# Precipitation of morphology-controlled uranium(VI) peroxide in nitric acid media

Lucas Muller, Paul Estevenon, Christelle Tamain, Nicolas Dacheux, Nicolas Clavier

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Lucas Muller, Paul Estevenon, Christelle Tamain, Nicolas Dacheux, Nicolas Clavier. Precipitation of morphology-controlled uranium(VI) peroxide in nitric acid media. Dalton Transactions, 2025, 54, pp.6847-6857. 10.1039/D4DT03467H. hal-05024010

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# Precipitation of morphology-controlled uranium(VI) peroxide in nitric acid media

Lucas Muller<sup>a,b</sup>, Paul Estevenon<sup>*a</sup>, Christelle Tamain<sup>a</sup>, Nicolas Dacheux<sup>b</sup>, Nicolas Clavier<sup>b</sup>

Corresponding author: Paul Estevenon (paul.estevenon@cea.fr)

<figure><img src="image_2.png" /></figure>

This study demonstrates a clear correlation between the initial saturation index and the final morphology of U(VI) peroxide precipitates, highlighting the potential to control particle characteristics through this parameter.

<footnote><sup>a</sup> CEA, DES, ISEC, DMRC, Univ Montpellier, Marcoule, France</footnote>

<footnote><sup>b</sup> ICSM, Univ Montpellier, CEA, CNRS, ENSCM, Marcoule, France</footnote>
Abstract

The precipitation of U(VI) peroxide (UO₂(O₂)·4H₂O, known as studtite) has been extensively studied in mildly acidic media (i.e. pH > 1). However, few studies have investigated the influence of highly acidic conditions (i.e. pH < 1) on the precipitation of studtite, particularly regarding the morphology of the final powder. Therefore, the influence of high acidity (0.1 M < C(H⁺)ini < 2.0 M) and C(H₂O₂)ini/C(U)ini molar ratio (ranging from 10 to 70) on the precipitation kinetics, as well as the morphological and crystallographic properties of U(VI) peroxide was investigated in this study. Decreasing the acidity and increasing C(H₂O₂)ini/C(U)ini molar ratio have significantly enhanced both the final precipitation yield and the reaction kinetics. Furthermore, a correlation was found between the initial saturation index of the reaction mixture and the final morphology and crystallite size of the precipitated U(VI) peroxide, independent of the initial precipitation conditions. These results demonstrate that in highly acidic nitric media, it is possible to control the morphology of the precipitated U(VI) peroxide by simply adjusting the initial saturation index. This correlation opens up interesting perspectives for the synthesis of powders with specific morphologies, in particular uranium peroxide or corresponding oxide powders obtained after thermal treatment. This morphology control is of great interest in the field of nuclear fuel fabrication as the characteristics of oxide powder, such as flowability or sintering reactivity, have a strong impact on the manufacturing process and the properties of the final ceramic.

Keywords: Uranium, peroxide, precipitation, morphology, saturation index

1. Introduction

Uranium (VI) peroxide tetrahydrate, UO₂(O₂)(H₂O)₂·2H₂O, commonly called "studtite", from the name of the natural mineral form of this compound [1], is ubiquitous in the industrial uranium chemical treatment process. In the front-end of the nuclear fuel cycle, particularly during uranium extraction process, uranium (VI) peroxide can be precipitated and then calcined at low temperature to recover uranium from the in-situ leaching of uranium ore [2,3]: it is usually precipitated in sulfuric media at low acidity (pH 3-4) [4]. Peroxide precipitation can also be a purification step prior to isotopic enrichment [5]. Downstream, in advanced reprocessing of spent nuclear fuel, uranium can be recovered by peroxide precipitation [6]. In addition, in the context of the direct long term repository of spent nuclear fuel, the interactions between dissolved uranium and H₂O₂ produced by water radiolysis [7] likely lead to the formation of UO₂(O₂)·4H₂O that can passivate the dissolving surface [8–12].

There have been many studies on the precipitation of uranium (VI) peroxide. Some phenomenological ones have demonstrated quantitative precipitation of studtite at low acidity (pH 2-3) [13,14] then its conversion to pure U₃O₈ after calcination in air [15]. Several types of contactors have also been studied to precipitate the uranium (VI) peroxide like in continuous reactor [16] or fluidized-bed [17]. Nevertheless, only few studies dealt with a fundamental approach. As a matter of example, Planteur *et al.* have addressed the crystal growth of uranium (VI) peroxide at pH = 3 in sulfuric acid [18,19] while Kim *et al.* [20] and Schwerdt *et al.* [21] have established a relation between the particle size and initial conditions of precipitation.

