Fusion Power: Energy Generation

of the Future

John Norris

History: Nuclear Power• Conceived shortly after the discovery of radioactive

elements– Released huge amount of energy per energy-mass equivalence

• Initially dismissed as impractical – High energy radioactive elements corresponded to short half lives

• Overall it was an expensive proposition (mining/uncontrollable)

• Discovery of neutron led to more atomic experimentation– “Induced” radioactivity changed the perceptions of radioactivity

• Discovered by Frédéric and Irène Joliot-Curie– Made the production of radioactive elements cheaper (less mining)– Idea of slowing neutrons down contributed to higher success in

achieving induced radiation• Discovered in large part to work done by Enrico Fermi

[1]

• Tests were conducted on much heavier elements– In 1938, Otto Hahn, Fritz Strassmann, Lise Meitner, and Otto Robert

Frisch conducted experiments bombarding uranium with neutrons, to investigate Fermi's claims

– This resulted in the roughly equal split of the nucleus into two lighter nuclei

• Differed from previous experiments that only involved small mass changes to the nuclei (think α & β decay)

– Potential for immense energy release was immediately recognized– All occurred immediately prior to WWII

• Focus shifted to creating sustainable chain reactions– Effective Neutron Multiplication Factor: k– Energy Generation k = 1– Weaponization k > 1

• Experimentation and Production continued post-war• Cold War contributed to exponentially increased weaponization

– Also prompted further exploration into nuclear phenomenon– Hydrogen Bomb: First large scale man made fusion reaction

• Totally uncontrollable• Most common type was fission initiated

• Peace time development of nuclear technology has been largely in the realm of energy generation

[1]

Fusion vs. FissionFission• Splitting large nuclei into smaller

pieces• Energy release is very high• Both parent and daughter nuclei

are highly radioactive– Very long half lives– Irradiates both reactor

components and the water used for cooling and heat transfer

• Extremely dangerous– Meltdowns– Environmental Hazards– Inputs and Outputs can be used to

create weapons

Fusion• Hard to achieve

– Protons don’t like other protons– High temps and magnetic fields

are a must• More powerful than fission

reactions– Large nuclei have smaller binding

energies than small• Abundance of inputs• Only low levels of radioactive

wastes– Mostly just the activated interior

panels of the reaction vessel– Input radioactivity is non-

penetrative

Benefits of Fusion• Abundance of input fuels

– Deuterium can be extracted from seawater– Tritium can be made in the fusion reactor with lithium– Helium-3 can in theory be mined from immense deposits in the

lunar surface– As opposed to fission where uranium is rare and must be mined

• Safe– Only small amount of fuel required compared to fission reactors– Most reactors make less radiation than the natural background– Risk of accidental release is non-existent since plasma requires

incredibly precise control• Clean

– No combustion by products– No weapons grade nuclear by products

Difficulties• Must overcome the Coulomb barrier

– Requires incredibly high temperatures– Simple classical calculations imply temperatures on the order of

1011 K– Taking into account quantum effects decreases this maxima– Quantum Tunneling would lower threshold temperature to

roughly 107 K• QT is best described as the individual nuclei “leaking” through the

Coulomb barrier as opposed to overcoming it– This means it doesn’t have to technically overcome the energy

of the Coulomb force• Plasma Turbulence

– Coherent plasma streams are ideal– In reality plasma flows are incredibly complex requiring equally

complex control mechanisms and systems of stabilization

[2]

FUSION

METHODS

Magnetic Confinement• Pinch

– Uses plasma’s electrical conductivity• Induces a magnetic field around plasma• Force is directed inwards causing plasma to collapse inwards and increase in density• Chain reaction

– Denser plasma generates denser magnetic fields– External magnetic fields required to induce the current in the plasma– Drawbacks:

• Can produce chaotic plasma flow ranging from general instabilities and vortices to reversing the toroidal direction of flow

• Staged Z-Pinch– Developed to reduced the instabilities that occur in normal pinch type designs– Injects a linearly stable plasma stream that, upon reaching the critical temperature,

loses stability, but keeps the overall plasma flow stable• Thought to be due to the instabilities being absorbed and dissipated in the stable stream

• These approaches can be thought of as steady state fusion reactions• Requires long plasma containment time

– Confinement refers to the time τ the energy must be retained so that the fusion power released exceeds the power required to heat the plasma

[3],[14]

Tokomak• Invented in the 50’s by Soviet Physicists

– Transliteration means:• Toroidal chamber with magnetic coils• Toroidal chamber with axial magnetic fields

• Most common form of magnetic confinement reactor– Most studied and promising (currently)

• Walls “capture” the heat and pass it to a heat exchanger which produces steam to drive a turbine

• Utilizes two types of magnetic fields– Toroidal

• Causes plasma to travel around torus• Created by external magnets

– Poloidal• Causes circular plasma rotation in planar cross sections• Results from toroidal current flowing through plasma and

is orthogonal to it• ITER

– International Thermonuclear Experimental Reactor– Being built in France– First tokomak fusion reactor that will become

productive

[5],[18]

Plasma Turbulence: Edge Effects

Toroidal Coordinate System:

• Common in plasma physics

• Red arrow - poloidal direction (θ)

• Blue arrow - toroidal direction (φ)

[5]

[12]

Captured by an ultra-high-speed camera, a pellet of fuel is injected into a plasma at the ASDEX Upgrade Tokomak in Garching, Germany. Photo: EFDA.

