Anal. Chem. 1996, 68, 3204-3209
Direct Uranium(VI) and Nitrate Determinations in Nuclear Reprocessing by Time-Resolved Laser-Induced Fluorescence Christophe Moulin,* Pierre Decambox, and Patrick Mauchien
CEA, DCC/DPE/SPEA/SPS, Analytical Laser Spectroscopy Group, 91191 Gif sur Yvette, France Dominique Pouyat and Laurent Couston
CEA, DCC/DRDD/SEMP/SEAA, 30206 Bagnols sur Ce´ ze, France
Time-resolved laser-induced fluorescence (TRLIF) has been used for direct uranium(VI) determination in the nuclear reprocessing medium. To take into account the different phenomena that affect the uranyl fluorescence (absorption, quenching, and complexation), a model has been defined. A spectral deconvolution procedure allows uranium speciation in nitric acid and therefore nitrate determination with good accuracy. After normalization of the fluorescence intensity, it is then possible to reach the uranium concentration. Results obtained on synthetic solutions and, above all, on reprocessing samples by the application of this model are presented and compared with results from other techniques. The limit of detection for uranium analysis in reprocessing matrices is in the submilligram per liter range, and a ratio of Pu/U as high as 105 can be directly analyzed.
speciation studies19-21 using spectral and temporal features of TRLIF. Moreover, in addition to the previously quoted advantages, determination can be performed remotely with the use of fiber optics and optodes.22-24 Usually, uranium determination is carried out with classical complexing reagents (depending on the matrix) to enhance fluorescence, such as Fluran, phosphoric acid, or sulfuric acid,1-12 using the standard addition method. In these media, the limit of detection is in the subnanogram per liter range (4 × 10-13 M), and the analysis time is around 10-15 min. In the framework of on-line analysis, several conditions have to be fulfilled. First, determinations have to be performed directly (with no standard addition) in the reprocessing medium (HNO3), despite the drawbacks of this medium (absorption, quenching, and complexation). This requires a better knowledge of the uranium(VI) fluorescence in nitric acid.25-27 By the use of a deconvolution model, it has been shown that it is possible to
On-line analytical methods are required in the nuclear fuel cycle and especially in the reprocessing for process control. Elements present in solution, such as actinides, need to be analyzed at very different concentration ranges, depending on the stage of reprocessing. Moreover, the matrices are very complex (from concentrated nitric acid solutions to organic phases) and radioactive (presence of plutonium, uranium, fission products, etc.) which makes radiometric methods difficult to use. A purely optical method such as time-resolved laser-induced fluorescence (TRLIF) can be of great interest for such determinations. Hence, TRLIF is a very sensitive, selective, and fast method for fluorescent actinides and lanthanides analysis that has been extensively used in various fields of the nuclear fuel cycle (geology, reprocessing, waste storage, medical, environment), mainly for uranium ultratrace analysis1-12 and, more recently, for complexation13-18 and
