Diane Rébiscoul

Zijie Lu

Joseph Lautru

Thomas Zemb

multi-lamellar vesicles of carboxylate based surfactant

Keywords: Colloidal Sol-gel, UO2 nanoparticle, multi-lamellar vesicle, carboxylate surfactants

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Nuclear fuel preparation for future generations of nuclear reactor is a challenging task regarding the safety conditions. Generally, fuels such as UOx (UO2) [START_REF] Kaufmann | Nuclear Reactor Fuel Elements: Metallurgy and Fabrication[END_REF][START_REF] Hasegawa | Process for manufacturing uranium dioxide powder[END_REF][START_REF] Yemel'yanov | The metallurgy of nuclear fuel: properties and principles of the technology of uranium, thorium and plutonium[END_REF], ThO2 [START_REF] Yemel'yanov | The metallurgy of nuclear fuel: properties and principles of the technology of uranium, thorium and plutonium[END_REF][START_REF] Shiratori | Fabrication of very high density fuel pellets of thorium dioxide[END_REF] and MOx (UO2-PuO2 mixed oxides [START_REF] Caldwell | PLUTONIUM--URANIUM DIOXIDE POWDER AND PELLET FUEL MANUFACTURE[END_REF][START_REF] Bataller | MIXED URANIUM--PLUTONIUM OXIDE FUEL FABRICATION FOR RAPSODIE[END_REF][START_REF] Mattys | PLUTONIUM METALLIC AND CERAMIC FUEL FABRICATION AND DEVELOPMENT AT THE EUROPEAN INSTITUTE[END_REF][START_REF] Vauchy | Ceramic processing of uranium-plutonium mixed oxide fuels (U1-yPuy)O2 with high plutonium content[END_REF]) fuels are synthesized by powder metallurgical routes [START_REF] Cunningham | United States Experience in Powder Metallurgy for Nuclear Applications[END_REF]. Such preparation presents several disadvantages such as potential contamination problem due to UO2 microparticles powder and also for MOx preparation [START_REF] Vaidya | Sol-gel process for ceramic nuclear fuels[END_REF] where the ultimate mixing of U/Pu has to be homogenous at a nanoscale. Low temperature chemical processes in aqueous solution such as colloidal sol-gel process leading to the possible formation of an organized mesoporosity in actinides oxides matrix, may be a real interesting alternative for several reasons [START_REF] Ganguly | Sol-Gel Microsphere Pelletization: A Powder-Free Advanced Process for Fabrication of Ceramic Nuclear-Fuel Pellets[END_REF][START_REF] Vaidya | Status of sol-gel process for nuclear fuels[END_REF][START_REF] Sood | The role sol-gel process for nuclear fuels-an overview[END_REF][START_REF] Aparicio | Sol-gel processing for conventional and alternative energy[END_REF]. First, from a safety point of view, the aqueous colloidal sol-gel route avoids dried powder manipulation by operators and the formation of hotspot due to the repulsions of actinides oxides particles in solution in the case of UO2/PuO2 mixture. Second, the presence of organized mesoporosity in the UO2 final material provides advantages such as (i) the possibility of an homogenous PuO2 incorporation into the mesopores avoiding hotspot formation, (ii) the fission gas release during the fuel burning in nuclear reactor through the porous network if the pores are connected and, (iii) due to its high specific surface area, the increase of its dissolution rate during its reprocessing [START_REF] Corkhill | Role of Microstructure and Surface Defects on the Dissolution Kinetics of CeO2, a UO2 Fuel Analogue[END_REF][START_REF] Quiñones | Modelling of the spent fuel dissolution rate evolution for repository conditions. Matrix Alteration Model results and sensitivity analysis[END_REF]. Third, the homogeneity and the ultimate morphology of such materials can be easily adjusted to fit with different types of nuclear reactor.

Since early 1960s, several colloidal sol-gel routes (Internal [START_REF] Kumar | Studies of the Preparation of Thoria and Thoria-Urania Microspheres Using an Internal Gelation Process[END_REF][START_REF] Vaidya | A Study of Chemical-Parameters of the Internal Gelation Based Sol-Gel Process for Uranium-Dioxide[END_REF] and External Gelation Process [START_REF] Kovacheva | Mechanochemically induced synthesis of UO2+x and uranium-thorium mixed oxides from sol-gel produced precursors[END_REF], Total Gelation Process [START_REF] Fu | Preparation of UO2 kernel for HTR-10 fuel element[END_REF] and Complex Sol-Gel Process [START_REF] Deptula | Synthesis of uranium and thorium dioxides by Complex Sol-Gel Processes (CSGP)[END_REF]) have been developed for the synthesis of oxides or mixed oxides of actinides (Uranium, Thorium, Plutonium…). These kinds of process yield actinide oxides microspheres. However, there is a lack of investigation regarding the preparation of mesostructured actinide materials. Recently, Zhao et al. reported the synthesis of ordered mesoporous uranium dioxide via a complex multi-step "nanocasting" route [START_REF] Zhao | Synthesis of ordered mesoporous uranium dioxide by a nanocasting route[END_REF]. Using ordered mesoporous silicates as hard templates, the authors have successfully synthesized UO2 powder having a rod-like morphology at nanoscale. Nevertheless, to our best knowledge, no study of the colloidal sol-gel route coupled with soft template molecules with a view to synthetize actinide oxides presenting organized porosity has been reported. This is the goal of this study focusing on the understanding of the sols to, one day, be able to prepare highly organized UO2 materials. Indeed, after the discovery of ordered mesoporous silica in 1990s, the synthesis of mesoporous material has been largely extended to other non-siliceous elements such as transition metals, rare earth elements, etc. [START_REF] Schuth | Non-siliceous mesostructured and mesoporous materials[END_REF][START_REF] Gu | Synthesis of non-siliceous mesoporous oxides[END_REF][START_REF] Reitz | Facile and General Synthesis of Thermally Stable Ordered Mesoporous Rare-Earth Oxide Ceramic Thin Films with Uniform Mid-Size to Large-Size Pores and Strong Crystalline Texture[END_REF]. Generally, two types of sol-gel processes are used based on different forms of precursors: the polymeric route using alkoxides and the colloidal route using metal salts. Nevertheless, the synthesis of actinides alkoxides is fastidious due to the high reactivity of these species and thus, difficult to control from an industrial point of view [26,[START_REF] Clark | New Development in Actinide Alkoxide Chemistry[END_REF]. Therefore, the colloidal route using metal salts presents an interesting alternative.

In the colloidal sol-gel route using metal salts, the main difficulty is the control of the reactivity, i.e the relative kinetics of nucleation, growth and coagulation of the colloids and their condensation leading to the network formation around the templating agent. Nucleation and growth must be balanced, while coagulation in the form of microparticles must be avoided [START_REF] Jolivet | Metal oxide chemistry and synthesis: from solution to solid state[END_REF]. Depending on the metal salt, the pH has to be controlled and the addition of a complexing agent can be required. [START_REF] Soler-Illia | Chemical strategies to design textured materials: From microporous and mesoporous oxides to nanonetworks and hierarchical structures[END_REF] Recently with U(IV) solution at 6<pH<8, Leblanc and al. [START_REF] Leblanc | Stable uranium sols as precursors for the elaboration of nanostructured nc-UO2 materials[END_REF] have used carboxylate surfactants with a non-ionic polyethoxylated part between its anionic group and its hydrophobic alkyl chains as complexing and templating agent for the preparation of structured mesoporous UO2 materials. The final UO2 materials synthesized from stable sols of UO2 nanocrystallites was only partially lamellar structured as observed by Transmission Electron Microscopy.

Based on the same colloidal system, we have intended to better control the processes driving the formation of this lamellar structure and the distributions of uranium species in the sol. Thus, we have studied thoroughly the distribution of U(IV) species in the sols and the competition between the UO2 formation and the complexation of soluble uranium species by the carboxylate group of the surfactants. To reach this goal, the surfactant solutions and the sols prepared by indirect precipitation from U(IV) solution were characterized by Small and Wide Angle X-ray Scattering (SWAXS), and Infrared Spectroscopy (IR).

MATERIALS AND METHODS

2.1 Materials. AKYPO RO 90 VG (Polyoxyethylene(9) oleyl ether carboxylic acid, C16/18:1E9COOH, Mw = 722 g mol -1 ) and AKYPO RLM 100 (Polyoxyethylene(10) lauryl ether carboxylic acid, C12E10COOH, Mw = 685 g mol -1 ) from Kao Chemicals were used as received.

2.2 Surfactant solutions preparation. C16/18:1E9COOH and C12E10COOH were dissolved in Mili-Q water to prepare solutions at 10 -1 M and referred as C16/18E9 and C12E10. This concentration is higher than the critical micellar concentration (10 -4 M) [START_REF] Micheau | Specific Salt and pH Effects on Foam Film of a pH Sensitive Surfactant[END_REF] and the surfactants form an isotropic micellar phase. The pH of the surfactant solutions were adjusted from an initially pH20°C=2.3 to 6, 7, 8 and 9 by adding a 3 M NH4OH solution. At these pH values the carboxylic groups of surfactant molecules are deprotonated (apparent pKa value is around 5.3 ± 0.2 at 10 -1 M [START_REF] Micheau | Polyethoxylated carboxylic surfactant for ion foam flotation: fundamental study from solution to foam; Tensioactif carboxylique polyethoxyle pour la flottation ionique: etude fondamentale de la solution a la mousse[END_REF]) and the precipitation of UO2(am) is favorable since its saturation index is positive and constant when the pH value is higher than 5 [START_REF] Saravia | Synthesis of carbide fuels from nano-structured precursors: impact on carbo-reduction and physico-chemical properties[END_REF]. The prepared surfactant solutions were then stocked in the dark to avoid solution alteration until their analyses [START_REF] Leblanc | Stable uranium sols as precursors for the elaboration of nanostructured nc-UO2 materials[END_REF].