Only two papers have measured precipitation yields in acidic media. Kornikov *et al.* [22] assessed the precipitation yield after 24 hours in 1 to 3 M HNO₃ solutions and for different peroxide ion concentrations. The higher the acidity, the lower the yield was for a given initial peroxide concentration. Hibert *et al.* studied the precipitation of U(VI) and demonstrated the co-precipitation of U(VI) and Pu(IV) (potential production of MOx fuel directly from U-Pu flow) working in 0.5 M HNO₃ [23].
Still, there is very little data on the morphology adopted by U(VI) peroxide, especially in very acidic media (0.1 to 2 M) despite the fact that UO₂(O₂)·4H₂O precipitation is a way of conversion and fabrication of nuclear fuel [23] with preservation of the peroxide morphology during the calcination step. With this objective, the shape of the particles is of great importance as it is the main parameter that will impact the powder flowability [24] and sinterability [25].

In this work, the precipitation of uranium (VI) peroxide has been studied in nitric acid concentration between 0.1 M and 2.0 M. To this end, several precipitation reactions were performed with a fixed uranium concentration (0.07 M) over a wide acidity range going from 0.1 to 2.0 M H⁺ and molar ratios of C(H₂O₂)ₗᵢᵢ/√C(U)ₗᵢᵢ ranging from 10 to 70. The precipitated U(VI) peroxide was characterized by X-ray diffraction and observed using scanning electron microscopy. The evolution of the precipitation yield was also measured under specific conditions.

## 2. Material and methods

### 2.1. Precipitation of Uranium (VI) peroxide

*Caution!* ²³⁸U is an α-emitter and is considered as a health risk. Experiments involving actinides require appropriate facilities and persons trained in the handling of radioactive materials.

Commercial powdered uranyl nitrate, UO₂(NO₃)₂·6H₂O, (Prolabo purity 99%) was used as the uranium source for all the experiments. Acidity was fixed using concentrated analytical grade nitric acid solution at 70 wt.% from CARLO ERBA. The hydrogen peroxide used was an analytical grade commercial solution at 30 wt.%, supplied by CARLO ERBA (the solution was measured at C(H₂O₂) = 10.05 mol·L⁻¹ by potentiometric titration with Ce(IV)).

1.00 mL of uranium (VI) solution with an uranyl concentration of 0.46 M was prepared in nitric media by dissolving around 235 mg of uranyl nitrate hydrate in diluted nitric acid solution with an acidity ranging from 0.1 to 2.0 M. 5.80 mL of hydrogen peroxide solution was prepared by diluting concentrated H₂O₂ in nitric acid media with the same acidity than the uranium solution. The uranyl concentration of 0.46 M was arbitrarily set so that the final uranyl concentration after mixing these two solutions was 0.07 M. Concentrations of hydrogen peroxide were determined so that C(H₂O₂)ₗᵢᵢ/√C(U)ₗᵢᵢ molar ratio was between 10 and 70 (Table S1) after the mixing of the two solutions.

Uranyl peroxide precipitation was achieved by adding the uranyl solution dropwise to the corresponding hydrogen peroxide solution in 10 mL glass beakers with a diameter of 2.5 cm, with a 1.2 cm magnetic stirring rod at 500 rpm. The resulting mixture was stirred for a few hours, resulting in the formation of a yellow precipitate, with kinetics varying strongly upon chemical conditions considered. The progress of the reaction at the end of the experiment was calculated from the ratio of the effective yield to the theoretical yield at equilibrium. The reaction was arbitrarily considered to be at equilibrium when its progress reached 95%.

At the end of the precipitation, the suspension was collected and centrifuged at 4500 rpm (i.e. 3260 g) for 5 min in 15 mL tubes. The powder and the supernatant were then separated. A washing step was carried out twice with 2 mL of a 50:50 (vol:vol) water/ethanol mixture. The solid was isolated from the supernatant by centrifugation and finally dried overnight at room temperature.

It is worth noting that under the experimental conditions considered, although nitrous acid (HNO₂) was autocatalytically generated in the nitric medium [26] and can catalyze the decomposition of hydrogen peroxide, this reaction remains negligible for HNO₃ concentrations below 8 M [27]. In addition, since the reactions were carried out at room temperature, the decomposition rate of H₂O₂ was further limited. Additionally, the reaction durations are relatively short compared to the
characteristic decomposition kinetics of H₂O₂. Consequently, the amount of H₂O₂ added can be considered nearly constant throughout the experiment.