Plasma image following the injection of a frozen deuterium pellet

[8]

[9] [11]

[13]

Spherical Tokomak

ITER Reactor: Cross Section [15]

Inertial (laser) Confinement• Implosion of micro-capsules of fuel by high power laser beams

– Lasers cause instantaneous sublimation to plasma– Plasma envelope collapses under the radiative pressure– Collapse sends a shockwave through the fuel heating it to its critical temperature

• Final stage the interior fuel reaches 20 times the density of lead and 108 K• Instead of having to confine the plasma for long periods, IC confines plasma in very short bursts• Exposed “reactor” core making energy easier to remove from the system• No magnetic fields also allows for a wider range of materials for construction

– Carbon Fiber– More resilient which decreases levels of neutron activation

• Two types:– Direct drive – Lasers focused directly on target fuel

• Hard to initiate uniform implosion• Suffers turbulence effects similar to magnetic confinement techniques

– Indirect drive – Fuel pellet is placed in a hollow cylindrical cavity (a hohlraum)• Lasers strike the metallic surface creating x-rays which are used to heat the pellet• Causes a much more symmetric implosion

• More stable due to its uniformity• Still not as efficient as magnetic forms • Improvements in laser technology and honing the general technique could actually make it more

efficient in the long run– Short plasma confinement times– Less energy overall to initiate the reaction

[3],[5]

[5],[7]

D-T micro-balloon fuel pellet

Gold Hohlraum

Hohlraum Reactions

[10],[16],[17]

[3],[4]

Reaction

Types

D-T: Deuterium-Tritium• Easiest and currently the most promising• Reaction employed with the ITER fusion

plant• Requires breeding of tritium from lithium

– Advanced reactor designs utilize liberated neutrons within the plasma to do this internally

– n + 6Li → T + 4He– n + 7Li → T + 4He + n

• Drawbacks– Produces lots of high energy neutrons– Only ≈ 20% energy yield in the form of charged

particles• Rest is lost to neutrons

– Limits direct energy conversion– Requires handling of the radioisotope tritium

(τ1/2=12.32 yrs) (write down the other facts and note card and bring up)

– Neutron Flux is 100 time higher than current fission reactors

[3], [4]

D-D: Deuterium-Deuterium• More difficult to achieve than D-T

– Initiation energy is only slightly higher, but confinement times are usually 30 times longer

• Reaction has two branches:1. D + D → T (1.01 MeV) + 1H (3.02 MeV)2. D + D → 3He (0.82 MeV) + n (2.45 MeV)

– Occur with nearly equal probability– Some D-T fusion will occur but no input tritium is required– Neutrons released from (2) will have 5.76 times less kinetic energy than

from D-T reactions• Advantages

– 18% decrease in energy lost to neutrons– Lower average neutron flux to internal components

• Decrease material stresses/damage• Reduces the range of isotopes that may be produced within internal components

– No input lithium or tritium required• Disadvantages

– Power produced can be as much as 68 times lower than D-T

[3], [4]

Aneutronic Fusion• Many potential candidate reactions

– Most can be ruled out due to very high input energies• Two Main Types:

– D - 3He– H -11B

• Fusion power where neutrons are ≤ 1% of the total energy released• D-T & D-D reactions can release up to 80% of their energy as high velocity

neutrons• Would significantly reduce the damage to reactor wall components• Decreases the need for measures taken to protect against ionization damage

– Specifically the need for protective shielding and remote handling safety procedures• Pros:

– Tremendously more efficient– Dramatic cost reductions (inputs & safety measures)– Conversion directly to electricity (no steam turbines necessary)

• Cons:– Incredibly difficult to initiate the reactions

[3], [4]

D-3He: Deuterium-Helium3

• D + 3He → p (14.7MeV) + 4He (3.7MeV) + 18.4 MeV

• Reaction products comprised mostly of charged particles thus minimal damage to reactor components

