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# Impact of firing temperature and atmosphere on the chemical reactivity of UO$_{2}$ + x powders in nitric acid

Thomas Barral, Laurent Claparede, Nicolas Dacheux

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Thomas Barral, Laurent Claparede, Nicolas Dacheux. Impact of firing temperature and atmosphere on the chemical reactivity of UO$_{2}$ + x powders in nitric acid. Hydrometallurgy, 2025, 238, pp.106569. 10.1016/j.hydromet.2025.106569. cea-05248763

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Submitted on 10 Sep 2025

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# Impact of firing temperature and atmosphere on the chemical reactivity of UO₂+x powders in nitric acid

Thomas Barral<sup>a,b</sup>, Laurent Claparede<sup>a</sup>, Nicolas Dacheux<sup>a,*</sup>

## ARTICLE INFO

Keywords:

Dissolution kinetics

Uranium oxide

Reprocessing of nuclear fuel

## ABSTRACT

The aim of this work is to study the dissolution kinetics of a series of powdered UO₂+x samples with different structural and microstructural properties. For this purpose, UO₂+x powders were prepared by hydroxide precipitation and then heat-treated at different temperatures under argon and reducing atmospheres. The calcined UO₂+x samples were first investigated ex-situ by several physicochemical techniques in order to highlight the dependence of the normalized dissolution rates on various parameters. The PXRD experiments showed the preservation of the fluorite structure under reducing atmosphere over the whole temperature range studied, while the formation of a U₃O₈ phase was highlighted under argon at T ≤ 1100 °C. The study of the dissolution of UO₂+x samples first highlighted the effect of increasing the calcination temperature (decrease of S<sub>SA</sub>), which significantly improves the chemical durability of the solids. The higher the calcination temperature, the lower the reactivity of the sample and the longer the time required to reach full dissolution. Secondly, the presence of a U₃O₈ fraction in some samples calcined under argon resulted in a higher normalized dissolution rate. For comparison, the normalized dissolution rate of a pure U₃O₈ sample reached R<sub>L</sub> = (5.6 ± 1.1) × 10⁻¹ g m⁻² d⁻¹, a higher value than that of UO₂+x, R<sub>L</sub> = 5.5 × 10⁻² g m⁻² d⁻¹ on average. Furthermore, these samples showed no change in kinetic regime during dissolution, which could be explained by the blocking by U₃O₈ of the transition to a kinetic dissolution regime autocatalyzed by nitrogen species.

## 1. Introduction

The dissolution of uranium-based spent nuclear fuels (SNF) is a critical step in the reprocessing of uranium and plutonium. Actinide reprocessing is therefore a major challenge and should help to reduce the long-term radiotoxicity of nuclear waste, increase resistance to plutonium proliferation and improve the sustainability of nuclear energy in the future by recycling fission products and minor actinides including neptunium, americium and curium (Salvatores et al., 1995; Tommasi et al., 1995; Lebreton et al., 2012). Currently, the usual conditions for SNF dissolution consist in hot and concentrated nitric acid (4 mol L⁻¹ HNO₃ and 90 °C) and lead to the full dissolution of UO₂ matrix. However, in order to be more energetically and economically efficient, this key step needs to be optimized and the parameters driving the dissolution kinetics need to be well understood. The SNF consist of about 96 wt% of uranium dioxide and due to its very high specific activity, pure UO₂ has been widely used in literature as a model compound to study the dissolution kinetics of SNF in nitric acid solutions (Fukasawa

and Ozawa, 1986; Fukasawa et al., 1991; Ikeda et al., 1995; Shabbir and Robins, 1968; Shabbir and Robins, 1969; Taylor et al., 1963; Cordara et al., 2017; Zhao and Chen, 2008; Lalleman et al., 2019; Bertolotto et al., 2020; Marc et al., 2017). Obviously, the composition and the microstructure of SNF is much more complex and heterogeneous than that of pure and single phase UO₂ (Kleykamp, 1985; Konings et al., 2015; Burns et al., 2012), but it was usually admitted that the dissolution of the UO₂ matrix mainly drives the overall dissolution of the spent UOₓ fuel.

The dissolution mechanism of UO₂ in nitric acid solutions is based on the oxidation of uranium (IV) to uranium (VI) at the solid/solution interface (Ikeda et al., 1995). According to the literature (Fukasawa et al., 1991; Ikeda et al., 1995; Shabbir and Robins, 1968; Shabbir and Robins, 1969; Taylor et al., 1963; Cordara et al., 2017), the most likely mechanism is initiated by a redox reaction between UO₂ and HNO₃ at the solid/solution interface following Eq. (1):

$$UO_2(s) + HNO_3(aq) + 2H^+ \rightarrow UO_2^{2+}(aq) + HNO_2(aq) + H_2O \quad (1)$$

<footnote><sup>a</sup> ICSM, Univ Montpellier, CNRS, CEA, ENSCM, Site de Marcoule, 30207 Bagnols sur Cèze, France</footnote>