U(IV) solution preparation. Uranium chloride solution (U(IV) solution) was

prepared by dissolution of uranium metal in 6 M HCl solution. The uranium metal chips were firstly washed in 2 M HCl to dissolve the oxide impurities then rinsed with water and ethanol.

Finally, the pretreated uranium metal was dissolved in 6 M HCl to prevent the oxidation of uranium and maintain uranium at the oxidation state IV for several months [START_REF] Dacheux | Synthesis and Properties of Uranium Chloride Phosphate Tetrahydrate -Uclpo4-Center-Dot-4h2o[END_REF]. Based on the titration method presented in Dacheux et al. [START_REF] Dacheux | Synthesis and Characterization of Mixed-Valence Uranium Orthophosphate -U(Uo2)(Po4)(2)[END_REF], the uranium concentration of U(IV) solution was 0.515 ± 0.002 M.

Preparation of colloidal dispersions.

Uranium sols were prepared by indirect precipitation [START_REF] Saravia | Synthesis of carbide fuels from nano-structured precursors: impact on carbo-reduction and physico-chemical properties[END_REF] in the glove box under nitrogen to keep the oxygen concentration under 1 ppm. First, the surfactant solutions were degassed to remove dissolved oxygen. Then U(IV) solution was added dropwise into 8 mL of surfactant solution until the target concentration of U(IV) was reached. After each drop addition, the pH value of the colloidal dispersions was corrected by adding a degassed solution of 3 M NH4OH. Table 1 presents the references, the final pH and the molar ratio between uranium and surfactant U/S. Reflection (ATR) mode equipped with a DTGS/KBR detector. The samples were placed at the surface of the diamond. The spectra were recorded from 400 to 4000 cm -1 adding 32 scans with a 2 cm -1 of resolution correcting from the background spectrum for each substrate.

Baseline adjustments were performed using the Origin software. Small and Wide Angle X-ray Scattering (SWAXS) analysis were carried out in transmission geometry with a Xenocs setup equipped with a Mo anode (λ = 0.71 Å) using a MAR345 2D imaging plate detector.

Such short wavelength allows the probing of a scattering range from few angstroms to 30 nm and at the same time, is only weakly absorbed by the high-z elements such as uranium. The collimation was ensured by a Fox2D multilayer mirror and by a set of scatterless slits that delimitated the beam to a square section (0.8 mm side length at the sample position). The distance from the sample to the detector was about 750 mm and was calibrated using silver behenate powder. Sols were analyzed in glass capillaries of 2 mm of diameter. Azimuthal averaging of 2D-data recorded by a MAR345 imaging plate detector was performed using the FIT2D software taking into account the electronic background of the detector, the empty cell subtraction and an intensity calibration. The scattered intensity in absolute scale (in cm -1 ) was expressed versus the magnitude of the scattering vector q = (4πsinθ)/λ, where θ was the scattering angle. Experimental resolution was Δq/q = 0.02. An Environmental Scanning Electron Microscope (ESEM) FEI Quanta 200 FEG with a Scanning Transmission Electronic Microscope (STEM) stage is used to observe the vesicles. This stage is composed by a TEM grid holder fixed above a STEM detector, which is composed of two segments for Bright Field (BF) and Dark Field (DF) imaging. Indeed, this device allows getting an image with the transmitted electron in order to improve the resolution. After the vesicles deposition on TEM grid, the sample was mounted on the stage in the microscope chamber. Then the micrographs are done under 10 -3 Pa vacuum. The conditions for imaging are a high voltage between 10kV and 20kV, a working distance between 4mm and 6mm and a magnification between X10000 and X100000. Low resolution Transmission electron microscopy (TEM) analyses were conducted at 200 kV on a Jeol200CX TEM equipped with a Photonic-Science camera. In order to determine the impact of pH on the self-assembly of the surfactant and their structure in the surfactant solutions, SWAXS analysis have been carried out. Figure SI2 presents the SWAXS patterns of different surfactant solutions at different pH. Whatever the surfactant, the variation of pH from 2.3 to 9 has no distinguishable impact either on the size of micelles or on their globular structure. This low pH sensitivity is different than the behavior of fatty acid solutions which a slight pH variation leads to a phase transition (an example is given in Figure SI3 with octanoic acid).

RESULTS AND DISCUSSION

The results obtained from the fitting of the experimental SWAXS patterns at pH comprised between 6 and 9, show core-shell micelles having a radius of 3.2 nm and a number of aggregation ≈ 100 for the C16/18E9 and 2.9 nm and ≈ 90 for the C12E10 (see Figures SI4, Tables SI1 andSI2.). The decrease of forward intensity is mainly due to the electrostatic repulsion linked to the deprotonation of core-shell micelles modifying the intermolecular interaction.

Stability of the colloidal dispersions

We have systematically varied pH and U(IV) to surfactant molar ratio U/S using the one pot method described previously. After the addition of U(IV) solution and the pH adjustment, the stability of the colloidal dispersion was noted. When the dispersion was unstable, the precipitation was observed few hours after the addition of U(IV) solution. The colloidal dispersion with C16/18E9 remained stable for more than 1 month and the colloidal dispersion with C12E10 started to precipitate after 1 week. Since these two dispersions remained stable for a relative long time even in presence of heavy elements, they could be therefore considered as sols and were referred to C16/18E9 sols and C12E10 sols. A few hours after the stirring, the changes of color from green to dark brown for C16/18E9 sols and clear brown for C12E10 sols were observed indicating the formation of UO2 nanoparticles.

Structure of the sols at colloidal scale

In order to determine the modification of phase structure after the addition of U(IV) solution, SWAXS analysis was carried out. The SWAXS patterns of sols are presented on the Figure 2 and reveal that the structuration of these two types of sols are close. The medium chain C12 and the long chain C18 have similar but not identical behaviors.

The common point is liquid ordered structure producing the Lorentzian broad peak at q=20 nm -1 . In the intermediate q range, a lamellar structure is detected with two first orders of reflection. In the case of long chains, we observe disordered locally lamellar structures producing no second order except for the sample C16/18E9-6-0.25 producing rigid lamellar structure (Lβ). In the following part, we focus on the C16/18E9 sols which shows the best stability through the study of the effect of the pH and the U/S ratio on the self-assembly of the surfactant and their structure within the sols.

Effect of pH

The Figure 3 After the addition of the U(IV) solution, two main modifications which depend on the pH and which are characteristic of the presence of UO2 nanoparticles and/or aggregates of UO2 are visible on the SWAXS patterns. First, the large peak growing at 20 nm -1 is associated with the crystalline plane (111) of a fluorite-type structure of UO2 nanocrystallites (nc-UO2) [START_REF] Leblanc | Stable uranium sols as precursors for the elaboration of nanostructured nc-UO2 materials[END_REF][START_REF] Wang | Mobile uranium(IV)-bearing colloids in a mining-impacted wetland[END_REF][START_REF] Wu | Synthesis of colloidal uranium-dioxide nanocrystals[END_REF][START_REF] Bouala | Preparation and characterisation of uranium oxides with spherical shapes and hierarchical structures[END_REF]. This peak becomes more intense with the pH increase. Indeed, the increase of pH favors the formation of nc-UO2 and bigger aggregates of nc-UO2. The formation of objects having a size bigger than 20 nm is suggested by the increase of the signal at low q compared to the one of surfactant solutions. All of these results are attested by the TEM image of the sol C16/18E9-8-0.25 presented on Figure 3 confirmed by the SWAXS pattern showing a large peak at high q (insert figure in Figure 3 (a))

where the maximum corresponds to a distance of 0.309 nm. This is in agreement with the interplanar distance between crystallographic planes of the (111) face-center-cubic lattice of UO2 (0.3153 nm, 00-041-1422 ICDD [START_REF] Icdd | International Centre for Diffraction Data[END_REF]).

Second, as already observed in [START_REF] Leblanc | Stable uranium sols as precursors for the elaboration of nanostructured nc-UO2 materials[END_REF] where l is the average chain length and V is the molecular volume. Unlike classical surfactant system, the area a0 and therefore the spontaneous packing parameter are now a function critically controlled by the uranium to surfactant U/S. In other words, the L α is promoted when P0 is near 1.

Moreover, the repeating distance is smaller than the length of the average alkyl chain of one C16/18E9 molecule, which is about 6 nm according the Tanford's equation [START_REF] Tanford | Micelle Shape and Size[END_REF]. Even though taking into account that the alkyl chains are not fully extended, this is incompatible with the periodicity of the lamellar phase L α obtained in this study. Therefore, the alkyl chains of C16/18E9 molecule must be at least partially interdigitated with another C16/18E9 molecule and form the bilayers. Such interdigitation of alkyl chains can be observed on the bilayers of lipids with two alkyl chains [START_REF] Simon | Interdigitated Hydrocarbon Chain Packing Causes the Biphasic Transition Behavior in Lipid Alcohol Suspensions[END_REF][START_REF] Mcintosh | Induction of an Interdigitated Gel Phase in Fully Hydrated Phosphatidylcholine Bilayers[END_REF][START_REF] Mou | Alcohol Induces Interdigitated Domains in Unilamellar Phosphatidylcholine Bilayers[END_REF]. However, some single-chained surfactants having a sufficiently large headgroup area could also form an interdigitated thin bilayer [START_REF] Israelachvili | Intermolecular and surface forces[END_REF]. Indeed, close contacts between the charged headgroups are unfavorable. The alkyl chains are forced to interdigitated in order to keep the neighboring headgroups apart. Similar phenomena have been observed with dialkyldimethylammonium Bromides bilayers [START_REF] Ferreira | Structural Parameters of Lamellar Phases Formed by the Self-Assembly of Dialkyldimethylammonium Bromides in Aqueous Solution[END_REF][START_REF] Rozenfeld | Oligonucleotide Adsorption Affects Phase Transition but Not Interdigitation of diC14-Amidine Bilayers[END_REF][START_REF] Oliveira | Temperature-Dependence of Cationic Lipid Bilayer Intermixing: Possible Role of Interdigitation[END_REF]. In addition, the increase of the sol pH (C16/18E9-7-0.25 and C16/18E9-8-0. In conclusion, the increase of pH benefits the formation of nc-UO2 and their aggregation.

