THE EXTRACTION OF PLUTONIUM FROM NEUTRON-IRRADIATED URANIUM BY URANIUM TRICHLORIDE AND BY MAGNESIUM CHLORIDE1

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1 THE EXTRACTION OF PLUTONIUM FROM NEUTRON-IRRADIATED URANIUM BY URANIUM TRICHLORIDE AND BY MAGNESIUM CHLORIDE1 ABSTRACT The extraction of plutonium from neutron-irradiated uranium by uranium trichloride and by magnesium chloride has been examined in the temperature range 1150" to 1250" C. The results of the uranium trichloride experiments have been used to evaluate the standard freeenergy change of the equilibrium reaction UCls + PLI % PuCls + U. These free-energy values have been used to predict the plutonium estractton In the magnesium chloride esperiments on the assumption that the uranium trichloride, continuously formed from the non-equilibrium reaction 3/2MgCl? + U -+ UCIs + 3/2Mg, rapidly established the above equilibrium with the plutonium in the uranium. Agreement between predicted and experimental results supports this assumption. A preliminary study of the kinetics of the MgC1-U reaction showed that the initial rate of reaction is controlled by the distillation of magnesium metal from the reaction zone. INTRODUCTION In previous papers, the separation of plutonium from neutron-irradiated uranium by volatilization (I) and by liquid silver extraction (2) was described. Another possible technique for achieving this important separation is by reaction of plutonium with molten inorganic salts, as represented in the followiilg equilibrium expression, MX+ Pu%PuX + M. The plutonium salt formed by the reaction enters the salt phase and is thus separated from the uranium metal. This method has been examined by a number of investigators (3, 4) using uranium fluorides. The present paper describes an investigation of the extraction of plutonium by uranium trichloride and by magnesium chloride. High-temperature thermodynamic data may be used to predict the distribution of plutonium between the salt and metal phases. From the data on the free energy of formation, the standard free-energy change accompanying the reactions may be calculated for a given temperature. These values may be used to obtain the equilibrium constant for that temperature. Using the mass-action expression for the equilibrium constant, the distribution of plutonium between the two phases may be calculated. Since only estimated values for the free energies of formation of liquid uranium and plutonium chlorides are available (5, G), the distribution equilibria are a useful checlc on the reliability of the available data for these compouncls. Besides testing the available thermodynamic data, the above calculations have been reversed to give, from the experimental data, new equilibrium constants, and hence, standard free-energy changes accompanying the reactions. A similar procedure has been used by Buyers to determine the free energy of formation of plutonium trifluoride (7). These new values have been used to predict the distribution of plutonium in the experiments with magnesium chloride. EXPERIMENTAL Materials and Apparatus The neutron-irradiated uranium was cut from natural uranium fuel rods which had been irradiated in the NRX reactor and allowed to decay for 3 years. The concentration lmanuscript received May 8, Contribzltion from Chemistry and Metallurgy Division, Research Chetnistry Branch, Atomic Energy of Canada Limited, Chalk River, Ontario, Canada. Issued as A.E.C.L. No "resent address: Atomics International Division, North American Aviation Inc., P.O. Box 309, Canoga Park, California, U.S.A. Can. J. Chem. Vol. 36 (1968)

2 1234 ChN.4DIAN JOURNAL OF CHEMISTRY. VOL of pluto~~ium in this material was 0.025y0. Uranium trichloride was prepared from uranium metal by passing hyclrogen chloride over the hydride (8). Considerable care to exclude moisture and air was required to obtain a pure product. Anhydrous magilesium chloride was obtained by carefully heating C.P. grade ammonium magnesiu~n chloride hexahydrate in a current of dry hyclroge~z chloride to remove water, follo~ved by further heating in vacuum to remove ammonium chloride. 3lIagnesium analyses of the product gave and 23.35y0 Mg, calculated 25.54%. The chlorides were stored and handled in a dry nitrogen atmosphere. The uranium and salt, contained in an alumina crucible ("Triangle" recrystallized alumina, Morganite Carbon Products Canacla Limited, 195 Eastern Ave., Toroilto 2, Ontario), were heated bj- a tungsten wire coil surrounded by thermal and radiation shields. The assembly was mounted in a glass bulb which could be evacuated by diffusion and mechanical pumps (1). Temperatures were measured with a platinum, platinum - 10% rhodiuim thermocouple which was coiled in the heating zone as a seat for the crucible containing the charge. The thermocouple was calibrated to give the temperature of the charge by melting various pure metals in the apparatus. The temperature measureme~lts were accurate to 413' C. Procedn~re and Analyses For an experiment, the uranium was etched in acid to remove approximately 25% of its weight for a reference solution, then washed in water and acetone. Weighed amounts of the chloride and of neutron-irradiated uranium, in an alumina crucible, were placed in the apparatus. The glass bulb was pumped down to a pressure of 10-5 mm Hg or less and then the crucible was heated to 300' C for further degassing. After admitting welding grade argon to a pressure of 1 atmosphere, the charge was heated to temperature. The salt and uranium were contacted for 30 minutes in all runs except those where time was the variable to be studied. After contacting, the charge was cooled in an argon atmosphere, separated from the crucible, and the salt phase dissolved or washed away from the metal billet. In each experime~lthe complete phase was dissolved for analysis. The solutions were analyzed for uranium (9) and magnesium (10) spectropl~otometrically; plutonium was determined using standard radiochemical procedures. Plutonium recoveries of 95y0 or better were obtained in all runs. RESULTS AND DISCUSSIOS The Extraction of Plutoniz~m with Uranium Trichloride On contacting the liquid phases of uranium trichloride and neutron-irradiated uranium the following equilibrium is established: UCIB + Pu % PuC13 + u. The literature values of thermodynamic data listed in Table I were used to calculate the equilibrium constants for the above equilibrium. Tlle distribution of plutonium between salt and metal was then predicted on the assumptions that plutonium behaves ideally in molten neutron-irradiated uranium and that the product plutonium trichloride dissolves to form an ideal solution in uranium trichloride. Evidence at higher temperatures for the former assumption has been obtained in previous worli (1, 3), while the isomorphism of the trichlorides and the proximity of plutoni~~m and uranium in the actinide series are supporting evidence for the validity of the latter. The extraction of plutonium was examined for various U/UC13 mole ratios (Table 11)

