URANIUM ARC FISSION REACTOR FOR SPACE POWER AND PROPULSION. submitted by. Richard T. Schneider Isaac Maya Juan Vitali

Transcription

1 URANIUM ARC FISSION RACTOR FOR SPAC POWR AND PROPULSION submitted by Rihard T. Shneider Isaa Maya Juan Vitali DW XTflOf gfati Final Reprt fr SBIR Cntrat NAS-2618 Researh Supprted by Strategi Defense Initiative Organizatin/ Innvative Siene and Tehnlgy Managed by Natinal Aernautis and Spae Administratin, Lewis Researh Center Di'IG t-u. ytfjaiiiii ij^^rsi'uxiäüjj * RTS Labratries, In. 166 Tehnlgy Av. Alahua, FL 2615 PLAS RTURN TO: 8MD TCHNICAL INFORMATION CNTS* en BALLISTIC MISSIL DFNS ORGANIZATION 7100 DFNS PNTAGON WASHINGTON D.C Marh, ^$

2 Aessin Number: 484 Publiatin Date: Mar 01,1992 Title: Uranium Ar Fissin Reatr fr Spae Pwer and Prpulsin Persnal Authr: Shneider, R.T.; Maya, I.; Vitali, J. Crprate Authr Or Publisher: RTS Labratries, In., 166 Tehnlgy Ave., Alahua, FL 2615 Desriptrs, Keywrds: Uranium Ar Fissin Reatr Spae Pwer Prpulsin Pages: 000 Catalged Date: Mar 18, 199 Cntrat Number: NAS-2618 Dument Type: HC Number f Cpies In Library: Rerd ID: 2645

3 URANIUM ARC FISSION RACTOR FOR SPAC POWR AND PROPULSION submitted by Rihard T. Shneider Isaa Maya Juan Vital! Final Reprt fr SBIR Cntrat NAS-2618 Researh Supprted by Strategi Defense Initiative Organizatin/ Innvative Siene and Tehnlgy Managed by Natinal Aernautis and Spae Administratin, Lewis Researh Center RTS Labratries, In. 166 Tehnlgy Av. Alahua, FL 2615 Marh, 1992

4 PROJCT SUMMARY Cmbining the prven tehnlgy f slid re reatrs with uranium ar nfinement and nn-equilibrium dissiatin and inizatin by fissin fragments an lead t an attrative pwer and prpulsin system. The benefit ensues frm using the high quality direted energy f fissin fragments and assiated radiatin t btain wrking fluid/prpellant dissiatin and inizatin diretly, withut first degrading the energy t heat. The dissiatin and inizatin energies an be utilized in a nzzle, thruster r MHD aeleratr/generatr. Uranium ar tehnlgy is being develped fr use in spae nulear thermal and eletri prpulsin reatrs. In the Uranium Ar Fissin Reatr, ars are driven mainly by fissin energy and require little eletrial energy input. The ars perate at 10,000 K, and transfer energy t the prpellant r wrking fluid via ptial radiatin, thus aviding material temperature limitatins. The result is a prpulsin afterburner that an elevate fluid temperatures t levels abve the melting pint f any material (abve 4,000 K). This enables very high speifi impulse prpulsin r ultrahigh temperature pwer nversin. Fissin events in the nulear ar plasma prvide fr additinal dissiatin and inizatin f prpellant r wrking fluid via fissin fragments and inizing radiatin. This inizatin, plus assiated enhaned fluid prperties suh as eletrial ndutivity, lead t a redutin in the eletrial energy that must be invested t dissiate and inize the prpellant in an eletri thruster (MPD-ar) r MHD generatr. Fr spae pwer platfrms, the resultant redued eletrial pwer input requirement prvides fr imprved effiieny, redued and easier waste heat rejetin and thus a mre mpat, lwer mass reatr design. An experimental prgram was nduted t shwn that fissin energy an be used t inrease the temperature f an ar plasma, and thus inrease the radiative energy transfer frm the ar. Helium vrtex stabilized ars f uranium and brn were perated in a thermal neutrn flux f 10 6 n/m 2 se. The intensities f bserved spetral lines inreased during irradiatin. The spetrspially measured temperature inreased frm 8,100 K t 9,400 K (15%) fr uranium ars, and frm 9,500 K t 11,500 K (20%) fr brn ars. This effet is attributed t the mre effiient inizatin mehanism f high energy harged partiles mpared t eletri field/hmi heating. Additinal experiments were perfrmed t demnstrate that the radiative energy uld be absrbed in an external prpellant r wrking fluid. The experiments shwed that 90% f the emitted light was absrbed at a partile density f 2 x m' (1 atm and 60 K) and an ptial path length f 10 m. Subsequent experiments shwed that the uranium lss rates during ar peratin uld be redued t a few mg/m 2 s by prper eletrde design. The experimental results indiate the fundamental feasibility f uranium ar driven energy transfer. Fr thermal prpulsin, inreased temperatures mean higher speifi impulse. Fr spae pwer platfrms, the resultant imprved effiieny means redued and easier waste heat rejetin and thus a mre mpat, lwer mass reatr design. A mre effiient thruster als results in lwer pwer requirements r redued radiatr area and thus mass. The tehnlgy is als appliable fr mre effiient and safer grund-based eletri pwer generatin. A spinff develpment is the tehnlgy fr handling uranium and its mpunds in liquid and vapr frms. This tehnlgy will be required in advaned Gas and Vapr Cre Reatrs. ii

5 Table f Cntents PROJCT SUMMARY ii 1. INTRODUCTION Identifiatin and Signifiane f the Prblem r Opprtunity Phase I Tehnial Objetives 2. URANIUM ARC FISSION RACTOR NCPT AND NUCLAR-AUGMNTD THRUSTR NCPT Physis Basis Uranium Ar Fissin Reatr Cnept Desriptin 5 2. Nulear-Augmented Thruster Cnept 10. XPRIMNTAL ST UP RSULTS AND DISCUSSION Spetral Line missin Intensity Plasma Temperature Measurements 1 4. letrial Cndutivity Radiative nergy Absrptin and Mass Transfer NCLUSIONS 2 6. RFRNCS 2 m

