# Impact of metallic Mo nanoparticles on the dissolution kinetics of UO2 in nitric acid under representative PUREX process conditions

Mohammad Husainy, Stéphanie Szenknect, Renaud Podor, Xavier Le Goff, Philippe Moisy, Christoph Hennig, Nicolas Dacheux

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## ► To cite this version:

Mohammad Husainy, Stéphanie Szenknect, Renaud Podor, Xavier Le Goff, Philippe Moisy, et al. Impact of metallic Mo nanoparticles on the dissolution kinetics of UO2 in nitric acid under representative PUREX process conditions. Journal of Nuclear Materials, 2025, 615, pp.155922. 10.1016/j.jnucmat.2025.155922. hal-05168253

HAL Id: hal-05168253

https://hal.umontpellier.fr/hal-05168253v1

Submitted on 17 Jul 2025

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# Impact of metallic Mo nanoparticles on the dissolution kinetics of UO₂ in nitric acid under representative PUREX process conditions

Mohammad Husainy<sup>a,†</sup>, Stéphanie Szenknect<sup>a,¶</sup>, Renaud Podor<sup>a,§</sup>, Xavier Le Goff<sup>a</sup>, Philippe Moisy<sup>b</sup>, Christoph Hennig<sup>c,d</sup>, Nicolas Dacheux<sup>a,¶</sup>

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

## HIGHLIGHTS

* The impact of metallic Mo nanoparticles on the kinetics of UO₂ dissolution in HNO₃ is investigated.

* Mo(0) and UO₂ exhibit mutual inhibition during dissolution.

* The mechanism of Mo(0) corrosion in HNO₃ is re-examined.

* Competition between UO₂ and Mo(0) for the same reactive species is demonstrated.

* No mixed U and Mo precipitates were observed under the representative PUREX process conditions.

## ARTICLE INFO

### Keywords:

Molybdenum

Uranium dioxide

Dissolution mechanisms

Solid-liquid interface

Nuclear fuel recycling

## ABSTRACT

This study is dedicated to the dissolution of UO₂-based model compounds incorporating metallic Mo nano-particles in 4 mol.L⁻¹ HNO₃ at 353 K (representative for PUREX process conditions) following a dynamic protocol. Compounds of structural and microstructural properties similar to spent nuclear fuel allowed us to evidence the specific impact of Mo in the metallic form on the dissolution kinetics of UO₂. The dissolution of a pure metallic Mo reference sample under the same dissolution conditions was carried out to gain deeper insights into the oxidation behavior of Mo in nitric acid and to evaluate its influence on the UO₂ dissolution mechanism. The evolution of the U, Mo, and nitrous acid concentrations in solution was measured and the solid residues of dissolution were characterized. The results revealed a mutual inhibitory effect between UO₂ and metallic Mo during dissolution in nitric acid. No precipitates of mixed U and Mo were formed under the studied conditions. The dissolution processes of UO₂ and Mo were found to be interconnected and likely compete for the same autocatalytic nitrogen species necessary for their oxidation and solubilization in the +VI oxidation state. The results provided crucial insights into how metallic Mo reacts with nitric acid under the representative PUREX process conditions. The findings suggested that metallic Mo(0) is initially completely oxidized to an amorphous brown-red phase with an oxidation state between +IV and +VI. Further oxidation, then solubilization of this phase as Mo(VI) species appeared to be facilitated by the auto-catalytic nitrogen species.

## 1. Introduction

Pressurized water reactors (PWRs) are the most common type of electricity-generating nuclear reactors operating globally. The nuclear fuels used in the majority of PWRs are oxide-type ceramics (in the form of sintered, dense pellets) that are composed of either uranium dioxide

enriched in <sup>235</sup>U (UOX, UO<sub>2</sub>) or mixed oxides of uranium and plutonium (MOX, (U,Pu)O<sub>2</sub>) [1]. During irradiation, the fuel pellets are subjected to various constraints of neutron, thermal, and mechanical origins, which lead to major changes in their chemical, physical, and microstructural properties. The spent nuclear fuel (SNF) thus forms a complex heterogeneous system with >40 fission products (FPs) and minor actinides

<footnote><sup>a</sup> ICSM, University of Montpellier, CEA, CNRS, ENSCM, Site de Marcoule, Bagnols-sur-Cèze, France</footnote>

<footnote><sup>b</sup> CEA, DES, ISEC, DMRC, University of Montpellier, Site de Marcoule, Bagnols-sur-Cèze, France</footnote>

<footnote><sup>c</sup> Institute of Resource Ecology, Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany</footnote>

<footnote><sup>d</sup> The Rossendorf Beamline at ESRF, The European Synchrotron, Grenoble, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: stephanie.szenknect@cea.fr (S. Szenknect).</footnote>
(Np, Am and Cm). These FPs can be distributed among several phases (metallic or oxide precipitates, elements dissolving in the UO₂ fuel matrix, and volatile elements) depending on irradiation temperature, pressure, and burnup conditions [2]. The remaining UO₂ in the UOX-SNF (fuel of interest in this study) still represents about 95 % of its total weight, Pu (~ 1 wt. %), minor actinides (~0.1 wt. %), and the FPs (3–4 wt. %) [3,4]. The valuable elements, such as uranium and plutonium (~ 96 wt. %), could be recycled, thus ensuring the qualification of a ‘closed nuclear fuel cycle’, whereas other elements (FPs, activation products, and minor actinides) are vitrified and stored prior to deep geological disposal. The recycling option is an essential strategy devised by the nuclear programs of several countries, including France, for the purpose of self-sustained power production, reducing the toxicity of ultimate waste, and preserving natural uranium resources.

SNF reprocessing in France is performed by the PUREX (Plutonium, Uranium, Reduction, ELxtraction) process, which is the most commonly used hydrometallurgical process for separating U and Pu from the irradiated fuel and has been successfully operating on an industrial scale since the early 1950s [5–8]. This process is based on liquid-liquid solvent extraction and involves an initial step of dissolution of the used nuclear fuel in hot concentrated nitric acid (HNO₃). The co-extraction of U and Pu using tri-n-butyl phosphate (TBP) diluted with an organic solvent allows the chemical separation of U and Pu and the conditioning of the process wastes [5,6]. The raffinate solution of the extraction process contains the majority of FPs and minor actinides. U and Pu nitrates are purified in subsequent additional stages of the PUREX process and can be returned to the fuel cycle by converting them into solid forms, mainly oxides [7]. The dissolution of SNFs relies on the oxidation of U(IV) into U(VI) by nitric acid in order to obtain uranyl ions (UO₂⁺) for reprocessing [5,9–13]. This heterogeneous reaction is also accompanied by the formation of various nitrogen species in the aqueous phase as well as in the gas phase, where NO and NO₂ are the main gaseous species detected [13–16]. The most probable oxidation mechanism of UO₂ in HNO₃ is that proposed by Ikeda et al. [12] who showed using ¹⁷O NMR that it is most likely initiated by the redox reaction between UO₂ and HNO₃ at the solid/solution interface in the absence of a U–O bond break. For 3 M< [HNO₃] < 5 M, this mechanism is described by reaction (1):

$$UO_{2(s)} + 3HNO_{3(aq)} \rightarrow UO_{2(aq)}^{2+} + 2NO_{2(aq)} + 1/2NO_{2(aq)} + 1/2NO_{(aq)} + 3/2H_{2}O_{(l)} \quad (1)$$

The rate of this reaction is considered to be slow. Additionally, the UO₂ dissolution reaction involves the production of nitrous acid (HNO₂) as a reduction product of HNO₃. Fukasawa et al. [17] observed the generation of HNO₂ at a molarity equal to that of dissolved uranium. This was in agreement with Dalger et al. [18,19], who also demonstrated the formation of one mole of nitrous acid per mol of dissolved UO₂ in their dissolution experiments. Several studies support the fact that the UO₂ dissolution mechanism is autocatalyzed by nitrous acid according to the following reaction sequence proposed by [12,20,21]:

$$UO_{2(s)} + 2HNO_{3(aq)} + 2H_{2(aq)}^{+} \rightarrow UO_{2(aq)}^{2+} + 2NO_{(aq)} + 2H_{2}O_{(l)} \quad (2)$$

$$2NO_{2(aq)} + H_{2}O_{(l)} \rightleftharpoons HNO_{3(aq)} + HNO_{3(aq)} \quad (3)$$

$$HNO_{3(aq)} + 2NO_{(aq)} + H_{2}O_{(l)} \rightleftharpoons 3HNO_{2(aq)} \quad (4)$$

The rate of reaction (2) is considered much faster than reaction (1). Indeed, the autocatalyzed mechanism by nitrous acid was supported by several studies, in which an increase in the dissolution kinetics was reported when adding nitrite ions (NO₂) to nitric acid [9–11,22]. In contrast, the addition of nitrous acid scavengers, such as hydrazine or urea, was reported to decrease the dissolution kinetics [9,23]. Other tested species, such as uranyl nitrate, were also considered but seemed very unlikely to be the autocatalytic species [9,13,24]. Furthermore, the study reported by F. Charlier [25] through modeling the different scenarios for the formation of the gas species (NO and NO₂) and the catalyst

(called Z) in the dissolution medium suggests that the catalyst would result from the recombination of the dissolved NOx species within the diffusion layer. This implies that HNO₂ (if it plays the role of the catalyst) would not be directly produced from the reaction between UO₂ and HNO₃ at the solid/solution interface. Additional studies [26–28] show that the HNO₂ formation reaction from the recombination of NO and NO₂ would be thermodynamically more stable than its decomposition. Even if HNO₂ is consumed according to reaction (2), it can be regenerated from the disproportionation of NO₂ and/or reduction of HNO₃ by NO following the equilibrium reactions (3) and (4).

