DETERMINATION OF ELEMENTAL IMPURITIES IN U308 BY WAVELENGTH-DISPERSIVE X-RAY FLUORESCENCE SPECTROMETRY

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1 DETERMINATION O ELEMENTAL IMPURITIES IN U308 BY WAVELENGTH-DISPERSIVE X-RAY LUORESCENCE SPECTROMETRY Jozef Leon Parus, Wolfgang Raab International Atomic Energy Agency, Safeguards Analytical Laboratory Wagramerstrasse 5, P.O. Box 00, A- 400 Vienna, Austria Renata Mikolajczak Radioisotope Centre POLATOM, Swierk, Poland ABSTRACT Ten UsOs reference materials certified for impurities were used to develop the analytical procedure. A sequential wavelength dispersive X-ray spectrometer was used to perform the measurements. The measurement conditions were optimized to assure the best signal to background ratios and the minimum spectral interferences. Nineteen elements (Al, Bi, Ca, Co, Cr, Cu, e, Mg, Mn, MO, Na, Ni, Pb, Si, Ti, V, W, Zn, Zr) can be determined above the 0 to 20 ppm level in a measurement time equal to 50 s for single element (50 s line intensity and 2 x 50 s background). The precision and accuracy is assessed based on the counting statistics and the confidence limits of the certified element concentrations. INTRODUCTION The assessment of U308 purity from the point of view of its usefulness in nuclear fuel cycle is very important activity of IAEA Safeguards Analytical Laboratory. On the other hand, the gravimetric determination of U, which is the most accurate method for bulk quantities of U oxides analysis, requires correction for impurities. Due to its many advantages, XR analysis was a method of choice. We were aware of several limitations in using this method for impurities analysis in U materials. They are as follows: () low atomic number elements, e.g., Li, Be, B cannot be analyzed, (2) uranium matrix has a high absorption coefficient for all analyte lines, (3) U X-ray spectrum is rather rich consisting many L and M lines, of first, second and third order of diffraction, (4) scattered X-ray tube lines increase the possibility of spectral interferences, (5) some other instrumental lines originating in X-ray tube, sample holder and collimators cannot be avoided. The literature on the direct determination of trace elements in uranium oxides is very scarce. The Ca, Cr, Cu, e, Mn and Ni were determined in UOz using the standards prepared by adding known amounts of these elements to U308 followed by grinding with boric acid and pressing into pellets (). In pellet form the standards were prepared for determination of Ca, Y, Gd and Th (2). It was shown that the standards prepared by adding the impurities in solution to U308 were more homogenous. Copyright (C) JCPDS-International Centre for Diffraction Data 997 ISSN , Advances in X-ray Analysis, Volume 40 Copyright 0 JCPDS-International Centre for Diffraction Data 997

2 This document was presented at the Denver X-ray Conference (DXC) on Applications of X-ray Analysis. Sponsored by the International Centre for Diffraction Data (ICDD). This document is provided by ICDD in cooperation with the authors and presenters of the DXC for the express purpose of educating the scientific community. All copyrights for the document are retained by ICDD. Usage is restricted for the purposes of education and scientific research. DXC Website ICDD Website - ISSN , Advances in X-ray Analysis, Volume 40

