Thermal decomposition of

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Nov 9, 2016 - Studtite (UO4$4H2O) and Metastudtite (UO4$2H2O) are the only .... thermal decomposition of UO4$4H2O in reducing atmosphere, N2/.
Journal of Nuclear Materials 483 (2017) 149e157

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Thermal decomposition of (UO2)O2(H2O)2$2H2O: Influence on structure, microstructure and hydrofluorination R. Thomas a, b, M. Rivenet a, *, E. Berrier a, I. de Waele c, M. Arab b, D. Amaraggi b, B. Morel b, F. Abraham a Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unit e de Catalyse et Chimie du Solide, F-59000 Lille, France Hall de Recherche de Pierrelatte, AREVA NC, BP 16, 26701 Pierrelatte, France c Universit e de Lille, CNRS, UMR 8516 e LASIR - Laboratoire de Spectrochimie Infrarouge et Raman, F-59000 Lille, France a

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 1 July 2016 Received in revised form 11 October 2016 Accepted 7 November 2016 Available online 9 November 2016

The thermal decomposition of uranyl peroxide tetrahydrate, (UO2)O2(H2O)2.2H2O, was studied by combining high temperature powder X-ray diffraction, scanning electron microscopy, thermal analyses and spectroscopic techniques (Raman, IR and 1H NMR). In situ analyses reveal that intermediates and final uranium oxides obtained upon heating are different from that obtained after cooling at room temperature and that the uranyl precursor used to synthesize (UO2)O2(H2O)2$2H2O, sulfate or nitrate, has a strong influence on the peroxide thermal behavior and morphology. The decomposition of (UO2) O2(H2O)2$2H2O ex sulfate is pseudomorphic and leads to needle-like shaped particles of metastudtite, (UO2)O2(H2O)2, and UO3-x(OH)2x$zH2O, an amorphous phase found in air in the following of (UO2) O2(H2O)2 dehydration. (UO2)O2(H2O)2$2H2O and the compounds resulting from its thermal decomposition are very reactive towards hydrofluorination as long as their needle-like morphology is kept. © 2016 Published by Elsevier B.V.

1. Introduction Studtite (UO4$4H2O) and Metastudtite (UO4$2H2O) are the only known uranyl peroxide minerals found in nature, first identified by Walenta [1] and Deliens et al. [2], respectively. Studtite and Metastudtite growth in nature can result from the formation of H2O2 by alpha-radiolysis of water which explains why they can form as alteration products of spent nuclear fuel (SNF) [3]. The formation of these peroxides under different dissolution conditions and its implication for storage of SNF have motivated several studies in recent years[4e17]. Synthetic uranyl peroxides were previously obtained by precipitation from a solution of uranyl sulfate [18] or uranyl nitrate [19] by addition of hydrogen peroxide at ambient temperature for the tetrahydrate and at 80  C for the dihydrate. In each case a light yellow powder is obtained. The unit cell parameters of the two hydrates were first determined by Debets [19]. Determination of the structures of UO4$4H2O by single crystal Xray diffraction [20] and UO4$2H2O using density functional theory [21] led to larger monoclinic and orthorhombic cells, respectively

* Corresponding author. E-mail address: [email protected] (M. Rivenet). http://dx.doi.org/10.1016/j.jnucmat.2016.11.009 0022-3115/© 2016 Published by Elsevier B.V.

and to the structural formulas [(UO2)(m2-O2)(H2O)2]$2H2O and [(UO2)(m2-O2)(H2O)2]. The structures consist in [(UO2)(m2O2)(H2O)2]∞ chains built from [(UO2)(O2)2(H2O)2] hexagonal bipyramids linked via the peroxide groups. Hydrogen bonds further link the chains through the H2O groups located in the inter-chain spaces for the tetrahydrate and through the H2O groups belonging to one chain and the Oyl and peroxo oxygens of a neighboring chain in the dihydrate. Uranium peroxide is also of importance in the nuclear fuel cycle since it can be found as an intermediate compound both in the front-end and back-end of the nuclear fuel cycle. In the back-end of the nuclear fuel cycle, following the reprocessing of SNF by the PUREX process using dissolution in nitric acid, extraction and partitioning of the valuable elements, uranium can be recovered from solution as uranyl peroxide. This is later transformed into the most stable uranium oxide, U3O8, in order to avoid alteration, particularly hydration, during the transport and/or storage of uranium compounds. In the front end of the nuclear fuel cycle, uranium ores extracted in mining sites are ground, mashed and dissolved in concentrated sulfuric acid [22]. Uranium is recovered from solution either as uranates, by using alkaline solution (ammonium hydroxide or sodium hydroxide), or as an uranyl peroxide by using hydrogen peroxide [23], then calcinated into UO3