On the other hand, several studies disagreed with the hypothesis of an autocatalyzed reaction [13]. For instance, Fukasawa et al. [17] did not evidence any change in the UO₂ dissolution rate with the concentration of HNO₂. Although the exact role of HNO₂ in the autocatalytic mechanism for the dissolution of UO₂ in nitric acid remains questionable, it is evident that it plays a key role and is a clear indicator of the establishment of the autocatalytic mechanism.

The dissolution of a real SNF has been poorly studied on account of the complexity of this material, combined with its very high specific radioactivity and radiotoxicity. Given that the UO₂-based SNF is still composed of ~ 95 wt % UO₂, most studies have been devoted to investigating the kinetics of mainly pure UO₂ dissolution in nitric acid media, which is believed to govern the dissolution of the overall fuel. However, these studies do not currently allow for the prioritization of the contributions of the SNF constituents during dissolution in nitric acid. It seems therefore important to assess the effect of the main FPs and minor actinides on the dissolution kinetics of UO₂, as this could improve our ability to anticipate the behavior of the SNF during reprocessing.

In the framework of this study, we are particularly interested in investigating the impact of Mo on the dissolution kinetics of UO₂ in nitric acid under representative PUREX process conditions. In fact, this element is one of the most abundant FPs with a significant fission yield (~ 6.1 % per ²³⁵U fission). In the SNF, Mo has a complex speciation linked to its interaction with the fuel and other FPs [2]. For instance, it can be found in different oxidation states (0, +IV, and +VI) in metal inclusions or oxides under specific irradiation conditions [2]. Even after SNF dissolution in nitric acid, Mo remains a significant component of various undissolved residues, such as incompletely dissolved (Mo–Tc–Ru–Rh–Pd) metallic precipitates, reprecipitated multi-component (Mo–Tc–Ru–Rh–Pd–Te–Zr–Sn–U–Pu) oxide hydrates, and zirconium molybdate precipitates (ZrMo₂O₇(OH)₂·2H₂O), which complicate the recycling processes [5,29–31]. Based on post-irradiation analyses of SNF from PWR in nominal operation, the majority of Mo is retained in the metallic state (0) within the ε-particles, from which a part of Mo could be partitioned and oxidized into MoO₂, predicted to be dissolved in the Ba-perovskite phase of the general formula (Ba₁-x·y·SrₓCsᵧ)(U,Pu,Zr,Mo,RE)O₃ [2,32]. Given the abundance of Mo and its capacity to oxidize, it is admitted to serve as a sensitive indicator of the fuel oxidation state. However, the latter Mo(IV) phase is poorly observed in the case of UOX SNFs from PWRs, and the presence of simple MoO₂ is only supported from a thermodynamical point of view but has never been evidenced experimentally [2,33]. This work was thus dedicated to studying the specific impact of Mo in the metallic form at different contents on the dissolution kinetics of UO₂ in nitric acid medium under representative reprocessing conditions. A special attention was devoted to understanding separately the dissolution behavior of pure metallic Mo in nitric acid under the same conditions since its oxidation mechanism in nitric acid is controversial and not well-established.

## 2. Experimental

### 2.1. Preparation and characterization of UO₂-based model compounds incorporating Mo⁰

To reduce the complexity of the SNF material and the Mo chemical system, the use of simplified UO₂-based model compounds is necessary,
in addition to the major difficulties associated with handling radioactive materials. It is worth noting that the study of SNF dissolution, although essential, remains mainly empirical. The mechanism controlling the kinetics of UO₂ dissolution in nitric acid medium largely depends on the experimental conditions, particularly on nitric acid concentration and temperature [9–11]. The dissolution kinetics can also vary considerably depending on the solid properties of the dissolved sample, including density, porosity, geometry, reactive surface area, chemical composition, etc. [13]. Consequently, in order to be able to evidence the specific impact of Mo in the metallic form on the dissolution kinetics of UO₂, we used a series of UO₂ pellets incorporating different contents of nanosized metallic Mo particles between 0 and 9 mol % Mo, consistent in terms of geometry as well as structural and microstructural properties (Table 1).

These pellets were prepared in the previous study by Husainy et al. [34] starting from powders obtained by hydroxide precipitation followed by pressing and then sintering at 1873 K under a reducing atmosphere (Ar + 4 % H₂). The authors showed using PXRD analyses that the only phases constituting the pellets are nearly stoichiometric UO₂ of fluorite-type cubic structure (space group *Fm$\overline{3}$m*) and metallic Mo of body-centered cubic structure (space group *Im$\overline{3}$m*) and that the Mo solubility is very likely negligible in the UO₂ matrix [34]. Furthermore, these pellets have close relative densities (83–89 %), and the open porosity is dominant over the closed porosity in all of them (Table 1). Additionally, they exhibit a representative microstructure similar to that of the real SNF, as shown in Fig. 1, with the Mo metallic particles closely mimicking the ε-Ru (Mo, Tc, Rh, Pd) metallic particles in terms of particle size (19–26 nm), spherical geometry, and homogeneous distribution across the pellets, except that the UO₂ grains are smaller (0.4–1.9 µm) [34]. The smaller grain size was attributed to the inhibitory effect of Mo addition on UO₂ grain growth during sintering (solute drag effect) as well as the choice of the thermal treatment conditions during both calcination and sintering steps (mainly temperature and duration) [34].

The specific surface area of the solid, although difficult to estimate during the progress of dissolution, is a key parameter to be taken into account, especially when comparing the dissolution rates of samples exhibiting different microstructures. In this study, the initial specific surface area ($S_{SA}$ in m²·g⁻¹) developed by each sintered pellet was determined by the SESAM (Study of Evolving Surface Area by Microscopy) method [35]. This method was employed as the conventional geometrical determination strongly underestimates the real surface area of the solid/solution interface by several orders of magnitude due to the presence of pores, cracks, and roughness [18,35,36]. The analyzed pellets were first polished from both sides using a series of SiC papers of different grades wetted with water, followed by a final polishing step with 1 µm diamond paste and cleaning ultrasonically with ethanol for a few minutes. For each pellet, a total of 10 recorded images with a magnification of × 1000 and an accelerating voltage of 15 kV under high

<figure><img src="image_5.png" /><figcaption>Fig. 1. SEM micrographs of a prepared UO₂ + 4 mol % Mo (a) and of a real SNF (extracted from [1]) (b).</figcaption></figure>

vacuum from different selected zones (5 from each pellet side) were binarized using the Fiji software [37] to determine the surface area of the pores observed in each zone. Black pixels in the binary image were attributed to the pores, whereas white pixels corresponded to the grains. The pore size distribution was then evaluated from the binarized images using the “analyze particles” plugin implemented in the software. The analyzed pellet zones were considered to be representative of the whole sample, and the $S_{SA}$ value was thereafter calculated by assuming that the pores were cylindrical in shape and taking into account the volume of the open porosity as well as the mass and dimensions of the sample pellet. The determined $S_{SA}$ values are given in Table 1 and were used for the normalization of the weight loss of U and Mo from each dissolved sample pellet and, more importantly, for comparing the dissolution rates.

SEM micrographs recorded at high magnification and low acceleration voltage of 3 kV under high vacuum were also used to determine the size distribution of the Mo particles observed at the UO₂ pellet surface and calculate the specific surface area of the metallic Mo particles, $S_{SA}^{Corr}(Mo)$ in m²·g⁻¹ (Table 1). The method used to determine Mo particle size distribution from SEM micrographs analysis was described in details in [34]. Typically, 200 to 800 Mo particles were analyzed for each sample from various selected zones. The average Mo particle diameter, $G_{Mo}$ was calculated from the particle size distribution (Table 1). Given that these particles have spherical geometry and density of 10.2 g·cm⁻³, the total number (N) and the mass of the Mo metallic spheres in each pellet were first calculated, assuming that they all have the same diameter ($G_{Mo}$), and the $S_{SA}^{Corr}(Mo)$ was then calculated as follows:

$$S_{SA}^{Corr}(Mo) = N \times \frac{\pi G_{Mo}^2}{m_{(Total Mo)}} \quad (5)$$

Where $m_{(Total Mo)}$ is the total mass of Mo in the pellet before dissolution (in g).

**Table 1**

The initial mass ($m_0$) and microstructural properties of the prepared UO₂ + Mo pellets: relative density ($d_{geo}/d_{calc}$), closed porosity ($P_{closed}$), open porosity ($P_{open}$), specific surface area of the pellets ($S_{SA}$), average UO₂ grain size ($G_{UO2}$), average Mo particle size ($G_{Mo}$), Mo surface coverage, total number of Mo spheres ($N_{(total Mo spheres)}$), and total specific surface area developed by the Mo spheres ($S_{SA}^{Corr}(Mo)$). The microstructural properties are characterized in detail in [34].

