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Spectrophotometric determination of the oxygen to uranium ratio in uranium oxides based on dissolution in sulphuric acid
~3~91~~1
Edma, Vol. 38, No. 11, pp. 13354340, 1991. printedin Great Britain. Ail rights reserved
$3.00 + 0.00
Copyright Q 1991Pergamon Press pk
SPECTROPHOTOMETRIC DETERMINA~ON OF THE OXYGEN TO ~~NIUM RATIO IN U~~UM OXIDES BASED ON DISSOLUTION IN SULPHURIC ACID TIW and S. SYAMSUNDAR B. NARASIMHA MURTY, R. B. YADAV,C. K. RAMAMUR
Control Laboratory, Nuclear Fuei Complex, Hyderabad, India (Receiued 8 iMarch 1991. Revised 15 Aprii 1991. Accepted 30 April 1991)
Summary-The oxygen to uranium ratio in uranium oxides such as &OS, UOr +, powders and UO, fuel pellets has been determined by a new spectrophotometric metlmd. The method can be used for determination of O/U ratio in UOr pellets and powders on a routine basis. In the described method, uranium oxides in the powder form are dissolved in 2M sulphuric acid containing a few drops of HF. The ~n~ntmtions of II(IV) and U(W) are directly dete~n~ by means of the absorbances of these species at different wavelengths. For determination of the O/U ratio in U,O, powder samples, 630 and 310 nm are the wavelengths chosen for U(W) and U(W), respectively. For U02+ x powder, where the O/U ratio lies between 2.04 to 2.15, U(W) and U(W) are determined at 630 and 300 nm respectively, whereas for UOs fuel pellets, where the O/U ratio is less than 2.01,535 and 285 nm are used. The molar absorptivity of U(N) at 630 and 535 am is 21.4 and 6.8 l.mole-‘.cm-’ and that of U(W) at 310, 300 and 285 nm is 178.1, 278.6 and 585 1. mole-‘. cm-‘, respectively. Standard deviations of +0.002 O/U ratio units for pellets and &O&I4 O/U ratio units for powders have been achieved.
The uranium-oxygen system is one of the most complex systems known because of the multiplicity of stable oxidation states of uranium. Because of this, the exact stoichiometry of uranium dioxide is seldom attained, as it usually exists as a hyperstoichiometric oxide which includes excess oxygen in its cubic lattice. One of the most important parameters of nuclear grade uranium dioxide pellets is the O/U ratio, as it is important in the characterization of the sinterability of the powder and its performance as a fuel under reactor conditions. An oxygen-touranium ratio closely approaching 2.000 is a frequent requirement for nuclear fuel, so, as a quality control measure, it is essential to know the O/U ratio of the U02 powder. There are many instrumental and chemical methods available for the determination of the O/U ratio. Both destructive and non-destructive methods have been reviewed by Florence,’ Some of the instrumental methods are X-ray diffraction analysis,* electromotive force measurement of a Ni, NiO electrolyte-UO, +x cell= and X-ray photoemission spectrometry.7T8 A major disadvantage of these methods, is that they need reference materials having identical composition with that of the sample in order to calibrate the methods. Chemical methods include gravimetry,3v9 coulometry,‘O*l’ titrimetry3J2 and polarography. 13-is
All the wet chemical methods for the determination of the O/U ratio are based on the dete~ination of either U(IV) and/or U(W), since the degree of hyperstoichiometry can be directly related to the concentration of U(VI).16 Thus, the estimation of U(W) serves as a measure of the excess oxygen and, coupled with total uranium determination, enables one to determine the exact composition of the oxide. For near stoichiometric U02 pellets, where a minute quantity of U(V1) has to be determined in the presence of a large excess of U(W), polarographic, coulometric and biamperometric methods are preferred.” However, for U02,., powders, where U(V1) is also present in relatively higher concentration, titrimetric procedures’*‘**lgusing potentiometric and/or visual indicator techniquesZO are more convenient. However, these methods require that the sample is weighed accurately and dissolved completely. S~trophotomet~c methods for the determination of the O/U ratio do not require weighing and complete dissolution of the sample because both U(W) and U(W) are determined in the same aliquot. Our experiments demonstrate that Urals dissolves at the same rate in both oxidation states as evidenced by the same O/U ratio obtained irrespective of whether the sample is partially or completely dissolved. However, spectrophotometric methods which
