July, 1960
PHASE
EQUILIBRIA I N THE S Y S T E M S BEF~-THF* A N D LIF-BEF~-THF*
S65
unity obtained for the chloride solutes might result from complex formation in the liquid; the existence of solid compounds represents some evidence for this although the presence of a stable compound in the solid state does not necessarily establish its existence in the liquid. Also, the occurrence of color changes (yellow to violet) offers further evidence for the existence of different bismuth 600 species in these melts. Another mechanism that must be considered is the formation of bismuth chloride polymers. In this case a value of rt greater than two can be obtained; Table I11 shows such an fi a t a composition of 9.7y0KC1. This result is consistent with the formation of a glass which was found to occur in this system at KCl concentrations greater than 20 mole yo (see Fig. 3). This tendency toward glass formation may also hare an effect at loxv solute concentrations. If this is the case, the cryoscopic numbers found for the dilute melts may be the result of several processes: (1) the chloride ion acting as a common ion resulting in an rt of one, G L A S S Y __ ( 2 ) local ordering or the formation of complexes REGION yielding a's of one to two, and (3) polymeric associations producing a's greater than two. The present data do not provide sufficient information to distinguish which one or more of these processes are occurring at a given concentration. 1 Thus, in the molten BiCb system, freezing point data for the solutes used in this study do not yield a satisfactory heat of fusion except a t low solute concentrations. Although these results may not be typical of other molten salt systems, they do Acknowledgments.-The authors wish to acsuggest that heat8 of fusion values derived from phase diagrams should be accepted only with knowledge the assistance of Dr. G. M. Wolten in carrying out the X-ray determinations of the KC1great reservations. BiC13 mixtures and to thank Drs. S. J. Yosim and 1 3 5 ~were smaller t h a n thoqe at 108' and m a y indicate another D. E. McKeiizie for valuable discussions. eutectic, eutectoid OF solid phase change. I
1
PHASE EQUILIBRIA IS THE SYSTEMS BeF2-ThF4 AND LiF-BeF2-ThF, BY R. E. THOMA, H. INSLEY, H. A. FRIEDMAS -4ND C. F. WEAVER Oak Ridge Xational Laboratory, Reactor Chemistry Division,Oak Ridge, Tennessee Received December 18, 1969
As a part of a study of materials potentially useful as fluid fuels of high temperature nuclear reactors, equilibrium diagrams for the condensed systems BeF2-ThF4 and LiF-BeF2-ThF4 have been determined. Both thermal analysis and quenching techniques were used with phase identification accomplished by petrographic and X-ray diffraction analysis. The system BeFz-ThF4 contains a single eutectic a t 2.0 ThF4 (mole YO), melting point 527 Ifr 3'. In association with primary phase fields of the three components and five binary compounds there occur six ternary invariant points within the system LiFBeFa-ThF,. Of these, only one invariant point is a eutectic. Unusual solid miscibility occurs in the compound 3LiF.ThF4 which appears as a single phase solid solution within the area bounded by 75 LiF, 25 ThFp-58 LiF: 16 BeFp,26 ThF,-59 LiF, 20 BeFt, 21 ThF4 (mole %).
Introduction llolten fluoride mixtures containing uranium tetrafluoride have been shown to be useful as circulating fuels for high temperature nuclear reactors.2a*bConversion of thorium to U233can be accomplished in such a reactor if ThF4 is included in the fuel or if a molten blanket containing ThF4 (1) Operated for t h e United States .4tomic Energy Commission b y t h e Union Carbide Corporation. (2) (a) A. M. Weinberg a n d R. C . Bnant. Iv-dear Sea. and Eng., 2, 797 (1957). (b) E. 8. B e t h , J. I,. Neem, R. E. Affel, W. B. Cottrell a n d G. D. Whitman, abid., 2, 804 (1957).
surrounds the reactor. hlliali fluorides with beryllium fluoride can serve as solvent mixtures in which moderate concentrations of uranium tetrafluoride and,'or thorium tetrafluoride can be maintained in solution at 400-600". Such mixtures are among the very few fused salt, mixtures which exhibit high enough concentratioiis of thorium to be of interest in high temperature icnctor technology. Solutions of UF, and ThF4 in Li7k'BeF2 or in SaF-BeFt solvent mixtures have been proposed as fuels for such converter^.^
R.E. THOMS, H. INSLEY, H. A. FRIEDMAS -4ND C. F. WEAVEH.
866
THERMAL ANALYSIS RESULT
1400 r
w
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ThG (mole X),
Fig. 1.-The
90
(00 ThG
system BeF2-ThF4.
