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Distribution of plutonium, americium, and several rare earth fission product elements between liquid cadmium and LiClKCl eutectic
Journal of Alloys and Compounds, 199 (1993) 77-84 JALCOM 671
77
Distribution of plutonium, americium, and several rare earth fission product elements between liquid cadmium and LiC1-KC1 eutectic John
P. A c k e r m a n
and Jack L. Settle
Chemical Technology Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439 (USA)
(Received November 19, 1992; in final form January 21, 1993)
Abstract
Separation factors were measured that describe the partition between molten cadmium and molten LiCI-KCI eutectic of plutonium, americium, praseodymium, neodymium, cerium, lanthanum, gadolinium, dysprosium, and yttrium. The temperature range was 753-788 K, and the range of concentrations was that allowed by the sensitivity of the chemical analysis methods. Mean separation factors were derived for Am-Pu, Nd-Am, Nd-Pu, Nd-Pr, Gd-La, Dy-La, La-Ce, La-Nd, Y-La, and Y-Nd. Where previously published data were available, agreement was good. For convenience, the following series of separation factors relative to plutonium was derived by combining the measured separation factors: Pu, 1.00 (basis); Am, 1.54; Pr, 22.9; Nd, 23.4; Ce, 26; La, 70; Gd, 77; Dy, 270; Y, 3000. These data are used in calculating the distribution of the actinide and rare earth elements in the pyrochemical reprocessing of spent fuel from the Integral Fast Reactor.
I. Introduction
The recovery of practically all of the trans-uranium ( T R U ) actinide elements (plutonium, neptunium, americium and curium) from spent nuclear fuel is an essential part of the Integral Fast Reactor (IFR) advanced reactor concept [1, 2]. After recovery, the T R U elements are consumed as fuel in the reactor. The spent IFR fuel is treated by a pyrochemical process wherein the actinide fuel constituents are separated from structural cladding material and the fission product elements by partitioning between a metal phase and molten LiC1-KCI [3]. From a chemical perspective, the isolation of noble metals (zirconium and those metals that are less easily oxidized) and active metals (those more readily oxidized than the rare earth metals) is relatively straightforward. Under process conditions, noble metals are never perceptibly oxidized into the salt phase; they remain as metals throughout the process. The highly active alkali and alkaline earth metals are oxidized almost completely into the salt, where they remain throughout the process. Fission product iodine also remains in the salt. The lanthanide and actinide elements, however, are distributed between metal and salt phases. Differences in distribution between metal and salt are exploited to separate the actinides from the lanthanides. In IFR fuel processing, the actinide elements in the fuel are electrotransported from an anode, through the salt phase, and into a cathode, where the purified actinides are collected as metals. The anode contains
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chopped segments of spent metal fuel and the stainless steel cladding used to contain the fuel in the reactor. When the electrotransport current is applied, actinide metal is oxidized to actinide chloride at the anode. At the same time, an equivalent amount of actinide chloride is reduced to metal at the cathode. Actinide chlorides are required to be present in the salt for electrotransport to occur; the total amount of actinide chlorides does not change during electrotransport. If there are rare earth metals in the anode, they are oxidized in preference to the actinides, resulting in a reduction in actinide chloride concentration during electrotransport. To avoid this problem, an amount of oxidant, such as UC13 or CdCI2, equivalent to the estimated rare earth content is added prior to electrotransport. The actinide metal deposited at the cathode is not usually of the same composition as the metal oxidized at the anode; the difference is balanced by a change in the relative amounts of the actinide chlorides. Two kinds of cathodes are used: solid mandrel cathodes and liquid cadmium cathodes. Uranium is sufficiently more noble than the other actinides and the rare earths that only an essentially pure solid uranium product deposits onto solid cathodes. Recovery of pure uranium on solid cathodes is possible over a range of salt compositions much wider than is required by the process [4]. It is thus possible to adjust the salt composition by removal of uranium on a solid cathode; uranium chloride is reduced at the solid cathode, but both uranium and the transuranium ( T R U ) elements
© 1993- Elsevier Sequoia. All rights reserved
78
J. P. Ackerman, J. L. Settle~Distribution of fission product elements
(neptunium, plutonium, americium and curium) are oxidized to chlorides at the anode. In liquid cadmium, the T R U metals are stabilized by interaction with the cadmium, i.e. they have very low activity coefficients. Uranium is not stabilized in this way. The liquid cadmium cathode can thus be made to recover small amounts of uranium relative to the T R U elements; it will also contain small amounts of rare earths. The composition of the liquid cadmium cathode product depends on the salt composition and relative affinities of the actinide and lanthanide elements for the salt and metal phases; these affinities are measured by the separation factors. Extraction of T R U elements from the salt phase with excess uranium dissolved in liquid cadmium can be used for complete removal of the T R U actinides from the salt in preparation for periodic removal of the highly radioactive rare earth fission products. The bulk of the rare earths remain in the salt after uranium extraction and are transferred to a waste stream in a subsequent reduction step. Because the distribution of rare earths and actinides among the product and waste streams is largely determined by partition between salt and metal phases, knowledge of their partition behavior is essential for design, development, and operation of the pyrochemical fuel recovery process. In the present work, at least one separation factor was measured for each element, using techniques similar to those described in a previous paper on the partition of lanthanum and neodymium [5]. Chemically similar elements have small and easily measured separation factors; dissimilar elements have large separation factors that are difficult to measure directly with good precision. It is possible, however, to derive a set of separation factors for every pair of elements by "chaining" the separation factors of neighboring elements. This was done; the results are expressed as separation factors for each element relative to plutonium for convenience. Because the distribution of actinides has been well established by evaluation of a large body of concentration data obtained during IFR process development [6], it is possible to predict the simultaneous distributions of the actinides from uranium to curium and all the rare earths included in this study.
2. Theory The thermodynamics of partition of actinides and lanthanides between metal and salt phases has been discussed previously [5, 3]. The partition of the elements is largely determined by the stability of their chlorides [7] and the activity coefficients of the chlorides in LiCI-KCI and of the metals in cadmium solution. The activity coefficients of the rare earth and T R U actinide
metals in cadmium tend to be small [8, 9] and to differ considerably from each other. They may also be affected by interactions among the metals in multisolute cadmium solutions. Activity coefficients of the salts, however, tend to be closer to unity and less variable [10]. The elements in this study all form trichlorides under the conditions used (as do all the actinide elements under IFR processing conditions), so that the separation factors take a particularly simple mathematical form. The distribution of any pair of elements between cadmium and molten salt phases is described by [MC13] + [M'(cd)] '
' [M(cd)] + [M'CI3]
(A)
where M and M' are rare earth or actinide elements. The corresponding equilibrium constant expression is: [aM]JaM,c,3] Keq(A) =
-
[aM,][aMa3] [x.,][x,,,,ao] [ /M'I['YMCI3]
(1)
Here, X indicates mole fraction, a denotes activity, and 3, signifies the activity coefficient. The separation factor corresponding to reaction A is:
IX,,,,]IX,,,,,ad SF(A) = [XM,][X,c,3]
(2)
Throughout this paper, the chemical equation will be written with the more active metal on the left-hand side; the values of all separation factors will thus be greater than unity. Also, "the M'-M separation factor" is written with the more active metal first. Unlike the equilibrium constant, the separation factor is not necessarily constant over wide composition ranges unless the product of the activity coefficients (the second term on the right-hand side of eqn. (1)) is constant. The individual activity coefficients in eqn. (1) can be far from unity, especially in the metal phase [8, 9]. In this work, however, the separation factors appeared to be constant over the concentration ranges studied, as if the solutes obeyed Henry's law and each solute had only one oxidation state. A large value of separation factor means that at least one solute concentration must be small, because of limitations on solubilities and practical amounts of reagents. If the separation factor becomes large, the sensitivity of the chemical analysis for one or two of the solutes usually does not allow direct determination of the separation factor with acceptable precision. Small separation factors were measured wherever possible. Larger separation factors were derived by combining appropriate smaller factors, as follows. Constant separation factors can be multiplied and divided in the same way that equilibrium constants are combined when the corresponding chemical equations
J. P. Ackerrnan, J. L. Settle~Distribution of fission product elements are added and subtracted. For example, [M'C13] + [M"(ca)] ¢ ' [M'(cd)] + [M"C13]
(B)
79
after each temperature change or reagent addition. This process was repeated several times to measure separation factors over a range of oxidizing conditions.