<table><thead><tr><th>Pellet composition</th><th><i>m</i><sub>0</sub> (±1 mg)</th><th><i>d</i><sub>geo</sub>/<i>d</i><sub>calc</sub> (%)</th><th><i>P</i><sub>closed</sub> (%)</th><th><i>P</i><sub>open</sub> (%)</th><th><i>S</i><sub>SA</sub> (m<sup>2</sup>·g<sup>-1</sup>)</th><th><i>G</i><sub>UO2</sub> (µm)*</th><th><i>G</i><sub>Mo</sub> (nm)</th><th>Surface coverage (%)</th><th><i>N</i><sub>(total Mo spheres)</sub></th><th><i>S</i><sub>SA</sub><sup>Corr</sup> (Mo) (m<sup>2</sup>·g<sup>-1</sup>)</th></tr></thead><tbody><tr><td>UO<sub>2</sub></td><td>134</td><td>83 ± 1</td><td>1 ± 1</td><td>16 ± 1</td><td>(1.8 ± 0.2) × 10<sup>-2</sup></td><td>1.90</td><td>–</td><td>–</td><td>–</td><td>–</td></tr><tr><td>UO<sub>2</sub> + 0.6 mol % Mo</td><td>121</td><td>82 ± 2</td><td>1 ± 1</td><td>17 ± 1</td><td>(3.1 ± 0.3) × 10<sup>-2</sup></td><td>1.71</td><td>26 ± 14</td><td>0.14 ± 0.04</td><td>(4 ± 1) × 10<sup>12</sup></td><td>23 ± 2</td></tr><tr><td>UO<sub>2</sub> + 2 mol % Mo</td><td>171</td><td>87 ± 1</td><td>2 ± 1</td><td>11 ± 1</td><td>(4.7 ± 0.8) × 10<sup>-2</sup></td><td>0.44</td><td>19 ± 6</td><td>0.61 ± 0.01</td><td>(37 ± 3) × 10<sup>12</sup></td><td>31 ± 1</td></tr><tr><td>UO<sub>2</sub> + 4 mol % Mo</td><td>149</td><td>88 ± 2</td><td>1 ± 1</td><td>11 ± 1</td><td>(4.9 ± 0.5) × 10<sup>-2</sup></td><td>0.50</td><td>25 ± 10</td><td>0.97 ± 0.23</td><td>(26 ± 2) × 10<sup>12</sup></td><td>24 ± 1</td></tr><tr><td>UO<sub>2</sub> + 6 mol % Mo</td><td>147</td><td>89 ± 1</td><td>5 ± 1</td><td>6 ± 1</td><td>(4.1 ± 0.7) × 10<sup>-2</sup></td><td>0.46</td><td>19 ± 11</td><td>1.81 ± 0.25</td><td>(90 ± 16) × 10<sup>12</sup></td><td>31 ± 2</td></tr><tr><td>UO<sub>2</sub> + 9 mol % Mo</td><td>161</td><td>84 ± 1</td><td>1 ± 1</td><td>15 ± 1</td><td>(6.8 ± 2.3) × 10<sup>-2</sup></td><td>0.55</td><td>26 ± 16</td><td>2.44 ± 0.12</td><td>(59 ± 12) × 10<sup>12</sup></td><td>22 ± 2</td></tr></tbody></table>

N.B.*: uncertainty on $G(UO_2)$ is almost ½ $G(UO_2)$.
In addition, in order to understand separately the oxidation behavior of metallic Mo in nitric acid and to evaluate the influence of the metallic Mo particles on the UO₂ dissolution mechanism, a foil was cut from a pure metallic Mo crucible source (CeraQuest, 99 %) (Supporting Information, Fig. S1). The Mo crucible was initially annealed at 1873 K for 6 h under Ar + 4 % H₂ reducing atmosphere to ensure that it is completely metallic and to remove any industrial artifacts (e.g., contaminants, residual stresses, or other defects arising during crucible processing). The annealed crucible was completely free from pores and surface defects and showed coarse Mo grains (Supporting Information, Fig. S1). The Mo foil has the shape of a cuboid with a mass of 0.133 g, length of 0.93 cm, width of 0.8 cm, and thickness of 0.015 cm. In this case, the specific surface area used for the normalization of the weight loss of Mo was the geometrical surface area ($S_{SA}^{geo}$) of the foil in contact with the solution, which was found to be 1.1605 × 10⁻³ m².g⁻¹.

## 2.2. Dissolution protocol and analyses in the leachates

The dissolution mechanism of UO₂ in nitric acid is most likely autocatalytic and involves the participation of other solubilized reactive nitrogen species (HNO₂, NO₂, and NO), generated at the solid/solution interface. In the catalytic regime, the kinetics of the oxidation-reduction reaction can be very fast, and thus a dissolution protocol suitable for the study of rapid reactions is required while ensuring kinetic control by the chemical reaction. This can be achieved thanks to a “dynamic protocol” design that presents a continuous, strong renewal of the leaching solution, and sufficient stirring. This is useful for limiting the thickness of the diffusion boundary layer and the possible accumulation of catalytic species within this layer at the solid/solution interface (i.e., limiting the formation of concentration gradients between the homogeneous solution bulk and the surface of the solid), as well as limiting the occurrence of any saturation phenomena. One of the main advantages of the dynamic protocol is the possibility to track the evolution of the concentration of the species of interest as a function of the dissolution time without perturbation of the reaction medium. Indeed, this protocol greatly facilitates the study of rapid reactions without modifying the mechanism(s) involved during dissolution. Nevertheless, the main disadvantage lies in the large quantity of liquid waste produced.

### 2.2.1. Description of the dissolution protocol under dynamic conditions

At an industrial scale, the UOX SNFs discharged from LWRs are dissolved in HNO₃ of 3–4 mol.L⁻¹ at a high temperature (353–368 K) for a duration of a few hours (~ 2 h) to achieve a final target U concentration in the range of 200–250 g.L⁻¹ [5]. In this study, all the dissolution experiments were carried out under similar PUREX reprocessing conditions in 4 mol.L⁻¹ HNO₃ at 353 K following a dynamic protocol. The experimental setup of the used protocol is represented in Fig. S2 (Supporting Information).

The dissolution tests were performed in 25 mL polytetrafluoroethylene (PTFE) reactors. The sample (pellet or foil) was placed in the reactor on a perforated PTFE chair-like support (Supporting Information, Fig. S2). The reactor was then closed hermetically and connected using PTFE capillaries to the dissolution system. Afterwards, the reactor was placed in an aluminum heating block in order to maintain the temperature of the dissolution medium at 353 K. The bottle containing the feeding solution of 4 mol.L⁻¹ HNO₃ was heated in a sand bath on a heating plate, and the temperature was also maintained at 353 K. The inlet of the dissolution reactor was connected to a peristaltic pump (MINIPULS 3 Gilson), which allowed the continuous flow of the hot nitric acid solution into the reactor at a fixed rate of 2 mL.min⁻¹. The outlet of the reactor was connected to an automatic fraction collector (Lambda OMNICOLL), and the solution was regularly sampled at a fixed time.

### 2.2.2. Determination of elemental and nitrous acid concentrations during dissolution

The evolution of the elemental concentration of the solid dissolved at time (t) was determined by analyzing the collected samples during dissolution by ICP-OES. Sample aliquots were diluted in 0.2 mol.L⁻¹ HNO₃ prior to analysis. The intensity of the emitted spectral signal was recorded at three characteristic wavelengths for each element: λ = 202.030, 203.844, and 204.598 nm for Mo, and λ = 367.007, 385.958, and 409.014 nm for U to avoid any interferences between the emission spectra of the two elements. The measurements were calibrated by analyzing mixed U and Mo standard solutions prepared by diluting 1000 ppm reference certified solutions (PlasmaCal from SCP Science) in 0.2 mol.L⁻¹ HNO₃. The concentrations of U and Mo elements in solution were calculated from the average concentration of at least three replicates.

At the same time, to assess the influence of Mo on the production of auto-catalytic species during UO₂ dissolution in nitric acid, the evolution of nitrous acid concentration was determined by directly quenching sample aliquots collected from the dissolution reactor into an excess of 8 mol.L⁻¹ sodium hydroxide (NaOH). The concentration of the nitrite ions (NO₂) formed was subsequently analyzed by high performance ion chromatography (HP-IC, Dionex Integron HPIC). Indeed, the addition of excess NaOH was necessary to stabilize HNO₂ as NO₂ ions and to provoke the precipitation of the uranyl cations present in solution. This step also enabled us to anticipate the progress of the dissolution of the UO₂-based pellets and to monitor the duration of the experiment, which was stopped after approximately 1 to 2 h once the yellow uranyl precipitates were no longer observed.

After allowing sufficient time for the yellow uranyl precipitates to settle in each tube, a known volume of the supernatant was withdrawn, diluted with distilled water, and transferred into 1.5 mL sample vials. The pH of the prepared samples for HP-IC analysis was in the range between 13 and 14. Calibration was performed using NO₂ standard solutions of known concentrations (0–50 ppm), prepared by diluting a 1000 ppm NO₂ certified standard (CertIPUR® Merck) solution with distilled water and adjusting the pH to 13–14 using 8 mol.L⁻¹ NaOH. Fig. S3 (Supporting Information) shows the chromatogram of the known standards and the calibration curves established for the measurement of the concentration of nitrite ions. A characteristic peak corresponding to the NO₂ anions was detected after an elution time of approximately 4.27 min using 55 mmol.L⁻¹ potassium hydroxide (KOH) as an eluent. The NO₂ concentration was subsequently quantified from the peak area. The measurements were repeated 3 times (3 injections per sample vial) with a total run time of 15 min per sample.

### 2.2.3. Dissolution data treatment

The relative mass loss (Δmᵢ(t)/m₀ in %) for the solid dissolved at time (t) under dynamic conditions was calculated according to the following equation:

$$ \frac{\Delta m_i(t)}{m_0} = \int_0^t \frac{C_i(t) \times Q \times 100}{f_i \times m_0} dt \quad (6) $$

where mᵢ(t) (g) corresponds to the total amount of the element i (U or Mo) released in the solution at time t (h), m₀ (g) is the initial mass of the solid dissolved, Cᵢ(t) (g.L⁻¹) is the elemental concentration at t determined by ICP-OES, Q (L.h⁻¹) is the flow rate and was fixed at 0.12 L.h⁻¹ for all the dissolution tests performed in this study, and fᵢ is the mass fraction of the element i in the solid.

