# In situ Raman monitoring of studtite formation under alpha radiolysis in $^{18}$O-labeled water

Aurélien Perrot, Aurélien Canizares, Sandrine Miro, Laurent Claparede, Renaud Podor, Thierry Sauvage, Sylvain Peuget, Christophe Jegou, Nicolas Dacheux

► To cite this version:

Aurélien Perrot, Aurélien Canizares, Sandrine Miro, Laurent Claparede, Renaud Podor, et al. In situ Raman monitoring of studtite formation under alpha radiolysis in $^{18}$O-labeled water. Journal of Nuclear Materials, 2024, 600, pp.155267. 10.1016/j.jnucmat.2024.155267. hal-04704283

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In situ Raman monitoring of studtite formation under alpha radiolysis
in ¹⁸O-labeled water

Aurélien Perrot<sup>a</sup>, Aurélien Canizares<sup>b</sup>, Sandrine Miro<sup>a,*</sup>, Laurent Claparède<sup>c</sup>, Renaud Podor<sup>c</sup>, Thierry Sauvage<sup>b</sup>, Sylvain Peuget<sup>a</sup>, Christophe Jegou<sup>a</sup>, Nicolas Dacheux<sup>c</sup>

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**Abstract**

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Studtite [(UO₂)(O₂)(H₂O)₂]·2(H₂O) is a secondary phase precipitating during the alteration of uranium-
bearing materials. The mechanistic link between the formation of uranyl and peroxide bonds in the
solid and the nature of the oxidizing species produced by water radiolysis remains to be elucidated. In
order to improve our understanding of these mechanisms, an original experimental methodology in
the presence of ¹⁸O isotopes and Raman spectroscopy has been developed. It appears that there is a
direct chemical relationship between the peroxo ligands inside studtite and the peroxide entities of
H₂O₂ molecule into the solution. The link between H₂O₂ formation mechanism in solution by the
radiolysis of water, the nature of the radiation and the isotopy of the peroxo ligands inside studtite has
been described thanks to a coherent set of experimental data. For the uranyl UO₂²⁺ ions, the
characterization of its isotopy allows to specify the mechanism of oxidation at the UO₂ /water
interface. The isotopic configurations observed for the uranyl ion inside studtite, can be explained by
assuming an oxidation mechanism of UO₂ involving both a simple transfer of electrons by interaction
with H₂O₂ and the incorporation of oxygen atoms from the solution into the fluorite structure via OH°
radicals.

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**Keywords:** Nuclear fuel; Oxidative dissolution; Uranium dioxide; Irradiation; Hydrogen peroxide; Spectroscopy

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

<footnote><sup>b</sup> CEMHTI, Univ d'Orléans, Orléans, France</footnote>

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

<footnote>*sandrine.miro@cea.fr</footnote>
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# 1. Introduction

Water in interaction with heterogeneous nuclear materials such as spent fuel ($UO_2$ or ($U,Pu$)$O_2$ based ceramic), fuel debris or molten nuclear fuel (corium) produces hydrogen peroxide by radiolysis [1-4]. Interaction between the surface and hydrogen peroxide induces oxidative dissolution of uranium oxide and the precipitation of U(VI) secondary phases such as studtite [((UO$_2$)(O$_2$))(H$_2$O)$_2$]·2(H$_2$O), a common uranium peroxide that generally forms under strongly oxidizing conditions. It has been the subject of important studies over the past 20 years as it can potentially incorporate radionuclides, play a protective role during spent fuel alteration or even control uranium solubility in solution [3-9]. Studtite has also been observed in natural deposits such as barite solid solutions incorporating $^{226}$Ra and associated with other uranyl minerals such as rutherfordine or uranophane [10]. The crystallographic structure of this uranium peroxide is well known [11]. It is composed of ribbons of U-coordination polyhedra with shared edges, which are composed of $O_2^{2-}$ bridging peroxo ligands.

There are also numerous studies on the stability of uranium peroxides as a function of solution chemistry, irradiation conditions and temperature [12-16]. For example, with regard to solution chemistry, the presence of carbonate ligand and $H_2O_2$ in groundwater increase studtite dissolution and uranium (U) release [12]. The stability under $\gamma$ and $\alpha$ irradiation of studtite, and more generally of uranyl peroxide-based solid materials, with various structural topologies, has also been studied by XRD and Raman spectroscopy [13, 14]. Studtite is particularly affected by $\alpha$ irradiation which induces the formation of amorphous uranyl peroxide [14]. In addition, an increase in temperature leads to the loss of water molecules and the formation of metastudtite ($UO_2$(O$_2$)(H$_2$O)$_2$, or even that of an amorphous compound [15, 16].

Despite of all these studies, the link between the radiolytic products of water (i.e. radicals and molecular species) and the formation of the uranyl ion and peroxo ligands within studtite needs to be better understood, requiring further investigation. Recently, a study coupling Raman spectroscopy with $\gamma$ irradiation of $^{18}$O labelled water has shown the possibility of linking the formation of radiolysis products to the main structural entities of studtite, namely peroxo ligands and uranyl ion [17]. The goal of the present work is to perform this study under $\alpha$ irradiation by monitoring studtite precipitation using in-situ Raman spectroscopy and $^{18}$O labelled water irradiated by 5 MeV He$^{2+}$ particles. This can be achieved using the in-situ irradiation Raman set-up implemented at the CEMHTI cyclotron beamline, which has already been used in previous studies [18, 19]. The originality of this work lies in the use of $^{18}$O labelled water. Indeed, the gathering of Raman spectra coupled with solution analysis is expected to provide new insights to propose studtite formation mechanisms. The respective roles of hydrogen peroxide and radicals in the formation of peroxo and uranyl bonds will be particularly evaluated. The nature of isotopic exchanges under irradiation will provide a better understanding of the effects of Linear Energy Transfer (i.e. high or low LET effect) in studtite formation and, more broadly, in the mechanism of oxidative dissolution of uranium dioxide.

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**2. Materials and methods**

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**2.1. Sample preparation**

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The target used for the alpha radiolysis experiment came from a sintered pellet of depleted UO₂, manufactured at CEA Cadarache, and was obtained after wire saw cutting and mirror polishing with diamond paste. The resulting UO₂ disk measured 8 mm in diameter and was 277 µm thick, stored in air within membrane storage boxes. Prior to leaching, three cycles of pre-leaching of the experimental device (cell and pellet) were carried out in a water solution (MilliQ Water) in the presence of 10⁻³ mol.L⁻¹ NaHCO₃, in order to ensure the stoichiometry at the sample surface by removing the pre-oxidized surface layer. The pre-leaching steps lasted 1 hour, with the exception of the last one, which was extended to 14 hours. This pre-leaching was followed by three washing stages in the presence of the leaching cell, two washing stages were performed with pure water (MilliQ Water) and the last one was performed with ¹⁸O-labelled water.