6 1. INTRODUCTION 1.1 Identifiatin and Signifiane f the Prblem r Opprtunity. Pwer and prpulsin systems utilizing nulear fissin energy pssess many advantages ver hemial prpulsin systems. These inlude: Higher ahievable perating temperatures Muh greater energy and pwer densities ffiient nn-thermal inizatin mehanisms These advantages lead t signifiant missin perfrmane advantages ver hemial systems. Hwever, in spite f these advantages, systems using nulear reatrs are still perfrmane limited by the nstraints impsed by material temperature limits and heat transfer via standard thermal presses. In this wrk, we seek t verme these limitatins by using radiative energy transfer mehanisms t avid material temperature limitatins and enhane heat extratin, and expliting the high quality inizatin f fissin fragments t inrease effiieny. T ahieve fissin's perfrmane ptential, many spae nulear pwer and prpulsin systems based n slid- and gaseus-re reatrs have been extensively studied during the past 40 years. The limitatins n the perfrmane f these systems, whether speifi impulse, thrustt-weight rati, r speifi mass, are impsed by the maximum effetive temperature available fr energy transfer, energy nversin and heat rejetin. Fr slid fueled reatrs, these limits are ditated by the fuel melting temperatures and assiated yle materials temperatures. With these nstraints, slid re reatr-based prpulsin systems still yield speifi impulses f up t 1000 t 1200 sends, ver twie the speifi impulse f hemial prpulsin systems prduing mparable thrust. Presently, the mst prmising apprah t verming these inherent limitatins is t use a fissile fuel in vapr, gaseus r mirn-size liquid drplet phase in the re at thusands f degrees K htter than the materials ntaining the system. The nversin and thermal management system, upled t the ultrahigh temperature reatr, have t prvide effiient energy nversin while still maintaining heat rejetin temperature in the 1600 t 2100 K range. With these nstraints, a gaseus re reatr's speifi mass an be as lw as ~1 kg/kw, and the speifi impulse an be as high as,000 t 5,000 sends due t the high wrking gas r prpellant temperature at high pressure. The majr prblems with gaseus re reatrs are nfinement f the fuel in the re regin while simultaneusly ahieving the requisite high temperature energy transfer frm the 1

7 fuel t the wrking fluid r prpellant In this wrk, we prpse a new reatr nept whih vermes these prblems by taking advantage f eletri field nfinement f the fuel gas the Uranium Ar Fissin Reatr (UAFR). nergy transprt presses in a fissining plasma uld enable ultrahigh temperature pwer nversin r very high speifi impulse prpulsin. nergy transfer frm the fissining fuel t the wrking fluid used in the pwer nversin system r prpellant in the ase f prpulsin is amplished by dissiating and inizing bth the fuel and the wrking fluid in and lse t the plasma regin. The highly effiient inizatin sure f the fissin fragments an be used t uple energy diretly int the ar. This is mbined with radiative upling f the inized wrking fluid with the bulk wrking fluid flwing external t the ar t ahieve effiient energy transfer. The wrking fluid an then be used fr pen yle diret thrust, r in lsed yle energy nversin. The perfrmane f this new system lies between thse f the slid and gaseus re systems in terms f speifi impulse. An embdiment f the UAFR nept is disussed in Setin 2. In suh a reatr the majrity f the nulear fuel is in the frm f slid material r a mlten uranium metal pl. Hwever, a minrity amunt f the nulear fuel is in the frm f an ar plasma (~8000 K) emanating frm the mlten uranium pl. Cntainment f this plasma is prvided by the eletrmagneti fres f an ar. Sine melting f the pl and inizatin f the ar is prvided by nulear fissin, the required eletrial pwer input is minimal. A signifiant advantage f this apprah is that the bulk f the ultrahigh temperature nulear fuel des nt irulate and is thus ntained in the re (slid r therwise) withut requiring hydrdynami ntainment. Ahievement f ritiality is therefre n lnger linked t the perating pressure. Dissiatin and inizatin are ahieved by interatins with the fissin fragments and assiated radiatin. Radiatin transfer urs at phtni speed withut having t rss material surfaes, as ppsed t nventinal heat transfer whih urs at phnni speed (speed f sund) and requires heat transfer surfaes. The effiient inizatin aused by fissin fragments an als be applied t advantage in a new thruster nept the nulear-augmented thruster. Previus neutrn irradiatin experiments perfrmed by us at Ls Alams Natinal Labratry's Gdiva reatr have shwn that fissin fragment inizatin an be used t inrease the ndutivity f a plasma by fatrs f greater than ten ver its thermal equilibrium level.[l,2] In the present nept, the eletrial energy nrmally supplied t an MPD thruster t inize the prpellant befre aeleratin and exhaust is replaed by mre effiient diret fissin fragment inizatin. This leads t signifiantly redued eletrial energy input requirements. Redued eletrial needs translate int lwer reatr pwer generatin requirements, less heat rejetin area and thus lwer verall system mass.