has a separation factor of S F ( B ) - [XM'][XM"cI3]
(3)
[XM-I[XM,c 3] Adding reactions A and B gives
[MC13] + [M"(c~)] , SF(C) -
, [M(c~)] + [M"C13]
[XM][XM-a,] [x q[X c,3]
(C) (4)
The separation factor for reaction C is the product of the separation factors for reactions A and B. Had the reactions been subtracted, the separation factors would have been divided. This method is used to derive large separation factors and, for convenience, to display a set of separation factors relative to one reference element.
3. Experimental details 3.1. Apparatus The techniques and apparatus were similar to those previously described in more detail [5]. The experiments were conducted in an isothermal zone (+0.5 K or better) of an externally heated furnace well that was located in the floor of a helium-atmosphere glovebox. Water and oxygen levels were less than 2 ppm by volume in the helium atmosphere. A solution of solute metal in cadmium (typically 4-500 g) was covered with salt (typically 175-200 g); salt and cadmium were held in a tantalum crucible. A tantalum shield tube extending through the salt allowed insertion of a sampling tube into the cadmium without gross contamination of the tube with salt; the cadmium was melted, the shield tube was placed, and the cadmium was refrozen with the shield in place prior to addition of salt to the crucible. A closed-end tantalum tube extending into the melts was provided with blades for stirring; it also served as a protection tube for a calibrated thermocouple. 3.2. Procedures Aliquots of the salt and metal phases were drawn through tantalum frits into preheated tantalum sampling tubes by means of a slight suction; the sample size averaged approximately 2 g. After samples were taken at 755, 773, and 785 K, cadmium chloride was added to the salt to oxidize some of the solute from the metal phase, forming the chloride of that solute metal in the salt phase; then another set of samples was taken. The melts were equilibrated with stirring at least overnight
3.3. Sampling and chemical analysis Both salt and metal samples were dissolved in dilute nitric acid and analyzed for potassium, lithium, and solute metals by atomic emission spectroscopy using an inductively coupled plasma source (ICP-AES). The cadmium content in the metal samples was determined by titration using ethylenediaminetetraactetic acid. The ICP-AES method was calibrated against standards traceable to NIST for each element. Chloride was determined gravimetrically by precipitation with silver nitrate. Plutonium and neodymium were also determined by isotope-dilution mass spectrometry, and relative concentrations of americium were measured by gamma counting using an intrinsic germanium detector. At the 95% confidence level, the precision of the ICP-AES technique is + 5% for substances present in concentrations above 0.1 wt.%, + 10% for concentrations in the range 0.01%-0.1%, and +20% for concentrations in the range 0.001%-0.01%. The precision of the counting techniques was similar to that of ICPAES, but the precision of the mass spectrometric technique was thought to be better by a factor of five. The reported concentrations were corrected for possible cross-contamination by using potassium as a tracer for salt contamination of metal samples and cadmium to measure contamination of the salt. The entire tracer concentration was regarded as resulting from phase admixture. The metal contamination of the salt phase was always negligible; corrections for salt contamination of the metal phase were quite small. 3.4. Materials LiCI-KC1 eutectic salt (58 mol% LiC1) was purchased from APL Engineered Materials. It was delivered in sealed glass ampoules and contained less than 50 ppm water (Karl Fischer titration by the vendor). CdClz (stated purity 99.999%) was obtained from AESAR, as were the rare earth metals (stated purity 99.99%, metal basis, vacuum remelted Reacton grade, lump form, in sealed glass ampoules) and cadmium shot (stated purity 99.9999%). Plutonium was obtained from the Special Materials Division of Argonne National Laboratory.
4. Results 4.1. Plutonium, americium, and neodymium Americium is formed in plutonium by /3-decay of znlpu; the relatively small amounts of americium in the salt and metal phase samples were determined by gamma
J. P. Ackerman, J. L. Settle/Distn'bution of fission product elements
80
counting. For the purpose of calculating separation factors, the ratio of the concentration of americium in the salt phase to that in the metal phase is all that is required; absolute concentrations are not needed. The concentrations of americium and neodymium in eight salt and metal sample pairs are presented in Table 1; the corresponding separation factors are given in Table 2. The data in these tables have been corrected for phase admixture as described in Section 3. The first set of three sample pairs is not included in these tables because it was measured before cadmium chloride was added to the salt, and metal ion concentrations in the salt phase were too low to allow a precision of + 20% or better in the chemical analysis. Meaningful separation factors could be calculated after the addition of cadmium chloride to the experiment, because concentrations fell within a range that allowed acceptably precise measurement. A total of 9.4867 g of cadmium chloride was added in three aliquots and was consumed by oxidation of solute metals. The next addition of cadmium chloride resulted in an excess of oxidant. Gamma counting was inadvertently not requested for samples 7 and 8. Data from the ninth sample pair are not included in Tables 1 and 2 because the Nd-Pu and Nd-Am separation factors associated with this pair differ from the mean of the other values by 69 standard deviations; the neodymium concentration in cadmium appeared to be in error. As shown in Table 2, the mean separation factors are 23.4+ 1.2 for Nd-Pu (1.6 units larger than uncorrected data), 15.5+ 1.1 for Nd-Am (2.2 units larger than uncorrected data) and 1.54 + 0.15 for Am-Pu (0.02 units larger than uncorrected data). All uncertainties quoted throughout this paper are + 2 times the standard deviation of the mean ( + 20"). The reliability of the Nd-Pu separation factor is the highest of those reported here because of the high neodymium and plutonium concentrations and the intrinsic reliability of the isotope dilution analytical technique. This separation factor provides a dependable
link between the rare earth separation factors measured in this work and the actinide separation factors compiled by Koyama et al. [6] through analysis of process data.
