Uranium Determination'by the Isotope DiIu ti o n Technique PAUL GORIS, WAYNE
E.
DUFFY', and FRED H. TINGEY
Afomic Energy Division, Phillips Petroleum Co., Idaho Falls, Idaho
b Complexity and radioactivity of feed solutions a t the Idaho Chemical Processing Plant limit methods used for total uranium. Isotope dilution analysis was investigated as a means of greater reliability with minimum radiation hazard. Remote handling facilities enable purification of small samples to which surface ionization mass spectrometry is particularly adapted. The best ratio of sample to spike is approximately 1 for total uranium. This ratio should b e greater than 0.1 for more accurate uranium isotope distribution results. An error component study shows that the error for a single determination a t the 95% confidence limits is *3.7% for total uranium and *0.13% for the major isotope present. The method offers advantages of total uranium and uranium isotope distribution results from a single sample which need not b e quantitatively recovered following purification. Total uranium can be determined from as low as 0.001 to over 350 mg. of uranium per ml. Approximately 50 y of the spiked uranium are required for analysis.
T
function of the Idaho Chemical Processing Plant (ICPP) is to recover unburned uranium of high 235 content from spent reactor fuel elements. The process essentially consists of dissolution of the fuel elements in a suitable solvent, followed by separation of the uranium from the fission products and inert cladding material by liquid-liquid extraction. This process, even though complex and expensive, is more economical than ore processing to obtain enriched uranium. The solution resulting from dissolution of a spent fuel element represents a very complex system for the analysis of uranium, as it contains a high concentration of fission products as well as the cladding material and its impurities. I n addition, radioactive hazards due to fission products require handling by remote control methods. The major fission products are elements 36 (krypton) to 46 (palladium) and 52 (tellurium) to 62 (samarium). Included are HE
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yttrium, zirconium, niobium, molybdenum, technetium, ruthenium, rhodium, barium, and the rare earths. These elements, complex in nature and present in multioxidation states and cladding materials-such as aluminum, zirconium, and stainless steel-interfere with conventional analytical methods. The analysis for the dissolution sample must be highly reliable, because the uranium input figure for the plant is established by it. France1 (4)and Duffy and Tingey (1) initially showed that the isotope dilution method with a mass spectrometer was particularly adapted for this analysis. I n brief, the method consists of adding to the sample a known amount of a uranium isotope not originally present, homogeneous mixing, semiquantitative separation of the uranium from the fission products and other diverse ions, and measurement of the relative abundance of the uranium isotopes with a mass spectrometer. The intensity of the ion beam for each isotope as measured with a mass spectrometer is proportional to the isotope in the sample. The relationship used to determine concentration of total uranium by the isotope dilution technique is
\There C, = concentration of total uranium in the unknown solution C, = concentration of uranium in the added isotope (spike) solution V , = volume of unknown solution T', = volume of spike solution 4 = fractional isotopic purity factor for the spike isotope P = fraction of the spike isotope in the mixture of sample and spike
The constant, A , is evaluated from an isotope analysis for the particular spike material in use. P is determined from an isotope analysis for each spiked sample. The uranium isotopes ordinarily present in the plant feed are uranium-234, uranium-235, uranium-236, and ura-
nium-238. Uranium-233, available in quantity from the Oak Ridge National Lab., was chosen as the spike isotope. Isotope analysis of this material shows that small fractions of uranium-234, uranium-235 and uranium-238 are present; hence the need for A in Equation 1. These isotopic impurities present in the spike must also be corrected for in the isotope distribution analysis of uranium in the sample. Special corrections depending on a mass assay prior to addition of spike must be made from samples originally containing the spike isotope; however, such a condition does not apply for this study. SAMPLE PREPARATION
The complete remote handling procedure for separation of uranium from fission products and the cladding material is described by Shepherd and Rein (6). High recovery of uranium after purification is desirable though not always essential, as the homogeneous mixing of sample and spike takes place before the separation. Recovery has been found to be proportional for the isotopes present, and the ratio between sample and spike remains unaltered. Procedure. Pipet 0.500 mi. of a spike solution o$ known concentration into a container suitable for use with a stirrer. Add 1 ml. of a solution which is 1.5M in aluminum nitrate, 5% citric acid, and 1 N in nitric acid. Add the known sample volume (the total uranium due to the spike and sample should be approximately 2 mg.) Stir vigorously for 2 minutes. Add a volume of 3% tributylphosphate in nhexane such that the organic and aqueous volumes are nearly equal. Stir vigorously for 2 minutes. Separate the organic phase containing the uranium. To this phase add an equal voIume of 30% hydrogen peroxide and stir for 2 minutes. Centrifuge, decant, and then dissolve the precipitate in a minimum volume of 12N nitric acid. Evaporate this solution onto a heated filament for mass analysis. I
SURFACE IONIZATION MASS ANALYSIS
Fundamentally the instrument used in this investigation is a 12-inch radius, 60-degree direction-focusing mass spec-
Table I.