8 1.2 Phase I Tehnial Objetives The researh desribed herein is direted t the demnstratin f the sientifi feasibility and tehnlgy develpment f the key aspets f nn-thermal mehanisms f energy transfer, dissiatin, and inizatin via fissining ars. The bjetives f the experiments desribed here were t demnstrate that fissin energy an be upled t the inizatin and radiative emissin f the ar, that the enhaned ar emissin an subsequently be radiatively upled t a wrking fluid r prpellant, and that this augmented heat transfer urs withut signifiant lss f the nulear fuel. Nulear effets aused by fissin fragment and gamma irradiatin were btained bth diretly by using uranium, and simulated by using the harged fissin fragments frm the 10 B(n, 4 He) 7 Li nulear reatin. In a nventinal ar, eletrial energy is nverted int heat via hmi heating and llisin-dminated mehanisms. A nulear-augmented uranium ar derives mst f its pwer diretly frm the fissin energy f the fuel and requires little eletrial energy input. Suh a uranium ar perates at 10,000 K, and transfers its energy t the prpellant r wrking fluid via ptial radiatin, in additin t the nventinal heat ndutin and nvetin. The result is similar t a prpulsin afterburner that an elevate fluid temperatures t levels abve the melting pint f any material (abve 4000 K) sine energy transfer urs withut rssing slid material bundaries. This enables very high speifi impulse prpulsin,[2,] ultrahigh temperature pwer nversin, r mre effiient thruster designs. In a nventinal eletrial ar, mst f the energy prvided by the pwer supply is used t inize the partiipating ar speies. After this latent heat has been prvided, additinal energy is then spent in heating the plasma. Depending n the ar design, sme energy is als required t prvide the latent heat neessary t evaprate atms frm the slid interfae f the eletrdes. In this reprt we explre an alternate methd f prviding inizatin t the ar. Inizatin an be ahieved by expsing the ar t an external sure, suh as an eletrn beam, r by using a fissile plasma in a neutrn envirnment. In the latter, fissin events at r near the eletrde surfae prvide fr latent heat, dissiatin and inizatin f plasma speies via fissin fragments and inizing radiatin. This results in diret energy input int the ar via eletrn llisins with the ar plasma. An additinal energy sure is the fissining that urs diretly in the plasma lumn. In ur experiments we augment a nventinal eletrial ar with a fissile mpnent. As the fissile plasma is expsed t neutrns, fissin urs and a new sure f inizatin is reated. The eletrial pwer that was previusly used t inize is nw available fr ther purpses, prvided that the eletrial pwer input is held nstant (i.e., via a nstant urrent pwer supply). In suh an ar, this newly available pwer prvides additinal hmi heating f the plasma, thereby inreasing its verall temperature and radiative emissin. The experimental verifiatin f this effet is reprted here. In additin, we demnstrate that the radiated energy an be absrbed by a send medium, and that the press urs withut signifiant lss f the fissining material.

9 2. URANIUM ARC FISSION RACTOR AND NUCLAR-AUGMNTD THRUSTR NCPTS 2.1 Physis Basis The high pwer apabilities and mpatness f nulear reatrs ffer the ptential fr extremely high pwer densities. Hwever, even thugh the energy utput per uranium fissin event f 200 MeV is 10 8 times the typial 1 ev utput f hemial systems, the state-f-the-art in nulear energy nversin has nt advaned beynd degradatin f the released fissin fragment energy t heat The maximum temperature f this nversin press is thus nstrained by material limits. The net result is mat nulear pwer has been energy extratin limited, and has nt even nearly apprahed its generatin limit. T take advantage f this harateristi f high pwer density, nulear pwer uld thus mst benefit frm the use f advaned pwer extratin mehanisms. In a traditinal heat engine, the dminant heat extratin mehanisms are heat ndutin and nvetin, with usually minr ntributins frm radiative heat transfer. Hwever, heat ndutin and nvetin are amplished by phnn transfer at the speed f sund f the material, while radiative transfer is amplished by phtns mving at the speed f light. Thus, if fast energy extratin as desired with fissining sures, radiative heat transfer is preferred. This nlusin was readily understd by the early prpnents f Gas Cre Reatrs, e.g., the Nulear Light Bulb ngine. In additin t radiative heat transfer, there are additinal heat transfer mehanisms that uld be used fr rapid heat transfer. When ht gases intermix at different temperatures, e.g., a fissining gas in an ar plasma, energy interhange urs nt via phnn interatins, e.g., lattie vibratins, but by llisins suh as atm-atm and eletrn-in, whih may inlude rembinatin and re-emissin f radiatin. Cllisin frequenies in high temperature gases and plasmas are relatively high, and therefre the energy transfer rate is rrespndingly high. The high inizatin rates in a fissining plasma make these mdes f heat transfer very effiient. In additin t the limitatins n heat extratin, traditinal heat sure perfrmane is limited by the tradeff between Carnt effiieny and waste heat rejetin area. In the envirnment f spae, heat rejetin is amplished via radiatin. Sine effiieny, given a speified material-limited sure temperature, is inversely prprtinal t the first pwer f the heat rejetin temperature, whereas heat rejetin area (mass) varies inversely with the furth pwer f the heat rejetin area, minimum mass in rbit fr traditinal heat sures is ahieved by maximizing the heat rejetin temperature and aepting lwer effiienies. By rejeting heat at highest temperature, nulear reatrs ffer a neptually straightfrward methd fr minimizing radiatr mass. Hwever, the mehanisms prpsed herein fr radiative and llisinal energy transfer withut rssing material surfaes then ffer the ptential fr the highest sure temperature, thus maximizing effiieny, plus the ptential fr the highest heat rejetin temperatures, minimizing radiatr mass.