This phenomenon causes a partial deformation of the L α structure. Thus, C16/18E9-6-0.25 being the sol presents the most organized lamellar-like structure, it has been chosen to determine the effect of the U/S ratio. confirmed the existence of vesicles in the colloidal system. In addition, compared with the study of the pH effect (Figure 3 (a)), no significant change is observed in the high q region with the increase of U/S ratio. This attests that the complexation of U(IV) species is a preferential mode compared to the nc-UO2 precipitation. Apart from the change of structuration of sols showing by SWAXS analysis, FTIR spectra of these samples present also some modifications with the U/S ratio. Figure 4 (c) shows that peak of the C-H stretching bonds increases with U/S ratio probably due to the phase transformation from globular micelles to multi-lamellar vesicles. Such modification of C-H bands have been already associated to a phase transformation [START_REF] Jia | Mechanism of PEO-PPO-PEO micellization in aqueous solutions studied by two-dimensional correlation FTIR spectroscopy[END_REF]. In addition, Figure 4 (d) shows that the C-O-C stretching band around 1080 cm -1 presents a decrease of the contribution of the vibration at low wave number leading to a decrease of the peak width with a higher U/S ratio. Dehydration of ether backbone due to the interdigitation of hydrophobic chain of the surfactant may be responsible for this peak modification [START_REF] Jia | Mechanism of PEO-PPO-PEO micellization in aqueous solutions studied by two-dimensional correlation FTIR spectroscopy[END_REF]. The SWAXS patterns show that both, the supernatants and the dense gel present the two same microstructure observed previously (Figure SI2 and Figure 2), i.e. a micellar phase and a L α phase in multi-lamellar vesicles respectively. Besides, the pattern of C16/18E9-6-0.25 S presents the same core-shell structure as for the surfactant solution but with an increase of the intensity of the scattering signal in the small q region with a slope close to q -2 . This may be due to the presence of nc-UO2 aggregates in the supernatant.

Effect of U/S ratio on sols U

To refine quantitative results, the SWAXS patterns of C16/18E9-6-0.25 S and C16/18E9-6-0.25 G were simulated on absolute scale. First, for the simulation of the experimental SWAXS pattern of C16/18E9-6-0.25 S, we have taken into account two contributions: (i) a fraction of core-shell micelles having the electron density of the C16/18E9

surfactants and (ii) a fraction of dispersed aggregates of nanoparticles with an exponential mass fractal model [START_REF] Breßler | SASfit: a tool for small-angle scattering data analysis using a library of analytical expressions[END_REF]. The fitting parameters and the simulated SWAXS patterns corresponding to the best fit are presented on the Table SI3 and Figure 5 (b) respectively.

Consequently, the supernatant of the C16/18E9-6-0.25 mainly consists of UO2 aggregates and core-shell micelles of C16/18E9 surfactants. From this simulation, the fraction of surfactant complexing U(IV) species was calculated. Indeed, knowing the number density of core-shell micelles existing in the initial C16/18E9 solution from the simulation presented in Table SI2, and the number density of core-shell micelles in the C16/18E9-6-0.25 S, we can deduce that 21% of C16/18E9 surfactant molecules are still present as globular micelles and 79% of them are complexed by U(IV) species. Moreover, by interpreting the forward scattering I0, the fraction of UO2 aggregates was determined and count for about 1 % of the total U(IV) species (see on the Figure 4 (b). The image reveals that some vesicles having a size of around 200 nm are stuck together. As proposed previously, this may be due to the presence of UO2 aggregates at the surface of the vesicles linking the vesicles between each other.

As so far, we are able to determine the distributions of U(IV) species as well as C16/18:1E9COOH molecules partitioned as different forms in the system U-C16/18:1E9COOH (see Table 2).

Multi-scale structure of the final system

The multi-scale structure of the system is presented on Figure 7 and shows a schematic overview of the solution-sol transition of the C16/18E9-6-0.25 at different scales. Added to an C16/18E9 solution at 0.1 M presenting core-shell micelles, U(IV) solution leads to the formation of an homogeneous dark green colloidal uranium sol. The complexation of U(OH) 4-n n+ (1≤n≤4) by deprotonated carboxylic function of C16/18E9 molecules induced the shape transformation of a part of micelles (79 %) to multi-lamellar vesicles having around 200 nm size (See Figure 13) having interdigitated alkyl chains. Thus, 21% of C16/18E9 surfactant having no interaction with uranium species as counter-ions remains as micelles. In addition, during the indirect precipitation process, 11% of U(IV) species precipitate in the form of nc-UO2 forming aggregates adsorbed on the surface of the vesicles.

These UO2 aggregates may act as nodes from one vesicle to another leading to the formation of heaps. Around 28% of U(IV) species form nc-UO2 with a size about 1 nm are also present within the vesicles. The small size of these nanocrystallites may be due to: (i) the limited concentration of free U(IV) species (U(OH) 4-n n+ (1≤n≤4)) which directly precipitate to form nc-UO2 at the equilibrium with the U(IV) species complexed by C16/18E9 surfactants or/and (ii)

the limited amount of U(IV) species in the volume between the surfactant layers estimated to be an order of magnitude of -19 in mole.

While at 25 °C, log k 1 is known and comprised between 45 and 47 according to different studies [START_REF] Rai | The solubility of Th (IV) and U (IV) hydrous oxides in concentrated NaCl and MgCl2 solutions[END_REF][START_REF] Rai | Uranium (IV) hydrolysis constants and solubility product of UO2. cntdot. xH2O (am)[END_REF][START_REF] Yajima | Uranium(Iv) Solubility and Hydrolysis Constants under Reduced Conditions[END_REF][START_REF] Neck | Solubility and hydrolysis of tetravalent actinides[END_REF], log k 2,n is unknown. However, the complexation constants of U(IV)

3. 1

 1 pH sensitivity of surfactant solutions Surfactant solutions were characterized using ATR-FTIR. The results of C16/18E9 and C12E10 (Figure SI1 (a) and (b) respectively) do not show any significant modification as a function of pH except on the C-O-C stretching band of ether backbone of C16/18E9 samples. Indeed, the C-O-C stretching band (around 1070 cm -1 ) presents a slight increase of the absorbance.

Figure 1 .

 1 Figure 1. Stability ranges of pH and molar ratio U/S for colloidal dispersions prepared with (a) C16/18E9 and (b) C12E10 (red stars represent sols and black spheres represent unstable dispersions).

Figure 2 .

 2 Figure 2. SWAXS patterns of the sols of (a) C16/18E9 and (b) C12E10.

  (a) presents the SWAXS patterns of the C16/18E9 sols at different pH with a molar ratio U/S=0.25.

Figure 3 .

 3 Figure 3. (a) SWAXS patterns of C16/18E9 sols with a molar ratio U/S=0.25 at different pH. The SWAXS pattern of water was subtracted to the sols one; (b) TEM image and electron diffraction pattern of UO2 aggregates from C16/18E9-8-0.25 sol; (c) FTIR spectra focus on the C-O-C stretching band of sols with a molar ratio U/S=0.25 at different pH.

  (b) showing the presence of aggregates of nanocrystallites around ten nanometers having a size comprised between 3 nm and 12 nm.Moreover the electron diffraction pattern of several nanoparticles (Figure3 (b)) displays the (111), (200), (220), (222), (311), (024) and (511) planes of a face-center-cubic lattice. This is

  , a bilayer-based L α phase is present in C16/18E9-6-0.25 sols. The addition of U(IV) solution has changed a part or all the initial core-shell structure existing in the surfactant solution to a L α structure having 4.8 nm of repeating distance. The high electronic density of U(IV) complexed by C16/18E9COOH surfactants leads to a decrease of the head area occupied by each surfactant (a0). This induces an increase of the spontaneous packing parameter: P0(U/S)= V a 0 (U/S)l

  Figure SI1(a); (ii) the increase of pH causes an aggregation of nc-UO2 attaching to the ether

Figure 4 (

 4 Figure 4 (a) presents the SWAXS patterns of the sols with U/S molar ratio from 0.16 to

Figure 4 .

 4 Figure 4. (a) SWAXS curves of sols U with different U/S ratios at pH=6 (The SWAXS pattern of water was subtracted to the sols one); (b) STEM image of the vesicles presented in C16/18E9-6-0.25; (c) FTIR spectra of stable sols U at pH=6 with different U/S ratio focus on the C-H (a) and C-O-C and (d) stretching bands. C16/18E9-6-Ref is referred to 0.1 M C16/18E9 surfactant solution at pH=6.

3. 4

 4 Qualitative and quantitative analyses of sol C16/18E9-6-0.25In order to have a clear description of the components (U(IV) species, nc-UO2, surfactant) distribution and of the system organization, the sol C16/18E9-6-0.25 was centrifuged at 14000 rpm until the obtaining of clear supernatant (S) and a dense gel (G). The SWAXS patterns of the two phases after separation are presented on the Figure5(a).

Figure 5 .

 5 Figure 5. (a) SWAXS patterns of the supernatants C16/18E9-6-0.25 S and the dense gel C16/18E9-6-0.25 G of the sample C16/18E9-6-0.25; (b) Experimental and simulated SWAXS patterns of C16/18E9-6-0.25 S; (c) Experimental and simulated SWAXS patterns of C16/18E9-6-0.25 G.

Figure 7 .

 7 Figure 7. Schema of U-C16/18:1E9COOH system.

Figure 8

 8 Figure 8 presents the distribution of U(IV) species as a function of the different forms in

Figure 8 .