Precision of Concentration and Mass Assay Results at Variable SampleSpike Ratios
Ratio,
Sample lvt./SPike W L 1000 100 10 1 0.1 0.01 0.001
Concn., AI!& c / a I l . 350 100 10
1 0.1 0.01 0.001
% of concn.
Standard Deviation % of most abundant isotope
2.12 1.16 0.67 0.56 1.21 2.40 4.70
0.093 0.082 0.094 0.087 0.368 2.14 7.66
CHEM I ST-TESTER
DECONTAMINATION AND PREPARATION
T I M E OF ANALYSIS
DUPLICATES
Figure 1. Typical crossed classification group for the error component study
Mass instrument A. Mass operator A. Pipet A trometer equipped with a vacuum lock for the introduction of solid samples without undue loss of vacuum. A procedure for the mass analysis of uranium is given (2) for this instrument. This type of instrument is described by Stevens and Inghram (7). Echo and Morgan (3) describe an improved model constructed at the National Reactor Testing Station. The permanent magnet field is varied by a shunt which is positioned by a worm and gear drive. Scanning is accomplished by auxiliary field coils on the magnet. Mercury diffusion pumps and Welch mechanical pumps provide the vacuum. The signal from the ion collector is amplified by use of an Applied Physics vibrating reed electrometer and Brown Electronik recorder. The source, attached to the carriage of the vacuum lock, is designed for solid samples. Purified uranium in the form of a concentrated uranyl nitrate solution is pipeted onto a heated filament of tantalum ribbon and a uranium oxide (USOs) deposit is formed. After introduction into the instrument through the vacuum lock arrangement, the deposit is evaporated by electrically heating the filament. The ionization produced depends on the fact that the filament has a higher affinity for electrons than the material applied to it. For this reason the filament must be made of material with a high work function. Acceleration of the charged particles into the magnetic field is accomplished
through a potential of approximately 6000 volts. Beams for U*. UO+. and UO$+ ions have been observed: however, the UOz+ beams are the most sensitive, and this spectrum is used for uranium analysis. The individual ion beams are recorded as a series of separate peaks from which the relative abundance of each isotope is determined. Corrections are necessary for the isotopic distribution of oxygen-16, oxygen17, and oxygen-18 in the UOz+ ions received as signal upon collector plate. EXPERIMENTAL
Precision Study. Inspection of Equation 1 shows t h a t C, is an inverse function of the variable P for given values of the factors A , C,. V,, and V,. The usable concentration range for C, with respect t o these factors then depends on the reproducibility of results for variable sample-to-spike ratios. For the study summarized in Table I, a four-isotope standard representative of the uranium processed a t the I C P P was used throughout. At the higher ratio levels the spiked mixture was prepared by direct mixing of pure uranyl nitrate solutions. At the lower ratio levels the additional step of precipitation with ammonium hydroxide was used to concentrate the spiked mixture before depositing on the filament for analysis. The expressed standard deviations
in Table I are based on nine or more complete scans from a single preparation a t each ratio level. Variances due to sample and spike-volume measurements were thereby eliminated. Moreover, the completed sets of peaks for each level were obtained without interruption by one experienced instrument operator using the same mass spectrometer throughout. Variances due to time of analysis, different instrument operators, and different instruments were accordingly eliminated. The precision obtained under these ideal conditions should indicate more clearly whether an optimum relation exists between sample and spike, and how the precision can be expected to vary throughout the usable ratio levels. The approximate maximum and minimum concentration levels for which analysis is required a t the I C P P are listed in column two, Table I. Precision study data for these limits and intermediate levels were obtained from variable sample-to-spike ratios through a range of 106 as shown in column one. This study shows that maximum variance is approached for isotopic analysis as