1335
B. NARASIMHA MURTY et al.
1336
use phosphoric acid21mz4 reported so far for the determination of O/U ratios of UOz pellets are not convenient on a routine basis because they suffer from many disadvantages such as high viscosity, long ~s~lu~on time, poor ~nsiti~ty and elaborate treatment procedures.25 The present method is based on the dissolution of the sample in 2M sulphuric acid in the presence of a very small quantity of HF, followed by measurement of absorbances at specific wavelengths for U(IV) and U(V1). Earlier, Takeuchi et 01.‘~ have reported the dissolution of uranium oxide in 1M sulphuric acid containing a few drops of HF and subsequent determination of the U(V1) coulometritally by titrating with electrogenerated Ti(II1). Stronhilli9 determined ~anium(~ titrimetritally with Fe2+ or T?+ based on sulphuric acid dissolution. EXPERIMENTAL Reagents
Uranium dioxide pellets, UOz powders and U,Os powders produced at the Nuclear Fuel Complex, Hyderabad, India were used as samples. All dissolution reagents were prepared with reagent grade materials and distilled water. Instrumentation Absorbance measurements were carried out on a Shiiadzu Model W-240, Graphicord Recording Spectrophotometer. W-visible Matched quartz cells of l-cm path length were used. The base line and digital absorbance readings were adjusted to 0.000 for a reagent blank (2M sulphuric acid) before each set of measurements. Procedure In order to obtain a reliable value of the O/U ratio in U02+=, it is essential that the oxidation states of uranium in the solid be kept unchanged throughout procedures such as the grinding of the UOZfx pe llet and the dissolution of the powder obtained. Keeping this in view, the following strategy was adopted for grinding the pellets and dissolution of the powder. UO, pellets. The U02+X pellet was placed in a cylindrical container and the air was displaced with a jet of argon. After suEcient flushing the lid was tightened, and the sample was ground for about 10-15 set in a Spex Mixer Mill to provide a mesh size no smaller than 150. If the particles are very fine some oxidation may
occur. Nearly 10-15 g of the ground powder was then dissolved under argon atmosphere in 100-l 10 ml of 2M sulphuric acid containing l-2 drops of HF, on a water bath. The dissolution was carried out for about one and half hours. The absorbances for U(IV) and U(VI) were measured at 535 and 285 nm, respectively. iYO,/U, OS powders. About 80-100 mg of powder was dissolved in 40-50 ml of 2M sulphuric acid containing one drop of HF. Dissolution was carried out on a water bath in ambient air for about half an hour. For UOz powders, the absorbances of U(IV) and II(VI) were measured at 630 and 310 nm, respectively, while for U,O, powders they were measured at 630 and 300 nm. ~eterm~ation of molar absorptivity of U(W) at 630 and 535 nm. Nuclear grade uranium metal turnings were degreased with carbon tetrachloride, pickled with 1: 1 nitric acid, washed with 1:3 hydrochloric acid followed by distilled water and finally acetone. A weighed quantity (300-350 mg) of the cleaned and dried uranium metal turnings was dissolved under argon atmosphere in 2M sulphuric acid containing a few drops of HF on a water bath. After complete dissolution of the turnings, the solution was cooled to room temperature under inert atmosphere and transferred to a loo-ml standard flask and diluted to volume with 2M sulphuric acid. The absorbances were recorded immediately. ~eterrn~~tio~ ofmolar absorptivity of U(H) at 310, 300 and 285 nm. An accurately weighed amount of cleaned uranium metal turnings was dissolved in concentrated nitric acid and evaporated to dryness. A few milliliters of concentrated sulphuric acid were added and the solution was heated to dense white fumes. This residue was then dissolved in 2M sulphuric acid and diluted to 100 ml. This solution was used as a stock solution and absorbances were recorded after appropriate dilution of this solution.
From the absorption spectra of U(IV) and U(V1) in 2M sulphuric acid medium (Fig. I), it is seen that U(IV) has maxima at 535 and 630 nm with no absorption by UPI) at these waveleng~s. U(IV) does not absorb at all in the wavelength region 280-380 nm. Below 275 nm, however, there is a continuous absorption by U(IV) as well as U(V1). U(V1) absorbs in the wavelength region 360-480 nm. Below 340 nm,
Spectrophotometric determination of the O/U ratio in uranium oxides
1337
inert gas during dissolution of uranium oxide powders is not necessary. However, in the case of uranium dioxide pellets, the O/U ratio is an important parameter and a precise O/U ratio determination is required. (The O/U ratio specification for powders is normally in the l.O: ;b range 2.05-2.15 compared with 2.000-2.015 for 4 : pellets. The powder ratio is important for char; : acterizing sinterability of the pellets.) The rate : ‘\ of dissolution in the case of pellet powders is ‘\ slow compared to uranium oxide powders and 250 300 400 500 600 700 therefore a dissolution period of about 1.5 hours Wavelength, nm is required to dissolve sufficient U(W). During Fig. 1. Absorption spectra of U(W) and U(W) in 2M such extended dissolution periods, oxidation of sulphmic acid medium. (a) U(W), 14.6 mg/ml (b) U(W) 0.32 mg/ml. U(N) by air becomes significant (l-1.5%) and would result in an increase in the O/U ratio of 0.015 O/U ratio units. Hence, an inert atmosthere is no wavelength of maximum absorption phere is essential during dissolution of the pellet for U(W), but rather is a continuous absorpsamples. The volume of 2M sulphuric acid to be tion. However, the Beer-Lambert law is obeyed for solutions of U(W) at 310, 300 and 285 nm used for the dissolution purpose should be from 0.06-1.2 absorbance units. The molar ab- 100-110 ml with 0.3-0.5% (v/v) HF so that sorptivities of U(VI) at 310,300 and 285 nm are precipitation of uranium(N) sulphate does not take place, even after slight evaporation during 178.1, 278.6 and 585 l.mole-‘.cm-‘, respectdissolution. This precipitation would cause a ively, whereas the molar absorptivities of U(N) at 630 and 535 nm are 21.4 and 6.8 higher O/U ratio because of the preferential precipitation of the uranium(W) sulphate by 1.mole-‘. cm-‘, respectively. The higher molar absorptivity value of U(W) at 285 nm and lower virtue of its limited solubility in sulphuric acid. At room temperature, oxidation of U(W) by molar absorptivity value of U(Iv) at 535 nm enables one to determine the O/U ratio in U02 air in 2M sulphuric acid medium is not signifipellets close to 2.001, a measurement which is cant within 5-6 hr. However, this rate of oxidation of U(IV) at elevated temperatures difficult in phosphoric acid medium. The extent of oxidation of U(W) in 2M (N 100’) decreases with an increase in sulphuric sulphuric acid by air at N 100” within half an acid concentration because of the increased stability of U(IV) in a higher concentration of hour is 0.1-0.2%, an amount which contributes sulphuric acid. The effect of air oxidation in to a positive bias of 0.001-0.002 O/U ratio units various concentrations of sulphuric acid at (well within the required precision). Secondly, N 100” are shown in Fig. 3. At a sulphuric acid since stringent O/U specifications are not required in the case of powder samples, purging of concentration greater than 5M, the oxidation may not be significant and at this acidity an inert atmosphere may not be needed during dissolution of pellet powders also, thus making the procedure still convenient. The drawback of dissolving the uranium dioxide powders in such high concentrated sulphuric acid is the 2.0 j I limited solubility of uranium(N) sulphate. I , Hence, 2M sulphuric acid is taken as the jb E < medium of dissolution, using inert gas purging i l.O1 during dissolution. It is observed that gadolinium in 2M sulphuric acid medium does not have significant absorbance in the wavelength range 700-250 nm. Hence, the present method is well suited even for determining the O/U ratio of uranium Fig. 2. Absorption spectra of U(N) and U(W) in 13M dioxide pellets containing gadolinium as a bumphosphoric acid medium. (a) U(N), 9.53 mg/ml (b) U(W), 1.53 mg/ml. able poison.