The normalized weight loss (Ntᵢᵢ(t) in g.m⁻²) for the solid dissolved at time (t) under dynamic conditions was also calculated from the elemental concentrations after normalization by the specific surface area (SA in m².g⁻¹) of the solid in contact with the solution according to the following equation:
$$N_{L,i}(t) = \int_{0}^{t} \frac{C_i(t) \times Q}{f_i \times S_{SA} \times m_0} dt \quad (7)$$

It is worth noting that $S_{SA}$, $f_i$, and $Q$ were considered as constant parameters; therefore, $N_{L,i}(t)$ is proportional to $\Delta m_i(t)/m_0$. The normalized dissolution rate ($R_L(i)$ in g.m$^{-2}$.h$^{-1}$) for the solid dissolved at time (t) under dynamic conditions is defined as the time derivative of the normalized weight loss and was determined from the slope of the linear part of the $N_{L,i}(t)$ dissolution curves by linear regression as follows:

$$R_L(i) = \frac{d(N_{L,i}(t))}{dt} \quad (8)$$

Another estimation method was taken into account for correcting the normalized weight loss of Mo ($N_{L,Mo}^{Corr}(t)$ in g.m$^{-2}$) as well as the Mo dissolution rate ($R_L^{Corr}(Mo)$ in g.m$^{-2}$.h$^{-1}$) by the total surface area of the metallic Mo particles in each pellet ($S_{SA}^{Corr}(Mo)$ in m$^2$.g$^{-1}$) (Table 1).

These equations were used to compare the kinetics of dissolution of the different samples dissolved in the same hot, concentrated nitric acid medium (4 mol.L$^{-1}$, 353 K). The pure $UO_2$ sintered pellet and the Mo metallic foil were considered reference materials for comparison.

## 2.3. Characterization of the remaining residues following the dissolution of $UO_2 + Mo^0$ pellets and $Mo^0$ foil

Scanning electron microscope (SEM) observations of the remaining residues following the dissolution of the samples were performed by an environmental scanning electron microscope (ESEM, FEI Quanta 200 ESEM FEG) using a backscattered electron detector (BSE) or a secondary electron detector (SE) in vacuum conditions at an optimal working distance of about 10 mm. A part of the residues was taken out of the acid and directly deposited on a carbon dish without any additional sample preparation. The SEM micrographs of the residues were recorded with a low accelerating voltage of 5–10 kV. In order to verify if any mixed U and Mo precipitates were formed under the studied dissolution conditions, the chemical composition of the residues was checked by energy dispersive X-ray spectroscopy (EDS) coupled with SEM using the same operating conditions.

In addition, transmission electron microscope (TEM) micrographs of the residues were recorded at the MEA Platform in the University of Montpellier, using a JEOL 2200 FS-200 kV high-resolution microscope (HR-TEM) equipped with a CCD camera (GATAN USC 4092 × 4092 px$^2$). Before image collection, a part of the residues was separated from the acid by centrifugation and stored in ethanol. The residues were then placed on a support grid made of copper and covered with lacey carbon films. Furthermore, EDS analyses were also obtained in the scanning transmission electron microscopy (STEM) mode by an Oxford Instrument X-MaxN 100 TLE detector with a spot size of 0.7 nm.

In order to identify the precise nature of the brown-red Mo precipitates observed following the dissolution of metallic Mo (pure or mixed with $UO_2$) in nitric acid and to determine the oxidation state of Mo, synchrotron X-ray diffraction (XRD) measurements were performed in combination with X-ray absorption near edge spectroscopy (XANES) measurements at the Rossendorf beamline (ROBL-II, BM20) of the European Synchrotron Radiation Facility (ESRF, Grenoble, France) on the same XRD-2 station [38]. The precipitates were withdrawn with nitric acid from the reactor and then placed in a thin borosilicate glass capillary of about 0.5 mm in diameter. For radioprotection reasons, the fragile capillary was afterwards confined in a protective Kapton polyimide tube of 1 mm in diameter and sealed from both sides with epoxy glue. The resulting capillary of about 30 mm in length was fixed in a hollow brass pin sample holder with glue. Fig. S4 (Supporting Information) shows a prepared sample ready for analysis with the corresponding dimensions. The XRD data were recorded using a Pilatus 2 M detector and monochromatic radiation of 17 keV ($\lambda$ = 0.7276 Å) conditioned by two Rh-coated mirrors and a silicon (111) double crystal

monochromator [38]. Measurements were collected in an angular range 2θ between 0 and 47°, with an acquisition time of 30 s per scan. NIST SRM660 lanthanum hexaboride (LaB$_6$) was used to calibrate the detector geometry parameters.

The XRD measurements were combined with XANES measurements. They were performed using a single-element Si drift detector (Vortex X90 CUBE) with a FalconX1 processor [38]. The XANES spectra were collected at the Mo K-edge (20,000 eV) in the fluorescence mode, with continuous scanning in the range between 19,940 and 20,200 eV using a step size of 1 eV and a total acquisition time of 15 min per scan. Several acquisitions (3 to 6 scans) were recorded for each sample and summed up in order to improve the signal-to-noise ratio. Data averaging and normalization were done in the same range using a specific procedure developed by the beamline scientists [38]. A series of Mo reference sample powders of high purity (≥ 99%, Sigma Aldrich) having different oxidation states: Mo (0), $MoO_2$ (IV), and $\alpha-MoO_3$ (VI) were similarly prepared in capillaries and analyzed for comparing the Mo spectral signatures.

# 3. Results and discussion

## 3.1. Dissolution results of the $UO_2 + Mo^0$ pellets

### 3.1.1. Mass loss trends and evaluation of the normalized dissolution rates of U and Mo

The evolution of the relative and normalized mass losses corresponding to the dissolution of the pure $UO_2$ reference sample pellet and the $UO_2$ pellets containing metallic Mo particles between 0.6 and 9 mol % in $4 \text{ mol.L}^{-1} \text{ HNO}_3$ at 353 K are represented in Fig. 2. Each dissolution experiment was carried out over a period of 3 to 5 h, and the mass loss curves relative to both U and Mo from all the experiments showed the same sigmoidal “S-like shaped” curve trends.

According to Fig. 2, two kinetic regimes could be identified from the curves of U and Mo mass losses. The first observed kinetic regime corresponds to the so-called “induction period”, during which the rate of mass loss for both $UO_2$ and Mo was relatively slow at the beginning of dissolution. The duration of this step was short on account of the aggressive dissolution conditions and was maintained for about 20 to 30 min in all the experiments. This period was reported in several studies [10,11,18,39,40] related to the dissolution of pure $UO_2$ compounds in similar conditions and was referred by Bertolotto et al. [40] to the non-catalyzed oxidation mechanism (represented by Eq. (1)), during which the autocatalytic nitrogen species are accumulating in the solution until their amount becomes sufficient enough to start interacting effectively with $UO_2$ and initiate the autocatalytic mechanism following the previously stated dissolution equations (Eqs. (2)–(4)). Cordara et al. [39] determined the end of the induction period at a $UO_2$ dissolved mass loss of about 0.5 %.

The second observed kinetic regime (after 30 min) is characterized by a sharp increase in the rate of mass loss of both U and Mo from the pellets, and consequently a sharp increase in the U and Mo concentrations in the solution. This could be mainly attributed to the transition from the non-catalyzed to the autocatalyzed dissolution mechanism as well as to the increase in the reactive surface area during the dissolution of the pellets [36,39]. Eventually, the rates of U and Mo mass losses slowed down again and reached a plateau, suggesting that most of the reactive material has already been dissolved, and thus the remaining material dissolves much more slowly.

In addition, the relative mass loss of $UO_2$ was approximately twice higher than that of Mo (Fig. 2). This indicated that the $UO_2$ matrix was more soluble than the nanosized metallic Mo particles in the nitric acid solution for all the dissolved sample pellets. The dissolution of the $UO_2$ matrix was almost complete with a U mass loss ≥ 90%, whereas the Mo mass loss was only between 40 and 60% (Table 2 and Supporting Information, Fig. S5). Both U and Mo mass losses reached the plateau phases at the same time, and then Mo ceased further dissolution.
<figure><img src="image_6.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>(e)</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>(f)</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 2.** Evolution of the relative and normalized mass losses of U and Mo (by the $S_{SA}$ of the pellets) during dissolution of the pellets in 4 mol.L⁻¹ HNO₃ at 353 K under dynamic conditions: (a) UO₂ (pure), (b) UO₂ + 0.6 mol % Mo, (c) UO₂ + 2 mol % Mo, (d) UO₂ + 4 mol % Mo, (e) UO₂ + 6 mol % Mo, and (f) UO₂ + 9 mol % Mo. The normalized dissolution rates of U and Mo were determined from the slopes of the blue lines.</figcaption></figure>

Therefore, the synchronized dissolution behavior of UO₂ and Mo indicated that the dissolution processes for both elements are interconnected. Some level of interaction between the dissolving UO₂ matrix and Mo particles was suspected.

In order to highlight the impact of the metallic Mo particles on the

dissolution of UO₂ in nitric acid, the normalized dissolution rates $R_L(U)$ (g.m⁻².h⁻¹) of all the pellets were determined from the normalized weight loss regions corresponding to the autocatalyzed dissolution mechanism following the induction period by linear regression, as illustrated in Fig. 2. It should be noted that this method provides an
<caption>**Table 2**</caption>

Comparison of the determined relative mass losses and the normalized dissolution rates of U and Mo from the pellets with those of the pure UO₂ and Mo⁰ reference samples, following dissolution in 4 mol·L⁻¹ HNO₃ at 353 K under dynamic conditions.