 8 Figure 8. Distribution U(IV) species in the system. SCS is referred to surfactants forming core-shell micelles; SL is referred to surfactants forming multi-lamellar vesicles.As we mentioned before, during the indirect precipitation, there is a competition between

Table 1

 1 References, final pH and molar ratio U/S of the colloidal dispersions.

	Reference	pH	U/S	Reference	pH	U/S
	C16/18E9-6-0.16	5.9	0.16	C12E10-6-0.13	6.0	0.13
	C16/18E9-6-0.18	6.0	0.18	C12E10-6-0.21	6.2	0.21
	C16/18E9-6-0.25	6.0	0.25	C12E10-6-0.3	5.9	0.3
	C16/18E9-6-0.33	6.1	0.33	C12E10-7-0.13	6.8	0.13
	C16/18E9-6-0.5	5.9	0.5	C12E10-7-0.21	7.1	0.21
	C16/18E9-7-0.16	7.0	0.16	C12E10-8-0.13	8.0	0.13
	C16/18E9-7-0.18	7.0	0.18
	C16/18E9-7-0.25	7.4	0.25
	C16/18E9-8-0.16	8.0	0.16
	C16/18E9-8-0.18	7.9	0.18
	C16/18E9-8-0.25	7.9	0.25
	C16/18E9-9-0.16	8.8	0.16

2.5 Characterization. Solutions were analyzed using Fourier Transformed Infrared spectroscopy (FTIR) with a Perkin Elmer Spectrum 100 spectrometer in Attenuated Total

Table 2 .

 2 Distributions of U(IV) species and C16/18:1E9COOH molecules in the system of C16/18E9-6-0.25.

	Components	Forms	Portions
		U-C16/18:1E9COOH MLV	61%
	U(IV) species	nc-UO2	28%
		UO2 aggregates	11%
	C16/18:1E9COOH	U-C16/18:1E9COOH MLV	79%
	molecules	Globular micelles	21%

U(OH) 4-n n+ 1≤n≤4 4.8 nm 500 nm UO 2 nanoparticles

ACKNOWLEDGMENT

The authors thank Xavier Le Goff for his assistance in TEM experiments and data analysis, the LIME for the use of the facilities and Stéphanie Szenknect for the fruitful discussion. This work is a part of the PhD project of Zijie LU, which is financially supported by China Scholarship Council (CSC).

supporting information Table SI2 and section D). Now, the full q range experimental SWAXS pattern of C16/18E9-6-0.25 G can be safely simulated by combining three contributions: (i) a fraction of vesicles having a multi-lamellar structure and the density of the C16/18E9 surfactant; (ii) a fraction of spherical nanoparticles having the density of UO2; (iii) a fraction of dispersed aggregates of nanoparticles with an exponential mass fractal model [START_REF] Breßler | SASfit: a tool for small-angle scattering data analysis using a library of analytical expressions[END_REF]. The fitting parameters and the simulated SWAXS pattern corresponding to the best fit are presented in Table SI4 and Figure 5 (c) respectively.

The contributions of the multi-lamellar vesicles, core-shell micelle and of aggregates used in the simulated SWAXS patterns are also presented. The fitting results allowing the acquisition of a density of spherical nc-UO2 (see Table SI3). Hence, we can deduce that about 28% of U(IV) species has formed nc-UO2. Furthermore, these nc-UO2 are also present inside the multi-lamellar vesicles. That is why they can be separated from the supernatant C16/18E9-6-0.25 S during the centrifugation. In fact, the competition between hydrolysis-condensation of uranium species and the complexation U(IV) and surfactants leads to the liberation of U(IV) species in solution allowing the formation of nc-UO2. In addition, by interpreting the forward scattering intensity of the aggregates fraction in the fitting data (see Tables SI2 andSI3), we deduce that about 10% of U(IV) species has formed UO2 aggregates with a gyration radius around 7.5 nm. The calculations are presented in supporting information (section D). Furthermore, it is possible that the aggregates present in C16/18E9-6-0.25 are attached on the surface of the vesicles. That is why they are present in the C16/18E9-6-0.25 S after centrifugation. Moreover, the size and the morphology of the vesicles were determined by analyzing one drop of C16/18E9-6-0.25 by STEM. The image is presented with others carboxylic acids can be found in the literature. For simple acids like acetic acid, based on Andra thermodynamic database [START_REF] Giffaut | Andra thermodynamic database for performance assessment: ThermoChimie[END_REF], the complexation constant of U 4+ with acetate - is log k = 5.64 ± 1.00. For more complex acids such as α-isosaccharinic acid and gluconic acid, Warwick et al. reported a stability constants of U(IV) at 25 °C around log k 2,1 = 50 [START_REF] Warwick | Stability constants of uranium(IV)-alpha-isosaccharinic acid and gluconic acid complexes[END_REF].

In case of C16/18:1E9COOH surfactants, which present strong ion-complexing polar heads, the depolarization of alkyl chains may enhance the complexing ability and makes them stronger complexing agents comparable to complex acids [START_REF] Micheau | Polyethoxylated carboxylic surfactant for ion foam flotation: fundamental study from solution to foam; Tensioactif carboxylique polyethoxyle pour la flottation ionique: etude fondamentale de la solution a la mousse[END_REF][START_REF] Micheau | Ion foam flotation of neodymium: From speciation to extraction[END_REF]. In that case, log k 1 and log k 2,1 should be close, explaining the distribution of U(IV) species as U-C16/18:1E9COOH complexes and as nc-UO2 (U(OH4) is not stable and directly forms UO2(am) and then UO2(cr)). Consequently in this system, strong electrostatic interaction between U(IV) and C16/18:1E9COOH surfactants in competition with U(IV) complexation by OH -avoids a fast nc-UO2 formation and their uncontrolled aggregation.

CONCLUSION

A family of metastable colloidal sols of uranium using U(IV) as precursor and carboxylic surfactants (C16/18:1E9COOH and C12E10COOH) has been successfully prepared by indirect precipitation for 6≤pH≤8 and molar ratio of U/S<0.3. The system consists of core-shell micelles, U(IV) species complexed by surfactant molecules forming multi-lamellar vesicles with interdigitated bilayers, nc-UO2 within the vesicles and nc-UO2 forming aggregates attached on the surface of the vesicles.

The competition between the U(IV) species complexation by surfactant molecules and their hydrolysis prevents the rapid condensation of U(OH) 4-n n+ (1≤n≤4) and makes the sol-gel process controllable.

Furthermore, this hierarchical structured colloidal system containing nanoparticles of 1 to 3 nm diameter as well as the bigger particles made from aggregated nanoparticles described here shows great interest for the nuclear fuel fabrication. The surfactant removal of this system would result in the packing of the nanoparticles of the system. This would lead to the formation of empty volumes between the nanoparticles from 0.3 to 3 nm diameter [START_REF] Lu | Elaboration of microporous CeO 2 thin layers having nanocrystallites network controlled by Pluronic P123: Impact of key experimental parameters[END_REF]. Such pores may be typically in the range of mesopores (2 to 50 nm [START_REF] Mcnaught | Compendium of chemical terminology[END_REF]).

An ideal mixed oxide fuel for fast neutron reactors would be made by the filling of such mesoporous UO2 with a PuO2 concentrated solution. Thus, the problem of hot spot due to local density fluctuations of U/Pu ratio existing with the current nuclear fuel could be avoided.


Dr Marvin Edeas
email: marvin.edeas@inserm.fr

Jumana Saleh

Carole Peyssonnaux

Iron: Innocent bystander or vicious culprit in COVID-19 pathogenesis?

Keywords: Hyper-Ferritinemia, Hypercoagulability, Iron Homeostasis, Ferroptosis, Oxidative Stress, Mitochondria

diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The coronavirus 2019 (COVID-19) pandemic has taken the world by surprise as it viciously spread through the continents with rapidly increasing mortality rates. Current management of COVID-19 is based on that respiratory failure is the leading cause of fatalities [START_REF] Zhou | Clinical course and risk factors for mortality of adult inpatients with COVID-19 in Wuhan, China: a retrospective cohort study[END_REF]). Nevertheless, mounting evidence points to drastic systemic events taking place that contribute to accelerated COVID-19 pathogenesis. The "cytokine storm" is a notion that is reportedly hailed as the hallmark of the COVID-19 hyper-inflammatory state [START_REF] Mehta | COVID-19: consider cytokine storm syndromes and immunosuppression[END_REF]. Consecutive studies linked COVID-19 related hyper-inflammation to systemic events including; hypercoagulability, oxidative stress and altered iron metabolism [START_REF] Mehta | COVID-19: consider cytokine storm syndromes and immunosuppression[END_REF][START_REF] Phua | Intensive care management of coronavirus disease 2019 (COVID-19): challenges and recommendations[END_REF]). These events were linked to accelerated pathogenesis in gravely ill COVID-19 patients as highlighted in a recent perspective [START_REF] Moore | Cytokine release syndrome in severe COVID-19[END_REF]. Several components of the heightened inflammatory state have been proposed as therapeutic targets particularly IL-6 blockers as drugs of more relevance in COVID-19 management than steroids, however concerns of prolonging viral clearance were stated [START_REF] Moore | Cytokine release syndrome in severe COVID-19[END_REF].

Hyper-ferritinemia, has been described as a cardinal feature that predicted with high significance the increased mortality risk [START_REF] Mehta | COVID-19: consider cytokine storm syndromes and immunosuppression[END_REF][START_REF] Phua | Intensive care management of coronavirus disease 2019 (COVID-19): challenges and recommendations[END_REF]. These studies demonstrated serum ferritin levels in COVID-19 non-survivors that exceeded "twofold" the levels in the survivors. In spite of the strong association with mortality, it is not yet clear if hyper-ferritinemia in COVID-19 patients is merely a systemic marker of disease progression, or a key modulator in disease pathogenesis.

Recently we showed that hepcidin, the key iron regulatory molecule, plays a major role during inflammatory processes [START_REF] Bessman | Dendritic cell-derived hepcidin sequesters iron from the microbiota to promote mucosal healing[END_REF]. However, the role and management of a dysregulated iron state in COVID-19 pathogenesis has not yet been addressed.

Is iron a key strategic player in COVID-19 pathogenesis?

Increasing evidence shows that inflammation, oxidative stress and altered iron homeostasis are inevitably linked at a systemic level [START_REF] Kernan | Hyperferritinemia and inflammation[END_REF]. This perspective elaborates on the potential aspect of altered iron homeostasis, marked by hyper-ferritinemia, and its potential role in COVID-19 pathogenesis and management strategies.