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In order to carry out a complementary experiment to study the alteration of UO₂ without irradiation but in the presence of ¹⁸O-labelled hydrogen peroxide, a second UO₂ sample was synthesized. This sample was prepared according to the process described by Martinez et al., i.e. by precipitating uranium (IV) in hydroxide form [20]. After washing, the precipitate obtained was deagglomerated then heated at 700°C for 4h under Ar/H₂ atmosphere. Finally, the powder was pelletized and sintered at 1550°C for 6h under Ar/H₂. The resulting sintered UO₂ pellet was fragmented. One of the fragments was annealed under Ar/H₂ atmosphere at 1100°C for 5h to ensure U/O stoichiometry. After synthesis and prior to leaching, the sample was characterized by Raman spectroscopy. It then underwent a 4 stages pre-leaching cycle in the presence of 10⁻³ mol.L⁻¹ NaHCO₃. Pre-leaching lasted 1 hour, with the exception of the last one, which was extended for 12 hours. These steps were followed by two washing stages with deionized water and a last one with ¹⁸O labelled water.

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**2.2. Leaching experiments**

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**2.2.1. Leaching of UO₂ sample under α irradiation in presence of H₂¹⁸O**

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**Irradiation conditions.** An external helium irradiation beam was used to irradiate the UO₂/H₂¹⁸O system. This irradiation was performed at the CEMHTI cyclotron, using 45 MeV ⁴He²⁺ ions. The He²⁺ ion beam was collimated to obtain a 6 mm diameter centred on the UO₂ target. The 277 µm thickness of the UO₂ target was chosen according to the results of the SRIM software [21], which evaluated the trajectory and the dissipated energy of ions passing through a material. The SRIM-calculated energy of ⁴He²⁺ ions at the UO₂ disk exit was 7.7 ± 0.9 MeV, with UO₂ density of 10.74 g.cm⁻³ (98 % theoretical density) and taking into account the presence of three 26.8 µm-thick titanium foils at the beam controller. Atom displacement energies have been set at 20 eV for oxygen and 40 eV for uranium, based on calculation of Soullard et al. [22]. The calculated projected range of these 7.7 MeV ⁴He²⁺ ions, which induce water radiolysis in H₂¹⁸O, was 71.8 ± 0.8 µm. The irradiation lasted 6 hours at an intensity of 10 nA, only during the first 35 minutes, then the intensity was decreased to 5 nA to limit the appearance of dihydrogen bubbles. The corresponding fluxes were 5.5 × 10¹⁰ α.cm⁻².s⁻¹ for 5 nA and 1.1 × 10¹¹ α.cm⁻².s⁻¹ for 10 nA, to reach a final fluence of 1.3 × 10¹⁵ α.cm⁻². Based on fluence and SRIM simulation data [21], the displacement per atom (dpa) within the uranium dioxide layer probed by Raman spectroscopy (~1 µm) was calculated at 4.0 × 10⁻⁴ dpa. This value corresponds to a measure of the number of atoms ejected from their initial position in the crystal lattice, following ballistic collisions. It provides us a quantitative means of assessing the damage inflicted on the material by irradiation. However, it does not take into account defect recombination.

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**Experimental set-up in the irradiation chamber.** This set-up allowing in-situ Raman monitoring of the UO₂/H₂¹⁸O interface during and after irradiation is presented in **Figure 1**. It consists of a leaching cell, manufactured in a polymer material resistant to radiolytic species attacks (PCTFE) in which the UO₂ target is placed and held in place by a system of screwed rings to ensure watertightness. The leaching
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solution used was ¹⁸O-enriched deionized water (97.17% ¹⁸O, 0.34% ¹⁷O, 2.49% ¹⁶O, INNOVA-CHEM, France). During irradiation, the α beam is transmitted through the UO₂ specimen inducing water radiolysis. Finally, a 40 mm diameter quartz window located on the opposite side of the UO₂ sample provides optical access for in-situ Raman monitoring of the UO₂/H₂¹⁸O interface.

<figure><img src="image_1.png" /></figure>

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<caption>**Figure 1.** In situ experimental set-up combining a leaching cell under α-irradiation with Raman monitoring: a. picture of the instrumental set-up; b. schematic diagram of the set-up, inspired from [18].</caption>

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**In-situ Raman spectroscopy.** Raman measurements were performed continuously on the direction parallel to the ⁴He²⁺ beam direction (Figure 1), using a Renishaw RA100 spectrometer equipped with a He-Ne excitation laser (633 nm). The laser beam was delivered to the irradiation cell via a multimode optical fiber and a Renishaw's RP10 probe, equipped with a Mitutoyo long working distance (30.5 mm) objective which offers a 0.28 numerical aperture and with a 20x magnification. The 7 mW laser power measured at the sample surface allows a very good compromise between UO₂ disk laser damage resistance and Raman efficiency. The spectra were recorded with a 1800 groove/mm holographic grating, allowing 4 cm⁻¹ spectral resolution and offering simultaneous observation from 310 to 1244 cm⁻¹, and an accumulation time of 2 minutes for each spectrum. Raman monitoring was carried out during a 21-hour leaching period, enabling the continuous observation of studtite precipitation, including post-irradiation precipitation induced by radiolytic products remaining in solution after a 6-hour irradiation period.

When the leaching test was stopped, the solution was drained, followed by 3 successive washes of the cell with pure water. The pellet was then taken off and carefully dabbed with Kimtex© paper to remove water and stop surface alteration, ensuring no rubbing occurred during the process.

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### 2.2.2. Leaching of UO₂ in presence of H₂¹⁸O₂

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To understand the role of oxidizing species during water radiolysis, an additional leaching experiment was carried out by replacing the presence of these radiolytic species produced in ¹⁸O-labelled water with a H₂¹⁸O₂ solution. The 10⁻³ mol.L⁻¹ H₂¹⁸O₂ solution was prepared from a commercial solution (Sigma-Aldrich Hydrogen Peroxide - ¹⁸O₂ Solution - 2-3% in H₂O, 90 atom % of ¹⁸O) diluted with H₂¹⁸O water. The H₂O₂ concentration was verified in the resulting solution by the Ghormley method [23]. The sample was weighed and placed in a 10 mL leaching tube for 48 hours in the presence of 5 mL of 10⁻³ mol.L⁻¹ H₂¹⁸O₂ solution. After leaching, the samples were characterized by Raman spectrometry, while the leachates were filtered to 3 kDa by centrifugation before being analysed by ICP-OES.

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### 2.3. Post-mortem characterization

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**ESEM.** ESEM (Environmental Scanning Electron Microscopy) characterization was carried out to observe the morphology of studtite precipitated on the pellet surface, using a ThermoFischer Scientific
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Quattro S SEM. Micrographs were recorded using a voltage varying between 1 and 10 kV and a pressure in the chamber between 10⁻³ and 10⁻⁴ Pa.

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Following an initial observation of the pellet's surface, an analysis of its cross-section was conducted. Prior to this second ESEM observation, the irradiated sample was vacuum-coated in resin, cut with a Buehler IsoMet 1000 milling cutter, then polished with a Buehler Beta polishing machine using diamond paste 1 µm to achieve a mirror-polished surface. To prevent charge accumulation onto the surface, the cross-section was coated with carbon prior to SEM observation.

