considerable scatter, as is inevitable in a n experiment trying to measure an error, i t I S apparent that whichever the column, the ratio increases in the direction of the first component as successively larger samples are used. The lines drawn in Figure 4 represent the theoretical estimates of the effect, using Equation 1 3 as a basis. It appears that the observed effect considerably exceeds the theoretical expectation, but, by the nature of the experiment we cannot tell whether or not the
difference is significant. It suffices to show that there are theoretical reasons and experimental evidence that detector calibrations depend on absolute sample sizes in the conditions indicated.
RECEIVED for review December 12, 1966. Accepted February 13, 1967. We acknowledge with gratitude a grant to N.D. from Unilever Research Ltd.
Separation of Lanthanide Fission Products from Nuclear Fuels by Extraction Chromatography and Cation Exchange for lsotope Dilution Mass Spectrometric Analysis S . Fredric Marsh Iduho Nucleor Corporation, Idaho Falls, Idaho A rapid (4-hour), specific, three-column sequential separation procedure has been developed for the carrier-free isolaition of the individual lanthanidesfrorn spent nuclear fuels for their measurement by isotope dilution mass spectrometry. The first column separates the lanthanide group from the trivalent actinides and all other fission products by extraction chromatography. The extractant is di-(2-ethylhexyl)phosphoric acid (D2EHP) in diisopropylbenzene supported on chromatographic-grade Chromosorb; the elutriant is lactic acid-sodium diethylenetriaminepentaacetate. The second column uses undiluted D2EHP on Chromosorb to selectively retain tetravalent cerium from strong nitric acid-potassium bromate, while the trivalent lanthanides elute. On the final column (cation exchange) the individual lanthanides are separated by chromatographic elution with ammonium a-hydroxyisobutyrate at controlled pH. All three columns operate at roomi temperature. After the first two columns, which require only 1 hour, the separated trivalent lanthanilde fraction is sufficiently free of radioactivity for transler from remotely operated facilities to a conventional laboratory. Mechanical innovations in the procedure allow a semiautomated system for more convenient operation.
MOST OF THE ELEMENTS in the lanthanide series have been known for over a century; however, it is only since 1945, when the identification of promethium completed the series, that techniques have been developed for high punty separations of the individual elements. Because of the prominence of the lanthanide group within the fission products and the unusually high nt:utron cross-sections of some of the individual nuclides, the growth af lanthanide separations knowledge has been greatly stimulated by the increasing demands of nuclear twhnology. Because of the high specific activity of fission product samples, a procedure was desired for the lanthanides a.t the microgram levels. The determination of individual lanthanide fission produ\:!\ may bc divided into three distinct parts: separarion of the lanthanide group from other elements; separation of the
1 Present address, The Babcock and Wilcox Co., P. 0. Box 1260, Lynchburg, Va.
individual lanthanide elements; and concentration analysis and isotopic analysis of the individually separated elements. There are many techniques for the separation of the lanthanide group from mixed fission products, including sequential precipitations (I) and solvent extraction (2). In nuclear fuels which have undergone the high levels of burnup typical of power reactors, however, the levels of americium and curium are sufficient to also require their separation. Americium in its hexavalent state is easily separated from the lanthanides ( 3 ) , but curium, exhibiting only trivalency in solution, quantitatively follows most lanthanide group separations. Because these two actinides constitute a serious biological hazard to laboratory personnel, their separation, preferably a t the beginning of the procedure, is highly desira ble. Existing techniques for separating the trivalent actinides from the lanthanide group include anion exchange ( 4 . 5) and solvent extraction (6) from concentrated lithium chloride solutions. These techniques are poorly suited to precede mass analysis because of the high salt concentrations involved. Mineral acid-organic solvent mixtures also have been used for the separate elution of certain lanthanides and trivalent actinides from ion exchange columns, ( 7 , 8) but curium is insufficiently separated from the Lighter, high fission yield lanthanides. In a recent process for recovering multigram levels of the transplutonium elements, a high separation factor between ( I ) M. M. Woyski and R. E. Harris, ‘Treatise on Analytical Chemistry,” Part 11, Vol. 8 , I. M. Kolthoff, P. J. Elving, Eds..