Iron is an essential trace element that plays a role in systemic oxygen transfer, and acts as an electron donor or acceptor in many biological functions. Ferritin is the primary site of iron storage in the cell mainly in its ferric state (Fe3+). Ferritin can carry up to 4500 iron molecules in its core [START_REF] Kell | Serum ferritin is an important inflammatory disease marker, as it is mainly a leakage product from damaged cells[END_REF]. Generally, systemic inflammations are associated with increased serum ferritin levels. During a heightened inflammatory state, cytokines, particularly, IL-6, stimulate ferritin and hepcidin synthesis [START_REF] Mcdermid | Host iron redistribution as a risk factor for incident tuberculosis in HIV infection: an 11-year retrospective cohort study[END_REF][START_REF] Daher | Iron metabolism and the role of the iron-regulating hormone hepcidin in health and disease[END_REF].

Hepcidin, the key iron regulatory hormone, sequesters iron in the enterocytes and macrophages, leading to increased intracellular ferritin, and preventing iron efflux from enterocytes and macrophages [START_REF] Daher | Iron metabolism and the role of the iron-regulating hormone hepcidin in health and disease[END_REF]) (Figure 1). Thereby, we speculate that increased serum ferritin levels as a result of COVID-19 related hyper-inflammation signifies a vicious cycle of events where increased ferritin levels may lead to further tissue damage [START_REF] Kell | Serum ferritin is an important inflammatory disease marker, as it is mainly a leakage product from damaged cells[END_REF].

Excess intracellular iron interacts with molecular oxygen, generating reactive oxygen species (ROS) [START_REF] Kell | Serum ferritin is an important inflammatory disease marker, as it is mainly a leakage product from damaged cells[END_REF]. This may largely contribute to oxidative damage of cellular components of different organs (lungs, liver, kidney, heart). Mounting evidence link increased ferritin levels to various inflammatory pathologies including cardiovascular events [START_REF] Knovich | Ferritin for the clinician[END_REF]. Moreover, the complex interplay between iron metabolism and reactive nitrogen species (RNS) and reactive sulfur species (RSS) in addition to ROS suggests a clear interaction between iron metabolism and the newly defined reactive species interactome (Cortese-Krott et al. 2017) (Figure 1).

Interestingly, recent studies implicated that ferroptosis, which is the process of programmed cell death mediated by iron-dependent peroxidation mechanisms (Ursini and Maiorino 2020) in inflammatory pathologies, involve multiple organs including liver, kidney, heart and lung [START_REF] Sun | The emerging role of ferroptosis in inflammation[END_REF]. Ferroptosis was found to be linked to neurological disturbances including cognitive impairment [START_REF] Sun | The emerging role of ferroptosis in inflammation[END_REF], agueusia and anosmia (taste and smell loss) [START_REF] Osaki | Clinical and physiological investigations in patients with taste abnormality[END_REF][START_REF] Dinc | Does iron deficiency anemia affect olfactory function?[END_REF]) that are regular manifestations of COVID-19 disease [START_REF] Vaira | Anosmia and Ageusia: Common Findings in COVID-19 Patients[END_REF]. Iron chelators and ferroptosis inhibitors had protective effects by inhibiting intracellular iron induced lipid peroxidation [START_REF] Kernan | Hyperferritinemia and inflammation[END_REF]. The impact of iron overload on extra and intracellular mitochondria dysfunction [START_REF] Rouault | Mitochondrial iron overload: causes and consequences[END_REF], on microbiota dysbiosis (lungs and gut) [START_REF] Yilmaz | Gut microbiota and iron: the crucial actors in health and disease[END_REF]) and on others pathogens, may be strongly implicated as shown in the Figure 1. Furthermore, serum coagulability is a major concern in COVID-19 pathogenesis, and rapidly recognized as a key risk factor in susceptible patients [START_REF] Giannis | Coagulation disorders in coronavirus infected patients: COVID-19, SARS-CoV-1, MERS-CoV and lessons from the past[END_REF][START_REF] Lodigiani | Venous and arterial thromboembolic complications in COVID-19 patients admitted to an academic hospital in Milan, Italy[END_REF][START_REF] Oxley | Large-Vessel Stroke as a Presenting Feature of Covid-19 in the Young[END_REF][START_REF] Zhang | Coagulopathy and Antiphospholipid Antibodies in Patients with Covid-19[END_REF]). In the context of the cellular iron overload, it has long been documented that coagulopathy is a hallmark of iron toxicity.

Oxidized iron accelerates serum coagulation by interacting with proteins of coagulation cascade [START_REF] Jankun | Unusual clotting dynamics of plasma supplemented with iron(III)[END_REF].

Coagulation and cardiac biomarkers have been described to be elevated in COVID-19 patients; reflecting an inflammatory status characterized by coagulation activation and vascular endothelial dysfunction, found to be predictors of mortality [START_REF] Giannis | Coagulation disorders in coronavirus infected patients: COVID-19, SARS-CoV-1, MERS-CoV and lessons from the past[END_REF][START_REF] Lodigiani | Venous and arterial thromboembolic complications in COVID-19 patients admitted to an academic hospital in Milan, Italy[END_REF]. More evidence points towards stroke incidents in COVID-19 patients, including younger adults [START_REF] Oxley | Large-Vessel Stroke as a Presenting Feature of Covid-19 in the Young[END_REF]. A recent report demonstrated acute formation of large vessel strokes in young adults infected with COVID-19 [START_REF] Oxley | Large-Vessel Stroke as a Presenting Feature of Covid-19 in the Young[END_REF]. The inflammation in the blood vessel walls and platelets mitochondria alteration may be driving thrombosis formation [START_REF] Giannis | Coagulation disorders in coronavirus infected patients: COVID-19, SARS-CoV-1, MERS-CoV and lessons from the past[END_REF][START_REF] Lodigiani | Venous and arterial thromboembolic complications in COVID-19 patients admitted to an academic hospital in Milan, Italy[END_REF][START_REF] Oxley | Large-Vessel Stroke as a Presenting Feature of Covid-19 in the Young[END_REF][START_REF] Zhang | Coagulopathy and Antiphospholipid Antibodies in Patients with Covid-19[END_REF].

Conclusion

Hyper-ferritinemia observed in COVID-19 patients may be induced in response to inflammation. However, its role in COVID-19 disease progression has not been fully established. It has been reported that hyper-inflammation in association with altered iron homeostasis may play a key role in pathogenesis of disease including viral infections [START_REF] Drakesmith | Viral infection and iron metabolism[END_REF][START_REF] Schmidt | The role of iron in viral infections[END_REF]) (Figure 1). It may be postulated that hyper-ferritinemia is associated with a state of iron toxicity which may result from increased ferritin leakage from damaged tissue releasing free iron in the process. There is no established consensus to exclude this possibility. Therefore, it is crucial to investigate coexisting iron parameters in COVID-19 patients including transferrin saturation, plasma iron levels, non-transferrin bound iron (NTBI) as well as hepcidin. The association of hyper-ferritinemia with increased transferrin saturation may reflect a state of iron overload.

In this case, we suggest that targeting the intracellular iron overload may be a strategy of vital importance needed to be taken into consideration in future controlled clinical trials.

We suggest, in addition to treatment of the inflammatory state (Moore and June 2020), to envisage the application of approved iron chelators, ferroptosis inhibitors, hepcidin modulators and erythropoietin [START_REF] Monti | Does chloroquine really act through oxidative stress?[END_REF][START_REF] Pinto | Erythropoietin mediates hepcidin expression in hepatocytes through EPOR signaling and regulation of C/EBPα[END_REF][START_REF] Hossaini | Association between serum levels of hepcidin and ferritin in patients with thalassemia major and intermedia, the role of iron chelator[END_REF][START_REF] Bessman | Dendritic cell-derived hepcidin sequesters iron from the microbiota to promote mucosal healing[END_REF][START_REF] Hadadi | Does recombinant human erythropoietin administration in critically ill COVID-19 patients have miraculous therapeutic effects?[END_REF]. This promising therapeutic approach can be associated with drugs that specifically target extra and intracellular mitochondria dysfunction or even the reactive species interactome production and ferroptosis (Cortese-Krott et al. 2017; Kernan and Carcillo 2017) (Figure 1).

Figure 1 :

 1 Figure 1: COVID-19 infection and Iron dysregulation

Acknowledgements

We thank Dr. Laurent Chatre, ISTCT-CNRS, Université de Caen-Normandie, CEA, for his valuable discussions.

Funding source

None.

Ethical approval

Approval was not required.

Conflict of interest

No conflict of interest to declare.


C Onofri

C Sabathier

C Baumier

C Bachelet

D Drouan

M Gérardin

M Legros

Extended defect change in UO2 during in situ TEM annealing

Predicting the nuclear fuel microstructure at each moment of its irradiation cycle (nominal, power transient, accidental conditions) is a significant nuclear safety issue. For that, it is necessary to understand the impact of irradiation parameters on the microstructure. This study provides insight about the temperature effect on dislocations. In situ thermal annealing up to 1400 °C on pre-irradiated polycrystalline UO2 thin foils was performed inside a TEM for the first time. The aim of the current study is to establish the kinetic and the mechanisms of thermal recovery of extended defects induced by irradiation.

Whatever the initial irradiation conditions, extended defect recovery was observed around 1000-1100 °C, in good agreement with literature data. Dislocation line disappear mainly by climb and dislocation loops move by pencil glide along the <110> Burger vector directions. Defect growth by coalescence of dislocation loops is also observed.