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

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: nicolas.dacheux@umontpellier.fr (N. Dacheux).</footnote>
The rate of this reaction is considered to be slow (Ikeda et al., 1995). According to this reaction, the increase in the concentration of nitrous acid HNO₂ is proportional to the concentration of uranyl nitrate in solution (Fukasawa et al., 1991; Dalger et al., 2018; Dalger et al., 2021). A second reaction between UO₂ and HNO₂ is then assumed to be much faster than Eq. (1):

$$UO_2(s) + 2HNO_2(aq) + 2H^+ \rightarrow UO_2^{2+}(aq) + 2NO(aq) + 2H_2O \quad (2)$$

$$2NO(aq) + HNO_3(aq) + H_2O \rightleftharpoons 3HNO_2(aq) \quad (3)$$

In fact, nitrous acid first forms during the oxidation of UO₂ in nitric acid (due to the disproportionation of NO₂) and then oxidizes UO₂ in return. It can be formed again from Eqs. (2) and (3). Therefore, this reaction sequence can be considered as an autocatalytic mechanism. Dalger et al. (Dalger et al., 2018; Dalger et al., 2021) also reported that the effect of HNO₂ was noticeable only at high progresses of dissolution. This result suggested the probable existence of a threshold concentration of HNO₂, beyond which the acceleration of the dissolution rate was significant. At this stage of the dissolution, the reactive surface area developed by the solid was greatly increased, making it difficult to quantify the effect of HNO₂ on the normalized dissolution rate. Although the direct catalytic role of HNO₂ could still be questioned, it is undeniable that this species plays a key role in the rapid dissolution of UO₂ in nitric acid, since HNO₂ is in equilibrium with the catalytic species NO and NO₂ according to the Eq. (4):

$$2 HNO_2(aq) \rightleftharpoons NO_2(aq) + NO(aq) + H_2O \quad (4)$$

Most studies devoted to the dissolution of UO₂ have focused on highly concentrated nitric acid solutions and elevated temperatures (typically C₅HNO₃ > 4 mol L⁻¹ and T > 60 °C). These conditions systematically lead to the complete and rapid dissolution of UO₂ powders or pellets. Under these conditions, the dissolution mechanism quickly evolves into a catalyzed reaction that occurs at the liquid/solid interface in a triphasic system involving gaseous species (Marc et al., 2017; Dalger et al., 2021). Consequently, it is difficult to compare the kinetic data obtained from different experimental setups (static vs. dynamic conditions, with or without stirring, open vs closed vessels ...) and especially from UO₂ samples with various structural and microstructural characteristics. Thus, even if the dissolution rate is normalized by the reactive surface, the kinetics determined for UO₂ samples of different origins can be very different. Moreover, since the main factors influencing the dissolution rate have been found to evolve strongly during dissolution, the period over which the rate was determined is also of great importance, but has not always been described in published works.

However, few studies have shown the consequences of the physicochemical modifications induced by the sample history (synthesis route, heat treatment ...), on its dissolution kinetics. In fact, several parameters of the dissolved solid, such as its reactivity (related to the crystallization state and specific surface area), its purity (presence of secondary phases) or even the oxygen stoichiometry, could have a significant impact on the normalized dissolution rates of mixed oxide compounds (Hingant et al., 2009; Heisbourg et al., 2003; Heisbourg et al., 2004; Claparede et al., 2011a; Claparede et al., 2015; Claparede et al., 2011b). These parameters have been poorly investigated compared to the well-known effects of temperature or pH in the literature. For example, the oxygen over-stoichiometry of the oxide, indicated by the x value in UO₂+x and informing about an excess of oxygen compared to the ideal formula UO₂₀₀, has never been considered as an important parameter to study. Yet this slight excess in oxygen can lead to the partial oxidation of uranium (from U⁴⁺ to U⁵⁺ or U⁶⁺) to balance the charges, and thus modify certain physicochemical properties of the material, such as its reactivity.

Therefore, the aim of this work is to study the dissolution kinetics of a panel of powdered UO₂+x samples with different structural and microstructural properties. For this purpose, the powders are heat treated at different temperatures (700 °C < T < 1500 °C) and under two types of

atmospheres, reducing (Ar-4 %H₂) and argon.

## 2. Material and methods

### 2.1. Preparation of the samples and thermal conversion

The uranium dioxide UO₂+x sample was prepared by wet chemistry route, using the hydroxide precipitation already reported in the literature (Martinez et al., 2015; Clavier et al., 2017). In this method, a solution of uranium (IV) (Cₐ = 0.7 mol L⁻¹) in hydrochloric acid was obtained by dissolving U metal chips (kindly provided by CETAMA) in concentrated HCl, according to the protocol reported in the literature by Dacheux et al. (Dacheux et al., 1995). The uranium solution was then mixed with a large excess of ammonia (400 %) at room temperature with stirring, resulting in immediate precipitation. The precipitate was then allowed to stand for 30 min with stirring. The freshly prepared precipitate was washed by centrifugation at 12000 rpm, twice with water and once with ethanol to remove all traces of ammonium. The supernatant collected after the precipitation step was analyzed by ICP-AES to determine the precipitation yield of uranium. Several wavelengths (i.e. λ = 393.203 nm, 385.958 nm and 385.466 nm) were used to avoid any interference. From the results obtained, the precipitation appeared to be reproducible and quantitative, with yield value ≥99.9 %.

The resulting hydroxide precursor (also called hydrated oxide UO₂±x·nH₂O due to its greater stability in this form (Martinez et al., 2015; Neck and Kim, 2001)) was further dried and stored under an inert atmosphere (argon) to limit rehydration of the highly reactive powder prior to the conversion stage. Finally, it was converted to uranium dioxide UO₂+x by heating at 700 °C for 4 h under a reducing atmosphere (Ar-4 %H₂). This protocol allowed the elimination of the water present in the sample, as previously described by Martinez et al. (Martinez et al., 2015). The batch of UO₂+x powder was then divided into two parts. The first part was calcined by heat treatments under reducing atmosphere (Ar-4 %H₂, pO₂ = 6 × 10⁻²⁹ atm), while the other part was calcined under argon atmosphere (pO₂ = 3 × 10⁻⁶ atm). These heat treatments were carried out in a tubular furnace at a constant rate of 2 °C min⁻¹ from 700 °C to 1500 °C with a dwell time of 4 h. The calcined powders were then characterized ex situ.

