Fusion Power: Energy Generation of the FutureJohn Norris

History: Nuclear PowerConceived shortly after the discovery of radioactive elementsReleased huge amount of energy per energy-mass equivalenceInitially dismissed as impractical High energy radioactive elements corresponded to short half livesOverall it was an expensive proposition (mining/uncontrollable)Discovery of neutron led to more atomic experimentationInduced radioactivity changed the perceptions of radioactivityDiscovered by Frdric and Irne Joliot-CurieMade the production of radioactive elements cheaper (less mining)Idea of slowing neutrons down contributed to higher success in achieving induced radiationDiscovered in large part to work done by Enrico Fermi[1]

Tests were conducted on much heavier elementsIn 1938, Otto Hahn, Fritz Strassmann, Lise Meitner, and Otto Robert Frisch conducted experiments bombarding uranium with neutrons, to investigate Fermi's claimsThis resulted in the roughly equal split of the nucleus into two lighter nucleiDiffered from previous experiments that only involved small mass changes to the nuclei (think & decay)Potential for immense energy release was immediately recognizedAll occurred immediately prior to WWIIFocus shifted to creating sustainable chain reactionsEffective Neutron Multiplication Factor: kEnergy Generation k = 1Weaponization k > 1Experimentation and Production continued post-warCold War contributed to exponentially increased weaponizationAlso prompted further exploration into nuclear phenomenonHydrogen Bomb: First large scale man made fusion reactionTotally uncontrollableMost common type was fission initiatedPeace time development of nuclear technology has been largely in the realm of energy generation

[1]

Fusion vs. FissionFissionSplitting large nuclei into smaller piecesEnergy release is very highBoth parent and daughter nuclei are highly radioactiveVery long half livesIrradiates both reactor components and the water used for cooling and heat transferExtremely dangerousMeltdownsEnvironmental HazardsInputs and Outputs can be used to create weaponsFusionHard to achieveProtons dont like other protonsHigh temps and magnetic fields are a mustMore powerful than fission reactionsLarge nuclei have smaller binding energies than smallAbundance of inputsOnly low levels of radioactive wastesMostly just the activated interior panels of the reaction vesselInput radioactivity is non-penetrative

Benefits of FusionAbundance of input fuelsDeuterium can be extracted from seawaterTritium can be made in the fusion reactor with lithiumHelium-3 can in theory be mined from immense deposits in the lunar surfaceAs opposed to fission where uranium is rare and must be minedSafeOnly small amount of fuel required compared to fission reactorsMost reactors make less radiation than the natural backgroundRisk of accidental release is non-existent since plasma requires incredibly precise controlCleanNo combustion by productsNo weapons grade nuclear by products

DifficultiesMust overcome the Coulomb barrierRequires incredibly high temperaturesSimple classical calculations imply temperatures on the order of 1011 KTaking into account quantum effects decreases this maximaQuantum Tunneling would lower threshold temperature to roughly 107 KQT is best described as the individual nuclei leaking through the Coulomb barrier as opposed to overcoming itThis means it doesnt have to technically overcome the energy of the Coulomb forcePlasma TurbulenceCoherent plasma streams are idealIn reality plasma flows are incredibly complex requiring equally complex control mechanisms and systems of stabilization[2]

FUSIONMETHODS

Magnetic ConfinementPinchUses plasmas electrical conductivityInduces a magnetic field around plasmaForce is directed inwards causing plasma to collapse inwards and increase in densityChain reactionDenser plasma generates denser magnetic fieldsExternal magnetic fields required to induce the current in the plasmaDrawbacks:Can produce chaotic plasma flow ranging from general instabilities and vortices to reversing the toroidal direction of flowStaged Z-PinchDeveloped to reduced the instabilities that occur in normal pinch type designsInjects a linearly stable plasma stream that, upon reaching the critical temperature, loses stability, but keeps the overall plasma flow stableThought to be due to the instabilities being absorbed and dissipated in the stable streamThese approaches can be thought of as steady state fusion reactionsRequires long plasma containment timeConfinement refers to the time the energy must be retained so that the fusion power released exceeds the power required to heat the plasma [3],[14]

