Comparison of Instrumental Methods Used in the Determination of Uranium and Plutonium

Carmen S. Sabau, Delbert L. Bowers, and Florence P. Smith

Analytical Chemistry Laboratory, Chemical Technology Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439-4837

INTRODUCTION

The determination of plutonium and uranium in nuclear materials is important not only in the fuel recovery process, but also in the waste generated by reprocessing. The recovery of plutonium and uranium from all types of scraps and residues, as well as the removal of plutonium and other transuranic elements (TRU) from radioactive wastes, are very pressing problems from an economical and ecological point of view. These processes must be highly efficient so that the elements that are reused in fuel production are essentially pure, and the other process effluents, can be classified as non-TRU waste .

At Argonne National Laboratory, liquid metals and molten salts are being used as reagents and solvents to effect the necessary separations of actinides from nuclear waste. These separations are possible because of differing stabilities of the compounds in the molten salt relative to the liquid metal. To assure a high efficiency of each separation process, it is important to have reliable methods for analysis of the feed material. Depending upon the nature of each sample, different pi-ocedures have to be applied. Usually, the concentration of the element of interest and the composition of the samples to be analyzed vary. The precision desired and the time requirements also vary. Therefore, the analytical methods must be adaptable to the specific needs.

In our laboratory, process samples of salts and metals are first dissolved, and then the elements of interest (such as Al, By Bay Cay Cd, Ce, Cu, Fey K, Li, Mg, Mo, Na, Nd, Pu, Si, Sm, Sr, Ta, Ti, U, Y, Zn, and 2) are determined by inductively coupled plasma/atomic emission spectrometry (ICP/AES). For quick determinations of a-emitters, 2n: a-counting is helpful in making decisions on sample size or dilution. Where high precision is required for the uranium and plutonium determinations, mass spectrometric isotope dilution (MSD) is

In most cases, these analyses required determination of low levels of an element in a short amount of time with maximum precision and accuracy. This requires a very sensitive and rapid method, but in reality one needs to apply more than one method to the same sample to meet these criteria.

' employed.

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PREPARATION OF SAMPLE

The samples to be analyzed were received in a variety of forms, Le., metal tubes, powders, grains, dendrites, large and small chunks, or a combination of the above. The sample matrix usually consisted of a process salt or metal.

To use 2n: a-counting, ICP/AES, or MSID, the samples must first be dissolved. The samples containing plutonium were dissolved by using strong mineral acids and heat in gloveboxes. For dissolution of salt samples, water and 12 M HCl were primarily used while metal samples were dissolved in water and concentrated HNO,. The other waste samples were usually dissolved with a mixture of acids (HNO, + HCl) or concentrated HNO,. In a few cases, when the matrix was primarily aluminum, concentrated HC1 was preferred. When zirconium was known to be present, HF was added before the HNO,. Once the dissolution was accomplished, the starting solutions were either weighed or measured for volume and then an aliquot was taken and removed from the glovebox for subsequent separations and analyses. Next, depending upon their elemental content and the analysis method chosen, the samples were either simply diluted or treated by separation procedures, such as solvent extraction or ion exchange.

SAMPLE ANALYSIS

Alpha-counting

For a quick evaluation of the amount of plutonium present in a particular sample, small aliquots (usually 10 pL) of a diluted solution were evaporated on a tantalum planchette and a-counted. To withm an uncertainty of *lo%, one can obtain valuable information about the concentration level of plutonium such that rapid decisions can be made as to sample treatment in subsequent analysis.

The a-counting equipment was a 2n ionization chamber with associated electronics and used P10 counting gas (10% CH, + 90% Ar). The instrument a-activity background was 5-6 cpm, and the efficiency of the instrument was slightly over 50%.

Inductively Coupled PlasmdAtomic Emission Spectrometry

Most of the ICP/AES analyses were carried out with diluted aliquots of the initial dissolutions. However, in some cases when an element such as Cd, U, or Li was in

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excess and could spectrochemically interfere with the analysis, a prior separation was necessary. These separations were done with ion exchange columns (Bio-Rad AGlX8, 100 - 200 mesh, 0.8 cm dia-4 cm length). By varying the concentration and nature of the initial solutions and eluants, one can remove the interfering matrix element quite efficiently.

The ICP/AES instrument used in the analysis is a 3520 Quantometer W O E S (Applied Research Laboratory, Bausch and Lomb, now Fisons) scanning spectrometer, which is housed in a glovebox facility. This arrangement allows us to analyze radioactive materials containing high levels of a-emitting isotopes and moderate levels (<lo0 mR/h) of p- and y-emitting radionuclides.

Mass Spectrometric Isotope Dilution

The MSID method generally requires that all unwanted elements be separated from the analytes of interest, namely, PU and U. The sample must be free of interfering nuclides and chemical impurities, because they might introduce variations in the mass spectrometer background or in the fractionation behavior of the element of interest. Based upon the results from a-counting, appropriate aliquots are taken for the mass spectrochemical determinations. Generally, we did not exceed 10-12 pg of plutonium for reasons of procedural requirements regarding radiation safety and mass spectrometry. Because of potential interferences, uranium did not exceed 100 times the amount of plutonium. The sample medium is generally 4 M HNO,. The sample is first spiked with known amounts of 2”Apu and 233U and then taken to dryness in the presence of HF (to destroy the plutonium polymers) and HClO, (to oxidize all the plutonium to the VI oxidation state). The residue is dissolved in 4 &j HNO, and then extracted with 2.8 M Al(NO,), and hexone. The organic phase is then stripped with 0.05 M HNO,; this step separates the Pu and U from all the other elements. Lately, this actinide separation has been done with ion exchange columns, a sample medium of 8 @ HNO,, and an eluant of 0.1 &l HC1.