(10) Moulin, C.; Decambox, P.; Mauchien, P. Appl. Spectrosc. 1991, 45, 116118. (11) Brina, R.; Miller A. G. Anal. Chem. 1992, 64, 1413-1418. (12) Moulin, C.; Decambox, P.; Trecani, L. Anal. Chim. Acta 1996. 321, 121126. (13) Dobbs, J. C.; Susetyo, W.; Knight, F. E.; Castles, M. A.; Carreira, L. A.; Azarraga, L. V. Anal. Chem. 1989, 61, 483-488. (14) Moulin, C.; Decambox, P.; Mauchien, P.; Moulin, V.; Theyssier, M. Radiochim. Acta 1991, 52/53, 119-125. (15) Bidoglio, G.; Omenetto, N.; Robouch, P. Radiochim. Acta 1991, 52/53, 5761. (16) Kim, J. I.; Wimmer, H.; Klenze, R. Radiochim. Acta 1991, 54, 35-41. (17) Moulin, V.; Tits, J.; Moulin, C.; Decambox, P.; Mauchien, P.; de Ruty, O. Radiochim. Acta 1992, 58/59, 121-128. (18) Reiller, P.; Lemordant, D.; Moulin, C.; Beaucaire, C. J. Colloid Interface Sci. 1994, 163, 81-86. (19) Meinrath, G.; Kato, Y.; Yoshida, Z. J. Radioanal. Nucl. Chem. 1993, 174, 299-305. (20) Moulin, C.; Decambox, P.; Moulin, V.; Decaillon, J. G. Anal. Chem. 1995, 67, 348-353. (21) Eliet, V.; Bidoglio, G.; Omenetto, N.; Parma, L.; Grenthe, I. J. Chem. Soc., Faraday Trans. 1995, 91, 2275-2285. (22) Malstrom, R. A.; Hirschfeld, T. Anal. Chem. Symp. Ser. 1984, 19, 25-30. (23) Varineau, P. T.; Duesing, R.; Wangen, L. E. Appl. Spectrosc. 1991, 45, 16521655. (24) Moulin, C.; Rougeault, S.; Hamon, D.; Mauchien, P. Appl. Spectrosc. 1993, 47, 2007-2012. (25) Matsui, T.; Suzuki, K.; Sakagami, M.; Kitamori, T. Appl. Spectrosc. 1991, 45, 32-37. (26) Deniau, H. De´veloppement de la Spectrofluorime´trie Laser a` Re´solution Temporelle pour le controˆle en ligne de l'uranium dans les solutions du proce´de´ PUREX. Ph.D. Thesis, Universite´ Pierre et Marie Curie, Paris, France, 1992. (27) Deniau, H.; Decambox, P.; Mauchien, P.; Moulin, C. Radiochim. Acta 1993, 61, 23-28.
(1) Robbins, J. C. CIM Bull. 1978, 71, 61-67. (2) Campen, W.; Bachmann, K. Mikrochim. Acta II 1979, 159-170. (3) Zook, A.; Collins, L. H.; Pietri, C. E. Mikrochim. Acta II 1981, 457-461. (4) Mauchien, P. Dosage de l’uranium par Spectrofluorime´trie a` Source d’Excitation Laser. CEA Report R-5300; Centre d’Etudes Nucle´aires: Fontenay aux Roses, France, 1985. (5) Young, J. E.; Deason, P. T. Anal. Chem. Symp. Ser. 1984, 19, 7-12. (6) Berthoud, T.; Decambox, P.; Kirsch, B.; Mauchien, P.; Moulin, C. Anal. Chem. 1988, 60, 1296-1299. (7) Fujimori, H.; Matsui, T.; Suzuki, K. J. Nucl. Sci. Technol. 1988, 25, 798804. (8) Moulin, C.; Beaucaire, C.; Decambox, P.; Mauchien, P. Anal. Chim. Acta 1990, 238, 291-296. (9) Moulin, C.; Decambox, P.; Mauchien, P. J. Phys. IV 1991, 1, C7:677-680.
