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Options for development of space fission propulsion systems

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Fission technology can enable rapid, affordable access to any point in the solar system. Potential fission-based transportation options include high specific power continuous impulse propulsion systems and bimodal nuclear thermal rockets. Despite
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  OptionsforDevelopmentofSpaceFissionPropulsionSystems MikeHouts,MelissaVanDyke, Tom Godfroy,KevinPedersen,JamesMartin,RickyDickens,PatSalvail,IvanaHrbud NASAMSFC,TD40,MarshallSpaceFlightCenter,Alabama,35812michael.houts@dnsfc.nasa.gov/ 256)544-7143/Fax: 256)544-5926 Abstract.Fissiontechnologycan enable rapid,affordableaccesstoanypointinthesolarsystem.Potentialfission-based tnmspmafionoptions includehighspecificpower continuous impulsepropulsionsystemsandbimedalnuclear thermal rockets.Despitetheirtremendouspotentialfor enhancing orenablingdeepspaceandplanetarymissions,todatespacefissionsystemshaveonlybeenusedinEarthorbit.Thefirststeptowardsutilizingadvancedfissionpitonsystemsisdevelopmentofasafe,near-term, affordable fission system thatcan enhance or em_ near-termmissionsof interest. An evolutionaryapproach fordevelopingspacefissionpmgmlsionsystemsisproposoctINTRODUCTIONThefissionprocesswasfirstreportedin 1939, and in1942the world's first tram-madeserf-sustaining fission reactionwasachieved. Creating a self-suslaining fissionchain reaction isconceptuallyquite simple. Allthatisrequiredisforthefightmaterialstobeplaced in therightgeometry-no extreme temperaturesorpressuresrequired - andthesystemwilloperate.Fissionsystemsoperateindependentlyofsolar woximity ororientation,andarethuswellsuitedfordeepspaceorplanetarysurfacemissions.Inaddition,thefuelfor fissionsystems (highlyenriched m) isvirtually non-radioactive,containing 0.064curies/kg. This comparesquitefavorablytocurrentnuclearsystems(Pu-238 in radioisotopesystems contains 17,000cuties/kg)andcertainfuturisticpropulsion systems (tritiuminD-Tfusionsystemscontains 10,000,000 curies/kg). An additional comparison isthatathuncha typical spacefission Wopulsion systemwould contain anorderof magnitude lessonboardradioactivity than did Mars Pathfinder'sSojournerRover,whichusedradioisotopesforthermal control. Theprimarysafetyissuewithfission systemsisavoklinginadvertentsystemstart-addressingthisissuethroughpropersystemdesign is quite straightforward.The energy densityoffissionis compembleto thatofD-Dfusionand higher thanthechargedparticleenergydensityofD-Tfusion.Thepotentialcapabih'tyoffissionpropulsionsystems is compar_withthatofexistingandfuturisticpropulsionsystemsinTable 1. AsshowninTable 1, the e_rgy densityin fissile fuelissevenordersofmagnitudegreaterthanthatofthebestchemicalfuels, t,m anotherway,completelyfissioningapieceofuraniumthesizeofacokecanwouldyield 50 timesmore energy thanburningallofthe chemicalfuelcontained inthespaceshuttlemaintank.Ifproperlyharnessed,theenergy density infissilefuelfar exceeds thatrequiredtoenablerapidaccesstoanypoint in thesolarsystem.Fissionsystemsarethenearest-termoptionfor high efficiency,highthrust in-space propulsion.Althoughseveral hundred thousandkilograms ofhighly enricheduraniumhasbeendeclared  excess , thereisstill significant expenseinvolvedwithihbricatingfeelpins for space fission systems.The cost estimategiven inTable 1 was providedbyChidester,2000,and includes allcostsassociated with providingfissilefuelpins for a solidcore spacefission system. Likewise, it may be possibleto extract upto 30 kgoftritiumfromwastethathasaccumulatedover the pastseveraldecadesfromheavy-watercooledterrestrialnuclearpowerplants. Although thiswastetritiummaybeavailableforon the order of  30M/kg 0Vilms 2000),producingnew tritium isprojectedto be muchmoreexpensive. The Departmem of Energy'songoingtritiumproductionprogramhasacostgoalof 100Mperkgoftritiumproduced(Lisowski,1998).  