474
Anal. Chem. 1984, 56,474-478
Dynamic Ion Exchange Chromatography for Determination of Number of Fissions in Thorium-Uranium Dioxide Fuels C. H. Knight, R. M. Cassidy,* B. M. Recoskie, and L. W. Green General Chemistry Branch, Atomic Energy of Canada Limited Research Company, Chalk River Nuclear Laboratories, Chalk River, Ontario, Canada KOJ 1JO
Hlgh-performance llquld chromatography and the use of “La as a Hsslon monitor have been examined for the detennlnatlon of the number of flsslons and burnup In (Th,U)OI fuels. The flsslon product IaoLaIn a solutlon of lrradlated fuel was separated on a reversed phase dynamlcally modlfled wlth l-octanesulfonate, and the eluted metal Ions were monltored by an “on-llne” postcolumn reactlon system. The relatlve standard deviation of the peak h e m s for lanthanm In 13 fuel solutlons over a 1 month perlod was 0.96%, and the agreement between the HPLC burnup results and those obtalned by standard mass spectrometrlc technlques was wlthln 0.1 %; the largest source of uncertalnty for the determlnatlon of the number of flsslons was that associated wlth flsslon yleld vglues. Other uses of thls chrornatographlc system were also examlned briefly.
A number of nuclear research laboratories are engaged in studies of advanced nuclear-fuel cycles in an effort to conserve the energy resources of the world. One of the options is the thorium fuel cycle being studied in our laboratories. An important parameter required for studies of nuclear fuels is burnup, which is the number of fissions per 100 heavy nuclide atoms (mass 1 225) initially present in the fuel (I). Most conventional methods used for burnup determination involve dissolution of the irradiated fuel in a shielded facility, separation and determination of a suitable fission monitor, and separation and determination of the major actinides ( 2 , 3 ) . To date the analytical technique of choice has been isotopedilution mass spectrometry, because of its accuracy and its ability to determine individual isotopes. However, the overall procedure required for the determination of burnup by mass spectrometry is time-consuming, and the total costa of a single determination can be very high. Consequentlythere is a need for alternate analytical techniques and some possibilities have been examined (2, 3), but these methods are usually not as accurate and/or are susceptible to interferences. One problem common to most techniques used for burnup determination is the separation step (or steps) used to isolate the fission monitor and actinides from the fission products. These separations are normally done by slow and inefficient classical column techniques, which are particularly tedious when performed in “hot-cell” facilities. Modern high-performance liquid chromatography (HPLC) can give rapid and high-resolution separations and sensitive detection for many metal species (4), including the lanthanides, some of which fulfill the requirements of a fission monitor (2,3).Two HPLC methods have been described for the determination of a fssion monitor (5-8, but both involved the collection of fractions for subsequent analysis by mass spectrometry. One of these techniques required a conventional preliminary separation (6), and the other (5, 6) used a bonded-phase anion exchanger. Some of our recent studies (8) showed that dynamic ion exchangers gave superior resolution of transition-metal ions and greater flexibility with regard to choice of separation conditions than conventional bonded ion exchangers; dynamic 0003-2700/84/0356-0474$01.50/0
exchangers are formed when ionic hydrophobic modifiers are sorbed onto the hydrophobic surface of a reversed phase to produce a charged double layer at the surface where ion exchange can occur. Consequently we decided to investigate the application of HPLC techniques to the determination of the number of fissions and the burnup in thorium-uranium fuels. A prerequisite for this application is the identification of a suitable fission monitor; the criteria for selection of a fission monitor have been given elsewhere (2, 3 ) . Two possible monitors were identified. The first was the sum of the major fission-productlanthanides (La, Ce, Pr, Nd, Sm), which has been used ( 2 , 3 )in conjunction with conventional analytical techniques. The second was fission-product 139La;La is present almost entirely as this isotope in decayed fuels and thus can be determined by chemical methods instead of isotopic methods. The fission monitor 13gLahas been used successfully for the mass spectrometric determination of burnup in uranium oxide, uranium-plutonium oxide, and uranium metal fuels (9-13). The results from these applications and a review of ita properties showed that 139Lasatisfied the major requirements of a fission monitor: it is stable; it does not migrate appreciably in the fuel, nor does it have a long-lived precursor that migrates (14); it and the fission product of one mass less (138Ba)have relatively small cross sections for thermal-neutron capture (15);the fission yield is large and does not vary greatly with neutron energy (15);and the fission yields from 2a3Uand 236U(the major fissioning isotopes in the (Th,U)02fuel) are almost equal. This paper gives the resulta of an investigation on the application of HPLC for the determination of the number of fissions in (Th,U)02fuel and compares these results with mass spectrometric results.