## 2.2. Determination of uranium concentrations

Uranium concentration measurements were performed using an ICP-OES (Inductively Coupled Plasma Optical Emission Spectroscopy) Spectro Arcos apparatus. λ = 367.007 nm, λ= 385.958 nm, λ= 263.553 nm and λ = 393.203 nm wavelengths were considered to determine the uranium concentration. Calibration was performed using a series of standard uranium solutions (0.5 to 20 ppm), prepared by dilution of a 1000 ppm standard solution supplied by SCP Science. Uranium concentration was also determined by Photon Electron Rejecting Alpha Liquid Scintillation (PERALS) considering the protocol developed by Dacheux et al. [28]. The extraction was performed by contacting 6 mL of analyzed aqueous solutions with 1.5 mL of Alphaex™ scintillating cocktail purchased from ORDELA Inc. (Oak Ridge, USA). Alphaex™ is an organic cocktail containing HDEHP (bis-(2-ethylhexyl)phosphoric acid) as extractant, PBBO (2-(4'-biphenylyl)-6-phenylbenzoxazole), PPO (2,5-diphenyloxazole) and NPO (2-(1-Naphthyl)-5-phenyloxazole) as scintillator, naphthalene as energy transfer agent and toluene as solvent. The mixture was shaken for approximately 2-3 minutes and then centrifuged at 2000 rpm (i.e. 643 g) for 5 minutes. 1.3 mL of the organic phase was then collected and the ²³⁸U + ²³⁴U activity was measured, leading to uranium concentration in the leachate.

## 2.3. Characterization of the prepared solids

Solids were analyzed using a Bruker D8 diffractometer equipped with a Lynxeye detector adopting the reflection geometry with Göbel mirror and using Cu Kα₁,₂ radiation (λKα₁ = 1.540593 Å ; λKα₂ = 1.544427 Å). For each acquisition, PXRD (Powder X-Ray Diffraction) pattern was recorded on the 10° < 2θ < 60° range with a step size of 0.027° (2θ) and a counting time of 1 s·step⁻¹. Powders were prepared on flat sample holders with top-loading, placed in a dome-shape specimen holder aiming to avoid radioactive contamination (Bruker airtight specimen holder with knife edge, acting as an anti-scattering blade). Note that this PMMA (Poly(Methyl MethAcrylate))dome can be the source of a large diffusion peak in the range of 10° < 2θ < 20°. Le Bail refinements [29] of these patterns were performed using the Fullprof_suite program [30] with Cox-Hastings pseudo-Voigt profile function [31]. The instrumental function was obtained by refinement of a LaB₆ reference. The backgrounds were manually selected.

For SEM characterization, the samples were deposited on a carbon adhesive tape and measured directly without any metal coating step. A FEI Quanta 200 scanning electron microscope, equipped with an Everhart-Thornley detector (ETD) and a backscattered electron detector (BSED), was used to record images at an acceleration voltage of 15 kV under high-vacuum conditions (10⁻⁶ Pa).

Specific surface area (SSA) were measured on a Micrometric ASAP 2020 apparatus with BET method (nitrogen adsorption at 77 K).

## 2.4. Determination of precipitation yield, saturation index and progress of the reaction

The precipitation yield at time t was calculated from the initial and final uranium concentrations in the supernatant, by using Eq. 1. The final uranium concentrations were measured either by ICP-OES or PERALS (depending of experimental availability), after dilution by 100 to avoid
any precipitation in the analyzed tube. The initial uranium concentration was calculated from the mass of uranyl nitrate added.

$$Eq. 1 \quad Yield_t = \frac{[UO_2^{2+}]_{ini} - [UO_2^{2+}]_t}{[UO_2^{2+}]_{ini}} \%$$

The initial Saturation Index (SI ini) was calculated from the U(VI) peroxide precipitation reaction (Eq. 2) according to Eq. 3, i.e. from the logarithm of the ratio between the reaction quotient out of equilibrium, Q ini (Eq. 4) and the solubility constant, K (Eq. 5). The saturation index is useful here, as it expresses the driving force of the precipitation reaction. Indeed, when SI > 0, the solution is supersaturated with respect to the precipitated phase, whereas it remains undersaturated for SI < 0. In the latter case, no phase precipitation is expected in the solution. For the solubility constant of studtite (Eq. 5), the recommendations of the Nuclear Energy Agency were followed [32] and the value from Kubatko et al. [33] was selected, i.e. log(K) = -2.87.

