Investigation of the polymorphs and hydrolysis of ... - Springer Link

J Radioanal Nucl Chem (2013) 296:105–110 DOI 10.1007/s10967-012-2063-9

Investigation of the polymorphs and hydrolysis of uranium trioxide Lucas E. Sweet • Thomas A. Blake • Charles H. Henager Jr. Shenyang Hu • Timothy J. Johnson • David E. Meier • Shane M. Peper • Jon M. Schwantes

•

Received: 19 July 2012 / Published online: 17 August 2012 Ó Akade´miai Kiado´, Budapest, Hungary 2012

Abstract This work focuses on the polymorphic nature of the UO3 and UO3–H2O system, which are important materials associated with the nuclear fuel cycle. The UO3– water system is complex and has not been fully characterized, even though these species are key fuel cycle materials. Powder X-ray diffraction, and Raman and fluorescence spectroscopies were used to characterize both the several polymorphic forms of UO3 and the certain UO3-hydrolysis products for the purpose of developing predictive capabilities and estimating process history; for example, polymorphic phases of unknown origin. Specifically, we have investigated three industrially relevant production pathways of UO3 and discovered a previously unknown low temperature route to the production of b-UO3. Several phases of UO3, its hydrolysis products, and key starting materials were synthesized and characterized as pure materials to establish optical spectroscopic signatures for these compounds for forensic analysis. Keywords Uranium trioxide Uranium oxide X-ray diffraction Raman spectroscopy Fluorescence spectroscopy Uranium oxide hydrolysis

Introduction Uranium trioxide (UO3) is produced on the front end of the nuclear fuel cycle in the mining, milling, refinement and conversion processes that precede isotope enrichment [1]. In

L. E. Sweet (&) T. A. Blake C. H. Henager Jr. S. Hu T. J. Johnson D. E. Meier S. M. Peper J. M. Schwantes Pacific Northwest National Laboratory, 902 Battelle Blvd., Richland, WA 99352, USA e-mail: [email protected]; [email protected]

addition, spent nuclear fuel usually undergoes a refinement process that produces the intermediate species UO3 from the denitrification of uranyl nitrate hexahydrate (UNH) after purification (typically) by solvent extraction [1, 2]. The uranium trioxide species, UO3, is then reduced to UO2 and subsequently converted to UF4 (and ultimately to UF6) by fluorination prior to isotopic enrichment. The structural phase of UO3 that is formed as a result of these different production routes is characteristic of the process. With an understanding of the polymorphic nature of UO3 and with the structural forms of the end products, one could use such knowledge to discern the production method. Thus, a detailed knowledge of the UO3 system can be useful for nuclear forensic applications. Previous work has demonstrated that different structural forms of UO3 are achieved depending on the starting materials and on the reaction conditions as summarized in Table 1. The structures of the various polymorphs of UO3 have also been well characterized in many studies using X-ray and neutron diffraction (these studies are referenced in Table 1). In addition to six different polymorphic forms of pure UO3 that can be made under atmospheric pressures of O2, there are also several different hydrolysis products of UO3 that can be formed depending on the conditions to which UO3 is subjected [3]. X-ray diffraction (XRD) patterns have been established for all isomorphs of the pure UO3 material (see Table 1 for references); however, this technique is incapable of fully capturing the complexity of this system, which often represents mixtures of polymorphs, the amorphous material and various hydrolyzed species. In these instances where the limitations of XRD to detect amorphous species and minor components to less than *5 % abundance are most evident, the application of optical techniques can be very advantageous. Some of the optical methods can also readily characterize amorphous species. But the application of optical spectroscopies to solid

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J Radioanal Nucl Chem (2013) 296:105–110

Table 1 Preparation methods for the different phases of UO3 Phase

Starting material

Conditions

References

A-UO3

UO42H2O, UO2C2O43H2O, (NH4)4UO2(CO3)3

400 °C

[9]

a-UO3

UO42H2O, NH4UO2(NO3)3

400–470 °C

[29, 30]

b-UO3

UO2(NO3)26H2O, ‘‘(NH4)2U7O22’’

Heat rapidly to 450–500 °C

[2]

c-UO3

UO2(NO3)26H2O, NH4UO2(NO3)2H2O

400–600 °C

[15, 30, 31]

d-UO3

b-UO2(OH)2

375 °C

[9]

e-UO3

U3O8

350 °C in a flow of NO2 gas

[9]