4.2. Lanthanum and cerium Table 3 gives the results of nine independent measurements of the La-Ce separation factor. In the first set of three sample pairs, which was taken prior to addition of cadmium chloride to the salt, there was insufficient lanthanum and cerium in the salt phase for analysis. A total of 9.2879 g of cadmium chloride was added in three batches and consumed by oxidation of solute metals, but the next addition of cadmium chloride resulted in an excess of oxidant. The mean of the La-Ce separation factor measurements is 2.72+0.37; this is 0.007 units larger than the value calculated from uncorrected data. 4.3. Yttrium, lanthanum, and neodymium Separation factors among yttrium, neodymium and lanthanum were measured simultaneously; solute concentrations are given in Table 4. Samples 1-3 were taken prior to the addition of cadmium chloride, when the rare earth chloride concentrations were too small to be measured reliably. Neodymium and lanthanum concentrations in the metal became too small for reliable measurement after sample 17; yttrium concentrations were too small after sample 16. The 14 independent determinations of the La-Nd separation factor shown in Table 5 have a mean value of 3.66 + 0.76; this is 0.06 units larger than the value based on uncorrected data. This independent determination agrees with the previously published value [5] of 2.5+ 1.1. Combining the La-Nd separation factors from Table 5 with the previously published values gave a set of 32 values; the mean of these is 3.01+0.71. Based on 13 measurements, the mean Y-La separation factor is 42.5+9.3 (7.1 units larger than uncorrected data), and the mean Y-Nd separation factor is 140 + 21 (23 units larger than uncorrected data).
T A B L E 1. Solute concentrations for Pu, Am, and Nd Sample
4 5 6 7 8 10 11 12
Temperature (K)
790 770 754 775 773 790 772 755
Weight CdCI 2
Milligram of element per gram of sample
cum. (g)
Pu in salt
Pu in Cd
Nd in salt
Nd in Cd
A m in salt
Am in Cd
2.9265 2.9265 2.9265 6.2474 6.2474 9.4867 9.4867 9.4867
3.38 3.32 3.35 15.4 14.7 45.1 45.3 45.6
29.6 25.4 20.8 30.9 30.0 22.1 20.5 20.2
16.3 16.6 18.0 33.2 33.7 40.3 40.2 40.6
6.29 5.37 4.32 2.93 2.93 0.961 0.797 0.704
0.150 0.107 0.131
0.724 0.616 0.446
1.13 1.12 1.13
0.431 0.368 0.341
Counts (109 dpm)
J. P. Ackerman, J. L. Settle~Distribution of fission product elements TABLE 2. Separation factors among Pu, Am, and Nd Sample
Temperature
Separation factor
(K) 4 5 6 7 8 10 11 12
790 770 754 775 773 790 772 755
Mean Uncertainty ( ± 2 ~ )
Nd-Pu
Nd-Am
Am-Pu
22.7 23.7 25.9 22.6 23.4 20.5 22.8 25.6
12.5 17.9 14.2
1.81 1.33 1.83
16.0 16.6 16.0
1.28 1.37 1.59
23.4 1.2
15.5 1.1
1.54 0.15
4.4. Praseodymium, gadolinium, and dysprosium At the onset of this work, a scoping experiment involving cerium, praseodymium, neodymium, lanthanum, gadolinium, dysprosium, samarium and europium was conducted in much the same way as the experiments described above. The objective was to establish the approximate order of the separation factors. In this experiment, however, cadmium and salt were added to the apparatus at various times, and experimental and analytical techniques were being developed and evaluated. Results showed that separate experiments under carefully controlled and highly reducing conditions are required to establish separation factors for samarium and europium; those experiments are nearly complete. Oxidizing conditions were suitable for the approximate determination of separation factors of praseodymium, gadolinium and dysprosium from several of the sample pairs. Analytical results for all sample pairs in the usable range are presented in Table 6, and separation factors are given in Table 7. Data for lanthanum and neodymium are included to link these three
lanthanides to the other lanthanides. Because the experimental and analytical techniques were under development, the separation factors derived in this experiment may be less reliable than others described in this paper. Separation factors of 1.02 + 0.04 for Nd-Pr, 1.10+0.33 for Gd-La and 3.8+1.4 for Dy-La were used (below) to establish the position of these three elements in the separation factor series.
5. Discussion
No trend in separation factors with oxidizing condition, i.e. with the amount of cadmium chloride added, was observed in any of the experiments reported here. This invariance is evidence that all chlorides were present in only a single oxidation state. Significant stability of a low-oxidation state chloride of an element would have resulted, under reducing conditions, in relatively large amounts of that element in the salt phase, and hence in an apparent dependence of its separation factors on oxidizing condition. In this work, no significant variation of separation factor with temperature was ever seen; any such variation is smaller than experimental uncertainties within the temperature range that was investigated. Apparently, the temperature coefficients of the activity coefficients and free energies balance each other to such an extent that experimental uncertainties obscure any temperature effects over the fairly short temperature range investigated. Sakata et al. [11] measured separation factors for lanthanum, cerium and yttrium at 773 K and 873 K and of these plus praseodymium and neodymium at 673 K using unfiltered samples and a quartz and alumina apparatus. They described a trend for separation factors to approach unity slowly as the temperature increased, but the scatter in their data was not reported.
TABLE 3. Solute concentrations and separation factors for the L a ~ e Sample
4 5 6 7 8 9 10 11 12
Temperature (K)
774 753 787 773 788 753 789 774 753
*Mean 2.72; uncertainty ( + 2or), 0.3.
81
pair
Weight CdC12 cum. (g)
Milligram of element per gram of sample La in salt
La in Cd
Ce in salt
Ce in Cd
1.2412 1.2412 1.2412 5.2012 5.2012 5.2012 9.2879 9.2879 9.2879
0.544 0.586 0.757 2.900 4.700 4.640 10.000 10.200 10.200
0.496 0.290 0.937 0.948 1.29 0.500 1.04 0.980 0.470
2.00 2.25 2.29 10.3 10.3 10.3 17.6 18.4 18.7
5.61 3.74 7.35 4.85 6.95 3.31 5.13 4.95 2.58
Separation factor a
3.08 3.36 2.60 1.44 2.40 2.98 2.80 2.80 2.99
82
J. P. Ackerman, J. L. Settle~Distribution of fission product elements
TABLE 4. Solute concentrations for Y, La, and Nd Sample
4 5 6 7 8 9 10 11 12 13 14 15 16 17
Temperature (K)
Weight CdCI2 cum. (g)
773 788 753 773 753 788 773 788 753 773 753 788 788 773
Milligram of element per gram of sample
0.7457 0.7457 0.7457 2.2277 2.2277 2.2277 17.4437 17.4437 17.4437 27.6641 27.6641 27.6641 35.2330 35.2330
Nd in salt
Nd in Cd
La in salt
La in Cd
Y in salt
Y in Cd
0.025 0.030 0.019 0.091 0.036 0.033 0.766 0.747 0.431 3.05 1.79 3.09 19.3 17.8
11.0 11.7 7.39 9.55 7.25 10.2 11.5 13.4 8.44 13.1 9.83 16.0 13.6 10.5
0.017 0.028 0.011 0.024 0.017 0.029 0.229 0.317 0.124 0.650 0.170 0.920 5.58 5.63
1.26 2.35 0.680 1.510 0.751 2.50 1.25 2.06 0.516 0.820 0.319 1.70 2.22 1.07
1.80 1.67 1.56 3.95 3.98 3.89 33.4 34.1 35.1 53.4 55.3 52.3 57.8 55.8
5.29 6.09 5.02 5.85 5.25 6.44 5.34 5.08 4.59 5.57 1.52 1.52 0.333 -
TABLE 5. Separation factors among Y, La, and Nd Sample
4 5 6 7 8 9 10 11 12 13 14 15 16 17
Temperature (K) 773 788 753 773 753 788 773 788 753 773 753 788 788 773
Mean Uncertainty (±2~)
Separation factor LaNd
Y-La
Y-Nd
5.98 4.69 6.36 1.67 4.58 3.62 2.75 2.75 4.71 3.57 2.92 2.81 1.77 3.10
25.3 23.0 19.2 43.3 33.5 52.1 34.1 43.7 31.8 46.4 68.3 63.5 69.1
151 108 122 71.1 153 189 93.9 120 150 165 200 178 122
3.66 0.76
42.5 9.3
140. 21.