<table><thead><tr><th>Sample</th><th>Final<br>Δm<sub>U</sub>/m<sub>0</sub> ( %)</th><th>Final<br>Δm<sub>Mo</sub>/m<sub>0</sub> ( %)</th><th>R<sub>L</sub> (U)<sup>z</sup><br>(g·m<sup>-2</sup>·h<sup>-1</sup>)</th><th>R<sub>L</sub><br>(Mo)<sup>*</sup><br>(g·m<sup>-2</sup>·h<sup>-1</sup>)</th><th>R<sub>L</sub><sup>Corr</sup> (Mo)<sup>†</sup><br>(g·m<sup>-2</sup>·h<sup>-1</sup>)</th></tr></thead><tbody><tr><td>UO<sub>2</sub></td><td>88.4</td><td>–</td><td>39.1 ± 0.4</td><td>–</td><td>–</td></tr><tr><td>Mo<sup>0</sup> foil</td><td>–</td><td>86.3</td><td>–</td><td>1074 ± 25</td><td>1074 ± 25</td></tr><tr><td>UO<sub>2</sub> + 0.6<br>mol % Mo</td><td>89.3</td><td>39.6</td><td>38.9 ± 1.8</td><td>18.8 ± 0.6</td><td>(229 ± 11) × 10<sup>-4</sup></td></tr><tr><td>UO<sub>2</sub> + 2 mol<br>% Mo</td><td>97.5</td><td>45.7</td><td>18.7 ± 0.3</td><td>8.7 ± 0.1</td><td>(134 ± 2) × 10<sup>-4</sup></td></tr><tr><td>UO<sub>2</sub> + 4 mol<br>% Mo</td><td>96.7</td><td>49.2</td><td>13.2 ± 0.1</td><td>6.6 ± 0.1</td><td>(139 ± 1) × 10<sup>-4</sup></td></tr><tr><td>UO<sub>2</sub> + 6 mol<br>% Mo</td><td>91.1</td><td>58.3</td><td>6.2 ± 0.1</td><td>4.0 ± 0.1</td><td>(53 ± 1) × 10<sup>-4</sup></td></tr><tr><td>UO<sub>2</sub> + 9 mol<br>% Mo</td><td>99.5</td><td>62.6</td><td>8.4 ± 0.3</td><td>4.8 ± 0.1</td><td>(147 ± 3) × 10<sup>-4</sup></td></tr></tbody></table>

apparent dissolution rate since it is based exclusively on the elemental concentrations of the solid dissolved in solution and does not consider the potential formation of secondary solid phases. The results were compared to the R<sub>L</sub> (U) determined for the pure UO₂ pellet used as a reference material and are given in Table 2.

The results presented in Table 2 showed that the dissolution rate of UO₂ in nitric acid decreases significantly depending on the content of the metallic Mo particles in the pellets. For 0.6 mol % Mo in UO₂, the normalized dissolution rate R<sub>L</sub>(U) was 38.9 ± 1.8 g·m⁻²·h⁻¹, close to that obtained for pure UO₂ of 39.1 ± 0.4 g·m⁻²·h⁻¹. Under the same dissolution conditions, the UO₂ dissolution rate decreased with the increase in the Mo content beyond 0.6 mol %. Specifically, R<sub>L</sub>(U) for UO₂ with 2, 4, 6 and 9 mol % Mo decreased by a factor of 2, 3, 6.3 and 4.7, respectively. Consequently, the presence of Mo particles inhibits the dissolution of UO₂ in nitric acid. This result was in accordance with previous studies reported by T. Cordara [36] and T. Kaczmarek [41], who also reported an inhibitory effect of Mo metallic particles on UO₂ dissolution in nitric acid at a content of 3 mol %. It is important to note that the inhibitory impact of Mo on UO₂ dissolution would have been even more significant if the grain size of the pure UO₂ reference sample was smaller.

In addition, the obtained dissolution rates of Mo, R<sub>L</sub>(Mo) also seemed to be influenced by the Mo content in the pellets. However, these values seemed to be numerically incorrect because the specific surface area used to determine the normalized weight loss for Mo in each case (Fig. 2) was that of the pellet and not that developed by the Mo particles themselves. In order to correct this bias, the normalized weight losses of Mo, N<sub>L.Mo</sub><sup>Corr</sup>(t) (in g·m⁻²), were recalculated using the total surface area of the metallic Mo particles in the pellets (S<sub>SA</sub><sup>Corr</sup>(Mo) in m²·g⁻¹) (Table 1). The apparent normalized dissolution rate of Mo, R<sub>L</sub><sup>Corr</sup>(Mo) (in g·m⁻²·h⁻¹), was similarly determined for each pellet from the N<sub>L.Mo</sub><sup>Corr</sup>(t) region corresponding to the autocatalyzed dissolution mechanism by linear regression. The results are presented in Fig. S6 (Supporting Information) and Table 2. According to Table 2, the dissolution rate of the metallic Mo particles in nitric acid after correction seemed to be unaffected or varied insignificantly with the Mo content introduced in each UO₂ pellet. In any case, the results showed that the Mo particles dissolved slower than the embedding UO₂ matrix.

This led us to postulate that the inhibitory effect of Mo on UO₂ dissolution could be attributed to several possible mechanisms. For

instance, the physical presence of Mo particles or the formation of less soluble neoformed Mo compounds could possibly act as a barrier and slow down the dissolution rate of the embedding UO₂ matrix. This hypothesis is supported by the fact that only 40–60 % of the metallic Mo particles dissolved despite the aggressive experimental conditions and the strong, continuous renewal of nitric acid in the reaction medium, and the remaining insoluble Mo compounds should be strongly refractory to dissolution. Likewise, the presence of Mo could also possibly interfere with the autocatalytic dissolution mechanism of UO₂ through reacting with the autocatalytic nitrogen species or altering the chemical environment in a way that reduced the efficiency of the dissolution process, for instance by consuming protons. Hence, investigating the nature of the remaining residues from the dissolution experiments and the specific interactions between Mo and the autocatalytic species could provide deeper insights on the exact mechanism(s) by which Mo inhibits UO₂ dissolution.

### 3.1.2. Influence of Mo on the production of autocatalytic species during the dissolution of the pellets

Fig. 3 shows the evolution of the elemental concentrations of U and Mo as well as that of nitrous acid for the considered dissolution experiments. The examination of the shapes of the U and Mo concentration curves obtained for each sample pellet showed similar sequences in the release of U and Mo during the dissolution in the nitric acid medium. Such synchronized release patterns may imply that the Mo metallic particles dissolve in HNO₃ following a mechanism similar to that of UO₂. Another possible significance could be the existence of a physical or chemical interaction between U and Mo during the dissolution of the pellets, as previously indicated.

At the same time, analysis of the nitrous acid concentration was carried out in order to verify if Mo has an influence on its production during UO₂ dissolution in nitric acid. In the case of pure UO₂ dissolution in Fig. 3(a), the HNO₂ concentration was almost equal to that of the U concentration, suggesting the generation of HNO₂ at a molarity equal to that of dissolved uranium. This was in agreement with several studies [12,13,17,18,22] that reported the formation of one mole of nitrous acid per mole of dissolved UO₂ according to Eq. (9). Once again, it is worth noting that HNO₂ is unlikely to be produced directly from the reaction between UO₂ and HNO₃ at the solid/solution interface but rather from the reduction of HNO₃ by NO (Eq. (4)) and NO₂ disproportionation (Eq. (3)) [26–28]. In this study, the use of dynamic experimental conditions, which do not require the opening of the reactor, prevented the elimination of the NOx gases and favored their retention within the nitric acid solution and their recombination into nitrous acid.

$$UO_{2(s)} + 3 HNO_{3(aq)} \rightarrow UO_{2(aq)}^{2+} + 2NO_{2(aq)} + HNO_{2(aq)} + H_{2}O_{(l)} \quad (9)$$

In the case of the UO₂ pellets containing metallic Mo particles at different contents (Fig. 3(b), (e), and (f)), the ratio between the U and HNO₂ concentrations was also nearly equimolar, similarly to that for the reference UO₂ dissolution, and remained unchanged with the increase of Mo content in the pellets in the studied range (0.6–9 mol %). This suggested that the presence of the metallic Mo particles did not significantly alter the production of HNO₂ during the dissolution process and therefore did not induce any perturbation of the nitrogen oxide species participating in the autocatalytic mechanism of UO₂ dissolution in the nitric acid medium. Even in the case of the largest Mo content (i.e. 9 mol %, Fig. 3(f)), where the dissolved Mo concentration represented about 5 % of the U concentration and thus, could induce significant perturbations of the HNO₂ production rate, the HNO₂ concentration was equal to that of dissolved uranium. Therefore, the Mo inhibition of UO₂ dissolution could not be directly explained by a lack of the autocatalytic species in solution. On the other hand, maintaining an equimolar concentration of HNO₂ and U in solution could indicate that Mo also contributes to the production of HNO₂ when dissolving. If both UO₂ and Mo are seemingly oxidized through the same dissolution mechanism, they

<footnote>* calculated using S<sub>SA</sub> of the pellet.</footnote>

<footnote>† calculated using the specific surface area of the Mo particles, S<sub>SA</sub><sup>Corr</sup>(Mo).</footnote>
<figure><img src="image_12.png" /><figcaption>(b)</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>(c)</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>(d)</figcaption></figure>

<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>Fig. 3. Evolution of the U, HNO₂ (left y axis) and Mo (right y axis) concentrations during the dissolution of the sintered sample pellets in 4 mol.L⁻¹ HNO₃ at 353 K under dynamic conditions: (a) UO₂ (pure), (b) UO₂ + 0.6 mol % Mo, (c) UO₂ + 2 mol % Mo, (d) UO₂ + 4 mol % Mo, (e) UO₂ + 6 mol % Mo, and (f) UO₂ + 9 mol % Mo.</figcaption></figure>

might be competing for the available oxidizing reagents in the solution, thus leading to a decrease of UO₂ dissolution rate.