3 ANALYZED MATERIALS AND SAMPLE PREPARATION Two sets of Standard Reference Materials (SRM) for impurities in UsOs prepared and distributed by the New Brunswick Laboratory (NBL), USA and 2 SRM, named Chanterelle and Morille, distributed by CETAMA, rance, were used in this study. Each of the 2 NBL sets consists of 6 standards and matrix sample. The standards were prepared by addition of single impurity solutions followed by drying, blending and milling. The NBL 23 set contains 8 elements and the NBL 24 contains 24 elements. It was rather obvious at the beginning of this study that the element concentrations below 5 to 0 mg/g will not be measurable using XR, hence only the first 4 SRMs of each series were used for measurements. The concentration ranges for elements measured and not measurable are shown in Table. One gram of SRM was transferred to a pel- Table. Impurity elements in UaOs reference let die of 32 mm diameter into which a brass materials of 23 and 24 series. cylinder of 28 mm internal diameter was inserted. The powder was spread out uniformly Element Approximate and was pressed by hand with a brass plunger I cont. range, ppm closely fitted to the brass cylinder. Both these Possible to be analyzed tools were then carefully withdrawn from the Na die and the remaining pellet-shaped material Al,Ca, e, Ni, Si, W, Zn, Zr was covered with 4 g of finely powdered boric Cr, Mg, MO 00-0 acid. The die disk in contact with U308 was made of tungsten carbide. The pellet was fabri- Bi, Cu, Pb, Mn, Ti, V 50-5 cated under vacuum in a hydraulic press with force of 25 tons. The sample layer in the pellet was of the infinite thickness for all element lines measured. Three pellets were prepared from each SRM and 2 from each Us08 matrix. Before weighing, the CETAMA SRMs which were in the form of mm diameter granules, were ground in plastic containers for 0 minutes in a mixer mill (Chemplex) containing 2 grinding balls. One pellet was also made from boric acid to check for a possible contamination during the pellet preparation procedure and to confirm the presence of instrumental lines in the spectrum. The pellets produced in this way are very durable and did not show significant changes in X-ray line intensities during the period of 2 years, except of SiKa which significantly increased its intensity due probably to contamination with the air dust. INSTRUMENTATION AND MEASUREMENT PARAMETERS The PW 480 Philips sequential spectrometer with rhodium X-ray tube has been used. Very extensive studies using LI 200, LI 220, PET and PXl (multilayer) crystals in the vicinity of analyte lines shown in Table 2 have had been performed. The purpose of these studies was twofold: () to select the best values for voltage and current from the point of view of the excitation efficiency and the peak to background ratio, (2) to identify the spectral interferences and to select the optimal angles for background measurements. It was found that UL lines are most efficiently excited with 65 kv and 40 ma (without exceeding the maximum available generator power of 3 kw). At these voltage and current settings the element lines from Table 2 do not show the best peak to background ratio because the background continuum is relatively high. To improve this ratio the voltage had to be decreased to 40 kv and the current increased to ma. With dominant U matrix even the very low inherent intensity lines of this element are comparable or exceed analyte lines and additionally, second and third diffraction order U lines are still relatively intense. The Rh-tube lines also interfere to some extent. Taking this into account the background angles shown in Table 2 are the best compromise be- tween the minimum of spectral interference and maximum approximation to the background continuum. Copyright (C) JCPDS-International Centre for Diffraction Data 997 ISSN , Advances in X-ray Analysis, Volume 40 Copyright 0 JCPDS-International Centre for Diffraction Data 997

4 Table 2. PW 480 measurements parameters. Ele- Line kv ma Colli- Diff. Angle Bg+ Bg- Det. Discr. level nent mator ord. 20 low up Al C L Bi Ca co Cr cu e Mg Mn MO Na Ni Pb Si Ti V W Zn Zr La 40 La 40 La 40 La 40 La 40 C C C C C ~ ~ S L S L S S L L L L S S L L L S S L Table 3. Spectral interferences. Analyte 20 line CaKo TiKa 86.4 VKa CrKa MnKa WLa ZnKa 4.80 PbLa BiLa MoLa 76.4 MgKa NaKa UMLN ULa, III TiKP VKP 69.3 CrKP ULP, II 4.9 ULP, II UL& II 4.9 ULYll II ULYl SKa RhLl RhLn 22.7 U Mz, II U Ma, III U Ma2 III The spectral interferences are shown in Table 3. Some of them, e.g., ZnKa, MgKa and BiLa are rather strong and impair seriously the detection limits and the precision of measurements. The interference of VKa, CrKa and MnKa with TiK& VKP and CrKP respectively, was studied in detail. The intensities of KB lines of V, Cr and Mn are between 4 to 8% of the respective Ka lines. The angle differences between background (Table 2) and respective peak are from 0. to 0.6 of 20, so the contribution of interfering KP lines to the background measurement in the concentration range considered is rather negligible. We carried out the measurements for these three elements also with the Li 220 crystal which confirmed our estimates. We could not find acceptable conditions for the measurement of Sn either for SnKa and SnLa lines at the concentration level of about 50 ppm. In both cases the Sn lines cannot be distinguished from background and the dominating UM lines, respectively. MO can only be analyzed using its L lines, the K lines overlap with ULp-+ ZrKa lines are very close to ULI line. Also the measurements of AgL, CdL and KK lines are impossible due to strong overlapping with UM lines. Copyright (C) JCPDS-International Centre for Diffraction Data 997 ISSN , Advances in X-ray Analysis, Volume 40 Copyright 0 JCPDS-International Centre for Diffraction Data 997