Separation factors are useful for relating the concentrations of solutes in liquid phases, but the compositions of any coexisting solid phases must be determined separately. R e d u c e d solubilities in cadmium were observed in the course of this work whenever concentrations of two or m o r e rare earths a p p r o a c h e d their binary solubilities. This effect has been described quantitatively and attributed to interactions among the c a d m i u m - r a r e earth intermetallic compounds that precipitate from saturated solution [5]. The effect was invoked by Sakata et al. [11] to explain mass-balance discrepancies. Koyama et al. [6] compiled separation factors relative to uranium for neodymium, cerium, curium, americium
and plutonium by statistical analysis of a large volume of process d a t a taken at 773 K. The scatter is reported for all their results. R e c o m m e n d e d values from this work are listed in Table 8, along with a comparison with the results derived from Sakata et al. and Koyama et al. It was necessary to calculate equivalent separation factors, since the data of Sakata et al. are reported as separation factors relative to cerium, and Koyama et al. report their results as separation factors relative to uranium. The agreement among the three data sets is good.
6. Conclusions The following series of separation factors relative to plutonium was derived from the r e c o m m e n d e d separation factors of Table 8: Pu, 1 (basis); Am, 1.54; Pr, 22.9; Nd, 23.4; Ce, 26; La, 70; Gd, 77; Dy, 270; Y, 3000. Koyama etal. give the following series of separation factors relative to uranium: U, 1 (basis); Pu, 1.88; Np, 2.12; Am, 3.08; Cm, 3.52. We extend that series as follows: Pr, 43.1; Nd, 44.0; Ce, 49; La, 130; Gd, 150; Dy, 500; Y, 6000. The order of elements in this series is quite different from their order in a series arranged by the free energy of formation of their chlorides, namely U, Np, Pu, Dy, Y, Gd, Am, Nd, Ce, Pr and La [7]. Evidently, the activity coefficients, especially of the metals in cadmium solution, act to reorder the series. Examination of the separation factors shows that separation of the T R U actinides from uranium by partition between cadmium and molten salt is difficult, although a process for separating usable uranium-rich and TRU-rich products in this way can be devised [12]. Separation of actinides from even the heavy rare earths
J. P. Ackerman, J. L. Settle~Distribution of fission product elements
83
T A B L E 6. Solute concentrations for Pr, G d , a n d D y Sample
7 8 9 10 11 12 13
M i l l i g r a m o f element per g r a m o f s a m p l e N d in salt
N d in Cd
L a in salt
L a in Cd
Pr in salt
Pr in Cd
D y in salt
D y in Cd
G d in salt
G d in Cd
0.228 2.17 2.28 15.1 10.2 6.84 10.4
1.18 1.31 1.48 1.02 0.965 0.688 0.180
0.189 4.01 5.20 25.0 10.5 10.0 11.5
0.711 0.730 0.939 0.402 0.425 0.308 0.060
0.258 2.42 2.63 18.6 12.0 8.40 13.2
1.24 1.48 1.71 1.30 1.23 0.888 0.240
3.09 10.3 10.2 18.7 18.4 7.28 8.30
1.74 0.490 0.529 0.165 0.125 0.085 0.020
0.907 6.37 6.09 17.5 12.9 7.35 8.50
1.73 1.06 1.14 0.406 0.125 0.084 0.020
T A B L E 7. S e p a r a t i o n factors a m o n g La, N d , Pr, G d , a n d D y Sample
7 8 9 10 11 12 13
Temperature
Weight
Separation factor
(K)
CdC12 c n m . (g)
La-Nd
La-Pr
Gd-La
Dy-La
Nd-Pr
Gd-Nd
Dy-Nd
1.8728 4.0685 4.0685 8.4107 8.4107 8.4107 10.006
1.37 3.32 3.58 4.19 2.34 3.26 3.32
1.27 3.36 3.59 4.35 2.55 3.43 3.48
1.98 1.09 0.97 0.69 1.32 0.93 0.74
6.67 3.83 3.48 1.82 5.95 2.64 2.17
0.93 1.01 1.00 1.04 1.09 1.05 1.05
2.72 3.63 3.46 2.90 3.09 3.02 2.45
9.16 12.7 12.5 7.62 13.9 8.62 7.18
3.06 0.70
3.15 0.74
1.10 0.33
3.79 1.41
1.02 0.04
3.04 0.31
10.2 2.1
755 756 767 776 785 755 792
Mean
Uncertainty ( + 2tr)
T A B L E 8. S e p a r a t i o n factors f r o m this work, Sakata et al. [11] a n d K o y a m a et al. [6]
Am-Pu Nd-Am Nd-Pu Ce-Pu Nd-Pr Gd-La Dy-La La-Ce La-Nd Y-La Y-Nd
Derived from S a k a t a et al.
Derived f r o m K o y a m a et al.
T h i s work
-
1.64 a 12.7. a 21.0 a 24.0 a _ _
1.54 _+0.15 15.5 + 1.1 23.4:1:1.2 26.0 a 1.02 + 0.04 1.10___0.33 3.79 -¢-1.41 2.72 + 0.37 3.01 + 0 . 7 1 42.5 + 9.3 140.0 + 21.0
1.2 ~' b 2.7 a 3.1 a'b 33.6 ~ 115 ~"b
Acknowledgments We wish to thank Edmund Huff, Delbert Bowers, Alice Essling, Irene Fox, Kenneth Jensen, Steve Newnam, Everett Rauh, Carmen Sabau, and Florence Smith for providing the chemical analyses. This work was supported by the US Department of Energy, Nuclear Energy Research and Development Program, under contract W-31-109-Eng-38. The submitted manuscript has been authored by a contractor of the US Government under contract No. W-31-109-ENG-38. Accordingly, the US Government retains a non-exclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for US Government purposes.
aValue derived f r o m stated results. bValue at 723 K.
References
is easier than separations among the actinides. The set of separation factors reported above is suitable for calculating element distributions in the IFR electrorefiner and in countercurrent extractors being developed for removing traces of TRU elements from electrolyte salt in the presence of larger amounts of rare earths.
1 C. Till a n d Y. C h a n g , E v o l u t i o n o f t h e liquid m e t a l reactor: the Integral F a s t R e a c t o r ( I F R ) concept, Proc. Am. Power Conf., Chicago, IL, April 24-26, 1989, Vol. S1, A m . Power Conf., Chicago, IL, 1989, pp. 688-691. 2 L. Burris, R. S t e u n e n b e r g a n d W. Miller, T h e application o f electrorefining for recovery a n d purification o f fuel disc h a r g e d from the I n t e g r a l F a s t R e a c t o r , A I C h E Syrup. Ser. No. 254, 83 (1987) 135-142.