10 Finally, the effiient inizatin aused by fissin fragments an be used fr mre than just t enable fast llisinal energy transfer. It an be used t redue eletrial energy requirements f eletri prpulsin engines. In suh engines, the majrity f the eletrial energy input is used t inize the prpellant. A smaller prtin f the eletrial energy is then used t elevate the plasma temperature befre exhaust Inizatin f the prpellant is aused primarily by atm llisins with eletrns whse energies are abve the prpellant's inizatin energy. The number density f these eletrns in the tail f the Maxwellian distributin f the plasma eletrns is relatively lw - less than 1% f the bulk eletrn number density. The eletrial energy requirement fr inizatin is high beause eah inizatin event requires ev's per atm, e.g., 5.4 ev fr lithium r 8. ev fr brn, and beause the eletrial energy transfer press must heat the entire eletrn ppulatin distributin in rder t ppulate the high energy eletrn tail f the Maxwellian distributin f the plasma eletrns. Diret fissin fragment inizatin, hwever, prvides large numbers f high energy eletrns f energies abve the inizatin ptential f all atms. Fr a plasma at 4000 K, the number density f these eletrns has been estimated at 10 t 10 4 times higher than the rrespnding number density in the Maxwellian distributin. [5] This is equivalent t the ppulatin density f a 10,000 K plasma. In ther wrds, a 4000 K fissining plasma prvides the inizatin equivalent f a thermal plasma at 10,000 K. The gain in effiieny mes frm replaing the eletrial energy that wuld therwise had t have been prvided t heat the plasma t the higher temperatures. The derease in eletrial energy input further results in a derease in the thermal energy t be rejeted and a rrespnding derease in the size and mass f the spae radiatrs. The neutrn apture reatins available with whih t enable the abve features inlude the fllwing: He(n,p)T 765 kev 6 Li(n,T) 4 He 4.8 MeV 10 B(n, 4 He) 7 Li 2.8 MeV ^UffOff 200 MeV The uranium fissin reatin listed is representative f that pssible with ther fissinable istpes, e.g., ^U. 2.2 Uranium Ar Fissin Reatr Cnept Desriptin The UAFR bridges the gap between slid re nulear reatrs and gas re reatrs. It further relaxes the high materials temperature limitatins f slid re reatrs and allws ntat f ultrahigh temperature prpellant with materials that are nn-nulear, nt mehanially stressed, and have high utilizatin temperature, e.g., tungsten and graphite, r are atively led, suh as

11 the nzzle. It als minimizes the radiative heat transfer and mass lss prblems f gas re reatrs by using eletri ar nfinement f the fuel, radiative and llisinal presses fr rapid energy transfer at ultrahigh temperatures withut rssing material surfaes, and diret inizatin t minimize the eletrial energy requirements. A shemati diagram f the reatr is shwn in Figure 1. The figure shws a reatr very similar t the typial NRVA-type slid re, with the additin f a fissining ar setin at the aft end f the re. Prpellant is first heated in the slid re in the nventinal manner. Then, instead f disharging int the exhaust hamber befre entering the nzzle, the prpellant is heated t ultrahigh temperatures in the uranium ar regin. An individual ar element is shwn in Figure 2. The uranium fuel is ntained in a tungsten tube that serves as ne f the eletrdes in the ar iruit. The tube is led by graphite fins in ntat with the prpellant. A uranium ar is established between the liquid uranium surfae (menisus) and a tungsten r graphite eletrde pin. (The radiatin standard fr ptial radiatin measurements perates a graphite pin at 4000 C.) The uranium ar perates at 8,000 K t 10,000 K. An end view f the array f ar elements at the aft f the reatr is shwn in Figure. The diamnd shaped spaes frmed by nneting the ling fins serve as flw hannels fr the prpellant. The nulear-augmented eletri ar serves t heat the prpellant and deter uranium lss. The heat transfer mehanism frm the ar plasma t the prpellant is via radiative transfer. In a nventinal ar, the eletrial input t the eletrdes is used t heat the eletrdes, inizatin, and elevating the temperature f the wrking fluid. letrde heating and inizatin nstitute the bulk f the input energy requirements, yet d nt lead t inreased plasma temperature. After these tw requirements have been satisfied, additinal energy, the smallest fratin f the three, is then used t inrease the plasma temperature and nverted exlusively int radiatin whih an be transferred t a send medium. Hwever, eletri field/hmi heating by itself is a very ineffiient way t heat the plasma. In the nulear-augmented uranium ar, the energy required fr heating the eletrdes and fr inizing (several ev) at the uranium surfae and in the ar, are prvided diretly by fissin fragments. This displaes the prtin f eletrial energy that des nt lead diretly t higher plasma temperature. Thus nearly all eletrial energy input, plus the fissin energy released in the plasma lumn, are used t heat the plasma lumn t ar temperatures, typially ~1 ev. Furthermre, this energy is reemitted as radiatin and an be used t heat the prpellant withut rssing slid material surfaes. The absrptin f energy by a seeded prpellant was experimentally demnstrated and reprted in Referenes and 6. Older gas re nepts relied n radiative transfer assuming a uranium plasma emitting blakbdy radiatin. This was required by the need t transfer all the reatr pwer in this manner, and the assumptin that nly the furth pwer dependene f blakbdy radiatin n temperature uld amplish this. This may have been true at the perating pressures previusly prpsed fr these devies, e.g., 500 atm. A plasma will indeed appear as a blakbdy radiatr

12 SHILD DOM ND PLNUM PRSSUR VSSL RFLCTOR OUTLT PLNUM R INLT PLNUM RFLCTOR LATRAL SUPPORT SYSTM RFLCTOR INLT PLNUM Ar Regin FLOW Figure 1. Uranium Ar Fissin Reatr (UAFR) Shemati

13 xhaust Nzzle Figure 2. Fuel lements in Ar Regin

14 xhaust Nzzle Wall Figure, Fuel-Ar lement Array at Reatr Aft.

