Report No ..UNCLASSIFIED
*■’ Rm c Wm - Sp* ( MHHltii WOMt «k| m RADIATION EFFECTS ON BORON-CONTAINING COMPOUNDS by Donald J. Hamman Paul Schell \-JC 1$ ±Ji. i lkfc^ L.nso». **• u January 6, I960 RESTRICTED DATA - Thu document 4au m in the Atomic fneegy Act of U6*. UUl ot 4l»rfc*urc of lu content! in any iQtuittt uthovlMd porton u pcahibltrU. 4 BATTELLE MEMORIAL INSTITUTE 505 King Avenue Ceilumbu« 1, Ohio confidential • • • • • UNCIASSIFIFD •• • • • 0• I • •m • • • e • CONFIDENTIAL Jane! 4 TABLE O f CONTENTS Page ABSTRACT...... 5 INTRODUCTION...... 5 EXPERIMENTAL PR O C E D U R E S...... 5 Selection of Materials ...... 5 Irradiation C onditions...... S Prcirradiatlon and Poatlrradiation Examinations ...... IS R E SU L T S...... 16 Helium R e l e a s e ...... 16 Motallographic E xam ination...... 21 CONCLUSIONS...... 25 R E F E R E N C E S ...... 25 CONFIDENTIAL • • •• t e es CONFIDENTIAL 5 RADIATION EFFECTS ON BORON CONTAINING COMPOUNDS Don*Id J. H inunin and Paul Schall f /ruMlW rarkide. aud iifrMUM dihartde were irradiated at reaetor amtuent lemprraiurt (ISO F t mud elevated Irmprrulurt a (MO to ?S0 F) to kmmafi r « |M | /»©» 20 to 95 per rial • / lAr boreulO iaatmpa /irrM im dtkmtide toaiainiug hath MtorW m W enriched ametMt af baron JO «hm evaluated The amount of helium releeued during irradiate* mo* mtaruttd, and X-eay diffractio*. mtialloftapkic, mod tkamtrmi eiamtaaitoai va n for form od hath kaforr amd after irradiation. 7ir tout urn dtbonde root atuiug natural hotou appeared to be tke mo*t p ro w 1*4 material from a keliumreteuho* il—dpomt. hone of tke material* eakikited a*ee atioe par heir fragmentation or dimeuaionml tkaugea dunug ike taur*e of ike etpenment* even though there mas evidente of temperature* math higher than dettgu cord it tout \ . INTRODUCTION Since helium ie produced during the irradiation of boron, the ability of the mate rial to retain this helium it of prime importance. Also important are the dlmon- •ional stability and all other properties consistent with reactor operating conditions. The program reported here on boron-containing compound* was fr>ng oT several to aesistjfhrknolls Atomic ^ower Laboratory )in the selection of alternate control ma- teriaUJand was supported by the Naval Reactor Branch of the United States Atomic) i Energy Commission* In the program, elemental boron, boron carbide, and airconium cfrWride were irradiated in the MTR at reactor ambient temperature (150 F) and elevated temperatures (500 to 750 F) to burnupe ranging from 20 to 95 per cent of the boron-10 isotope. Two capsules containing hafnium diboride are etill under irradiatiot at the ETR. EXPERIMENTAL PROCEDURES Selection of Materials The material# used for this program were powdere of known meeh site. This form was chosen to eliminate limitations imposed by dispersions,• compacts, etc., or specific geometries of control-rod construction. Selection of the materials to be studied during the program wss based on several factors. First, they were chosen to be representative of the most likely forms of CONFIDENTIAL L CONFIDENTIAL $ t>oron-containing material* which would bo confide rod (or control malarial* on tha bant of chemical and physical pro partial, The malarial* **iactad w*r* alamantal boron, boron carblda, and matal boride* of both th* diboride and hexaborid* typ**, Other m aul boride* such *• tha monoboridei, tstraborldes, and dodecaboridea are known but ar* usually harder to prapara and ara relatively unstable compared with the diboride* and hexaboridas. Tha matal borides selected were alrconlum and hafnium diboride* and dysprosium and europium Hexaborides. Of these materials, data have bean obtained only on the slrconiuin diboride. The hafnium diboride is currently being irradiated, and effort on the hexaborides hae been limited to preparation of eetiefactory materials. Several different crystal structures are represented by the materials selected for thie program. The