All reactions are carried out in air unless otherwise stated. Excluded are the routes to the different phases that require a pressure of O2 [ 1 atm or H2 atmosphere

phases presents challenges of its own. Optical spectroscopy databases for these types of characterizations are limited in scope. While there exists several IR and Raman databases [4–7] for solids, these largely contain no phase-specific information, and little or no information for lanthanides or actinides. Some of the techniques are moreover sensitive to surface roughness and/or the particle size of the individual samples, making comparisons between different samples of the same materials difficult. In this study we attempt to utilize the well established method of powder XRD to help assign the optical spectra of UO3 phases, starting materials and hydrolysis products. We then use the assigned optical spectra to identify spectroscopic signatures of different UO3 production methods. It is hoped that the phase-specific and trace-component information afforded from the optics measurements will allow insight into nuclear forensic applications; for example, the origin and process history of such materials. The three routes of UO3 production we have chosen to investigate include synthesizing UO3 from either UO2(NO3)2 6H2O [8], (NH4)4UO2(CO3)3[9] or UO42H2O [10]. Making UO3 from UO2(NO3)26H2O is commonly done when reprocessing spent fuel [1]. Another potential route for producing UO3 during ore processing is through the precipitation and conversion from (NH4)4UO2(CO3)3. UO3 production from UO42H2O (meta-studtite) or UO44H2O (studtite) is also a common route of obtaining UO3 when processing and refining uranium from ores [11, 12]. Each of these routes results in a different polymorphic phase of UO3 (see Table 1) as described below. Experimental The (NH4)4UO2(CO3)3 used to make UO3 was prepared by adding 1.1965 g of (NH4)2CO3 (Sigma-Aldrich, 99.9 %) to

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2.17 mL of a 0.96 M aqueous solution of UO2(NO3)2 (solid UO2(NO3)26H2O from International Bio-Analytical Industries, Inc., 99.9 %). The yellow precipitate was allowed to settle and the water was decanted off. The wet solid was baked in a furnace at 80 °C for 3 h to remove the remaining water. By comparing the XRD powder pattern of the product to the powder pattern calculated from the crystal structure of (NH4)4UO2(CO3)3 [13], the product was confirmed to be pure. The UO42H2O (meta-studtite) used to make UO3 was synthesized by adding 3 mL of 30 % H2O2 (SigmaAldrich) to 5 mL of a 1 M aqueous solution of UO2(NO3)2 (solid UO2(NO3)26H2O from International Bio-Analytical Industries, Inc., 99.9 %). A pale yellow precipitate formed upon addition of the hydrogen peroxide. The solution was heated to 80 °C for 24 h to dry the sample. Powder XRD of the final product confirmed the pale yellow powder was in fact UO42H2O. Separate samples of UO2(NO3)26H2O (International Bio-Analytical Industries, Inc., 99.9 %), (NH4)4UO2(CO3)3 and UO42H2O were individually heated to a variety of temperatures ranging from 350 to 500 °C for 60 h. Powder XRD patterns were collected on all starting materials and products using a Rigaku Ultima IV powder diffractometer equipped with a graphite monochromated Cu Ka source and a D/Tex silicon strip detector. The samples were also analyzed using an ExamineR 785 Raman spectrometer (Delta-Nu), which uses a 785 nm excitation laser. The ExamineR module was attached to an Olympus BX51 compound microscope equipped with a 109 objective. Raman spectra were collected in a dispersive configuration with a spectral range of 200–2,000 cm-1 Raman shift and a resolution of 5 cm-1. In addition, fluorescence spectra of samples were collected using a prism and reflector imaging spectroscopic system (PARISS made by LightForm Inc.) using a 130 W mercury arc lamp source and a filter cube that allowed for a 375–425 nm excitation band and a collection of fluorescence spectra from 475 to 900 nm. The excitation source and spectrometer were attached to a Nikon Eclipse 50i microscope equipped with a 109 objective. The spectra were collected in an epifluorescence mode. The spectral resolution of the described system was 1.35 nm. Along with investigating different routes in the formation of pure UO3, we also examined the formation of certain UO3 hydrolysis products under ambient conditions (humid air at room temperature) by separately synthesizing and characterizing some of the hydrolyzed species. The two common hydrolysis products of UO3 are a-UO2(OH)2 and (UO2)4O(OH)65H2O (meta-schoepite) [3, 14]. Pure a-UO2(OH)2 was prepared by stirring a sample of c-UO3 in water (forms a slurry) and then heating to 80 °C for 24 h to drive off the excess (unreacted) water. (UO2)4O(OH)65H2O,


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a further hydrated form, was prepared by stirring a sample of c-UO3 in water and allowing the water to evaporate at room temperature. Each of these samples was analyzed using powder XRD, Raman microscopy and fluorescence microscopy.