15 at these pressures. Hwever, at the uranium pressures urrently prjeted in the ar regin, i.e., 25 t 50 arm, the plasma will be a line radiatr. In a nventinal eletri ar, lal thermdynami equilibrium (LT) an be assumed, and the line radiatin pwer density is a funtin f the plasma temperature and density f the radiating partiles. The nulear-augmented ar may prvide fr additinal nnequilibrium radiatin, i.e., nn-thermal radiatin in that the radiatin density is nt due t a temperature effet. This uranium line radiatin an be absrbed by the seeded prpellant. 2. Nulear-Augmented Thruster Cnept The inizatin aused by fissin fragments an be used t inrease the effiieny f eletri thrusters. Typially, these devies use a large fratin f their eletrial energy input t inize the prpellant befre exhaust. The fratins range frm ~10% t 15% fr lithium prpellant (inizatin ptential 5.4 ev) t ~0% t 45% fr argn (inizatin ptential 15.8 ev). The required energy and inizatin an be effiiently prvided by fissin fragments. This an be amplished by lating the thruster in lse prximity t the nulear reatr. Then, by seeding the thruster prpellant with the materials listed abve, e.g., 6 Li, 10 B, et., neutrns esaping frm the reatr re an be aptured t prvide the inizatin f the prpellant diretly. If the prpellant is lithium, the prpellant serves bth purpses f thrust and inizatin enhanement via the 6 Li(n,T) 4 He neutrn apture reatin. Lithium is a slid at earth rm temperature, and thus des nt require a refrigeratin system. It als minimizes the required arg spae. In additin, missin safety nerns f gas leakage are minimized, and peratinal reliability is inreased by aviding the refrigeratin unit. In additin t prviding the inizatin f the prpellant, fissin fragments will inrease the ndutivity f the plasma as desribed in Referenes 1, 2 and 5. Fissin fragments brn at high energy (100's f kev t MeV) transfer their energy t eletrns during slwing dwn. As these eletrns then slw dwn via llisins, there will be an inrease in the ppulatin f eletrns apable f inizing the plasma. The fissining plasma will be inized t a degree rrespnding t thse f plasmas at muh higher thermal temperatures. In effet, this dereases the temperature t whih the prpellant needs t be heated t ahieve the neessary aeleratin. The net result f the "free" inizatin prvided by neutrns that wuld have nrmally esaped, and the enhaned ndutivity f the plasma is t signifiantly inrease thruster perfrmane effiienies. 2. XPRIMNTAL ST UP The fissin reatin in the initial experiments was simulated with the 10 B(n,alpha) 7 Li reatin. xperiments were perfrmed t bserve the effet f the neutrn flux n the brn nm dublet. T amplish this, the athde pin was filled with brn and the emissin spetra f the ar were bserved. Subsequent experiments were perfrmed with uranium, and the uranium atm (U I) lines at 65. nm and 65.9 nm, and uranium in (UII) line at nm were bserved. 10

16 A helium vrtex-stabilized ar was used in the study. A shemati view f the assembly is shwn in Figure 4. Helium is injeted at an angle thrugh the bttm plate f the assembly, and exit thrugh the tp int a ath tank. The athde pin is made f arbn due t its inherent stability in ar frmatin. A bred hle in the enter allws fr the intrdutin f brn r uranium during the ar. The bttm plate thrugh whih the athde emerges is internally waterled. A tungsten ande is attahed t the internally water-led pper bak plate. The ar setin is enlsed by a ylindrial quartz envelpe, and the verall ttal length f the ar hamber is 0.1 m. Spetrspi analysis was ahieved by setting up an inherent fiber pti waveguide whih wuld transprt ptial infrmatin frm the ar t a spetrmeter lated in the ntrl rm 60 feet away. The waveguide nsisted f 64 quartz fibers f 100 mirmeter in diameter. Attenuatin in the waveguide is wavelength dependent and alibratins were perfrmed using tungsten ribbn alibratin standards. The middle f the ar plasma was fused n the fiber pti bundle. A Czerny-Turner type spetrmeter with an verall fal length f 48 inhes was used fr spetrmetri analysis. The grating used was 1190 grves per millimeter with a blazing wavelength f 500 nm. The linear dispersin was measured t be nm per millimeter spread. The entrane slit f the spetrmeter was set at 20 mirmeters. The spetrmeter uld be run bth in multiline r mnhrmatr mde. In the multiline mde, a series f waveguides were psitined at the exit plane t detet tw r mre predetermined spetral lines. In bth bservatin mdes, eah line was deteted by a pht-multiplier tube (PMT) (Hammamatsu Mdel R76) and phturrent signals were transfrmed int vltage signals using preamplifiers (Hammamatsu Mdel C105). The experiments were perfrmed at the Plasma Irradiatin Faility using the Kaman A- 711 Neutrn Generatr. The Kaman A-711 generates neutrns by means f the DT reatin whih prdues 14.7 Mev neutrns. Prper mderatin is used t thermalize the spetrum, resulting in a fast t thermal neutrn rati f 1:10. The thermal neutrn flux was measured t be 10 6 /m 2 se. The perating predure was as fllws. A neutrn flux was established in the test setin. The ar was then initiated remtely and brn emissin line intensity data were rerded fr abut 0 t 40 sends. The neutrn sure was then shut dwn while the ar ntinued peratin, and data were rerded fr anther 0 sends. The neutrn sure was then instanüy brught bak t the initial flux value, and data rerded an additinal 0 sends. The ar vltage and urrent nditins were maintained nstant thrughut the experiment. Line emissin intensities were measured with pht-multiplier tubes at the exit slits f the spetrmeter in the ntrl rm. xperiments were als perfrmed with uranium ars t measure the energy transfer frm the ar t uranium hexafluride gas in an absrptin ell. A pwer radimeter sensitive t the 11

17 T Vauum Outer Cerami -Tube letrde Penetratin Inner Cerami Tube Water ut W-Nzzle Water in Bre fr Charge f B,U Helium in Water ut ^ Water in Quartz nvelpe Carbn letrde Figure 4. Shemati Diagram f Vrtex Stabilized Ar