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**X-Rays Diffraction.** Structural analysis of the solids was carried out by X-ray diffraction (XRD) using a Bruker D8 ADVANCE diffractometer (Cu-Kα₁ radiation, λ = 1.5406 Å, 40 kV, 40 mA, LinxEye detector). XRD patterns were collected at room temperature over the 5-100° (2θ) range, using a step size of 0.009° and a duration per step equal to 1 s.

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**ICP-OES/ICP-MS.** Concentrations of uranium released in the leachate were first determined using a iCAP 7000 series optical plasma emission spectroscopy (ThermoFischer Scientific). The analysis was performed considering the results obtained at λ = 385.958 nm. In a second step, solutions with uranium concentrations below the limit of quantification by ICP-OES (LOQ (U) ~ 100 ppb) were analysed by ICP-MS (LOQ (U) ~ 50 ppt). The elemental concentrations of uranium was calculated as the average of three replicates. The instrument used was a ThermoFischer Scientific iCAP RQ. For all elemental analyses, a calibration curve was produced using a range of diluted samples prepared from a certified solution, framing the concentration range under consideration. All samples were diluted with 0.2 mol.L⁻¹ HNO₃ prior to analysis, to limit uranium adsorption onto the walls of the vials.

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**Raman spectroscopy.** Post-mortem µ-Raman analyses were carried out using the LabRam ARAMIS spectrometer (Horiba Jobin-Yvon) to study the alteration of UO₂ sample leached in H₂¹⁸O₂ solution. The instrument is equipped with He-Ne excitation laser (632.8 nm) and a Nd:YAG laser (532 nm). The laser beam was focused onto the sample surface using an Olympus BX 41 microscope equipped with a 100× SLMP objective. The resulting spot size was of the order of 1 µm². During spectra acquisitions, the laser (532 nm) was filtered down to 0.3 mW (measured at the sample surface) to avoid any sample modification. The spectra were recorded using the 1800 grooves.mm⁻¹ holographic grating and taking into account accumulation time of 4 minutes per spectrum. These parameters were chosen to ensure a good signal-to-noise ratio. For the sake of reproducibility, different remote areas were analysed to confirm the homogeneity of the sample surface. Once the spectra have been recorded, they were processed using OriginPro (Version 2023b. OriginLab Corporation, Northampton, MA, USA) software. A baseline correction was performed and the spectra were then normalized to the band of maximum intensity.
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# 3. Results

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## 3.1. In-situ Raman monitoring of studtite precipitation under alpha irradiation

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Uranium dioxide presents a fluorite-type crystal structure (CaF₂), characterized by a space group *Fm$\overline{3}$m (O₈). In this structure, the U⁴⁺ cations adopt a face-centred cubic (f.c.c.) arrangement, while the anions (O²⁻) occupy the tetrahedral positions [24]. In accordance with group theory, a triply degenerate Raman-active vibrational mode at 445 cm⁻¹ is observed in the initial Raman spectrum. This band, called T₂g, results from the antiphase oscillation of stretching oxygen atoms around uranium [25]. The initial spectrum admits other bands, including defect bands between 500 and 700 cm⁻¹ (Figure 2). A broad band is also observed at 1150 cm⁻¹. This band has been assigned to a second-order longitudinal optic phonon (2LO) [24]. Finally, a band at 911 cm⁻¹ was also identified, generally attributed to the 2TO₈ mode [24, 26]. All spectra recorded during the leaching test were normalized in intensity with T₂g and then used to acquire a temporal Raman map (Figure 2). During the first few minutes of irradiation, dihydrogen was produced by radiolysis of water within the cell. This led us to make short interruptions of the irradiation at 10 nA to evacuate the bubbles present and to lower the beam intensity. During these stops, the recording of Raman spectra were stopped. These stops correspond to the grey lines observed in Figure 2.

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<figure><img src="image_2.png" /><figcaption>**Figure 2.** 2D time-resolved Raman spectra alteration with the pristine UO₂ signal (bottom) and the final spectrum showing studtite precipitation (top).</figcaption></figure>

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The characteristic bands of the UO₂ compound were observed in all the Raman spectra recorded during the entire experiment. The final spectrum was still composed of the UO₂ fingerprint, in which no position and FWHM variation was detected in the T₂g and 2LO bands compared to the initial spectrum. No significant evolution of the FWHM was also observed by Gutierrez et al. for a dpa lower than 0.03 [27]. However, a significant evolution of the band associated with defects was observed between 500 and 700 cm⁻¹. The intensity of this defect band composed of three contributions: U₁, U₂ and U₃ is reported in **Figure 3a**. The increasingly pronounced appearance of these three defect bands during irradiation is clearly evidenced. Their growth was due to the ballistic impact of the He²⁺ ions in the fluorite structure. The latter is composed of 3 defect bands: Firstly, the U₁ band is present at 530 cm⁻¹, which assignment is still discussed [18, 28]. Indeed, it is sometimes associated with the distortion of the cubic lattice by interstitial oxygen. Secondly, the U₂ band at 577 cm⁻¹, which is assigned to the first-order longitudinal optical phonon (1LO) mode, active in IR and observable in Raman in the presence of defects in the fluorine structure, due to loss of symmetry. This loss of symmetry resulted from the appearance of oxygen clusters and the change in ionic radius between U (IV) and U (V) during oxidation [29, 30]. Lastly, the U₃ band at 634 cm⁻¹ which is characteristic of the presence of oxygen cuboctahedra, is observed [28, 29, 31]. When irradiation was stopped, the increase of bands intensity stopped and the phenomenon was not reversible. No saturation appeared to have been reached before the end of irradiation. This observation is in good agreement with the results obtained by G. Guimbretière et al. Indeed, saturation was not yet observed for fluence values of 1.3 × 10¹⁵ α·cm⁻² [32]. The displacement per atom (dpa) previously calculated tells us that alpha particles are transmitted with a very low impact on UO₂, revealing the extreme sensitivity of defect bands to irradiation-induced damages. In addition, in **Figure 2**, the growth of 3 bands at 774, 790 and 817 cm⁻¹ can be noted. These bands correspond to the [(UO₂)(O₂)(H₂O)₂]·2(H₂O) studtite compound and were already detected on ¹⁸O labelled studtite [11, 17]. The observation of this phase under our conditions is consistent with the results obtained by Corbel et al. [33]. The intensity of the studtite characteristic bands is plotted against time in **Figure 3b**. The increase in their intensity clearly shows the formation of this phase from the very beginning of the experiment, and its subsequent growth during the experiment. The evolution of the Raman response of this phase allowed us to establish that studtite precipitation occurred according to a linear kinetic. It was observed that the linear evolution of the Raman response continues while the pellet is no longer irradiated, until around 1000 minutes. This result is in good agreement with the observations by R. Mohun who has reported that studtite kept growing despite the stop of irradiation [34]. After 1000 minutes dissolution experiment, precipitation slowed down.