• More efficient than Neutronic Fusion– Higher Energy Output

• In reality though some D-D reactions occur in the plasma– Releases neutrons decreasing

efficiency and overall energy gain

– Still produces “wear” on internal components

H-11B: Hydrogen-Boron

• 1H+ + 11B → 3 4He + + 8.7 MeV • More efficient in practice than

D-3He– Side reactions result in ≤0.1%

loss in energy through neutron release

– Almost no damage to internal components

• Required temperature is 10 times higher than pure hydrogen fusion (star fusion)

• Confinement time is roughly 500 times that of D-T

[3], [4]

[4]

Deep Space Applications

• NASA is currently looking into developing small-scale fusion reactors for powering deep-space rockets

• Fusion propulsion has a nearly unlimited source of fuel

• More efficient and would ultimately lead to faster rockets– 7 orders of magnitude (107) times more energetic than

the chemical reactions

References:1) "Nuclear Power." Wikipedia. Wikimedia Foundation, 21 Sept. 2012. Web. 21 Sept. 2012.

<http://en.wikipedia.org/wiki/Nuclear_power>.2) "How Nuclear Fusion Reactors Work." HowStuffWorks. N.p., n.d. Web. 18 Sept. 2012.

<http://science.howstuffworks.com/fusion-reactor.htm>. 3) "Fusion Power." Wikipedia. Wikimedia Foundation, 18 Sept. 2012. Web. 18 Sept. 2012.

<http://en.wikipedia.org/wiki/Fusion_power>.4) "Nuclear Fusion." Wikipedia. Wikimedia Foundation, 22 Sept. 2012. Web. 22 Sept. 2012.

<http://en.wikipedia.org/wiki/Nuclear_fusion>. 5) "Nuclear Fusion." , Fusion Reactors. N.p., n.d. Web. 21 Sept. 2012. <http://www.splung.com/content/sid/5/page/fusion>.6) "Reversed Field Pinch." Wikipedia. Wikimedia Foundation, 18 Sept. 2012. Web. 22 Sept. 2012.

<http://en.wikipedia.org/wiki/Reversed_field_pinch>.

7) "Laser Fusion." --Â Kids Encyclopedia. N.p., n.d. Web. 23 Sept. 2012. <http://kids.britannica.com/comptons/art-124938/Laser-fusion-is-an-experimental-method-for-harnessing-the-energy>.

8) "The TFTR Project at Princeton Plasma Physics Laboratory." TFTR Public Home Page. N.p., n.d. Web. 23 Sept. 2012. <http://w3.pppl.gov/tftr/>.

9) "ITER - the Way to New Energy." ITER - the Way to New Energy. N.p., n.d. Web. 23 Sept. 2012. <http://www.iter.org/sci/plasmaheating>.

10) "Peering Inside an Artificial Sun -- Science & Technology -- Sott.net." SOTT.net. N.p., n.d. Web. 23 Sept. 2012. <http://www.sott.net/articles/show/202102-Peering-Inside-an-Artificial-Sun>.

11) "High Frequency Pellet Injector Project." EFDA. N.p., n.d. Web. 23 Sept. 2012. <http://www.efda.org/jet/jet-iter/high-frequency-pellet-injector-project/>.

12) "Plasma Research: Fusion Research, Plasma Confinement, Plasma Turbulence, Plasma Waves." Plasma Research: Fusion Research, Plasma Confinement, Plasma Turbulence, Plasma Waves. N.p., n.d. Web. 23 Sept. 2012. <http://www.ipf.uni-stuttgart.de/gruppen/pdd/pdd_driftwaves.html>.

13) "Noscope." Fusion Power. N.p., n.d. Web. 23 Sept. 2012. <http://noscope.com/2004/fusion-power>. 14) "Aneutronic Fusion." Wikipedia. Wikimedia Foundation, 18 Sept. 2012. Web. 23 Sept. 2012.

<http://en.wikipedia.org/wiki/Aneutronic_fusion>. 15) "Image Gallery - Chairman of National People's Congress of China Visits ITER." Image Gallery - Chairman of National

People's Congress of China Visits ITER. N.p., n.d. Web. 23 Sept. 2012. <http://www.iter.org/gallery/pr_2010_07_bangguo>.

16) "S&TR | September 2005: How One Equation Changed the World." S&TR | September 2005: How One Equation Changed the World. N.p., n.d. Web. 23 Sept. 2012. <https://www.llnl.gov/str/September05/Aufderheide.html>.

17) Sample, Ian. "California Fires up Laser Fusion Machine." The Guardian. Guardian News and Media, 28 May 2009. Web. 23 Sept. 2012. <http://www.guardian.co.uk/environment/2009/may/28/national-ignition-facility-fusion-energy>.

18) "Tokomak." Wikipedia. Wikimedia Foundation, 09 Dec. 2012. Web. 23 Sept. 2012. <http://en.wikipedia.org/wiki/Tokomak>.