Chemical separation and spectrographic estimation of rare earths and yttrium in PuO2 and (U,Pu)O2

143

SHORT COMMUNICATIONS Talanta, Vol 24. pp 143-145

Pergamon

Press, 1977 Pnnted m Great Bntam.

CHEMICAL SEPARATION AND SPECTROGRAPHIC ESTIMATION RARE EARTHS AND YTTRIUM IN PuO, AND (U,Pu)02 A. G. I. Radiochemistry

Division,

DALVI,

Bhabha

C.

Atomic

(Received 30 June

S.

DFODHAR and B. D. Research

Centre,

OF

JOSHI

Trombay,

Bombay

400085, India

1976. Accepted 26 September 1976)

Summary-An emission spectrographic method for the estimation of rare-earth impurities in plutonium oxide and the mixed oxide (U,Pu)O, (30% PuO,-70% U02) has been developed. Rare-earth impurities are separated from the matrix by solvent extraction with tri-n-octylamine in xylene and are estimated after concentration. Thulium is used as internal standard and LiF-AgCl as carrier. The spectra are excited in a d.c. arc. The detection limits lie in the range OXMWO.1 pg and the coefficient of variation ranges from 5 to 23:;.

Plutonium oxide and the mixed oxide of uramum and plutonium have become important in recent years as nuclear fuel for Fast Breeder Test Reactors (FBTR). The concentrations of the metallic impurities in the fuel must be below the maximum permissible levels specified to obtain the required density for the pellets and also to reduce the loss of neutrons because of absorption by impurities having high neutron cross-sections. The estimation of metallic impurities in the fuel thus becomes an essential part of the chemical quality control programme. Estimation of common impurities in these materials is carried out routmely by carrier distillation techniques, first developed by Scribner and Mullin for uranium samples. The rare earths, however, are estimated after their chemical separation from the matrix.‘-’ Josh1 et al.’ and Faris used ion-exchange techniques for separation of rare earths from plutonium, whereas solvent extraction methods were used by Vantuy14 and Dhumwad et al.’ Ko used solvent extraction techniques for separation of the rare earths in the mixed oxide and estimated them by the copper spark method.6.7 This paper describes a d.c. arc spectrographic method for estimation of rare earths in plutonium oxide and the mixed oxide after separation of rare earths by extraction of uranium and plutonium into tri-n-octylamine (TOA) in xylene. Thulium is used as internal standard and 1s assumed to be absent from the samples. All the elements except Tb could be determined at less than 1 ppm, the lower limit for Tb being 1 ppm.

EXPERIMENTAL

Reagents All the chemicals used for the preparation of individual stock solutions were of spectroscopically pure grade (Johnson Matthey). Analytical grade concentrated nitric acid and hydrochloric acid and water were distilled twice in a quartz distillation unit before use. Tri-n-octylamine (K & K Laboratories) was used without further purification. The xylene was guaranteed reagent grade [Sarabhai (M) Chemicals, India]. “Ultrapurity” graphite powder U.C.P.-2 (200 mesh) supplied by Ultra Carbon Corporation, was used for preparation of the carrier. Preparatron

of stundards

Individual stock solutions for rare-earth elements and the internal standard thulium solution, with a concentratlon of 5 mg/ml, were prepared by dissolving the oxides (ignited at 800’ for 1 hr). in 3M hydrochloric acid, and used to make a master solution with composition Y, Yb,

0.1 mg/ml; Eu, Gd, Dy, Er, Ho, Lu, La, 0.25 mg/ml; Ce, Nd. Sm. Tb 1 mg/ml. A set of seven standards in 6M hydrochloric acid was prepared by taking different volumes of the master solution to give the concentration range 0.2-200 pg/ml for different elements. The concentration of Tm in all standard solutions was 25 pg/ml. Preparatwn of carrier. A carrier mixture of LiF, AgCl and graphite powder in the ratio 1:1:2 was prepared by grinding weighed amounts in an agate mortar. Chemical separation procedure A solvent extraction technique (with 20% TOA in xylene) was used for the separatibn of Pu02 0; the mixed oxide W.Pu)O, (3O”‘XPuO,-709/, UO,) from rare earths. The mixed oxidk br $utoni;m oiide, free from rare earths, was used for the standardization of the chemical procedure. The mixed oxide or the Pu02 pellet was dissolved by heating in concentrated nitric acid that was 0.05M in hydrofluoric acid. After complete dissolution of the sample the solution was evaporated almost to dryness and taken up in 6M hydrochloric acid; this was repeated several times to remove traces of nitric and hydrofluoric acids. Known amounts of impurities and 2.5 pg of thulium were added to a solution containing 1 g of PuO, or (U,Pu)O, m 6M hydrochloric acid. This solution was evaporated to dryness and the residue dissolved in 10 ml of 6M hydrochloric acid. The plutonium was made quadrivalent by addition of a few drops of hydrogen peroxide (30%) and the excess of peroxide was destroyed by heating under an infrared lamp. The resulting solution was stirred vigorously (magnetic stirrer) with 10 ml of 20% TOA in xylene, pre-equilibrated with 6M hydrochloric acid. The Pu and U were extracted. Five such extractions gave complete removal of U and Pu. The aqueous phase was then rinsed twice with 10 ml of xylene. Finally the aqueous phase containing the rare earths was evaporated nearly to dryness, 2 ml of a 1:l mixture of concentrated nitric acid and perchloric acid were added and the solution was evaporated to dryness. The residue was heated at 400” for 1 hr to destroy any organic matter and was then used for spectrographic analysis. The Pu and U were stripped by vigorously stirring the organic phase with an equal volume of SM perchloric acid. Two back-extractions gave complete removal of Pu and U from the organic phase. The plutonium and uranium were then precipitated as the hydroxides which were then dissolved in nitnc acid for the separation of U and Pu by anion-exchange.