elemental boron is known to exist in an amorphous gftjjf™ end in two (hexagonal and tetragonal) crystalline states^). Therefore, boron might undergo changes in either crystal structure or degree of crysUllinlty during irradiation as the result of displacements of boron atoms and the production of lithium and helium. How ever, boron is quits stable over a wide range of lattice imperfections, and this property may denote reasonably good radiation stability. Preliminary experiments indicated that boron carbide was between elemental boron and the metal boridee with respect to expected structural radiation stability. The D4C crystal^) structure or B ^C ) consists of a linear chain of 3 carbon atoms with 12 boron at^ms at the points of a nearly regular icosahedron and tha whole arranged in * a rhombohedral structure. The matal diboridee and metal hexaborides normally have separate and distinct * crystalline structures of good stability. The two tyjpee of structure vary considerably. The diboride* have a primitive hexagonal structured similar to graphite in that they are layered with the boron and metal forming alternate layers. Th* hexaborid*• form a complex structured in which the six boron atoms are at the points of a regular octahedron, and each octahedron i* linked to six other octahadra, forming s continuous network of cubic symmetry with the metal atoms In the interstices. Radiation-stability differences are suggested by the large volume of the hexaborid# octahedra, which may effectively contain the lithium or helium atoms. If there are dimensional changes in the crystal structure, they would be expected to be anisotropic in the case of the dl- borides and isotropic in the hexaborides. Nuclear properties were also considered In the selection of the compounds to be studied. Elemental boron has an absorption cross section of 750 barns for the neu trons having a velocity of 2200 m per sec, and, being a 1/V absorber, ie effective over a fairly wide neutron-energy range. By using a compound of boron in which the second component la also a strong absorber, especially in th-, resonance-energy region, its effectivsness as a control material can be significantly improved. On this basis hafnium diboride was selected, and tho hexaborides chossn were those of dysprosium and europium. The results obtained for several boron-containing materials on helium release during irradiation and during postirradiation heat treatment and on structural stability (1) Reference! it end of text. CONFIDENTIAL • • • ... • • 1*. • «. .* • • • • • • • • • • • • • •. r • • .« • >* ...... • • • . ... • • * ■ 1 « . • ... • • . • » | . • • • • . ■ .. CONFIDENTIAL T « • w a te r mined by meuUo graphic examination and X-ray diffraction arc presented in the following diacusaion. The experimental program consisted oI tha irradiation of two aariaa of c epaulet fiat TabU 1) and pra irradiation and pot tir radiation a nomination a. Thar a war a ala capsules in tha fir at aariaa with two aach containing alamantal boron, boron carbide, and airconium diborida. Tha aacond tarlaa conaiatad of two capaulaa of airconiiam diborida containing natural boron, ona capaula of airconium diborida conUlating boron anrichad SO par cant in tha boron-10 taotopa, and ona of boron carbida containing natural boron. TABLE l . LIST OF CAPSULES Number Target Design of Boron Temperature, Cycles Where Burnup, Capsule C o n te n ts^ F Irradiated ]Irradiated per cent Thermocouple BMI-7-1 B(N) ISO 1 MTR 70 No HMI-7-2 B(N) ISO ) MTR 90 No BMI-7-J Z rB 2 (N) 150 1 MTR 70 No BMI-7-4 Z rB 2 (N) 150 1 MTR 90 No 11 MI-7-5 B4C (N) 150 1 MTR 70 No BMl-7-6 B4C (N) ISO 1 MTR 90 No BMI-7-14 Z rB 2 (N) 150 a MTR SO No BM1-7-15 Z rB 2 (E) 500-750 16 MTR 60 Yea BMI-7-16 Z rB 2 (N) 500-750 19 MTR 80 No BM1-7-17 b 4c (N) 500-750 20 MTR so No BMI-7-20 HfB2 (E) 500-750 19(h) ETR 90 Yea BMI-7-21 HfB2 (N) 500-750 19(b) ETR 90 No (a) (N) denote I natural baton, (t) denote! boron enriched