### 2.2. Sample characterization

The powders obtained after conversion of the hydroxide precursor to oxide and subsequent calcination steps were characterized by Powder X-Ray Diffraction (PXRD) using a Bruker D8 diffractometer equipped with a Lynx-eye detector and Cu Kα₁,₂ (λ = 1.54184 Å) adopting the reflection mode (parallel beam). The XRD patterns were collected at room temperature in a special sealed sample holder to avoid radioactive contamination, in the angular range 5° ≤ 2θ ≤ 70°, with a step size Δ(2θ) of 0.03° and an average total counting time of about 3 h per sample. The collected XRD patterns were then refined by the Rietveld method using the General Structure Analysis System software version II (GSAS-II) (Toby and Von Dreele, 2013), where the instrument parameters were obtained using pure silicon as an external standard. The background was modeled by the Chebyshev function with 20 coefficients. For the refined pattern, the following parameters were allowed to vary: zero shift, unit cell parameters, scale factors, global thermal displacement. In addition, the intrinsic microstructure parameters were modeled using an anisotropic size model.

The morphological characterization was performed directly on the raw samples by Environmental Scanning Electron Microscopy (ESEM) using a FEI Quanta 200 with a backscattered electron detector (BSED) or a secondary electron (SE) detector in vacuum conditions with an acceleration voltage of 5 kV. Because the powder experiments did not require a metal coating step, the SEM characterizations were non-destructive.

The specific surface area (SₐSA) values of the powdered samples were
measured by adsorption/desorption of N₂ or krypton (for values less than 5 m² g⁻¹) at 77 K (BET method) using a Micromeritics Tristar 3020 apparatus. Prior to the analyses, the sample was outgassed at 100 °C for 12 h to ensure the release of water molecules adsorbed on the surface of the solid.

## 2.3. Dissolution tests

Dissolution tests were performed under static conditions using polytetrafluoroethylene containers placed in an oven at 60 °C. For each dissolution test, approximately 100 mg of powder was placed in contact with 20 mL of 0.1 mol L⁻¹ HNO₃ solution until complete dissolution of the material. These experimental conditions were chosen in order to distinguish any difference in the dissolution mechanism or kinetics of the samples studied, although they are far from the usual conditions of spent fuel reprocessing (4 mol L⁻¹ HNO₃ and T = 90 °C). In fact, previous dissolution tests of UO₂ samples carried out by Cordara et al. (Cordara et al., 2017) in more concentrated nitric acid did not allow to detect these second-order differences due to too aggressive dissolution conditions. During the dissolution experiment, 1 mL aliquots were periodically removed and replaced with the same volume of fresh nitric acid solution to maintain a constant solution volume. Depending on the dissolution conditions, i.e. the concentration of released uranium, the aliquots were diluted with 0.2 mol L⁻¹ HNO₃. The total uranium concentrations were then determined by ICP-AES using the same wavelengths as above. Analyses were calibrated using several U standard solutions prepared by dilution of a certified standard solution of 1000 mg L⁻¹. Concentrations in solution were determined from three replicates. Under these conditions, the limit of detection was 0.1 mg L⁻¹.

The ICP-AES analyses of the aliquots taken from the dissolution solution made it possible to determine the evolution of the U concentration in solution. The mass of dissolved element *i* at time *t*, Δm(*t*)/m₀ (expressed in %), was calculated from the ICP-AES measurements as follows (Eq. 5):

$$ \frac{\Delta m(t)}{m_0} = \frac{m_i(t)}{f_i \times m_0} \times 100 = \frac{C_i(t) \times V}{f_i \times m_0} \times 100 \quad (5) $$

where mᵢ(t) (g) is the mass of element *i* released in solution for a given time *t*, calculated from the elemental concentration, Ci(t) (g L⁻¹), and the volume of solution in contact with the solid, V (L); fᵢ is the mass fraction of element *i* in the material and m₀ is the initial mass of the pellet (g).

The normalized weight losses (N₉(i,t) expressed in g m⁻²) were then calculated from the elemental concentrations according to Eq. 6. This included normalization by the initial reactive surface area S (m²) of the sample in contact with the solution and by the mass ratio of the element present in the solid, fᵢ (i.e. the ratio between the mass of the element considered and the total mass of the sample).

$$ N_{L}(i, t) = \frac{m_{i}(t)}{f_{i} \times S_{SA} \times m_{0}} \quad (6) $$

Finally, the normalized dissolution rates R₉(i,t)(g m⁻² d⁻¹), were derived from the normalized weight losses according to Eq. 7:

$$ R_{L}(i, t) = \frac{dN_{L}(i, t)}{dt} \quad (7) $$

Based on several studies carried out on the dissolution of uranium-based oxide samples, the normalized dissolution rates R₉(i,t) were determined from the linear domain of dissolution, usually called steady state, i.e. the second step observed during the steady state described by Cordara et al. (Cordara et al., 2017) and Dalger et al. (Dalger et al., 2018). In fact, the first step (initial pulse) reported by these authors was not observed in this work, surely due to the preparation of the samples by hydroxide precipitation. During this dissolution time, no decrease in the leaching rate was observed, suggesting the absence of a saturation

process such as precipitation of neoformed phases at the solid/liquid interface. It is worth noting that the R₉ values were determined at the very beginning of the sample dissolution, so fᵢ and SSA (m² g⁻¹) were assumed to be constant in Eq. 6, which could only introduce a very small bias.

