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Why fusion is essential


Why fusion is essential and why it is so difficult Francis F. Chen, University of California, Los Angeles K-Star, Daejeon, S. Korea, April 29, 2011 – PowerPoint PPT presentation

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Title: Why fusion is essential

Why fusion is essential and why it is so
Francis F. Chen, University of California, Los
K-Star, Daejeon, S. Korea, April 29, 2011
Renewable and clean energy sources
  • Hydro and geothermal
  • Wind
  • Solar
  • Nuclear (fission)
  • Fusion

Backbone power
  • Central-station power has to be dependable and
    always available. This is backbone power.
  • Intermittent or variable power cannot be stored
    and therefore can only be auxiliary power.

Backbone power Fossil fuels (coal, oil, gas),
fission, and fusion Auxiliary power Solar, Wind,
and Water (including hydroelectric)
Hydroelectricity and geothermal
  • These are very useful where they occur
  • But they occur in only a few places

Use of wave and tide energy has not been tested
and they would be small auxiliary sources anyway
Wind power
This is the most promising of the auxiliary
Modern, large turbines are efficient
But wind is variable
The power is proportional to the CUBE of the wind
Wind energy has to be stored
Winds energy payback time is short
  • The energy used for windpower
  • Mining and making the materials
  • Manufacture of the turbines and transmission
  • Operation and maintenance
  • Inspection and replacements

The energy used is recovered by energy produced
in 6.8 months
Bottom line on wind
  • Wind is renewable and non-polluting
  • Wind can recover its own energy cost in 2/3
  • Wind can supply only a small percentage of
    electrical power because switching from one
    source to another requires accurate prediction
    and can destabilize the grid.
  • Wind turbines are also unsightly in some

Solar energy
  • There are four kinds of solar energy
  • Rooftop solar
  • solar thermal
  • solar photovoltaic
  • Central-station solar
  • solar thermal
  • solar photovoltaic

Rooftop thermal
This is very simple and is already in use in many
places. It does not require much space and
should be used everywhere. Storage is automatic.
Rooftop photovoltaic
CdTe has lowered the cost of solar cells to
almost 1 per watt. Including mounting and
installation, it costs about 5 per watt. A U.S.
home uses 1.2 kW average and needs 5 kW for peak
hours. Thus, the cost is 25,000 minus rebates
and lasts 25 years. The system needs a converter
to AC and Pb-acid batteries for storage. This is
auxiliary power only, and the cost is not yet
Dont fall off the roof!
Central station solar thermal (1)
There are two main types solar tower and
parabolic mirrors
Mirrors focused on boiler at the top produces
steam, which turns turbines to generate
electricity. The mirrors do not move.
Central station solar thermal (2)
These mirrors do not move
These mirrors follow the sun
The mirrors focus onto a long pipe, in which a
liquid carries the heat to the ends. The heat can
be stored in a building filled with molten salt,
for instance. However, if the salt ever gets
cold, it solidifies and cannot be melted again
(no power).
Central station solar photovoltaic
  • It takes 100 km2 of area to replace one coal
    plant. This large plant in Spain is 1 of that.
  • The cost has to come down to below that of
    window glass.
  • There is no storage, so long transmission lines
    need to be built to connect to where the sun is

Central station solar takes a large area
A large coal plant generates 1000 MW of
electricity. A solar plant of this capacity
would require 2/3 the area of Manhattan Island in
New York
Energy payback time for solar
  • For either solar thermal or solar photovoltaic
  • The energy payback time is 12.5 months, about
    twice that for Wind.
  • Use of concentrators (expensive Fresnel lenses)
    can bring this
  • down to 6.7 months.
  • There is no proven way to store day-to-night
  • Underground storage loses heat and is
  • Such caverns do no exist under deserts.
  • Hydro pumping is possible only in a few places.

Bottom line on solar energy
  • Rooftop thermal should be universally adopted.
  • Rooftop solar cells are economical only in
    sunny places and must use batteries at night.
  • Central station solar thermal has storage, but
    requires large areas.
  • Central station photovoltaic needs large area
    and cannot be stored.

Both Wind and Solar can only be supplementary
sources. They must be backed up by central power
from Fossil, Fission, or Fusion.
Nuclear (fission) power
  • Uranium has to be mined, but it will last 100s
    of years.
  • But it has its three well known problems
  • Accidents that release radioactivity
  • Storage of long-lived radioactive waste
  • Danger of nuclear proliferation

Civilian nuclear power is very safe
  • Chernobyl This cannot happen in a well
    regulated society
  • Three Mile Island Total deaths 0
  • Previous deaths in history 5
  • Fukushima 2, so far

Many countries use nuclear for gt35 of
electricity. France 75 There have been no
Everything we do has risks
  • Cars 40,000 deaths per year in the U.S.
  • Coal mining 3000 per year in China
  • Oil drilling 11 deaths from Deepwater Horizon

Actuarial estimates of deaths per 100,000 people
per year (U.S.)
  • Motor vehicles 16
  • Falls from ladders, roofs 5.15
  • Airplanes 0.41
  • Accidents like TMI 0.00007

