i £ TECEVED BY nre gnp aa . o OAK RIDGE NATIONAL LABORATORY operated by UNION CARBIDE CORPORATION W NUCLEAR DIVISION for the U.S. ATOMIC ENERGY COMMISSION ORNL- TM- 2486 MASTER SOME ASPECTS OF THE THERMODYNAMICS OF THE_EXTRACTION OF URANIUM, THORIUM, AND RARE EARTHS FROM MOLTEN - LiF-BeF, INTO LIQUID Li-Bi SOLUTIONS =~ L. M. Ferris NOTICE This document contains information of a preliminary nature and was prepared primarily for internal use ot the Oak Ridge Nationai Laboratory. It is subject to revision or correction and therefore does not represent o final report. WISTRIBUTION OF THIS DOCUMENT [ UNLIMITED — s [ LEGAL NOT|CE e e i e mwal man o e meem ..__.__l This report was prepared as an account of Govarnment sponsored work., Neither the United States, nor the Commission, nor any person acting on behalf of the Commission: A, Makes any warranty or representation, expressed or implied, with respect to the accuracy, completeness, or usefulness of the information contained in this report, or that the use of any information, apparatus, method, or process disclosed in this report may not infringe privately owned rights; or B. Assumes ony liabilities with respect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this report. As used in the above, ‘‘person acting on behalf of the Commission®’ includes any employee or contractor of the Commission, of employese of such contractor, to the extent that such employee | or contractor of the Commission, or employese of such contractor prepares, disseminutes, or provides access to, any information purswvant te his employment or contract with the Commission, or his employment with such contractor, | U e e = s e e e - e e e e ORNL-TM-2486 Contract No. W-7405-eng-26 CHEMICAL TECHNOLOGY DIVISION Chemical Development Section B OF THE EXTRACTION SOME ASPECTS OF THE THERMODYNAMICS OF URANIUM, THORIUM, AND RARE EARTHS FROM MOLTEN LiF-BeF2 INTO LIQUID Li-Bi SOLUTIONS L. M. Ferris LEGAL NOTICE This report was prepared as an account of Government sponsored work. Neither the United States, nor the Commission, nor any person acting on behalf of the Commission: A. Makes any warranty or representation, expressed or implied, with respect to the accu- racy, completeness, or usefulness of the information contained in this report, or that the use of eny information, azpparatus, method, or process digclosed in this report may not infringe privately owned rights; or B. Assumes any liabilities with reapect to the use of, or for damages resulting from the use of any information, apparatus, method, or process disclosed in this report, Ag used in the above, “‘person acting on behaif of the Commission includes any em- ployee or contractor of the Commission, or employee of such contractor, to the extent that such employee or contractor of the Commission, or employee of such contractor prepares, disseminates, or provides access to, any information pursuant to his employment or contract with the Commission, or his employment with such contractor. MARCH 1969 OAK RIDGE NATIONAL LABORATORY Oak Ridge, Tennessee operated by UNION CARBIDE CORPORATION for the U. S. ATOMIC ENERGY COMMISSION Abstract 1. 2. CONTENTS IRrOdUCH O vt s vt e et ee e e e e e e e e e e e General Thermodynamic Treatment . The Thermodynamics of Uranium Extraction . The Thermodynamics of Thorium Extraction . .. .. .. .. ... ... .. ... . The Thermodynamics of Lanthanum Extraction . .. .. .......... .. ... The Thermodynamics of Sodium Extraction. . .. .. .. .. .. ... The Thermodynamics of Europium Extraction Discussion. . .. .. .. .. .. References . .... .. .. ---------------------------------------------- 10 10 11 12 SOME ASPECTS OF THE THERMODYNAMICS OF THE EXTRACTION OF URANIUM, THORIUM, AND RARE EARTHS FROM MOLTEN LiF--BeF2 INTO LIQUID Li-Bi SOLUTIONS L. M. Ferris ABSTRACT Expressions for the equilibrium distribution of uranium, thorium, lanthanum, and other solutes between LiF-BeF, solutions and lithium- bismuth solutions at 600 to 700°C were calculated, using thermody- namic data from the literature. The results obtained experimentally for uranium were in reasonably good agreement with the calculated values. However, the results for thorium and lanthanum reflect the high degree of uncertainty that exists in the available thermodynamic data for these solutes. It is concluded, therefore, that an accurate measure of the relative extractability of the various solutes can be obtained only by experimental means. 