# 3. Results

## 3.1. Characterization of the UO₂+x powdered samples

### 3.1.1. PXRD characterization

The calcined UO₂+x powders were characterized by ex-situ XRD to gain a better understanding of their crystallographic structure as a function of temperature and calcination atmosphere. The ex-situ XRD patterns were then refined by the Rietveld method to determine the lattice parameter and average crystallite size associated with each XRD pattern. XRD patterns of UO₂+x powders obtained after heat treatment in a reducing atmosphere are shown in Fig. 1 and demonstrate that the face-centered cubic fluorite structure (Fig. SI 1) is preserved over the entire calcination temperature range studied. According to these

<figure><img src="image_3.png" /><figcaption>Fig. 1. XRD patterns recorded after heat treatment in a reducing atmosphere (Ar-4 %H₂) of UO₂+x (700 °C ≤ T ≤ 1500 °C) (a) and zoom centered on the 26° < 2θ < 34° zone (red box) with the most intense peaks (b). The characteristic XRD lines of the UO₂ reference are shown in red [PDF-00-041-1422]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
analyses, no secondary phase is observed in the sample, which remains
single phase. A slight shift in the XRD peaks is observed when compared
with reference material. At this stage, this shift can be related to small
differences in oxygen stoichiometry within the samples, as confirmed by
the Rietveld refinements performed on the recorded XRD patterns.

The refined unit cell parameters and crystallite sizes are shown in Fig. 2 and Table SI 1. The lattice parameter values obtained for calcination temperatures below 1100 °C suggest a slight deviation from stoichiometry in oxygen, which can be explained by air reoxidation of the sample after removal from the furnace after calcination. The value of the lattice parameter *a*, obtained for *T* < 1100 °C, is lower than that obtained at higher temperatures for *T* > 1200 °C. Above this temperature, the value of the lattice parameter reaches a plateau, resulting in a value (*a* = 5.469(1) Å) in very good agreement with the value reported in the literature for stoichiometric UO<sub>2.00</sub> (i.e. 5.471(1) Å) (Leinders et al., 2015). This underlines the fact that the stoichiometric deviation has been corrected at this temperature. At the same time, the line width at half height of the XRD lines decreases significantly with increasing calcination temperature, indicating a clear improvement in the crystallization state of the sample. This observation is confirmed by the significant growth of the crystallites with increasing calcination temperature, especially from 900 °C onwards, reaching up to 172 ± 6 nm at 1500 °C (Fig. 2).

The XRD patterns of UO<sub>2+x</sub> powders obtained after heat treatment under argon atmosphere are shown in Fig. 3. They suggest a completely different behavior from that observed under reducing atmosphere. First, it appears that the face-centered cubic fluorite structure is not preserved over the entire temperature range. In fact, a significant fraction of the orthorhombic U<sub>3</sub>O<sub>8</sub> phase (α-U<sub>3</sub>O<sub>8</sub> form, space group C2mm) coexists with the fluorite phase up to about 1100 °C. The fractions of each phase (U<sub>3</sub>O<sub>8</sub> and UO<sub>2+x</sub>) were estimated from the Rietveld refinement results (Fig. SI 2). While the U<sub>3</sub>O<sub>8</sub> fraction reaches 42 wt% at 700 °C, it increases to 100 wt% at 900 °C as the fluorite phase is completely converted to U<sub>3</sub>O<sub>8</sub>. The U<sub>3</sub>O<sub>8</sub> phase then decreases rapidly with increasing temperature, from 15 wt% at 1000 °C to only 8 wt% at 1100 °C, and finally disappears at 1200 °C. In addition, the wide-angle shift of the characteristic fluorite phase lines underlines the fact that the sample is slightly overoxidized (in the form of UO<sub>2+x</sub>). This is confirmed by Rietveld analyses (Fig. 4 and Table SI 2), which show a lattice parameter value below the reference value (a = 5.471(1) Å (Leinders et al., 2015)) between 700 °C and 1100 °C.

<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** XRD patterns of UO<sub>2+x</sub> powders recorded after heat treatments (700 °C ≤ *T* ≤ 1500 °C) under argon atmosphere (a) and zoom centered to the 26° < 2θ < 34° zone (red box) with the most intense peaks (b). The characteristic XRD lines of the UO<sub>2</sub> and α-U<sub>3</sub>O<sub>8</sub> references are shown in red and blue, respectively [PDF-00-041-1422 and PDF-00-31-1424]. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Variation of unit cell parameter *a* (a) and average crystallite size of UO<sub>2+x</sub> powders (b) after heat treatment at different temperatures (700 °C ≤ *T* ≤ 1500 °C) under reducing atmosphere (Ar-4 %H<sub>2</sub>). The reference lattice parameter for UO<sub>2.00</sub> is taken from (Leinders et al., 2015).</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Variation of unit cell parameter *a* (a) and average crystallite size of UO<sub>2+x</sub> powders (b) after heat treatment at different temperatures (700 °C ≤ *T* ≤ 1500 °C) under argon. The reference lattice parameter for UO<sub>2.00</sub> is taken from (Leinders et al., 2015).</figcaption></figure>