144

SHORT

Table Spectrograph Grating Reciprocal linear dispersion Wavelength region Excitation unit Arc current (d.c.) Exposure time Electrodes Anode Cathode Arc chamber atmosphere Analytical gap Slit width Slit height Photographic material Developer and development Fixer Plate cahbratron Microphotometer Calculator

Tracer

Recovery,

‘We

results

1. Equipment

for tracers

%

Average recovery, ‘4

99.8 100.0

99.9

1%-154Pu

99.5 97.0

98.2

r7’Tm

99.8 99.8

99.8

97.3 96.4

96.8

9’Y

and operating

The residue containing the rare earths, after chemical separation of Pu and U, was dissolved in 200 ~1 of 6M hydrochloric acid. An aliquot of 20 ~1 of each standard

Table

Ce DY Er Eu Gd Ho

La Lu Nd Sm Tb Y Yb

and 40 ~1 of sample were loaded in duplicate on 10 mg of carrier mixture in the electrode crater. The electrodes were dried under an infrared lamp. The samples and standards were arced in a glass chamber inside a glove box. The spectra of the standards and the samples were photographed on the same plate in duplicate under the conditions given m Table 1 RESULTS

Sample preparation and spectrographic conditions

Element

conditions

Hilger 3.4-m Ebert plane grating spectrograph 30,000 lines/in. blazed at 1 pm in first order 0.8 A/mm in third order, 1.25 A/mm in second order 3570-4130 A in second order, 315&3500 A in third order Hilger source unit FS 131 13 A 45 set-third order, 35 set-second order Graphite rods (Ultra Carbon Corporatron, U.S.A.) “Ultrapurity” graphite ST-45, 4.76 mm dia. 2 cm long, 2.5 mm crater depth and 3.17 mm crater diameter Ultrapurity graphite ST-40, 3.17 mm dia. and 2.5 cm long. pointed 80% argon + 20% oxygen at a flow-rate of 20 l./min for second-order setting 4 mm (not constant during arcing) 15 pm (third order) and 20 pm (second order) 2.5 mm Two 10.2 x 25.4 cm Kodak plates (SA-1) Kodak D-19 at 18” for 3 mm Kodak acid fixing salt solution for 3 mm Seven-step rotating sector with iron arc exposure of 20 sec. with current 4A Hilger non-recording microphotometer Respectra

time

Table 2. Recovery

COMMUNICATIONS

3. Precision

AND

DISCUSSION

The solvent extraction technique using 2096 TOA in xylene removed all traces of the matrix elements, uranium and plutomum, as required for the spectrographic estrmation of impurities in PuO, and (U,Pu)O, samples. This was established by the absence of alpha-activity in the aqueous phase after the five extractions. The recovery of the rare-earth impurities was checked by using radioactive traces. Known amounts of the tracers, 1s2-154E~. “‘Tm. 141Ce and ‘rY were added to the solution containmg I g of sample, then separated by the method discussed above and estimated, 152-154E~, “‘Tm

and accuracy

Analytical line. A

Amount added,

Amount recovered,

Icg

Pg

Recovery, %

3942.7 4015.9 3264.78 4007.9 3971.99 3422.5 3768.9 3456.0 3891.02 4015.4 3337.5 3359.8 4012.4 3634.27 3324.4 3242.28 3289.37

0.4 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.4 0.4 0.4 0.04 0.04

0.36 0.096 0.106 0.095 0.098 0.107 0.099 0.108 0.095 0.094 0.121 0.106 0.406 0.39 0.42 0.049 0.041