<figure><img src="image_3.png" /><figcaption>**Figure 3.** Monitoring the time-dependent Raman response of bands normalized with T₂g during the leaching time. Evolution of the area of defect bands (a) and evolution of band area associated with studtite (b).</figcaption></figure>

## 3.2. Analysis of the solutions and post-mortem characterization of the studtite layer
The study of UO₂ alteration by water radiolysis is well documented in the literature [4, 17, 33, 35]. Consequently, studtite precipitation under these experimental conditions can be anticipated and should occur as soon as saturation of this phase in solution is reached. To correlate uranium release with studtite precipitation, the concentration of uranium in solution during the leaching test was measured. First, the pre-leaching steps ensured that the oxidized surface layer of the UO₂ pellet was removed. Thus, the effectiveness of the washing steps carried out after this pre-leaching stage was monitored, verifying that all the uranium present in the leach cell was recovered. This then enabled a balance to be drawn up for the uranium in solution. For all these reasons, the uranium concentration was determined after each stage (Table 1).

A low concentration of uranium released during the pre-leaching stage was noted, indicating a weak pre-existing oxidized layer of U(VI). This observation seems to be in agreement with the initial Raman response of the pellet, which did not show significant defect bands (in particular the U₃ band).

After the leaching test, the uranium concentration of the leaching solution was measured at (1.1 ± 0.1) × 10⁻⁴ mol·L⁻¹, from which a quantity of uranium released into solution equal to (5.9 ± 0.5) × 10⁻⁷ mol was deduced. These release values are in agreement with the release previously obtained by Corbel et al. [33].

A speciation calculation was carried out using CHESS software [36] and the ThermoChimie database [37]. At the end of the leaching stage, the saturation index in uranium was determined to be 2.58 at a pH of 3. This value confirms that the conditions in the solution were oversaturated with respect to studtite during leaching, resulting in the precipitation of this phase.

The thickness of the studtite layer was estimated using the data reported by Corbel et al. under similar conditions [33]. Knowing the output energy of the alpha particles (7.7 MeV) and the fluence at the end of the experiment (1.3 × 10¹⁵ α·cm⁻²), it can be calculated that the total energy deposited in the solution is 456 J. Corbel et al. have established a relationship between this deposited energy and H₂O₂ concentration, allowing us to estimate that the H₂O₂ concentration present at the end of irradiation was CₐH₂O₂ ≈ 6 × 10⁻³ mol·L⁻¹ [33]. From the H₂O₂ concentration and a second relationship established by Corbel et al, a layer growth rate of 100 nm·h⁻¹ was able to be estimated, enabling the determination of the precipitated studtite layer's thickness at 2.1 µm. With the knowledge of studtite's density (3.64 g·cm⁻³) [38], it can be deduced that the mass of studtite that was precipitated on the pellet's surface amounts was approximately 1 mg.

<table><caption>Table 1. Uranium concentration determination in solution during pre-leaching, washing and leaching stages when leaching UO₂ irradiated with α particles</caption><thead><tr><th>Sample</th><th>Duration (h)</th><th>Solution</th><th>Volume (mL)</th><th>C<sub>U</sub> (mol·L<sup>-1</sup>)</th></tr></thead><tbody><tr><td>Pre-leaching 1</td><td>1</td><td rowspan="3">NaHCO<sub>3</sub></td><td rowspan="3">4.5 ± 0.1</td><td>(1.7 ± 0.1) × 10<sup>-7</sup></td></tr><tr><td>Pre-leaching 2</td><td>1</td><td>(1.5 ± 0.1) × 10<sup>-8</sup></td></tr><tr><td>Pre-leaching 3</td><td>14</td><td>(1.8 ± 0.1) × 10<sup>-9</sup></td></tr><tr><td>Washing 1</td><td>--</td><td rowspan="2">H<sub>2</sub>O</td><td rowspan="2">4.5 ± 0.1</td><td>(1.6 ± 0.1) × 10<sup>-8</sup></td></tr><tr><td>Washing 2</td><td>--</td><td>(2.0 ± 0.3) × 10<sup>-10</sup></td></tr><tr><td>Washing 3</td><td>--</td><td>H<sub>2</sub><sup>18</sup>O</td><td>12.5 ± 0.2</td><td>(2.5 ± 0.1) × 10<sup>-9</sup></td></tr><tr><td>Leaching</td><td>21</td><td>H<sub>2</sub><sup>18</sup>O + α*</td><td>5.5 ± 0.1</td><td>(1.1 ± 0.1) × 10<sup>-4</sup></td></tr></tbody></table>

Once the radiolysis experiment was completed, the UO₂ disk was first characterized by X-ray diffraction (XRD) to identify crystal phases. The X-ray pattern obtained (Figure 4) in reflectivity mode with normal incidence, revealed the coexistence of two different phases: UO₂ and studtite

<footnote>*6 hours of irradiation</footnote>
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[(UO₂)(O₂)(H₂O)₂]·2(H₂O). This compound crystallizes in a monoclinic structure with space group C2/c

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[11].

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The recorded diffractogram is compared with the studtite diffractogram reported by Burns et al. [11].

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Thus, the XRD diffractogram revealed a strong anisotropy in the intensity of the lines, marked by a

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crystal growth surface favoured along the (-202) plane, associated with a position of 21.12° (2θ). The

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ratio of peak amplitudes (1,1,0)/(-2,0,2) is ~5.34, whereas it is ~0.13 for our sample [11]. Thus, it can

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be deduced, as shown in **Figure 4**, that studtite chains are oriented parallel to the axis of needle

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growth, this axis corresponding to the c-axis. The orientation of studtite within a needle is illustrated

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in **Figure 4a**, where the (-202) plane (**Figure 4b**) is arranged perpendicular to the growth direction while

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the (200) plane (**Figure 4c**) is aligned parallel along the growth axis. The work of Corbel et al. did not

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identify any preferential orientation in the formation of studtite under alpha irradiation [33], however,

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our results are consistent with the observations of Schlegel and Jegou who reported the presence of a

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preferential orientation of crystal growth along (-202) in the case of precipitation without irradiation

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[39]. Two hypotheses may be put forward to explain this contradiction. On the one hand, if irradiation

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does indeed inhibit studtite formation with preferentially oriented textures, our finding could result

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from the fact that, in our experiments, studtite growth mainly occurred in the absence of irradiation,

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during the last part of the experiment, when the beam was off. This could lead to a diffractogram that

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is predominantly representative of studtite that grew without the influence of irradiation. On the other

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hand, it's important to point out that studtite growth duration reported by Corbel et al. was

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significantly shorter than that observed in our study, with a maximum of 6h for continuous irradiation.

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However, research by Schlegel and Jegou has shown that the orientation effect along the (-202) plane

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is only perceptible after six hours of precipitation [39]. This time difference could therefore play a

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crucial role in whether or not orientation effects are observed, providing a second hypothesis to

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explain the observed contradiction.

297
<figure><img src="image_4.png" /><figcaption>**Figure 4.** XRD diagram and structural analysis of studtite growth: a. XRD pattern of UO₂ pellet after leaching, showing the formation of studtite at the solid/solution interface (left) and schematic illustration of the structure of studtite during crystal growth (right); uranium atoms being at the center of the coordination polyhedra; b. View of the plans (-202) and (-404); c. View of the plan (200)</figcaption></figure>

ESEM observations of the UO₂ disk reported in **Figure 5**, confirmed the formation of a homogeneous crystalline layer characteristic of the studtite phase formed during the oxidative dissolution of UO₂ in presence of H₂O₂ [33]. The area that was not in contact with the solution (**Figure 5a**) does not show any secondary phase or alteration marks on the surface. Only marks due to the polishing process such as scratches and grain tears are observed, whereas a layer covered with needle-like studtite crystals was formed on the area in contact with the solution (**Figure 5b**).