Introduction

Uranium dioxide (UO2), a ceramic compound that crystallizes in the fluorite structure (Fm-3m space group), is the standard material used as a nuclear fuel for pressurized water reactors. During fission of uranium nuclei, radiation damage and fission products accumulate in UO2 fuel pellets, in the form of point defects, extended defects (dislocation loops and lines), precipitates and fission gas bubbles. The fuel microstructure is therefore strongly modified during irradiation [1][2][3] affecting its mechanical [4] [START_REF] Henry | Irradiation effects on the fracture properties of UO2 fuels studied by micro-mechanical testing[END_REF] and thermal [6] [START_REF] Deng | Effects of edge dislocations on thermal transport in UO2[END_REF] properties which, in return, limit its lifetime in reactor [START_REF] Baron | Fuel Performance of Light Water Reactors (Uranium Oxide and MOX)[END_REF]. The links between microstructure changes during irradiation and fuel properties are partially known and still currently studied. Additional characterizations of the mechanical and thermal properties of irradiated fuels to better assess their evolution upon irradiation are therefore needed. For instance, during normal irradiation conditions, subdivision of initial grains into smaller sub-domains, a phenomenon generally attributed to dislocation rearrangement, was recently observed in the pellet center [START_REF] Noirot | Focused ion beam-scanning electron microscope examination of high burn-up UO2 in the center of a pellet[END_REF]. Fission gas atoms (Xe, Kr) may diffuse faster along the paths provided by dislocations [10] [START_REF] Murphy | Xe diffusion and bubble nucleation around edge dislocations in UO2[END_REF] and be released into free volumes between the fuel pellets and the cladding. If the gases are released from the fuel, they contribute to the internal pressure increase, which may 2 lead to the failure of the cladding. Therefore, in order to gain a better understanding of the complex fuel evolution, extended defects and their behavior under external and internal stimuli must be characterized.

The evolution of extended defects is the result of several simultaneous and correlated phenomena which are not yet fully understood: radiation damage accumulation, temperature (which changes with the radius of the pellets from ~500 to ~1000 °C), doping by fission products, and mechanical load. A well-suited way to study separately these different phenomena consists in performing ion irradiations on polycrystalline UO2 samples. In addition to allow a precise dose control, this technic offers the possibility to handle low activity samples. Many out-of-pile studies, i.e.

using ion [12] [13] [14] [15] [16], electron [17] [18] [19] or neutron

[20] irradiations, have therefore been performed to characterize extended defects in UO2 or CeO2, which is proposed as a good non-radioactive surrogate of UO2. However, the impact of these different parameters was not systematically studied. This work takes place within the scope of a broad separate effect study. Our previous works [21] [START_REF] Onofri | Influence of exogenous xenon atoms on the evolution kinetics of extended defects in polycrystalline UO2 using in situ TEM[END_REF] presented in detail the impact of radiation damage accumulation, temperature combined with radiation damage and doping by xenon atoms on extended defects change. The aim of the current study is to determine the effect of temperature on these defects. In situ thermal annealing was performed using TEM on pre-irradiated samples with 4 MeV gold ions at different temperatures (-180, 25 and 600 °C) and fluences (5 × 10 13 to 1 × 10 15 Au/cm 2 ) in order to follow the evolution of different extended defect populations (dislocation loops exclusively or lines and loops concomitantly).

Experimental

Polycrystalline UO2 discs were cut from sintered UO2 pellets. They were mechanically thinned on one face down to a thickness of 300 µm. They were subsequently annealed at 1400 °C during 4 hours under Ar/5 % H2 to relax strain induced by mechanical grinding and remove the last damages induced by polishing. To obtain TEM transparent thin foils, mechanical thinning was performed on the rough face using the tripod polishing technique. The final chemical etching consists of a solution of nitric acid, glacial acetic acid and orthophosphoric acid at 120 °C [23], allowing for reaching electron transparency. These UO2 thin foils were irradiated with 4 MeV gold ions using the Tandem accelerator ARAMIS [24] at the CSNSM laboratory (Centre de Sciences Nucléaires et de Sciences de la Matière in Orsay). Irradiation conditions were gathered in Table 1. Gold ions at the energy of 4 MeV were chosen to simulate the energy lost in the nuclear regime of fission products, which means at the end of their path. In this study, we focus on the effect of temperature on extended defects induced by irradiation, without any effect of exogenous atom implantation. As presented in detail in our previous study [START_REF] Onofri | Evolution of extended defects in polycrystalline Au-irradiated UO2 using in situ TEM: Temperature and fluence effects[END_REF], the large majority of the gold ions at this energy traverse the thin foil without stopping there. In other words, very few gold ions are implanted into the thin foil whereas damage is promoted. All the fluences achieved in this study (between 5 × 10 13 and 1 × 10 15 i/cm 2 ), represent only a few days in reactor. Thus, they will be rather compared with the beginning of the first irradiation cycle. Irradiation temperatures range between -180 and 600°C. These different irradiation temperatures combined with fluences allow us to study various initial defect populations: only dislocation loops or loops and lines concomitantly. The low temperature irradiation is also supposed to promote isolated point defects. The mean irradiation flux was around 1.9 × 10 11 Au.cm -2 .s -1 to limit sample heating during irradiation.

During in pile irradiation, the irradiation flux is very low compared to the one obtained in ion irradiations; about 1 dpa per day instead of about 10 2 dpa/day in our conditions. The effect of temperature on preexisting defects is therefore an important parameter to understand the microstructure evolution of the fuel in pile. In situ thermal annealing was performed on these samples using a Jeol 2010 TEM at the CEMES The sample thicknesses were determined using the fringe method [START_REF] Williams | Transmission Electron Microscopy: A Textbook for Materials Science[END_REF]. The formula used to determine the thickness t with the fringe method is given as followed:

with ξeff the effective extinction distance, n the number of thickness fringe, s the exciting error (Bragg deviation, close to 0 in our cases) and ξg the extinction distance. The thicknesses of the studied areas range between 40 and 80 nm.

The analysis of dislocation loops and lines (size and density) was performed manually. Between 50 and 250 loops were characterized at each condition. Since dislocation loops have often an elliptical shape, their size was defined as the length of their longest axis. The dislocation line density was obtained by measuring the length of all the dislocation lines, divided by the volume they are contained in.

The uncertainty on the loop density is given by the following formula:

∆ 1 √ ∆
were is the loop density, N is the number of counted loops and t is the thickness of the studied area.

The uncertainty on the thickness ∆t is set to 10 nm.

The uncertainty on the loop diameter is set to 15 % of the measured value, mainly due to the width of the loop contrast.

The uncertainty on the line density is obtained from: ∆ ∆

Results

Thermal evolution of dislocation loops

3.1.a. Isochronal annealing

Isochronal thermal annealing, as described in Table 1, was performed on a polycrystalline UO2 thin foil irradiated with 4 MeV Au at -180 °C and 5 × 10 13 Au/cm 2 (sample 1a). Figure 1 presents a typical example of microstructure evolution with increasing annealing temperature. The corresponding loop density and average loop diameter changes as a function of annealing temperature are given in Figure 2 while Figure 3 shows the loop size distribution after different annealing temperatures. Before annealing (Figure 1 Then up to 1000 °C, there is no microstructure evolution. The loop density, average loop diameter (considering uncertainties), and loop size repartition remain stable (Figure 2 and Figure 3).

Above 1000 °C, as illustrated by Figure 1 (d-g) and Figure 2, a strong decrease of the dislocation loop density (more than on order of magnitude between 1000 and 1350 °C) and an increase (multiplying by about 8) of their average size are observed simultaneously. Figure 3 shows an offset of the loop size repartition toward large sizes with increasing annealing temperature, which suggests a loop growth by coarsening mechanisms (Ostwald ripening and/or coalescence). At the final temperature (1350 °C), the microstructure only consists of a few large loops and some lines. However, around 1200 °C surface contamination settles over the thin foil. EDX measurements performed on the particles deposited on the foil surface indicated that this contamination comes from partial sublimation of the W resistances that were implemented on this holder. Using the Thermodynamics of Advanced Fuels -International Database (TAF-ID) [28], the calculated thermodynamic equilibrium of our samples at 1200°C gives a stoichiometry close to 2.0049. If the initial stoichiometry of our samples is higher than this value, the O2 released from the samples could oxidize the W resistances, inducing the WO2 particle deposition. Oxygen could also come from desorption from the walls or the sample holder. In all cases, calculations show that this surface deposition is not expected to influence significantly the stoichiometry of the sample intern microstructure. Thus, in order to increase the quality of the final image (Fig. 3g), a gallium ion beam with glazing incidence (16 keV/43 pA and 5 keV/15 pA during 3 minutes) was used after the annealing to remove this contamination. We checked that the microstructure was not altered by this operation.

3.1.b. Isothermal annealing

In order to study the effect of the annealing time, isothermal annealing was performed on another sample (sample 1b) irradiated with 4 MeV Au at the same fluence (5 × 10 13 Au/cm 2 ) and temperature (-180 °C).

Figure 4 presents the microstructure evolution and Figure 5 shows the loop size and density change as a function of annealing time. Before annealing (Figure 4(a)), the thin foil displays the same microstructure as sample 1a (Figure 1(a)). The density of the small defect clusters and their average diameter are similar as those presented in Figure 2: 1.0 × 10 15 cm -3 and 6.9 nm. First, the thin foil was annealed at 500 °C during 15 min. The same additional black dots appear, as observed previously (Figure 4(b)), inducing a significant density increase (2.7 × 10 16 cm -3 ). Then, annealing at 1150 °C was performed during 5 min. All the major changes take place during the first minute: the density is divided by a factor 3 and systematic loop growth is observed. Then, with increasing time, there is no more microstructure evolution.

Thermal evolution of dislocation loops and lines concomitantly

Three samples (number 2, 3 and 4) irradiated with 4 MeV Au at high fluence (0.5 and 1 × 10 15 Au/cm 2 )

were annealed as described in Table 1. Figure 6, Figure 7 and Figure 8 present their microstructure evolution as a function of the annealing temperature for samples 2, 3 and 4 respectively. All samples display very close microstructure evolution with increasing temperature. First, using video monitoring during the in situ TEM annealing, for temperatures ranging from 500 to 1000 °C, many dislocation lines recombination were recorded. These movements take place in the early few minutes at each new temperature plateau and are particularly striking around 800 °C. However, densities are not significantly affected, as the total line length remains unchanged. Thus, there is no line or loop density evolution between 25 and 1000-1100 °C (Figure 9) in good agreement with the results of Nogita et al.

obtained for fuel irradiated in pile [START_REF] Nogita | Thermal Recovery of Radiation Defects and Microstructural Change in Irradiated UO2 Fuels[END_REF]. It is important to underline that line densities obtained in this study after irradiation (up to 1000°C) are of the same order of magnitude than the one found by I.L.F. Ray et al.