### 3.1.3. Characterization of the remaining residues following the dissolution of the UO₂ + Mo pellets

Upon opening the PTFE reactors at the end of the dissolution

experiments, the remaining residues observed with the naked eye from the dissolution of the pure UO₂ and UO₂ + Mo pellets with contents lower than 6 mol % Mo in nitric acid were black in color (similar to those shown in Fig. 4(a)). On the other hand, brown-red precipitates were also clearly visible along with black precipitates following the dissolution of the UO₂ + Mo pellets with higher Mo contents, as shown in Fig. 4(b),
<figure><img src="image_18.png" /><figcaption>**Fig. 4.** Images of the remaining residues at the bottom of the PTFE reactors following the dissolution of UO₂ + 4 mol % Mo (a) and UO₂ + 9 mol % Mo (b) in 4 mol.L⁻¹ HNO₃ at 353 K under dynamic conditions.</figcaption></figure>

indicating the formation of Mo-related compounds.

Fig. 5(a) and (b) show the micrographs of the black and brown-red precipitates, respectively, collected from the dissolution of UO₂ + 9 mol % Mo pellet, with their corresponding EDS spectra. The black

residues in Fig. 5(a) correspond to the undissolved UO₂ matrix of the pellet. No metallic Mo particles were observed or detected in these precipitates, thus indicating that they were removed from the UO₂ matrix during dissolution. The microstructure developed at the end of the dissolution was consistent with that observed for the black residues of the attacked pure UO₂ pellet in Fig. 6 and showed UO₂ grains with dissolution facies characteristic of certain crystallographic orientations along with corrosion pits dug in the form of triangular and square-based pyramidal etch pits as well as holes and cavities. These observations were similar to those reported by Bertolotto et al. [40,42] during the dissolution of UO₂ single crystals with (100), (110), and (111) oriented faces in 2 and 4 mol.L⁻¹ HNO₃ at room temperature.

The topographical features observed in Figs. 5 (a) and 6 suggested a heterogeneous progress of the dissolution reaction at the solid/solution interface for the different UO₂-based pellets and the existence of preferential dissolution zones, such as grain boundaries and pores. It is possible that the formation of cavities is related to the progressive digging and expansion of pre-existing pores that increase the vulnerability of the pellets to acid attack. The accumulation of reactive species in certain zones at the solid/solution interface could also possibly provoke

<figure><img src="image_19.png" /><figcaption>**Fig. 5.** SEM-BSE micrographs showing the microstructure of the black precipitates (a) and brown-red precipitates (b) collected following the dissolution of an attacked UO₂ + 9 mol % Mo pellet in 4 mol.L⁻¹ HNO₃ at 353 K under dynamic conditions, with the corresponding EDS spectra.</figcaption></figure>
<figure><img src="image_20.png" /><figcaption>**Fig. 6.** SEM-EDS micrographs showing the microstructure of an attacked pure UO₂ pellet (black residues) at the end of the dissolution experiment in 4 mol.L⁻¹ HNO₃ at 353 K under dynamic conditions. The arrows indicate: (1) square-based pyramidal etch pit, (2) triangular etch pit, and (3) holes and cavities.</figcaption></figure>

stronger surface drilling, thus leading to the formation of corrosion pits and cavities.

In addition, Fig. 5(b) indicated that the observed brown-red precipitates correspond to oxidized Mo compounds that do not contain uranium. Consequently, it can be deduced that no mixed U and Mo precipitates were formed during the dissolution of the UO₂ + Mo pellets in the 4 mol.L⁻¹ HNO₃ at 353 K. This was supported by STEM-EDS mapping images (Fig. 7) of residues recovered from the dissolution of UO₂ + 4 mol % Mo and UO₂ + 9 mol % Mo pellets, which clearly showed the presence of uranium-free oxidized Mo compounds and molybdenum-free UO₂ residues, in agreement with the results presented in Fig. 5. Hence, the brown-red oxidized Mo phases were present at the end of the dissolution of the UO₂ pellets with Mo contents lower than 6 mol % but were not as clearly visible as in the case of the dissolved pellets with higher Mo contents (Fig. 4).

The brown-red Mo oxidized precipitates represent 40–60 % of the undissolved metallic Mo particles in the pellets (Supporting Information, Fig. S5), which appeared to be strongly resistant to further dissolution in nitric acid under the dynamic experimental conditions. On the other hand, the remaining Mo soluble species (also 40–60 %) in the considered nitric acid medium (pH < 0) exist in the prevailing +VI oxidation state

and six fold coordination, in the form of monomers and cationic dimers only (no polyoxomolybdates) [43]. The predominance of these species depends mainly on the pH and Mo concentration [43–46]. For Cₘ₀ < 10⁻⁴ mol.L⁻¹ (which is the case in Fig. 3), the mononuclear species are predominant and can be: molybdic acid MoO₃(H₂O)₃ (uncharged) and the cations: [MoO₂(OH)(H₂O)₃]⁺ and [MoO₂(H₂O)₄]²⁺. For Cₘ₀ > 10⁻³ mol.L⁻¹, the dinuclear Mo cations are predominant and can be: [Mo₂O₅(OH)(H₂O)₅]⁺, [Mo₂O₅(H₂O)₆]²⁺, and [Mo₂O₄(OH)(H₂O)₆]³⁺. It is worth noting that the mono- and di-nuclear Mo species can coexist with each other in different proportions within the same medium, once again depending on pH and Mo concentration [43–46]. Besides, the complexation of Mo(VI) with the nitrate ions is stated to be unimportant [47]. The majority of the E-pH diagrams established for the Mo-H₂O system show only the MoO₄²⁻ and HMoO₄ monomer ions and disregard or do not consider many other existing Mo species, especially at low pH. Such Mo(VI) ions are known to coprecipitate with Zr(IV) during SNF dissolution, thus leading to the formation of zirconium molybdate de-posits (ZrMo₂O₇(OH)₂·2H₂O) in the industrial dissolvers [5].

To our knowledge, the nature and composition of Mo oxide phases stable in hot concentrated nitric acid are not well established. Mo could exist within passive films in the form of oxides and/or oxide hydrates in

<figure><img src="image_21.png" /><figcaption>**Fig. 7.** STEM-EDS mapping images of residues collected after the dissolution of UO₂ + 9 mol % Mo (a) and UO₂ + 4 mol % Mo (b) pellets in 4 mol.L⁻¹ HNO₃ at 353 K in dynamic conditions.</figcaption></figure>
one or mixed oxidation states ranging from +III to +VI [48,49]. Fig. S7 (Supporting Information) shows a list of solid and dissolved Mo compounds of different oxidation states with their corresponding colors, compiled by Pourbaix et al. [48], where only those noted as being considered were represented in the Mo Pourbaix diagram established by the authors. By comparison, the brownish color of the Mo precipitates observed following the dissolution of the UO₂ + Mo pellets in this study could imply that Mo contained in these residues could be present as an oxide and/or as an oxide hydrate with an oxidation number between +IV and +VI. Only MoO₂ (+IV), dihydrated MoO₂ (+IV), and molybdenum oxo-hydroxide MoO(OH)₃ (+V) exhibit brown shades in this list (Supporting Information, Fig. S7).

However, according to several studies [48,50,51], the concentrated nitric acid is supposed to cover metallic Mo with a protective oxide layer of MoO₃ (+VI) that should prevent further acid attack and slow down the dissolution of the metal, but anhydrous MoO₃ as well as its hydrated forms are white or yellowish and not brownish (Supporting Information, Fig. S7) [48,52]. Additionally, MoO₂ is not expected to form because nitric acid is also supposed to oxidize MoO₂ into MoO₃ [48]. Therefore, investigating the oxidation mechanism of pure metallic Mo in nitric acid under the same experimental conditions is crucial in order to assess the inhibitory impact of the metallic Mo particles on the dissolution mechanism of UO₂ and is presented in the following paragraphs.

## 3.2. Dissolution results of the pure metallic Mo foil

### 3.2.1. Mass loss trends and determination of the normalized dissolution rate of Mo

Fig. 8 shows the evolution of the relative and normalized mass losses obtained during the dissolution of the pure metallic Mo foil in 4 mol.L⁻¹ HNO₃ at 353 K under dynamic conditions. The mass loss curve of Mo showed a sigmoidal “S-like shaped” curve trend, similar to the U and Mo mass loss trends obtained in the case of pure UO₂ and UO₂ + Mo pellets, which were dissolved under the same experimental conditions.

It was previously illustrated that the synchronized dissolution behavior of UO₂ and Mo suggested similar dissolution mechanisms in nitric acid. It is frequent to find in the literature [41,50] that the dissolution mechanism of metallic Mo in nitric acid is described by the following balanced chemical equation:

$$ Mo(s) + 2 HNO_{3}(aq) \rightarrow MoO_{3}(s) + 2 NO(g) + H_{2}O(l) \quad (10) $$

However, this equation does not seem to be correct, as the dissolution of the metallic Mo particles from the UO₂ + Mo pellets resulted in 40–60 % soluble Mo species in nitric acid. To add, the brown-red color of the

<figure><img src="image_22.png" /><figcaption>Fig. 8. Evolution of the relative mass loss of Mo from the dissolution of the pure Mo metallic foil in 4 mol.L⁻¹ HNO₃ at 353 K under dynamic conditions.</figcaption></figure>

undissolved oxidized Mo precipitates does not suggest that they are MoO₃ or its hydrates (mono or di). In the bibliographic study by E. Esbelin [50], the reaction of metallic Mo with nitric acid was reported to be autocatalytic based on the fact that the addition of sodium nitrite salts (NaNO₂) caused an increase in the Mo dissolution kinetics, while the addition of thiourea as nitrous acid scavengers slowed down the dissolution of Mo. Hence, by analogy to UO₂ dissolution in nitric acid, it can be deduced that the Mo oxidation could also involve the participation of nitrogen species (e.g., HNO₂, NO₂, and NO).