3 , I I I I McKEXZIE ET.-\L.: PLUTONIUM 1235 and also at various temperatures (Table 111). The proportioils of the original plutonium remaining in the salt and in the metal phase are listed in the tables. The calculations using the thermodyilamic data give directly the proportion of the original plutonium remaining in the uranium phase and these are listed in the last column of each table. TABLE I THERMODYKIJI~C D.11.i FOR KCIJ.IND PllC13 (AF - AH?98)/T(calories/degree) Reference AlI?ss LIIelting point Compound No 500" I< 1000" I< 1500" I< (lical) (" 10 UC (s) (s) 49 (1) PuC (s) 53 (s) 50 (1) Although the calculated values (Table 11) for plutonium remaining in the uranium phase are close enough to the experimeiltal values to be useful in malcing predictions, the experimental results at 1200' C are collsistently lower (i.e. less plutonium in the uranium phase) than the predictions. Also, in Table 111, there is excellent agreement for experiments at 116O0 C but the data show a trend of increasing extraction with temperature, which is the reverse of that predicted by the thermodyilamic data. In view of these discrepancies and the fact that the thermodynamic data were estimated, it seemed worth while to use the experimental data and the assumptions of ideality to calculate equilibrium constants and standard free-energy changes accompanying this reaction. The results of these calculations are shown in Table IV. The table shows for each temperature the "average" value for the equilibrium constant from which the standard free-energy TABLE I1 EFFECT OF MOLE RATIO U/UC13 ox EXTRACTIOX OF PLUTONIUM AT 1200 C Pu distribution (% original) Exptl. Calc. Wt U \;\it UC1, Mole ratio (g) (g) U/UCla Salt U U Temperature Mole ratio TABLE I11 Mole ratio U/UC13 = 14 Pu distribution (yo original) Esptl. (" c) U/UC13 Salt U U Calc.

4 CANADIAN JOURNAL OF CHEMISTRY. VOL. 3G TABLE IV REACTION 6cla f PU k I'LIC~~ f U ---- Temperature Eq~~ilibrium 4F0 (kcal) AF" (I~cal) (" c) constant (K) K,,. (Esperirnental) (Brewer el al.) FIG. 1. The effect of temperature on the experimentally determined equilibrium consta~lts for the reaction UC13 f Pu k PuC13 + U.

5 MCKENZIE ET.4~.: PLUTONIUM 1237 change for that temperature has been evaluated. This AFO is then compared with the previous estimate of Brewer et al. (5, 6). (For this compariso~l the estimates are listed to three significant figures although Brewer et al. give only two.) As a test of the internal consistency of the data, a plot of log K vs. l/t should yield a straight line. This plot is shown in Fig. 1. Although the result at 1250' C is low, the other data are reasonably consistent and may be represented, approximately, by the equation log I< = (5.42~ 103/T). From the slope of this line the enthalpy change accompanying the reaction UC13 + Pu % PuC13 + U at about 1220" C is calculated to be 24.8 lccal. The Reaction of Uran,ium Metal and Magnesium Chloride Molten magnesium chloride in contact with molten uranium reacts according to the following equation : 3/2h(IgCl2 + U + UCI, + 3/2Mg. Since magnesium chloride has a higher free energy of formation (i.e. more negative) than uranium trichloride, it would be expected that the reaction proceeds because the experiments were performed under conditions which permitted continuous distillation of magnesium from the reaction zone. It might be anticipated, further, that the initial rate of reaction will depend upon the surface area of uranium exposed to the salt. As the reaction proceeds, however, the rate will decrease owing to the accumulatioll of the reaction product, uranium trichloride, in the salt phase. These general expectations have been found to be correct. The initial reaction rates have been measured using both neutron-irradiated uranium and natural uranium at a number of temperatures; the data are summarized in Table V and plotted as log rate (grams of U reacted/hr cm2 U surface) vs. l/t in Fig. 2. The results have been treated by the least squares method to give the equation: log rate (grams of U reacted/hr cm2 U surface) = (7.16X 103/T). From the slope of this line an activation energy of 32.8 lccal has been calculated for the rate-controlling step of the initial process. This activation energy may be compared with the value of 32.3 kcal per mole, the heat of vaporization of magnesium metal at 1200 C (11). This agreement is good evidence that the initial rate of the reaction of uranium metal and magnesium chloride is controlled by the distillation of magnesium metal from the reaction zone. TABLE V THE EFFECT OF TEMPERATURE ON THE INITIAL REACTION RATE OF MgCI? AND URANIUM METAL -- Temperature Wt U Wt MgClr U surface Time U reacted U reacted (" c ) (6) (6) area (crn2) (min) (g) (g/hr) Natural uraniulil