Results and discussion c-UO3 from UO2(NO3)26H2O The first UO3 synthesis that we explored was the conversion of UO2(NO3)26H2O to c-UO3. After heating 1 g of UO2(NO3)26H2O at 350 °C for 60 h, the XRD powder pattern of the orange powder sample matched that of the crystal structure reported for pure c-UO3 [15] and the sample had the deeper orange characteristic of the c-phase of this material. The collection of the XRD patterns took 30 min per sample whereas the collection of the optical spectra took 1 min per sample for the Raman spectra and 1 s per sample for the fluorescence spectra. No impurities were detected by XRD. The sample was then heated at 400 °C for 60 h. The XRD powder pattern of the sample after heating at 400 °C still appears to be that of the c-UO3 phase. This behavior is consistent with what has been previously reported [9]. While Raman spectra of UO3 have been reported previously [16], the Raman and fluorescence spectra shown in Fig. 1 (bottom traces) have been confirmed to be of pure c-UO3 by an orthogonal technique, powder XRD. This UO3 isomorph is known to readily form hydrolysis products under ambient conditions (humid air at room temperature) [3, 14, 17–19]. The resulting Raman and fluorescence spectra for the pure materials a-UO2(OH)2 (middle trace) and (UO2)4O(OH)65H2O (meta-schoepite, top trace) as prepared from the c-UO3 sample described above are thus also shown in Fig. 1. While the distinction between the spectra of c-UO3 and the hydrolysis products is clear, the spectroscopic distinctions between the two hydrolysis products is more subtle; this is true both for the fluorescence and Raman spectra. The Raman band at 840 cm-1 for a-UO2(OH)2 and 841 cm-1 for (UO2)4O(OH)65H2O represent the well-known m1 symmetric stretching frequency of the uranyl (O=U=O)2? cation, and are typical UO22? frequencies for such species, as both have discrete uranyl ions in the crystal [20–22]. However, for both the uranyl m1 symmetric stretch and the m3 anti-symmetric stretch it has long been known that the peaks shift strongly depending on the local environment of the UO22? cation [20, 23]. The peaks at 840 and 841 cm-1 of the two hydrolysis products are typical of the uranyl symmetric frequency in many uranyl-bearing compounds and minerals such as the 815 cm-1 mode reported for kamotoite [24], the *840 cm-1 frequency of euxenite [22],

Fig. 1 Raman (left) and fluorescence (right) spectra of (UO2)4O(OH)65H2O (top), a-UO2(OH)2 (middle) and c-UO3 (bottom)

and the 865 cm-1 value reported for UO2F2 [16, 25]. On the other hand, the c-UO3 does not have clearly isolated uranyl groups as there are different uranium site symmetries with different degrees of coordination to neighboring oxygen atoms. Its vibrational spectrum is thus not as well understood, but has as its strongest band the peaks at 767, 484 and 339 cm-1 [16, 26]. We surmise that as the parent c-UO3 hydrolyzes, it is marked by the disappearance of these bands and emergence of a symmetric uranyl Raman band in the typical 830–870 cm-1 range as seen in Fig. 1. In a somewhat analogous fashion, the fluorescence spectra of the two hydrolysis products have blue shifted from the parent c-UO3 compound to 531 and 523 nm, though there is some evidence of a vibronic progression (the splitting of the fluoresce emission by the uranyl vibrational modes as seen more clearly resolved in the fluorescence spectrum of UO42H2O in Fig. 3b) in the meta-schoepite spectrum. In contrast, the fluorescence spectrum of the c-UO3 parent compound has a peak maximum further to the red at 549 nm [21]. Here, we have demonstrated two techniques (Raman and fluorescence spectroscopies) that can rapidly identify hydrolysis products of UO3 with non-destructive measurements that take only seconds. The changes observed in the Raman spectra and/or the fluorescence spectra between the pure and hydrolyzed UO3 may be utilized to deduce the storage condition of a sample. To test such a hypothesis, a set of prepared samples of pure UO3 was analyzed over time. A sample of c-UO3, initially made pure and confirmed by XRD as only c-UO3, was analyzed again by XRD after sitting in a capped vial at

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room temperature (23 °C) and humidity (20–30 %) for 34 days. The resulting XRD powder pattern indicates that the composition was already 25 % a-UO2(OH)2 and 75 % c-UO3 only 34 days after production. Ongoing studies are aimed at determining how temperature and humidity affect the rate hydrolysis.

unknown route to relatively pure b-UO3 production. After further heating the sample to 450 °C, the resulting product was 18 % a-UO3 and 82 % b-UO3 as determined by Rietveld refinement using the TOPAS software package [27].