According to Fig. 8, two kinetic regimes could also be identified based on the shape of the Mo mass loss curve. The first observed kinetic regime corresponds to the steady state or induction period, during which the rate of mass loss for Mo was relatively slow at the beginning of dissolution. This step was maintained for around 5 min, which was shorter than the induction period observed during the dissolution of the metallic Mo particles from the UO₂-based pellets of about 20–30 min, even under the same conditions (Fig. 2). This step could similarly be described as a non-catalyzed Mo oxidation mechanism, during which the autocatalytic nitrogen species are accumulating in the solution until their amount becomes sufficient enough to start interacting effectively and initiate an autocatalytic mechanism. The relative mass of Mo dissolved at the end of the induction period was <1 %.

The second observed kinetic regime (after 5 min) is characterized by a sharp increase in the rate of mass loss of Mo and consequently by a sharp increase in the Mo concentration in the solution. This could be mainly attributed to the transition from the non-catalyzed to the autocatalyzed dissolution mechanism as well as to the increase in the reactive surface area during the dissolution of the foil. Finally, the rate of Mo mass loss slowed down again and reached a plateau, suggesting that most of the reactive material has already been dissolved, and thus the remaining material dissolves much more slowly.

Within only 1 h, the metallic Mo foil was attacked appreciably by HNO₃, and the relative mass loss of Mo reached 86 %. This indicated that Mo dissolved faster and more efficiently in the absence of UO₂. As an illustration, the relative mass loss of Mo for UO₂ + Mo pellets was lower and did not exceed 60 %, although the samples were dissolved for a longer duration of 3 to 5 h with continuous renewal of nitric acid. In addition, the normalized dissolution rate of the Mo was similarly determined from the normalized weight loss region corresponding to the autocatalyzed dissolution mechanism following the induction period by linear regression, as illustrated in Fig. 8. The obtained value was found to be 1074 ± 25 g.m⁻².h⁻¹. In terms of absolute weight loss, this translates to approximately 0.165 g.h⁻¹, which aligns closely with the value reported by E.L. Mühr-Ebert [53] of about 0.258 g.h⁻¹, who conducted similar dissolution experiments with pure Mo pellets in 4 mol.L⁻¹ HNO₃ at 363 K. Notably, regardless of the normalization method used to determine the dissolution rates of Mo in the case of the Mo metallic particles within the UO₂ + Mo composite pellets, the dissolution rate of pure metallic Mo remained significantly higher by several orders of magnitude under the considered experimental conditions (Table 2). Furthermore, UO₂ and Mo, when mixed together, showed synchronized dissolution behavior and reached their plateau phases at the same time. The synchronized dissolution behavior of UO₂ and Mo, when mixed, supports the idea of mutual inhibition of the dissolution processes.

### 3.2.2. Influence of pure metallic Mo on the production of auto-catalytic species during dissolution in nitric acid

The analysis of the nitrous acid concentration was carried out in order to verify whether pure metallic Mo is able to produce autocatalytic species during dissolution in nitric acid. Fig. 9 shows a synchronized evolution of the concentrations of Mo and nitrous acid during dissolution. The concentration of nitrous acid was approximately three times higher than that of Mo, indicating the formation of 3 mol of nitrous acid per mole of dissolved Mo (VI) species in nitric acid under the studied experimental conditions. This aligns perfectly with the expected HNO₂:Mo (VI) ratio derived from the combination of Eqs. (10) and (4)
<figure><img src="image_23.png" /><figcaption>**Fig. 9.** Evolution of the Mo and HNO₂ concentrations during the dissolution of the pure metallic Mo foil in 4 mol.L⁻¹ HNO₃ at 353 K under dynamic conditions.</figcaption></figure>

and corresponded to the exchange of 6 e⁻ between NO₃⁻/HNO₂ and Mo⁶⁺/Mo⁰ couples, as shown in Table 3.

This finding suggested that the metallic Mo particles within the UO₂ + Mo pellets should have contributed significantly to the increase in UO₂ dissolution rates. However, this expected increase in UO₂ dissolution was not observed. In the case of the UO₂ + Mo pellets, the concentration of HNO₂ remained equal to the molarity of the dissolved uranium (Fig. 3), and the perturbation of the nitrogen oxide species participating in the autocatalytic mechanisms of both UO₂ and Mo dissolutions in nitric acid was not clearly observed. This can be attributed to the very low content of dissolved Mo in the solution compared to that of U (Fig. 3). Furthermore, the dissolution results of the pure metallic Mo foil indicated that Mo is more reactive than UO₂ in nitric acid and capable of producing more HNO₂, or equivalently, more autocatalytic species. However, the presence of UO₂ seemed to neutralize the potential acceleration effect of Mo, which could possibly be attributed to a competitive consumption of all the species necessary to produce the auto-catalytic nitrogen species (including, H⁺ and HNO₃) and of these autocatalytic species themselves. Indeed, this was indicated by a significant inhibitory impact of UO₂ on Mo dissolution, as the dissolution rate of Mo radically decreased in the presence of UO₂.

In parallel, the slowdown in Mo dissolution also impeded the dissolution of the underlying UO₂ matrix. As a result, it can be deduced

<caption>**Table 3**</caption>

Comparison between the expected and measured (HNO₂, Mo⁶⁺) ratios during Mo (0) and Mo (IV) oxidation by nitric acid. The expected ratios were determined by the redox balance involving the reduction half-reaction of the couple NO₃⁻/HNO₂ and the redox reactions of the couples Mo⁶⁺/Mo⁰ and Mo⁴⁺/Mo⁴⁻. Data mainly collected from [54]; data from [55] are denoted by (*).

<table><thead><tr><th>Couple</th><th>Redox reaction</th><th>E₀ (V/SHE)</th><th>HNO₂: Mo⁶⁺ (expected)</th><th>HNO₂: Mo⁴⁺ (measured)</th></tr></thead><tbody><tr><td rowspan="4">Mo⁶⁺ / Mo⁰<br>(0 → +6)</td><td>Mo + 4 H₂O ⇌ MoO₄²⁻ + 8 H⁺ + 6 e⁻</td><td>0.154</td><td></td><td></td></tr><tr><td>Mo + 4 H₂O ⇌ HMoO₄ + 7 H⁺ + 6 e⁻</td><td>0.095</td><td>3: 1</td><td>3: 1</td></tr><tr><td>Mo + 4 H₂O ⇌ H₂MoO₄ + 6 H⁺ + 6 e⁻</td><td>-0.002</td><td></td><td></td></tr><tr><td>Mo + 3 H₂O ⇌ MoO₃ + 6 H⁺ + 6 e⁻</td><td>0.075*</td><td></td><td></td></tr><tr><td rowspan="2">NO₃⁻ / HNO₂<br>(+5 → +3)</td><td>NO₃⁻ + 3 H⁺ + 2 e⁻ ⇌ HNO₂ + H₂O</td><td>0.934*</td><td>-</td><td>-</td></tr></tbody></table>

that the dissolution rates of Mo and UO₂ when mixed are interdependent and very likely controlled by the availability of the same species in the medium. Considering that the dissolution processes of UO₂ and Mo are similar, competition between them is thus very likely to occur as they might be commonly auto-catalyzed by the same nitrogen species, which eventually leads to slowing down the dissolution of one another. In this case, UO₂ has also an inhibitory impact on Mo dissolution in nitric acid, and the converse is true (mutual inhibition).

### 3.2.3. Characterization of the remaining residues following metallic Mo foil dissolution

At the end of the dissolution experiment of the pure metallic Mo foil under dynamic conditions, the remaining residues (14 wt %) observed in the PTFE reactor were also brown-red in color (Fig. 10(a)), which was consistent with the color observed for the remaining Mo precipitates in the case of the dissolution of the UO₂ + Mo pellets. The reaction of metallic Mo with nitric acid is a vigorous reaction accompanied by the release of brown-red NOx by-product gases, as shown in Fig. 10(b). This photo was taken when opening the PTFE reactor following the dissolution of another metallic Mo foil in 20 mL of 4 mol.L⁻¹ HNO₃ at 353 K under static conditions (without any HNO₃ renewal).

The SEM micrograph in Fig. 10(c) represents the microstructure of the brown-red Mo precipitates with the corresponding EDS spectrum. According to the literature [48,50,51], the concentrated nitric acid was supposed to cover the metallic Mo foil with a protective oxide layer of MoO₃ (+6) that should prevent further acid attack and slow down the dissolution of the metal. However, the dissolution of the Mo foil in 4 mol.L⁻¹ HNO₃ solution at 353 K, whatever the protocol (static or dynamic), was rather quick. To add, the remaining precipitates correspond to an oxidized Mo phase only, and no metallic Mo was even detected, in agreement with the previous results of the remaining Mo precipitates from the dissolution of the UO₂ + Mo pellets. The big debris parts left at the bottom of the reactor of relatively smooth surfaces, as shown in Fig. 10(c), also indicated that the attacked foil had undergone complete oxidation before dissolution. Again, the brownish color did not match that of anhydrous MoO₃ and its hydrates, which are white or yellowish [48,52].

In addition, the synchrotron XRD measurements revealed that the remaining Mo oxidized precipitates in nitric acid are amorphous, as shown in Fig. 11(a). Furthermore, the XANES spectra collected at the Mo K-edge of the Mo precipitates and the Mo⁰, Mo⁴⁺O₂ and α-Mo⁴⁺O₃ reference compounds are shown in Fig. 11(b). The shape of the spectrum obtained for the Mo precipitates was close to that of Mo⁴⁺O₂ but with post-edge features similar to those of α-Mo⁴⁺O₃. The characteristic pre-edge peak feature of Mo(VI) at about 20,007.6 eV (1s(Mo) → 4d(Mo) + 2p(O) [56–58]) was however not observed. Furthermore, the absorption energy at the edge position (taken at the inflection point) was approximately 20,014 eV for the precipitates, which was between 20, 012 eV for Mo⁴⁺O₂ and 20,016 eV for α-Mo⁴⁺O₃. This indicated that the remaining Mo precipitates have an oxidation state between +4 and +6 and may indicate either several species or a precipitate with mixed oxidation state.