In addition, the observation of the cross-section of the pellet (**Figure 5c**) allowed us to image the alteration layer and to determine the morphology of the crystal growth. A single homogeneous layer of studtite, formed by needles grown perpendicularly to the UO₂ surface, with an average height of about 1.4 µm, was observed. The studtite thickness measured is in accordance with that which has been previously estimated by leachate analysis (i.e. 2.1 µm).

The SEM observations of the studtite layer clearly indicate that the needles have grown according to a preferential orientation. This result is in good accordance with XRD data, where a texture marked by preferential orientation of crystal growth was observed (Figure 4).
<figure><img src="image_5.png" /><figcaption>**Figure 5.** ESEM micrographs of UO₂ leached pellet: a. Non-leached peripheral zone; b. Altered surface covered with a layer of studtite needle-like crystals; c. Cross-section of the pellet.</figcaption></figure>

### 3.3. Analysis of studtite isotopic enrichment

The use of ¹⁸O enriched water and Raman spectroscopy offer a precise method to determine the origin of the oxygen present in studtite and to deduce its formation mechanisms. In order to analyse the enrichment of the different bonds in studtite, it was first necessary to determine the different Raman band positions of ¹⁸O enriched studtite. To evaluate the theoretical vibration frequencies and deduce the wavenumber associated with each ¹⁸O enriched studtite bond, it was considered that an
intramolecular stretching vibration can be described by a harmonic oscillator model in which the
binding force is described by the Hooke's law [40].

$$
\tilde{v} = \frac{1}{2 \pi c} \sqrt{\frac{k}{\mu}} \qquad (1)
$$

where $\tilde{v}$ is the wavenumber (in cm$^{-1}$), c is the speed of light (in cm.s$^{-1}$), k is the binding force constant (in kg.s$^{-2}$) and μ corresponds to the reduced mass (in kg). There is a direct relationship between mass and bond vibration frequency. Isotopic labelling thus induces a shift in the Raman bands by modifying the mass of the original atoms. To determine the position of the bands after isotopic enrichment, the experimental position of the isotopically unlabelled bands was used and then the calculated shift was applied [17].

In the case of studtite, its response to isotopic labelling is characterized by a behaviour similar to that of a free molecule, where each possible isotopic configuration is associated with a specific Raman response. This is in contrast to solids, which generally show a single band per bond, integrating the different contributions [17]. The calculated vibration frequencies associated with each of the different oxygen configurations within the peroxide and uranyl bonds of studtite are presented in Table 2.

<table><caption>Table 2. Calculated Raman band positions associated with the contribution of the various bonds for O¹⁸ enriched H₂O₂, studtite and UO₂²⁺</caption>
  <thead>
    <tr>
      <th>Bond</th>
      <th>Compound</th>
      <th>Configuration</th>
      <th>Experimental position (cm<sup>-1</sup>) [17]</th>
      <th>Calculated Raman band position for <sup>18</sup>O enriched compound (cm<sup>-1</sup>)</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td rowspan="6">Peroxyde</td>
      <td rowspan="3">H<sub>2</sub>O<sub>2</sub></td>
      <td><sup>16</sup>O-<sup>16</sup>O</td>
      <td>876 (1)</td>
      <td></td>
    </tr>
    <tr>
      <td><sup>16</sup>O-<sup>18</sup>O</td>
      <td></td>
      <td>851</td>
    </tr>
    <tr>
      <td><sup>18</sup>O-<sup>18</sup>O</td>
      <td></td>
      <td>826</td>
    </tr>
    <tr>
      <td rowspan="3">Studtite</td>
      <td><sup>16</sup>O-<sup>16</sup>O</td>
      <td>863 (1)</td>
      <td></td>
    </tr>
    <tr>
      <td><sup>16</sup>O-<sup>18</sup>O</td>
      <td></td>
      <td>838</td>
    </tr>
    <tr>
      <td><sup>18</sup>O-<sup>18</sup>O</td>
      <td></td>
      <td>814</td>
    </tr>
    <tr>
      <td rowspan="6">Uranyl</td>
      <td rowspan="3">UO<sub>2</sub><sup>2+</sup></td>
      <td><sup>16</sup>O = U = <sup>16</sup>O</td>
      <td>870 (3)</td>
      <td></td>
    </tr>
    <tr>
      <td><sup>16</sup>O = U = <sup>18</sup>O</td>
      <td></td>
      <td>845</td>
    </tr>
    <tr>
      <td><sup>18</sup>O = U = <sup>18</sup>O</td>
      <td></td>
      <td>820</td>
    </tr>
    <tr>
      <td rowspan="3">Studtite</td>
      <td><sup>16</sup>O = U = <sup>16</sup>O</td>
      <td>817 (2)</td>
      <td></td>
    </tr>
    <tr>
      <td><sup>16</sup>O = U = <sup>18</sup>O</td>
      <td></td>
      <td>793</td>
    </tr>
    <tr>
      <td><sup>18</sup>O = U = <sup>18</sup>O</td>
      <td></td>
      <td>770</td>
    </tr>
  </tbody>
</table>

**Figure 6** shows the Raman spectra of studtite from three leaching experiments together with the reference spectrum of studtite. Pseudo-Voigt function fits of the Raman spectra presented in **Figure 6** were performed on the bands present between 700 and 900 cm⁻¹ associated with studtite, in order to assign the corresponding bonds to each band (**Table 2**) and thus deduce the isotopic enrichment of studtite.

As mentioned above, the Raman spectrum of pellet subjected to leaching by alpha irradiated labelled
water shows a cluster of 3 bands between 700 and 900 cm⁻¹ (Figure 6a). The first band at 774 cm⁻¹ is
associated with the uranyl bond composed of two ¹⁸O oxygen (¹⁸O = U = ¹⁸O). The second band at 790 cm⁻¹ is associated with the uranyl bond composed of one ¹⁸O and one ¹⁶O (¹⁶O = U = ¹⁸O). Finally, the third band at 817 cm⁻¹ is attributed to the sum of the contributions of the uranyl bond composed of two ¹⁶O oxygen (¹⁶O = U = ¹⁶O) and the peroxide bond composed of two ¹⁸O oxygen (¹⁸O – ¹⁸O). No bands at 840 cm⁻¹ and 860 cm⁻¹ attributed respectively to the peroxide bond ¹⁸O - ¹⁶O and ¹⁶O - ¹⁶O were observed despite the presence of ¹⁶O (3 atom %) present in water.

