R. Pampin (2007). State of the art of fusionmaterial recycling and remaining issues. Fusion Engineering and Design 82(15-24): 2844-2849.
[9] El-Guebaly, L., Massaut, V., Tobita, K. and L. Cadwallader (2008). Goals, challenges, and successes ofmanaging fusion activated materials. Fusion Engineering and Design 83(7): 928-935.
[10] US Nuclear Regulatory Commission. Radiological Assessments for Clearance of Materials from Nuclear
Facilities (U.S. Nuclear Regulatory Commission Main Report NUREG-1640, Washington, D.C, 2003).
[11] IAEA. Application of the Concepts of Exclusion, Exemption and Clearance, Safety Standards Series, No.
RS-G-1.7 (International Atomic Energy Agency, Vienna, 2004).
[12] Zucchetti, M., Di Pace, L., El-Guebaly, L., Kolbasov, B. N., Massaut, V., Pampin, R., & Wilson, P. (2009). The
Back-End of Fusion Materials Cycle: Recycling, Clearance and Disposal. Fusion Science and Technology, 56(2),781-88.
[13] Kooda, K.E., Galloway, K., McCray, C.W. and D.W. Aitken. INEEL lead recycling in a moratorium
environment. (WM ’03 Conference, Tucson, AZ, 2003). http://www.wmsym.org/archives/2003/pdfs/615.pdf[accessed March 2016].
[14] El-Guebaly, L., Massaut, V., Tobita, K. and L. Cadwallader. Evaluation of Recent Scenarios for Managing
Fusion Activated Materials: Recycling and Clearance, Avoiding Disposal (Fusion Technology Institute, Universityof Wisconsin, Madison, WI, 2008). http://fti.neep.wisc.edu/pdf/fdm1333.pdf [accessed March 2016].

27

[15] Cook, I., Marbach, G., Di Pace, L., Girard, C., Rocco, P. and N.P Taylor (2000). Results, conclusions, andimplications of the SEAFP-2 programme. Fusion Engineering and Design 51-52: 409-417.
[16] Brenner, D.J. and E.J. Hall (2007). Computed tomography - An increasing source of radiation exposure. New
England Journal of Medicine 357(22): 2277-2284.
[17] Gulden, W., Ciattaglia, S., Massaut, V. and P. Sardain (2007). Main safety issues at the transition from ITER
to fusion power plants. Nuclear Fusion 47(9): 1391-1398.
[18] Cook, I., Marbach, G., Di Pace, L., Girard, C., Rocco, P., and N.P Taylor (2000). Results, conclusions, andimplications of the SEAFP-2 programme. Fusion Engineering and Design 51-52: 409-17.
[19] US National Research Council (2002). Nuclear and radiological threats, in Making the Nation Safer: The
Role of Science and Technology in Countering Terrorism: 39-64. http://www.nap.edu/catalog/10415.html[accessed March 2016].
[20] Ferguson, C.D. and W.C. Potter (Eds.) The Four Faces of Nuclear Terrorism. (New York, NY: Routledge,
2005).
[21] Petti, D.A., Merrill, B.J., Moore, R.L., Longhurst, G.R., El-Guebaly, L., Mogahed, E., et al. (2006). ARIES-AT
safety design and analysis. Fusion Engineering and Design 80(1-4): 111-137.
[22] Stewart, S. (2010). Dirty bombs revisited: Combating the hype. Security Weekly - Stratfor Global
Intelligence. https://www.stratfor.com/weekly/20100421_dirty_bombs_revisited_combating_hype [accessed
February 2015].

28

Article 6: Politics and Progress
Commercializing fusion power will be an expensive
and long-term undertaking, requiring progress at
both national and international levels. Alternate waysof advancing the technology compete for limited
budgets, and there is debate about the relative meritsof national and international projects. This article
reviews the history of political interest in magnetic
confinement fusion and then discusses several key
existing and planned fusion experiments.