18 full spetrum f emitted light was arranged t detet the ttal light intensity transmitted thrugh the absrptin ell. The final aspet f feasibility examined in the urrent effrt was ntainment f the uranium. Fr these measurements, the athde pin was laded with a measured quantity f uranium. A uranium ar with a helium vrtex gas flw was perated fr a speified perid f time. The uranium remaining in the pin was weighed, and the uranium mass transfer ut f the ar was determined. 4. RSULTS AND DISCUSSION 4.1 Spetral Line missin Intensity The emissin spetra f brn, arbn, helium and uranium were measured t establish the baseline nditins. These are shwn in Figures 5a thrugh 5d. Frm the measured spetra, andidate emissin lines were seleted n whih t bserve the influene f neutrn irradiatin. An initial series f brn seeded helium ar experiments were perfrmed t determine the effet f neutrn irradiatin. The spetrmeter was arranged t detet in a mnhrmatr mde. The nm brn dublet line intensities as a funtin f neutrn irradiatin are shwn in Figure 6. Neutrn irradiatin is perfrmed during the early sequene, irradiatin is stpped during the middle sequene, and irradiatin is again perfrmed during the last part f the sequene. The figure learly shws a tw-fld inrease in the line intensity when the neutrn sure is n, bth during the initial and final irradiatins. This effet was bserved repeatedly under varius ar urrent-vltage nditins. The effet was als bserved fr the adjaent nm arbn line emitted by vaprized athde material, as shwn in Figure 7. The inreased line emissin intensity is interpreted as an inrease in the ar temperature made pssible by a mre effiient inizatin mehanism prvided by the high energy nulear reatin prduts. The nstant-urrent pwer supply ntinues t supply the same pwer t the ar during irradiatin, as shwn in Figure Plasma Temperature Measurements The temperature f the ar with and withut irradiatin was btained frm helium line rati measurements. The tehnique used is the relative line intensity temperature methd [7-9]. The tehnique is based n the measurement f line intensities and their rati gives the temperature arding t the fllwing relatinship: h=n 0 (ga/u(t)) hv e -ilkt where n 0 is the partile density f the grund state f the partiular speies, g ( is the statistial weight f the upper state, A^ is the transitin prbability fr the partiular emissin frm state i t state j, U(T) is the partitin funtin, hv is the energy f the emitted phtn, j is 1

19 H U \ r~~ ON JA Figure 5a. Spetral Line missin f Brn (249.7 nm) and Carbn (247.9 nm).

20 <u 1 1 H-( = nj 1 ) <U 0 < z n: C t= e t= l~- :C t-. CM r» IT) r> r t <T> 0 00 en CT> Ol 0 r r ** Figure 5.b shwn). Spetral Line missin fr Helium (88.8 nm and nm analytial lines

21 M H g en a\ wyu iäsf*» : *«;:» : t*>«*.s:ss:**&«-ä JSSÄÄ^sÄäÄäÄÄ HHMf g^s^gjgi I«P^#«$ $P$P^$F *^?'&l^2^*;^ j ^ ^^ ^^^^^ A }i f ' : v ^M^i^ÄÄiärJsÄ K ^?M<& ^«^P 1 ^-^l*'^^^l Figure 5. Spetral Line missin fr Uranium (65. nm and 65.9 nm lines shwn).

22 H H 6 Figure 5d. Spetral Line missin fr Uranium (417.1 nm shwn).

23 u (0 CM < < T" II X e a> z CM CM ii X li_ C ' a) z (s)iun AjeJiiqjv) A»jsueiu eun ujg IO IO es zs C IS awmm OS (0 6fr 8fr l_ h. Zfr 9» Sfr h. frfr et z fr IP «*- Ofr 66 8S It 96 SS UL te (ft ee 0 t- CM uen > s 2" 6 (0 fl) 8 Z 5 9 S? fr & 6 w O) rr CM VZ ) 81. Zl <D SI ]Q frl 61 O Q ZV H C 01 k. 6 m 8 Z (Q 9 t S k. P 6 > u_ 1.

24 ZOl 901 SOI»01. C 801 ZOl (0 k. 66 v. 86 Z6 96 S6» h Z * k. C u. (0 (0 u (0 08 6Z 0) 8Z ^m 11 ** 91 SZ h. frz a> h«i ez a> LU 2Z t-z OZ Z fr8 a G k. 09 ffl n ZS d ts D) MB u. (sjjun AjBJiiqjv) Xlisuajui aun ujg

25 CM u CM <.0 t < T" II X v. 0) z II X GZ Z i^b H 1 l L^^^^^^^^^ ^^f"" ^B ^ ^ ^ ^ ^ ^H - 1 ^^^^^ I H 1 ^H 1 ^^ 1 h^ ^H 1 H 1 mm (sjiun ÄJBJisqjv) A)!sua)u eun uqje 1 Sfr fr* e* Zfr C O Lfr Of T 6 (0 86 a- ZC C 9 k. ** se <D PZ ee z z «- 16 p=i e Ü 6Z 8Z Li. zz g 92 & CM CM CM imen iz S. C k. ** OZ UJ ^^ > LI 91 CM SI. s ' 0) PI Zl Zl C LI. n 01. 1_ 6 O 8 Z h- 9 <u k. 9

26 ü 0) CM < t < II X 2 0) z O II X 1_ O z Z » Z 8Z LL 91 SL VL ZL 0 Zg izs 01 & 89 «Z S9 uj fr S IS 9S 88 frs -88 ZS IS OS 6* 8fr Zfr 8* IB XI (0 hi IM LL -1 1 >. O 0) la O) 8 in t (sjiun Ajej«qjv) Ajjsuejui eun uqje