3204 Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
S0003-2700(96)00257-0 CCC: $12.00
© 1996 American Chemical Society
directly determine free uranyl (UO22+) and uranyl nitrate complexes (UO2(NO3)n(2-n)+, n ) 1 or 2) percentages (i.e., speciation), allowing determination of nitrate concentration.28 This procedure is essential in order to normalize fluorescence intensity and for further uranium concentration determinations. Second, for industrial applications, the classical nitrogen laser (λ ) 337 nm) has to be replaced by a more reliable laser technology, such as the tripled Nd-YAG laser (λ ) 355 nm), which, despite a poorer uranium absorption coefficient at this wavelength compared to that at 337 nm, has several advantages: 29 solid state technology, better beam quality (for fiber-optic injection), and less nitric acid absorption dependence. This paper describes a model taking into account the various effects that influence the uranyl fluorescence for direct uranium determination in nitric acid and, for the first time, considers its application to real samples at various stages of the reprocessing. EXPERIMENTAL SECTION Reagents. Standard solutions of uranium(VI) in nitric acid are obtained from suitable dilution of a solution prepared by dissolution of high-purity metal with nitric acid (Merck). Uranium concentration of the initial standard solution is verified by mass spectrometry. Reprocessing samples are directly obtained from the Atelier Pilote of Marcoule. Instrumentation. Time-Resolved Laser-Induced Fluorescence. A Brilliant Nd-YAG laser (Quantel), operating at 355 nm and delivering about 5 mJ of energy in a 10 ns pulse with a repetition rate of 50 Hz, is used as the excitation source. The beam diameter is 4 mm, and the divergence is around 1 mrd. The laser output energy is monitored by a laser power meter (Scientech). The beam is directed into a 4 mL quartz cell (Hellma). In the case of inactive samples, the laser beam is focused into the cell of the spectrofluorometer FLUO 2001 (Dilor) by a quartz lens. The radiation coming from the cell is focused on the entrance slit of the polychromator. Taking into account dispersion of the holographic grating used in the polychromator, a 200 nm measurement range in the visible spectrum is obtained. The detection is performed by an intensified photodiodes (710) array, cooled by Peltier effect (-30 °C), and positioned at the polychromator exit. For radioactive samples determination, the cell holder is placed in a classical glovebox (unshielded). The radiation from the cell is focused on the entrance slit of the polychromator HR 320 (Jobin Yvon). The detection is performed by an OMA III (EGG-PAR). For both systems, recording of spectra is performed by integration of the pulsed light signal given by the intensifier. The integration time adjustable from 1 to 99 s allows for variation in detection sensitivity. Logic circuits, synchronized with the laser shot, allow the intensifier to be active with determined time delay (from 0.1 to 99 µs) and during a determined aperture time (from 1 to 999 µs). The whole system is controlled by a microcomputer. All measurements are made at room temperature. The standard addition method used in TRLIF has been carried out directly in nitric acid solution after suitable dilution, depending on the matrix. Determination of the initial fluorescence spectrum F0 (at t ) 0) is performed by extrapolation of fluorescence spectra (28) Couston, L.; Pouyat, D.; Moulin, C.; Decambox, P. Appl. Spectrosc. 1995, 49, 349-353. (29) Moulin, C.; Decambox, P.; Couston, L.; Pouyat, D. J. Nucl. Sci. Technol. 1994, 31, 691-699.
obtained at different gate delays (0.1, 0.2, 0.3, 0.5, 0.7 , 0.9, 1.3, 1.7, 2.5, 3.5, and 5 µs) with a fixed gate length (3 µs). RESULTS AND DISCUSSION In the case of a short pulsed excitation, the fluorescence signal expression previously described9 can be generalized to take into account several fluorescent species (at a defined excitation wavelength) and expressed as follows:
1 F(λ)(t) ) kI0tirr l( τ0
∑
[
iCie-∆t/τi)[e-l∑jj′cj′]
i
]
1 - e-l′∑kk′′ck′′
∑ ′′c ′′ k
k
k