1. INTRODUCTION One method that has been considered for separating uranium and rare-earth fission products in the processing of the fuel carrier salt, LiF-BeF2 (66-34 mole %), from a two-fluid molten-salt breeder reactor is reductive extraction of the respective elements into liquid bismuth. -3 During the course of process development, we measured the equilibrium distribution of uranium, thorium, sodium, and certain rare earths between LiF- BeF2 solutions and Li-Bi solutions at 600 to 700°C to determine the relative ease of extraction of the various e|emeni's.3 It was possible to predict the exiraction behavior of several of the solutes by using the system of thermodynamics developed by Boes4’5 for LiF-BeF2 systems, and the activity coefficients reported for the various metals in liquid bismuth. In this report, these calculated results are com- pared with those obtained experimentally with two salts: LiF-BeF2 (66=34 mole %) and LiF-BeF, (56.9-43.1 mole %). Activity coefficients for ThF4 and LaF3 in the latter salt at 600°C were also computed from the experimental data. 2. GENERAL THERMOD YNAMIC TREATMENT The extraction of a solute MF , which is present in low concentration in molten LiF-Ber, into liquid bismuth containing lithium can be expressed in terms of the general reaction MF , . +n Li + n LiF (n n(d) (Bi) in which the subscripts (d) and (Bi) denote the salt and bismuth phases, respectively. =M (Bi) (d) ’ This reaction is actually the sum of the two half-reactions ME @ T M =MpytnF g (2) nLi=nLi++ ne . (3) Discontinuing the use of the subscripts, we can write the equilibrium constant for Eq. (1) as nFAE a, a. —_— K =_M_L‘_i e R (4) TMF L n in which a is the activity, F is the Faraday constant, R is the gas constant, T is the absolute temperature, and AEO =E From Eq. (4), we obtain o,M Eo,Li ' FAE : "% . °MOLF WiF M =ln ———— =nln + In : (5) RT o o A O MF MF L ‘ N Let a = XY, where X = mole fraction and Y is the activity coefficient; then X, . Y, . X Y AE =R|;r|nxLlF+EI-:I|n—-L£+-E%In-)Z-M-—+-§FllnYM . (6) ° Li 1 MF_ ME_ If we define the distribution coefficient for component M as D z-.)f_M_.. M XMF Eq. (6) can be written as s =Rl p & jpp Ry LRy M o F Li nF M F Yi;g MF Y ME ' n Mc:>u|1'on6 has defined the quantity Ec: for component M as RT | M n—_ oM nF YMFn ! = Eo,M E Rearranging Eq. (8), we get Y Y . RT M RT Li RT RT E -—In —— =-(E ,,-—=In — }=—=InD,,-— 1InD,,. o,M nF YMFn (O,Ll F YLiF ) nF M F Li . / = / - / If we define AEO,M Eo,M Eo, L 7 Eq. (10) becomes RT RT ’ = ——— - g—— S MM Dy In Dy or Y Y, . RT M RT Li AEY =F -E .-= In + 2= In . The experimental determination of distribution coefficients allows values of (8) (%) (10) (1) (12) AE’ to be calculated from Eq. (11). The use of reported activity coefficients for o,M metals in bismuth, and the activity coefficients and standard reduction potentials for the metal fluorides as given by Baes '5 using Eq. (12). In Baes' treatment, LiF--BeF2 (66~34 mole %) was used as the permits an independent calculation of AE; M I reference salt, and partial molal free energies of formation in this salt were calculated for various solutes from the available thermochemical and equilibrium data. Standard reduction potentials were then computed from the free energy data. The activity coefficient for each solute (at low concentration) was defined as unity in this salt; however, the activity coefficients for LiF and BeF2 were defined as 1.5 and 3, respectively. The changes in the values of these activity coefficients as the