$$Eq. 2 \quad UO_2(O_2) \cdot 4H_2O_{(s)} + 2H^+_{(aq)} \rightarrow UO_2^{2+}_{(aq)} + H_2O_{(aq)} + 4H_2O_{(aq)}$$

$$Eq. 3 \quad SI_{ini} = \log \left( \frac{Q_{ini}}{KS} \right)$$

$$Eq. 4 \quad Q_{ini} = \frac{a_{H_2O_2}^{ini} \times a_{UO_2^{2+}}^{ini}}{(a_{H^+}^{ini})^2}$$

$$Eq. 5 \quad K = \frac{a_{H_2O_2}^{eq} \times a_{UO_2^{2+}}^{eq}}{(a_{H^+}^{eq})^2}$$

To simplify the calculations, the reaction progress has been calculated by taking into account only the U(VI) peroxide precipitation reaction (Eq. 2). The first step is to calculate the reaction quotient Q t for a time t.

$$Eq. 6 \quad Q_t = \frac{a_{H_2O_2}^t \times a_{UO_2^{2+}}^t}{(a_{H^+}^t)^2}$$

Taking into account the initial reactant concentration and the progress ε, Q t can be expressed as follows:

$$Eq. 7 \quad Q_t = \frac{(C(U)_0 - \varepsilon) \times (C(H_2O_2)_0 - \varepsilon)}{(C(H^+)_0 + 2\varepsilon)^2}$$

The reaction reaches equilibrium when Q t equals K. The progress of the reaction is thus:

$$Eq. 8 \quad \varepsilon = \frac{(C(U)_0 + C(H_2O_2)_0 - 4 \times K \times C(H^+)_0) - \sqrt{(C(U)_0 + C(H_2O_2)_0 - 4 \times K \times C(H^+)_0)^2 - (4 - 16 \times K) \times (C(U)_0 \times C(H_2O_2)_0 - K \times C(H^+)_0)^2}}{(2 - 8 \times K)}$$

It is then possible to calculate the theoretical yield of precipitation which would correspond to:

$$Eq. 9 \quad Yield_{theo}^{eq} = \frac{\varepsilon}{C(U)_0} \%$$

And then to access the rate of progress of the reaction by comparison with the final experimental yield:

$$Eq. 10 \quad \tau = \frac{Yield_{exp}^{final}}{Yield_{theo}^{eq}} \%$$
<figure><img src="image_3.png" /></figure>
# 3. Results

## 3.1. Kinetic study associated to the precipitation of studtite

Two sets of precipitation experiments have been carried out. For the first one, the initial nitric acid concentration was varied from 0.1 to 2.0 M and the C(H₂O₂)ₗₐₗₐ/C(U)ₗₐₗₐ molar ratio was set to 10 (Fig 1a). On the contrary, for the second set, C(H₂O₂)ₗₐₗₐ/C(U)ₗₐₗₐ was varied from 10 to 30 while the initial nitric acid concentration was set to 0.5 M. For both sets, the evolution of the precipitation yield (Eq. 1) was monitored (Fig 1b) since it is clear from Eq. 2 that both the initial concentrations of nitric acid and peroxide have an effect on the precipitation of UO₂(O₂)·4H₂O.

Simulations were performed using the Phreeqc software and the Thermochimie v12 database [34] to determine the uranium speciation under these conditions (Fig SI 1 and Table SI 3). These simulations exclusively relied on thermodynamic constants validated by the latest NEA (Nuclear Energy Agency) review [32]. In addition to the various species included in the Thermochimie database, H₂O₂, HO₂⁻, and the complexes [(UO₂)(O₂)(OH)]⁻ and [(UO₂)₂(O₂)₂(OH)]⁻ were added.

However, these simulations do not match the experimental data, as the precipitation of U(VI) peroxide is significantly overestimated. This discrepancy is most likely due to the omission of higher-order uranyl nitrate complexes, for which no constants have been validated by the NEA, except for the mono-nitrate species.

Nevertheless, it remains possible to qualitatively estimate that the only expected solid phase under these conditions is U(VI) peroxide, while the predominant species in solution are uranyl nitrate and the U(VI) hydroxide complex. Regarding peroxo complexes, they only form under highly basic conditions and are therefore not expected in our system.