TokomakInvented in the 50s by Soviet PhysicistsTransliteration means:Toroidal chamber with magnetic coilsToroidal chamber with axial magnetic fieldsMost common form of magnetic confinement reactorMost studied and promising (currently)Walls capture the heat and pass it to a heat exchanger which produces steam to drive a turbineUtilizes two types of magnetic fieldsToroidalCauses plasma to travel around torusCreated by external magnetsPoloidalCauses circular plasma rotation in planar cross sectionsResults from toroidal current flowing through plasma and is orthogonal to itITERInternational Thermonuclear Experimental ReactorBeing built in FranceFirst tokomak fusion reactor that will become productive [5],[18]

Plasma Turbulence: Edge EffectsToroidal Coordinate System: Common in plasma physics Red arrow - poloidal direction () Blue arrow - toroidal direction () [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) ConfinementImplosion of micro-capsules of fuel by high power laser beamsLasers cause instantaneous sublimation to plasmaPlasma envelope collapses under the radiative pressureCollapse sends a shockwave through the fuel heating it to its critical temperatureFinal stage the interior fuel reaches 20 times the density of lead and 108 KInstead of having to confine the plasma for long periods, IC confines plasma in very short burstsExposed reactor core making energy easier to remove from the systemNo magnetic fields also allows for a wider range of materials for constructionCarbon FiberMore resilient which decreases levels of neutron activation Two types:Direct drive Lasers focused directly on target fuelHard to initiate uniform implosionSuffers turbulence effects similar to magnetic confinement techniquesIndirect 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 pelletCauses a much more symmetric implosionMore stable due to its uniformityStill not as efficient as magnetic forms Improvements in laser technology and honing the general technique could actually make it more efficient in the long runShort plasma confinement timesLess 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-TritiumEasiest and currently the most promisingReaction employed with the ITER fusion plantRequires breeding of tritium from lithiumAdvanced reactor designs utilize liberated neutrons within the plasma to do this internallyn + 6Li T + 4Hen + 7Li T + 4He + nDrawbacksProduces lots of high energy neutronsOnly 20% energy yield in the form of charged particlesRest is lost to neutronsLimits direct energy conversionRequires handling of the radioisotope tritium (1/2=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-DeuteriumMore difficult to achieve than D-TInitiation energy is only slightly higher, but confinement times are usually 30 times longerReaction has two branches:D + D T (1.01 MeV) + 1H (3.02 MeV)D + D 3He (0.82 MeV) + n (2.45 MeV)Occur with nearly equal probabilitySome D-T fusion will occur but no input tritium is requiredNeutrons released from (2) will have 5.76 times less kinetic energy than from D-T reactionsAdvantages18% decrease in energy lost to neutronsLower average neutron flux to internal componentsDecrease material stresses/damageReduces the range of isotopes that may be produced within internal componentsNo input lithium or tritium requiredDisadvantagesPower produced can be as much as 68 times lower than D-T[3], [4]

Aneutronic FusionMany potential candidate reactionsMost can be ruled out due to very high input energiesTwo Main Types:D - 3HeH -11B Fusion power where neutrons are 1% of the total energy releasedD-T & D-D reactions can release up to 80% of their energy as high velocity neutronsWould significantly reduce the damage to reactor wall componentsDecreases the need for measures taken to protect against ionization damage Specifically the need for protective shielding and remote handling safety proceduresPros:Tremendously more efficientDramatic 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 MeVReaction products comprised mostly of charged particles thus minimal damage to reactor componentsMore efficient than Neutronic FusionHigher Energy OutputIn reality though some D-D reactions occur in the plasmaReleases neutrons decreasing efficiency and overall energy gainStill produces wear on internal components

H-11B: Hydrogen-Boron

1H+ + 11B 3 4He + + 8.7 MeV More efficient in practice than D-3HeSide reactions result in 0.1% loss in energy through neutron releaseAlmost no damage to internal componentsRequired 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 ApplicationsNASA is currently looking into developing small-scale fusion reactors for powering deep-space rockets Fusion propulsion has a nearly unlimited source of fuelMore efficient and would ultimately lead to faster rockets7 orders of magnitude (107) times more energetic than the chemical reactions

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