LiF/BeF2 ratio in the salt varies were also estimated by Baes. The standard states for the bismuth solutions are the pure metals; the activity coefficients, which are actually Henry's law constants, are practically constant when the solute is present in bismuth in low concentrations. The activity coefficient at infinite dilution is the one used throughout this report when referring to the metal phase. Activity coefficients for solutes can be calculated for an LiF-BeF2 composition other than 66-34 mole % if distribution coefficient data for the solute in both the new salt and the reference salt are obtained. Since activity coefficients for the metals in bismuth change only slightly with concentration, we get from Eq. (12) for LiF-BeF2 compositions 1 and 2: Y Y, . RT M RT Li AE’ = , -E .-=In + =— In (13) o,M, 1 o,M o,Li nF YMFn ] F YLiF,T Y Y,. RT M RT Li AE’ =E -E ,-— In ———— + — In (14) n,2 Subtracting Eq. (14) from Eq. (13), we get Y Y, . IMF RT LiF,2 RT n,2 MAE'Y = AE' - AE’ = In 2 - |p ——2 (15) o o,M,1 o,M,2 F YLiF,l nF YMFn ] Let composition 1 be LiF-BeF2 (66~34 mole %), where Y of Eq. (15) yields = 1; then, rearrangement MF n / nF & AEO) log v =nlog —~ - s Man.2 YLiF,l 2.303 RT (16) 3. THE THERMODYNAMICS OF URANIUM EXTRACTION Early in the development of the reductive extraction process, uranium was 3,6,7 believed to exist primarily as a tetravalent species in the salt. However, the . . 8,9. . C results of recent experiments ' indicate that the uranium is actually, for the most part, in the trivalent state in the salt, especially when the distribution coefficient is greater than about 0.1. These experimental results can be compared with those ’ calculated by using the standard potentials given by Baes "~ and reported values for the activity coefficient for lithium in bismuth. Consider the reaction UF + Li = UF + LiF 4(d) T (Bi) (d) ’ in which the subscripts (d) and (Bi) refer to LiF - BeF2 (66-34 mole %) and bismuth 3(d) solvents, respectively. From Baes' treatment of the system we get, at 600°C: e =% E_=-1.1465 v Li=Li +¢ E_ = +2.6453 v ot ati=ut 0t E_= 14988y The equilibrium constant _ oLy 3 K X U4+’Y Ut cah be written os [XU4+] [D ] [YLI ] X, and use Y if we defineD,. = L X Rearrangement gives [0 (5 ) Li From the standard potential of the reaction (given above), we find that K = 4.51 X 108. According to Baes' convention, Y i+ = 1.5; and from data reported by Argonne National chorotory,lo we gety, . = 5.9 x 10-5. Thus, X 3+ -5 U _ 5.9x 10 A Yt (255 ), 3+ U _ 4 [X ]w(l.774x10)DLi . The average uranium valence in the salt is S(XU4+ )+4 Values of n at different values of DLi are shown in the following table. Li Conc. Li Conc. U3+ in Bi in Bi 5 [x ] ~ (at. %) (ppm) Li n 0.0037 1.23 5.6x 107 1.0 3.50 0.004 1.33 6.06 x 107> 1.08 3.48 0.01 3.32 1.515 x 1074 2.69 3.27 0.02 6.64 3.03 x 1074 5.38 3.16 0.1 33.2 1515 1075 26.9 3.04 Using T 9.8 x 10 > a’r 600°C, as determined at Oak Ridge National Lc:borclfory,6 the calculated U /U values are slightly lower than those shown in the table. However, over the range where we can experimentally determine distribution co- efficients for uranium and lithium (lithium concentration in the bismuth of greater than 1 ppm), we would expect the uranium in the salt to be primarily trivalent. The agreement between distribution coefficients that were obtained experimentally and those that were calculated from the available thermochemical and equilibrium data is easily seen by comparison of the respective AE values. In calculating AE for U3+, we used Baes' values4' for YiiE and E o Li’ fhe activity coefficient for uranium in bismuth as calculated from the expression log YU =0.7107 - (3995/7) , and either ANL or ORNL values for Y (the values from i-he ANL work 10 at various temperatures, and the value at 600°C reported by ORNL ) In the following table the calculated values are compared with those determined experimentally: Calculated AE § (volt) Experimental Temp. From ORNL From ANL AEc'> (°C) YLi YLi (volt) 600 0.66 0.62 0.66 675 - 0.60 0.66 The agreement is surprisingly good, considering the inherent inaccuracies involved and the variety of sources from which the data were obtained. 