History of global
cooperation
Fusion research has received decades of publicfunding in the U.S., Europe, and Russia; recently
Japan, South Korea, and China have become
significant contributors. The British, U.S., and Soviet
governments conducted controlled fusion research
efforts in secret laboratories in the years following
World War II. Then, in the mid-1950’s, secrecy gave
way to openness. President Dwight D. Eisenhower’s
1953 “Atoms for Peace” speech signaled increased
political interest in peaceful uses of nuclear
technology. By 1958, magnetic confinement fusion
research was declassified in the U.S.
In 1971, the United Nations’ International Atomic
Energy Agency (IAEA) established the International
Fusion Research Council (IFRC) to coordinate its
response to proposals for experimental fusion
reactors so large and costly that they required
international collaboration. This forum was part ofa broad effort to strengthen East-West ties throughcollaborative scientific ventures. In 1978 the Soviet
Union proposed to the IAEA that an International
Tokamak Reactor should be built, larger than anyof the anticipated national reactors. At the time,
the cost of this international collaboration was
expected to exceed $1 billion [1,2]. The vision for
an international research reactor has evolved overthe decades into the International Thermonuclear
Experimental Reactor, now known simply as ITER [3].
The collaboration currently has seven members: theEuropean Union, China, India, Japan, South Korea,
Russia, and the United States.

ITER, Latin: “the way”
ITER is the largest fusion research project in the world
and is under construction in Cadarache, France,near the Rhone River -- with high-tech componentsthat are being fabricated around the world. In Figure
6.1 the size and expected performance of ITER
are compared with the achievements at the Joint

European Torus (JET), currently the world’s largest
tokamak. The ITER facility seeks to produce 500
megawatts of fusion power with a D-T plasma with50 megawatts of input power, a ten-fold return onenergy input. The substantial fusion power of ITER is
expected to be sustained for at least 400 seconds
and to be repeated about once per hour. Achieving
these performance parameters would allow thetesting of a number of key issues that bridge the gap
between small national experiments and projects
addressing fusion at near-commercial scales. ITER
is also expected to test plasma control, continuous
fueling, tritium breeding, and other engineering issues
associated with a commercial-scale reactor.

ITER
JET

Plasma volume (cubic meter): 80
Fusion power (megawatt): 16
Pulse duration (second): 2

Plasma volume (cubic meter): 830
Fusion power (megawatt): 500
Pulse duration (second): 400

Figure 6.1: Expectations for ITER are compared withthe achievements of the Joint European Torus (JET, at
Culham, U.K.) [4].

Delays have caused official milestone dates to slip
repeatedly. ITER’s design was approved in 2001,
and a cooperative agreement was signed in 2006.
Construction of ITER began in 2008. At the timeof site selection in 2005, ITER was expected to bein operation by 2016 but this was moved back to2018 and then 2019. At present ITER is expected
to be operational in 2026. The estimated total costof ITER initially ranged from $5 to $10 billion, thensurpassed $15 billion and now exceeds $20 billion.
Such estimates are difficult; in addition to engineering
uncertainties, ITER members make most of theircontributions through “in-kind” equipment.

Sustaining politicalinvestment in ITER
Rising estimates of ITER costs have caused members
to reassess their commitment to the collaboration
repeatedly [5,6]. Despite high-level interest in the29

long-term economic, environmental, and technologicalprospect of commercial fusion power, domestic
decision-making is commonly shorter-term. Scientificprestige and economic stimulus have been important
for the domestic political viability of ITER funding [7],which in several countries consumes a substantial
portion of the fusion budget. Sustained domestic
political justification for ITER is buoyed by contracts
for equipment awarded to domestic industry as wellas complementary domestic fusion experiments.
The protracted conflict over which country would
host ITER suggests that many countries expectedthat having ITER on their soil would nurture theirdomestic research programs in fusion and beyond.
First came the search in the early 1990s for a single
headquarters to coordinate engineering design for
ITER; the result was three sites – in Germany, Japan,
and the U.S.
Siting the reactor itself was even more contentious.
In early negotiations, both France and Germany
indicated they would not bid to host the ITER site,
because the anticipated costs were too high. Spain
meanwhile offered to double its contribution to ITER
if its site was chosen. Japan’s political commitment
to ITER was volatile. In the late 1990s Japan, facing
domestic cuts to public science expenditures,
encouraged ITER to delay its schedule by several
years so as to avoid being seen as a marginalized
ITER partner [8]. Once its domestic budget shortfall
was resolved, Japan offered to bear the entire costsof the reactor core if ITER was sited in Japan [9].
Eventually, France changed its position and cameforward with a bid for building the project. Canada
proposed a site as well – at Clarington, a suburb of
Toronto. The U.S. and Russia considered making bids
but refrained because of high costs.