(1) with k the apparatus factor, I0 the laser intensity, tirr the laser pulse duration (s), τ0 the natural fluorescence lifetime (s), i and Ci respectively the molar absorption coefficients (M-1 cm-1) and concentrations (M) of the fluorescent species (in our particular case, free uranyl and uranium nitrate complexes), ∆t the time between the laser excitation and the fluorescence measurement (s), τi the fluorescence lifetimes of the fluorescent species i (s), l the optical pathlength for laser excitation (cm), l′ the optical pathway for the fluorescence emission collection (cm), j′ and cj′ respectively the molar absorption coefficients (M-1 cm-1) and concentrations (M) of species absorbing at the fluorescence emission wavelength, and k′′ and ck′′ respectively the molar absorption coefficients (M-1 cm-1) and concentrations (M) of species absorbing at the excitation wavelength. This equation takes into account the different phenomena that can affect the fluorescence, i.e., (i) the quenching, including static quenching (formation of a less fluorescent or nonfluorescent complex) and dynamic quenching (collisional), affecting the lifetime (τi) of the fluorescent species and given by the SternVolmer equation,
τwq τ) 1+
∑ l
(2)
kQl[Q]lτwq
where τwq is the lifetime without quenchers (s), kQl is the quenching constant (M-1 s-1), and Ql is the concentration of quenching species (M); (ii) the absorption, including prefilter effect [species (k′′, ck′′) absorbing at the laser excitation wavelength (in our case, 355 nm)] and postfilter effect [species (j′, cj′) absorbing at the fluorescence emission wavelength (in the case of uranyl, 450-600 nm)]; and (iii) the complexation, which could affect i or τi (in our case, formation of uranyl nitrate complexes). Complexation can lead to diminution and even extinction of fluorescence (assimilated to static quenching) or fluorescence enhancement due, for instance, to shielding (avoiding quenching by water molecules) or to energy transfer. Since nitric acid is the main constituent of the matrix, it is very important to know the influence of nitric acid on the uranyl fluorescence. As previously mentioned, the different effects that will influence the fluorescence signal are absorption, quenching, and complexation.22,25-27 The first step in the application of the model defined by the general equation (eq 1) is to determine the uranium speciation (i.e., concentration of the different species) Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
3205
Figure 2. Speciation diagram of uranium in nitric acid from the OECD data base31 (plain curve), together with results obtained with the new spectral deconvolution. 9, % UO22+; 2, % UO2NO3+; b, % UO2(NO3)2.
Figure 1. Uranium fluorescence spectrum in nitric acid (plain curve), together with the theoretical spectrum (dotted line) obtained for the contribution from free uranyl and the first and second nitrate complexes. [U] ) 1 mg/L.
directly from fluorescent measurements. In nitric acid (concentration up to 5 M), besides free uranyl UO22+, the uranium nitrate complexes that have been identified by spectral deconvolution using TRLIF28 are UO2NO3+ and UO2(NO3)2. Up to now, this spectral deconvolution procedure28 has considered that free and complexed nitrate uranium have similar spectra, only differentiated only by the shift in wavelength. Hence, it is not possible to isolate by temporal resolution the different uranium nitrate complexes from the free uranyl since they have very similar lifetimes (2-3 µs)29 with monoexponential decays. However, the fitting spectrum systematically presents slight differences from the experimental one. One recent work using direct temporal resolution20 has allowed isolation spectroscopically of the hydroxyl complex UO2OH+ (lifetime, 80 µs; shift, 10 nm; and broadening of the peaks by a factor 2 (17 nm) compared to free uranyl), and another using chemical selectivity30 (uranium concentration and partial CO2 pressure) has shown that, despite the fact that it is difficult to isolate spectroscopically UO2(OH)2 from UO2(OH)+, their spectra seem to have the same broadening. Therefore, since nitrate groups are as likely to be in equatorial position as hydroxyl groups, it was decided to improve the spectral deconvolution procedure by considering that nitrate complexes have a similar broadening. Figure 1 presents typical uranium fluorescence spectra in 1 and 2 M nitric acid with the contribution of each species to the spectrum determined using this new (30) Moulin, C.; Laszak, I.; Moulin, V.; Tondre, C. Appl. Spectrosc., to be submitted.