For both sets of experiments, a plateau seems to be reached more or less quickly depending on the initial conditions. The maximum precipitation yield is different for each precipitation and depends on the acidity or the C(H₂O₂)ₗₐₗₐ/C(U)ₗₐₗₐ molar ratio. The higher the initial acidity, the lower the plateau value. From the mass action law, it is clear that the precipitation reaction is favored by increasing the initial uranium or peroxide concentrations and disfavored by increasing the initial acidity. This explains why the plateau reached at the end of the precipitation reaction increases as the initial nitric acid concentration decreases (Fig 1a) and as the C(H₂O₂)ₗₐₗₐ/C(U)ₗₐₗₐ molar ratio increases (Fig 1b), which agrees well the Le Chatelier's principle. Kornikov et al. obtained similar results for similar initial acidities of 0.5 and 1.0 M, molar ratio C(H₂O₂)ₗₐₗₐ/C(U)ₗₐₗₐ of ten, and initial uranium concentration of 0.1 M after 24 hours. In 2.0 M H⁺, a yield of 90% was measured after 24h while in this work it did not exceed 60% after 7 hours, without reaching equilibrium. This means that the precipitation in 2.0 M H⁺ has not reached an equilibrium after 7 hours.

In terms of kinetics, the precipitation rate can be written considering Eq. 11 where k₁ and k₂ represent the straight forward and the indirect kinetic constants, respectively. α, β, and γ stand for the partial orders related to uranium, hydrogen peroxide and proton concentrations respectively, and s is the surface of the particles. From this equation, it can be confirmed that the precipitation rate increases with the initial uranium or peroxide concentration (i.e C(H₂O₂)ₗₐₗₐ/C(U)ₗₐₗₐ molar ratio for a given uranium concentration), (Fig 1b), and decreases when increasing the initial proton concentration (Fig 1a).

$$Eq. 11 \quad v = \frac{d[U]}{dt} = k_1 \times [U]^\alpha \times [H_2O_2]^\beta - k_{-1} \times f(s) \times [H^+]^{2\gamma}$$
<figure><img src="image_4.png" /></figure>

Fig 1 : Evolution of uranium (VI) precipitation yield (a) for various initial nitric acid concentrations (0.1 M in black squares, 0.5 M in red circles, 1.0 M in blue up triangles, 2.0 M in green down triangles) and considering C(H₂O₂)ₗᵢᵣ/С(U)ₗᵢᵣ = 10 and C(U)ₗᵢᵣ = 0.07 M and (b) for various initial C(H₂O₂)ₗᵢᵣ/С(U)ₗᵢᵣ molar ratios (10 in red circles, 20 in brown diamonds and 30 in purple hexagons) and with C(H⁺)ₗᵢᵣ = 0.5 M and C(U)ₗᵢᵣ = 0.07 M. Dashed line is a visual help and does not represent any mathematical fit.

3.2. Morphological characterization of the studtite precipitates

The influence of the initial nitric acid concentration (i.e. 0.1, 0.5, 1.0 and 2.0 M) and the
C(H₂O₂)ₗᵢᵣ/C(U)ₗᵢᵣ molar ratio (i.e. 10, 20, 40, and 68) on the powder morphology was also studied.
SEM images are presented in Fig 2. It is clear that larger particles are formed when increasing the
initial nitric acid concentration for a given C(H₂O₂)ₗᵢᵣ/C(U)ₗᵢᵣ molar ratio. A similar effect is also
observed when decreasing C(H₂O₂)ₗᵢᵣ/C(U)ₗᵢᵣ molar ratio for a given initial nitric acid concentration.

As it has already been described for other systems, the morphology of the precipitate can be
also correlated with saturation conditions in the solution [35,36]. For each sample, the initial
saturation index associated to the corresponding precipitation conditions was calculated. A
correlation is clearly evidenced, as the particle size decreases when increasing the starting SI value.
Furthermore, three different types of particle morphologies was observed depending on the initial SI
values. For an initial SI<sub>ini</sub> < 2.9, the powder is composed of needle-like crystals of less than 1 µm wide
and few micrometers long, which become longer for lower SI<sub>ini</sub> (around 10 µm for SI = 0.9). These
needles are agglomerated to form pseudo-spheres for 2.4 < SI<sub>ini</sub> < 2.9 (from few micrometers for SI =
2.4 and less than 1 µm for SI = 2.9). It should be noted that, to our knowledge, these particular
shapes have never been observed for UO<sub>2</sub>(O<sub>2</sub>)·4H<sub>2</sub>O. Nanoparticles are obtained for SI<sub>ini</sub> > 2.9.
<figure><img src="image_5.png" /><figcaption>Fig 2: SEM pictures of uranium (VI) peroxide powders obtained for various initial C(H₂O₂)ₗᵢᵣ/С(У)ₗᵢᵣ molar ratios and various H⁺ concentrations and for C(У)ₗᵢᵣ = 0.07M. Initial saturation index are in yellow. Framed pictures in matching colors correspond to two samples prepared with different initials conditions but with an identical initial saturation index. *Reaction was stopped before reaching 95% of progress.</figcaption></figure>

The influence of saturation index on the morphology can be explained by the impact of saturation on the nucleation phenomenon. The higher the saturation index, the more primary nucleation is promoted and the less uranium in solution is available for crystal growth. Consequently, as S<sub>l ini</sub> increases, the crystal size formed at equilibrium decreases.