We can never be protected from natural disasters
like volcano eruptions or earthquakes!
Present-day reactors (Generation II)
These are light-water reactors using ordinary
water to moderate (slow down) the neutrons, which
come out too fast to cause the next fission.
Fuel rods are made of thousands of fuel pellets
containing uranium. Control rods (boron
absorbers) are lowered to slow down the reaction
rate if the fuel gets too hot. This requires
active control of the control rods.
Future fission reactors
Generation III reactors also use light water as
moderator, but they are designed to be safe
against control rod failure. Generation IV
reactors will have novel, safe designs. For
instance, the pebble-bed reactor
The fuel and graphite moderator are made into
balls. Each can take the maximum temperature of
an accident and will not melt. The fuel can be
refreshed without shutting down the reactor.
Waste storage
Spent fuel still generates heat and has to be
cooled in Swimming pools. It is then
transferred to secure canisters and stored above
ground. No underground storage has yet been
built, though it has been considered in the U.S.,
Finland, and Sweden.
This temporary storage is OK if fission is not
forever. Eventually it will be replaced by fusion
  • Uranium 238 has to be enriched with U235 for a
    chain reaction.
  • This usually requires a huge diffusion plant,
    but the new centrifuge method is smaller, and it
    can easily be configured to enrich uranium
    further to make bombs.
  • Plutonium can be used directly for bombs.
    Reactors that make plutonium, like liquid-metal
    breeder reactors, can be raided.
  • Plutonium also occurs in uranium waste and in
    reprocessing of uranium fuel (to make storage

Bottom line on nuclear
  • Nuclear fission supplies an important fraction
    of our electricity.
  • The safety and proliferation problems can
    probably be solved, but the waste storage problem
    will remain.
  • We need to support nuclear power because oil
    and gas will run out in 40 years, and then we
    will have only coal for backbone power.
  • Even if the emissions from coal can be
    captured, we still dont have a proven method to
    store them.
  • We need fission for backbone energy until
    fusion is developed.

Why is fusion so difficult?
It comes down to the fact that the
deuterium-tritium (DT) reaction is so much easier
to use than all the others. This reaction has two
problems the tritium and the neutron.
  • The tritium has to be bred
  • The neutron causes damage

By default, we discuss the tokamak
  • Fusion requires a thermal plasma at
    gt100,000,000 degrees
  • This can be confined only by a magnetic field
  • The magnetic field has to be shaped like a
    torus, not a sphere
  • The toroidal system that has been studied the
    most is the tokamak

Confinement physics is understood well enough
Rayleigh-Taylor instabilities
Kink instabilities
Confinement physics is understood well enough
Drift-wave instabilities
Banana orbits
These are stabilized by
Good curvature
Computers can handle 3 dimensions
New tokamaks have advanced features that work
Reversed shear can be produced by controlling the
plasma current profile with auxiliary heating.
This results in an internal transport barrier
which can be added to the H-mode barrier at the
Only a few problems remain in the physics
ELMs (edge-localized modes)
The H-mode mechanism
The problems are in engineering
  • The first wall material
  • Blanket design
  • Tritium breeding
  • Divertors
  • Auxiliary heating and current drive

The main parts of a tokamak reactor
The first wall material
The first wall is bombarded by neutrons, plasma,
and radiation. It must be low-Z,
high-temperature, and neutron-resistant
The best material we know is fiber-reinforced
silicon carbide (SiC/SiC). But this has never
been manufactured in large quantities.

The tritium-breeding blankets
Tritium is bred from lithium, an abundant
element. The blankets contain lithium.
The number of neutrons from fusion is not enough.
Beryllium can be used as a neutron multiplier.
This also makes helium, which is in short supply.
The coolant can be helium or liquid Pb-Li
  • Blankets absorb the fusion energy coming out in
    neutrons and transfer the heat to a coolant.
  • They must have a first wall that faces the
  • They must contain a neutron multiplier.
  • They must contain lithium to breed tritium
  • They must capture the tritium without losing

A holder for blanket modules
Blanket designs depend on the coolant
Here are some examples
Helium Cooled Ceramic Breeder (HCCB)
Dual-Cooled Lithium Lead (DCLL)
Helium-Cooled Lithium-Lead (HCLL)
Behind each blanket section is a large room
ITER has only 3 ports for test blankets
We need another tokamak to test blanket designs.
Test blanket port
Breeding tritium is a very slow process
It takes more than 5 years to double the tritium
Feasable ratios
A divertor receives the plasma heat
High-Z materials can be used in divertors

A water-cooled divertor design (not good enough
for reactors)

Conceptual helium-cooled divertor
Heating and current drive
  • Types of heating power
  • NBI (neutral beam injection). Bigger, but not
    an obstacle
  • ICRH (ion cyclotron resonance) 50 MHz, 20 MW
    in ITER
  • Straightforward, but needs an internal
  • ECRH (elec. cycl. Resonance) 20 MW _at_ 170 GHz
    in ITER
  • Crucial for current drive, and needs
  • LHH (lower hybrid heating) 5 GHZ, needs
    grill antenna

Size of a 1-MW gyrotron
A 2-MW, 170-GHz gyrotron design, with
superconducting magnets
Gyrotrons have a lot of lost heat, and they need
vacuum windows of synthetic diamond.
Lower-hybrid antennas
A ¼ size model of a grill antenna for
lower-hybrid heating in ITER
Unsolved problem ELMs
Design of ELM suppression coils for ITER
These coils cover the surface of ITER, but they
will be incompatible with blankets.
A real reactor has many parts that need
engineering development
A Fusion Development Facility
Engineering can be done simultaneously with
ITER. Small, normal-conducting tokamaks can be
Cost of large projects and wars
  • The estimate for fusion includes
  • 21B for ITER
  • 12.6B for three FDFs at 45
  • the size of ITER
  • 42B for a DEMO reactor

  • Developing fusion is slow, expensive, and
  • We have made great progress in the last 50
  • For central-station power, fusion is the best
  • for Year 2050
  • For central-station power, fusion is the ONLY
  • for Year 2100

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