:Lmeans of understanding the phase relations existing in LiF-BeFz-ThF4-UF4 nuclear reactor fuel mixtures, a study has been made of the binary and ternary systems limiting the LiF-BeF2ThF4-CF4 quaternary system. Except for the system BeF2-ThF4-TJF4, which is presently under investigation a t this Laboratory, phase diagrams of each of the other limiting systems have been r e p ~ r t e d . * ~ I- n~ this paper detailed phase diagrams are presented for the systems BeFZ-ThF4 and LiF-BeFz-ThF4. Experimental Ab
Materials.--The mixtures used in these phase equilibrium studies were prepared from reagent grade lithium fluoride, beryllium fluoride and thorium fluoride. Lithium fluoride was obtained from Foote Mineral Company and from May1%-oodChemical TTorks, thorium fluoride from Iowa State College and from Sational Lead Company, and beryllium fluoride from the Brush Beryllium Company. N o impurities iTere found in the thorium tetrafluoride by X-ray diffraction or microscopic analysis. Less than 0.25 weight cc impurities mere foiind bv spectroscopic analysis of this material. The phase equilibria data were obtained by thermal anal> sis of slowly-cooled melts and by identifying the phases present in mixtures which had been equilibrated and quenched. Bec:ruse thorium fluoride is easily converted to oxyfluorides or oxide a t elevated temperatures5 it was necessary t o remove small amounts of water and oxygen as completely as possible from the starting materials. To facilitate the removal of these Rubstances ammonium bifluoride was added to the mixtures of BeFz-ThFI and LiFB e F r T h F l before initial heating in the thermal analysis experimmtii. As mixtures were heated the water was (3) J. A. Lane, H. C. MacPherson and Frank Maslan. editors, “Fluid Fuel Reactors,” Addison-Wesley Publishing Co., Inc., Reading, Mass., 1958, Chapters 11 and 12, pp. 567-594. (4) (a) C. J. Barton, H. .%.Friedman, W. R . Grimes, H. Insley, R. E. Moore and R . E. Ttionia, J . A m . Ceram. Soc., 41, ti3 (1958); (b) A . T. Sovoselova. Yu. 1’. Simanov and E. I. Yaremhash, Zhur. F i z . Khim., 26, 1244 (1952); (c) L). AI. Roy, R . Roy and E. F. Osborn, J . A m . Ceram. Soc., 37, 171 300 (1954); (d) L. J. Wittenberg, ibid., 42, 209 (1959); (e) R. E. Thoma, H. Insley, B. S. Landau, H. A . Friedman and W. R . Grimes, THISJ O U R N J L , 63, 1266 (1959); (f) C. F. Weaver, R. E. Thoma, H. Insley and H. A. Friedman, J . A m . Ceram. Soc., 43, 213 (1960); (g) L. B. Rinehammer, P. A. Tucker a n d E. F. Joy, “Phase Equilibria in the System BeFz-UFI,” in “Phase Diagrams for Ceramists: Part 11,” b y E. b l . Levin and H. F. lZcAlurdie, The American Ceramic Society, Columbus, Ohio, 1959, p. 98; (h) L. V. Jones, I). E. Etter. C. R . Hudgens, A . A . Huffman, L. B. Rinehammer. N. E. Rogers, P. A . Turker and L. J. Wittenberg, “Phase Equilibria in t h r I.il“-Bel~~--VF* Ternary E’iised Salt Systpm.” MLJI-1080, A I I P . 2 4 , 1959. L5) R . W. M. II’Eye, .I. Chsm. SOC.,196 (1958).
Vol. 64
evaporated from the system. The oxides were converted by reaction with the ammonium bifluoride to products which have not been identified but which are likely to be ammonium fluorometallates.6 Upon further heating the “ammonium fluorometallates” decomposed to form the metal fluorides. These same mixtures were used later in the quenching experiments. Apparatus and Methods.-The terhriicrues used for me&&rement of the temperatures which difine the phase diagrams as well as the X-ray and microscopic techniques for identifying phases have been discussed previously.4~,i,~ The accuracy of the temperature measurements reported in these studies is limited by the characteristics of the chromelalumel thermocouples used. Manufacturers estimate this accuracy to be within 5” in the temperature range 400-800”. Except in the LiF and ThF, apices of the system LiF-BeF2ThFa, so large an amount of reproducible phase data was accumulated that statistically reliable cslculations of standard temperature deviations could be made. These deviations in the measurement of invariant temperatures are Itlo. Reproducible phase data were accumulated from experiments performed using six sets of quenching and temperature measurement apparatus, whose cjesign and operation is nearly identical. In each of these IS Incorporated 18 thermocouplefi whose independent readings were used to determine a temperature cnlibrat,ioii curve of the thermal gradient. within the furnace. This arrangement causes error in it single thermocouple t o be readily apparent.
Discussion of Results The System BeF2-ThF4.--A preliminary diagram of the system BeF2-ThF4has been reported from this Laboratory.9 The phase equilibrium diagram reported here (Fig. 1) representas a synthesis of thermal analysis and t,hernial gradient’ quenching data obt’ained from mixtures of BeF-ThF4, LiF-BeF2-ThF4, and SaF-BeF2-ThFa. The system cont,ains a single invariant, point,, the eutectic at 527” and at, 2.0 mole % ThFi. llolteii i n i s t u r e s of BeF2 and ThF, cont,aining mow tjhan : h u t ’ ’73 mole % BeF2 are so 1-iscous at, temperat,ures near the liquidus that, equilibrium is not rmchecl after heating for as long as three weeks. Rather than employing very long annealing treatmttitr list was made of the fact that small additioiis of or LiF will so reduce the 1-iscosity of melts that q u i librium can be reached in less than three weelis. Thus, solid-liquid transitions ere tipt,ermiiied for mixtures having compositioiis iiear thc Rei.'? apex of the SaF--Be1.‘2-ThF4 trm anti estrapolations based on these data JIn m ’ e to the limiting binary system Hel:2-Thl’4. l’lir, results so obtained for the range 0 t o 25 mole 5; rrhl:4 ill the syst,em BeF2-ThF4 are showi in Tahlc 1’”:\lid the data in tJhe system SaF-BeI’2-ThI’,3 oil which these are based are given in Tablc 11. The eutectic invariant composit,ioii and t rmperat ure. and the liquidus values from 0 to 23 mole yc ThF, were determined iit this way. The licluidul; temperatures for the les:: I ouh rcgioii from 23 t o 100 mole Yc ?‘h1.’4 were tlettwniiietl hy t h r ~ i ~ n i a l ( 6 ) B. J. Sturui, Oak Riiirre Sational Laburnto,-. lwrsonal c w i ~ t munioation. (7) C . J. Barton, W. R . Grimes. H. Insley. 11. E . 31