6 CANADIAN JOURNAL OF CHEIMISTRY. VOL NATURAL URANIUM - A NEUTRON - IRRADIATED URANIUM- FIG. 2. The effect of temperature on the initial rate of reaction between magnesium chloride and uranium metal. The Extraction of Plutonium in the illagnesium Chloride Experiments For these experiments, the following reactions occur on contacting magnesium chloride with molten neutron-irradiated uranium: 3/2MgCl? + U 4 UCI, + 3/2Mg 1 3/2MgC12 + Pu -, PuC13 + 3/2R/Ig [21 UC13 + Pu = PuC12 + U. r3 I The experiments were performed under a pressure of one atmosphere of argon and at a temperature above the boiling point of magnesium metal. Thus, since the temperature of the glass bulb surrouilding the reaction crucible was coilsiderably below the melting point of magnesium, the magnesium continuously distilled from the reaction zone and equilibrium with respect to reactions [I] and [2] was not reached. To calculate the extraction of plutonium using thermodyilamic data, it was assumed that reaction [3], which does no involve tvolatile components, was in equilibrium at all times, and that uranium trichloride was dissolved in an inert diluent, magnesium chloride.

7 McKEXZIE ET AL.: PLUTONIUM 1239 On this basis, the equilibrium constants, calculated from the expression derived for the uranium trichloride experiments, were used to predict the extraction of plutonium in the magnesium chloride experiments. At the conclusion of an experiment the concentration of uranium trichloride in the salt phase was determined (by chemical analysis) and substituted into the mass-action expression to calculate the plutonium distribution, again, assuming ideal solution behavior in the salt and inetal phases. The extraction of plutonium has been examined at three temperatures (Table VI) and for various times (Table VII). The tables show the mole fraction of uranium trichloride in the salt and the proportions of original plutonium in the salt and in the metal at the end of each experiment. The latter is compared with the calculated value for plutonium remaining in the uranium. TABLE VI EFFECT OF TEMPERATURE ON EXTR4CTION OF PLUTONIUM BY MAGSESIUJI CHLORIDE Contacting time = 30 minutes - Pu distribution (yo original) Final UCI3 in salt Tempera- Esptl. Calc. ture \\'t U Wt ibigci? % ;\iio!e (" c) (a) (a) Oriai~lal U fraction Salt U U TABLE VII EFFECT OF CONT~LCTING TIME ON EXTRACTION OF PLUTONIUM BY JIAGNESIUII CHLORIDE AT 1200" C -- - Final UCIJ in salt I'u distribution (% original) Esptl. Time.l.l!t 6 \.lit higci? C/o Mole (min) (8) (s) Original U fraction Salt U U GO I The reasonable agreement of the calculated and experimental results for contacting times of thirty minutes or longer provides some support for the assumptions used in malcing the calculations. Thus, the distribution of plutonium can be predicted from the concentration of uranium trichloride in the salt phase by assuming that the reaction between plutonium and uranium trichloride is at equilibrium even though magnesium chloride and uranium are still reacting. The results for the first experiment listed in Table VII indicate that this assumption does not hold for contacting times of less than thirty minutes. - T\vo additional experiments were performed at 1140 C and an argoii pressure of 100 p.s.i.g. The objective in these esperiinents \\?as to prevent the formation of uranium trichloride by preventing the distillation of magnesium metal. The data are summarized in Table VIII. While the application of pressure to this sl-stem did not completely prevent distillation, the results show that the rate of distillation mas reduced, since there was considerably less uranium trichloride formed than in other 30-minute experiments Calc.

St. John s College High School Mr. Trubic AP Midterm Review Packet 1

St. John s College High School Mr. Trubic AP Midterm Review Packet 1 Name Date Directions: Read each question carefully and write your response in the space provided following each question. Your responses to these questions will be scored on the basis of the accuracy and