in the baon-10 tiotope. (b) There ceptulci are m il being Inaduud. The aeriaa of six capaulaa was intended to provide data on tha comparative radia tion stability of tha three material*, and the aariaa of four waa intended to provide more complete data on tha stability during irradiation at elevated temperatures of boron carbide and airconium diborida. In addition to theae there are two capsules of hafnium diboride, one containing natural boron and the other containing boron enriched to SS. 6 per cant in the boron-10 isotope, being irradiated in the ETR. , CONFIDENTIAL co*ru>t>nt t trro4l*«un» t . | jj| ? 3 | AU (omj»4*4i »«r« lrr«4Ui*4 la c epaulet H |»ifvU« • |t»n tir» a*«h » that iMrinMm uniformity of W r« |fia U N ebiainod M i KiU |r«vti» enough im ittut (or peetlr radiation »umui«!U»«, Tt»* cevitie* u.u> wll(l> the *#« bo4*4 had • depth mi 0.01$ u ., m i —linliitoo 4 ladlaatad dot Mo n » (iy io ^ m M ail pto» duc« M U*ititl4ui| •(lieu in #«(«•• oI (0 par coot from the p N tr eorlate t* the pew* dor canter. Theet ukuU uow era probably mo boCUv lion oil per cob lovert* c Kongo a took place in iKo trrad4ation*c*pauU detign oa the (aaiUlaai of irradiation progrooood (roan unKinl temperature «o elevated temperature* maintained by gamma hooting m 4 the location of tha irradiation* chan go 4 (rom tho MTR to the ITK Figures I aid l show tho doolgn 4rawing and photograph, respectively, of tKa cipawla components lor tho (trot aorioo oi tlx capsules doaignotod oa BUI*I*I Ihrf^fh DM1* 7-4. Tha powder* oraro contained In tKo lunar aluminum holder which *a* ****** ttally a apllt cylinder Kol4 together by two drive collar* . TKo cavttlaa were 0.01$ in deep* with tho oval croaa taction bomg 0.4 In. long by 0. II la. wide, Tboao capaulaa oar* not ovacuatod. Thoy oporatod at a maairnum tomparaturo tilw la b i to bf 100 F In tha MTR. Tho design drawing and photograph at compononta lor capsules daaignatod to UM1- 7 -1A through © Ml IT art shown In Figures $ and 4, respectively, The Innar rora oI theta capaulaa waa coppor and waa hold In place by copper * up* no ton coupling! which oloo provided an adjustable boat loak. Tha to capsule« war* daalgnad $o oporata in tho tango ol $00 to 7$0 f in lh« MTH hy uao ol gamma hotting, and tboao tempera* turoa war# maintalnod by evacuating tha annulw* botwoan tho innar and outor cylinder*. Aa tho drawing in Figaro 1 shows, Uio powder cavities in tbaao capaulaa wort changed to 0. $0 by 0.90 In. Tha original 0.0l$* in. dopth dimonaton wo* ratal nod. Tho doolgn drawing and photograph ol component# ol Copauloa ft Ml-7-10 and It Ml - 7 - z I, containing HIBg ($S. 4 par cant anrichod la boron* 10) and MIBg (natural), roepoctlvalyi and currently being irradiated In tha ETR, are ahowa in Figures land 4. In thaea capaulaa tho innar core la again aluminum to accommodate the hlghor gamma hoating in tha ETR. and a bolt through tho cantor ol the Innar cor a provtdaa additional clamping action lor botttr powdor rotantlon. Thoaa capaulaa woro alao daalgnad to oparato In tha ranga ol $00 to 710 F. Two ol tho elevated-temperature capaulaa wort llttod with tharmocouplaa ao notad in Tablo I to provide a contlnuoua monitor el tho Irradiation lomporaturo. Aa la diacuaaad later, tha dtiign tamporaturai wara exceeded during at Waal part ol tha irradiation*. All of tha capaulaa wara daalgnad to accommodate a recoil plate neat to tha boron compound powdor. Tho malarial used aa rocoll plates varied with the di( • larant capaulaa. ) CONFIDENTIAL • • » • • •• • : • • ♦ ji :n • »*• : I • :: : : • * * • . • 4MMI O 8 r r *: r I ::r> * m*«i »»>**»>*r»8 8 9>»M *»**■»< noun* i m^ioh or cahujmbi aut ?