[31] for fuel irradiated at high burn-up (2.2 10 10 cm -2 ). This value does not change significantly between the periphery and the center of the pellet. Then, from 1000-1100 °C, both dislocation line and loop densities

Density

Size decrease strongly (Figure 9), but even after annealing at 1400 °C the extended defect recovery is not complete especially in the thickest areas, as observed by Nogita et al. [START_REF] Nogita | Thermal Recovery of Radiation Defects and Microstructural Change in Irradiated UO2 Fuels[END_REF]. It is important to remember, that in the case of fuel irradiated in pile, many exogenous atoms (in solid solution, precipitates, bubbles…) are present and should affect the dislocation mobility. The effect of exogenous atoms on extended defect annealing will be discussed in another paper. A loop growth was also highlighted from 1000-1100°C. For instance, in sample 3 the average loop diameter increases from 4.6 to 10.6 nm between 900 and 1000 °C and from 10.6 to 19.8 nm between 1000 and 1400 °C. (Burn-up of 20 GWj/tU, power per unit of length of 250 W/cm, 235U enrichment of 4.48%).

Discussion

Annealing mechanisms below 1000-1100 °C

4.1.a. Dislocation loops

In the case of sample 1(a-b), which initially contains exclusively small dislocation loops (mean size of 7 nm), the apparition, at the TEM scale, of a second set of small dislocation loops (diameter < 5 nm) was Between one and three steps of relaxation were highlighted according to the studies. An elastic strain relaxation step was observed between 500 and 800 °C and was attributed by previous studies to the uranium vacancy migration [37] [START_REF] Turcotte | Alpha radiation damage in the actinide dioxydes[END_REF]. The point defects recovery can occur by different mechanisms, such as recombination, clustering and absorption by sinks, such as dislocations. Thus, the second set of dislocation loops could result from the point defect clustering in new dislocation loops and/or absorption by defect clusters already present but too small to be detected using the TEM. It is important to mention that in our experiments, we see only clusters of diameter larger than about 1-2 nm (depending on the thin foil quality).

Smaller clusters probably exist but are not able to be observed directly.

4.1.b. Dislocation lines

In the case of samples 2, 3 and 4, which initially contain dislocation lines and small loops, many dislocation line rearrangements were observed in the temperature range of [500 -1000 °C], and particularly around 800 °C (part 3.2). These movements are very fast (on the order of a second) and jerky as shown in video 1 (Supplement -Video 1). Since they take place at relatively low temperature, they could be attributed to slip.

Figure 10 illustrates some of these dislocation line slip motion. However, since dislocation lines are anchored and their movements limited, it is not possible to obtain slip traces that are long enough to determine the corresponding crystallographic slip planes.

The origin of dislocation glide is probably to be found in the mutual dislocation-dislocation interactions.

Dislocations induce strain field around them, which magnitude decreases with distance to dislocation and which can be attractive or repulsive depending on their geometry (parallel or anti-parallel Burgers vectors, same or different slip planes) [START_REF] Friedel | Dislocations[END_REF]. The high dislocation density in our samples actually promote these movements. This obviously worked as some dislocation rearrangements involving slip were observed, but their amplitude were too small to be able to clearly identify the crystallographic slip planes involved. In all cases, this is the first time that dislocation line movements are observed and reported in real time in UO2 during annealing. where dislocations are still present in the sample even after several hours at high temperatures (1500 -1800 °C).

4.2.a. Dislocation loops

Dislocation loops move along particular directions without any significant size variation. These movements are very fast (less than 1s) and jerky, and correspond typically to movement by pencil glide of prismatic loops along their Burgers vector direction As explained for the line glide in part 4.1.b, repulsive or attractive interaction between dislocation loops or with free surfaces (image force) could also be a motor for the pencil glide. Thus due to elastic interaction dislocation loops can disappear at surfaces or interact with other loops and anneal themselves or grow by coalescence, as shown in Figure 12. ). However, a detailed study of the Burgers vectors and habit planes of the colliding loops will be necessary to give more insight into the growth mechanisms.

4.2.b. Dislocation lines

Dislocation lines move slowly (few tens of seconds). The total length of the lines decreases. They can, in some configurations, disappear with surfaces. Lines motion is clearly observable in video 3 (Supplement -Video 3). The change of the line trace (intersection with surface) is not straight, as highlighted in Figure 14. This indicates that this trace does not correspond to a crystallographic one and that dislocation climb became active at this high temperature. Either lines may climb by absorption of point defects, clusters not observable at the TEM scale, or resolvable loops (see video 3). It is likely that point defects from free surfaces could also participate to the line climb. We will comment this point in the following part.

Vacancy changes with respect to temperature is also an important parameter to fully understand the change in dislocation mechanisms. nanometer size and a second one of about 10 nanometer size near dislocations. A link between dislocations, gas atoms and vacancies is thus clearly established. These cavities could pin dislocations lines, preventing their climb for example. We expect a different thermal recovery kinetics of the dislocation lines when exogenous gas atoms are implanted in the samples, as we will discuss in another work.

4.2.c. Bulk sample

In order to study the effect of free surfaces during annealing, we performed an annealing on a bulk sample (disk of 500 µm of thickness) implanted with 27 MeV Kr at 600 °C and 5 × 10 15 i/cm 2 (see [START_REF] Onofri | Evolution of extended defects in polycrystalline UO2 under heavy ion irradiation: combined TEM, XRD and Raman study[END_REF]). This implantation condition, inducing a damage layer over 5 µm with a maximum at 3.5 µm, drastically reduces the effect of surfaces during annealing (compared to thin foils). Thin foils were obtained with a FEI Helios 600 NanoLab dual beam FIB at the 'Centre Pluridisciplinaire de Microscopie Electronique et de Microanalyse' (CP2M), before and after the annealing at 1400 °C during 1 h under Ar/5%H2, as shown in Figure 15. Initially the sample presents a very dense dislocation line network and small dislocation loops.

After annealing, it is clearly visible that most dislocation lines disappeared and large dislocation loops are also present. This microstructure is in good agreement with those of the thin foils directly annealed in the TEM.

This suggests that the elastic interaction between dislocations is the main motor for the microstructure evolution during annealing, even if the effect of free surfaces could affect the kinetics in case of thin foils.

Indeed, during thin foil irradiations, areas near surfaces (thickness of 10-30 nm) are denuded of dislocations, as previously observed by L.F. He et al. [START_REF] He | In situ TEM observation of dislocation evolution in Kr-irradiated UO2 single crystal[END_REF] in Kr irradiated UO2 or in our previous work [START_REF] Onofri | Evolution of extended defects in polycrystalline Au-irradiated UO2 using in situ TEM: Temperature and fluence effects[END_REF]. This phenomenon was attributed to a surface sink effects and seems to be related to the irradiation temperature. In our case, it is important to note that free surfaces could provide vacancy defects which could strongly participate to dislocation climb. However, as all samples are observed in similar conditions, this potential effect should be leveled.

Summary and conclusion

Thermal recovery of extended defects (dislocation lines and loops) induced by ion irradiations in polycrystalline UO2 thin foils was examined through in situ TEM observations. Four samples, irradiated A first annealing stage was highlighted between 500 and 1000 °C, with different outcomes depending on initial microstructure. When only dislocation loops are initially present due to a low temperature and fluence irradiation, a second set of small dislocation loops (diameter < 5 nm) was observed around 500 °C.

This could be attributed to point defect recovery on sinks such as dislocations. When both dislocation lines and irradiation loops are initially present, heavy line rearrangements by slip took place, particularly around 800 °C.

Then, whatever the initial irradiation conditions, a second annealing stage corresponding to the extended defect recovery was observed above 1000-1100 °C. Both dislocation loop and line densities decrease with increasing temperature. The total length of dislocation lines is reduced by climb and the dislocation loops move by pencil glide along the <110> Burger vector directions. The coalescence and annihilation of dislocation loops could occur through pencil glide, but probably also involved climb in some cases.

However, these movements are only observed in the early few minutes at each temperature plateau. Thus, even after several hours of annealing some dislocations remain, in good agreement with post mortem literature observations.

The current study about mobility of extended defects with temperature constitutes a substantial work to improve the modelling of UO2 under irradiation, especially the formation of sub-grains in central area.

3 .

 3 (a)), the microstructure exhibits few black dots identified as small dislocation loops [12] [20] [21]. Around 500 °C, monitoring the video captured during the annealing, additional small black dots become visible at the TEM scale (diameter < 5 nm), as shown in Figure 1 (b). These black dots are small dislocation loops too. So from 500 °C, Figure 2 reports an increase of the loop density (almost one order of magnitude) and a decrease of the loop average size correlated with a strong increase of the smallest (]0-5] nm) loop proportion, as shown in Figure This second set of dislocation loops was also observed by Whapham et al. [20] in UO2 irradiated with neutrons and further annealed at 1100 °C during 1 hour.

Figure 1 :

 1 Figure 1: Sequential bright field (BF) TEM images of polycrystalline UO2 thin foil irradiated with 4 MeV Au at -180°C and 5 × 10 13 Au/cm 2 : (a) as implanted, (b, c) annealed at 900 °C (two areas), (d) 1000 °C, (e) 1150°C, (f) 1200 °C, (g) 1350 °C. The observations were carried out with g = 200 reflection. The insets show the diffraction patterns.

Figure 2 :

 2 Figure 2 : Loop density and average loop diameter as a function of annealing temperature in UO2 irradiated with 4 MeV Au at -180 °C and 5 × 10 13 Au/cm 2 . Doted lines are guide for the eyes.

