Introduction

Temporal changes in the morphology of uranium oxide materials could provide insight into process and/or storage history [1–5]. Process variables inherent in the synthesis of nuclear materials are known to influence the morphology of uranium oxide products [4, 6–15]. In particular, comparison of the morphology of precipitates with those of subsequent calcination products suggests that microstructures of the precursor precipitates are retained through heat treatment [6–8, 13–16]. A number of studies have shown that U3O8, a common intermediate in the production of nuclear material, generates schoepite phases (UO3·xH2O) during storage under higher relative humidities presumably following oxidation and/or hydration [2, 17–19]. It would be valuable to understand the variables contributing to temporally-induced morphology changes, and if any changes occur, the prospect for using the temporal changes in morphology that arise during storage under different conditions for forensic analyses.

The objective of this study is to characterize images of three α-U3O8 samples—one phase-pure α-U3O8 sample and two phase-impure U3O8 samples—prepared from reaction routes used in nuclear conversion processes. These images were collected using scanning electron microscopy (SEM). Sub-samples of the materials were stored under controlled temperature and relative humidity conditions for 2 years. The chemical speciation of these materials and any chemical changes in the materials following storage were detailed previously [2]. The morphology of the materials are characterized using a lexicon of descriptors for assessing any temporal changes [20]. These results are compared with powder X-ray diffraction (p-XRD) patterns and Extended X-ray Absorption Fine Structure (EXAFS) spectra [2].

Experimental

Materials

Uranium U960 Standard Reference Material obtained from the National Bureau of Standards was used in the synthesis of α-U3O8 [2]. Ammonium hydroxide (Trace Metal grade), concentrated hydrochloric acid (Trace Metal grade), hydrogen peroxide (A.C.S. certified, 30 %), and platinum foil (Alpha Aesar 00261) were purchased from Fisher Scientific and used as received. Lithium iodide and potassium nitrate were purchased from Fisher Scientific and used as received.

Syntheses

The preparation of α-U3O8 (S1) is described elsewhere [2, 16]. The phase purity was confirmed by p-XRD analysis. This material is a black powder. Legacy materials were retrieved from LANL stock of uranium oxides synthesized on a 10 g scale using reaction routes often used in nuclear fuel processes. Legacy sample 1 (L1) and legacy sample 2 (L2) were made from these reaction routes:

$$\begin{aligned} {\text{UO}}_{ 2} {\text{F}}_{ 2} \cdot x{\text{H}}_{ 2} {\text{O}} {\mathop{\longrightarrow} \limits^{{20}^{\,\circ}{\text{C}}}_{{\text {pH}}\,8.4}} \left( {{\text{NH}}_{ 4} } \right)_{ 2} \left( {{\text{U}}_{ 2} {\text{O}}_{ 7} } \right) {\mathop{\longrightarrow}\limits^{{800\,}^{\circ}{\text{C}}}} {\mathbf{L1}} \end{aligned}$$
(1)
$$\begin{aligned} {\text{UO}}_{ 2} \left( {{\text{NO}}_{ 3} } \right)_{ 2} \cdot x{\text{H}}_{ 2} {\text{O}} {\mathop{\longrightarrow} \limits^{{20}^{\,\circ}{\text{C}}}_{{\text {pH}}\,1}} {\text{UO}}_{ 2} \left( {{\text{O}}_{ 2} } \right) \cdot x{\text{H}}_{ 2} {\text{O}} {\mathop{\longrightarrow} \limits^{{975}^{\,\circ}{\text{C}}}} {\mathbf{L2}} \end{aligned}$$
(2)

Both legacy samples are also black powders. The chemical speciation of L1 and L2 is detailed elsewhere [2].

Material storage under controlled temperature and relative humidity

Aging vessels fabricated from Swagelok fittings are described elsewhere [1, 2, 21]. The four storage conditions of temperature and relative humidity used in these experiments were: Condition 1 (5 °C and 25 %), Condition 2 (37 °C and 15 %), Condition 3 (5 °C and 97 %), and Condition 4 (37 °C and 89 %). These conditions are listed in Table 1 with respective water vapor densities. A saturated lithium iodide aqueous solution was used to obtain lower relative humidities. A saturated potassium nitrate aqueous solution was used to achieve higher relative humidities. Time 0 for S1 was the time at which the synthesis of the material was completed. Time 0 for L1 and L2 was the time at which each sample was retrieved from storage. Average conditions in our laboratory were 20 °C and ~20 % relative humidity.

Table 1 Conditions (temperature, relative humidity) and resultant water vapor density used for storage of materials [1, 2, 21]
Full size table

Scanning electron microscopy (SEM)

Samples were prepared by dusting the surface of a carbon adhesive disc adhered to an SEM mount with uranium oxide material and then lightly tapping the loose particles away. Images of the samples were collected on an FEI Quanta 200F Field Emission SEM using accelerating voltages of 15–30 kV.