A second separation is sometimes necessary if the concentration ratio for the two elements of interest is too high (e.g., uranium more than 200 times the amount of plutonium). This separation is carried out using a 0.4 cm ion exchange column (Bio- Rad AGlX8,100-200 mesh). The sample medium is 9 &j HBr (48%), which reduces plutonium to the III oxidation state, and allows plutonium to pass through the resin, whereas uranium is adsorbed. The uranium can be removed from the column with 0.1 @ HC1. The final step is the conversion of the samples to a nitrate medium. Following these procedures, the samples are ready for M S D analysis.

The instrument used in the analysis is a thermal ionization mass spectrometer (TIMS) (V.G. Isotopes, Ltd., Model Isomass 54R). The mass spectrometer features a 90”-sector magnetic mass analyzer and a deep Faraday cup detection system. Two

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rhenium filaments are required per plutonium sample (25-50 ng) and two tantalum filaments per uranium sample (1-2 pg). The ionizing filament is also rhenium. The sample solution is deposited on one side of the filament and is then evaporated to dryness and oxidized by passing a current through the filament. The loaded sample filament and the blank sample filament are incorporated into a triple filament ionization assembly for subsequent insertion into the TIMS source. Up to 16 ionization assemblies can be inserted into the source chamber of the mass spectrometer.

COMPARISON OF RESULTS

Some characteristics of the three instrumental methods used in our laboratory to analyze multielement samples are given in Table 1. Table 2 presents the plutonium results obtained using MSID and a-counting for metal and salt samples. Table 3 presents the uranium obtained using MSID and ICP/AES for different kinds of samples. Table 4 shows the plutonium data for all three methods: ICP/AES, MSID, and a-counting.

Table 1 .: Comparison of Methods Tested for Multielement Analyses

Parameter MSID a-Counting ICP

Accuracy * 0.5%

Instrument Analysis 1.5 h/sample Time

Preliminary Chemistry Dissolution Dilution Oxidation Extraction or lon-exchange Evaporation

f 1-5%

1-1 00 min/sample (depending on the activity of the sample)

Dissolution Dilution Ion Exchange

* 5-10% 2-5 minkample (depending on the number of elements to be determined)

Dissolution Dilution Sometimes, Ion Exchange

Number of Analyses Lowest per Unit Time

Low Highest

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CONCLUSION

For fast, reliable methods to determine uranium and plutonium concentrations in samples, ICP/AES and alpha counting can be used. The data in Tables 2-4 clearly show that ICP/AES or a-counting can be used to obtain high- quality results for the determination of plutonium. When high accuracy is needed for mass balance, however, MSID is the better choice. Also, ICP/AES is a poor method to use for plutonium determination, when samples contain large amounts of spectrally interfering elements, such as uranium, rare earths, and zirconium. In these cases, if ICP/AES is used, chemical separation of plutonium is needed prior to analysis, or the a-counting method can be used. Because of the poor sensitivity for uranium in the ICP/AES technique, use of MSID is the only practical method to analyze samples that are low in uranium.

ACKNOWLEDGMENTS

We gratefully acknowledge Everett G. Rauh who performed the MS determinations at the beginning of this project, and John J. Heiberger, Zygmunt Tomczuk, Raymond D. Wolson, Gerald K. Johnson, and Michele A. Lewis for providing us with samples.

Table 2. Pu-data: Comparison of MSID and a-Counting

Sample ID Sample Weight, g MSID a-Counting

Pu, % Pu, %

PuI-139 S 10.0726 0.271 0.299

Pul-150 M 1.3317 0.199 0.210

Pul-152 S 6.4435 0.079 0.072

PuI-170 M 3.5907 0.622 0.645

hl-177 M 4.1810 0.548 0.540

Pul-190 S 3.5654 2.120 2.230

Pul-509 S 5.3908 0.013 I 0.014

DISCLAIMER

This report was prepared as an account of work sponsored by an agency of the United States , Government. Neither the United States Government nor any agency thereof, nor any of their

employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

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Table 3. U-data: Comparison of MSID and ICP in different samples

Sample ID

Pul-212

Pul-321A

Pul-331

Sample Weight, g MSID ICP

u, % u, %

0.3390 3.370 3.280

0.4381 1.643 1.690

1.1115 0.575 0.520 ~ ~ ~

0.2442 7 0.762 - 1 0.820

Pul-364

Pul-386

Pub398 M

5.4054 0.097 0.100

5.6641 0.078 0.086

2.7844 0.177 0.160

.

4.0732 I 3.005 I 3.300 Pul-399 M

Pul-401 M

I I

4.0675 I 3.004 I 3.300

Pul-407 M I 15.8126 I 3.189 I 3.480

Pu, %

8.8

18.8

Table 4. Pu-data: Comparison of MSID, ICP, and a-Counting

Pu, % Pu, %

8.317 9.0

18.113 19.7

~~ 11 Sample ID I Sample Weight, g

Pul-494

Pub507

Pul-510

Pul-511

Pul-513

Pul-491 6.8927

Pul-493 0.6465

6.0917

3.5639

1.2845

2.2430

4.2560

13.85

0.71

1 Pul-514 1 , MM;: Pul-517 S

Pul-518 S 1.9325

12.249 13.3

0.858 0.856

ICP

0.37

0.54

I

0.357 0.38

0.609 0.636

MSID I a-Counting

1.03 I 0.933 I 0.97

~ 2.8: I 2.82 1 3.021

0.093 0.105 0.102

0.124 0.127