5th International Nuclear Chemistry Congress Gothenburg, Sweden 27 August – 1 September 2017
Isotopic composition of commercially available uranium chemicals and elemental analysis standards M. Peńkin*, S. Boulyga, B. Dabbs, D. Fischer, M. Humphrey, A. Kochetkov, A. Köpf, M. Sturm International Atomic Energy Agency — Department of Safeguards
Introduction
Discussion on material origin
Over decades of commercial operations, major producers of nuclear fuel worldwide have accumulated large stocks of tails from uranium isotope enrichment. Since such tails are of relatively high purity and have little value in nuclear industry, they are commonly used to produce uranium materials for non-nuclear applications. Therefore, it is no surprise that commercially available uranium chemicals and U-doped standards may have non-natural isotopic composition – depleted in 235U, in some cases with traces of 236U.
For all DU materials, the substantial presence of 236U suggests that they originate from uranium recycled from irradiated reactor fuel. We attempted to identify the fuel original enrichment and burnup using open publications and computer codes which model isotopic changes occurring in the nuclear fuel cycle: irradiation in a nuclear reactor and uranium isotope enrichment. One can exclude the scenario of simple irradiation of NU fuel to a high burnup, followed by reprocessing of the spent fuel (without further re-enrichment): a hypothetical irradiation of NU down to 0.2–0.4% 235U (e.g., in a CANDU reactor) generates material with 234U and 236U at levels significantly higher than our measurement data. On the other hand, NU fuel irradiated to a low burnup, such as from the production of weapons-grade plutonium, is only slightly depleted in the 235U isotope.
Early surveys of isotope ratios in commercially available uranium chemicals employed radiometric techniques [1–3]; the results had relatively large uncertainties and only included two or three isotopes. The most recent systematic study of uranium isotopic composition in commercially available chemicals employed high-precision mass spectrometric measurements of the three naturally occurring uranium isotopes and 236U [4]. Reference information on uranium isotopic composition in various materials, determined with high accuracy and precision, is recognized to be of essential value both in the fields of nuclear safeguards and nuclear forensics. The obvious interest of practitioners of these disciplines for updated reference datasets motivated the current work. In addition, an accurate knowledge of uranium isotopic composition is important when isotope-specific measurement techniques such as ICP-MS are employed for elemental analysis of uranium materials. Ten samples were acquired for the current study from commercial suppliers of chemicals and laboratory standards. Multiple purchases of similar products from the same vendor were generally avoided in the assumption that uranium isotopic composition of such items is likely to be the same due to the single stock material used, as demonstrated by Richter et al. [4]. Uranium isotopic reference materials were also deliberately excluded from the current study as their isotopic composition is well characterized and documented.
Sample
Supplier
1
Molekula (Germany)
2
Product code
Product name
Unit size
57713876
Uranyl nitrate hexahydrate (powder)
5g
IBI Labs (USA)
94270
Uranyl nitrate hexahydrate (powder)
25 g
3
Aname (Spain)
22400
Uranyl acetate (powder)
25 g
4
Perkin-Elmer (Austria)
5
Merck (Germany)
6
Inorganic Ventures (USA)
7
N9303844
Uranium single-element standard, 2% HNO3
125 mL, 1000 µgU/mL
Uranium ICP standard in 2–3% HNO3
100 ml, 10 µgU/mL
MSU-100PPM
Uranium for ICP-MS, 1.4% HNO3
125 ml, 100 µgU/mL
Inorganic Ventures (USA)
CGU1-125ML
