Status of the Nuclear-Induced Conductivity Experiment (NICE) Project

4 Pages·2001·0.22 MB·English

by  Bitteker, Leo

StatusoftheNuclear-InducedConductivityExperiment(NICE)Project
LeoBitteker,ShannonM. Bragg-Sitton,RonJ.Litchford
NASA
MarshallSpaceFlightCenter
Huntsville,AL
Nuclear-basedmagnetohydrodynam(icMHD)energyconversionhasbeenpursuedin
variousformssincethe1950's.Themajorityofthisworkwasmotivatedbythe
compatibilityofMHD generatorswiththehightemperatureachievablewith anuclear
reactorandtheassociatedpotentialforveryhighcycleefficiency. Asaresultofthis
perspectivem, ethodsforenhancingtheelectricalconductivityoftheMHD flow have
primarilyfocussedontraditionalthermalionizationprocessese,speciallythoseutilizing
alkalimetalseeds.However,electricalconductivityenhancemenvtiathermal
interactionsimposessignificantlimitationsontheflowexpansionthroughthegenerator,
andhenceontheultimatepowerdensity.Furthermoret,heintroductionofanalkalimetal
seedintotheflow significantlycomplicatestheengineeringdesignandincreasesthe
potentialforsystemfailuresduetoplatingoftheevaporatedmetaloncoldsurfaces.
Inordertoavoidtheselimitationsanddifficultiesinherenttomostpracticalthermal
ionizationtechniquesv,ariousnon-thermaln, on-equilibriumionizationprocessehsave
beenconsideredoverthepast50years.Theabilitytoachievesufficientelectrical
conductivity,typicallygreaterthani rnho/m,independenotfflow bulktemperatureand
theassociatedimplicationstorenhancedflowexpansionthroughthegeneratorhas
motivatednumerousinvestigations.Conceptsfornon-thermailonizationsourceshave
includedchargedparticlebeamsl,asers,rf energy,andelectricalarcs,amongothers.The
effectivenessoftheseconceptshasbeenlimitedinlargepartbythenecessitytoutilize
somefractionofthegeneratoroutputpowertodrivetheionizationsource.Difficulties
associatedwithintroducingtheionizingenergytotheflow havealsolimitedthesuccess
andinterestin thesealternativestothermalionization.
Oneconceptthathasshowngreatpromiseinrecentyearsistheutilizationofenergy
releasedinnuclearinteractionstoenhanceconductivity.Specifically,thekineticenergy
liberatedininteractionsbetweenreactorneutronsandisotopeswith alargeneutron
absorptioncross-sectionisusedtoionizethesurroundinggasviacollisions.Some
examplesofpromisinginteractionsandtheassociatedkineticenergyliberatedper
interactioninclude3He(n,p)3H(760keV),I°B(n,_)VLi(2.3MeV),6Li(n,003He(4.78
MeV),andthefissioningofheavyelementssuchasuranium(-200 MeV). Conceptually,
eitherthereactorcoolantgasoraparallelworkingfluidflowingthroughtheregionof
highneutronflux isthermodynamicallydriventhroughaMHD generator.Theneutron
interactionenergycanbeintroducedtothefloweitherbyutilizingaflow composedof
theinteractingisotope(3He, 235UF6), seeding a bulk flow with the isotope (10% 3He +
90% 4He), or lining the flow channels with a solid form of the isotope (_°B, 235U). By
utilizing the reactor neutrons to enhance the electrical conductivity, none of the generated
power is needed to drive the ionization source, resulting in higher overall cycle
efficiency. Furthermore, the nuclear-induced ionization is volumetric in nature resulting
inamoreuniformionizationthanispossiblewith variousbeamorsurfaceionization
processes.
By eliminatingthedependencoeftheelectricalconductivityongastemperaturet,he
rangeof applicationfornuclear-baseMdHD cyclesisdramaticallyincreased.Duetothe
temperatureindependenntatureofthenuclear-inducedelectricalconductivity,theflow
canbeexpandedtolowertemperatureandhighervelocitythroughthegeneratorresulting
inhigherpowerdensityandhence,amorecompactgeneratorforagivenpoweroutput.
Also,cyclesoperatingatlowerultimatetemperaturebecomefeasible,makinglow power
MHD cyclesmorepractical.Withthesametechnology,veryhightemperatureh,ighly
efficientcyclesarealsopossibleforlarge-scalepowerproduction.Theexpandedrange
ofoperationofMHD cyclesutilizingnuclear-enhanceedlectricalconductivitymayhave
applicationinhigh-capacitycommercialpowerproduction,compactpowerforremote
locations,space-basepdowerandpropulsion,andnavalpower.InanearlierreportI, the
potentialbenefitandrequirementsforapplicationtospacepowerandpropulsionwas
considered.Inparticular,thisreportshowedthatnuclear-MHDenergyconversionisa
feasibleoptionforachievingasystemmasstopowerratioontheorderof 1kg/kW. A