the ratio is decreased below 1. At the low ratios the isotopic impurities added with the spike may be present to a much greater extent than isotopes composing the sample. The accuracy with which correction factors are applied then becomes highly significant in determining isotope peak values. The lower limit for usable ratios can probably be extended much further if the spike does not contain isotopic impurities for the element in question. One of the important objectives in the precision study was t o show approximately the ratio below which variance in isotope analysis beconies significant. This is shown in column four, Table I, as occurring between ratios of 0.1 and 1. Knowledge as t o the approximate concentration of the sample before analysis can then be helpful in adjusting this ratio t o a value near 1. Minimum variance can then be expected for both total uranium and isotope distribution analyses. At the Ion-er ratios the precision of total uranium results is probably better than can be obtained with other microtechniques. I n practice this method is sometimes applied in the analysis of solutions containing approximately 1 y of uranium per ml. t o obtain estimates of the major isotope concentration, Khen a high degree of precision is required the uranium from these solutions must be concentrated before the spike is added. Error Component Study. T h e majority of isotope dilution analyses a t I C P P are carried out a t a sample spike ratio of approximately 20-i.e., 0.5 ml. of sample X 2.0 mg. U/ml. = 2o 0.5 ml. of spike X 0.1 mg. U/ml. VOL. 29,
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At this ratio, laboratory contamination from the relatively high uranium-233 alpha activity is easily kept below tolerance limits, and the precision is still satisfactory. Contamination hazards definitely increase for routine analysis as the sample-spike ratio is decreased. I n practice, however, occasional use of ratios as low as 0.001 is necessary and reasonably safe with radiation protection equipment in use. At a ratio of 20 a knowledge m-as desired of the magnitude and significance of effects from the many sources of variance associated with the method (6). Therefore an error component study was made with 15 chemisttesters, two remote pipetters, tJyo surface ionization mass spectrometers, and four mass spectrometer technicians. The study was so designed that seven sources of variability could be identified and their corresponding magnitudes estimated: 1. Chemist-tester variation 2. Pipet variation 3. Variation introduced in the decontamination and sample preparation step 4 &lassinstrument operator variation 5 . Differences between mass instruments 6. Day to day variation in instrument performance 7 . Reproducibility of replicate scans or measurement error
It was assumed that the mass instrument, mass instrument operator, and pipet were crossed variablesthat is, the effect attributable to a given level of each of these variables mas the same regardless of the other variables involved. I n contrast, the chemisttester, decontamination and sample preparation, time of analysis, and measurement variables were considered as being nested or subgrouped within each combination of levels of the crossed variables. Identified with each combination of mass instrument, mass spectroscopist, and pipet, it was originally proposed that two cheniisttesters selected in a random manner be assigned to decontaminate in duplicate a n identical sample using the given pipet. These samples were then to be analyzed in duplicate, each a t two different times by a given mass instrument operator on the given mass spectrometer. Hence a typical group would be as shown in Figure 1 for the given crossed classification and similarly for each combination of mass instruments, mass instrument operators, and pipets. Since four operators, two instru-
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Table II. Percentage Error of Components for Sources of Variance Mass Concn. -4ssay
Chemist-tester Negl. Mass instrument operator Negl. Sample preparation Negl. Day to day variation 28 Variation between mass spectrometers 7 Variation between pipets 21 Reproducibility in mass 44 laboratory _ . Totals 100
Negl. Negl. Xegl. Negl.