The XRD patterns recorded for $T \ge 1200$ °C show the coexistence of two face-centered cubic fluorite phases (Fig. 5). This phenomenon could be explained by the simultaneous presence of two phases of formula UO<sub>2+x₁</sub> and UO<sub>2+x₂</sub>, which exhibit different uranium speciation (hence different values of *x*). The most intense XRD peak observed around 2θ = 28.6° for *T* = 1200 °C (Fig. 5) could be associated with a more oxidized phase. This UO<sub>2+x₂</sub> phase would be reduced at higher temperatures to form the UO<sub>2+x₁</sub> phase (associated with a line around 2θ = 28.4° for *T* = 1500 °C). However, a shift of this line relative to UO<sub>2.00</sub> is observed, which could reflect a slight overoxidation of the samples, as evidenced by the lattice parameter values at the end of the refinements (Fig. 4), which are lower than those expected between 1200 and 1500 °C (Leinders et al., 2015). Thus, the UO<sub>2+x₂</sub> phase, which dominates at *T* = 1200 °C (81 wt%), becomes a minority compared to UO<sub>2+x₁</sub> at *T* = 1500 °C (36 wt%). This latter phase could be directly related to the reduction of the U<sub>3</sub>O<sub>8</sub> phase. At the same time, the crystallite growth at lower temperatures (700 °C - 1000 °C) seems to be more significant compared to the crystallite growth observed under reducing conditions. However, the crystallization state of the sample at *T* = 1500 °C appears

to be slightly poorer than that calcined in a reducing atmosphere, which is confirmed by the lower crystallite growth (148 ± 5 nm at 1500 °C, Fig. 4).

### 3.1.2. SEM characterization

The morphology of the prepared powders was observed by SEM at the end of each heat treatment as a function of temperature and calcination atmosphere. The micrographs obtained are shown in Fig. 6. Particle growth within the agglomerates is clearly observed on the micrographs for both atmospheres, but appears at higher temperatures under reducing atmosphere. In fact, grains of several microns in diameter are already visible at 900–1100 °C under argon, whereas calcination at 1500 °C is required to observe grains of the same size under reducing atmosphere. These results are in good agreement with those obtained from *ex-situ* XRD studies. Crystallite growth under argon occurs at lower temperatures than under reducing atmosphere. At 1500 °C, the grains reach a few µm under reducing atmosphere, a much smaller value than that measured under argon (about ten µm).

<figure><img src="image_8.png" /><figcaption>**Fig. 5.** XRD patterns of UO<sub>2+x</sub> powders recorded after heat treatment under argon at different temperatures (1200 °C ≤ *T* ≤ 1500 °C) (a) and zoom centered on the first two XRD lines (b). The characteristic lines of the UO<sub>2</sub> reference are shown in red [PDF-00-041-1422], while the stars (*) are associated with the XRD peaks of the sample holder used. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>Fig. 6. SEM micrographs recorded on UO$_{2+x}$ powders calcined between 700 °C and 1500 °C under argon (right) or reducing (left) atmosphere (Ar-4 %H$_{2}$).</figcaption></figure>

### 3.1.3. Determination of specific surface area

The variation of the specific surface area $S_{SA}$ of UO$_{2+x}$ powders was studied as a function of temperature and calcination atmosphere. The results are presented in Table 1 and in Fig. 3. The temperature and the type of calcination atmosphere show a significant influence on the

evolution of the $S_{SA}$. The values measured in a reducing atmosphere remain mostly higher than those measured in argon. In fact, when calcined in a reducing atmosphere, the $S_{SA}$ stabilizes at about $1 \text{ m}^2 \text{ g}^{-1}$ at 1100 °C and then decreases to about $0.1 \text{ m}^2 \text{ g}^{-1}$ above 1300 °C due to powder sintering. Powder calcination under argon, on the other hand,
<caption>**Table 1**</caption>

Variation of specific surface area $S_{SA}$ measured for $UO_{2+x}$ powders calcined between 700 °C and 1500 °C in argon or reducing atmosphere (Ar-4 %H₂).

<table><thead><tr><th rowspan="2">Heating temperature (°C)</th><th colspan="2">Specific surface area (m² g⁻¹)</th></tr><tr><th>Argon</th><th>Ar-4 %H₂</th></tr></thead><tbody><tr><td>700</td><td>3.3 ± 0.2</td><td>17 ± 1</td></tr><tr><td>900</td><td>0.6 ± 0.1</td><td>14 ± 1</td></tr><tr><td>1000</td><td>0.2 ± 0.1</td><td>6.7 ± 0.3</td></tr><tr><td>1100</td><td>0.1 ± 0.1</td><td>1.0 ± 0.1</td></tr><tr><td>1200</td><td>0.1 ± 0.1</td><td>1.2 ± 0.1</td></tr><tr><td>1300</td><td>0.1 ± 0.1</td><td>0.3 ± 0.1</td></tr><tr><td>1500</td><td>0.1 ± 0.1</td><td>0.1 ± 0.1</td></tr></tbody></table>

results in a drastic drop in $S_{SA}$ at 700 °C, then rapidly stabilizes at about 0.1 m² g⁻¹ for temperatures above 1000 °C.

This behavior can be explained by grain growth at lower temperatures under argon, as suggested by the SEM characterization of the samples. Thus, porosity elimination certainly occurs at lower temperatures under argon. At higher temperatures, where grain growth predominates, there are no major differences. However, it is possible that at high temperatures ($T \ge 1300$ °C) the specific surface area of the powders becomes slightly lower than 0.1 m² g⁻¹. However, it was not possible to obtain a more precise value of the $S_{SA}$ for these heating temperatures, as it becomes lower than the quantification limit of the instrument.

## 3.2. Dissolution of the powdered $UO_{2+x}$ powders

To demonstrate the influence of temperature and calcination atmosphere on the normalized dissolution rate $R_L$ of $UO_{2+x}$ powders, dissolution tests were performed under static conditions in 0.1 mol L⁻¹ HNO₃ at 60 °C. Fig. 7 and Fig. 8 show the evolution of the relative mass loss $\Delta m/m_0$ (%) of uranium determined during the dissolution of all the $UO_{2+x}$ powders studied (calcination from 700 °C to 1500 °C under argon or reducing atmosphere (Ar-4 %H₂)).