and geopolitical [13]. After a year of negotiations, theEuropean Union threatened to build the Cadarache
reactor even without the support of the other ITER
members. This threat forced supporters of theJapanese bid to consider the risk of the international
enterprise splintering into multiple projects or fallingapart entirely [14].
Japan relinquished its bid in 2005 in exchange forsharing costs with the Europeans on a package offacilities located in Japan that would support both
ITER and an envisioned successor international
facility, called DEMO. The bargain included asupercomputing design center and a materials
testing facility. Japan would also make an outsized
contribution to ITER of in-kind equipment and project
scientists, and ITER’s first director general would be
expected to come from Japan [15]. Figure 6.2 shows
the ITER construction site in Cadarache, France as ofFebruary 2016.

Figure 6.2: After years of delay and continued

Figure6.2: After yearsof delaycontinued uncertainty
in cost and timeline,
uncertainty
in costandandtimeline,
ITER construction
has
ITER construction has begun. The circular structure is the beginning of the
begun.
circularstructureis thebeginningof thebuilding
thatThewill housethe tokamak.
The photois from
February 2016[16].

building that will house the tokamak. The photo is fromFebruary 2016 [16].

Balancing domestic and international priorities

The U.S. left the ITER collaboration in 1998 for
Exceptions for Japan aside, the burden-sharing agreement for ITER’s “in-kind” subthree years as a result of budget concerns [10]. Fivesystem contributions is quite straightforward: the European Union bears five elevenths(45.45 percent) of the overall project cost including all on-site buildings, and the other six
years later in January 2003, the U.S. re-enteredpartners contribute one eleventh (9.09 percent) each [17]. Before India’s inclusion in
ITER, reflecting renewed interest in Congress and ITER, the EU had taken on 50 percent and other five members 10 percent each.
diplomatic pressure from both Japan and Canada The objective of ensuring that investments in ITER serve domestic industrial andcommercial interests has driven every stage of the ITER negotiations. To source[11]. The U.S. rejoined just as China and South Korea
Exceptions
for Japan
aside,
the burden-sharing
equipment
for ITER, the members
crafteda cost-sharing
agreement that divided theover one hundred
packages”
of in-kind equipment,
became members and launched their own domesticproject into
agreement
for “procurement
ITER’s “in-kind”
sub-systemsummarized in Figure 6.3. Through these discrete contracts, countries individuallyfusion programs. Canada withdrew from ITER entirely
is andquite
straightforward:
thetoEuropean
maximizecontributions
the present scientificeconomic
benefits of the project
justify their shortterm costs
and haveless needpoint to the long-term– anduncertainof– payoff
in December 2003 after it became clear that it would
Union
bears
fivetoelevenths(45.45percent)
the [18].
help of Figures
2.1 and3.2, including
we can elaborate
the distributionof effortand
shown
lose its bid to host the project. The project picked upWith the overall
projectcost
all onon-sitebuildings,
in Figure 6.3. The EU is contributing a substantial share to all system categories, but
momentum nonetheless, with India joining ITER in particularly
thesixpartners
contribute
onetheelevenththeotherbuildings
andmachine core.
It is also splitting
production (9.09of thetoroidal fieldcoils witheach
Japan. [17].
Japan additionallyis supplying
the conductors
for the2005.
percent)
Before India’s
inclusionin ITER,
central solenoid, and other elements of the machine core and control equipment. China
theEUinvolved
had taken
on 50
percent
and themain
otherfivehandlinghas mainly
been
in the machinecore(magnet conductor,
vessel,France was the favored location for the project
and transfer systems) and external auxiliaries (electrical circuitry); India is the largest
members
10
percent
each.
within Europe (over sites in Germany, Spain, andBalancing domestic andinternational priorities

Sweden) [12]. The European Commission then  had to persuade the Japanese to abandon theirbid to site ITER at Rokkasho. The high-level politicalstalemate between Japan (supported by the U.S. andSouth Korea) and France (supported by Russia, theEuropean Union, and China) was technical, financial,