With respect to the spherical agglomerates, the morphology of the particles was followed during precipitation with C(H⁺)ₗᵢᵣ = 0.5 M and C(H₂O₂)ₗᵢᵣ/C(U)ₗᵢᵣ = 20 by regularly sampling the powder throughout the process (Fig 3). SEM images show that at the very beginning of the reaction, when only about 20% of the uranium has precipitated, needles of 1 – 2 µm are formed. They quickly agglomerate into a parallelepiped shape with nearly aligned needles. The needles and agglomerates stop growing when the yield exceeds 80%. This could mean that the number of agglomerated particles is likely to increase.
<figure><img src="image_6.png" /><figcaption>Fig 3 : SEM micrographs of uranium (VI) peroxide obtained during precipitation for C(H⁺)ini = 0.5 M, C(H₂O₂)ini/C(U)ini = 20 and C(U)ini = 0.07 M. Dashed line is a visual help and does not represent any mathematical fit.</figcaption></figure>

3.3. Structural characterization of the studtite precipitates

PXRD patterns were recorded for all the samples prepared. All powders correspond to studtite, UO₂(O)₂·4H₂O (monoclinic, space group C2/c [37]). For C(H₂O₂)ini/C(U)ini = 10, UO₂(O)₂·4H₂O precipitates obtained at different acidities are compared in Fig 4a. While there is no obvious difference between 0.5, 1.0 and 2.0 M H⁺, the lower acidity studied (0.1M) clearly resulted in much larger diffraction peaks. Similarly, the effect of the C(H₂O₂)ini/C(U)ini molar ratio was investigated in 0.5 M H⁺ (Fig 4b). Here, the studtite synthesized for a molar ratio of ten has sharper peaks than the other samples.
<figure><img src="image_7.png" /><figcaption>Fig 4 : PXRD patterns of studtite precipitates obtained for various H⁺ concentrations (0.1 M in black, 0.5 M in red, 1.0 M in blue, 2.0 M in green), with C(H₂O₂)ₗᵢₗ/C(U)ₗᵢₗ = 10 and C(U)ₗᵢₗ = 0.07 M (a) and for different initial C(H₂O₂)ₗᵢₗ/C(U)ₗᵢₗ molar ratios (10 in red, 20 in brown and 30 in purple) with C(H⁺)ₗᵢₗ = 0.5 M and C(U)ₗᵢₗ = 0.07 M (b) ; PXRD lines in red correspond to uranium (VI) peroxide from Burns et al. [37].</figcaption></figure>

The difference in PXRD line width reflects a difference in crystallite size. Crystallite size and unit cell volume were determined by Le Bail refinement from the PXRD patterns (Fig 5). The data confirm that the crystallite size increases as the initial acidity increases. This crystallite size also decreases as the C(H₂O₂)ₗᵢₗ/C(U)ₗᵢₗ molar ratio increases.

Except for the uranium (VI) peroxide synthesized at 0.1 M H+, no changes in the volume of the unit cell (681 Å³) are observed. The volume of 684 Å³ observed for the uranium (VI) peroxide synthesized in 0.1 M H+ can be explained by nanocrystalline particles. This lattice parameter expansion for nanocrystallites is regularly reported [38,39] and can be attributed to (i) the accumulation of sorbed species on the nanoparticle surfaces which has a tensile effect on the crystal lattice [40], and/or (ii) the accumulation of defects such as vacancies at the boundaries of nanocrystals, which locally increase the lattice parameters [41].
<figure><img src="image_8.png" /><figcaption>Fig 5: Unit cell volume of uranium (VI) peroxide for different initial $H^+$ concentrations (a) and for different initial $C(H_2O_2)_{ini}/C(U)_{ini}$ molar ratios (b); Crystallite size of uranium (VI) peroxide for different initial $H^+$ concentrations (c) and for different initial $C(H_2O_2)_{ini}/C(U)_{ini}$ molar ratios (d).</figcaption></figure>