EXPERIMENTAL SECTION HPLC System. The HPLC system consisted of a Spectra Physics pump (Model 8700, Spectra Physics, Santa Clara, CA), a Rheodyne sampling valve (Rheodyne, Berkley, CA) with a 100-pLsample loop, and a variable-wavelengthabsorbance detector (Tracor, Austin, TX). The signal from the detector was monitored with a strip chart recorder, a disk data-storage system (Model 8110BR, Bascom Turner Instrumenta, Newton, MA), and a computing integrator (SP 4270, Spectra Physics). The eluted metal ions were monitored after a postcolumn reaction with 4-(2-pyridylazo)resorcinol(PAR) or 3,6-bis[(o-arsenophenyl)azo]-4,5-dihydroxy-2,7-naphthalenedisulfonicacid (Arsenazo111). The postcolumn reagenta were added to the eluate via a low volume mixer ( 4 ) with a syringe pump (ISCO M314, Lincoln, NE) at a flow rate of 1.5 rnl-min-’; our recent studies have shown that less expensive FMI pumps (FMI, Oyster Bay, NY)can also be used for reagent addition with no increase in base line noise due to irregular mixing caused by pump pulsations. The PAR solutions (2 x IO-‘ mo1.L-l) were 2 mo1.L-l in ammonia and 1mo1.L-’ in ammonium acetate, and the concentration of Arsenazo I11 was 1.5 X mol.L-’. The wavelengths used for detection were 535 nm for PAR and 653 for Arsenazo 111. The columns used were as follows: 5 pm Supelcosil LC-144.6 X 150 mm (Supelco Inc. Bellefonte, PA); 10 bm Hamilton PRP-1, 4.1 X 150 mm (Hamilton Co., Reno, NV); 10 fim Brownlee, AN, 4.6 X 250 mm (TechnicalMarketing Associates, ON, Canada); 0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56,NO. 3, MARCH 1984 475
and 10 pm Partisill0 SAX,4.6 X 250 mm (Whatman Inc., Clifton, NJ). Reagents. The a-hydroxyisobutyric acid (HIBA) (Aldrich Chemical Co., Milwaukee, WI) eluent was purified with a 3 X 200 cm AG 50W-X4 cation-exchangecolumn. The modifiers used for the reversed-phase columns were 1-octanesulfonate (Regis Chemical Co., Martin Grove, TN) and Aliquat 336 (tricapyrylmethylammonium chloride, Aldrich Chemical Co.). All other reagents were reagent grade, and all aqueous solutions were prepared from triple-distilled water. Standards. Standard lanthanide solutionswere prepared from Spex Industries standard solutions; these solutions were also calibrated with lanthanide oxides (K&K Laboratories, Johnson Matthey & Co., Spex Industries) that had been ignited at 950 "C to a constant weight (16),and for lanthanum a further check was made with a lanthanum metal standard. Standards for the direct analysis of fuel solutions were prepared to simulate actual fuel samples by using reagent-grade thorium nitrate, a NBS-960 natural-uranium standard (NBS, Washington, DC), and Spex lanthanide standards; the thorium nitrate did not contain any detectable amounts of lanthanides. Sample Preparation. The fuel samples were taken from a 36-element (Th,U)02bundle (17) that had an initial composition of 97.3 wt % Thoz and 2.7 w t % UOz (93% z35U enriched) and was irradiated for 2 years in the National Research Universal reactor at the Chalk River Nuclear Laboratories. The fuel was allowed to decay for at least 2 years prior to dissolution and analysis; much shorter decay times could be used. Samples of the fuel (20 to 25 g) were transferred to the 'hot cell", weighed, and dissolved in 13 mobL-' "03-0.05 mo1.L-l HF (17). An aliquot of the solution ( 1mL) was weighed and gravimetrically diluted with 0.5 mol-L-' HN03 to give a thorium concentration mol.L-'. For burnup determinations by the 139La of -4 X method, -1 mL of this solution was placed in a polyethylene microvial and transferred out of the "hot cell". For the determination of the major rare earths, 1g of diluted fuel solution was spiked with a rare earth recovery monitor (Er, Tb, or Gd) and then the actinides were removed on