91.6 96.5 105.6 95.0 95.4 107.0 99.5 108.6 94.9 94.2 121.1 105.8 101.5 99.4 105.7 123.3 102.5

Coeffictent of variation, 0 0 7 8 10 9 3 11 9 7 7 5 9 10 6 6 6 23 16

145

SHORT COMMUNlCATIONS

Table 4. Analytical line pairs and estimation ranges Analytical line, A

Element Cerium Dysprosium Erbium

3942.1 4015.9 3264.78 4007.9 3971.99 3422.5 3768.9 3456.0 3891.02 4015.4 ~3359.8 3337.5 4012.25 3634.27 3324.4 3242.28 3289.37

Europium Gadolinium Holmium Lutetium Lanthanum Neodymmm Samarium Terbium Yttrium Ytterbium

Table 5. Comparison

of detection oxide

Internal standard line, A 3949.28 3949.28 3425.6 3949.28 3949.28 3425.6 3734.12 3425.6 3949.28 3949.28 3235.5 3235.5 3949.28 3734.12 3235.5 3235.5 3235.5

limits for the mixed

Estimation limit, ppm Rare earth Ce

Nd Sm Eu Gd Tb DY Ho Er Tm Yb Lu Le Y

Literature method6*

Present method?

50 50

0.5 0.5 0.2 0.05 0.05 1.0 0.125 0.125 0.125 hit. std. 0.02 0.05

5 5 25 2.5 5 5 2.5 5 3 Int. std.

0.05 0.02

* No details given of amount of sample used for separation and amount of separated impurities arced. t Values based on arcing : of the impurities separated from 1 g of sample. and 14iCe by gamma-counting with an NaI(T1) detector, and “Y by the liquid scintillation technique. The results obtained are shown in Table 2. The recovery was also obtained by the spectrographic method after chemical separation from the matrix. These results are shown in Table 3. It can be seen from the results in Tables 2 and 3 that the recoveries are quantitative within experimental error.Analytical lines and internal standard lines used are given in Table 4. Working curves were drawn by plotting the ratio of the intensities of the sample and standard lines against the logarithm of the sample concentration and were found to be linear.

Estimation range, ppm

Estimation limits, H

0.5-20.0 0.125-2.5 0.125-5.0 0.125-1.25 0.05-1.25 0.05-5.0 0.125-2.5 0.125-5.0 0.125-2.5 0.125-2.5 0.05-5.0 0.05-5.0 0.5-5.0 0.2GlO.O l&20.0 0.02-2.0 0.02-2.0

0.1 0.025 0.025 0.025 0.01 0.01 0.025 0.025 0.025 0.025 0.01 0.01 0.1 0.04 0.2 0.004 0.004

The coefficient of variation for the estimation of rareearth impurities (12 replicate samples with duplicate determinations) is shown in Table 3 and is below 16% except for yttrium, for which it is 23%. The lowest concentration at which the working curve is linear is considered to be the lower estimation limit (Table 4). Table 5 gives a comparison of the estimation limits for the present method with others reported for mixed oxide samples. The limits obtained in the present work are all lower than those so far reported. The maximum permissible levels of rare earths specified for plutonium oxide and mixed oxide fuel used for FFTR reactars’ are given as 100 ppm for the sum of four elements, oiz. Sm. Eu. Cd and Dv. The low estimation limits obtained suggest that the sample size may be reduced below 1 g for the chemical quality-control analysis of rare earths in FBTR fuel. Acknowledgement-The

authors are grateful to Dr. M. V. Ramaniah, Head of the Radiochemistry Division, for his encouragement and keen interest in the progress of the work. REFERENCES

1. B. F. Scribner and H. R. Mullin, J. Res. Nat/. Bur. Stds., 1946, 31. 379. 2. B. D. Joshi. B. M. Pate1 and A. G. Page. Anal. Chim. Acta, 1971,‘57, 379. 3. J. P. Faris, U.S. At. Energy

Comm. Rept., TID-7655, 1962. 4. H. H. Vantuyl, Report, H.W. 28530, 1953; Nucl. Sci. A&r.. 1958, 12, 10386. 5. R. K. Dhumwad, M. V. Joshi and A. B. Patwardhan, Anal. Chim. Acta, 1970, 50, 237. 6. R. Ko, U.S. At. Energy Comm. Rept., WHAN-SA-44,

1970. 7. J. E. Rein, G. M. Matlack, G. R. Waterbury, R. T. Phelps and C. F. Metz, U.S. At. Energy Comm. Rept., LA-4622,

1971.