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Lecture #24

Fusion

ENGR 303I

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Outline

• Fusion→ Definition→ Atoms usually used

• Previous attempts at fusion

• Current attempts at fusion→ International Thermonuclear Experimental

Reactor (ITER)

• Summary→ Advantages→ Disadvantages

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Fusion - Definition• 2 lighter weight atoms bond together to form

1 heavier atom• As the 2 lighter atoms become close

together they repel each other with natural electrostatic forces

→ At very high temperatures these forces will be overcome and fusion will occur (100,000,000°C)

• The mass of the resulting atom is slightly less than the sum of the original 2 atoms

→ The difference in mass turned into energy→ E = mc2

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Fusion - Definition

Most commonly used atoms for fusion are isotopes of hydrogen:

→ Hydrogen→ Deuterium → Tritium

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Fusion - ReactionThe most common fusion reaction:

Deuterium + Tritium => Helium + neutron + energy

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http://www.iter.org/default.aspx

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Fusion Reaction Requirements• Need to heat small amounts of deuterium

and tritium atoms to 100,000,000°C• Need to contain and push these atoms

together long enough so they fuse• Need to recover the heat• Use the heat to make steam and then

electricity→ Part of that electricity could be used to make

more deuterium and tritium atoms from water→ If it works it is considered renewable because

of large amounts of deuterium and tritium in water

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Containment

Containment must be non-material

→ Any material will melt at the required temperatures

→ Inertial containment: High powered lasers keep atoms inside vessel

→ Magnetic containment: Magnetic fields hold the atoms inside vessel

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Inertial Containment• Laser beams

hold plasma in center (away from cooler walls)

• This method not used as much as next

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Magnetic Containment• Magnetic fields

keep plasma in center (away from walls)

• Total shape is a doughnut

• Difficult to keep center hot but magnets cold

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Magnetic Containment – Tokamak• Toroidal (doughnut-shaped) magnetic field for containing

plasma• Tokamak is an acronym from Russian words that mean: toroidal chamber in magnetic coils

• Made by Russian scientists who achieved much higher temperatures than US or British working with different shapes

Doughnut shape allows smooth, continuous magnetic field to be made from electric field

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Magnetic Containment – Tokamak

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Magnetic Containment – TokamakEarly 1980’s, Princeton University built Tokomak Fusion Test Reactor (TFTR)

→ Goal was to produce more power than put into magnetic fields, pumps, equipment - Never achieved the goal

http://www.pppl.gov/projects/pages/tftr.html

Also early 1980’s Joint European Torus (JET) was built in England

→ Q = power produced by fusion / input power→ Q = 1 is self–sustaining but no net production→ In 1997 produced Q = 0.7, best so farhttp://www.jet.efda.org/index.html

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TFTR at Princeton

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Picture of Joint European TorusInside reactor before and during operation

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Japan Torus JT-60

• Did experiments with deuterium-deuterium fuel

• If used deuterium-tritium fuel they would have produced more power than power input

http://www-jt60.naka.jaea.go.jp/english/index-e.html

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International Thermonuclear Experimental Reactor (ITER)

To be built in France (2006 agreement)

→ Research fusion and show feasibility of full-scale fusion power reactor

→ 10 years of construction; 20 years operation→ $12.1 billion to construct→ Goal is to make 500 MW for up to 500

secondsPrevious results of TFTR, JET, JT were usually less than a second

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Drawing of ITER

Much larger than a human

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Advantages of Fusion

• Tritium is the only radioactive material used→ It only emits weak radiation→ It does not concentrate or linger in living organisms

• No long term waste products

• The temperature is high but no danger of nuclear melt down because very little radioactive material inside

• Considered renewable because of significant amount of water available to obtain deuterium and tritium

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Disadvantages of Fusion• Requires superconducting magnets at -270°C

• Very high temperature in center (100,000,000+°C)

• Chamber must be in complete vacuum

• Reactor cooled with liquid lithium that explodes on contact with air or water

• Inner wall of reactor replaced every 2-10 years

• Much larger and more costly than nuclear fission plants

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Shiva

Nova

NIF