Interscience, New York (1963) pp. 28-9. (2) P. C. Stevenson and W. E. Nervik, “The Radiochemistry of the Rare Earths, Scandium, Yttrium, and Actinium,” National Academy of Sciences, Nuclear Science Series, NAS-NS 3020 (February 1961). (3) F. L. Moore, ANAL. CHEM., 35, 715 (1963). (4) M. H . Lloyd and R. E. Leuze, Nucl. Sci. Etig., 11, 274 (1961). (5) E. K. Hulet. R. G. Gutmacher, and M. S. Coops, J . Itiorg. Nucl. Chern., 17, 350 (1961). (6) F. L. Moore, ANAL.CHEM., 33, 748 (1961). (7) R. A. Penneman and T. K. Keenan, “The Radiochemistry of Americium and Curium,” National Academy of Sciences, Nuclear Science Series, NAS-NS-3006, (1960) pp. 4&8. (8) D. C. Stewart C . A. A. Bloomquist, and J. P. Faris, U.S. AI. Energy Comrn. Rept., ANL-6999 (February 1965). VOL 39, NO. 6, MAY 1967
641
the lanthanide and actinide groups is reported (9). The process,known as TALSPEAK, depends on preferential extraction of the lanthanides into hydrocarbon solutions of monoacidic $1-ganophosphates,while the trivalent actinides are complexed with a suitable aminopolyacetic acid to remain in the aqueous phase. This process also offers the potential advantage of recovering the lanthanides in a salt-free medium. Smith and Hoffman (10) describe the chromatographic elution of the individual lanthanides from Dowex 50-X4 resin with ammonium a-hydroxyisobutyrate solutions. The comcross-linkage bination of small-particle-size resin and 4 gives room temperature separations comparable to those previously obtained only with heated columns. Optimum elution conditions for each lanthanide element are reported. The accepted technique for high-accuracy measurements of stable isotopes is mass spectrometry, using isotope dilution, wherein the isotopes sought are compared to an added isotope of the same element. To obtain reliability, the concentration of the added isotope solution must be known accurately. The added isotope solution is ordinarily calibrated against a solution of the natural element, usually prepared from highpurity lanthanide oxides. Yamamura (11) reports as high as 15% disagreement in the stoichiometry of these materials. It is essential, therefore, that the solutions used for calibration be accurately standardized by chemical analysis.
r I -lI Strip Char’t Recorder
B
Dowex 5 0 - X 4 ( A a -HIE Elutriant)
i Count - Rate Meter
Drop Counter 0 0
0
0
0 Fraction Collector
0
EXPERIMENTAL
Apparatus. The apparatus used to automate the cation exchange separation is shown in Figure 1. The column effluent is monitored by a flow-through type NaI(T1) scintillation crystal coupled through a 4-decade logarithmic count rate meter to a strip chart recorder. The effluent is collected in a commercial fraction collector which is controlled by a photocell-actuated drop counter. The numerical controller is preset for the desired number of drops. A tandem, double dispersion, mass spectrometer (Nuclide Corp., Model TDD-I, State College, Pa.), with a triple filament source, was used for all mass spectrometric measurements. Reagents. Quartz-distilled water, nitric acid, hydrochloric acid, and ammonium hydroxide were used ^for all reagent preparation, as well as in the separation procedure. All glassware was boiled in hydrochloric acid, rinsed with distilled water, and rinsed with quartz-distilled water before use. With the exception of the chromatographic cationexchange columns, all glassware was discarded after a single use to minimize the possible cross-contamination of samples. Alpha-hydroxyisobutyric acid (AHIB) (Fairmount Chemical Co.) was recrystallized three times from benzene and was dried to constant weight under vacuum. Two 0.50M solutions, one at pH 3.50 and the other at pH 3.90, were prepared with ammonium hydroxide for pH adjustment. Diethylenetriaminepentaacetic acid (Geigy Chemical Corp.) was recrystallized three times from water, dried to constant weight under vacuum, and dissolved in the stoichiometric amount of sodium hydroxide solution to just form the pentasodium salt (Na,DTPA). Di(2ethylhexyl)phosphoric acid (Union Carbide Chemicals) was purified by the procedure of Schmitt and Blake (12). The Dowex 50-X4 (minus 400 mesh, hydrogen form) cationexchange resin was water-graded, and the heaviest 10% and lightest 10% fractions were discarded. The re-
-
(9) B. Weaver and F. A . Kappelmann, U. S. A ! . Energy Comm. R e p . , ORNL-2559 (August 1964). (10) H. L. Smith and D. C . Hoffman, J’. Inorg. Nucl. Chem., 3, 243 (1956). (11) S . :s. iamamura, ANAL.CHEM., 36, 2515 (1964). (12) 2 M. Schmitt and C. A Blake, U. S. A!. Enegy Cornin. P~.D*., ORNL-3548 (February 1964).