Lexicon

A lexicon was employed to describe the morphology of the materials. The lexicon provides a taxonomic path schematic for the description of particle/subparticle morphology including the surface textures and features observed in an SEM image [20]. Terminology from the lexicon is denoted by italic type face.

Results and discussion

Phase-pure α-U3O8

Images of the phase-pure α-U3O8 (S1) were collected and assessed to benchmark temporal changes in legacy materials L1 and L2. The S1 was prepared by calcination of UO2(O2)·xH2O as described in references [2] and [16], and the p-XRD analysis of S1 revealed that it was phase-pure α-U3O8. An image collected from S1 is presented in Fig. 1. We found the lexicon useful for describing the morphology of the materials. Freshly-prepared S1 is characterized as having clumped/massive agglomerates composed of sub-particles having a rounded/sub-rounded habit of semi-rounded grains arranged in irregular clumps that are somewhat rough and have a similar appearance as the solution-grown UO2(O2)·xH2O (see Table 2) (vide supra). The sub-particles in this image are >0.5 μm, but an image of α-U3O8 from a replicate synthesis showed the same morphology in sub-particles that were <0.5 μm [16]. Comparison of the images of the precursor UO2(O2)·xH2O with those of S1 indicated that morphology was maintained following calcination of the uranyl peroxide precipitate to the uranium oxide product. Further work should be carried out to determine if the particulate size of the precursor is carried over into the calcination product, as well.

Fig. 1
figure 1

SEM images of α-U3O8 (S1) at time 0 [16] (left) and after storage for 2 years under Conditions 1–4 (center and right)

Full size image
Table 2 Lexicon descriptions and chemical speciation of S1 at time 0 and after 2 years of storage under Conditions 1–4 [20, 22]
Full size table

Sub-samples of S1 were stored under Conditions 1 through 4 for 2 years after which time images were collected by SEM and characterized using the lexicon (See Table 2) [22]. The SEM images of the aged materials are given in Fig. 1. These images of material stored under lower relative humidity (Conditions 1 and 2) do not reveal apparent changes in morphology. Little change in chemical speciation of S1 was measured by p-XRD or EXAFS, either [2]. However, storage of S1 under higher relative humidity (Conditions 3 and 4) resulted in an apparent flattening of the habit to an eroded surface appearance (Condition 3) or eroded or dissolved surface appearance (Condition 4) (See Table 2). Likewise, the p-XRD patterns of S1 stored under high relative humidity (Conditions 3 and 4) showed the presence of both α-U3O8 and schoepite-like species (UO3·xH2O), likely due to oxidation and hydration of the initial material [2]. The larger size of the particles in S1 following storage under Condition 4 is likely due to significant absorption of water. These results are further supported by the EXAFS data which showed that S1 stored under Condition 4 exhibited the greatest degree of disorder in the local structure compared to material stored under the other conditions (Conditions 1–3.)

Legacy samples

Two legacy samples retrieved from LANL stock were selected for this study because these materials contained chemical impurities that potentially could be incorporated in uranium oxide materials prepared in nuclear processes. For example, the p-XRD pattern of L1 confirmed the presence of α-U3O8 and a measurable amount of [UO2F2(H2O)](H2O)0.571, which most likely was starting material that prevailed through the precipitation of ammonium diuranate and subsequent calcination to α-U3O8 [2]. The p-XRD pattern for L2 indicated the presence of α-U3O8 and a small quantity of schoepite phases, likely due to oxidation and hydration before the sample was retrieved from storage for study.

The SEM images of L1 and L2 at time 0 are given in Fig. 2, and the descriptors for L1 and L2 are provided in Tables 3 and 4, respectively. The SEM image of L1 reveals clumped/massive agglomerates with a rounded/sub-rounded mineral habit of granular, semi-rounded grains arranged in irregular clumps composed of particles of up to 5 μm diameter, but in comparison with S1, the particle surfaces are smooth with surface decorations of fines that are <0.5 μm diameter. Although these morphologic differences could be dependent upon chemical impurities, it is also possible that they are a result of the morphology of the precursor complex. No images of the precursor complexes were available for comparison, and so conclusive attributions cannot be assigned.

Fig. 2
figure 2

SEM images of L1 (α-U3O8 and [UO2F2(H2O)](H2O)0.571) (left) and L2 (α-U3O8 and UO3·xH2O) (right) at time 0

Full size image
Table 3 Lexicon descriptions and chemical speciation of L1 (α-U3O8 and [UO2F2(H2O)](H2O)0.571) at time 0 and after 2 years of storage under Conditions 1, 2 and 4
Full size table
Table 4 Lexicon descriptions and chemical speciation of L2 (α-U3O8 and UO3·xH2O) at time 0 and after 2 years of storage under Conditions 1–4
Full size table

The descriptor for the SEM image of L2 is different from that for S1. The SEM image of L2 displayed clumped/massive agglomerates with irregular clumps of particles <0.5 μm diameter.