InTable1, engineering Q isdefinedastheratioofthetotal en_gy generatedinthereactionchamber/volumetothe energy that must berecycledoutsidethechambertosustainthereactionAn enoneeri_ Q of 1.0wouldthusrepresentthecasewhereallofthe energy generatedinagivenreactionisusedtosustainthereaction,andtheonlyenergyleftoverforpropulsionislow-qual/tywaste heatfromsystem inefficiencies. An engineeKug  Q muchgreaterthan1.0isthusrequiredforalxepolsionsystemtobepotentiallyattractive.Pulsedfusionsystemsdrivenbyfissionprimariesareconsideredtobefissionsystems.Becauseofthe early stageoffusionpropulsionresearch,noattempthasbeenmadetodemonstrate even alow(Q>0.0001) e_n_g Qinnon-fission-drivenfusionsystems. Both fissionandfusionsystemshaveadequatefuelenergydensitytotheoreticallyenable mlid accesstoanypointinthe solarsystem.Although the clmrgcd-paflicleenergy densityoffissioniscomparabletothatofD-TorD-Dfusion,it hasproven diffkadttodesignfissionsystemsthatusechargedparticlesdirectlyas pre_llant. Theprimary potentialadvantageof fusion systems is thus thetheoretical ability to more easilyusechargedparticlesdirectly aspropellant Primary advantages offission systemscompared tofusion systemsarethe hightechnologyreadinesslevelofcertain fission systemsandtherelative ease ofgeneratingaserf-sustainingfissionreaction.ThescarcityofHelium-3andthescarcity/hazards of tritiumwilllimittheiruseasfueKManyproposedD-Tspacefusionpropulsionconcqmcannot breedadequate tritiumonbtmdtosustainoperation,and thus tritiumwouldhavetobelaundtedfromEarth.Inaddition,wasteheatgeneratedbytritiumproduction in D-Tfusionpropulsionsystemsthatdo _ breedingtritiumonboardseverelylimitsthepotentialperfommn_ofthosesystems.Fromanoverallarchiteoturestandlx_D-D fusion may thus bethebestpotentialfusionoigion,assumingalight-weight, highengineering  Q D-Dfi_onpropulsionsystemcanbedevised.TABLE1. Coml_rison ofFission Pro[mlsion toEMsting(Chemical)andFuturistic(Fusion)PropulsionOptions. ...... ParameterLoX/H2D-DFus/onD-TFusionD-He3 F/ss/on Fusion ,........................ , .... ..... TheoreticalFuelEnergyDensity(J/kg)DemonstratedFuelEnergyDensity (OperationatSystem, J/kg) Charged ParticleEnergyDensity (I/kg) NeutronEnergyDensity(Po_ntialfor RadiationDamage,JAg) DemonstratedEnginecfi_Q (F.nergyOut / EnergyRecycletoSustainRx)Fuel Cost Fuel Availability FuelHeat Generation During Storage RadioactivityatLaunch1.6x10 _ 1.6xlO  r 00 O0 LOW mgh 00 7.8xI0231.0xl013NotLow High 00Not Attempted _$100M/kg TLow 325 W/kgTlxl07 Ci/kg T 3.5x10_403.5x1014 0+(D-D)NotAttempted me Low 0 0 oo MJkg <0.001 W/kg 0.064Ci/k APOTENTIALAPPROACHTOFISSIONPROPULSIONSYSTEMDEVELOPMENTDespitetherelative simplicityand tl_mendouspotentialof spacefission systems,thedevelopmentandutilizationofthese systems hasprovenelusive.Thefirstuse of fissiontechnology in spaceoccurred 3 April1965withtheUSlaunfhoftheSNAP-IOAreactor.TherehavebeennoadditionalUSusesofspacefissionsystems. Whilespace fissionsystemswereusedextensivelybytheformerSovietUnion,theirapplioationwaslimitedto earth-orbital missions.Early space fission systems mustbe safelyand affordablyutilizedif weare toreap the benefitsofadvancedspacefissionsystems.Table2givesal_aliallistofmajorUS space fissionprogramsthathavefailedtoresult in Rightof asystem (Angelo,1985).Therearea variety ofreasonswhytheseprogramsfailedtoresult in a flight. Thefactthatsomany  programsaveailedindicateshatasignificantlyifferentaplxoachmustbetaken f futureprogramsaretosucceed.Inmanycases,spacereactorprogramswerecancelledbecausethe _ missionwascancelled.However,inmanyofthosecasesmissioncancellationwaspartiallyduetothefactthatthereactorrequiredbythemissionwastakingtoolongandcostingtoomuchtodevelop.Terrestrial