642
ei
ANALYTICAL CHEMlSfRY
Figure 1. Apparatus for cation-exchange separation of individual lanthanides
maining resin was washed sequentially with excess 12M HCI, HzO, 8 M “01, H20, 4M HCI, H20, NH40H (to convert the resin to the ammonium form); and finally was rinsed with water until the effluent was neutral to pH paper. The Dowex 50-X8 (100-200 mesh, hydrogen form) resin was not water-graded, but the washing treatment was identical with the exception of ammonium hydroxide, as this resin was used in the hydrogen form. All other reagents were A. R. grade and were used without further purification. Enriched isotopes of the lanthanide oxides, obtained from the Isotope Sales Division of the Oak Ridge National Laboratory, were dissolved and diluted to the microgram/gram level. These were calibrated against standardized solutions of the natural elements. The natural element standards were prepared at the milligram/gram level from the oxides, after ignition to stoichiometry as recommended by Duval (13): and additionally standardized by EDTA titrimetry. These were diluted on a weight basis to the microgram/gram level. The enriched isotope solutions were calibrated against the natural element standards by isotope dilution mass spectrometry. The material for the first column was prepared by mixing two parts of acid-washed DMCS Chromosorb W, 1OG22i! mesh (obtained from Wilkens Instrument and Research, Walnut Creek, Calif.), and one part of 1.2M di(2-ethylhexy1)phosphoric acid (D2EHP) in diisopropylbenzene with sutticient acetone to saturate the Chromosorb. The acetone was evaporated, first over a steam bath and then by air, until the resulting material was completeiy dry in appearance; o‘r‘crnight drying usually is sufficien:. A 5- X 150-mm column with a glass woo! plug in the restricrion was packed with the ____
___
(13) C. Duva!, “Inorganic Thermograwmetric kr,a!ysis,” Fisevici . New York, 1953.
prepared material to a height of 60-mm which was tamped to restrict the flow rate to 0.25 ml/minute. The second column had the same dimensions as the first and was prepared in the same way, except that one part of undiluted D2EHP :s mixed with five parts of Chromosorb. The third (cation-exchange) column is described in the procedure. Procedure. Place an aliquot of the sample containing at least 1 pg of each of the lanthanides to be measured in a 40-1111 centrifuge tube, add the appropriate enriched isotope spikes, and mix well by swirling the solution. (The use of remote facilities is ordinarily advised.) Coprecipitate the lanthanides with Ni(OH)* (0.5 ml of 1 mg/ml high-puri.ty nickel carrier solution) by adding 6 M NaOH. If aluminiJm is present, add sufficient 6M NaOH to dissolve the aluminum which initially precipitates. Warm the solution for 5 minutes in a heated water bath to help coagulate the precipitate. Centrifuge and decant the supernatant solution. 7Nash the precipitate with water. Again centrifuge and deca.nt the supernatant solution. Dissolve the Ni(OH)2 precipitate in 1 ml of 1M lactic acid-0.05M Na5D1'PA by warming the mixture in a heated water bath. Transfer the dissolved sample to the first column. Wash the column with three separate 1-ml portions of the 1 M lactic acid-0.05'M Na5DTPA elutriant which removes the trivalent actinides and extraneous fission products. Wash the column with two 1-ml portions of water to remove the elutriant. Elute the lanthanide group with 3 ml of 8M "03. Add approximately 0.3 mi of 1 M K B r 0 3 solution and warm the solution for several minutes either in a water bath or under a heat lamp. Transfer this solution to the second column, which previously has been equilibrated with 8M "O3-O.1M KBr03. Collect the effluent, which contains the trivalent lanthanides. Elute the trivalent lanthanides completely with another 2 ml of 8 M H N 0 3 , combining the effluent with this first. (Set aside temporarily). Wash the column with 2 ml of water and discard this effluent. Elute cerium from the second column into a 5-ml centrifuge tube with 3 ml of 6.44 HC1-1 HzOz. Evaporate this effluent to a small volume for the mass analysis. This effluent contains lr4Ceand requires shielding. Add another 0.5 ml of the nickel carrier and coprecipitate the trivalent lanthanides by adding an excess of 6M NaOH to the HN03-KBr03 effluent solution of the trivalent lanthanides. (For aged fission products the activity level is sufficiently low that little or no shielding is required for the remainder of the p.rocedure.) Centrifuge the precipitate and decant the supernatant solution. Dissolve the precipitate in a minimum