«* imoutut ami t * CONFIDENTIAL Assembled unit Retaining j y s k mring if * > Sealing nut tmmt -l I wII End plug Recoil plate Powder half Opposite half of core of core ■ Retaining ring Outer shell N60310 FIGURE 2. TYPICAL COMPONENTS FOR CAPSULES BMI-7-1 THROUGH BMI-7-6 CONFIDENTIAL #• • CONFIDENTIAL ) Section 0*0 Section A Month A ond Mid thhttd (Irtl Ihowtt It tamo tho o» obovt) (Romolndor of COpMilt not (Romolndor of COpMilt Copsulo ThormocoupiM Without -— Copwlt Thormocotipltf With SocttuT C-C SocttuT Section Section AA FIGURE 3. DESIGN OF CAPSULES 14 *3MI-7- THROUGH DMI-7-17 Section Section B-B W*4 ond mochtno tmooth both Ofidt CONFIDENTIAL CONFIDENTIAL Coupling Expansion mm coupling SI* m 1 H1 L 1IB g S-S v - Dosimeter \ ■ i i f i 1 "-'1 Powder hoi of core half of core Lock nut Expansion _ coupling v Lock screws End plug N37791 FIGURE 4. TYPICAL COMPONENTS FOR CAPSULES BMI-7-14 THROUGH BMI-7-17 CONFIDENTIAL DENTI L IA T N E ID F N O C Steflon A* A SetilM B*B Stef Ion 0*0 « MM Hour lot <*oMm s Htti m Um tpocHM FIGURE 5. DESIGN OF CAPSULES BMI-7-20 AND BMI-7-2I * CONFIDENTIAL Bolton Ptug^ Oufor Shod N55054 FIGURE 6. TYPICAL COMPONENTS FOR CAPSULES BMI-7-20 AND BMI-7-21 CONFIDENTIAL CONFIDENTIAL Pretrrsdlation and Post irradiation Examinations The examination of the unlrradUte 1 powders consisted oft (1) Chemical and aptctfographlc analyse* to determine purity and composition | (2) X-ray diffraction examination (M Metallographlc axa mi nation. The postirradiation examination consisted of: (1) Recovery and analysis of |aa released during irradiation (2) Recovery and analyala of gaa ralaaaed during haat treatment (3) X-ray diffraction examination (4) Metallographlc examination (5) Cobalt dosimeter-wire analysis (6) Analysis for lithium to determine burnup of boron* 10 (7) Analysis for retained helium to provide additional check on burnup. The last three items provide separate means for the determination of the amount of boron-10 burnup. The cobalt-wire technique is considered inferior to the direct analyala for lithium because of the probable 25 per cent uncertainty in calculation of flux depression and attenuation in tne powders. Ths initial analyais for rstainod helium was accomplished by measuring the amount of helium released after hasting the powders at 2200 F until no additional gaa raises# could be detected. This tempersture (2200 F) was the maximum attainable with the apparatus used. Since it was not cer tain that all of the retained helium was released during this treatment, a procedure was developed which involved dissolution of the airconlum diboride powder in molten iron. Since the diboride dissolved in molten iron, fusion of the syetem could be accomplished at a lower temperature than the melting point of the boride powdtr. The flame spectrophotometric method waa used to accomplish the lithium analy sis using dissolved powder samples. This method was preferred over ths analysis for helium since it was assumed that all the helium was not rtcovered. This was partially verified by the fact that the helium analyeia gave lowsr values of burnup than the lithium analyeia in all cases. Accuracy within *0.5 per cant on analysis of simulated samples of dissolved powder containing known quantities of lithium showed this method to be quite reliable. All of the burnup values quoted in the following diacuanion are based on the lithium analysis with the exception of the material from Capsule n BMI-7-17 which was too radioactive for lithium analysis. CONFIDENTIAL M 0 ••• • # » « • * • i • • • 0 • I M II cowrtPKNnAi io RMUUI Helium Ki Im m The results on helium release during irradiation and posttrradiaUon heat treat ment toy the first tin capsules designated as Capsules BM1-7-I through B Ml-7-4 and eeetatnlng boron, boron carbide, and eircontum d&bortde aie iummtru«4 In Table t. TAltLE 1. HELIUM RELEASE FROM IRRADIATED BORIDES IN THE FIRST SERIES O f CAPSULES P a r ti c le B u rn u p of He Bum-He lease Data!*) i . j g t j - 1- l i a s , B o r o n - 10, As 1 Week at 2 W ee k s a C a p s u le C o n te n ts U . 1 . m e s h p e r cen t R eceived^ TOO f<«> 700 B M I-7 -1 II •6 0 *«0 10 1 < 0 ,0 1 . 