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

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 sources

Modern, large turbines are efficient

But wind is variable

The power is proportional to the CUBE of the wind speed!

Wind energy has to be stored

Wind’s energy payback time is short

The energy used for windpower: • Mining and making the materials • Manufacture of the turbines and transmission lines • 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 year • 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 locations.

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 competitive.

Don’t 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 km 2 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 shining.

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 energy.

• Underground storage loses heat and is inefficient.

• 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

F

ossil,

F

ission, or

F

usion.

Nuclear (fission) power

Uranium has to be mined, but it will last 100’s 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 >35% of electricity.

France: 75% There have been no accidents

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 alone Actuarial estimates of deaths per 100,000 people per year (U.S.) • Motor vehicles: 16 • Falls from ladders, roofs 5.15

• Airplanes: • Accidents like TMI: 0.41

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.

take the Each can maximum temperature of an accident and will not melt.

The reactor.

fuel refreshed shutting can be without down the

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.

storage No underground

has yet been built, though it has been considered in the Sweden.

U.S., Finland, and This temporary storage is OK if fission is not forever. Eventually it will be replaced by fusion

Proliferation

• 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 unnecessary).

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 don’t have a proven method to store them.

• We need fission for backbone energy until fusion is developed.

THIS IS WHY FUSION IS ESSENTIAL!

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 10 8 6 4 2 0 0

D-T D-He 3 D-D

100 200 300 Ion temperature (keV)

p-B 11 p-Li 6

400

He 3 -He 3

500

By default, we discuss the tokamak

• Fusion requires a thermal plasma at >100,000,000 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

Shear

These are stabilized by

Good curvature

Computers can handle 3 dimensions

New tokamaks have advanced features that work

30 q q 20 10 0 0 current hole 0.2

0.4

r / a 0.6

J 0.8

1 Reversed shear can be produced by controlling the plasma current profile heating.

with auxiliary This results in an internal transport barrier which can be added to the H-mode barrier at the edge.

Only a few problems remain in the physics

The H-mode mechanism DISRUPTIONS ELMs (edge-localized modes)

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 plasma.

• They must contain a neutron multiplier.

• They must contain lithium to breed tritium • They must capture the tritium without losing any.

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 inventory.

PERCENT TRITIUM BURNUP

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 antenna • ECRH (elec. cycl. Resonance): 20 MW @ 170 GHz in ITER Crucial for current drive, and needs development • LHH (lower hybrid heating): 5 GHZ, needs “grill” antenna

Gyrotrons

Size of a 1-MW gyrotron Gyrotrons have a lot of lost heat, and they need vacuum windows of synthetic diamond.

A 2-MW, 170-GHz gyrotron design, with superconducting magnets

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 used.

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

Conclusion

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

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