27 .2 (0 Q < Q. C Q. (/> k. 0) Q. (0 0) 00 D) (SHBM) JaMOd

28 the energy level f the upper state, and \ is the intensity density f the emitted line in (watts/m ). If ne takes tw lines frm the same speies, prvided that their differene in exitatin energies is large enugh t prdue the desired sensitivity, and prvided that the lines are nt self-absrbed, the rati f the intensities f the tw lines is given by: ^_v 2 5r 2^. 2/exp( _-^ A Vl^l^l-l' kt frm whih a temperature an be derived. The lines hsen fr the temperature measurement were the nm and the 88.8 nm helium lines. The graph shwing the sensitivity f this line in the range between 5000 K and 20,000 K is shwn in Figure 9. The temperatures during the brn test runs are shwn in Figure 10, and the uranium run temperatures are shwn in Figure 11. The results shw that the brn ar temperature inreased frm ~9,500 K withut irradiatin, t ~ 11,500 K during irradiatin with a neutrn flux f 10 6 n/m 2 se. This is a 20% inrease in plasma temperature. The uranium ar run temperatures inreased frm ~8,100 K t ~9,00 K, an inrease f 15%. The inrease in ar temperature under the influene f neutrn irradiatin an be explained thrugh an energy balane. An ar, as a rule is a self-sustaining disharge with a relatively lw athde fall ptential (f the rder f the inizatin ptential f atms, abut 10 ev). This small athde fall results frm athde emissin mehanisms that are apable f supplying a urrent whih is indeed the ttal ar urrent. Ar athdes emit eletrns frm presses suh as thermini, field, and thermini field emissin, et. Ar athdes use a large fratin f the eletrial energy frm the urrent t reah a suffiiently high temperature, either ver the entire area r ver a small lalized area. In priniple, the athde fall (r athde layer), reates nditins fr the selfsustainment f the urrent. The number f pairs f harges prdued in the athde layers f an ar is suh that their reprdutin is ensured by sendary emissin due t thermini influx. In the ase f thermini emissin the eletrns that are emitted reate suffiient inizatin t maintain a self-sustained disharge. When the brn filled athde is expsed t neutrns, fissining f the brn atm prdues tw highly inizing partiles - He 4 and Li 7 with a psitive Q value f 2.79 MeV. Uranium fissin prdues inizing harged partiles with a ttal kineti energy f ~180 MeV. This energy is then used t inize the plasma in the ar prper replaing the energy that ne was needed t heat the athde t prdue eletrns fr ar self-maintenane. Sine the pwer supply maintains a fixed urrent, as shwn in Figure 8, the exess energy then ges 2

29 CM O (0 1 X a> g a Ö </> k_ <*- ** (0 a ** 4) DC 1- (0 T" Q) C i» O (888C/600fr) ijeu aun uim. H

30 (X) ajnjbjaduiai

31 m O CM CD a> O" 0> 00 W i_ 0) a I- (0 (0 a Q. < MM (0 a» O K ^ ** a X UJ «*- t UJ (0 u> «CM (X) 9jn}ejadiuei

32 int heating and thus raising the temperature f the plasma. Thus, bth the uranium, brn and arbn line intensities were bserved t inrease rrespndingly. The uranium atm and in densities an be btained frm the measured line intensities, the helium line rati temperature measurements, and the abve equatins. These values an be mpared t the densities btained by assuming thermal equilibrium and using Saha equatins. This methdlgy is desribed in Referene 7. The results are shwn in Figure 12. The alulated densities fr the uranium ar runs withut irradiatin, based n the measured line intensities, are shwn as the data pints with the apprpriate errr bars. The urves represent the results f Saha equilibrium alulatins fr the range f uranium pressures during the experiments. The results shw that the data lie in the regin braketed by the Saha urves. This prvides a gd benhmark hek f the measurements and alulatin^ methdlgies. 4. letrial Cndutivity Figure 1 shws the VI harateristis fr uranium ars with and withut neutrn irradiatin. The VI urve withut irradiatin is mparable t that btained in Referene 9. The perating nditins suh as pressure and helium flw rates were the same fr experiments with and withut irradiatin. Figure 1 shws that fr a given vltage, a fissining plasma will supprt a larger urrent. This is equivalent t a derease in the plasma resistane, i.e., an inrease in the ndutivity, and is nsistent with the general findings in Referenes 1 and 2. The temperature in the urrent experiments was ~8,000 K, mpared t ~2,500 K in Referenes 1 and 2. Thus, the fratinal inrease an be expeted t be lwer in the urrent experiments. The measured uranium ar ndutivities derived frm VI harateristis are shwn in Figure 14. Fr a given plasma ndutivity, a fissining plasma an supprt mre pwer beause it an mre effiiently uple it int the ar and thus radiate it away. At a given pwer level hwever, a fissining plasma will require a signifiantly lwer temperature t balane eletrial pwer input and radiated pwer utput. Due t the lwer temperature at a given pwer level, its ndutivity will be rrespndingly lwer. 4.4 Radiative nergy Absrptin and Mass Transfer T ahieve ultrahigh temperatures, ptially thin prpellants suh as hydrgen will need t be seeded with material whih will absrb and transfer ar-emitted radiatin. Fr this reasn, the next series f experiments were perfrmed t investigate if the ptial radiatin emitted by the ar plasma uld be absrbed and thus used t inrease the prpellant temperature. T amplish this, the same experimental setup was used as desribed abve, the ar perated with uranium, and ttal radiatin intensity passing thrugh a test ell was measured with a radimeter. The radimeter measures the ttal spetrum f light inident n its 26

33 (Cvui/j.) Ajjsuaa i)jed

34 1 < i <. 1 " II Ü e s s Cft O z z < < -5 i < # «>! 1 i i IO > Ü «I a> (0 k. (B O 0) a. > < : 1 / ' Q> k. O) IBS u. < II 4 «u> K (sajaduiv) luajjn

35 (0 U hm < C (0 > C Ü (0 u u D u. (ui/qui) AiiAjjnpu

36 phtell, in ntrast t the single line spetrmeter measurements desribed abve. T simulate the maximum absrptin ptential f seed material, a UF 6 absrptin test ell was plaed between the ar and the radimeter, and the uranium partial pressure in the test ell was varied. The results f the transmissivity measurements with varying UF 6 partile pressure in the absrptin ell are shwn in Figure 15. The UF 6 partile density in the absrptin ell was btained by mnitring the temperature and pressure in the reservir. The temperature was maintained in the range f 50 K t 60 K, while the UF 6 pressure was inreased frm vauum (1-2 trr) t 1 atm. The figure shws an inverse dependene f transmissivity n the UF 6 partile density. At a partile density f 2 x m" (1 atm and 60 K) and an ptial path length f 10 m, the UF 6 absrbed 90% f the emitted light One f the key aspets f the uranium ar fissin reatr is the ability t minimize fuel lss frm the reatr via the eletri field nfinement. Data frm the uranium mass flw rate experiments are presented in Figure 16. The experiments were perfrmed fr a range f urrents frm 20 t 0 amp. In additin, the physial nfiguratin f the uranium within the athde pin was als examined. Thus, the uranium was inserted at varius depths int the pin t examine the impat n fuel lss rate. Due t limitatins in the spe f the effrt, ptimizatin f perating parameters uld nt be perfrmed. Hwever, Figure 16 indiates that the mass lss rate frm the eletrde an be dereased t very lw levels (few mg/m 2 s), by prper eletrde design. Uranium reirulatin and revery was als perfrmed by perating the uranium pin as the athde. In these experiments, uranium atms evaprating frm the pl surfae are returned t the pl by the effet f the eletri field. Uranium lss frm the eletrdes uld thus ndense n ld surfaes near the eletrde within the ar assembly. Thus, uranium either exhausted with the helium, r was revered within the ar assembly. The results f these experiments are shwn in Table 1. The results shw that up t 50% f the lst uranium an be revered in this manner. Table 1. Summary Uranium Reirulatin and Revery xperiments. xperiment # 1 xperiment # 2 Average Insertin Distane (mm) Ttal Lss (grs) 0.220(100%) 0.91(100%) Lsses thrugh Helium Flw (grs) 0.19(88%) 0.179(46%) Reirulated Uranium Revered (grs) 0.026(12%) 0.211(54%) 29