Amorphous, b-UO3, and a, b-UO3 from (NH4)4UO2(CO3)3

A pure (by XRD) sample of a-UO3 was prepared by heating meta-studtite (UO42H2O). We could not obtain a reasonable Raman spectrum of a-UO3 because of the resulting fluorescence when using a 785 nm laser for excitation; it is hoped that 1,064 nm excitation could abate this problem [7]. Interestingly, the bulk of the a-UO3 sample did not fluoresce in the visible/near IR region when exposed to excitation bands ranging from 375 to 560 nm. However, small particles of meta-studtite and a-UO2(OH)2 could be observed in a product that was thought to be pure a-UO3 based on the powder XRD pattern. In Fig. 3 the bright spots circled and labeled A and B are the results of sample fluorescence when excited with a 375–425 nm band. The fluorescence spectrum of A (Fig. 3) match well the spectrum of a-UO2(OH)2 obtained earlier (see Fig. 1) and this particle is ascribed to trace a-UO2(OH)2 in the sample. The spectrum of the B particle in Fig. 3 matches the spectrum of the UO42H2O starting material, a second impurity.

A series of preparation temperatures was tried for the formation of UO3 from the pure (NH4)4UO2(CO3)3 starting material. Figure 2 shows the XRD patterns and Raman spectra that resulted from the sequential heating of a sample at 350 and 450 °C that began as (NH4)4UO2(CO3)3. It has previously been reported that (NH4)4UO2(CO3)3 forms amorphous UO3 at 400 °C [9]. As seen in Fig. 2, however, the products that formed after heating (NH4)4UO2(CO3)3 to 350 °C are largely amorphous. The sample preparation, quantity and measurement parameters were consistent for the sample shown in Fig. 2, so we can directly compare the signal to noise ratio in each one of the XRD patterns. The signal to noise ratio on the sample prepared at 350 °C was very low indicating a large degree of amorphous product. However, the discernable peaks for the sample prepared at 350 °C match [2] the crystal structure of b-UO3 quite well. This is a previously

a-UO3 from UO42H2O

450°C 450°C

350°C

350°C

(NH4)4UO2(CO3)3 (NH4)4UO2(CO3)3

10

20

30

40

50

60

Degrees 2θ

70

80

90

200

560

920

1280

1640

2000

-1

Raman Shift (cm )

Fig. 2 Powder XRD patterns (left) and Raman spectra (right) of the products that are formed after heating (NH4)4UO2(CO3)3 (bottom) to 350 °C (middle) and 450 °C (top)

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109

A B

A

B

Fig. 3 A picture of a sample of a-UO3 through a 9 10 optical microscope that covers an area of 1.75 mm 9 0.65 mm is shown at the top. The bright spots circled and labeled as A and B are a result of fluorescence from a 375–425 nm illumination source. The fluorescence spectrum of these two particles are shown below and labeled with the corresponding A and B labels. The spectrum of particle A matches that of a-UO2(OH)2. The spectrum of particle B matches that of UO42H2O

While we could not obtain direct spectroscopic signatures from the a-UO3 phase itself, we were able to identify small particles of starting material and hydrolysis product. There is also interesting anecdotal evidence that the meta-studtite sample contains distinct uranyl ions; in studtite it is known that the uranium U6? cation occurs in the form of a linear chain of uranyl ions (UO2)2? and with the U ion further bonded to additional O atoms [28]. A similar (uranyl ions) structure can be expected for meta-studtite. Figure 3 lends credence to this as the spacing of the six vibronic peaks seen in the figure is *835 cm-1, typical of the uranyl symmetric stretching frequency. The (UO2)2? cation is known for emission spectra structured with vibronic progressions in this frequency, which are often used at cryogenic temperatures for detection of uranyl [21]. The sample of a-UO3 was stored in a capped vial for 45 days. From Rietveld analysis of the resulting powder XRD pattern it was determined that the sample consisted of 90 % aUO2(OH)2 [14], 5 % a-UO3 [29] and 5 % (UO2)4O(OH)6 5H2O [14]. Efforts are currently underway to establish the rate of hydrolysis under a given set of conditions.

Conclusions We have shown here that a combination of XRD powder patterns, Raman spectra, and fluorescence spectra collected

on the starting materials, the polymorphs of UO3, and the hydrolysis products of UO3 can be used to determine the process history—production and fate—of UO3. Determining the spectral signatures of the relevant compounds and how they might change over time due to environmental factors is an important first step in developing a forensics toolbox for the analyst. Due to the complex polymorphic nature of UO3, the form of the end product is indicative of the preparation method used and the conditions in which the product has been stored. The ability to determine the production method by using rapid and relatively inexpensive (compared to XRD) spectroscopic methods like Raman and fluorescence can be a powerful tool for nuclear forensic types of applications. Acknowledgments This research was supported in part by the National Technical Nuclear Forensics Center (NFNFC, a department of the U.S. Department of Homeland Security), and was co-funded by NA-22 in the National Nuclear Security Administration/Office of Nonproliferation & Verification Research and Development. The work was conducted at the Pacific Northwest National Laboratory, a multiprogram national laboratory operated by Battelle for the U.S. Department of Energy under Contract DE-AC05-76RL01830. We thank our sponsors for their support.

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