B. NARWMHAMaw
1338
4
6 5 4 3i
IM
ZM
3M
4M
5M
6M
lH,sO,l. M Fig. 3. Effect of concentration of sulphuric acid on air oxidation of U(IV). A(O/U) indicates the difference of O/U ratio values with and without inert atmosphere during
Absorption spectra in 2M sulphuric acid. The
absorption spectra of U(W) and U(V1) in 2M s~ph~c acid and strong (13M) phosphoric acid (SPA) are shown in Fig. 1 and Fig. 2, respectively. It is clear from the spectra of U(W) and U(W) in 2M sulphuric acid and SPA for expected concentrations of U(IV) and U(W), that the spectral pattern in both media is essentially similar, indicating the salary between the molecular structure of U(IV) and U(W) and the nature of transitions involved% in these two media. In both cases there is a shift towards lower wavelengths and significant difference in I.4 1.3 -
-
d
1.21.11.0 0.9 _ 0.8-
-’
et al.
the molar absorptivity, causing better sensitivity in sulphuric acid medium when compared to phosphoric acid medium. The absorbances of II(IV) and U(W) at the wavelengths considered are ind~end~t of the inundation of sulphuric acid between 1 and 5M, which is clear from Fig. 4. The reported molar absorptivity of U(IV) at 544 mn is 11.66 l.mole-‘.cm-’ and that of U(W) at 290 rrm is 215.9 l.mole-l.cm-’ in SPA.23 Whereas in the present 2M sulphuric acid medium, the molar absorptivity of U(W) at 535 nm is 6.8 1.mole-’ .cm-’ and that of U(W) at 285 nm is 585 l,mole-‘.cm-‘. Hence, in the present medium there is an almost five-fold increase in sensitivity with respect to O/U ratio units. This has enabled the work to be extended to the determination of O/U ratio in UOz pellets closer to 2.001 O/U units with good accuracy. The rate of dissolution of UO,,, is greater in 2M sulphuric acid containing a few drops of HF compared to SPA, which makes the present method of analysis more rapid. HF seems to act as a catalyst in the dissolution, as is the case with PuO, and ThO, dissolution in HNO,-HF mixture2’ where the nature of the intermediate complex is assumed to be MF:+, Most of the HF vaporizes out of solution with the argon stream by the end of the dissolution period, Therefore, the concentration of the residual HF, if any, is too low to attack quartz cells at room temperature during the absorbance measurements. It has been observed that a small quantity of free HF in 2M sulphuric acid does not alter the absorbance values of U(IV) and U(V1) at their respective wavelengths. The data tabulated in Table 1 shows the precision of the method for UO1 fuel pellets and UOt +xpowders. It is clear from Table 2 that the O/U ratios obtained by the s~rophotomet~c method are in close agreement with those
0.7 ’
0.6”
Table 1. Data justifying the precision of the method
0.5 0.4 0.3 -
====z
0.2-
Fa
0.1 -
Fig. 4, Absorbances of a mixture of 14.6 mg/ml U(N) and 0.32 mg/ml U(VI) at varying concentrations of sulphuric acid. U(W) at (a) 310 nm, (b) 300 nm, (c) 285 nm. II(IV) at (d) 630 nm, (e) 535 nm.
Pellets
Powders
2.005 2.005 2.004 2.004 2.005 2.008 2.007 2.007 2.006 2.005 Mean 2.006 Precision *0.002
2.069 2.068 2.069 2.067 2.070 2.077 2.072 2.070 2.076 2.070 2.071 ItO.
Spectrophotometric
determination of the O/U ratio in uranium oxides
Table 2. O/U ratios in UO, and U,Os powders S. No. 1 2 3 4 5 6 7 8 9* 10*
O/U ratio value spectrophotometry volumetry
Sample Code No. M 370 M 365 M 30 M31 M 32 M 39 M 36 M 38 M 33 M 29
2.077 2.058 2.120 2.085 2.148 2.103 2.093 2.078 2.683 2.677
2.078 2.056 2.118 2.086 2.149 2.101 2.095 2.076 2.682 2.679
*U,OB powder samples
obtained by the volumetric method. This indicates the stability of tetravalent uranium in 2M sulphuric acid medium. The O/U ratios calculated with possible combinations of wavelengths for U(W) and U(V1) give the same value of O/U ratio for a given sample (Table 3). It is concluded that the combination 535 and 285 nm for U(W) and U(VI), respectively, is *well suited to the determination of the O/U ratio in UOz pellets where the O/U ratio lies between 2.001 and 2.010 units. Comparison of O/U ratios of uranium dioxide pellets obtained by the spectrophotometric method based on dissolution in SPA and 2M sulphuric acid are shown in Table 4. The fair agreement between these results emphasize that the stability of U(N) towards oxidation in both media is of the same order. The values of O/U
1339
Table 5. Comparison of O/U ratios in uranium dioxide fuel pellets by spectrophotometric and polarographic methods oxygen to uranium ratio S. No. Code No. : 3 4 : ; 9 10
Spectrophotometry
CFFP 21 CFFP 47 CFFP 30 CFFP 26 CFPP 51 CFFP 49 CFFP 44 CFFP 45 2604 2586
2.002 2.003 2.082 2.004 2.004 2.003 2.082 2.000 2.801 2.001
2.002 2.002 2.082 2.002 2.083 2.003 2.001 2.002 2.001 2.001
ratios of UOz pellets obtained by polarography and the present spectrophotometric method have been compared and the results are shown in Table 5. This table shows the good agreement between the two methods. The fact that the O/U ratio values obtained by the two independent methods are in good agreement confirms the non-oxidising nature of the present medium, i.e., 2M sulphuric acid, and also the accuracy of the present spectrophotometric method. CALCULATIONS
Let the absorbances of U(N) at I, nm be Q units and that of U(W) at A2nm be b units and the corresponding molar absorptivities be L, and Lo respectively. Then, the O/U ratio of the
Table 3. O/U ratio values of UOr pellets calculated with various combinations wavelength S. No. Wavelength combination for UOV), U(vI), (nm) 630,310 630,300 630,285 535,310 535,300 535,285