Le Bail refinements were also performed on the other precipitates (Table S1). From these data, the crystallite size and lattice parameters of the studtite crystals can then be expressed as a function of the $S_{I_{ini}}$ value (Fig 6). First of all, regarding the lattice parameters, *b* and *c* do not seem to vary with $S_{I_{ini}}$. On the contrary, a and $\beta$ increase with $S_{I_{ini}}$ above 2.2, i.e. for conditions where spherical agglomerates are obtained. A plateau seems to be reached for $S_{I_{ini}} > 2.7$, i.e. for conditions where nanoparticles are formed. The cell volume logically follows this tendency. The evolution of these parameters is probably related to the three morphologies obtained. For nanoparticles, the effect of the particle surface is isotropic and greater than for the other two morphologies. On the contrary, for the needle-like morphologies, the effect of the surface on the lattice parameter is more likely to be anisotropic. This could then explain the variation of *a* and $\beta$ as a function of particle morphology.

Finally, there is a good correlation between the $S_{I_{ini}}$ value and crystallite size. Indeed, as the $S_{I_{ini}}$ increases, the crystallite size decreases from about 150 – 200 nm to about 10 – 50 nm. For $S_{I_{ini}} < 2.7$ which corresponds to the formation of studtite needle-like crystals, the crystallite sizes are about 150 nm which is smaller than the size of the crystals observed in SEM images for these $S_{I_{ini}}$ values (Fig 2). This implies that the needle crystals are composed of multiple crystallites. On the contrary, it is likely that the crystallite sizes obtained for $S_{I_{ini}} < 2.7$ correspond to the nanoparticles observed in the SEM images.
<figure><img src="image_9.png" /></figure>

Fig 6: Lattice parameters (a, b, c and β), unit cell volume and crystallite size of studtite crystals as a function of initial saturation index. Grey points are U(VI) peroxide sample from precipitation which have not reached equilibrium. Refinement results are available in Table SI 2.
## 4. Discussion

In the context of the high importance of powder morphology for fuel fabrication, saturation index seems to be the key parameter controlling the morphology and the microstructure of the precipitate. The higher the $Si_{ini}$, the smaller the crystallite and the crystal size of the agglomerates. These observations suggest that neither the initial concentration of nitric acid nor that of peroxide ions, considered individually and for a same initial $Si$, influence the morphology of the precipitate. Instead, the morphology appears to be primarily influenced by the value of the saturation index in the solution. Indeed, it appears to be identical for a given saturation index value (images within frames of matching colors in Fig 2), even though the initial acidity and peroxide concentration are different. The relative lack of effect of the initial acidity and $C(H_2O_2)_{ini}/C(U)_{ini}$ ratio on the particle characteristics compared to the saturation index is somewhat surprising, since the kinetics of precipitation would be expected to be unlike for different initial chemical conditions. Therefore, the nucleation processes should be different and thus the final powder morphology should be distinct. Two additional precipitation experiments with kinetic monitoring were performed with the same initial saturation index (i.e. 1.7) but different initial reactant concentrations (i.e. $C(U)_{ini}$ = 0.07M, $C(H^+)_{ini}$ = 1.0 M and 2.0 M, $C(H_2O_2)_{ini}/C(U)_{ini}$ = 14.7 and 59.4) to confirm the predominant influence of the initial saturation on the final morphology (Fig 7).

First, the morphologies at the end of the precipitation are indeed identical (Fig 7). However the evolution of the yield does not follow the same curve although, the plateau reached by the two precipitations is almost the same. For the precipitation in 1.0 M $H^+$ (Fig 7, red squares), a first plateau seems to be formed at the beginning of the experiment, unlike the other conditions which precipitate faster. This difference is probably explained by a different evolution of the saturation index during precipitation. The precipitation in 2.0 M $H^+$, with more peroxide, and no plateau at the beginning, presents a higher saturation index that is maintained in the reacting system during the precipitation compared to the first experiment. This agreement of the morphologies but not of the kinetics for the same $Si_{ini}$ could be explained by the fact that the evolution of these two precipitations is not sufficiently different to form different morphologies or that the difference between the particles is not visible at the SEM scale. Nevertheless, it seems that in very acidic media, particle morphologies are controlled only by supersaturation. A wide variety of particle morphologies from nanoparticles to needles or spherical agglomerates can then be synthesized on demand.
<figure><img src="image_10.png" /><figcaption>Fig 7: Uranium (VI) precipitation yield evolution and morphologies for different initial reactant concentrations ($C(U)_{ini} = 0.07$ M, $C(H^{+})_{ini} = 1.0$ M, $C(H_{2}O_{2})_{ini}/C(U)_{ini} = 14.7$ in red squares and $C(U)_{ini} = 0.07$ M, $C(H^{+})_{ini} = 2.0$ M, $C(H_{2}O_{2})_{ini}/C(U)_{ini} = 59.4$ in blue circles) but same initial supersaturation index (1.7). Dashed line are a visual help and do not represent real values. The scale bars on SEM images represent 1 µm.</figcaption></figure>

Finally, given that thermal treatment preserves the morphology of U(VI) peroxides [21], oxides with corresponding morphologies can be produced. However, it remains difficult to determine whether a specific morphology is optimal for nuclear fuel fabrication. As mentioned above, the two key parameters to consider are powder flowability and sinterability.