1 .0 1 .4 B M 1 -7 -1 n : -6 0 4 6 0 64 1 4 .1 1 .1 y B M I-7 -J E r B g -1 0 460 17 < 6 .0 «• < 0 .4 B MI • 7 - 4 E r B . • -JO 4 6 0 41 < 0 .1 0 .0 2 < 0 .0 1 B M I-7 -1 BgC •9 0 4 6 0 21 < 0 .1 < 0 .0 2 < 0 .0 1 B M 1 -7 -6 BgC -SO 4 6 0 66 p S .6 •» B <•) N m i ol m u ! M on « gi*M Wwf, (b Cipmle •• Mc«fM hum Uh MT» Mi po m or I « mm* M ifriM i, (c) Ibetsd m On t4cwM W W« DM* l # ef KM(«r y e»*t. Tho capsules of lower burnup of each material n rrt Irradiated for one MTR cycle end (he higher burmtpe for three MTR cyclee. The irradiation temperature of all six capeulee was reactor ambient, the materiel from Cepeule BMI-7-6 wee exposed to moieture during irradiation and eo wee not subjected to heat-treftment etudiee. Due to the urgency of theee irradiations, chemical analyeea and X-ray diffrac tion examinations ears not competed prior to encapeulation. Later examination showod that the boron had poor crystallinity. The reeults obtained, therefore, may not be indicative of the helium-retention capabUitiea of good crystalline material. Chemical and spectroscopic analytes, X-ray diffraction studios, and metal- lographic examinations of the aircoaium diboride powder showed that the material had a alrcontum-to-boron ratio of approximately unity and that the powder co:«aiated of a ■irconium phase and a xirconium diboride phase. The distribution of the phaees was such that it is not expected that the xirconium phase had any appraciable effect on the retention of helium and that the reeulta were probably representative of the helium- * retention capabilities of the xirconium diborlde. / The second series of Irradiations included one capsule of xirconium dlboridc irradiated at reactor ambient temperature to serve as a check on the previous data, ' SStX&KBXlftk o m o |>imU el ttrcMlvm dibertde (rnll» TABLE S, HELIUM RELEASE DURING IRRADIATION OF «K)KU)U IK riUt SECOND SERIES O f CAPSULES Helium Release Irradiation B erea-10 During Irradiation, Temperature, ttu r n u p , per tettl ef C s s f c t a u M C a p s u le d . l i w p t r f e n t helium produced n MI. 7- 14 ZfBg(N) n o 47,1 0.6 BMI-7-15 ZrBg (E) 500 5 1 .0 57.0 BM1-7-16 ZrBg (N) 500 •4.7 1.1 BM1-7-I7 B4C (N) 500 • 4 (4) use, UM« M plw IOO mmOu (N) 4mm* n « « *1 two*. (I) 4«m « Mvw mi Cm femm- I* t**np«. I Tho 500 F temperatures given for Iho last throo capsules in Table 5 irt the nominal <1« sign temperatures. At mentioned earlier tho temperatuie* were to bo mo In to l nod by raoctor gamma haot. Capsule BMI-7-15 was fitted with thermocouples and is known to havt maintained o temperature in tha range of 475 to 525 F during tho firot MTU cycle, However, tho thormocoviplao wore Inadvertently cut at thl* tima ond the comploto temperature hiatory la not ovollabio. Postirradiation examination of tho interior of tha capauiaa revealod reaction* between aavoral of tho dieeimiiar metal* of tho inner capsule, cobalt-aluminum doeimator wires, and recoil plates. From tho observed conditions in tho capsule in teriors and tho known phase diagram* of tho various metallic systems, it was con cluded that tho following maximum temperature* were probably attained during at least part of tho irradiation time: (1) BMI-7-15: close to but not exceeding about 1000 F (2) BMI-7-16: in tho range 1000 to 1200 F » O) BMI-7-17 : In the range 1440 to 1760 F. . Several important points may be noted from the data in Table 5. Natural ZrB^ exhibits good helium retention even under elevated-temperature Irradiation. However, ' the enriched ZrBg exhibit* poor helium retention compared with natural material. It should be noted that for both the enriched and natural ZrBg irradiated at elevated CONFIDENTIAL • *• •• » ♦ • •• * • *• • *• • ••• • • • • • * •#• - » ... • ••• ••• i •* • •• ••• • • MMaMRfHMfcMI T l I CONFIDENTIAL temperature, the burnup* based on total atomic per cent boron are about equal (16.5 and i*).1* per rent, respectively). Furthermore, all evidence from poatirradiaUon ex amination *»f ’he capsule indicates that the natural material attained a higher maximum temperature during irradiation than did the enriched material. This effect of enrich ment is in agreement with the results of experiments on dispersions of enriched and natural dihoridr • • i; j . .r.ds.