1 2 3 4 5
of
1
2
3
4
5
6
BR202
M418
BR248III
BR2381
BR252
MB160
2.004 2.005 2.004 2.004 2.804 2.005
2.006 2.006 2.005 2.006 2.006 2.005
2.003 2.003 2.003 2.004 2.003 2.003
2.084 2.004 2.005 2.004 2.004 2.004
2.010 2.010 2.009 2.010 2.010 2.009
2.007 2.007 2.097 2.087 2.007 2.007
Table 4. O/U ratios in UOs pellets
S. No.
Oxygen to Uranium ratio 13M phosphoric acid medium 2M sulphuric acid medium (544 mn and 290 nm are used [535 mn and 285 nm are used for U(IV) and U(VI)] for U(IV) and U(VI)] 2.010 2.007 2.006 2.008 2.008
Polarography
;zz 2:005 2.006 2.007
B. NARASIMHA MURTYet al.
1340
various kinds of uranium oxides can be calculated as follows: C OU = 2 + cq,) T&,~ 1
=2+
1 + @z) (a) @I) (b)
However, in the case of pellets where Cu(W)* G(W) 3 the above expression reduces to
(4 @I --
o’u=2+(62) (a)
If the observed absorbance for U(W) at 535 nm is a units and that of U(W) at 285 nm is b units, then the O/U ratio is given by O/U = 2 + 0.0116 b/u. Acknowledgements-The authors wish to express their deep gratitude to Dr. T. S. Krishnan, Deputy Chief Executive (Q.A.) for his keen interest and encouragement in the present work. The authors also acknowledge Shri H. R. Ravindra for useful discussions during the course of the present investigation.
7. B. M. Veal and D. Lam, J. Phys. Lerr. A, 1974, 49, 466. 8. G. C. Allen, J. A. Crofts, M. T. Curtis, P. M. Tucker, D. Chadwick and P. J. Hampson, J. Gem. Sot. Dalton Trans., 1974, 1296. 9. E. A. Shaefer and J. 0. Hibbits, Anal. Chem., 1969,41, 254. 10. R. W. Stromatt and R. E. Connally, ibid., 1961,33,345. 11. S. Kihara, Z. Yoshida, H. Mutto, H. Aoyagi, Y. Baba and H. Hashitani, ibid., 1980, 52, 1601. 12. J. J. Engelsman, J. Knaape and J. Visser, Talanfa, 1968, 15, 171. 13 R. M. Burd and G. M. Goward, USAEC Rep. WAPD’ 205, 1959. 14. H. Kubota, Anal. Chem., 1960, 32, 610. 15. K. Matojima and A. Hoshino, J. Ar. Energy Sot. Jpn., 1960, 2, 1. and M. Zambianchi, 16. P. L. Buldini, D. Ferri, E. Palti Analyst, 1984, 109, 225. 17. V. Kuvik, J. Kritl and A. Moravec, Radiochem. Radioanal. Lert., 1982, 54, 209.
18. S. R. Dharwadkar and Chandrasekbaraigb,
Anal. Chim.
Acta, 45, 545, 1969.
ii’
L. G. Stronhill, Can. J. Chem., 1959, 37, 454.
’ R. B. Yadav, B. Narasimha Murty and S. Syamsundar,
National Workshop on Testing and Charactertiation of Materials TACOM-90, Bombay, March 15-16, 1990. E. Kuhn, G. Baumgartel and H. Schmieder, 2. Anal.
21’ Chem., 1973, 264, 103. -1 LL. S. Kihara, T. Adachi and H. Hashitani, ibid., 1980,303, 28.
REFERENCES 1. T. M. Florence, IAEA Symp. Anal. Meth. in the Nucl. Fuel Cycle, Vienna, 1972. 2. R. Conti, Euratom Rep., EUR 4518f, 1970. 3. Hellmann K. W. U., Erlangen, W. Germany, 1982, Private communicalions.
4. K. Kiukkola and C. Wagner, 1. Elecrrochem. Sot., 1957, 104, 379. 5. S. Aronson and J. Bell, J. Chem. Phys., 1958, 29, 151. 6. T. M. Florence, Analytical Methodr in the Nuclear Fuel Cycle, IAEASM--149164, IAEA: Vienna, 1972, p. 145.
23. H. Takeishi, H. Muto, H. Aoyagi, T. Ada&i, K. Ixawa, Z. Yoshida, H. Kawamura and S. Kihara, AMY. Chem., 1986, S& 458. 24. M. K. Ahmed and N. L. Sreenivasan, ibid., 1986, 58, 2479.
25. A. Tolk and W. A. Lingerak, Proceedings of a peel on Analytical Chemistry of Nuclear Fuel; IAEA: Vienna, 1972; STI/PUB/337, pp. 51-58. 26. T. Takeuchi and T. Yoshimori, Banseki Kagaku 1963, 12, 840. 27. 0. K. Tallent and J. C. Mailen, Nuclear Technology, 1977, 32, 167.