mass-to-powerratioequaltoorlessthan1kg/kWisessentiaflorfulfilling manyofthe
envisionedNASA missionstotheouterSolarSystemaswell asmannedexplorationof
theplanets.
Inordertoevaluatetheconceptofnuclear-inducedconductivityenhancementt,he
fundamentapl hysicsoftheweaklyionizedgasmustbeunderstood.Computational
studiesofnuclear-induced3Heplasmaindicatethatusefullevelsofconductivity
enhancemenmt aybeattainableindependenotftemperature.Thecomputationaml odel,
CSOLVE,andpredictedresultsforastaticgashavebeendescribedindetailinpast
publications2.Inbrief,CSOLVEusesatwo-temperaturekineticmodelof3HerHe
mixturesin aneutronfieldtocalculateelectricalconductivity.Themodelcalculatesthe
numberdensityofatomicandmolecularheliumions,atomicandmolecularmetastable
heliumneutrals,andfreeelectronsaswell aselectrontemperaturefromasetofkinetic
rateequationsthatincludes22differentinteractions.Theconductivitymodelusesa
standardseriesresistancemodelthatcombinestheeffectsofelectron-neutrale,lectron-
ion,andelectron-electroncollisionsallowingreasonableapplicationoverawiderangeof
chargeandneutraldensities.
TheresultsofthesecomputationsaresummarizedinFigure1forthesteady-statecaseof
pureSHeoverarangeoftemperatured,ensityandneutronflux inaninfinitevolume.
Thisdataindicatesthatatthermalneutronfluxesgreaterthan-10Iz/cm2sanddensityless
thanstandarddensity,conductivitygreaterthan1mho/misachievable.Thisdataalso
showsthatthebulktemperaturehasasecondaryeffectonconductivitywhilethedensity
isofprimaryinfluence.Inthispaper,thepredictedbehaviorof3Heplasmawill be
furtherdiscussed.Thisdiscussionwill includebriefdescriptionsofthebehaviorofthe
plasmaspeciest,ransitionfromelectron-neutratloelectron-chargecollisiondominance,
anddecayoftheconductivityinabsenceofthesource.
Inordertoconfirmthesecomputationarlesults,NASAMarshallSpaceFlightCenter's
Nuclear-InducedConductivityExperiment(NICE)projectisperformingaseriesof
experimentstomeasuretheelectricalconductivityandrelatedplasmaparametersinpure
heliuminbothstaticandMHDflow conditions. Therearethreephasesofexperiments
invariousstatesof progress.Inthefirstphase,thediagnostictoolsneededtoaccurately
characterizethenuclear-inducedplasmaarebeingdevelopedandtested.Forthese
experimentsa, non-equilibriumheliumplasmaiscreatedby meansofafloodofenergetic
electrons.Upto 1mAofelectronsspreadovera10cmdiameterwindowareintroduced
intotheheliumgasatanominalenergyof50keV. Collisionsbetweenelectronsand
heliumatomsproducesanapproximatelyvolumetricionizationverysimilartothat
causedbyneutroninteraction.Forcomparisont,heenergydepositedin thegasby the
electronsisequivalenttothepredictedeffectofathermalneutronflux oforder1012
/cm2s. The second phase of these experiments uses the same diagnostic tools to fully
characterize the nuclear-induced helium plasma for comparison to the computational
results. These nuclear experiments are designed to cover the full range of density and
neutron flux included in Figure 1. Finally, in the third phase of experiments, a sub-scale
MHD flow utilizing non-thermal ionization is characterized to examine the effect of flow,
magnetic field, and related parameters on the plasma kinetics. The MHD flow will make
use of a 3 Tesla split coil superconducting magnet currently being fabricated. This paper
primarily describes the objectives, design, and current results of these experiments.
Conductivity v. Relative Density iI '"_-"kl"-'1100^^1lt0
1.E+03 '--ii-"10^12
--,p.,-_0^t3
,-.N,--10^14
,i1-,-10^15
1,E+02
: + 10^10, 500K-250
! i :. = c=:zl_ 10^11, 500K-250
J & 10^12, 500K-250
o 10^13, 500K-250
1.E.02
1.E-03 -
1.E-04
1.E-04 1.E-03 I.E-02 1.E-01 1.E+00 1.E+01
Relative Density
Figure 1: Electrical conductivity versus relative density in pure 3He as a function of
neutron flux (/cm2s) and bulk temperature (K) as calculated by the CSOLVE
computational model. Continuous line corresponds to 300 K, symbols plotted at each
decade cover the range from 500 K to 2500 K.
References
1- R. J. Litchford, L. Bitteker, J. Jones, "Prospects for Nuclear Electric Propulsion using
Closed-Cycle Magnetohydrodynamic Energy Conversion," AIAA Paper No. 2001-0961,
39'hAIAA Aerospace Sciences Meeting and Exhibit, Reno, Jan. 2001.
2 - L. Bitteker, "Nuclear induced electrical conductivity in 3.H..e.., J. App. Phys., Vol. 83,
No. 8, 1998, pp. 4018-4023.