In terms of flowability, and based on the correlations established by Arianer et al. [24], the most influential morphological factor appears to be the particle size distribution for particle size below 75 µm. While spherical morphologies (such as those observed for $SI_{ini}$ values between 2.4 and 2.9) are generally expected to improve flowability, this advantage is offset by the strong cohesion induced by small particle sizes.

From a reactivity standpoint, which directly influences sinterability, the nanometric powder obtained at $SI_{ini}$ values above 2.9 appears to be the most favorable. Specifically, the specific surface area (SSA) of four representative morphologies from this study (Table 1) was determined via the BET method, revealing that the nanometric powder synthesized at $SI_{ini} = 3.5$ exhibits the highest SSA. In general, a higher specific surface area translates into lower required pressing pressure and sintering temperature, which ultimately leads to a higher final pellet density [42].

<table><caption>Table 1: Specific surface area (SSA) of powders obtained for different $SI_{ini}$.</caption><thead><tr><th>IS<sub>ini</sub></th><th>3.5</th><th>2.4</th><th>2.1</th><th>1.8</th></tr></thead><tbody><tr><td>C(H<sup>+</sup>)<sub>ini</sub> (mol·L<sup>-1</sup>)</td><td>0.1</td><td>0.5</td><td>1.0</td><td>2.0</td></tr><tr><td>C(H<sub>2</sub>O<sub>2</sub>)<sub>ini</sub>/C(U)<sub>ini</sub></td><td>10</td><td>20</td><td>40</td><td>68</td></tr><tr><td>SSA (m<sup>2</sup>·g<sup>-1</sup>)</td><td>21 ± 1</td><td>7 ± 1</td><td>5 ± 1</td><td>5 ± 1</td></tr></tbody></table>
**5. Conclusion**

The effect of initial chemical conditions on the precipitation of studtite in acidic media has been studied. Initial acidity and peroxide concentration (expressed as the molar ratio of initial peroxide concentration to uranium concentration at constant initial C(U)) both affect the kinetics of precipitation. Increasing acidity and decreasing peroxide concentration both slow down the precipitation of studtite.

Secondly, a correlation was found between the initial saturation index and the final morphology and crystallite size of the powder. The higher the $S_{Iini}$ value, the smaller the particles and the smaller the crystallite size. Conversely, low $S_{Iini}$ values promote the formation of larger crystals with larger crystallite sizes. An intermediate morphology consisting of spherical needle clusters is also observed for $2.4 < S_{Iini} < 2.9$. This correlation is explained by the fact that at high saturation index the nucleation phenomenon is favored to the detriment of crystal growth, which increases the formation of small particles. At low values of $S_{Iini}$, larger crystals are formed because the crystal growth phenomenon is promoted. In the conditions studied, the saturation conditions seem to be the only parameter controlling the morphology. In fact, for the same $S_{Iini}$ value, but different initial concentrations of the reactants, the final morphologies were found to be similar. Although the obtained morphologies cannot be considered as optimal for fuel fabrication at this stage, these results show that the morphology of U(VI) peroxide powder precipitated from an acidic uranyl solution can be controlled by adding the appropriate amount of peroxide ions, corresponding to the correct initial saturation index.
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# AUTHOR INFORMATION

## Corresponding authors

*P. E.: phone. +33 4 66 79 65 66; e-mail: paul.estevenon@cea.fr

## ORCID

Lucas Muller, Paul Estevenon, Christelle Tamain, Nicolas Dacheux, Nicolas Clavier

Lucas Muller: 0000-0002-5366-2337

Paul Estevenon: 0000-0001-8517-4744

Christelle Tamain: 0000-0002-3492-6536

Nicolas Dacheux: 0000-0003-1636-1313

Nicolas Clavier: 0000-0001-8010-2661

## Notes

The authors declare no competing financial interest.

## Acknowledgments

The authors are grateful to Stéphanie Szenknect for her help with Phreeqc simulations and Renaud Podor and Joseph Lautru for SEM micrographs. The authors would like to thank CEA for funding the PhD work of Lucas Muller.