Uranium for ICP, 2% HNO3
125 ml, 1000 µgU/mL
8
GFS Chemicals (USA)
23290-1829
Uranium ICP standard, 5% HNO3
100 ml, 1000 µgU/mL
9
High Purity Standards (USA)
100064-1
Uranium single-element standard, 2% HNO3
100 ml, 1000 µgU/mL
10
SPEX CertiPrep (USA)
CLU2-2Y
Uranium ICP-MS standard in 2% HNO3
125 mL, 1000 µgU/mL
170360
Results of the current study compared to two NU irradiation scenarios. The arrows indicate isotopic changes in a nuclear reactor. However, if uranium recycled from such low-burnup fuel was mixed with NU and then enriched to produce low-enriched uranium (LEU), then the resulting tails may fit our DU measurement data. In particular, the isotopic composition found in Group A is similar to the isotopic values of the tails from the Paducah gaseous diffusion uranium enrichment plant during several periods of time [8]. The majority of recycled uranium (RU) used as feed at Paducah (in a mixture with NU) originated from the Hanford NU-fuelled reactors that produced plutonium for military purposes. Assuming the 93.8% 239Pu target in the fuel at the time of its discharge from the reactor [9] and using WIMS-FISPIN calculations for Pu production at the most advanced Hanford N-pile, we estimate that the NU fuel was irradiated to approximately 860 MWd/tU, and the resulting uranium contained 0.0052% 234U, 0.64% 235U, and 0.014% 236U. This composition is close to the reported average RU component of the Paducah feed during the 1953–1976 period. Further, taking into account that the Paducah plant enriched uranium to about 2.75% 235U [8] and assuming the tails composition corresponds to the isotopic values of Group A, we modelled a gaseous diffusion enrichment cascade using MSTAR code. This allowed us to estimate the ratio of NU and RU in the enrichment feed at about 3:2.
Group A U-235
Experimental All samples were converted to 3 M nitric acid solutions; uranium concentration was adjusted to 2.5 mg/g. For the measurements of U isotope ratios, Triton MC-TIMS instrument (Thermo Scientific) was used in the modified total evaporation mode [5–7]. Portions of sample solutions containing ~2.5 µg U were delivered by pipette onto tungsten evaporation filaments and oxidized by applying a current of 1.0 A for 300 sec, and then 2.4 A for 60 sec. Each sample-loaded filament was partnered with a degassed rhenium ionization filament in a double filament assembly. An automated sequence of measurements was run for 21 filament positions per turret. On a single turret, five pre-defined positions were loaded with a mass-fractionation standard and four positions – with a QC standard. The remaining positions were loaded with two samples, each in six pre-defined positions. An amplifier gain calibration was performed for each turret; an amplifier baseline calibration – for each filament. Once the ionization filament reached a target intensity of 50 mV, focusing on the Re signal was performed, and the peak was centred on Re mass. The evaporation filament was then ramped up to 500 mV to focus on 238U signal; 236U mass peak was centred on the middle Faraday cup. Data collection started at a summed target voltage intensity of 2 V; the MTE was run with a target intensity of 30 V. To improve the accuracy of the 234U and 236U isotope determinations, the background and peak tailing were measured within each filament analysis. The secondary electron multiplier signal for 236U was also corrected for internal yield. For each measured isotope ratio, an expanded combined uncertainty was calculated according to the GUM recommendations, using a coverage factor ≥ 2 corresponding to the 95% confidence interval. Detection threshold for the 236U isotope (5 × 10–7 at.%) was established as three standard deviations of the 236U/238U ratio measured in reference material NBL CRM 112-A.