For the stoichiometric UO₂ pellet submitted to a leaching by a 1mM H₂¹⁸O₂ + H₂¹⁸O solution (Figure 6b), the Raman spectrum showed a band at 772 cm⁻¹, corresponding to the uranyl ¹⁸O = U = ¹⁸O bond. A second band was seen at 788 cm⁻¹, corresponding to the uranyl ¹⁶O = U = ¹⁶O bond. Finally, a last band located at 811 cm⁻¹ was associated with the contribution of the uranyl ¹⁶O = U = ¹⁶O bond, but also with that of the peroxide ¹⁸O – ¹⁸O bond. A slight band at 860 cm⁻¹ attributed to the peroxide ¹⁶O - ¹⁶O bond seemed to be present. This band is confused with the noise and the 2TOR band at 925 cm⁻¹ and was expected in these proportions due to the presence of ¹⁶O (10 atom% ¹⁶O) in the H₂¹⁸O₂ enrichment. Sarrasin had noted for a pellet having undergone leaching by a gamma-irradiated labelled water (Figure 6c) the presence of a significant band associated with the ¹⁶O - ¹⁶O peroxide bond while the band at 817 cm⁻¹ was weak [17]. Finally, the reference spectrum of studtite without isotopic labelling (Figure 6d) showed two bands at 817 cm⁻¹ and 860 cm⁻¹ corresponding to the uranyl (¹⁶O = U = ¹⁶O) bond and the peroxide (¹⁶O - ¹⁶O) bond, respectively.

<figure><img src="image_6.png" /><figcaption>**Figure 6.** Results of deconvolution of studtite Raman spectra recorded for UO₂ after 1 day of α radiolysis (a), UO₂ after 2 days of leaching with H₂¹⁸O₂ (b) and UO₂ after 1 day of γ radiolysis, based on the results of [17](c) and studtite without any isotopic labeling (d)</figcaption></figure>

In order to quantify the isotopic proportion of each band, the method proposed by Sarrasin was carried out [17], which results are presented **Figure 7**. The model proposed by Sarrasin is a binomial distribution in which *x* corresponds to the isotopic abundance of ¹⁶O while 1 – *x* corresponds to the isotopic abundance of ¹⁸O. Thus, for the uranyl bond, the respective probabilities of the different
configurations are as follows: $x^2$ for $^{16}\text{O}=\text{U}={}^{16}\text{O}$, $2x(1-x)$ for $^{16}\text{O}=\text{U}={}^{18}\text{O}$ and $(1-x)^2$ for $^{18}\text{O}=\text{U}={}^{18}\text{O}$.

The intensity of the three uranyl peaks is directly proportional to the probability of a given isotopic configuration. By knowing the probabilities of the configurations, the isotopic abundance can be determined from the intensity ratios of the uranyl bands. Applying this model at the end of the leaching test of the $\text{UO}_2$ sample under α irradiation in $\text{H}_2\text{^{18}O}$, an isotopic fraction of 79% $^{18}\text{O}$ in the uranyl bond and 97% $^{18}\text{O}$ in the peroxide bond was obtained. Refinement of the recorded spectrum of studtite precipitated on stoichiometric $\text{UO}_2$ in the presence of $\text{H}_2\text{^{18}O}_2$, gave an isotopic fraction of 85% $^{18}\text{O}$ in the uranyl bond and 97% $^{18}\text{O}$ in the peroxide bond. No $^{16}\text{O}$-$^{18}\text{O}$ band was found at 840 cm$^{-1}$. For these experiments, the process of studtite formation seems to have produced only oxygen groups of identical isotopy, and this feature has persisted over time.

<figure><img src="image_7.png" /><figcaption>**Figure 7.** Isotopic labeling fit of studtite bands by lorentzian function: a. $\text{UO}_2$ after 1 day with α radiolysis; b. $\text{UO}_2$ after 2 days of leaching with $\text{H}_2\text{^{18}O}_2$. The isotopic enrichment of uranyl (blue) and peroxide (red) bonds is shown on each graph.</figcaption></figure>

During leaching, a significant change in the isotopic ratio of the uranyl bond was observed, as shown in **Figure 8**. During irradiation, a band corresponding to $^{18}\text{O}=\text{U}={}^{16}\text{O}$ was observed, with an intensity comparable to that of the band associated with the $^{18}\text{O}=\text{U}={}^{18}\text{O}$ bond (**Figure 8a**). When irradiation was stopped, a change in the isotopic response occurred: the $^{18}\text{O}=\text{U}={}^{16}\text{O}$ band became progressively weaker compared to that of the $^{18}\text{O}=\text{U}={}^{18}\text{O}$ band (**Figure 8b**). The observation of an isotopic change when the beam is stopped suggests an evolution in studtite formation mechanism, with or without irradiation, which will be discussed later. The shape of the curve showing a gradual isotopic transition following the end of the irradiation can be attributed to the fact that the studtite layer as a whole influences the Raman response. As a result, the isotopic impact of studtite formed under irradiation remains despite the beam being stopped. The result is a direct link between the curve profile and the
403
404
405

kinetics of studtite growth after the beam is stopped (Figure 3b). Thus, the inflexion observed in the curve after 1000 minutes can either be interpreted as the attainment of a spectrum characteristic of studtite formed without irradiation, or as the result of a slowdown in precipitation.

406
407
408
409

Finally, it was not possible to obtain a satisfactory fit with the binomial distribution model, suggesting the existence of a mechanism for the formation of the specific uranyl bond during alpha irradiation. This mechanism seems to favour the formation of the ¹⁸O=U=¹⁶O bond during irradiation. The peroxide bond, on the other hand, is not affected by these changes during leaching.

410
411
412

<figure><img src="image_8.png" /><figcaption>**Figure 8.** Evolution of uranyl bond isotopy during leaching experiment a. spectrum recorded at the end of irradiation; b. intensity ratio of 790 cm⁻¹ (¹⁸O=U=¹⁶O) to 774 cm⁻¹ (¹⁸O=U=¹⁸O) bands.</figcaption></figure>
413 **4. Discussion**

414 In light of the results obtained, the first open question is to understand the chemical relationship
415 between the peroxide bond of the H₂O₂ radiolytic product and the bridging peroxo ligand inside
416 studtite.

417 Firstly, the reference leaching experiment of the UO₂ pellet in the presence of H₂¹⁸O₂ shows that there
418 is isotope exchange with the solution and secondly that the isotopy of the bridging peroxo ligand in
419 studtite is directly linked to that of H₂O₂ in solution. Indeed, **Figure 7b** shows that in the presence of
420 H₂¹⁸O₂, the bridging peroxo ligand in studtite is predominantly ¹⁸O-¹⁸O. In other words, the stability of
421 the peroxo ion is important and the peroxide bond is not broken when studtite precipitation occurs.
422 Based on this observation, it is interesting to discuss the isotopy of the peroxide bond in studtite in
423 relation to the radiolytic processes leading to the formation of H₂O₂ in solution under high and low LET
424 radiation. **Figure 6** clearly shows that under alpha irradiation (high LET) the peroxide bond is
425 predominantly ¹⁸O-¹⁸O, whereas it is mainly ¹⁶O-¹⁶O under gamma irradiation (low LET). Under water
426 radiolysis, primary hydrogen peroxide is produced mainly by recombination of OH° radicals during the
427 heterogeneous chemistry step (< 10⁻⁶ s) in the spurs generated by the energy deposition of alpha
428 particles and gamma rays. However, under gamma irradiation, over longer periods and during the
429 homogeneous chemistry stage (> 10⁻⁶ s), dissolved oxygen (¹⁶O₂) in solution controls the final H₂O₂
430 concentration at steady state and is strongly involved in its formation processes [41-45]. Under gamma
431 irradiation, dissolved oxygen leads to the formation of H₂¹⁶O₂ by scavenging the H° radical and the
432 aqueous electron according to the following reaction scheme:

$$
433 \qquad H^{\circ} + {}^{16}\text{O}_2 \rightarrow H^{16}\text{O}_2^{\circ} \qquad (2)
$$

$$
434 \qquad e^{-}_{aq} + {}^{16}\text{O}_2 \rightarrow {}^{16}\text{O}_2^{\cdot-} \qquad (3)
$$

$$
435 \qquad H^{16}\text{O}_2^{\cdot-} + {}^{16}\text{O}_2^{\cdot-} + (\text{H}_2\text{O}) \rightarrow \text{H}_2^{16}\text{O}_2 + {}^{16}\text{O}_2 + \text{OH}^{-} \qquad (4)
$$

$$
436 \qquad H^{16}\text{O}_2^{\cdot-} + H^{16}\text{O}_2^{\cdot-} \rightarrow \text{H}_2^{16}\text{O}_2 + {}^{16}\text{O}_2 \qquad (5)
$$

437 This process, related to the presence of dissolved oxygen, has been the subject of several comparisons
438 between radiolysis calculations and experimental measurements and is now well known and reported
439 in the literature [41-45]. Under alpha irradiation, the primary production of H° radicals and aqueous
440 electron is much lower than under gamma irradiation, therefore the production of H₂O₂ is not very
441 sensitive to the presence of dissolved oxygen in the homogeneous solution. The main process of H₂O₂
442 formation in H₂¹⁸O labelled water remains the recombination of two ¹⁸OH° radicals according to:

$$
443 \qquad {}^{18}\text{OH}^{\circ} + {}^{18}\text{OH}^{\circ} \rightarrow \text{H}_2 {}^{18}\text{O}_2 \qquad (6)
$$

444 The isotopic signatures of the peroxo ligands in studtite for the experiments carried out under alpha
445 and gamma irradiation are therefore fully consistent with the formation processes of hydrogen
446 peroxide in solution. A ¹⁶O-¹⁶O chemical bond is observed under gamma irradiation, whereas an ¹⁸O-
447 ¹⁸O bond is favoured under alpha irradiation. The presence of a peroxo ¹⁸O-¹⁸O bond under alpha
448 irradiation also confirms the stability of water with respect to isotopic exchange over the duration of
449 the experiment, which is consistent with the long-term studies conducted by Bonales [46].

450 The second objective is to obtain some information on the oxidative dissolution mechanism from the
451 use of isotopes. For the uranyl bond, the isotopic composition of the oxygen atoms inside UO₂²⁺ ions
452 can provide information on the oxidative dissolution mechanisms of uranium dioxide. However, in
453 order to ensure that the actinyl species keeps the memory of the dissolution mechanism, the residence
time of the oxygen atoms on the UO₂²⁺ ions must be long enough since its release from the surface to the studtite to be analysed by Raman spectroscopy. The isotopic exchange of oxygen atoms within the uranyl ion has been studied since the early 1960's. These studies [47, 48] have shown that the uranyl ion does not exchange its oxygen atoms with the solution for a period of 2000 hours, much longer than the duration of the experiments presented in this work. Similarly, studies of the nature of isotopic exchange between studtite and ¹⁸O-enriched water have shown the absence of exchange with the peroxo and uranyl bonds within studtite [47]. These bonds are strong and not very sensitive to isotopic exchange. The only fast exchanges are with the water molecules of studtite. Therefore, it is reasonable to assume that the oxygen isotope of the uranyl ion provides relevant information about the nature of the oxidative dissolution mechanism at the UO₂ / water interface.

The reference leaching experiment of the UO₂ pellet in the presence of H₂¹⁸O₂ clearly shows that the uranyl bond contains a significant amount of ¹⁶O (15%) while the peroxo bond mainly contains ¹⁸O (97%) (Figure 6). The uranyl bond well integrates oxygen ¹⁸O coming from the labelled water (H₂¹⁸O + H₂¹⁸O₂ media), but also ¹⁶O. The latter certainly comes from the solid and reflects an oxidation mechanism at the surface of the pellet. Indeed, the hypothesis of an initial release of the U⁴⁺ ion from the surface into the solution, followed by oxidation in the solution (H₂¹⁸O + H₂¹⁸O₂ system), would have led to an ¹⁸O enrichment close to 100%. Furthermore, ¹⁶O₂ coming from the air is 5 times less concentrated than H₂¹⁸O₂ and its kinetic oxidation constant is 200 times lower [49], which does not support its participation in the uranium oxidation process.

With regard to the surface oxidation process in the presence of H₂O₂, it is generally described in two stages, with the formation of U(V) first, followed by the formation of the uranyl ion. A plausible mechanism proposed by Ekeroth and Jonsson [50] for the reaction between UO₂ and H₂O₂ is a primary slow one-electron transfer stage producing U(V) and OH° radicals according to the following reaction:

$$477 \qquad UO_2 + H_2O_2 \rightarrow UO_2^{+\text{surf}} + OH^{\circ} + OH^{-} \qquad (7)$$

Which becomes by taking into account the isotopy of the studied system:

$$479 \qquad U^{16}O_2 + H_2^{18}O_2 \rightarrow U^{16}O_2^{+\text{surf}} + ^{18}OH^{\circ} + ^{18}OH^{-} \qquad (8)$$

According to such a process, the formation of U(V) from H₂O₂ would not necessarily involve the incorporation of oxygen atoms into the fluorite structure. The formation of the uranyl ion can then be written [50]:

$$483 \qquad U^{16}O_2^{+\text{surf}} + U^{16}O_2^{+\text{surf}} \rightarrow U^{16}O_2^{2+\text{surf}} + U^{16}O_2 \qquad (9)$$

As the ¹⁸OH° radicals formed, they can contribute to catalytic decomposition of H₂O₂ according to the following mechanism [51-53]:

$$486 \qquad H_2^{18}O_2 + M \rightarrow ^{18}OH^{-} + ^{18}OH^{\circ}_{ads} + M^{+} \qquad (10)$$

$$487 \qquad ^{18}OH^{\circ}_{ads} + H_2^{18}O_2 \rightarrow H^{18}O_2^{\circ} + H_2^{18}O \qquad (11)$$

$$488 \qquad M^{+} + H^{18}O_2^{\circ} \rightarrow H^{+} + ^{18}O_2 + M \qquad (12)$$

$$489 \qquad 2 H_2^{18}O_2 \rightarrow ^{18}O_2 + 2 H_2^{18}O \qquad (13)$$

They can even oxidize the UO₂ surface. The relative contribution of these two processes (catalytic decomposition versus oxidation) depends in particular on the amount of OH° radicals scavenged by hydrogen peroxide [52]. Sunder [54] has proposed an interaction process between OH° and the surface
involving the incorporation of oxygen atoms into the fluorite structure. OH° radicals, produced during the first oxidation step with H₂O₂ are smaller in size than the H₂O₂ molecule and can also oxidize the surface. The UO₂ surface can incorporate oxygen atoms in an interstitial position according to the following process [54]:

$$497 \qquad U(IV) + ^{18}\text{OH}^{\circ} \rightarrow H^{+} + ^{18}\text{O}_{1}^{2-} + U(V) \qquad (14)$$