84
J. P. Ackerman, J. L. Settle~Distribution of fission product elements
3 J. Ackerman, Chemical basis for pyrochemical reprocessing of nuclear fuel, lnd. Eng. Chem. Res., 29 (1991) 142-145. 4 Z. Tomczuk, J. Ackerman and W. Miller, Transport to solid electrodes in pyrochemical reprocessing of nuclear fuel, J. Electrochem. Soc., 139 (1992) 3523-3528. 5 J. Ackerman and J. Settle, Partition of lanthanum and neodymium metals and chloride salts between molten cadmium and molten LiC1-KCI eutectic, Z Alloys Comp., 177 (1991) 129-141. 6 T. Koyama, T. Johnson and D. Fischer, Compilation of actinide distribution data for molten chloride salt-cadmium metal systems, J. Alloys Comp., 189 (1992) 37--44.
7 L. Pancratz, Bulletin 674, 1984 (United States Department of the Interior, Bureau of Mines). 8 I. Johnson and R. Yonco, Metall. Trans., 1 (1970) 905-910. 9 I. Johnson, M. Chasanov and R. Yonco, Trans. Met. Soc. AIME, 233 (1965) 1408-1414. 10 J. Lumsden, Therrnodynamics of Molten Salt Mixtures, Academic Press, London, 1966. 11 M. Sakata, M. Kurata, T. Hijikata and T. Inoue, J. Nucl. Mater., 185 (1991) 56-85. 12 J. Ackerman and W. Miller, Single electrorefining cell for separate recovery of uranium and uranium/plutonium from spent fuel, US Patent 4,880,506, 1987.
77
Distribution of plutonium, americium, and several rare earth fission product elements between liquid cadmium and LiC1-KC1 eutectic John
P. A c k e r m a n
and Jack L. Settle
Chemical Technology Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439 (USA)
(Received November 19, 1992; in final form January 21, 1993)
Abstract
Separation factors were measured that describe the partition between molten cadmium and molten LiCI-KCI eutectic of plutonium, americium, praseodymium, neodymium, cerium, lanthanum, gadolinium, dysprosium, and yttrium. The temperature range was 753-788 K, and the range of concentrations was that allowed by the sensitivity of the chemical analysis methods. Mean separation factors were derived for Am-Pu, Nd-Am, Nd-Pu, Nd-Pr, Gd-La, Dy-La, La-Ce, La-Nd, Y-La, and Y-Nd. Where previously published data were available, agreement was good. For convenience, the following series of separation factors relative to plutonium was derived by combining the measured separation factors: Pu, 1.00 (basis); Am, 1.54; Pr, 22.9; Nd, 23.4; Ce, 26; La, 70; Gd, 77; Dy, 270; Y, 3000. These data are used in calculating the distribution of the actinide and rare earth elements in the pyrochemical reprocessing of spent fuel from the Integral Fast Reactor.
I. Introduction
The recovery of practically all of the trans-uranium ( T R U ) actinide elements (plutonium, neptunium, americium and curium) from spent nuclear fuel is an essential part of the Integral Fast Reactor (IFR) advanced reactor concept [1, 2]. After recovery, the T R U elements are consumed as fuel in the reactor. The spent IFR fuel is treated by a pyrochemical process wherein the actinide fuel constituents are separated from structural cladding material and the fission product elements by partitioning between a metal phase and molten LiC1-KCI [3]. From a chemical perspective, the isolation of noble metals (zirconium and those metals that are less easily oxidized) and active metals (those more readily oxidized than the rare earth metals) is relatively straightforward. Under process conditions, noble metals are never perceptibly oxidized into the salt phase; they remain as metals throughout the process. The highly active alkali and alkaline earth metals are oxidized almost completely into the salt, where they remain throughout the process. Fission product iodine also remains in the salt. The lanthanide and actinide elements, however, are distributed between metal and salt phases. Differences in distribution between metal and salt are exploited to separate the actinides from the lanthanides. In IFR fuel processing, the actinide elements in the fuel are electrotransported from an anode, through the salt phase, and into a cathode, where the purified actinides are collected as metals. The anode contains
0925-8388/93/$6.00
chopped segments of spent metal fuel and the stainless steel cladding used to contain the fuel in the reactor. When the electrotransport current is applied, actinide metal is oxidized to actinide chloride at the anode. At the same time, an equivalent amount of actinide chloride is reduced to metal at the cathode. Actinide chlorides are required to be present in the salt for electrotransport to occur; the total amount of actinide chlorides does not change during electrotransport. If there are rare earth metals in the anode, they are oxidized in preference to the actinides, resulting in a reduction in actinide chloride concentration during electrotransport. To avoid this problem, an amount of oxidant, such as UC13 or CdCI2, equivalent to the estimated rare earth content is added prior to electrotransport. The actinide metal deposited at the cathode is not usually of the same composition as the metal oxidized at the anode; the difference is balanced by a change in the relative amounts of the actinide chlorides. Two kinds of cathodes are used: solid mandrel cathodes and liquid cadmium cathodes. Uranium is sufficiently more noble than the other actinides and the rare earths that only an essentially pure solid uranium product deposits onto solid cathodes. Recovery of pure uranium on solid cathodes is possible over a range of salt compositions much wider than is required by the process [4]. It is thus possible to adjust the salt composition by removal of uranium on a solid cathode; uranium chloride is reduced at the solid cathode, but both uranium and the transuranium ( T R U ) elements
© 1993- Elsevier Sequoia. All rights reserved
78
J. P. Ackerman, J. L. Settle~Distribution of fission product elements
(neptunium, plutonium, americium and curium) are oxidized to chlorides at the anode. In liquid cadmium, the T R U metals are stabilized by interaction with the cadmium, i.e. they have very low activity coefficients. Uranium is not stabilized in this way. The liquid cadmium cathode can thus be made to recover small amounts of uranium relative to the T R U elements; it will also contain small amounts of rare earths. The composition of the liquid cadmium cathode product depends on the salt composition and relative affinities of the actinide and lanthanide elements for the salt and metal phases; these affinities are measured by the separation factors. Extraction of T R U elements from the salt phase with excess uranium dissolved in liquid cadmium can be used for complete removal of the T R U actinides from the salt in preparation for periodic removal of the highly radioactive rare earth fission products. The bulk of the rare earths remain in the salt after uranium extraction and are transferred to a waste stream in a subsequent reduction step. Because the distribution of rare earths and actinides among the product and waste streams is largely determined by partition between salt and metal phases, knowledge of their partition behavior is essential for design, development, and operation of the pyrochemical fuel recovery process. In the present work, at least one separation factor was measured for each element, using techniques similar to those described in a previous paper on the partition of lanthanum and neodymium [5]. Chemically similar elements have small and easily measured separation factors; dissimilar elements have large separation factors that are difficult to measure directly with good precision. It is possible, however, to derive a set of separation factors for every pair of elements by "chaining" the separation factors of neighboring elements. This was done; the results are expressed as separation factors for each element relative to plutonium for convenience. Because the distribution of actinides has been well established by evaluation of a large body of concentration data obtained during IFR process development [6], it is possible to predict the simultaneous distributions of the actinides from uranium to curium and all the rare earths included in this study.