Edma, Vol. 38, No. 11, pp. 13354340, 1991. printedin Great Britain. Ail rights reserved
$3.00 + 0.00
Copyright Q 1991Pergamon Press pk
SPECTROPHOTOMETRIC DETERMINA~ON OF THE OXYGEN TO ~~NIUM RATIO IN U~~UM OXIDES BASED ON DISSOLUTION IN SULPHURIC ACID TIW and S. SYAMSUNDAR B. NARASIMHA MURTY, R. B. YADAV,C. K. RAMAMUR
Control Laboratory, Nuclear Fuei Complex, Hyderabad, India (Receiued 8 iMarch 1991. Revised 15 Aprii 1991. Accepted 30 April 1991)
Summary-The oxygen to uranium ratio in uranium oxides such as &OS, UOr +, powders and UO, fuel pellets has been determined by a new spectrophotometric metlmd. The method can be used for determination of O/U ratio in UOr pellets and powders on a routine basis. In the described method, uranium oxides in the powder form are dissolved in 2M sulphuric acid containing a few drops of HF. The ~n~ntmtions of II(IV) and U(W) are directly dete~n~ by means of the absorbances of these species at different wavelengths. For determination of the O/U ratio in U,O, powder samples, 630 and 310 nm are the wavelengths chosen for U(W) and U(W), respectively. For U02+ x powder, where the O/U ratio lies between 2.04 to 2.15, U(W) and U(W) are determined at 630 and 300 nm respectively, whereas for UOs fuel pellets, where the O/U ratio is less than 2.01,535 and 285 nm are used. The molar absorptivity of U(N) at 630 and 535 am is 21.4 and 6.8 l.mole-‘.cm-’ and that of U(W) at 310, 300 and 285 nm is 178.1, 278.6 and 585 1. mole-‘. cm-‘, respectively. Standard deviations of +0.002 O/U ratio units for pellets and &O&I4 O/U ratio units for powders have been achieved.
The uranium-oxygen system is one of the most complex systems known because of the multiplicity of stable oxidation states of uranium. Because of this, the exact stoichiometry of uranium dioxide is seldom attained, as it usually exists as a hyperstoichiometric oxide which includes excess oxygen in its cubic lattice. One of the most important parameters of nuclear grade uranium dioxide pellets is the O/U ratio, as it is important in the characterization of the sinterability of the powder and its performance as a fuel under reactor conditions. An oxygen-touranium ratio closely approaching 2.000 is a frequent requirement for nuclear fuel, so, as a quality control measure, it is essential to know the O/U ratio of the U02 powder. There are many instrumental and chemical methods available for the determination of the O/U ratio. Both destructive and non-destructive methods have been reviewed by Florence,’ Some of the instrumental methods are X-ray diffraction analysis,* electromotive force measurement of a Ni, NiO electrolyte-UO, +x cell= and X-ray photoemission spectrometry.7T8 A major disadvantage of these methods, is that they need reference materials having identical composition with that of the sample in order to calibrate the methods. Chemical methods include gravimetry,3v9 coulometry,‘O*l’ titrimetry3J2 and polarography. 13-is
All the wet chemical methods for the determination of the O/U ratio are based on the dete~ination of either U(IV) and/or U(W), since the degree of hyperstoichiometry can be directly related to the concentration of U(VI).16 Thus, the estimation of U(W) serves as a measure of the excess oxygen and, coupled with total uranium determination, enables one to determine the exact composition of the oxide. For near stoichiometric U02 pellets, where a minute quantity of U(V1) has to be determined in the presence of a large excess of U(W), polarographic, coulometric and biamperometric methods are preferred.” However, for U02,., powders, where U(V1) is also present in relatively higher concentration, titrimetric procedures’*‘**lgusing potentiometric and/or visual indicator techniquesZO are more convenient. However, these methods require that the sample is weighed accurately and dissolved completely. S~trophotomet~c methods for the determination of the O/U ratio do not require weighing and complete dissolution of the sample because both U(W) and U(W) are determined in the same aliquot. Our experiments demonstrate that Urals dissolves at the same rate in both oxidation states as evidenced by the same O/U ratio obtained irrespective of whether the sample is partially or completely dissolved. However, spectrophotometric methods which