amount c'f IMHCI. Add about 0.1 CI: of a slurry of the purified Dowex 50-X4 resin to the trivalent lanthanides solution. Wait several minutes for the lanthanides to adsorb and for the resin to settle. Withdraw the resin with a transfer pipet, taking as little solution as possible, and transfer it to the liquid above an 8- X 150-mm clslumn of the purified Dowex 50-X4 resin, Swirl the added resin to ensure uniform settling. Repeat these adsorption 2nd transfer steps with additional small portions of resin, i f necessary, until the comparative activity levels indicate that most of the lanthanides have been transferred to the column. Note: The most critical part of an efficient chromatographic column separation is the initial loading of the sample. It is essential that the sample be sorbed on a minimum amount of resin to form a narrow, uniform band. From this initial band at the top of the column, the individual elements are chromatographically separated. Drain the column liquid t o the level of the resin. Carefully add 0.5 ml of pH 3.50 AHIB solution. Insert a quartz wool plug above the resin bed to protect it from disturbance by inflowing elutrknt. Add another 14.5 ml of the same elutriant solution to the ion exchange column reservoir.
Connect an air pressure line (having a bleeder-valve to maintain constant pressure) to the reservoir and adjust the pressure to maintain an effluent flow rate of 10 ml/hour. Gamma-monitor the effluent to indicate the position of the various lanthanide activities. Set the automatic fraction collector to collect individual fractions not larger than 0.25 ml. When the pH 3.50 eluting solution is nearly consumed, add 15 ml of pH 3.90 AHIB solution. (The apparatus provides for the automatic delivery of this second elutriant.) When the desired lanthanides have been eluted, as indicated by gamma monitoring, gamma-spectrum scan the individual tubes and retain those fractions which contain the central portions of the desired elements. Combine the retained fractions of each element and dilute with an equal volume of water. Add 1 drop of 6 M HC1 to further weaken the a-hydroxyisobutyrate complexation. Pass this solution through a 5- X 15-mm column of purified Dowex 50-X8 resin. (A small eye dropper is suitable for this column.) Rinse the column with 1 ml of water and 3 ml of 1M HCl. Discard the effluent. Elute the lanthanide element with 3 ml of 6M HCI. Evaporate the eRluent to small volume, under a heat lamp, for the mass analysis. RESULTS AND DISCUSSION
A three-column sequential separation procedure has been developed for isolating the individual lanthanide fission products from dissolved nuclear fuel samples. The separation of the lanthanide group from the trivalent actinides, usually lengthy and difficult, is accomplished on a highly selective extraction chromatography column. This same column also separates the lanthanide group from other fission products. The second column separates tetravalent cerium from the trivalent lanthanides. On the third (cation-exchange) column, the remaining lanthanides are chromatographically eluted with a-hydroxyisobutyric acid (AHIB) at controlled pH. This third column is automated by the addition of an automatic elutriant dispenser, a gamma-monitoring system for the effluent, and an automatic fraction collector. All three columns operate a t room temperature, and the entire separation procedure requires only 4 hours. After the first two columns, which require only 1 hour, the separated trivalent lanthanide fraction is sufficiently free of radioactivity to allow its transfer from remotely operated facilities to a conventional laboratory. Lanthanide Group Separation. Because many fuels are aluminum clad, aluminum is often a major constituent in the samples. The lanthanide group is separated from aluminum by precipitation from strongly basic solution. However, when only microgram amounts of the lanthanides are present, a coprecipitant must be added to avoid large losses. The properties sought in the coprecipitant were efficient carrying of the lanthanide hydroxides from strongly basic solution and chemical dissimilarity in other respects to simplify its subsequent separation. Nickel fulfilled these requirements. The TALSPEAK process (9) was evaluated as a method for separating the lanthanide group from trivalent actinides and other fission products. TALSPEAK, developed a t the Oak Ridge National Laboratory as a multistage solvent extraction process for the recovery of transplutonium elements from irradiated reactor fuels, gave high separation factors between the lanthanides and trivalent actinides. The reported optimum conditions for recovering the trivalent actinides were: preferential extraction of the lanthanides into 0.3M di(2ethylhexy1)phosphoric acid (D2EHP) in diisopropylbenzene, from 1M lactic acid-0.1 M ammonium diethylenetnaminepentaacetate a t pH 3.0; the extraction of the actinides into VOL 39, NO. 6, MAY 1967