^) Some of the difference in helium retention of enriched and natural material might b«* attributed to the higher surface burnup attained for the enriched material for the same average burnup. However, variations in burnup as a function of depth calcu lated for the 15-mil layer of powders indicate that this effect cannot by itself account Inr th« largi* observed difference in helium retention. '• M r . " v - j ' . ' •' J g _ .... ’V | g y p . . «§S|§|.'. r ' " x - p ? 1- On the basis of the data tn Table 1, nb firm conclusion can be drawn with rugard ] to the relative merit* of D4 C and ZrB^ since the B^C apparently attained a much higher temperature than any of the ZrB^ specimens. Postirradiation heat treatments of /. rR» indic ate a "threshold" temperature of approximately 1000 F beyond which the helium rili4il rsts rase rapidly. 1 he results of post!rradiation heat treatment of ZrB^ (natural) from Capsules BMI - 7 * 14 and BMl-7-16 are shown in Table 4. The heat treatment for Capsule BMI-7* 14 w.is conducted under vacuum, and the gas samples were taken by periodically removing gas-sampling bulbs from a manifold external to the furnace. This apparatus f did not permit the instantaneous determination of gas-release rate. Observations dur ing th« course of the heat treatment of Capsule BMI-7-16 indicated that the gas was rfHeasori, primarily, in bursts corresponding to microcrack formation, particularly during periods of increasing temperature (sec Figure 7). The gas analyses from Crtpsulc Phil-7-14 showed no indication of any gas other than helium. The apparatus used for Capsule BMI-7-16 was modified to permit essentially continuous observation of the gts pressure, hut the gas was sampled only at the end of the heat treatment tint :« it was assumed from the previous capsule that no gas other than helium was evolved. Contrary to previous results, analysis of the gas showed that the majority of the gas was not helium. Hydrogen was the major constituent. Helium was present to the extent of 10 volume per rent, and the values given in Table 4 assume that the he Uum was a constant fraction of the total gas released. The "threshold" temperature is again evident in the heat treatment of Capsule BMl-7-16, although the actual tem perature is lower than In C apsule BMI-7-14. There was no thermal cycling during the t nurse of these neat treatments. Heat treatments were not made on the enriched ZrB^ from Capsule BMI-7-15 due t«> la< k of Sufficient powder or from the D4 C (natural boron) from Capsule BMI -7 17 h«?t .mu' of tin* highly radioactive state of the powder. The X ray data obtained on materials from Capsules BMI-7-14, BMI-7-15, and BMI-7- lb are tumiliAhlsd in Table 5. CONFIDENTIAL • 4» •• « I • • * « • • * S t • • • • • • • • • s ♦ * • • 4 • » n * • 4 * •• *• CONFIDENTIAL 19 TABLE 4. POST IRRADIATION GAS RELEASE ON ErBfc CONTAINING NATURAL BORON IN CAPSULES BMI-7-14 AND BMi-7-16 Temperature, Length of 1 rratment, Helium Recove red' *), Capsule • F hr per cent BMI-7-14 Room .. 0.6 500 72 0.04. 750 140 0.9 1000 196 1.2 1200 143 10.0 1400 96.5 6.0 1600 90. 25 It; 1 BMI-7-16 Room • • V 2 500 95 2; 3 750 96 1 6 1000 71 1.4 1200 71 14 | 1400 a<*> IB. f> («) Figuici given are pet cent ol total helium prodded in the tatnplr bated m lithium anal)tii. (b) T«M halted it thit point due to equipment lailute. TABLE 5. RESULTS OF X-RAY DIFFRACTION EXAMINATIONS OF ZIRCONIUM DIBORIDE tRRADlAIED IN CAPSULES BMI-7-14, HM1-7-I5. ANP B Ml - « • 1 BMI -7-14 BMI-7 - 15 13 Ml - 7 - l' (ZrBz, Natural) (Zr B2. Enriched) (Zrl3:> Natural) Prelrra- Poatirra- Preirra- Poatir ra- Preirra- Poatirra- diation diction diation diation diation diation Unit-Cell Dimension, A ao 3. 155 3. 220 3. 155 3. 1 S 3 155 3.218 Co 3. 508 3.493 3. 