The objective of ensuring that investments in ITER
serve domestic industrial and commercial interests
has driven every stage of the ITER negotiations. To
source equipment for ITER, the members crafted acost-sharing agreement that divided the project into

46  

30

Sauth
Del

over one hundred “procurement packages” of in-kindequipment, summarized in Figure 6.3. Through these
discrete contracts, countries individually maximize thepresent scientific and economic benefits of the projectto justify their short-term costs and have less need topoint to the long-term – and uncertain – payoff [18].
With the help of Figures 2.1 and 3.1, we can elaborateon the distribution of effort shown in Figure 6.3.
The EU is contributing a substantial share to all
system categories, but particularly the buildings andmachine core. It is also splitting the production of
the toroidal field coils with Japan. Japan additionallyis supplying the conductors for the central solenoid,
and other elements of the machine core and control
equipment. China has mainly been involved in themachine core (conductor cables for the magnets,
main vessel, handling, and transfer systems) andexternal auxiliaries (electrical circuitry); India is thelargest contributor to internal auxiliary systems andparticularly the heating and cooling systems; South
Korea contributes heavily to the vacuum vessel,heat shield and conductor cables; Russia provides
poloidal coils, electronics, and parts of the chamber;and the U.S. contributes to exhaust, fueling andcooling equipment, central solenoid, and plasma
heating and disruption mitigation technology –among other systems. The ITER Organization itself– using its operating budget from “in-cash” member
contributions – also directly procures a significant
Millions of 2010 dollars
1000
800

portion of the auxiliary, cooling, and control systems.
ITER has had to contend with intellectual property
rights, particularly each member’s interest in
keeping for itself any valuable information gained
through participating in ITER. To minimize exclusive
rights, ITER members in 2006 endorsed the “widest
possible dissemination” of ITER intellectual property
for most – but not all – technologies. They agreed
that the sharing of background intellectual property
and eventual experimental results should be on
an “equal and non-discriminatory” basis among
members [21]. The members have limited short-termcommercial incentives because of fusion’s multidecadal development path [22], and accordingly
they generally see the synergistic value of ITER
cooperation outweighing the benefits from exclusive
rights to ITER-related inventions.
It is still possible that ITER will not be finished.
Impatience with the continued delays in ITER’s
startup could imperil the participation of somecountries in its construction or subsequent operationand lead them to return to national programs basedon domestic experiments. The U.S. has threatened
to exit ITER many times – most recently, during theJune 2014 Senate budget discussions. However,according to the ITER agreement – which the U.S.
has signed -- all of the in-kind contributions pledgedby the U.S. would still be due in 2017. Even though
the ITER organization has no meansof enforcing member commitments,
incentives to drop out become
Machine core steadily weaker as construction
Internal Auxiliaries proceeds.

External Auxiliaries
Heating, Diagnostics, Control
Buildings

600
400
200
0
European
Union

China

India

Japan

Korea

RussiaUnitedStatesITER
organization

Figure 6.3: Value of the components and sub-systems contributed to ITER
by its seven members and by the ITER Organization [19]. Ninety percent
of each member's contribution is “in-kind” equipment (rather than direct
payments) as part of over one hundred discrete procurement packages.
The other ten percent of member contributions are made “in-cash” tofund the operational budget of the ITER Organization, which also directly
procures some packages. Data are from early 2015. (The original data are
in ITER Units of Account (IUAs), pegged to 2010 euros at 1552.24 euros= 1 IUA, which we have converted to 2010 dollars at US$1 = 1.33 euros)[16,20].

Key domestic
research programs
Alongside ITER are many singlenation research programs, ongoingand planned, whose objective iseither to build up to ITER or to lookbeyond it. We briefly discuss several
important experiments (threetokamaks and one stellarator),
acknowledging that there are manyothers that make their own uniquecontributions to global fusion
research.
Joint European Torus (JET): The Joint
European Torus (JET) is to date theworld’s largest and most powerfuloperational magnetic confinement
fusion device with the tokamak
configuration. It is a European
collaboration and was the first majorinternational fusion project. JET is31