4. THE THERMODYNAMICS OF THORIUM EXTRACTION Distribution coefficients for thorium have been meosured9 at 600°C, using both LEF-BeF2 (66~34 mole %), where YThF =1, and LiF—BeF2 (56.9-43.1 mole %); these coefficients gave AE; values of 0.43 and 0.475 volt, respectively. Baes S gives no value for the activity coefficient for ThF4 in the latter salt; however, it can be readily calculated from the measured AE; values, using Eq. (16): = 4 log (0.866/1.5) - (-0.045/0.0433) = 0.0849 log v ThF4 Y = 1.22. ThF4 It is interesting to note that the corr&spondmg activity coefficient for UF (the only tetravalent species treated by Boes ’ ) is about 1.7. Values of AE; were also calculated for thorium for both salts given above. Data r from Brookhaven National Laborafory] 3 yielded the following expression: log Y, = 1.289 - 7565/T , from which YTh is calculated to be about 4.3 x 10—8 at 600°C. The use of this activity coefficient, the value of ~1.77 volts for E0 T’ originally given by Bc:es,4'5 and the ANL and the ORNL values for ¥ at 600°C: resulted in the following values for AE; Li - Salt Composition Calculated AEO (volt) (mole %) From ANL From ORNL Experimental LiF BeF Y Y AEs . Li Li (volt) 66 34 0.432 0.470 0.43 56.9 43.1 0.476 0.514 0.475 The agreement among the calculated and experimental values is quite good. However, Baes now reporfs]4 Eo ThH fO be -1.89 volts at 600°C. The use of this new value, along with the activity coef;icients given above, results in calculated AE; values that are about 0.12 volt lower than those determined experimentally. If the new value for Eo,Th is really more accurate than the value reported originally, and if we assgme that all the other quantities used are reasonably valid, then Yy, o 600°C would have to be in the range of 7 x 10-” to 5 x 10_10 (instead of the value of 4.3 x 10-8, which was obtained from the BNL work). 5. THE THERMODYNAMICS OF LANTHANUM EXTRACTION Distribution coefficients for lanthanum were determined at 600°C with both LiF-—Ber (66-34 mole %), where YLaF3 =1, and LiF—BeF2 (56.9-43.1 mole %); these coefficients gave AE; values of 0.428 and 0.475 volt, respectively.9 Since 4,5 . . Baes '~ does not give a value for ¥ in the latter salt, it was calculated from the LGF3 AE; values by using Eq. (16): log Y, ¢ =3 log (0.866/1.5) - (-0.047/0.05773) = 0.098 3 =1.25 . 3 This value is slightly higher than the value that was reporfed4'5 for CeF3 in the YLcF same salt. Values of AE; at 600°C were calculated for both salts, using the original ,La E. g of -2.314 volts as given by Baes '~ and the activity coefficient for lanthanum ! 15 in bismuth as calculated from the expression r log Y, = 0.844 - (11070/1) , which gives Y, = 1.46 x 10-]2 at 600°C. The calculated AE; values were lower La than the experimentally determined values by about 0.2 volt. This rather poor agree- ment is probably the result of the large uncertainty in the value for E ola’ Assuming this to be the case, a new value of Eo Lo~ ~2.157 volts at 600°C was calculated by . / . ! o using the measured AEO for LiF BeF2 (66-34 mole %), the values of YLiF and Eo,Li ! as given by Kober et al., ™ and an average as given by Baes, "~ the value for vy La of the ORNL and ANL values for Yy If it is assumed that Baes' temperature co- efficient for E0 La (0.82 mv/°C) is valid, a value of =2.0955 volts is calculated for I E at 675°C. The revised values for E o,La o,La calculated and experimentally determined AE; values: lead to the following comparison of 10 Sali-((r:nclrlnepg;l)hon Calculated AE; (volt) Experimental | 2 From ANL From ORNL AE! Temp. LiF BeF y Y (vol}) (°C) 2 Li Li 600 56.9 43.1 0.46 0.49 0.475 675 66 34 0.41 - 0.43 As seen, the use of the revised standard potentials for lanthanum, along with the activity coefficient data cited above, gives much better agreement between the calculated and the experimentally determined quantities than was obtained with the original standard potential. 