2. Theory The thermodynamics of partition of actinides and lanthanides between metal and salt phases has been discussed previously [5, 3]. The partition of the elements is largely determined by the stability of their chlorides [7] and the activity coefficients of the chlorides in LiCI-KCI and of the metals in cadmium solution. The activity coefficients of the rare earth and T R U actinide
metals in cadmium tend to be small [8, 9] and to differ considerably from each other. They may also be affected by interactions among the metals in multisolute cadmium solutions. Activity coefficients of the salts, however, tend to be closer to unity and less variable [10]. The elements in this study all form trichlorides under the conditions used (as do all the actinide elements under IFR processing conditions), so that the separation factors take a particularly simple mathematical form. The distribution of any pair of elements between cadmium and molten salt phases is described by [MC13] + [M'(cd)] '
' [M(cd)] + [M'CI3]
(A)
where M and M' are rare earth or actinide elements. The corresponding equilibrium constant expression is: [aM]JaM,c,3] Keq(A) =
-
[aM,][aMa3] [x.,][x,,,,ao] [ /M'I['YMCI3]
(1)
Here, X indicates mole fraction, a denotes activity, and 3, signifies the activity coefficient. The separation factor corresponding to reaction A is:
IX,,,,]IX,,,,,ad SF(A) = [XM,][X,c,3]
(2)
Throughout this paper, the chemical equation will be written with the more active metal on the left-hand side; the values of all separation factors will thus be greater than unity. Also, "the M'-M separation factor" is written with the more active metal first. Unlike the equilibrium constant, the separation factor is not necessarily constant over wide composition ranges unless the product of the activity coefficients (the second term on the right-hand side of eqn. (1)) is constant. The individual activity coefficients in eqn. (1) can be far from unity, especially in the metal phase [8, 9]. In this work, however, the separation factors appeared to be constant over the concentration ranges studied, as if the solutes obeyed Henry's law and each solute had only one oxidation state. A large value of separation factor means that at least one solute concentration must be small, because of limitations on solubilities and practical amounts of reagents. If the separation factor becomes large, the sensitivity of the chemical analysis for one or two of the solutes usually does not allow direct determination of the separation factor with acceptable precision. Small separation factors were measured wherever possible. Larger separation factors were derived by combining appropriate smaller factors, as follows. Constant separation factors can be multiplied and divided in the same way that equilibrium constants are combined when the corresponding chemical equations
J. P. Ackerrnan, J. L. Settle~Distribution of fission product elements are added and subtracted. For example, [M'C13] + [M"(ca)] ¢ ' [M'(cd)] + [M"C13]
(B)
79
after each temperature change or reagent addition. This process was repeated several times to measure separation factors over a range of oxidizing conditions.
has a separation factor of S F ( B ) - [XM'][XM"cI3]
(3)
[XM-I[XM,c 3] Adding reactions A and B gives
[MC13] + [M"(c~)] , SF(C) -
, [M(c~)] + [M"C13]
[XM][XM-a,] [x q[X c,3]
(C) (4)
The separation factor for reaction C is the product of the separation factors for reactions A and B. Had the reactions been subtracted, the separation factors would have been divided. This method is used to derive large separation factors and, for convenience, to display a set of separation factors relative to one reference element.
3. Experimental details 3.1. Apparatus The techniques and apparatus were similar to those previously described in more detail [5]. The experiments were conducted in an isothermal zone (+0.5 K or better) of an externally heated furnace well that was located in the floor of a helium-atmosphere glovebox. Water and oxygen levels were less than 2 ppm by volume in the helium atmosphere. A solution of solute metal in cadmium (typically 4-500 g) was covered with salt (typically 175-200 g); salt and cadmium were held in a tantalum crucible. A tantalum shield tube extending through the salt allowed insertion of a sampling tube into the cadmium without gross contamination of the tube with salt; the cadmium was melted, the shield tube was placed, and the cadmium was refrozen with the shield in place prior to addition of salt to the crucible. A closed-end tantalum tube extending into the melts was provided with blades for stirring; it also served as a protection tube for a calibrated thermocouple. 3.2. Procedures Aliquots of the salt and metal phases were drawn through tantalum frits into preheated tantalum sampling tubes by means of a slight suction; the sample size averaged approximately 2 g. After samples were taken at 755, 773, and 785 K, cadmium chloride was added to the salt to oxidize some of the solute from the metal phase, forming the chloride of that solute metal in the salt phase; then another set of samples was taken. The melts were equilibrated with stirring at least overnight
3.3. Sampling and chemical analysis Both salt and metal samples were dissolved in dilute nitric acid and analyzed for potassium, lithium, and solute metals by atomic emission spectroscopy using an inductively coupled plasma source (ICP-AES). The cadmium content in the metal samples was determined by titration using ethylenediaminetetraactetic acid. The ICP-AES method was calibrated against standards traceable to NIST for each element. Chloride was determined gravimetrically by precipitation with silver nitrate. Plutonium and neodymium were also determined by isotope-dilution mass spectrometry, and relative concentrations of americium were measured by gamma counting using an intrinsic germanium detector. At the 95% confidence level, the precision of the ICP-AES technique is + 5% for substances present in concentrations above 0.1 wt.%, + 10% for concentrations in the range 0.01%-0.1%, and +20% for concentrations in the range 0.001%-0.01%. The precision of the counting techniques was similar to that of ICPAES, but the precision of the mass spectrometric technique was thought to be better by a factor of five. The reported concentrations were corrected for possible cross-contamination by using potassium as a tracer for salt contamination of metal samples and cadmium to measure contamination of the salt. The entire tracer concentration was regarded as resulting from phase admixture. The metal contamination of the salt phase was always negligible; corrections for salt contamination of the metal phase were quite small. 3.4. Materials LiCI-KC1 eutectic salt (58 mol% LiC1) was purchased from APL Engineered Materials. It was delivered in sealed glass ampoules and contained less than 50 ppm water (Karl Fischer titration by the vendor). CdClz (stated purity 99.999%) was obtained from AESAR, as were the rare earth metals (stated purity 99.99%, metal basis, vacuum remelted Reacton grade, lump form, in sealed glass ampoules) and cadmium shot (stated purity 99.9999%). Plutonium was obtained from the Special Materials Division of Argonne National Laboratory.
4. Results 4.1. Plutonium, americium, and neodymium Americium is formed in plutonium by /3-decay of znlpu; the relatively small amounts of americium in the salt and metal phase samples were determined by gamma
J. P. Ackerman, J. L. Settle/Distn'bution of fission product elements
80
counting. For the purpose of calculating separation factors, the ratio of the concentration of americium in the salt phase to that in the metal phase is all that is required; absolute concentrations are not needed. The concentrations of americium and neodymium in eight salt and metal sample pairs are presented in Table 1; the corresponding separation factors are given in Table 2. The data in these tables have been corrected for phase admixture as described in Section 3. The first set of three sample pairs is not included in these tables because it was measured before cadmium chloride was added to the salt, and metal ion concentrations in the salt phase were too low to allow a precision of + 20% or better in the chemical analysis. Meaningful separation factors could be calculated after the addition of cadmium chloride to the experiment, because concentrations fell within a range that allowed acceptably precise measurement. A total of 9.4867 g of cadmium chloride was added in three aliquots and was consumed by oxidation of solute metals. The next addition of cadmium chloride resulted in an excess of oxidant. Gamma counting was inadvertently not requested for samples 7 and 8. Data from the ninth sample pair are not included in Tables 1 and 2 because the Nd-Pu and Nd-Am separation factors associated with this pair differ from the mean of the other values by 69 standard deviations; the neodymium concentration in cadmium appeared to be in error. As shown in Table 2, the mean separation factors are 23.4+ 1.2 for Nd-Pu (1.6 units larger than uncorrected data), 15.5+ 1.1 for Nd-Am (2.2 units larger than uncorrected data) and 1.54 + 0.15 for Am-Pu (0.02 units larger than uncorrected data). All uncertainties quoted throughout this paper are + 2 times the standard deviation of the mean ( + 20"). The reliability of the Nd-Pu separation factor is the highest of those reported here because of the high neodymium and plutonium concentrations and the intrinsic reliability of the isotope dilution analytical technique. This separation factor provides a dependable
link between the rare earth separation factors measured in this work and the actinide separation factors compiled by Koyama et al. [6] through analysis of process data.