1335
B. NARASIMHA MURTY et al.
1336
use phosphoric acid21mz4 reported so far for the determination of O/U ratios of UOz pellets are not convenient on a routine basis because they suffer from many disadvantages such as high viscosity, long ~s~lu~on time, poor ~nsiti~ty and elaborate treatment procedures.25 The present method is based on the dissolution of the sample in 2M sulphuric acid in the presence of a very small quantity of HF, followed by measurement of absorbances at specific wavelengths for U(IV) and U(V1). Earlier, Takeuchi et 01.‘~ have reported the dissolution of uranium oxide in 1M sulphuric acid containing a few drops of HF and subsequent determination of the U(V1) coulometritally by titrating with electrogenerated Ti(II1). Stronhilli9 determined ~anium(~ titrimetritally with Fe2+ or T?+ based on sulphuric acid dissolution. EXPERIMENTAL Reagents
Uranium dioxide pellets, UOz powders and U,Os powders produced at the Nuclear Fuel Complex, Hyderabad, India were used as samples. All dissolution reagents were prepared with reagent grade materials and distilled water. Instrumentation Absorbance measurements were carried out on a Shiiadzu Model W-240, Graphicord Recording Spectrophotometer. W-visible Matched quartz cells of l-cm path length were used. The base line and digital absorbance readings were adjusted to 0.000 for a reagent blank (2M sulphuric acid) before each set of measurements. Procedure In order to obtain a reliable value of the O/U ratio in U02+=, it is essential that the oxidation states of uranium in the solid be kept unchanged throughout procedures such as the grinding of the UOZfx pe llet and the dissolution of the powder obtained. Keeping this in view, the following strategy was adopted for grinding the pellets and dissolution of the powder. UO, pellets. The U02+X pellet was placed in a cylindrical container and the air was displaced with a jet of argon. After suEcient flushing the lid was tightened, and the sample was ground for about 10-15 set in a Spex Mixer Mill to provide a mesh size no smaller than 150. If the particles are very fine some oxidation may
occur. Nearly 10-15 g of the ground powder was then dissolved under argon atmosphere in 100-l 10 ml of 2M sulphuric acid containing l-2 drops of HF, on a water bath. The dissolution was carried out for about one and half hours. The absorbances for U(IV) and U(VI) were measured at 535 and 285 nm, respectively. iYO,/U, OS powders. About 80-100 mg of powder was dissolved in 40-50 ml of 2M sulphuric acid containing one drop of HF. Dissolution was carried out on a water bath in ambient air for about half an hour. For UOz powders, the absorbances of U(IV) and II(VI) were measured at 630 and 310 nm, respectively, while for U,O, powders they were measured at 630 and 300 nm. ~eterm~ation of molar absorptivity of U(W) at 630 and 535 nm. Nuclear grade uranium metal turnings were degreased with carbon tetrachloride, pickled with 1: 1 nitric acid, washed with 1:3 hydrochloric acid followed by distilled water and finally acetone. A weighed quantity (300-350 mg) of the cleaned and dried uranium metal turnings was dissolved under argon atmosphere in 2M sulphuric acid containing a few drops of HF on a water bath. After complete dissolution of the turnings, the solution was cooled to room temperature under inert atmosphere and transferred to a loo-ml standard flask and diluted to volume with 2M sulphuric acid. The absorbances were recorded immediately. ~eterrn~~tio~ ofmolar absorptivity of U(H) at 310, 300 and 285 nm. An accurately weighed amount of cleaned uranium metal turnings was dissolved in concentrated nitric acid and evaporated to dryness. A few milliliters of concentrated sulphuric acid were added and the solution was heated to dense white fumes. This residue was then dissolved in 2M sulphuric acid and diluted to 100 ml. This solution was used as a stock solution and absorbances were recorded after appropriate dilution of this solution.
From the absorption spectra of U(IV) and U(V1) in 2M sulphuric acid medium (Fig. I), it is seen that U(IV) has maxima at 535 and 630 nm with no absorption by UPI) at these waveleng~s. U(IV) does not absorb at all in the wavelength region 280-380 nm. Below 275 nm, however, there is a continuous absorption by U(IV) as well as U(V1). U(V1) absorbs in the wavelength region 360-480 nm. Below 340 nm,
Spectrophotometric determination of the O/U ratio in uranium oxides
1337
inert gas during dissolution of uranium oxide powders is not necessary. However, in the case of uranium dioxide pellets, the O/U ratio is an important parameter and a precise O/U ratio determination is required. (The O/U ratio specification for powders is normally in the l.O: ;b range 2.05-2.15 compared with 2.000-2.015 for 4 : pellets. The powder ratio is important for char; : acterizing sinterability of the pellets.) The rate : ‘\ of dissolution in the case of pellet powders is ‘\ slow compared to uranium oxide powders and 250 300 400 500 600 700 therefore a dissolution period of about 1.5 hours Wavelength, nm is required to dissolve sufficient U(W). During Fig. 1. Absorption spectra of U(W) and U(W) in 2M such extended dissolution periods, oxidation of sulphmic acid medium. (a) U(W), 14.6 mg/ml (b) U(W) 0.32 mg/ml. U(N) by air becomes significant (l-1.5%) and would result in an increase in the O/U ratio of 0.015 O/U ratio units. Hence, an inert atmosthere is no wavelength of maximum absorption phere is essential during dissolution of the pellet for U(W), but rather is a continuous absorpsamples. The volume of 2M sulphuric acid to be tion. However, the Beer-Lambert law is obeyed for solutions of U(W) at 310, 300 and 285 nm used for the dissolution purpose should be from 0.06-1.2 absorbance units. The molar ab- 100-110 ml with 0.3-0.5% (v/v) HF so that sorptivities of U(VI) at 310,300 and 285 nm are precipitation of uranium(N) sulphate does not take place, even after slight evaporation during 178.1, 278.6 and 585 l.mole-‘.cm-‘, respectdissolution. This precipitation would cause a ively, whereas the molar absorptivities of U(N) at 630 and 535 nm are 21.4 and 6.8 higher O/U ratio because of the preferential precipitation of the uranium(W) sulphate by 1.mole-‘. cm-‘, respectively. The higher molar absorptivity value of U(W) at 285 nm and lower virtue of its limited solubility in sulphuric acid. At room temperature, oxidation of U(W) by molar absorptivity value of U(Iv) at 535 nm enables one to determine the O/U ratio in U02 air in 2M sulphuric acid medium is not signifipellets close to 2.001, a measurement which is cant within 5-6 hr. However, this rate of oxidation of U(IV) at elevated temperatures difficult in phosphoric acid medium. The extent of oxidation of U(W) in 2M (N 100’) decreases with an increase in sulphuric sulphuric acid by air at N 100” within half an acid concentration because of the increased stability of U(IV) in a higher concentration of hour is 0.1-0.2%, an amount which contributes sulphuric acid. The effect of air oxidation in to a positive bias of 0.001-0.002 O/U ratio units various concentrations of sulphuric acid at (well within the required precision). Secondly, N 100” are shown in Fig. 3. At a sulphuric acid since stringent O/U specifications are not required in the case of powder samples, purging of concentration greater than 5M, the oxidation may not be significant and at this acidity an inert atmosphere may not be needed during dissolution of pellet powders also, thus making the procedure still convenient. The drawback of dissolving the uranium dioxide powders in such high concentrated sulphuric acid is the 2.0 j I limited solubility of uranium(N) sulphate. I , Hence, 2M sulphuric acid is taken as the jb E < medium of dissolution, using inert gas purging i l.O1 during dissolution. It is observed that gadolinium in 2M sulphuric acid medium does not have significant absorbance in the wavelength range 700-250 nm. Hence, the present method is well suited even for determining the O/U ratio of uranium Fig. 2. Absorption spectra of U(N) and U(W) in 13M dioxide pellets containing gadolinium as a bumphosphoric acid medium. (a) U(N), 9.53 mg/ml (b) U(W), 1.53 mg/ml. able poison.