6. THE THERMODYNAMICS OF SODIUM EXTRACTION A AEC; value of 0.2 volt was determined experimenfallyS at 600°C for sodium with LiF-BeF, (66-34 mole %). Using this value, along with Baes' values4’5 for Eo L; and YLEF' the activity coefficient for sodium in bismufl'\]é_]8 as determined from the expression log YNg = 0.4892 - (3512/7) , and the activity coefficient for lithium in bismuth, yields a value of about -2.3 volts for E at 600°C. o,Na 7. THE THERMODYNAMICS OF EUROPIUM EXTRACTION A value of 0.33 volt has been defermined9 at 600°C using LiF-BeF2 (66-34 mole %). No comparison with calculated values can be made because of the lack of activity coefficient and standard potential data. 11 8. DISCUSSION The information presented in this report indicates that uranium in the salt exists primarily as UF3 during its extraction from LiF--BeF2 solutions with Li-Bi solutions. This is in accordance with the available thermodynamic data. Actually, the cal- culated distribution coefficients are in reasonable agreement with those that have been determined experimentally. The results of other experiments with LiF-Ber"‘ThF4 solufions? at 600 to 700°C strongly indicate that the potentials for the U4+ - U3 and 3+ o . . 1. . U~ ~ U half-cell reactions are about the same as those in L|F--Be:F2 solutions. A comparison of calculated and measured AEC: values for solutes such as ThF4 and L<:1F3 i [lustrates the high degree of uncertainty that exists in the standard potentials and/or the activity coefficients for these componentis in bismuth. As a result, the predicted relative extractobility of these two elements is greatly influenced by the particular sets of data used in the calculations. In general, the use of the available thermodynamic data gives only a rough indication of the relative extract- ability of the solutes of interest in the processing of molten-salt breeder reactor fuels, particularly in the case of LiF-Ber—ThF4 systems of high ThF4 concentration. Data for LiF--BeF2 systems are scarce and inaccurate; in addition, no reliable method for extrapolating these data to LiF--BeF2=-ThF4 systems has been devised. 1t is con- cluded, therefore, that an accurate measure of the relative extractability of the various solutes can be obtained only by direct experimentation. At present, a program to determine distribution coefficients for uranium, protactinium, zirconium, rare earths, and other fission products in a variety of molten fluoride salts and several liquid metal systems is under way ot ORNL. 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Crowley F. L. Culler D. R. Cuneo J. M. Dale D. G. Davis R. J. DeBakker J. H. DeVan S. J. Ditto A. S. Dworkin |. T. Dudley D. A. Dyslin . P. Eatherly . R. Engel . P. Epler . E. Ferguson . M. Ferris . P. Fraas . A. Friedman . H. Frye, Jr. . K. Furlong . H. Gabbard . B. Gallaher . E. Gehlbach . H. Gibbons . O. Gilpatrick . R. Grimes . G. Grindell W. Gunkel H. Guymon P. Hammond A. Hannaford H. Harley . G. Harman . O. Harms . Harrill . Haubenreich Zm 84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101. 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112. 113. 114. 115. 116. 117. 118. 119. 120. 121. 122. 123. 124. 125. 126. 127. 128. R. E. Helms P. G. Herndon D. N. Hess J. R. Hightower M. R. Hill H. W. Hoffman D. K. Holmes P. P. Holz R. W. Horton A. Houtzeel T. L. Hudson W. R. Huntley H. Inouye W. H. Jordan P. R. Kasten R. J. Kedl M. T. Kelley . R. Kennedy . W. Kerlin . T. Kerr . J. Keyes . V. Kiplinger . S. Kirslis W. Koger . B. Korsmeyer . |. Krakoviak S. Kress W. 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Colby, Jr., AEC, Washington éL..}UJfi E = Q = D ~ ale &;or?&gx E. L Youngblood F. C. Zapp Central Research Library Document Reference Section Laboratory Records Laboratory Records-RC DTIE D. J. Crouse R. G. Wymer EXTERNAL DISTRIBUTION