The total over-all error expressed as 95% limits of uncertainty for a single determination of uranium-235 was found to be i.0.13’%, expressed as relative percentage. V i t h regard to uranium235 percentage determination, none of the sources of variation contributed significantly to the over-all error when compared to analytical error or reproducibility within the mass laboratory.
Negl. Negl. 100 100
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TIME REQUIREMENTS
A careful survey of time requirements for each phase of the analysis as noted in the error component study indicates that approximately 1 hour and 50 minutes is required for the complete concentration and mass analyses as follows:
ments, and two pipets were involved, 4 X 2 X 2 = 16, such groupings would result. Furthermore, if comMinutes plete, 16 X 16 = 256 individual deRemote pipetting 15 terminations would be required. In Decontamination 25 view of the difficulty anticipated in adBack extraction 10 ministering such a testing program as Precipitation 10 Mass analysis 30 well as the prohibitive size of such an Calculations 20 experiment, an attempt was made to Total 1 hour, 50 minutes cut down the size by varying the number of replications a t a given step and ACKNOWLEDGMENT in some cases eliminating replications in a step completely. Thus in actuality The original authorization and all 62 individual analyses resulted. The preliminary arrangements for this work relative contributions of each source were made while the I C P P was operin terms of variance components to ated for the Atomic Energy Commission the over-all percentage of errors for by American Cyanamid Co. R. E. total uranium and mass assay are given Torley and R. J. France1 initiated the in Table 11. project, and the work was later comThe concentration error expressed as pleted under R. C. Shank and the the 9570 limits of uncertainty m-hich direction of T. D. Morgan. The reflects the effect of all the sources of authors are grateful to J. E. Rein for variance in the error component study technical assistance. listed was found to be k3.S70 for a single determination at the 9570 confiLITERATURE CITED dence level. The breakdown of the (1) Duffy, W. E., Tingey, F. H., U. S. total error into components and the Atomic Energy Commission, Rept. subsequent tests for significance indiIDO-14301(1955). cated that compared to the analytical (2) Duffy, W.E., Wheeler, G. V., Morgan, reproducibility within the mass laboT. D., eds., Zbzd., IDO-14318. (3) Echo. M.29, TI-..1595 Morgan, T. D., AIIAL. CHEX. (i95?). ratory, variation between pipets, variation between mass instruments, and (4) Francel, R. J., U. S. Atomic Energy day to day variation in instrument perCommission, Rept. IDO-15107 formance had a significant effect on (1953). ( 5 ) Goris, Paul, Tingey, F. H., Ibid., the over-all precision. The effects, IDO-14366(1956). if any, that could be attributed to the (6) SheDherd. I f , J.. Jr.. Rein, J. E., chemist-tester, decontamination step, Z6id.. IDO-14316(1955). or mass spectrometer operator were not (7) Stevens, C. RI., Inghram, 31. G., Zbid., ANL-5251 (1954). detectable in view of the reproducibility within the mass laboratory. RECEIVEDfor review October 6, 1956. Sensitivity scale factor calibration Accepted June 24, 1957. Work peron the mass spectrometers was found formed under contract No. AT(10-1)-205 with the Idaho Operations Office, U. S. to contribute entirely to the difference Atomic Energy Commission. Presented between the two instruments. With in part a t Fourth Annual Meeting, ASTM the correct factors applied, the over-all Committee E-14 on Mass Spectrometry, error decreased from i.3.8Oj, to ~ k 3 . 7 7 ~ . May 28 to June 1, 1956, Cincinnati, Ohio. \
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