B. NARWMHAMaw
1338
4
6 5 4 3i
IM
ZM
3M
4M
5M
6M
lH,sO,l. M Fig. 3. Effect of concentration of sulphuric acid on air oxidation of U(IV). A(O/U) indicates the difference of O/U ratio values with and without inert atmosphere during
Absorption spectra in 2M sulphuric acid. The
absorption spectra of U(W) and U(V1) in 2M s~ph~c acid and strong (13M) phosphoric acid (SPA) are shown in Fig. 1 and Fig. 2, respectively. It is clear from the spectra of U(W) and U(W) in 2M sulphuric acid and SPA for expected concentrations of U(IV) and U(W), that the spectral pattern in both media is essentially similar, indicating the salary between the molecular structure of U(IV) and U(W) and the nature of transitions involved% in these two media. In both cases there is a shift towards lower wavelengths and significant difference in I.4 1.3 -
-
d
1.21.11.0 0.9 _ 0.8-
-’
et al.
the molar absorptivity, causing better sensitivity in sulphuric acid medium when compared to phosphoric acid medium. The absorbances of II(IV) and U(W) at the wavelengths considered are ind~end~t of the inundation of sulphuric acid between 1 and 5M, which is clear from Fig. 4. The reported molar absorptivity of U(IV) at 544 mn is 11.66 l.mole-‘.cm-’ and that of U(W) at 290 rrm is 215.9 l.mole-l.cm-’ in SPA.23 Whereas in the present 2M sulphuric acid medium, the molar absorptivity of U(W) at 535 nm is 6.8 1.mole-’ .cm-’ and that of U(W) at 285 nm is 585 l,mole-‘.cm-‘. Hence, in the present medium there is an almost five-fold increase in sensitivity with respect to O/U ratio units. This has enabled the work to be extended to the determination of O/U ratio in UOz pellets closer to 2.001 O/U units with good accuracy. The rate of dissolution of UO,,, is greater in 2M sulphuric acid containing a few drops of HF compared to SPA, which makes the present method of analysis more rapid. HF seems to act as a catalyst in the dissolution, as is the case with PuO, and ThO, dissolution in HNO,-HF mixture2’ where the nature of the intermediate complex is assumed to be MF:+, Most of the HF vaporizes out of solution with the argon stream by the end of the dissolution period, Therefore, the concentration of the residual HF, if any, is too low to attack quartz cells at room temperature during the absorbance measurements. It has been observed that a small quantity of free HF in 2M sulphuric acid does not alter the absorbance values of U(IV) and U(V1) at their respective wavelengths. The data tabulated in Table 1 shows the precision of the method for UO1 fuel pellets and UOt +xpowders. It is clear from Table 2 that the O/U ratios obtained by the s~rophotomet~c method are in close agreement with those
0.7 ’
0.6”
Table 1. Data justifying the precision of the method
0.5 0.4 0.3 -
====z
0.2-
Fa
0.1 -
Fig. 4, Absorbances of a mixture of 14.6 mg/ml U(N) and 0.32 mg/ml U(VI) at varying concentrations of sulphuric acid. U(W) at (a) 310 nm, (b) 300 nm, (c) 285 nm. II(IV) at (d) 630 nm, (e) 535 nm.
Pellets
Powders
2.005 2.005 2.004 2.004 2.005 2.008 2.007 2.007 2.006 2.005 Mean 2.006 Precision *0.002
2.069 2.068 2.069 2.067 2.070 2.077 2.072 2.070 2.076 2.070 2.071 ItO.
Spectrophotometric
determination of the O/U ratio in uranium oxides
Table 2. O/U ratios in UO, and U,Os powders S. No. 1 2 3 4 5 6 7 8 9* 10*
O/U ratio value spectrophotometry volumetry
Sample Code No. M 370 M 365 M 30 M31 M 32 M 39 M 36 M 38 M 33 M 29
2.077 2.058 2.120 2.085 2.148 2.103 2.093 2.078 2.683 2.677
2.078 2.056 2.118 2.086 2.149 2.101 2.095 2.076 2.682 2.679
*U,OB powder samples
obtained by the volumetric method. This indicates the stability of tetravalent uranium in 2M sulphuric acid medium. The O/U ratios calculated with possible combinations of wavelengths for U(W) and U(V1) give the same value of O/U ratio for a given sample (Table 3). It is concluded that the combination 535 and 285 nm for U(W) and U(VI), respectively, is *well suited to the determination of the O/U ratio in UOz pellets where the O/U ratio lies between 2.001 and 2.010 units. Comparison of O/U ratios of uranium dioxide pellets obtained by the spectrophotometric method based on dissolution in SPA and 2M sulphuric acid are shown in Table 4. The fair agreement between these results emphasize that the stability of U(N) towards oxidation in both media is of the same order. The values of O/U
1339
Table 5. Comparison of O/U ratios in uranium dioxide fuel pellets by spectrophotometric and polarographic methods oxygen to uranium ratio S. No. Code No. : 3 4 : ; 9 10
Spectrophotometry
CFFP 21 CFFP 47 CFFP 30 CFFP 26 CFPP 51 CFFP 49 CFFP 44 CFFP 45 2604 2586
2.002 2.003 2.082 2.004 2.004 2.003 2.082 2.000 2.801 2.001
2.002 2.002 2.082 2.002 2.083 2.003 2.001 2.002 2.001 2.001
ratios of UOz pellets obtained by polarography and the present spectrophotometric method have been compared and the results are shown in Table 5. This table shows the good agreement between the two methods. The fact that the O/U ratio values obtained by the two independent methods are in good agreement confirms the non-oxidising nature of the present medium, i.e., 2M sulphuric acid, and also the accuracy of the present spectrophotometric method. CALCULATIONS
Let the absorbances of U(N) at I, nm be Q units and that of U(W) at A2nm be b units and the corresponding molar absorptivities be L, and Lo respectively. Then, the O/U ratio of the
Table 3. O/U ratio values of UOr pellets calculated with various combinations wavelength S. No. Wavelength combination for UOV), U(vI), (nm) 630,310 630,300 630,285 535,310 535,300 535,285