Sample
234
U, at.%
235
U, at.%
236
U, at.%
238
U, at.%
Remarks
1
0.00162 (6)
0.32582 (16)
0.00327 (4)
99.66929 (25)
2
0.000723 (26)
0.20046 (15)
0.002844 (19)
99.79597 (19)
Group A
3
0.000752 (27)
0.20388 (15)
0.002825 (18)
99.79254 (19)
Group A
4
0.000708 (26)
0.19929 (15)
0.002746 (18)
99.79726 (19)
Group A
5
0.00167 (6)
0.27369 (20)
0.01224 (12)
99.71240 (38)
6
0.002230 (25)
0.37488 (18)
0.00493 (5)
99.61796 (26)
7
0.000805 (29)
0.20985 (15)
0.002932 (19)
99.78641 (20)
Group A
8
0.005378 (23)
0.72002 (30)
not detected
99.27460 (32)
NU
9
0.005485 (22)
0.72378 (29)
0.000547 (2)
99.27019 (31)
10
0.00490 (4)
0.65920 (34)
0.000661 (2)
99.33524 (38)
Group A U-236
Group A U-234
Group A (estimated)
Minor isotopes in cascade tails of the Paducah plant [8]. Based on the isotopic data measured in this study and the reported composition of the Paducah tails one may deduce that the materials of Group A could have been generated as tails from enrichment operations which took place at the Paducah plant between 1966 and 1970 or in 1982. Similarly, isotopic composition of samples 1 and 6 is consistent with the Paducah tails originating from enrichment of feed containing a mixture of NU and RU from the Hanford reactors, although with different NU:RU ratios. Comparison of the measured isotopic data and the isotopic composition reported for the Paducah tails suggests that these materials could have been generated at Paducah around 1960–1964 and 1973, respectively.
Models of UF6 enrichment cascades fed with NU+RU mixtures, which may have generated tails consistent with the DU materials identified in the current work. The arrows show isotopic changes in cascades generating LEU product and DU tails.
Results As expected, the majority of the materials (samples 1–7) are based on uranium substantially depleted in 234U (0.0007–0.0022 at.%) and 235U (0.20–0.37 at.%), with a significant presence of 236U (0.0027–0.012 at.%). Several DU materials from different suppliers (samples 2, 3, 4, 7) have similar isotopic composition; in the discussion below they are called collectively as Group A. Sample 8 is based on natural uranium (NU). Sample 9 has natural concentrations of 234U and 235U, but contains a trace level of 236U; this composition is consistent with so-called ‘commercial NU’. Sample 10 is slightly depleted in 235U, with traces of 236U; it may originate from a small admixture of depleted uranium (DU) to NU. One caveat of the current work is that the 233U/238U isotope ratio was not measured. However, there is no reason to assume that the analysed materials would contain detectable levels of 233U as this isotope is only generated when thorium is irradiated in a nuclear reactor. In addition, the presence of the 232U isotope, also generated during irradiation of thorium, was not detected by the gamma spectrometry screening of the samples.
Isotopic composition of the uranium materials analysed in the current study. Red circle = Group A. The measurement uncertainties are negligible compared to the plot scale.
Composition of sample 5 is quite different from all other DU samples measured in this study, as it contains an appreciably higher level of 236U. Our initial working assumption was that such a composition might originate as tails from re-enrichment of a mixture of NU and RU originating from LEU fuel irradiated in a commercial power reactor with the 235U isotope remaining at levels slightly above natural. However, we failed to find a realistic scenario which would only involve two components.
Conclusions This survey confirmed that commercially available uranium chemicals and elemental analysis standards have a variety of isotopic compositions. Only one sample was found to have natural isotopic composition. As expected, most of the materials covered by this work are substantially depleted in the 234U and 235U isotopes, with a significant presence of 236U. We demonstrated that a plausible origin of materials used in uranium chemicals could be identified based on their isotopic composition and employing a combination of reactor codes and an enrichment cascade modelling tool. Scenarios involving the recovery of high burnup uranium, such as from power reactor spent fuel, could be excluded based on the relatively low levels of 234U and 236U measured in the chemicals. In general, the sources of the depleted uranium in the commercially available chemicals covered by this study were consistent with tails from enrichment of recycled uranium feed (primarily in a mixture with natural uranium). This recycled uranium likely originates from the low burnup of NU, such as the recovery of fuel irradiated for the production of weapons-grade plutonium.
References
Acknowledgments
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The authors are grateful to Ahmed El Gebaly for his efforts in identifying potential suppliers of uranium chemicals, and to Tobias Petersmann who arranged for procurement of the selected materials.
Corresponding author: Maxim Peńkin — Tel: +431-2600-21860 —
[email protected] — Vienna International Centre, PO Box 100, Vienna 1400, Austria