fission systemshavebeenutilizedbythegovernment,universities,industw,andutih'tiesforover 50 years.Inaddition,teclmologydevelopmentdirectlyrelatedtospacefissionsystemshasbeenprogressingforover 40 years.Near-termfissionsystemsmustcapitalizeonthis experience. ThedevelopmentofnewnucleartedmologyhashistoricallybeencostlyandtimeconsumingNucleartechnologydevelopedbyprevious wograms shouldthusbeutilized,andnonewnucleartechnologyshouldberequitedThismeansthatallin-corecomponentsshouldoperatewithindemonstratedfuelbtwaupcapabifityanddemonstratedneutrondamagelimitsforthegivenreactor environment (temperatme,chemistry,powerdensity, etc.), Theconstmctinnofnewnuclearfacih'tiesortheextensivemodificationofexistingfacilitieshashistoricallybeencostlyand timeconsuming. Near4erm fission systemsshouldthususeonly existing nuclearfacilitiesintheirdevelopmentNoneworsignificantlymodifiedfa_UtiesshouldberequiredFlightqualificationofanyspa_systemrequiresanextensivetestprogram. Near-term f_sionsystemflightunitsmustthusbehighlytestable.Because of theexpenseanddi_cultyasugimedwithperformingrealisticfull-powergroundnucleartests,p_-viompmgnunshaveconsidered the optionofforegoingfull-powergroundlagleartestinginfavorofa flight test.Forexatx_e, in Yoslofl_1993(referringtotheSP-100program)itisstatedthat'Wherehasbeenrecentinterestamonggovernmentagen_s in establishing an early flight missionthatwoeld wovide thecatalystneededto enableconfident planningforsubsequentoperationalmissions.Thisfirst flight wouldvalidatethetotalsystemperformance,obviatetheneedfor costly groundtngleartesting,demonstratesafetyfeaturesandfacilitatesafetyapwovalflxrough the INSRPprocessfor the subsequentoperationalmission_ Fullpowernucleargroundtestfacilityrequirengntsmayalsodictatethattheunittested on thegroundbesignificantlydifferentthantheactual flight unit.Anydifferencesbetweenwhatistestedandwhatis flown willlimitthebenefitfromfull-powergroundnucleartests.Highlytestable systems thatutilize established nucleartechnologyincortheleasttec,hnicalriskiffullpowergroundnucleartestingisnotperformed.Theability to quicklyandaffordably establish thesafetyandreliabilityofanyproposedspacefissionsystemwill be criticaltoitsprogrammaticsuccess.Additionalinnovativeapwo_heswill have to be usedtoensurethatthenext _ fission system developmentprogramresults in systemutiltzatio_Safetynmstbethe winmry focnsoftheprogram,butcostandschedulemustalsobesigniticam _s. Systempefforman_mustbeadequate,butthedesiretomakeperfommn_moretlmnadequateshouldnotbeallowed to drivesystemcostand _. Near=termspace fission systemsmust be safe,simple,andas inexpensiveto develop and utilizeaspossfi)le.TABLE2.PartiallistofmajorUSSpaceFissionProgramsthatHaveFailed to Result in FlightofaSystem.• Solid=C_re uclearRocket•SNAP-501SPUR Program•High-T_Gas- *Medium-PowerReactorCooled Electric ower Experiment(MIRE)Reactor (710 Reactor) •ThermionicTechnology*SPAR / SP-100Program(1963-1973)•FlightTopaz•SpaceNuclearThermalRocket*DOE40kWeThennionicProgramReactorProgram•SP-100 • AdvancedLiquidMetalCooledReactor • AdvancedSpaceN_learPowerProgram(SPR) • Multi-MegawattProgram•Thermionic Fuel ElementVerificationProgram•AirForceBimodaiStudyInitialresearchrelated to apotentialnear-term,low-costspacefissionsystemisunderwayatNASA'sMarshallSpaceFlight Center (MSFC). Contributors tothe effort includeDepartmemofEnergy National Laboratories,universities,industry,andotherNASAcenters.The witm_ near-termfissionsystemunder investigation istheSafeAffordableFission Engine (SAFE).ThreeSAFEsystemsarecurrentlybeing considered -the 30 kWtSAFEo30(stainlesssteel),the 300 kWtSAFE-300(molybdenum),and the 120kWtSAFE-300s(stainlesssteel).AllSAFEcoresusefuelpinsconductivelycoupledto
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