These precipitates indicated that the dissolution mechanism of metallic Mo in nitric acid consists of multiple steps that occur consecutively. According to the results, the first step is the complete oxidation of metallic Mo into the amorphous brown-red phase, which could be an oxide or an oxide hydrate, with a transition of the Mo oxidation state from 0 into a higher state between +IV and +VI. This step leads to the formation of NOx gas bubbles due to the reduction of the nitrate ions. The brown-red Mo oxidized precipitates represent an intermediate phase before full dissolution as Mo(VI) mono- and/or dinuclear species (highest oxidation state). Further oxidation of the brown-red Mo phase followed by solubilization as Mo(VI) species appeared to require the participation of auto-catalytic nitrogen species. To elucidate, as soon as the auto-catalytic nitrogen species were totally consumed at the end of the auto-catalytic kinetic regime (reflected by the drop in HNO₂
<figure><img src="image_24.png" /><figcaption>**Fig. 10.** (a), (b) Images of the remaining brown-red Mo precipitates from the dissolution of the metallic Mo foil in 4 mol.L⁻¹ HNO₃ at 353 K under dynamic and static conditions, respectively. (c) SEM micrograph showing the microstructure of the brown-red precipitates, with the corresponding EDS spectrum.</figcaption></figure>

concentration until becoming negligible), the dissolution of the brown-red Mo phase into Mo(VI) species stopped (plateau phase), and such an intermediate phase remained in solution. As a result, the amount of the brown-red Mo phase left in solution is directly related to the availability of the auto-catalytic nitrogen species in nitric acid. The total dissolution of Mo could thus be achieved by adding nitrite salts, for example, but was not tested. The metallic Mo foil dissolution experiment allowed us to propose a dissolution mechanism for the UO₂-based model pellets containing metallic Mo particles in nitric acid and is described in the following paragraph.

### 3.3. UO₂ dissolution mechanism in the presence of Mo (0)

The results presented in this work revealed, for the first time, a mutual inhibition relationship between UO₂ and Mo(0) during dissolution in nitric acid under representative PUREX process conditions. This mutual inhibition was evidenced by the significant decrease in the dissolution rate of UO₂ with the increase in the content of the metallic Mo particles present in the pellets. Likewise, the presence of UO₂ dramatically decreases the dissolution rate of Mo particles by several

orders of magnitude compared to pure metallic Mo. The dissolution behaviors of UO₂ and Mo were found to be synchronized, indicating that their dissolution processes are interconnected and likely compete for the same species necessary for their oxidation and solubilization in the +VI state. This interplay suggests that the dissolution of one species directly affects the dissolution kinetics of the other, as reflected in the observed mutual inhibition. The proposed mechanism for this mutual inhibition is shown schematically in Fig. 12.

The results suggested that Mo(0) is initially oxidized to an amorphous brown-red phase (indicated by the red spheres in Fig. 12(b) and (c)) with an oxidation state between +IV and +VI. This phase requires further oxidation to the Mo(VI) species, which is facilitated by the autocatalytic nitrogen species. When UO₂ is present, it competes with Mo for these nitrogen species, resulting in an important inhibitory effect on Mo dissolution. As UO₂ dissolution progresses, it consumes the available autocatalytic species, thereby preventing further oxidation and solubilization of the brown-red Mo phase, and this is why the plateau phase was reached for both at exactly the same time. This results in the accumulation of the brown-red Mo phase in solution (40–60 %), unlike in the case of pure metallic Mo dissolution, where no competitor is
<figure><img src="image_25.png" /></figure>

<figure><img src="image_26.png" /><figcaption>**Fig. 11.** (a) Synchrotron XRD of the remaining brown-red Mo oxidized precipitates following the dissolution of metallic Mo in 4 mol.L⁻¹ HNO₃ at 353 K. (b) Comparison of the normalized Mo K-edge XANES spectrum of the Mo precipitates with those of Mo⁰, MoIVO₂ and α-MoVIO₃ reference compounds.</figcaption></figure>

present and the autocatalytic species are sufficient to almost completely oxidize and solubilize Mo in a shorter time, leaving a smaller amount of the brown-red phase (only 14 %) in solution.

From a thermodynamic perspective, the reduction potentials of selected couples of interest (collected from [54,55,59]) in Fig. S8 (Supporting Information) support such a competitive inhibition mechanism driven by common autocatalytic species. All the couples including Mo(0) are below the U(VI)/U(IV) couple, favoring the initial oxidation of Mo by nitric acid. However, once Mo is oxidized into the brown-red phase, the couples including +IV, +V and +VI states are close to or above the U(VI)/U(IV) couple, favoring the further oxidation of UO₂ by the autocatalytic nitrogen species.

Furthermore, this study confirms that no mixed U and Mo precipitates were formed under experimental conditions considered. Therefore, U has a contradictory behavior compared to that of Zr and Pu, which coprecipitate with the Mo(VI) species in nitric acid [5,50]. We do not exclude the possibility that the inhibitory effect of metallic Mo particles on UO₂ dissolution could be more or less pronounced for the same content, depending on their number, size, and distribution within the UO₂ matrix. In the prepared pellets, the nano-sized Mo particles were uniformly distributed throughout the UO₂ matrix. This likely contributed to effective and consistent inhibition throughout the

<figure><img src="image_27.png" /></figure>

<figure><img src="image_28.png" /></figure>

<figure><img src="image_29.png" /><figcaption>**Fig. 12.** Proposed mechanism of dissolution of (a) UO₂, (b) Mo(0) and (c) UO₂ pellet incorporating Mo(0) particles in nitric acid. Acat-N refers to "auto-catalytic nitrogen species".</figcaption></figure>

material. Had they been clustered or unevenly distributed, there would have been variability in the overall dissolution rates. In addition, for the same content, larger Mo particles may be less uniformly distributed in the matrix and create localized areas of dissolution, while a greater number of smaller Mo particles may provide more surface area for interaction with the UO₂ matrix, potentially enhancing the inhibition of UO₂ dissolution in nitric acid.

## 4. Conclusion

This work has successfully investigated the effect of Mo(0) content on the dissolution kinetics of UO₂ in nitric acid under conditions representative of the PUREX process. The dissolution of a panel of UO₂ pellets containing between 0 and 9 mol % of nanosized metallic Mo particles, as well as a pure metallic Mo reference sample in 4 mol.L⁻¹ HNO₃ at 353 K was carried out. The results showed a mutual inhibition effect between UO₂ and metallic Mo during dissolution in nitric acid. The dissolution processes of UO₂ and Mo were found to be interconnected and likely to compete for the same auto-catalytic nitrogen
species necessary for their oxidation and solubilization in the +VI state. Specifically, the dissolution rate of UO₂ decreased by a factor of 2 to 6.3 with increasing Mo content, while the dissolution rate of Mo itself decreased by several orders of magnitude in the presence of UO₂.

The results obtained in this work showed that the competition for autocatalytic nitrogen species is clearly in favor of the UO₂ predominant phase. This observation leads to the conclusion that increasing the Mo content in the SNF, e.g. by increasing the burn-up, will only slightly delay the dissolution of the UO₂ matrix. However, reducing the reprocessing rate of spent fuel has a significant economic impact. Another aspect to consider is the increase in the amount of various un-dissolved residues such as incompletely dissolved metal particles and reprecipitated Mo(IV) and Mo(VI) phases. These dissolution residues contain various fission products and Pu and are deposited on the walls of the dissolvers where they can cause criticality problems. For all these reasons, the application of the PUREX process becomes challenging at high Mo contents. One possibility for process modification could be to subject the SNF to oxidative heat pre-treatment prior to dissolution in nitric acid. This alternative reprocessing scheme, often referred to as the “voloxidation” process, not only converts the UO₂ matrix into a soluble U oxide phase, but could also oxidize Mo (0) into Mo (VI) phases. This could mitigate the Mo interference and inhibitory effect on the SNF during the subsequent dissolution step.

Alternatively, the study of advanced dissolution process for Mo (IV to VI) based residues should be considered. The addition of nitrite salts or the bubbling of NO₂(g) could enhance the dissolution rate of the Mo residues, as their solubilization seemed to be facilitated by the presence of catalytic nitrogen species.

### CRediT authorship contribution statement

**Mohammad Husainy:** Writing – original draft, Methodology, Investigation, Data curation. **Stéphanie Szenknect:** Writing – review & editing, Writing – original draft, Validation, Supervision, Methodology, Conceptualization. **Renaud Podor:** Methodology, Investigation, Data curation. **Xavier Le Goff:** Methodology, Investigation, Data curation. **Philippe Moisy:** Writing – review & editing, Validation, Conceptualization. **Christoph Hennig:** Writing – review & editing, Methodology, Investigation, Data curation. **Nicolas Dacheux:** Writing – review & editing, Validation, 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

This work forms part of the PhD thesis of M. Husainy and has been funded by the CEA. The authors gratefully acknowledge the provision of the Rossendorf beamline (ROBL-II, BM20) at the European Synchrotron Radiation Facility (ESRF, Grenoble, France), where the combined XRD-XANES measurements were performed. Special thanks are also due to Cyrielle Rey (ICSM) for providing HP-IC training and assistance, to Joseph Lautru (ICSM) for recording SEM micrographs, to Guilhem Quin-tard (ICSM) for his assistance for laboratory XRD measurements and to Béatrice Baus-Lagarde (ICSM) for providing ICP-OES training and assistance.

### Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jnucmat.2025.155922.

### Data availability

The authors declare that the data supporting the findings of this study are available within the paper and its Supplementary Information files. Should any raw data files be needed in another format they are available from the corresponding author upon reasonable request.

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