located at the Culham Centre for Fusion Energy in
Oxfordshire, U.K. and has been operating since 1983[23]. Unlike ITER, JET does not use superconducting
magnets and is not suited for long-duration energy
generation. Nonetheless, JET has set two world
records: in 1991 it became the first device to produce
one megawatt of power for two seconds using a D-T
plasma with peak power of two megawatts [24]. In
1997, JET exceeded that record by producing 16.1
megawatts of peak fusion power, attaining 70 percent
of breakeven [25] and sustaining 10 megawatts formore than half a second [26]. For several years JET
has been serving as a test-bed for ITER technologies.
It now has a magnetic arrangement similar to ITER’sand tungsten and beryllium inner-vessel structuresthat will facilitate learning how to mitigate plasma
instabilities.
JT-60SA: Japan is constructing JT-60SA as an upgrade
to its flagship fusion research site operating since1985. The new tokamak – roughly the size of JET –will use superconducting coils and is another device
that will enable ITER-relevant research ahead of
ITER’s first experiments.
EAST: In the past decade China has developed
internationally important superconducting
fusion devices. China’s Experimental Advanced
Superconducting Tokamak (EAST) at the Institute of
Plasma Physics in Hefei was an early superconducting
experimental device [27]; it achieved pulses lasting
up to 1,000 seconds – though not with a D-T plasma[28]. Because its magnetic configurations and heating
schemes are similar to those at ITER, it is expected
to provide another experimental test bench for ITER,
especially for studying plasma stability.
Wendelstein 7-X (W7-X): The largest stellarator in
the world has just begun to operate in Greifswald,
Germany. Known as W7-X, it is tightly optimized.
Its superconducting magnets will enable testing ofcontinuous operability for 30 minutes, although notwith output power exceeding input power. W7-X wasyears over schedule and budget due to the daunting
technical precision necessary. A non-superconducting
stellarator predecessor to W7-X, called the National
Compact Stellarator Experiment (NCSX) and locatedat Princeton University, was cancelled in 2008 – a
U.S. budget decision driven by cost overruns andunderestimated technical difficulties [29].

become the stellarator’s strongest entry in thecompetition. The pivot toward the tokamak as a resultof its relative simplicity reveals the path dependency
in fusion research because of costs and long leadtimes in project construction. Tokamaks have a headstart toward commercialization and may stay in thelead because of the experience that is likely to begained from troubleshooting and then running ITER.
Conceivably, if ITER struggles and W7-X excels, thepost-ITER planning could involve a larger stellarator[30].

Post-ITER demonstration
reactors
Although it is possible that ITER’s technical goals will
not be realized because the burning plasma reveals
intrinsic complications, leaders of the fusion researchcommunity are planning facilities for the period
beyond 2030 that would create a bridge between
ITER and an eventual commercial reactor. The bridge,
called DEMO, would take the form of a demonstration
experiment – or series of experiments. According
to these inchoate but ambitious plans for 2030
and beyond, DEMO would feature near-continuous
operation, tritium breeding, 30-50 fold return onenergy input, and capabilities to convert fusion heatto electric power. There is no consensus on whether
DEMO would be an international collaboration like
ITER [31].
Simultaneously with the planning in the frameworkof DEMO, several nations are already making plans
for single-nation post-ITER experiments. In part,
they are responding to ITER’s long lead time and theinternational contestation over its costs and siting.
China is planning the Fusion Engineering Test Reactor(CFETR) as a stepping-stone between ITER and
DEMO [32], nearly at ITER’s scale [33] and intends
to integrate capabilities for generating power [34].
South Korea is planning to develop the K-DEMO, apost-ITER reactor aspiring to operate at commercial
scale [35]. The potential redundancy of these large
proposed experiments – with each other and with
DEMO – could create new political tension andunsustainable budgets. If at some point commercial
viability emerges, collaboration among nations may
fade in favor of competition for market share, but for
now international collaboration is the norm.