The images of L1 post-storage reveal changes in morphology (See Fig. 3) and chemical speciation. Following storage for 2 years under the low relative humidity Condition 1, the morphology took on clumped/massive sub-particles in a bimodal distribution of rounded/sub-rounded habit of granular, semi-rounded grains of irregular and smooth particles. The material stored under Condition 2 appears to be a conglomerate of sub-rounded clumps with a somewhat rough surface. A sample of L1 was not stored under Condition 3. The most visible change was in the images of material stored under Condition 4, which is best described by a different description path through the lexicon [20]. The morphology appears to be a complex conglomerate that was a mixture of sub-angular/angular and other particles. One portion of the particles looks to have high sphericity flattened and blocky irregular plates, with a smooth surface, while the other portion consists of subhedral and bladed plates with a smooth surface. The initial uranyl fluoride impurity was not measurable by p-XRD analysis after 2 years, perhaps through decomposition to another chemical species or due to masking by more predominant lines.

Fig. 3
figure 3

SEM images of L1 (α-U3O8 and [UO2F2 (H2O)](H2O)0.571) after storage for 2 years under Conditions 1, 2, and 4

Full size image

Images of the L2 material after storage for 2 years under Conditions 1–4 are given in Fig. 4. Powder-XRD analysis of these materials all show the presence of α-U3O8 and UO3·xH2O phases, likely inter-grown metaschoepite and schoepite patterns that cannot be resolved [19]. It is not possible to determine if either the schoepite phases are redistributing between different schoepite phases without changing the ratio of UO3:H2O due to dissolution and precipitation during storage over this time, or progressing from one schoepite phase to another. Material stored under the lower temperature conditions (1 or 3) appears to be clumped/massive conglomerates of sub-rounded and irregular clumps that included fines and scale decorations (Condition 1) or a dissolved surface appearance (Condition 3). The material stored under Condition 2 is a massive agglomerate of irregular clumps with a somewhat rough surface. In contrast, the image of material stored under Condition 4 exhibits a massive conglomerate of irregular clumps with a somewhat rough surface containing pored surface indentations. The formation of this texture could have formed from rapid absorption followed by desorption of water [1, 2].

Fig. 4
figure 4

SEM images of L2 (α-U3O8 and UO3·xH2O) after storage for 2 years under Conditions 1–4

Full size image

Image of S1, L1, and L2 reveal that changes in the morphology were particularly notable following storage under higher relative humidity and temperature (Condition 4). Although the same conditions were employed for storage of S1, L1, and L2 over 2 years, however, the changes in morphology of materials with impurities (L1 and L2) were more obvious than the changes in the morphology of phase-pure S1. It is possible that chemical impurities present in L1 and L2 at time 0 were responsible for the more significant changes in morphology. Research into uranium oxide materials has suggested that relatively minor shifts in localized environment affect the growth of crystals [23]. It also has been shown that doping more oxidized uranium oxides, such as U3O8, into UO2 powder influences grain size and packing during sintering [24, 25]. While not actinide systems, other studies of transition species in the Cd-I-Cu system have demonstrated that the impurities in copper compounds promote preferential growth directionality along surface planes, consistent with our results here, while other work has revealed the effect of chemical impurities on nucleation and growth of ettringite in a concrete environment. [26, 27]. Alternatively, materials L1 and L2 had already begun oxidation and hydration before they were removed from storage, which may have induced a more rapid change over this 2 year time period under high relative humidity.

Conclusions

An understanding of the potential for changes in morphology in uranium oxide materials over time and correlation of temporal changes in morphology with chemical speciation would be valuable for understanding the history of unknown analogues and/or evaluating morphology. Previous work to characterize U3O8 has shown that chemical signatures incorporated from a nuclear fuel process or produced following storage under controlled temperature and relative humidity over time may be measurable by p-XRD analysis. Additional characterization using p-XRD analysis and EXAFS spectroscopy has given a better understanding of the extent of oxidation and hydration in U3O8 following storage. Here, we collected images from the phase-pure material and two of the phase-impure materials from the previous study. The three materials were stored under different conditions of temperature and relative humidity over 2 years. The images of the phase-pure material showed little or no changes in morphology following storage under low relative humidity. While images of all materials revealed greater changes in morphology following storage under the higher relative humidity conditions, phase-impure materials yielded more discernable changes in mineral habit after storage for 2 years. These results suggest changes in surface area of the materials, which could provide another forensic signature of material storage history. Although these results do not elucidate these processes, these results suggest the need to conduct further studies to evaluate potential changes in morphology and particle sizes along with chemical speciation using uranium oxide materials with known quantities of impurities.