5 The background on both sides of the peak for 3 pellets of the same material fluctuated more than had been expected from the counting statistics. Therefore, it was decided to measure it on both sides of the peak. Probably it is caused by small changes of the surface of UsOs layer. These background changes resulted, in some cases, in corresponding changes of the peak height, but it was not always like that. rom repeated measurements of the same pellets at different times, it can be concluded that some changes of peak height had rather permanent character which can indicate inhomogeneous distribution of some elements in some reference materials. ig. shows the background for 9 elements measured. The crystals used and the 20 angle ranges are at the top of this igure. The light shaded bars represent the mean interpolated background under a peak, in kcps, measured on both sides of each analyte peak at the angles indicated in Table 2. The dark shaded bars are for the mean background in a peak position of each analyte measured on the 4 pellets of U308 matrix, two of them for series 23 and 24, respectively. or most of the elements the 2 values of background intensity differ. The higher background measured from matrix indicates the U line interference (Bi, Ti, Zn, Mg) or the presence of instrumental line (Cu, Ni, e, Mn, Cr, Si, Al). The higher value of interpolated background (Pb, W, Na) shows that the background does not change smoothly in the peak vicinity. The time of measurement was 50 s for the analyte lines and the background at both sides of each line. PEr 76-45O 0.6 ig.. Background for analyzed elements RESULTS The XR spectrometer was separately calibrated using the 23 and 24 series standards for 3 and 9 elements, respectively. The slope and intercept for a linear regression were calculated with a built-in regression software (Lucas-Payne algorithm with one variable). Then, the 23 set was measured as unknowns against the 24 calibration and in similar way the 24 set was measured against the 23 calibration. Additionally, Morille and Chanterelle were measured with both calibrations. The concentration results obtained for the series 23 and 24 are plotted in igures 2a, 2b and 2c for 3 elements. The horizontal bars represent the certified uncertainty limits at 95% confidence level. Copyright (C) JCPDS-International Centre for Diffraction Data 997 ISSN , Advances in X-ray Analysis, Volume 40 Copyright 0 JCPDS-International Centre for Diffraction Data 997

6 Cr./4 ig.2a. Relationship between concentration of impurities certified and found for SRM 23 and 24 series against each other; abscissa - concentration certified, ppm in U metal; ordinate - concentration found, ppm in U metal; certified uncertainity limits for 23 series; horizontal bar - certified uncertainity limit for 24 series; vertical bar - 26 calculated from counting statistics for 24 series; for 23 series these values are very similar Copyright (C) JCPDS-International Centre for Diffraction Data 997 ISSN , Advances in X-ray Analysis, Volume 40 Copyright 0 JCPDS-International Centre for Diffraction Data 997

7 zn n Si l-l e3 -/ $ /: zoo ca J 0 loo : ig.2b - the same as under ig.2a Copyright (C) JCPDS-International Centre for Diffraction Data 997 ISSN , Advances in X-ray Analysis, Volume 40 Copyright 0 JCPDS-International Centre for Diffraction Data 997

8 ig.2c - the same as under ig.2a The certified concentration, in ppm, is on the abscissa axis and the concentration determined against the respective calibration is on the ordinate axis. The vertical bars show 2 standard deviations, sd, resulting from counting statistics, only for 24 series measurements, expressed in ppm, calculated from the following formula: where:.& = & (B ;,): or sd = ; (IB &I$ () B - average number of background counts on both sides of a peak P - number of counts in a peak with background s - sensitivity-slope of regression line in cps/ppm n - number of independent measurements IB - average background intensity, cps IP - peak + background intensity, cps T - measurement time, the same for peak and background The detection limits (DL) are also shown in igures 2a-c and they were calculated from equation given below: where: DL = (2) BM - number of counts at analyte peak position for pure Us08 matrix IM - intensity at analyte peak position for matrix, cps ST - as in equation () The sensitivities (S), detection limits (DL), coefficients of variations of the linear regression (CV) for a mean concentration for both calibration sets are given in Table 4. Table 5 shows the results of 8 impurity element analysis in the Morille and Chanterelle SRMs against 24 and 23 series calibrations. The certified concentrations and the corresponding 95% confidence limits are also given. The lo is a standard deviation calculated from triplicate measurements. In Table 6 there are the concentration sums of certified analyzed and found element impurities (in ppm and in percent), sums and lists of impurities not analyzed and the total certified concentrations of all impurities. Copyright (C) JCPDS-International Centre for Diffraction Data 997 ISSN , Advances in X-ray Analysis, Volume 40 Copyright 0 JCPDS-International Centre for Diffraction Data 997