4.2. Lanthanum and cerium Table 3 gives the results of nine independent measurements of the La-Ce separation factor. In the first set of three sample pairs, which was taken prior to addition of cadmium chloride to the salt, there was insufficient lanthanum and cerium in the salt phase for analysis. A total of 9.2879 g of cadmium chloride was added in three batches and consumed by oxidation of solute metals, but the next addition of cadmium chloride resulted in an excess of oxidant. The mean of the La-Ce separation factor measurements is 2.72+0.37; this is 0.007 units larger than the value calculated from uncorrected data. 4.3. Yttrium, lanthanum, and neodymium Separation factors among yttrium, neodymium and lanthanum were measured simultaneously; solute concentrations are given in Table 4. Samples 1-3 were taken prior to the addition of cadmium chloride, when the rare earth chloride concentrations were too small to be measured reliably. Neodymium and lanthanum concentrations in the metal became too small for reliable measurement after sample 17; yttrium concentrations were too small after sample 16. The 14 independent determinations of the La-Nd separation factor shown in Table 5 have a mean value of 3.66 + 0.76; this is 0.06 units larger than the value based on uncorrected data. This independent determination agrees with the previously published value [5] of 2.5+ 1.1. Combining the La-Nd separation factors from Table 5 with the previously published values gave a set of 32 values; the mean of these is 3.01+0.71. Based on 13 measurements, the mean Y-La separation factor is 42.5+9.3 (7.1 units larger than uncorrected data), and the mean Y-Nd separation factor is 140 + 21 (23 units larger than uncorrected data).
T A B L E 1. Solute concentrations for Pu, Am, and Nd Sample
4 5 6 7 8 10 11 12
Temperature (K)
790 770 754 775 773 790 772 755
Weight CdCI 2
Milligram of element per gram of sample
cum. (g)
Pu in salt
Pu in Cd
Nd in salt
Nd in Cd
A m in salt
Am in Cd
2.9265 2.9265 2.9265 6.2474 6.2474 9.4867 9.4867 9.4867
3.38 3.32 3.35 15.4 14.7 45.1 45.3 45.6
29.6 25.4 20.8 30.9 30.0 22.1 20.5 20.2
16.3 16.6 18.0 33.2 33.7 40.3 40.2 40.6
6.29 5.37 4.32 2.93 2.93 0.961 0.797 0.704
0.150 0.107 0.131
0.724 0.616 0.446
1.13 1.12 1.13
0.431 0.368 0.341
Counts (109 dpm)
J. P. Ackerman, J. L. Settle~Distribution of fission product elements TABLE 2. Separation factors among Pu, Am, and Nd Sample
Temperature
Separation factor
(K) 4 5 6 7 8 10 11 12
790 770 754 775 773 790 772 755
Mean Uncertainty ( ± 2 ~ )
Nd-Pu
Nd-Am
Am-Pu
22.7 23.7 25.9 22.6 23.4 20.5 22.8 25.6
12.5 17.9 14.2
1.81 1.33 1.83
16.0 16.6 16.0
1.28 1.37 1.59
23.4 1.2
15.5 1.1
1.54 0.15
4.4. Praseodymium, gadolinium, and dysprosium At the onset of this work, a scoping experiment involving cerium, praseodymium, neodymium, lanthanum, gadolinium, dysprosium, samarium and europium was conducted in much the same way as the experiments described above. The objective was to establish the approximate order of the separation factors. In this experiment, however, cadmium and salt were added to the apparatus at various times, and experimental and analytical techniques were being developed and evaluated. Results showed that separate experiments under carefully controlled and highly reducing conditions are required to establish separation factors for samarium and europium; those experiments are nearly complete. Oxidizing conditions were suitable for the approximate determination of separation factors of praseodymium, gadolinium and dysprosium from several of the sample pairs. Analytical results for all sample pairs in the usable range are presented in Table 6, and separation factors are given in Table 7. Data for lanthanum and neodymium are included to link these three
lanthanides to the other lanthanides. Because the experimental and analytical techniques were under development, the separation factors derived in this experiment may be less reliable than others described in this paper. Separation factors of 1.02 + 0.04 for Nd-Pr, 1.10+0.33 for Gd-La and 3.8+1.4 for Dy-La were used (below) to establish the position of these three elements in the separation factor series.
5. Discussion
No trend in separation factors with oxidizing condition, i.e. with the amount of cadmium chloride added, was observed in any of the experiments reported here. This invariance is evidence that all chlorides were present in only a single oxidation state. Significant stability of a low-oxidation state chloride of an element would have resulted, under reducing conditions, in relatively large amounts of that element in the salt phase, and hence in an apparent dependence of its separation factors on oxidizing condition. In this work, no significant variation of separation factor with temperature was ever seen; any such variation is smaller than experimental uncertainties within the temperature range that was investigated. Apparently, the temperature coefficients of the activity coefficients and free energies balance each other to such an extent that experimental uncertainties obscure any temperature effects over the fairly short temperature range investigated. Sakata et al. [11] measured separation factors for lanthanum, cerium and yttrium at 773 K and 873 K and of these plus praseodymium and neodymium at 673 K using unfiltered samples and a quartz and alumina apparatus. They described a trend for separation factors to approach unity slowly as the temperature increased, but the scatter in their data was not reported.
TABLE 3. Solute concentrations and separation factors for the L a ~ e Sample
4 5 6 7 8 9 10 11 12
Temperature (K)
774 753 787 773 788 753 789 774 753
*Mean 2.72; uncertainty ( + 2or), 0.3.