The interstitial ¹⁸O oxygen atom can therefore also contribute to the formation of UO₂⁺surf and UO₂²⁺surf ions. More recently, studies on the interaction between water plasma and a UO₂ surface mention the incorporation of oxygen atoms into the fluorite structure, resulting in a decrease of the U4f/O1s surface area ratio measured by XPS [55]. Thus, the formation of the uranyl ion in the presence of H₂¹⁸O₂ alone (H₂¹⁸O + H₂¹⁸O₂ media), with both ¹⁸O and ¹⁶O isotopic contributions, is fully compatible with an oxidation mechanism involving both electronic transfer and incorporation of interstitial oxygen from the solution.

Therefore, it is interesting to discuss the isotopy of the uranyl bond for the alpha and gamma irradiation experiments. For the gamma irradiation experiment, the uranyl bond is highly enriched up to 95% in ¹⁸O. The primary yields of OH° radicals for gamma radiation (low LET radiation) are high [35] and these highly reactive species can rapidly interact with the UO₂ surface. The high reactivity of these ¹⁸OH° radicals produced by radiolysis of labelled water (H₂¹⁸O) would lead to a massive incorporation of ¹⁸O at the surface and explain the low ¹⁶O content of the uranyl ion. On the other hand, under alpha irradiation, H₂O₂ peroxide remains a major product of water radiolysis and the primary production of OH° radicals is 10 times lower than under gamma irradiation. Furthermore, the ¹⁸O enrichments obtained for the uranyl bond for the reference experiment with simple addition of H₂¹⁸O₂ and for the experiment under alpha irradiation with ¹⁸O labelled water are very similar (figure 6). This observation clearly argues for a predominant role of H₂¹⁸O₂, especially after the beam has been cut. It is then consistent to find the two isotopes (¹⁸O and ¹⁶O) of oxygen in the uranyl bond if processes involving both electronic transfer and surface incorporation occur simultaneously under alpha irradiation.

It should be noted that under alpha irradiation and before the beam is cut off, the ¹⁸O enrichment in the uranyl bond is lower and the ¹⁶O content is higher than after the beam is cut. This point may reflect an increased mobility of oxygen atoms (¹⁶O) in the solid due to the continuous creation of defects, but will require additional development in the future. Another hypothesis is that the high local content of hydrogen peroxide in the first seventy microns under alpha irradiation induces strong scavenging of ¹⁸OH° radicals.
## 5. Conclusion

An original experimental approach combining Raman spectroscopy to the use of H₂¹⁸O labelled water was developed to better understand the oxidizing dissolution of UO₂ and studtite precipitation mechanisms under alpha radiolysis of water. Two static leaching experiments were performed: the first involved the alteration of a UO₂ pellet under alpha radiolysis in the presence of ¹⁸O-labelled water with in-situ Raman monitoring of the altered layer, and the second one involved the alteration of UO₂ in the presence of ¹⁸O-labelled hydrogen peroxide. In these experiments, the precipitation of a studtite layer on the surface of the pellet was observed. It is clear that the bridging peroxo ligands in studtite originate from the peroxo species within the hydrogen peroxide molecule, which is itself produced by the radiolysis of water. Under alpha irradiation, the isotopic composition of the peroxide bond in studtite is essentially ¹⁸O-¹⁸O and originates from the H₂¹⁸O₂ molecule formed by recombination of ¹⁸OH° radicals in solution.

For the uranyl bond of the UO₂²⁺ ions, the presence of both ¹⁶O and ¹⁸O isotopes is observed. This can be explained by allowing an oxidation process involving both a simple electron transfer by interaction with H₂O₂ and the incorporation of oxygen atoms coming from the solution into the fluorite structure due to oxidation by OH° radicals. The production of ¹⁸OH° radicals, particularly by catalytic decomposition of hydrogen peroxide H₂¹⁸O₂ on the surface of UO₂, remains a preferred route leading to the insertion of oxygen atoms into the material structure at the surface.

Moreover, these results are consistent with previous studies, this time carried out under gamma irradiation and in aerated conditions. The chemical relationship between the peroxide bond of H₂O₂ and the O-O bridging bond in studtite is also confirmed, as is the important role of OH° radicals in the insertion of oxygen atoms into the fluorite structure.

In the future, it will be useful to further investigate the role of atomic defects in the dynamics of isotopic exchange at the reaction interface. In this context, the use of a solid doped with ¹⁸O, which allows the study of exchange dynamics within the solid under irradiation, would be relevant and is currently under extensive investigation.

## Acknowledgments

The authors would like to express their gratitude to William Hate, Dominique Baux and Paul Sigot, members of the CEMHTI particles beams platform, for their technical support in this study. We are grateful to acknowledge Rachelle Omnée from CEMHTI and Stéphanie Szenknect from ICSM for their technical assistance. We would also like to thank Véronique Broudic from LMPA, Nicolas Clavier from ICSM and David Chaulin from CEMHTI for organizing the transport of the sample as well as Joseph Lautru from ICSM for the SEM analyses and Maxime Lanyi for his careful proofreading. Finally, the authors would like to thank the project Transport & Entreposage of the French Institut Tripartite CEA-EDF-Framatome for supporting this study.

## CRediT

**Aurelien Perrot:** Writing – original draft, Methodology, Software, Formal analysis, Data curation, Investigation, Resources.

**Aurelien Canizares:** Methodology, Formal analysis, Data curation, Investigation, Resources, Writing – Review & Editing.
568 **Sandrine Miro:** Conceptualization, Validation, Formal analysis, Data curation, Investigation, Writing –
569 original draft, Writing – Review & Editing, Visualisation, Supervision, Project administration, Funding
570 acquisition.

571 **Laurent Claparede:** Conceptualization, Validation, Writing – Review & Editing, Visualisation,
572 Supervision.

573 **Renaud Podor:** Methodology, Formal analysis, Data curation, Investigation, Resources, Writing –
574 Review & Editing.

575 **Thierry Sauvage:** Conceptualization, Project administration, Writing – Review & Editing.

576 **Sylvain Peuget:** Writing – Review & Editing, Project administration, Funding acquisition.

577 **Christophe Jegou:** Conceptualization, Validation, Writing – original draft, Writing – Review & Editing,
578 Visualization, Supervision, Project administration, Funding acquisition.

579 **Nicolas Dacheux:** Conceptualization, Validation, Writing – original draft, Writing – Review & Editing,
580 Visualization, Supervision, Project administration, Funding acquisition.

581
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