Tokamaks and stellarators are being pursued inparallel. Each has its advocates, and W7-X has

32

References[1] Walgate, R. (1980). Thermonuclear fusion: Soviets bid half. Nature 286(5770): 198-99.
[2] Arnoux, R. (2008). INTOR: The international fusion reactor that never was. https://www.iter.org/
newsline/62/146 [accessed November 2015].
[3] Shapley, D. (1982). Keyworth on fusion: US seeks more foreign links. Nature 299(5881): 290.
[4] Campbell, D.J. (2014). ITER goals and project. ITER Organization. Slide 14, http://www.dpgphysik.de/dpg/
pbh/aktuelles/pdf/Campbell1.pdf [accessed March 2016].
[5] Butler, D. (1996). French report seeks to slow fusion project. Nature 381(6581): 356.
[6] Cho, A. (2014). U.S. Support for ITER Wavers as Costs Spiral. Science 344(6181): 243-44.
[7] Aldhous, P. (1991). ITER work is shared out. Nature 352(6332): 179.
[8] Nathan, R. and R. Triendl (1997). Spending cuts threaten ambitions to host ITER. Nature 387(6634): 643.
[9] Abbott, A. (1996). Funding forces fusion project out of Europe. Nature 382(6589): 288.
[10] Malakoff, D., Service, R.F. and E. Finkel (1998). Turn out the lights, the ITER's over. Science 282(5387):
209.
[11] Brumfiel, G. (2002). United States pledges to rethink rejection of fusion experiment. Nature 415(6869):
247-48.
[12] Abbott, A. (1994). Germany names candidate site for ITER fusion-reactor. Nature 369(6482): 595.
[13] Nature editorial (2004). Time for Japan to shine? Nature 427: 763.
[14] Giles, J. (2004). Stalemate over fusion project threatens to provoke split. Nature 432: 262.
[15] Butler, D. (2005). Japan consoled with contracts as France snares fusion project. Nature 435: 1142-43.
[16] Courtesy of the ITER Organization[17] ITER newsline (2015). Procurement sharing. http://www.iter.org/procurementsharing [accessed October
2015].
[18] Anderson, C. and P. Aldhous (1990). Fusion's turn again? Nature 347(6289): 114.
[19] Dulon, K. (2011). ITER Newsline 162: Money Talks. https://www.iter.org/newsline/162/576 [accessed
October 2015].
[20] ITER newsline (2015). Sharing the ITER Pie. http://www.iter.org/newsline/96/1318. [accessed October
2015].
[21] Official Journal of the European Union (2006). Agreement on the Establishment of the ITER International
Fusion Energy Organization for the Joint Implementation of the ITER Project. L358/62. http://fusionforenergy.
europa.eu/downloads/aboutf4e/l_35820061216en00620081.pdf [accessed October 2015].
[22] Dean, S.O. (1998). Lessons drawn from ITER and other fusion international collaborations. Journal of
Fusion Energy 17(2): 155-75.
[23] Nature opinion (1983). JET set fair: Europe's nuclear fusion is off the ground but has far to go. Nature
303(5920): 740.
[24] Pease, R. (1991). First use of tritium. Nature 354(6349): 95.
[25] Clery, D. (2013b). JET fusion reactor passes 30 and plunges into midlife crisis. Science 341(6142): 121.
[26] Dickson, D. (1997). Fusion results ‘confirm the scientific case for ITER’. Nature 390(6655): 5.
[27] Li, J., Guo H.Y., Wan B.N., Gong X.Z., Liang Y.F., Xu G.S., Gan K.F., et al. (2013). A long-pulse highconfinement plasma regime in the experimental advanced superconducting tokamak. Nature Physics 9(12):817-21.
[28] Normile, D. (2006). Waiting for ITER, fusion jocks look EAST. Science 312(5776): 992-93.
[29] Cho, A. (2008). Energy Department pulls plug on overbudget fusion experiment. Science 320(5880):
1142-43.

33

[30] Clery, D. (2015). Twisted logic. Science 350(6259): 369-71.
[31] ITER Organization (2015). “After ITER”. https://www.iter.org/sci/iterandbeyond.
[32] Clery, D. (2013). After ITER, many other obstacles for fusion power. Science Insider. http://news.
sciencemag.org/physics/2013/01/after-iter-many-other-obstacles-fusion-power [accessed October 2015].
[33] Wu, S.T., Li, J.G, Wan, B.N., Wan, Y.X., Fu, P., Ye, M.Y. et al. (2014) Concept design of CFETR tokamak
machine. Plasma Science, IEEE Transactions, 42(3): 503-09.
[34] Li, H, Ge L, Lu Q, and P. Fu (2014). Progress in CFETR power stations concept study. Journal of Fusion
Energy 33(6): 613-18.
[35] Park, S.B. (2013). South Korea makes billion-dollar bet on fusion power. Nature News 21.

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