1 2 3 4 5
of
1
2
3
4
5
6
BR202
M418
BR248III
BR2381
BR252
MB160
2.004 2.005 2.004 2.004 2.804 2.005
2.006 2.006 2.005 2.006 2.006 2.005
2.003 2.003 2.003 2.004 2.003 2.003
2.084 2.004 2.005 2.004 2.004 2.004
2.010 2.010 2.009 2.010 2.010 2.009
2.007 2.007 2.097 2.087 2.007 2.007
Table 4. O/U ratios in UOs pellets
S. No.
Oxygen to Uranium ratio 13M phosphoric acid medium 2M sulphuric acid medium (544 mn and 290 nm are used [535 mn and 285 nm are used for U(IV) and U(VI)] for U(IV) and U(VI)] 2.010 2.007 2.006 2.008 2.008
Polarography
;zz 2:005 2.006 2.007
B. NARASIMHA MURTYet al.
1340
various kinds of uranium oxides can be calculated as follows: C OU = 2 + cq,) T&,~ 1
=2+
1 + @z) (a) @I) (b)
However, in the case of pellets where Cu(W)* G(W) 3 the above expression reduces to
(4 @I --
o’u=2+(62) (a)
If the observed absorbance for U(W) at 535 nm is a units and that of U(W) at 285 nm is b units, then the O/U ratio is given by O/U = 2 + 0.0116 b/u. Acknowledgements-The authors wish to express their deep gratitude to Dr. T. S. Krishnan, Deputy Chief Executive (Q.A.) for his keen interest and encouragement in the present work. The authors also acknowledge Shri H. R. Ravindra for useful discussions during the course of the present investigation.
7. B. M. Veal and D. Lam, J. Phys. Lerr. A, 1974, 49, 466. 8. G. C. Allen, J. A. Crofts, M. T. Curtis, P. M. Tucker, D. Chadwick and P. J. Hampson, J. Gem. Sot. Dalton Trans., 1974, 1296. 9. E. A. Shaefer and J. 0. Hibbits, Anal. Chem., 1969,41, 254. 10. R. W. Stromatt and R. E. Connally, ibid., 1961,33,345. 11. S. Kihara, Z. Yoshida, H. Mutto, H. Aoyagi, Y. Baba and H. Hashitani, ibid., 1980, 52, 1601. 12. J. J. Engelsman, J. Knaape and J. Visser, Talanfa, 1968, 15, 171. 13 R. M. Burd and G. M. Goward, USAEC Rep. WAPD’ 205, 1959. 14. H. Kubota, Anal. Chem., 1960, 32, 610. 15. K. Matojima and A. Hoshino, J. Ar. Energy Sot. Jpn., 1960, 2, 1. and M. Zambianchi, 16. P. L. Buldini, D. Ferri, E. Palti Analyst, 1984, 109, 225. 17. V. Kuvik, J. Kritl and A. Moravec, Radiochem. Radioanal. Lert., 1982, 54, 209.
18. S. R. Dharwadkar and Chandrasekbaraigb,
Anal. Chim.
Acta, 45, 545, 1969.
ii’
L. G. Stronhill, Can. J. Chem., 1959, 37, 454.
’ R. B. Yadav, B. Narasimha Murty and S. Syamsundar,
National Workshop on Testing and Charactertiation of Materials TACOM-90, Bombay, March 15-16, 1990. E. Kuhn, G. Baumgartel and H. Schmieder, 2. Anal.
21’ Chem., 1973, 264, 103. -1 LL. S. Kihara, T. Adachi and H. Hashitani, ibid., 1980,303, 28.
REFERENCES 1. T. M. Florence, IAEA Symp. Anal. Meth. in the Nucl. Fuel Cycle, Vienna, 1972. 2. R. Conti, Euratom Rep., EUR 4518f, 1970. 3. Hellmann K. W. U., Erlangen, W. Germany, 1982, Private communicalions.
4. K. Kiukkola and C. Wagner, 1. Elecrrochem. Sot., 1957, 104, 379. 5. S. Aronson and J. Bell, J. Chem. Phys., 1958, 29, 151. 6. T. M. Florence, Analytical Methodr in the Nuclear Fuel Cycle, IAEASM--149164, IAEA: Vienna, 1972, p. 145.
23. H. Takeishi, H. Muto, H. Aoyagi, T. Ada&i, K. Ixawa, Z. Yoshida, H. Kawamura and S. Kihara, AMY. Chem., 1986, S& 458. 24. M. K. Ahmed and N. L. Sreenivasan, ibid., 1986, 58, 2479.
25. A. Tolk and W. A. Lingerak, Proceedings of a peel on Analytical Chemistry of Nuclear Fuel; IAEA: Vienna, 1972; STI/PUB/337, pp. 51-58. 26. T. Takeuchi and T. Yoshimori, Banseki Kagaku 1963, 12, 840. 27. 0. K. Tallent and J. C. Mailen, Nuclear Technology, 1977, 32, 167.