508 3.496 3.508 3 491 co^a 0 1. Ill 1.084 1. Ill 1. 110 1.111 1.086 Line Broadening, *26 • 001 ..U) 0. 30(c> .-U) 0. 57 FIGURE 7. GAS RELEASE DURING POST1RRADIATION HEAT TREATMENT OF NATURAL Z rB z IRRADIATED IN CAPSULE RMK-7-16 CONFIDENTIAL 21 Of the unit-cell dimensional changes only the changes in the aQ dimension for natural zirconium diboride are considered to be definitely larger than experimental error. The changes in this dimension have been tentatively attributed to a relaxation of the compressive effect of the boron on the zirconium-zirconium interatomic distance (this distance corresponds to the aQ dimension). In zirconium diboride the inter atomic distance between zirconium atoms is slightly less than would be predicted for zirconium atoms having a coordination number of 12, while the boron-boron inter atomic distance in the boron layer is "normal". It is thought that the reduced zircorium-rlrconium distance is due to a compressive effect by the boron layer. This compressive effect would be expected to relax during the weakening of the boron lattice due to burnup of the boron-10. The relaxation of this compressive effect might then allow the zirconium atoms to assume their "normal" interatomic distances, a value very close to the postirradiation a0 dimension for natural zirconium diboride. The agreement in the X-ray data for the two capsules containing the natural zirconium dlboride is excellent except for the line-broadening effects on Capsule BMI-7-16. This difference in line broadening between the material in Capsules BM W -14 and BMI-7-16 may be due to the higher irradiation temperature of Capsule BMI-7-16* The difference in X-ray data for the natural and enriched materials suggests a possible correlation between dimensional changes and the amount of helium retained during irradiation. No attempt has been made to obtain a quantitative correlation between line • broadening and radiation damage. Mctallographic Examination Although both the natural and enriched zirconium diboride showed some micro crack formation during the irradiation, neither of the two materials showed any appreciable fragmentation as evidenced by Figures 8 through 11. The enriched zirconium diboride in Capsule BMI-7-15 (Figure V) was consider ably mors porous, prior to irradiation, than the zirconium diboride containing natural boron. The void volume has been estimated to be about IS per cent. This porosity may explain, at least in part, the poor behavior of the enriched material from a helium-retention standpoint. The boron carbide irradiated in Capsule BM1-7-17 was badly permeated with microcrAcks after irradiation as can be eeen in Figure 11. However, again there was no evidence of appreciable particle fragmentation, with most microcracks being less than one-half of the smallest dimension of the particles. The greater degree of cracking no doubt contributes to the increased heliurrr release observed for this mate rial and may have been accentuated by the higher irradiation temperature estimated for this capsule. Photomicrographs of the recoil plates in Capsules DM1-7-14 and BMI-7-15 are shown in Figures 12 and 13. The edges shown represent a cross section of the face exposed to the boron-containing material, Capsule BMl-7-14 contained zirconium as a * recoil m aterial and Capsules BMI-7-15 and BMI-7-16 both contained niobium recoil plates. The recoil plate from BMI-7-17 was not recovered. The zirconium recoil CONFIDENTIAL • t ••• S • S IS M • • ss • II • •• i • • i i • • e • • t sees# s • • • ss as # • • • • • s • , • •• i I t r 25 OX RM 10055 250X H c m t a. Preirradiation b. Poatir radiation FIGURE 8. Zl»2 (NATURAL) FROM CAPSULE BMI-7-14 This material waa irradiated at 150 F. 500X RM11750 500X RM 11745 a, Preirradiation b. Pottir radiation FIGURE 9. Zn\&(50 PER CENT ENRICHED IN BORON-10) FROM CAPSULE BMI-7-15 Thia material wai irradiated at 500 F. CONFIDENTIAL CONFIDENTIAL 23 2S0X KM 10055 250X RMU74H a. Preirradiation b. Postirradiation FIOUKE 10. £ f | i (NATURAL) FROM CAPSULE I1M1-7-16 The irradiation temperature was variable but was greater than 500 K 250X RMI20I7 500X HC227 4 a. Preirradiation b. Postirradiation FIGURE II. B4 C (NATURAL BORON) FROM CAPSULE BMI-7-17 The irradiation temperature wa* variable but KigH. CONFIDENTIAL CONFIDENTIAL 150X HC 1340 FIGURE U . ZIRCONIUM RECOIL PLATE FROM CAPSULE BMI-7-14 The occurrence of what is probably sirconium hydride at the surface (top) of the plate adjacent to the ZrB^ sug gests that hydrogen was released during irradiation. 