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April 20, 2006: 22.012 Fusion Seminar (MIT)

Prof. Kim Molvig

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April 20, 2006: 22.012 Fusion Seminar (MIT)

D-T FusionDD--T FusionT Fusion

MeV nT D 6.17+++ MeV 5.3 MeV 1.14

What is GOOD about this reaction?

Highest specific energy of ALL nuclear reactions Lowest temperature for sizeable reaction rate

What is BAD about this reaction? NEUTRONS => activation of confining vessel and resultant

radioactivity Neutron energy must be thermally converted (inefficiently) to

electricity Deuterium must be separated from seawater Tritium must be bred

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April 20, 2006: 22.012 Fusion Seminar (MIT)

Consider Another Nuclear ReactionConsider Another Nuclear ReactionConsider Another Nuclear Reaction

MeV B p 7.8311 ++ What is GOOD about this reaction?

Aneutronic (No neutrons => no radioactivity!)

Direct electrical conversion of output energy (reactants allcharged particles) Fuels ubiquitous in nature

What is BAD about this reaction? High Temperatures required (why?) Difficulty of confinement (technology immature relative to

Tokamaks)

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4/20 April 20, 2006: 22.012 Fusion Seminar (MIT)

DT Fusion Visual PictureDT FusionDT Fusion Visual PictureVisual Picture

Figure by MIT OCW.

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5/20 April 20, 2006: 22.012 Fusion Seminar (MIT)

Energetics of FusionEnergeticsEnergetics of Fusionof Fusion

r

V kin

E

eV V Nuc 50

KeV R R

eV

T DCoul 400

2

+

409,076.1,10368.1,50200,95.45 542

321=====

A A A A A

QM tunneling required . . .

Empirical fit to data

Coefficients for DT (E in KeV, in barns)

2

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6/20 April 20, 2006: 22.012 Fusion Seminar (MIT)

Tunneling Fusion Cross Section and ReactivityTunneling Fusion Cross Section and ReactivityTunneling Fusion Cross Section and Reactivity

Gamow factor . . .

Compare to DT . . .

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7/20 April 20, 2006: 22.012 Fusion Seminar (MIT)

Reactivity for DT FuelReactivity for DT FuelReactivity for DT Fuel

Figure by MIT OCW.

( ) [ x 1 0 - 1

6 c

m 3 / s e c

]

T1 (KeV)

8

6

4

2

00 50 100 150 200

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8/20 April 20, 2006: 22.012 Fusion Seminar (MIT)

Reactivity for proton-Boron FuelReactivity for protonReactivity for proton --Boron FuelBoron Fuel

4

3

2

1

0

0 100 200 300 400 500 600 700

( ) [ x 1

0 - 1 6

c m

3 / s e c

]

T1 (KeV)

Figure by MIT OCW.

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9/20 April 20, 2006: 22.012 Fusion Seminar (MIT)

Comparison ReactivitiesComparisonComparison ReactivitiesReactivities

4

3

2

1

0 0 100 200 300 400 500 600 700

( ) [ x 1 0 -

1 6 c

m 3 /

s e c ]

T1 (KeV)

( ) [ x 1 0 - 1

6 c

m 3 / s e c

]

T1 (KeV)

8

6

4

2

00 50 100 150 200

Figure by MIT OCW.

Figure by MIT OCW.

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10/20 April 20, 2006: 22.012 Fusion Seminar (MIT)

Availability of Fuel Availability of Fuel Availability of Fuel

Protons?

=> Overwhelmingly available inseawater (deuterium extraction not evenrequired!)

Boron? Widely found in nature as alkali or

alkaline borates or as boric acid(Boron11 constitutes 80.2% of thenatural abundance)

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11/20 April 20, 2006: 22.012 Fusion Seminar (MIT)

Power Density ComparisonPower Density ComparisonPower Density Comparison

p - 11 B has almost 3 times the alpha energy of DT, so even with the reactivity itproduces LARGER alpha heating than DT => in that sense self-sustaining fusion

is easier to maintain if high temperatures can be stably confined.

Example Numbers:

Compare to losses (Bremstrahlung):

Whoops!

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12/20 April 20, 2006: 22.012 Fusion Seminar (MIT)

How to beat Bremstrahlung losses?How to beatHow to beat BremstrahlungBremstrahlung losses?losses?

Some Radiation can be recovered via wallaborption and conversion

But really must Run at lower electrontemperature:

Example:

Still marginal Power balance

ie T T

ie T T