643
I
! G
0.50
j
i 00 1.50 Gamma Energy (MeV)
I
2.00
Figure 2. Distribution of fission product activity in lanthanide group separation
D j E H P in Amsco(kerosene), a t pH 1.5;. and stripping of the actinides from the organic extractant into 1 M nitric acid. The reported high separation factor between the lanthanide and actinide groups was experimentally verified; however, the recovery of the lanthanide elements was low for a single contact. To increase the lanthanide recovery, the D2EHP extractant concentration was increased, the aqueous chelating agent concentration was decreased, and multistage extraction was obtained by adapting the extraction system to a n extraction chromatography column. The lanthanides were extracted into 1.2M D2EHP-diisopropylbenzene on an inert support of Chromosorb. Actinides and other contaminants were eluted with IM lactic acid4.05M Na,DTPA, the residual elutriant was removed with a water rinse, and the lanthanide group was eluted with nitric acid. The recovery of the lanthanide group with this modified system was now greater than 8 0 x (ample for mass spectrometric measurementj, and decontamination from all other !%>ion products was excellent. The high decontamination is illustrated by the comparative gamma spectra of the total sample and the separated lanthanide fraction in Figure 2. (The separated lanthanide fraction also was passed through the second column 13 remove cerium, to better illustrate the absence of nonlanthanide activities.) The decontamination from trivalent actinides, an important requirement of the separation, alsc was excellent. Less than 0.05% of the americium and curium activities remained with the lanthanide group after this separation. Cerium Separation. Because tetravalent cerium i s easily separated from the other lanthanides and contributes a large :he torai acrivii;; (greater than 3 9.z in 1 %year fractio,.
644 *
ANALYTICAL CHEMISTR?
cooled fission products), cerium was separated next in the procedure. The separation of cerium on the second column is based on its oxidation to cenum(1V) with potassium bromate in strong nitric acid, and the selective retention of tetravalent cations on a reversed-phase column of undiluted D2EHP on a Chromosorb support. The trivalent lanthanides, not retained from a strong acid medium, were again coprecipitated on nickel hydroxide to separate them from the bulk of the potassium bromate salt. Cerium then was eluted from the column by reducing it to nonretained cerium(II1) with a mixture of hydrogen peroxide and hydrochloric acid. The first and second columns, which remove the bulk of the original radioactivity and biologically hazardous actinides, are easily operated by remote techniques. Exposure of personnel to high radiation fields is therefore avoided. The cerium-free lanthanides fraction contains only a small portion of the original sample activity and is handled safely with little or no shielding. Individual Element Separation. The lanthanide fission products heavier than samarium have a combined fission yield of only 0.23% (14) and are of minor interest. Of the lighter lanthanides, lanthanum has only one isotope ( I 3 8 l . a ) that can be used as an isotopic diluent; however, because of its extremely low natural abundance (0.089 %), the highest isotopic enrichment currently available is only 1.9% (15). This enrichment is inadequate for highly-accurate mass spectrometric measurements. Praseodymium is monoisotopic, which precludes any possibility of isotope diiution. Promethium, with n o stable isotopes and only one fission product whose half life exceeds 50 hours (2.7-year 14;Pm),is likewise unsuited for mass spectrometric analysis. The procedure therefore has been developed for cerium, neodymium, and samarium, which d o lend themselves to isotope dilution measurement. The procedure of Smith and Hoffman (IO), a chromatographic elution of the individual lanthanide elements from a Dowex 50-X4 resin column with 0.50M AHIB solutions, gave satisfactory separations. Optimum eluting conditions for isolating lanthanide elements other than those described in this paper may be determined from their plot of element peak position cs. elutriant pH. Because of subtle