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Analytical Chemistry, Vol. 68, No. 18, September 15, 1996
spectral deconvolution procedure. A very good fit is obtained between convoluted and experimental spectra. The improvement in terms of accuracy for the determination of NO3- concentration between this new spectral deconvolution procedure and the previous one28 is close to 20%, especially at high nitric acid concentration (>2 M), where all three species are present. Figure 2 presents the speciation of uranium in nitric acid obtained using data from the OECD31 (the third uranium nitrate complex being neglected), together with data obtained from the new deconvolution procedure. Despite a slight discrepancy in the first part of the curves, where the free uranyl percentage is underestimated at the profit of the second uranyl nitrate complex, results obtained by this new deconvolution procedure are in very good agreement with data obtained from the Thermodynamical Data Base of the OECD. The second step in the application of the model relies on the ability of TRLIF to perform time-resolved measurements (i.e., at various ∆t), in order to extrapolate the initial fluorescence spectrum F0 (at t ) 0) and thus to eliminate dynamic quenching. In this way, eq 1 simplifies to eq 3,
∑
F(λ)0 ) K
i
iCi[e-∑jj′cj′l]
[
]
1 - e-∑kk′′ck′′l
∑ k
k′′ck′′
(3)
with K ) kI0ltirr/τ0 and i ) 0-2 (with i ) 0 for UO22+, i ) 1 for UO2(NO3)+, and i ) 2 for UO2(NO3)2). This initial fluorescence equation (eq 3) or a simplified form will be used for the rest of the study. The final procedure32 implies first the determination of the absorption coefficients (pre- and postfilter effects); then, by spectrum deconvolution of the initial fluorescence spectrum, speciation (determination of the percentage of free uranyl and nitrate complexes); and finally, determination of the uranium (31) Grenthe, I.; Fuger, J.; Konings, J. M.; Lemire, R. J.; Muller, A. B.; Nguyen Trung, C.; Wanner, H. Chemical Thermodynamics of Uranium; NEA-TDB, OECD Nuclear Energy Agency Data Bank; North Holland: Amsterdam, the Netherlands, 1992. (32) Couston, L. Mode´lisation de la me´thode d’analyse de l’uranium dans la phase aqueuse du proce´de´ de retraitement du combustible nucle´aire par Spectrofluorime´trie Laser a` Re´solution Temporelle. Me´moire CNAM, Montpellier, France, December 1994.
Table 1. Comparison between Prepared and Measured Nitric Acid and Uranium Concentrations by TRLIF on Synthetic Samples [HNO3] (M) prepared measured 1 2 2.5 2.5 3.5 3.5 4 4 5 5
1.04 2.03 2.54 2.51 3.6 3.31 4.24 4.05 5.72 5.31
deviation (%)
[U] (µg/L) prepared measured
4 1.5 1.6 0.4 2.8 5.4 6 1.2 14.4 6.2
500 750 1000 250 500 100 1000 250 750 500
445 685 898 221 425 94 877 208 568 422
deviation (%) 11 9 10 12 15 6 12 17 24 16
concentration by comparison with a calibration curve (previously obtained) in the case of synthetic samples in order to take into account the different constant experimental factors, represented by K. Uranium Determination in Synthetic Samples. The first validation of this procedure was performed on synthetic solutions containing various uranium and nitric acid concentrations. In such solutions, absorptions at the wavelength of the uranium fluorescence emission (450-650 nm) is negligible; when a tripled NdYAG laser is used, absorption by nitric acid at 355 nm can also be safely neglected.29 Therefore, for this particular case, eq 3 simplifies to
∑ C
F(λ)0 ) K
i i
(4)
i
Therefore, the procedure in this case simplifies to deconvolution of the initial fluorescence spectra for the uranium speciation that allows for nitric acid concentration determination and then uranium concentration determination by comparison with the calibration curve. Table 1 summarizes results obtained on various synthetic solutions with nitric acid concentration up to 5 M and uranium concentration from 100 to 1000 µg/L for the application of such a procedure. As can be seen, measured concentrations are in good agreement with theoretical ones for both determinations. Hence, deviation are e6% and 17% for nitric acid and uranium concentration determinations, respectively, in the 0-4 M nitric acid concentration range. However, and as expected, at very high nitric acid concentration (>4 M), the accuracy of the deconvolution procedure is affected by the loss of shape of the uranium fluorescence spectrum, and therefore, discrepancies between measured and theoretical concentrations are larger (