Figure 3 :

 3 Figure 3 : Dislocation loops diameter repartition as a function of annealing temperature (Color online).

Figure 4 :

 4 Figure 4 : Sequential BF TEM images of polycrystalline UO2 thin foil irradiated with 4 MeV Au at -180°C and 5 × 10 13 Au/cm 2 : (a) as implanted, (b) annealed at 500 °C 15 min, (c) annealed at 1150 °C 30 s, (d) 60 s, (e) 120 s, (f) 300 s . The observations were carried out with g = 220 reflection (diffraction pattern shown in inset).

Figure 5 :

 5 Figure 5 : Loop density and average loop diameter evolution as a function of the annealing time at 1150 °C.

Figure 9

 9 shows dislocation line and loop density change with increasing the annealing temperature.Initially, all samples present similar microstructure: dislocation lines and small dislocation loops (diameter <10 nm) between the lines as shown in our previous work[START_REF] Onofri | Evolution of extended defects in polycrystalline Au-irradiated UO2 using in situ TEM: Temperature and fluence effects[END_REF]. However, the sample irradiated at -180 °C (Figure6(a)) is very strained, thus it is difficult to clearly see individual dislocations and impossible to quantify them below 800 °C.

  Unfortunately, above 1100 °C samples are progressively covered by surface contamination too (visible as faceted dark particles and indicated by an arrow in Figure6(g)). This contamination is due to W redeposition, originating from the oven resistances that are overheating. It does not affect the internal dislocation structure of UO2.

Figure 6 :

 6 Figure 6 : Sequential BF TEM images of polycrystalline UO2 thin foil irradiated with 4 MeV Au at -180°C and 1 × 10 15 Au/cm 2 : (a) as irradiated, (b) annealed at 400 °C, (c) 600 °C, (d) 800 °C, (e) 950 °C, (f) 1100 °C, (g) 1250 °C, (h) 1300 °C. The observations were carried out with g = 111 reflection (a), with g = 200 reflection (b-d) and with g = 220 reflection (e-h). The insets show the diffraction patterns.

Figure 7 :

 7 Figure 7 : Sequential BF TEM images of polycrystalline UO2 thin foil irradiated with 4 MeV Au at 25 °C and 5 × 10 14 Au/cm 2 : (a) as implanted, (b) annealed at 400 °C, (c) 800 °C, (d) 1000 °C, (e) 1100 °C, (f) 1400 °C. The observations were carried out with g = 200 reflection. The inset shows the diffraction pattern.

Figure 8 :Figure 9 :

 89 Figure 8 : Sequential BF TEM images of polycrystalline UO2 thin foil irradiated with 4 MeV Au at 600 °C and 1 × 10 15 Au/cm 2 : (a) as implanted, (b) annealed at 600 °C, (c) 800 °C, (d) 950 °C, (e) 1000 °C, (f) 1100 °C, (g) 1150 °C, (h) 1200 °C, (i) 1300 °C. The observations were carried out with g = 220 reflection. The inset shows the diffraction pattern.

  observed around 500 °C (part 3.1.a). To understand this phenomenon, it is necessary to recall what is happening during low energy ion irradiation in UO2. According to Classical Molecular Dynamics (CMD) calculations, in the nuclear energy loss regime, atomic collisions induce displacement cascades, which lead to a thermal spike of a few picoseconds [33]. CMD simulations have shown that the overlap of displacement cascades induces the formation of cavities, interstitial dislocation loops and of many point defects [34]. These dislocation loops could be directly formed by a loop punching process [35] [34] or by trapping interstitials as the dense periphery of thermal spikes quenches [36] without the need for long-range diffusion. This could explain the presence of the first loop population. During the annealing, point defects induced by irradiation may become mobile above 500 °C. Indeed, in previous thermal annealing studies performed on UO2 irradiated with neutrons, alpha particles [37] [38] [30] or helium [39], elastic strain relaxation, related to point defects recovery, was observed in the temperature range of [200 -900 °C].

Figure 10 : 4 . 2 .

 1042 Figure 10 : Sequential BF TEM images of polycrystalline UO2 irradiated with 4 MeV Au at 600 °C and 1 × 10 15 Au/cm 2 during annealing at 800 °C, showing lines movement by glide. The arrows

[ 42 ]

 42 [43]. The loops induced by irradiation are prismatic with Burgers vectors along <110> directions, according to previous studies [13] [20] [44] [45]. Using stereographic representations, the <110> directions were identified as the glide directions. Therefore, these movements are in good agreement with dislocation loop characteristics in UO2. Figure 11 illustrates loop movement by pencil glide along the <110> directions.

Figure 11 :

 11 Figure 11: (a-b) Sequential BF TEM images of polycrystalline UO2 irradiated with 4 MeV Au at -180 °C and 5 × 10 13 Au/cm 2 , showing loop movements by pencil glide along the <110> directions during annealing at 1150 °C. The studied loops are circled. The observations were carried out with g = 200 reflection. The images on the left indicate the initial positions, the images on the middle correspond to the final positions and the images on the right are superposition of initial and final images (the colors of initial image were inverted and the studied loops appear in white). Arrows indicate the glide directions. (c) Stereographic representation associated to (a-b) [46]. The arrows are reported to indicated the glide direction, along the <101> directions.

Figure 12 :

 12 Figure 12 : Sequential BF TEM images of polycrystalline UO2 (a) irradiated with 4 MeV Au at -180 °C and 5 × 10 13 Au/cm 2 under thermal annealing at 1150 °C showing loop disappearance at surface (b) irradiated with 4 MeV Au at 25 °C and 5 × 10 14 Au/cm 2 under thermal annealing at 1200 °C showing loop coalescence by pencil glide. The studied loops are circled.

20 Figure 13 :

 2013 Figure 13 : Sequential BF TEM images of polycrystalline UO2 irradiated with 4 MeV Au at -180 °C and 5 × 10 13 Au/cm 2 under thermal annealing at 1200 °C showing loop coalescence by diffusion process. The studied loops are circled.

Figure 14 :

 14 Figure 14 : Sequential BF TEM images of polycrystalline UO2 irradiated at 600 °C and 1 × 10 15 i/cm 2 under thermal annealing, showing line movements by climb at (a) 1150 °C, (b) 1300 °C. The arrows indicate the studied lines. The images on the right (with red contours) illustrate the path of the intersection of the line with surfaces. Each black point corresponds to this intersection at different time (each 5s).

Figure 15 :

 15 Figure 15 : BF TEM images of polycrystalline UO2 irradiated with 27 MeV Kr at 600 °C and 5 × 10 15 i/cm 2 , (a) before [41] and (b) after annealing at 1400°C during 1h. Observations were carried out with g = 200 reflections.

with 4

 4 MeV gold ions at three temperatures (-180, 25 and 600 °C) and fluences (0.5, 5 and 10 × 10 14 Au/cm 2 ) were annealed up to 1400 °C in order to observe different initial microstructures. These initial levels of damage correspond to the early first time in reactor. Annealing up to high temperatures using successive temperature plateaus, allowed us to follow extended defects change for temperatures pellet periphery (400-500°C) up to the center (1000-1200°C), without exogenous atoms contribution.

Table 1 .

 1 laboratory (Centre d'Élaboration de Matériaux et d'Etudes Structurales in Toulouse). A custom-made dedicated sample holder, [25] [26], was used to perform observations at successive temperature plateaus, up to 1400°C. The subsequent annealing conditions were summarized in Video monitoring of the microstructure was ensured by a Megaview III CCD camera coupled with a hard drive at a rate of 25 images per second.

	Sample number	Energy and ions	Irradiation temperature ( °C)	Fluence (i/cm 2 )	Annealing temperature plateaus (°C)
	Sample 1a	4 MeV Au	-180	5 × 10 13	15 min at 500, 600, 800, 900 5 min at 1000, 1150, 1200, 1250, 1350
	Sample 1b	4 MeV Au	-180	5 × 10 13	15 min at 500 and 5 min at 1150
					15 min at 200, 400, 500, 600, 800, 950
	Sample 2	4 MeV Au	-180	1 × 10 15	5 min at 1100, 1150, 1200, 1250, 1300,
					1350
	Sample 3	4 MeV Au	25	5 × 10 14	5 min at 200, 400, 500, 600, 700, 800, 1000, 1100, 1200, 1300, 1400
					15 min at 400, 600, 800, 950
	Sample 4	4 MeV Au	600	1 × 10 15	5 min at 1000, 1100, 1150, 1200, 1250,
					1300

Table 1 : Irradiation and thermal annealing conditions.

 1

49][50][51].

  However, in available literature, only Xe or Kr irradiations at high fluences followed by annealing were performed [In these conditions, vacancy behavior could be affected by xenon or krypton atoms and could be strongly different from the one without exogenous gas atoms. Marchand et al.[START_REF] Marchand | Xenon migration in UO2 under irradiation studied by SIMS profilometry[END_REF] reported that after implantation at room temperature, Xe is mainly present under atomic state and no cavities (clusters of several vacancies that might contain gas atoms) could be

	reported small cavities with a diameter around 1 nm. These differences in the initial state could be
	explained, for the study of Michel, by the high out-of-focus values used for imaging bubbles. Indeed bubble
	size and density strongly depend on the out-of focus value [54], especially if the objects are very small, as
	observed in UO2 after ion irradiations [55]. In any case, a cavity growth was highlighted from 1000°C 12h

observed using PAS [52] or TEM [53]. At equivalent irradiation conditions, He et al. [49] and Michel [51] [51] or 1300°C 1h [49]. Two bubble populations are commonly observed: the first one of around 1

Acknowledgements

These experiments were supported by the French EMIR accelerators network. We thank the staff of SEMIRAMIS and in particular Sandrine Picard and Jérôme Bourçois (CSNSM/CNRS) for their help in performing irradiations. We also thank Martiane Cabié (CP2M) for performing FIB lamella and Chantal Martial (CEA Cadarache) for the estimation of the stoichiometry of our samples during the annealing using TAF-ID.