From these results, it appears that the temperature at which the samples are calcined has a significant influence on the time required to reach total dissolution (i.e. $\Delta m/m_0 \approx 100$ %). Indeed, under the experimental conditions considered (0.1 mol L⁻¹ HNO₃, $T = 60$ °C) and independently of the calcination atmosphere, the time required to dissolve a $UO_{2+x}$ powder heated at 700 °C is about 3 days, while this time reaches about 160 days for a calcination temperature of 1500 °C (Fig. 7 and Fig. 8). Furthermore, the mechanism of dissolution also seems to be affected by the calcination temperature of the samples. Thus, for calcination temperatures of 1200 °C or less and under a reducing atmosphere, a two-step dissolution mechanism is observed (Fig. 8a and b). The normalized dissolution rate $R_L$ was determined during the first stage, where the variation of the relative mass loss in U is low and the evolution of the normalized leaching is linear. This stage corresponds to the second kinetic regime described by Cordara et al. (Cordara et al., 2017) and Dalger et al. (Dalger et al., 2018). It is associated with a quasi-stationary regime known as the induction period. A sharp break in the slope then indicates a change in the kinetic regime. This change in kinetic regime is characteristic of a change in dissolution mechanism, from a “non-catalyzed” mechanism (Eq. 1) to one “catalyzed” by the nitrogen species accumulated in solution during the first stage (Eq. 2). For calcination temperatures in a reducing atmosphere of 1300 °C and 1500 °C, the evolution of the relative mass loss in U remains linear from the beginning to the end of the dissolution experiment (Fig. 8c). Thus, the concentration of catalytic species accumulated at the solid/solution interface is probably not sufficient to induce the catalytic mechanism of dissolution due to the lower reactivity of these solids ($S_{SA} \approx 0.1$ m² g⁻¹).

To overcome these large differences in terms of reactivity and thus compare their dissolution kinetics, the relative mass losses $\Delta m/m_0$ are normalized by the reactive surface area of the samples ($m_0 \times S_{SA}$). In this

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Evolution of the relative mass loss in U recorded during dissolution in 0.1 mol L⁻¹ HNO₃ and at 60 °C, of $UO_{2+x}$ powders heated at various temperatures ranging from 700 °C to 1500 °C in a reducing atmosphere (Ar-4 %H₂) (a) or in argon (b).</figcaption></figure>

way, it is possible to calculate the normalized dissolution rate of these samples, $R_L$ (in g m⁻² d⁻¹), during the period in which the reactive surface of the solid varies slightly, i.e. during which dissolution is not catalyzed. Beyond this period, the reactive surface may change significantly as a result of the catalyzed regime, which will affect the dissolution rate of the solid. The normalized weight losses $N_L$ values in the zone where the normalized dissolution rates $R_L$ were determined (steady state) are shown in Fig. 9. The normalized dissolution rates $R_L$ associated with each of the samples examined are shown in Table 2 and Fig. 10.

The normalized dissolution rates determined for the samples calcined in a reducing atmosphere are all similar, ranging from $R_L = (1.2 \pm 0.2) \times 10^{-2}$ g m⁻² d⁻¹ to $R_L = (6.8 \pm 1.3) \times 10^{-2}$ g m⁻² d⁻¹. The average normalized dissolution rate for this series of samples, shown by the gray dotted line in Fig. 10a, is therefore $R_L = 3.8 \times 10^{-2}$ g m⁻² d⁻¹ for samples heated in Ar-4 %H₂. This value is consistent with that calculated from the multi-parametric dissolution rate law proposed by Heisbourg et al. (Heisbourg et al., 2004), i.e. approximately $R_L = 5 \times 10^{-2}$ g m⁻² d⁻¹ in 0.1 mol L⁻¹ HNO₃ at room temperature.

In the case of the $UO_{2+x}$ samples calcined under argon (Fig. 7b), the dissolution kinetics again appear to be influenced by the calcination temperature. With the exception of the sample calcined at 700 °C (Fig. 8a), the evolution of the relative mass loss of uranium shows a single step dissolution profile. Indeed, the relative mass loss in U varies linearly from the onset of dissolution, without any induction period or break in the slope characteristic of a change in the dissolution mechanism. A slowdown is then observed, often associated with the approach of saturation conditions near the interface (thermodynamic equilibrium) or solid depletion. The sample calcined at 700 °C, on the other hand,
<figure><img src="image_11.png" /><figcaption>Fig. 8. Evolution of relative U mass loss when dissolving UO₂+x powders, in 0.1 mol L⁻¹ HNO₃ at 60 °C, previously calcined in argon or reducing atmosphere (Ar-4 %H₂) at 700 °C and 900 °C (a); at 1000 °C, 1100 °C and 1200 °C (b) and at 1300 °C and 1500 °C (c).</figcaption></figure>

shows a two-step dissolution, as in the case of samples heat treated in a reducing atmosphere. This difference in behavior could be related to the high reactivity of this sample (poorer crystallization state, high reactive surface and/or significant overoxidation). Contrary to what was observed under reducing atmosphere, the normalized dissolution rates R_L determined under argon atmosphere vary as a function of calcination temperature. In fact, R_L values of the samples prepared under argon range from R_L = (5.8 ± 1.4) × 10⁻² g m⁻² d⁻¹ to R_L = (5.6 ± 1.1) × 10⁻¹ g m⁻² d⁻¹ (Table 2 and Fig. 10). Normalized dissolution rates therefore appear to be higher for samples heated under argon. The weakening of chemical durability associated with the presence of U₃O₈ in the sample is confirmed (Fig. 10b). Indeed, the dissolution rate increases almost linearly with the measured U₃O₈ content.