differences in the chemical behavior of the lanthanides, their individual separation requires more care than most laboratory procedures. The most critical part of an efficient chromatographic ion-exchange separation is the initial loading of the column. The lanthanide fraction must be initially retained on a mininiiim amount of resin, forming a uniform, narrow band from which the chromatographic resolution into bands of the individua! elements is accomplished. To facilitate this uniform initial retention, the lanthanides in solution were equilibrated with a small portion of resir? which was transferred to the top i7.F the cation-exchange coiumn. A quartz wool plug, inserted above the resin bed, prevented disturbance of the bed by the intiowing elutriant. The cation-exchange proccd,xe used diEered little from the published form. but was clu:orn:ttrd for more convenien: operation as shown i n Figure :. The coiuinn effluent was monitored by a flow-through t,qe NaI(T!) scintillation c-;/stai coupled through a 4-decade logarithniic count rate meter t? a strip chart recorder. F r o v tix record of the 1an:hnidc
IO+
i Effluent
Volume
(rnl)
Figure 3. Gamma activity of cation-exchange column effluent effluent activity thus obtained, the positions of most lanthanide fission products were identified either by their own activity, or that of a n adjactmt element. This technique of monitoring the gamma activit,y of the chromatographic cation-exchange column effluent h2.s proven satisfactory even for samples of fission products several years old. The effluent was collected i n il commercial automatic fraction collector which was controlled by a photoc,ell-actuated drop counter. The numerical controller could be preset for the desired number of drops which usually was 10 (about 0.15 mi). Eluting conditio 7s were chosen LO obtain adequate separation of wmarium and neodymium from their adjacent ele'nents in a reasonable period of time. This was achieved *ith two elutrrant portions; ! 5 ml of 0.50M AHIB a t p H 3.50, followed Sc 15 nil of the same elutriant a t pH 3.90. A
coil of Tygon tubing with a measured volume of 15 ml, connecting the ion-exchange column reservoir and a second reservoir (Figure l), allowed both portions of the elutriant solution to be loaded initially, each in its respective reservoir. The second portion of elutriant thus was automatically dispensed into the column reservoir after the first portion had been nearly consumed, with n o premixing of the two portions. The degree of separation of the individual lanthanides, as recorded by the effluent gamma-monitor, is shown in Figure 3. Because quantitative recovery is unnecessary for isotope dilution mass-spectrometric analysis only the center fraction of the eluted lanthanide was isolated, to minimize contamination by adjacent lanthanides. To facilitate filament loading for mass spectrometric analysis, a final small cation exchange column was used to concentrate each separated lanthanide and to free it from the AHIB elutriant. After this column was washed free of contaminants with dilute hydrochloric acid, the retained lanthanide was eluted with 6 M hydrochloric acid and evaporated to a small volume for loading on the filament for mass spectrometric analysis. The recommended procedure has been used successfully to determine the concentration and isotopic distribution of microgram levels of fission product cerium, neodymium, and samarium in 233U, 235U, and 239Punuclear fuel. The standard deviation of the mean for four replicate analyses typically was less than 0 . 5 z on those isotopes a t the 1- to 5-pg level and was less than 0.8z at the 0.1- to 1.0-pg level. Advantages .ofthis procedure include the effective and rapid separation of the lanthanides from the trivalent actinides and gross fission products, the ease of remote handling for the initial portion of the procedure which must tolerate high activity levels, the semiautomated cation exchange system for convenience, and a n overall separation time of only 4 hours. ACKNOWLEDGMENT
The preparation and standardization of the mass isotopic spike solutions by Maxine E. Kussy and S. S. Yamamura, and mass spectrometric analyses by R. M. Abernathey, R. E. McAtee, and G . D. Workman are gratefully acknowledged.
RECEIVED for review January 9, 1967. Accepted March 13, 1967. Work performed under Contract AT(10-1)-1230 to the Idaho Operations Office of the U. S. Atomic Energy Commission. Division of Nuclear Chemistry and Technology, 153rd Meeting ACS, Miami Beach. Fla.
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