81
pair
Weight CdC12 cum. (g)
Milligram of element per gram of sample La in salt
La in Cd
Ce in salt
Ce in Cd
1.2412 1.2412 1.2412 5.2012 5.2012 5.2012 9.2879 9.2879 9.2879
0.544 0.586 0.757 2.900 4.700 4.640 10.000 10.200 10.200
0.496 0.290 0.937 0.948 1.29 0.500 1.04 0.980 0.470
2.00 2.25 2.29 10.3 10.3 10.3 17.6 18.4 18.7
5.61 3.74 7.35 4.85 6.95 3.31 5.13 4.95 2.58
Separation factor a
3.08 3.36 2.60 1.44 2.40 2.98 2.80 2.80 2.99
82
J. P. Ackerman, J. L. Settle~Distribution of fission product elements
TABLE 4. Solute concentrations for Y, La, and Nd Sample
4 5 6 7 8 9 10 11 12 13 14 15 16 17
Temperature (K)
Weight CdCI2 cum. (g)
773 788 753 773 753 788 773 788 753 773 753 788 788 773
Milligram of element per gram of sample
0.7457 0.7457 0.7457 2.2277 2.2277 2.2277 17.4437 17.4437 17.4437 27.6641 27.6641 27.6641 35.2330 35.2330
Nd in salt
Nd in Cd
La in salt
La in Cd
Y in salt
Y in Cd
0.025 0.030 0.019 0.091 0.036 0.033 0.766 0.747 0.431 3.05 1.79 3.09 19.3 17.8
11.0 11.7 7.39 9.55 7.25 10.2 11.5 13.4 8.44 13.1 9.83 16.0 13.6 10.5
0.017 0.028 0.011 0.024 0.017 0.029 0.229 0.317 0.124 0.650 0.170 0.920 5.58 5.63
1.26 2.35 0.680 1.510 0.751 2.50 1.25 2.06 0.516 0.820 0.319 1.70 2.22 1.07
1.80 1.67 1.56 3.95 3.98 3.89 33.4 34.1 35.1 53.4 55.3 52.3 57.8 55.8
5.29 6.09 5.02 5.85 5.25 6.44 5.34 5.08 4.59 5.57 1.52 1.52 0.333 -
TABLE 5. Separation factors among Y, La, and Nd Sample
4 5 6 7 8 9 10 11 12 13 14 15 16 17
Temperature (K) 773 788 753 773 753 788 773 788 753 773 753 788 788 773
Mean Uncertainty (±2~)
Separation factor LaNd
Y-La
Y-Nd
5.98 4.69 6.36 1.67 4.58 3.62 2.75 2.75 4.71 3.57 2.92 2.81 1.77 3.10
25.3 23.0 19.2 43.3 33.5 52.1 34.1 43.7 31.8 46.4 68.3 63.5 69.1
151 108 122 71.1 153 189 93.9 120 150 165 200 178 122
3.66 0.76
42.5 9.3
140. 21.
Separation factors are useful for relating the concentrations of solutes in liquid phases, but the compositions of any coexisting solid phases must be determined separately. R e d u c e d solubilities in cadmium were observed in the course of this work whenever concentrations of two or m o r e rare earths a p p r o a c h e d their binary solubilities. This effect has been described quantitatively and attributed to interactions among the c a d m i u m - r a r e earth intermetallic compounds that precipitate from saturated solution [5]. The effect was invoked by Sakata et al. [11] to explain mass-balance discrepancies. Koyama et al. [6] compiled separation factors relative to uranium for neodymium, cerium, curium, americium
and plutonium by statistical analysis of a large volume of process d a t a taken at 773 K. The scatter is reported for all their results. R e c o m m e n d e d values from this work are listed in Table 8, along with a comparison with the results derived from Sakata et al. and Koyama et al. It was necessary to calculate equivalent separation factors, since the data of Sakata et al. are reported as separation factors relative to cerium, and Koyama et al. report their results as separation factors relative to uranium. The agreement among the three data sets is good.
6. Conclusions The following series of separation factors relative to plutonium was derived from the r e c o m m e n d e d separation factors of Table 8: Pu, 1 (basis); Am, 1.54; Pr, 22.9; Nd, 23.4; Ce, 26; La, 70; Gd, 77; Dy, 270; Y, 3000. Koyama etal. give the following series of separation factors relative to uranium: U, 1 (basis); Pu, 1.88; Np, 2.12; Am, 3.08; Cm, 3.52. We extend that series as follows: Pr, 43.1; Nd, 44.0; Ce, 49; La, 130; Gd, 150; Dy, 500; Y, 6000. The order of elements in this series is quite different from their order in a series arranged by the free energy of formation of their chlorides, namely U, Np, Pu, Dy, Y, Gd, Am, Nd, Ce, Pr and La [7]. Evidently, the activity coefficients, especially of the metals in cadmium solution, act to reorder the series. Examination of the separation factors shows that separation of the T R U actinides from uranium by partition between cadmium and molten salt is difficult, although a process for separating usable uranium-rich and TRU-rich products in this way can be devised [12]. Separation of actinides from even the heavy rare earths
J. P. Ackerman, J. L. Settle~Distribution of fission product elements
83
T A B L E 6. Solute concentrations for Pr, G d , a n d D y Sample
7 8 9 10 11 12 13
M i l l i g r a m o f element per g r a m o f s a m p l e N d in salt
N d in Cd
L a in salt
L a in Cd
Pr in salt
Pr in Cd
D y in salt
D y in Cd
G d in salt
G d in Cd
0.228 2.17 2.28 15.1 10.2 6.84 10.4
1.18 1.31 1.48 1.02 0.965 0.688 0.180
0.189 4.01 5.20 25.0 10.5 10.0 11.5
0.711 0.730 0.939 0.402 0.425 0.308 0.060
0.258 2.42 2.63 18.6 12.0 8.40 13.2
1.24 1.48 1.71 1.30 1.23 0.888 0.240
3.09 10.3 10.2 18.7 18.4 7.28 8.30
1.74 0.490 0.529 0.165 0.125 0.085 0.020
0.907 6.37 6.09 17.5 12.9 7.35 8.50
1.73 1.06 1.14 0.406 0.125 0.084 0.020
T A B L E 7. S e p a r a t i o n factors a m o n g La, N d , Pr, G d , a n d D y Sample
7 8 9 10 11 12 13
Temperature
Weight
Separation factor
(K)
CdC12 c n m . (g)
La-Nd
La-Pr
Gd-La
Dy-La
Nd-Pr
Gd-Nd
Dy-Nd
1.8728 4.0685 4.0685 8.4107 8.4107 8.4107 10.006
1.37 3.32 3.58 4.19 2.34 3.26 3.32
1.27 3.36 3.59 4.35 2.55 3.43 3.48
1.98 1.09 0.97 0.69 1.32 0.93 0.74
6.67 3.83 3.48 1.82 5.95 2.64 2.17
0.93 1.01 1.00 1.04 1.09 1.05 1.05
2.72 3.63 3.46 2.90 3.09 3.02 2.45
9.16 12.7 12.5 7.62 13.9 8.62 7.18
3.06 0.70
3.15 0.74
1.10 0.33
3.79 1.41
1.02 0.04
3.04 0.31
10.2 2.1
755 756 767 776 785 755 792
Mean
Uncertainty ( + 2tr)
T A B L E 8. S e p a r a t i o n factors f r o m this work, Sakata et al. [11] a n d K o y a m a et al. [6]
Am-Pu Nd-Am Nd-Pu Ce-Pu Nd-Pr Gd-La Dy-La La-Ce La-Nd Y-La Y-Nd
Derived from S a k a t a et al.
Derived f r o m K o y a m a et al.
T h i s work
-
1.64 a 12.7. a 21.0 a 24.0 a _ _
1.54 _+0.15 15.5 + 1.1 23.4:1:1.2 26.0 a 1.02 + 0.04 1.10___0.33 3.79 -¢-1.41 2.72 + 0.37 3.01 + 0 . 7 1 42.5 + 9.3 140.0 + 21.0
1.2 ~' b 2.7 a 3.1 a'b 33.6 ~ 115 ~"b
Acknowledgments We wish to thank Edmund Huff, Delbert Bowers, Alice Essling, Irene Fox, Kenneth Jensen, Steve Newnam, Everett Rauh, Carmen Sabau, and Florence Smith for providing the chemical analyses. This work was supported by the US Department of Energy, Nuclear Energy Research and Development Program, under contract W-31-109-Eng-38. The submitted manuscript has been authored by a contractor of the US Government under contract No. W-31-109-ENG-38. Accordingly, the US Government retains a non-exclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, for US Government purposes.
aValue derived f r o m stated results. bValue at 723 K.
References
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