## 4. Discussion

As discussed in the powder characterization section, the physicochemical properties of the samples are strongly influenced by the heating temperature. In fact, the crystallinity of samples is significantly improved as the calcination temperature increases, resulting in lower dissolution. It has been reported in the literature that the state of crystallinity can have a moderate but significant effect on the dissolution rate of mixed oxides (Claparede et al., 2011a; Claparede et al., 2015; Claparede et al., 2011b). More importantly, the specific surface area SSA, and consequently the initial reactive surface area of the powders (m₀ × SSA), also has a significant effect on the chemical durability of the samples. As mentioned above, a progressive decrease in the specific surface area was observed with increasing calcination temperature, while the crystallinity of the samples was improved (mainly due to crystallite growth and elimination of crystal defects). Thus, the lower the reactivity of the sample, associated with a low specific surface area, the longer the time required to achieve the full dissolution of the solid. The catalysis of the dissolution reaction can be significantly delayed or even eliminated, as can be seen in the case of the UO₂+x sample heated at 1500 °C.

Second, some UO₂+x samples calcined under argon were found to be oxidized due to the higher partial pressure of oxygen in the argon atmosphere compared to the hydrogenated argon one (due to gas specification). These samples (for T = 700 °C, 1000 °C and 1100 °C) then present a greater or lesser fraction of U₃O₈. Furthermore, for a calcination temperature of 900 °C, the UO₂+x phase was shown to be completely oxidized to U₃O₈. In contrast, no trace of the U₃O₈ phase was detected in samples fired in a reducing atmosphere. When these results are correlated with those obtained during dissolution tests, it becomes clear that the presence of the secondary phase U₃O₈ has a significant impact on the dissolution mechanism and kinetics of the powdered samples (Fig. 10b). Indeed, the highest normalized dissolution rates R_L are observed for samples in which a fraction of U₃O₈ is present (e.g. for T ≤ 1100 °C, under argon atmosphere). Moreover, the sample calcined at 900 °C under argon and containing only a pure U₃O₈ phase is associated with the highest normalized dissolution rate in the study, namely R_L = (5.6 ± 1.1) × 10⁻¹ g m⁻² d⁻¹ (Fig. 10). Furthermore, this sample shows a linear evolution of relative uranium mass loss throughout the dissolution, in contrast to other samples calcined at similar temperatures (Fig. 8a), where dissolution occurs in two stages. This phenomenon can be explained by the absence of U(IV) in the solid and therefore by the probable absence of any strong autocatalytic dissolution mechanism involving nitrogen species (e.g. HNO₂). Thus, it appears that no change in the kinetic regime is observed during the dissolution of U₃O₈. This is also the case for samples calcined at 1000 °C and 1100 °C, which show a non-negligible fraction of U₃O₈. However, this is not the case for the sample calcined at 700 °C, for which the other parameters already mentioned in the previous paragraph, could influence the dissolution mechanism. The dissolution mechanism of U₃O₈ in nitric medium could correspond to the Eq. (8):

$$U_3O_8(s) + 2HNO_2(aq) + 6H^+ \rightarrow 3UO_2^{2+}(aq) + 2NO_2(aq) + 4H_2O \quad (8)$$

Finally, the normalized dissolution rate R_L of samples free of U₃O₈ as a secondary phase after heating (T = 700 to 1500 °C under Ar-4 % H₂ and T ≥ 1200 °C under Ar) are all similar, with an average dissolution rate equal to R_L = 5.5 × 10⁻² g m⁻² d⁻¹ (Fig. 10). Thus, there is a factor of about 10 between the dissolution rate of the U₃O₈ phase and the average dissolution rate of the samples free of U₃O₈. A small difference in dissolution rate remains between the samples calcined at high temperature (from 1200 °C to 1500 °C) under argon or reducing atmosphere. In fact, the average normalized dissolution rate for samples calcined under argon at high temperature, represented by the red dotted line in Fig. 10, is equal to R_L [Ar] = 6.4 × 10⁻² g m⁻² d⁻¹ compared to R_L [Ar-4 %H₂] = 3.8 × 10⁻² g m⁻² d⁻¹ in reducing conditions. This
<figure><img src="image_12.png" /><figcaption>**Fig. 9.** Evolution of normalized mass loss in U when dissolving UO<sub>2+x</sub> powders calcined in reducing atmosphere (Ar-4 %H<sub>2</sub>) or argon at 700 °C and 900 °C (a,b); 1000 °C, 1100 °C and 1200 °C (c,d); 1300 °C and 1500 °C (e,f), in 0.1 mol L<sup>-1</sup> HNO<sub>3</sub> at 60 °C. The presented zones correspond to the zones where the R<sub>L</sub> values were determined.</figcaption></figure>

discrepancy, by a factor of about 2, could be assigned to the slight overoxidation mentioned above for samples calcined under argon. Nevertheless, this deviation remains of the second order compared to that observed for lower calcination temperatures.

**5. Conclusion**

In order to study the effects of structure and microstructure on the chemical durability of UO<sub>2+x</sub> powders and to identify the associated dissolution mechanisms, the preparation of a panel of samples
<caption>**Table 2**</caption>

Normalized dissolution rates $R_L$ (expressed in $g m^{-2} d^{-1}$) obtained when dissolving $UO_{2+x}$ powders heated between 700 °C and 1500 °C under argon or reducing atmosphere (Ar-4 %H₂).