37 CM 0) C Q a> mmm O ^ i +* i- Q. e «(/> > u ^^» > i e» > > ** (0. O M " C (0 «0> Q (0 0> u (0 «rf k H (O LL t CD v. O) M LL AijAfssiiusueji

38 U) 0) 0) 0) > a> k. k. k. <u 0) Q. Q. Q. < < < in CM CM CM * *» ** C d> <u a> i_ k. a_ _ k. k> Ü a U u u k. k> k. < < < O < - T CM O Ü a Q tn (0 > tn ^ i i C => «*- r <D (0 J CD (0 u IÜ. < O) A T" (09S iu/ ßiu) ssi xny sse /\ lumuejn CM

39 5. NCLUSIONS The basi parameters supprting the Uranium Ar Fissin Reatr (UAFR) nept were measured with enuraging results. This new nept fr nulear spae pwer and prpulsin bsts prpellant exhaust temperatures via rapid radiative energy transfer at ultrahigh temperatures withut requiring slid heat transfer surfaes. A new effet was bserved supprting the transfer f energy int an ar plasma. In this effet, the line intensity emitted by an ar plasma ntaining r supprted by fissile material was inreased by neutrn irradiatin. A preliminary explanatin is that this is interpreted as a plasma temperature rise by the mre effiient inizatin mehanism prvided by the fast, harged fissin fragments. The spetrspially measured temperature inreased frm 8,100 K t 9,400 K (15%) with uranium ars, and frm 9,500 K t 11,500 K (20%) with brn ars. This new effet was bserved at surprisingly lw neutrn fluxes. The results are unexpeted beause the nulear energy released des nt explain the alulated enthalpy rise f the plasma. Thugh a detailed explanatin has nt been develped, the effet ptentially results frm the high inizatin effiieny f high energy harged partiles mpared t eletri field/hmi heating, thus allwing mre effiient use f the eletrial energy available t the ar fr raising plasma temperature. This effet an be applied in eletri thrusters t redue the eletrial pwer requirements needed fr inizatin f the prpellant. The researh effrt yielded new insight int the interatin f MeV fissin fragments with ev plasmas. The next level f researh will need higher fluxes and mre detailed measurements t establish a theretial basis. The ptial radiatin emitted by the ar plasma an be absrbed by an external medium. At a partile density f 2 x m' (1 atm and 60 K) and an ptial path length f 10 m, 90% f the emitted light an be absrbed. Mass lss rates frm the eletrdes an be redued t a few mg/m 2 s, a large fratin f whih an furthermre be revered. The experimental results indiate the fundamental feasibility f uranium ar driven energy transfer. Fr thermal prpulsin, inreased temperatures mean higher speifi impulse thrust. Fr spae pwer platfrms, the resultant imprved effiieny means redued and easier waste heat rejetin and thus a mre mpat, lwer mass reatr design. A mre effiient thruster als results in lwer pwer requirements r redued radiatr area and thus mass. The tehnlgy is als appliable fr mre effiient and safer grund-based eletri pwer generatin. A spinff develpment is the tehnlgy fr handling uranium and its mpunds in liquid and vapr frms. This tehnlgy will be required in advaned Gas and Vapr Cre Reatrs. 6. RFRNCS 1. Shneider, R.T., "Neutrn Indued Nn-Thermal Inizatin fr Nulear MHD Pwer Generatin," Seventh Sympsium n Spae Nulear Pwer Systems, January,

40 2. Shneider, R.T., I. Maya, and J. Vitali, "Nulear Driven MHD Pwer Generatin," RTS Labratries Final Reprt, Alahua, FL, Marh, Diaz, N.J., and I. Maya, "Ultrahigh Temperature nergy Transprt fr Spae Pwer and Prpulsin," Innvative Nulear Spae Pwer and Prpulsin Institute Final Reprt, INSPI-TR , September Watanabe, Y., et. al, "Uranium Ar Fissin Reatr fr Spae Prpulsin," Preedings f the 8th Sympsium n Spae Nulear Pwer Systems, Albuquerque, NM, January Maya, I., Y. Watanabe, J. Appelbaum, N.J. Diaz, and J. Vitali, "Fissining Plasma Physis," INSPI , Vl. 5, Innvative Nulear Spae Pwer and Prpulsin Institute Reprt, Otber, Williams, J.R., W.L. Partain, A.S. Sheny, and J.D. Clement, "Thermal Radiatin Absrptin by Partile-Seeded Gases," J. Spaeraft, 8(1971) Griem, H. R., Plasma Spetrspy, MGraw Hill, Gartn, W. R. S., "Survey f Reent Advanes in he Spetrspy f the Vauum Ultravilet," Preedings f the Fifth internatinal Cnferene n Inizatin Phenmena in Gases, Cl H, p. 1884, Randl, A. G., "A Determinatin f High Pressure, High Temperature, Uranium Plasma Prperties," Ph. D. Dissertatin, University f Flrida, 1969.