9 Table 4. Sensitivities, detection limits and coefficients of variation (CV) Ele- ment AI Bi Ca co Cr cu e Mg MU MO Na Ni Pb Si Ti V W Zn Zr r Series 24 S DL I Series 23 S DL x % MC S Sensitivity, cpslppm DL Detection limit, ppm MC Mean concentration, ppm DISCUSSION O RESULTS The sensitivity values, S, for series 23 and 24, shown in Table 4 differ significantly for most elements. Only for Cr, MO, Ni, V and Zn are these differences within about 0%. As a consequence of these differences the concentrations found, as presented in ig. 2a-c diverge visibly from the diagonal for both series of SRMs. Two divergent straight lines can be drawn through the points representing the concentrations found for each series. Particularly big differences are for highest concentrations and they are much higher than counting errors. The coefficients of variation, CV calculated from linear regression are also relatively high for many elements which can indicate both the inconsistencies in the concentrations certified and the inhomogeneities in the impurities distributions within materials. The difference in sensitivity also influence proportionally the values of the detection limits (DL), in some cases by about 50% (Ca, Mg, Zr). The values of DL are practically between 0 and 20 ppm for all elements considered except Na where this value is about 30 ppm. The results of impurities concentration in the Morille and Chanterelle SRMs against 24 and 23 SRh4s calibration in Table 5 are too low for a majority of elements. The standard deviations, lo, of 3 measurements are relatively high in a number of cases but this fact does not explain the differences observed. We also studied very carefully qualitative spectra of these two SRMs and we did not find significant differences in comparison to the spectra of 23 and 24 SRMs. The only difference was a 2 to 3 % higher background around the majority of elemental peaks which we attribute to a finer grain size distribution of these SRMs. Copyright (C) JCPDS-International Centre for Diffraction Data 997 ISSN , Advances in X-ray Analysis, Volume 40 Copyright 0 JCPDS-International Centre for Diffraction Data 997

10 Table 5.Results of impurity analysis in Morille and Chanterelle materials (in pg/g of U metal) Ele- zertitied 95% Morilie RM, ppm Certified 95% Chanterelle RM, ppm nent cont. conf. 24 calibration 23 calibration cont. conf. - limit cont. IS cont. s 24 calibration 23 calibration limit cont. s cont. Al 99 Bi 24.4 Ca 93 co 9.8 Cr 99 CU 50.2 e 2.6 Mg 9.3 Mn 24.5 MO 47 Ni 47 Pb 0 Si 00 Ti 49.2 V 48.7 W 00 Zn 98.6 Zr n.d. not determinable n.d. n.d large differencies between three measurements * only one result n.d n.d : n.d n.d n.d n.d n.d. 5.3 IS 2.7. _ n.d C 5.5 Table 6. Concentration of analyzed and non-analyzed elements certified and found, in ppm Ref. Material Certified Chanterelle Morille ound ound Impurity not analyzed Total cone: certified Cd Sn) (Ag, B, Be, Bi Cd, Co, Cu, Pb, Sn, Ti, W) (Ag, B, Be, Cd, Ga, Sn) (Ag, B, Ba, Be, Cd, RE, Li. Sn. Th. Zr /n.d.)) Copyright (C) JCPDS-International Centre for Diffraction Data 997 ISSN , Advances in X-ray Analysis, Volume 40 Copyright 0 JCPDS-International Centre for Diffraction Data 997

11 It was interesting to compare the total concentration of impurities determined and certified. It can be seen in Table 6 that the concentrations found for series 23 and 24 SRMs give quite reasonable values in the range from 82 to 3%. This is satisfactory for impurity correction in gravimetric determination of uranium. Only 75% of impurities found for the Morille and Chanterelle confirms the incompatibility of these two series of SRMs to other SRMs. SUMMARY The XR is a relatively simple analytical method for determination of many element impurities in U308 at the concentration level above 20 to 30 ppm. The XR can be a good complementary analytical technique for other methods of analysis which are not very reliable for higher impurities concentrations, particularly above 50 ppm level for an element. The availability of more SRMs, particularly at higher concentration levels is desirable. REERENCES ) V.L. Ribeiro Salvador, K. Imakuma, Anal. Chimi. Acta 88 (986) 67 2) R.M. Agrawal, S.K. Kapoor, X-Ray Spectrom. 6 (987) 8 I Copyright (C) JCPDS-International Centre for Diffraction Data 997 ISSN , Advances in X-ray Analysis, Volume 40 Copyright 0 JCPDS-International Centre for Diffraction Data 997