500X HC2275 FIGURE IS. NIOBIUM RECOIL PLATE FROM CAPSULE BMI-7-15 The reaction evident on the surface of the plate occurred ■ on the face next to the ZrB^ powder. CONFIDENTIAL • 4 • • • •• s ee • • e » • • • * • s • • • e • • • s 1 CONFIDENTIAL material from Capeule BMI-7-14 shows considerable variation in microstructure and the edges give the appearance of massive sirconium hydrides with microcracks per pendicular to the edge. It is possible that the powder in this capsule contained hydrogen which was released during irradiation and diffused into the sirconium plate. The niobium plate from Capsule BMI-7-1S shows a pronounced difference at the edges. It can be seen that there is considerable pitting or corrosion at the grain boundaries. There also seems to be a tendency for the grains to change from elongated to equiaxed at the outer edge. The face of the plate away from the powder showed no attack. The niobium recoil plate from Capsule BMl-7-16 had several defects apparently introduced during fabrication, and the information obtained from the photomicrographs had little till value other than giving an appearance of some precipitates or pits over both surfaces of the plate. SB a* I CONCLUSIONS The uee of boron compounds ie more promising than elemental boron as a means of attaining maximum helium retention of the control material. Natural sirconium diboride appears to be better able to withstand high burnup of the boron-10 than either elemental boron, boron carbide, or enriched sirconium diborlde. Natural sirconium diboride also withstood temperatures up to approximately 1000 Y without exhibiting excessive helium release. The differences observed between natural sirconium di boride and boron carbide may be due at less* partially to the maximum temperatures reached during the irradiation. None of the materials studied exhibited excessive particle fragmentation or dimensional changes during the course of the experiments. Variability A physical structure and purity of the materials make all the above conclusions subject to some question. The examinations of the unirradiated powders were not normally conducted until after the irradiated powders had been returned from the MTR. Some of the powders were in multiphase metallographic states, some wets more porous than others, and most contained several per cent impurities of various kinds. REFERENCES (1) Laubengayer, A. W., Newkirk, A. E., andBrandaur, R. L ., "Progress in the Preparation and Determination of the Properties of Boron", J. Chem. Ed. , 19, 182 (A ugust, 1942). (2) Laubengayer, A. W., Hurd, D. T. , Newkirk, A. E ., and Hoard, J. L. , "The i«a Crystal Structure of Boron Carbide", J. Am. Chem. Soc., 65, 1924 (October, •"* I94J). CONFIDENTIAL • • u • • • ••• • • • CONFIDENTIAL 26 0) Clark, H. K., and Hoard, J. L ., "Boron, I. Preparation and Properties of Pure Crystalline Boron", J. Am. Cham. Soc., 65, 2115 (November, 1943). (4) Pauling, L ., The Nature of the Chemical Bond and tho Structure of Molecule a and Crystals, Cornell University Prees (1939), p 400. (3) Pauling, L ., and Welnbaum, S ., Z. Krist, 87, IS1 (1934). Asanovicb, G ., Keller, D. J., Cunningham, G. W., Paul, D. H., Carlson, R. J. and Calkins, G. D., "Evaluation of Dispersions of Boron-Containing Materials in Titanium and Zircaloy 2", BM1-1164 (Confidential) February 25, 1957. DJH/PS:i*ns CONFIDENTIAL