a 5 X 60 mm anion exchange column with a HC1-acetone eluent (17).The lanthanide fraction was evaporated to dryness, disaolved in 1mL of 0.5 mol-L-' HN03, and transferred out of the "hot cell" in a polyethylene microvial. All weighings were made to 0.1 mg on a Mettler AK 160 (Mettler Instruments AG, Switzerland), which had been modified for remote operation in a "hot cell". Sample Analysis by HPLC. The 100-pL sample loop was flushed with 500 p L of 0.5 mol.L-' HN03 and then with 300 pL of the sample; the sample normally contained 50-500 ng of the lanthanides. The sample loop was then valved "in-line" with the eluent. For isocratic separations of lanthanum the eluent was 5X mol-L-' in 1-octanesulfonateand 0.178 mol-L-' in HIBA that had been adjusted to pH 4.6. For gradient elution the solvent program was run from 0.03 mo1.L-' HIBA to 0.3 mol-L-' HIBA over 15miq for both eluents the pH was 4.6 and the concentration of the 1-octanesulfonate was in the range of 5 x to 1 X mo1.L-l. The amount of the lanthanum present in the sample was determined by comparison of the peak heights with standard calibration curves of peak heights for known volume concentrations of lanthanum; duplicate aliquots were used for both standards and samples. These data and the density of the fuel solution were then used to calculate the weight concentration of la9La in the fuel sample. The number of fissions were then calculated by using this weight concentrationand the fission yield for 139La.
-
RESULTS AND DISCUSSION Anion Exchange Systems. Anion exchange in a HN03-methanol eluent was studied for the separation of the lanthanides because we had used it previously with conventional anion exchangers (1 7) and also because it had been used with a silica bonded-phase anion exchanger (5,6). A number of different bonded phase exchangers were examined but the results showed that the column-to-column reproducibility and column efficiencies were poor, column beds were unstable, and the exchange capacity of most columns was too low to permit efficient separation of the components of interest from the
I
I
I
I
4 1 0004
Tb
J
50s I
1
I
1
RETENTION TIME
I
(5)
Figure 1. Gradient separation of the lanthanides: Supelco LC18 column; linear program at pH 4.6 from 0.05 mol-L-' HIBA to 0.40 mobL-' HIBA over 10 min at 2.0 mlemin-'; modifier, l-octanesulfonate at 1 X IO-* rnobL-'; detection at 653 nm after postcolumn reaction with Arsenazo 111; sample, 5 pL of a solution containing 10 pg-mL-' of each lanthanlde.
-
solvent front. Because previous studies (8) had shown that dynamic anion exchangers gave efficient separations of inorganic anions, a similar system was applied to the separation of the lanthanides. The dynamic anion exchange system studied consisted of a 5-pm bonded reversed-phase column, a HN03-methanolLiN03 eluent, and Aliquat 336 [(C8H17)3CH3N+Cl-]as the anionic modifier. The separations obtained were similar to those reported for bonded-phase exchangers (5,6),but column capacities were low. Because of these separation difficulties and the poor sensitivity of both direct UV detection ( 5 6 ) and our postcolumn reaction system when large amounts of organic solvents were present in the mobile phase, cation exchange was investigated. Cation Exchange Systems. Results from our previous studies (8) showed that dynamic cation exchangers gave efficient separations of metal ions. Consequently this system was examined and the lanthanide separations that were obtained were superior to those obtained with bonded exchangers. An example separation is shown in Figure 1. The repeatability (relative standard deviation for nine injections) for peak heights was 0.5%, and column behavior was constant for at least 2 months. Day-to-day variations in retention times were