<table><thead><tr><th rowspan="2">R<sub>L</sub> (g m<sup>-2</sup> d<sup>-1</sup>)</th><th colspan="2">Heating atmosphere</th></tr><tr><th>Ar - 4 %H<sub>2</sub></th><th>Argon</th></tr></thead><tbody><tr><td>700</td><td>(1.2 ± 0.2) × 10<sup>-2</sup></td><td>(3.5 ± 0.7) × 10<sup>-1</sup></td></tr><tr><td>900</td><td>(1.5 ± 0.2) × 10<sup>-2</sup></td><td>(5.6 ± 1.1) × 10<sup>-1</sup></td></tr><tr><td>1000</td><td>(2.7 ± 0.4) × 10<sup>-2</sup></td><td>(2.8 ± 0.6) × 10<sup>-1</sup></td></tr><tr><td>1100</td><td>(5.9 ± 1.2) × 10<sup>-2</sup></td><td>(9.2 ± 2.3) × 10<sup>-2</sup></td></tr><tr><td>1200</td><td>(5.0 ± 1.0) × 10<sup>-2</sup></td><td>(5.8 ± 1.4) × 10<sup>-2</sup></td></tr><tr><td>1300</td><td>(6.8 ± 1.3) × 10<sup>-2</sup></td><td>(7.0 ± 1.7) × 10<sup>-2</sup></td></tr><tr><td>1500</td><td>(3.2 ± 0.8) × 10<sup>-2</sup></td><td>(6.3 ± 1.5) × 10<sup>-2</sup></td></tr></tbody></table>

<figure><img src="image_13.png" /><figcaption>**Fig. 10.** (a) Variation in normalized dissolution rate $R_L$ of $UO_{2+x}$ powders calcined in argon or reducing atmosphere as a function of heating temperature. The mean values $R_L$ [Ar-4 % H₂] and $R_L$ [Ar] are marked by the gray and red dotted lines respectively; (b) Variation of the normalized dissolution rate $R_L$ of the powders heated between 700 °C and 1500 °C in argon as a function of the wt% fraction of $U_3O_8$. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)</figcaption></figure>

containing hydroxide precursors was developed. The chosen synthesis method allowed the quantitative precipitation of the U(IV) cation in a high excess of ammonia ($\rho > 99.9 \%$). Structural and morphological characterization of calcined $UO_{2+x}$ powders (700 °C < T < 1500 °C) under argon or reducing atmosphere (Ar-4 % H₂) was also performed ex-situ. The variation of the properties of these samples was studied as a

function of temperature and calcination atmosphere. Ex-situ XRD experiments showed the preservation of the fluorite structure under reducing atmosphere over the whole temperature range studied, while the formation of a $U_3O_8$ phase was highlighted under argon at $T \le 1100$ °C. In addition, a drastic decrease in the specific surface area of the powders was observed with increasing calcination temperature, which was correlated with significant crystallite growth.

The study of the dissolution of various $UO_{2+x}$ samples calcined at different temperatures, under argon or reducing atmosphere, highlighted the effect of several parameters on the mechanism and kinetics of sample dissolution. First, increasing the calcination temperature significantly improves the chemical durability of the solids. Correspondingly, the higher the calcination temperature, the lower the specific surface area of the sample, thus prolonging the time required to reach full dissolution. In addition, the catalysis of the dissolution reaction may be delayed or even eliminated in the case of poorly reactive samples. Second, the presence of a fraction of $U_3O_8$ in certain samples heated under argon showed a significant effect on the mechanism and rate of normalized dissolution. Samples containing a $U_3O_8$ fraction showed no change in the kinetic regime during dissolution, with a linear evolution of the relative uranium mass loss observed until the end of dissolution. In addition, their normalized dissolution rate was higher than that of the other samples. This suggests that the presence of the $U_3O_8$ phase is likely to block the switch to a kinetic dissolution regime autocatalyzed by nitrogen species. For comparison, the normalized dissolution rate of a pure $U_3O_8$ sample was determined in $0.1 \text{ mol L}^{-1}$ $HNO_3$ and at 60 °C. It reached $R_L = (5.6 \pm 1.1) \times 10^{-1} \text{ g m}^{-2} \text{ d}^{-1}$, a value 10 times higher than that of $UO_{2+x}$, $R_L = 5.5 \times 10^{-2} \text{ g m}^{-2} \text{ d}^{-1}$.

In conclusion, the physicochemical properties of $UO_{2+x}$ samples, such as crystallinity, specific surface area and the presence of secondary phases, are strongly correlated with the temperature and calcination atmosphere conditions and have a significant influence on the chemical durability of the samples. Particular attention should be paid to the presence of $U_3O_8$ within $UO_2$ samples. Indeed, with an increase of one order of magnitude compared to $UO_{2+x}$, the presence of $U_3O_8$ within the solid is likely to modify the overall dissolution process of the starting material.

## Data statement

The data supporting this article have been included as part of the Supplementary Information.

## CRediT authorship contribution statement

**Thomas Barral:** Writing – review & editing, Writing – original draft, Visualization, Methodology, Investigation, Data curation, Conceptualization. **Laurent Claparède:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Methodology, Investigation, Conceptualization. **Nicolas Dacheux:** Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization.

## Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgements

Authors acknowledge the GENIORS project (H2020 Euratom Research and Innovation Program under grant agreement no. 755171) which provided financial support to this work. Authors would like to thank Beatrice Baus-Lagarde, Xavier Le Goff and Renaud Podor at ICSM
for the technical support during ICP-AES, XRD and SEM analyzes, respectively. PhD work of T.B. has been funded by CEA.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.hydromet.2025.106569.

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