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or U3O8. In the current process, the oxides from mines are purified by dissolution in nitric acid and extraction by tributylphosphate in dodecane, before hydrofluorination. A recent patent by AREVA has shown that UO3 or U3O8 coming from mines can be easily transformed into pure uranyl peroxide, UO4$4H2O, by adding H2O2 to an aqueous suspension of uranium oxides [24]. This process is advantageous in that it allows purification meanwhile limiting effluent treatments. The so-obtained UO4$4H2O can be easily hydrofluorinated into UF4 by reduction into UO2 and reaction with HF at high-temperature. So the knowledge of the thermal decomposition mechanisms, transformation temperatures and characterization of the intermediate phases, are very important for the industrial applications. Many studies were devoted to the thermal decomposition of uranyl peroxide under air either from synthetic [25e32] or natural studtite [1,33]. All the studies agree in that the first step is the dehydration of the tetrahydrate to the dihydrate at a temperature varying in a large range from 60  C [1] to 150  C [25] and the last one is the formation of U3O8 around 580  C. However, depending on the study, different intermediates are proposed. The present study aims to revisit the thermal decomposition of uranyl peroxide tetrahydrate by combining in situ analyses such as coupled Thermogravimetric and Differential Thermal Analysis, high temperature X-ray diffraction, Infra-Red and Raman Spectroscopies. Some characteristics of the so-called amorphous UO3, obtained by heating UO4$2H2O in air, are given. The influence of uranyl precursor on the morphology and thermal decomposition of the uranyl peroxide is also studied by means of using either uranyl sulfate or uranyl nitrate as a reagent. Ex situ analyses such as Scanning Electronic Microscopy have allowed to determine the extent to which the initial properties are inherited in the final products. Finally the thermal decomposition of UO4$4H2O in reducing atmosphere, N2/ H2, and its hydrofluorination are reported. 2. Experimental 2.1. Uranyl peroxide synthesis Uranyl peroxide was synthesized by adding 1 M hydrogen peroxide solution to a 0.5 M uranyl solution prepared by dissolving uranyl nitrate or uranyl sulfate in deionized water. In each case, a pale yellow powder precipitates immediately and powder X-ray diffraction analysis (XRD) confirm the synthesis of UO4$4H2O. The powders were filtered and washed with deionized water (100 mL of water for 1 g of powder) to remove nitrate or sulfate ions. Finally, the powder was dried at room temperature. According to the uranyl precursor used for synthesis, nitrate or sulfate, the samples will be hereafter named UO4$4H2O ex uranyl nitrate and UO4$4H2O ex uranyl sulfate, respectively. 2.2. Powder X-ray diffraction (XRD) analysis Room temperature XRD diagrams were obtained with a D8 Advance Bruker diffractometer (q-q mode, CuKa radiation) equipped with a Vantec1 linear position sensitive detector (PSD). Each diagram was recorded in the range 5e80 (2q), with a step 0.02 and with a speed of 0.4 s/step.

(2q), with a step 0.02 and a heating rate of 0.8  C min1 in air and 5  C min1 in N2/H2. 2.4. Scanning electron microscope SEM Scanning Electron Microscopy (SEM) was carried out by means of a Hitachi S4700 SEM FEG (field emission gun). 2.5. Thermogravimetric (TGA) and Differential Thermal Analyses (DTA) Thermogravimetric and Differential Thermal Analyses were carried out with the help of a SETARAM 92 thermal analysis system. The provided sample was placed in a platinum crucible and heated up to 80f0  C, in air, with a scan rate of 1  C.min1, or in N2/H2 (95/5) with a scan rate of 5  C min1. 2.6. Spectroscopic measurement In-situ Raman spectra were collected using the 488 nm line of an Arþ-ion laser (Melles Griot) in confocal mode. A 50 long working distance objective (Olympus) was used for both focusing the excitation beam on the sample and collecting the scattered light. The latter was dispersed using a 1800 grooves spectrometer grating after having passed through a confocal hole of 150 mm. A Peltiercooled CCD detector (Horiba Labram HR) was used for collecting the Raman spectra. 50 mg of uranyl precursor powder was placed in the crucible of an atmospheric chamber (Harrick Scientific) equipped with a plane dome and allowing a O2:He (1:4) gas mixture, corresponding to a total flow of 20 mL/min, to pass through the powder (from top to bottom). In-situ Infrared measurements were obtained in a Fourier Transform Infrared Spectrometer (Thermo-Nicolet, Magna 860) equipped with a MCT detector and using a Diffuse Reflectance (DRIFT) accessory and a high temperature reaction chamber (The Praying Mantis™ High Temperature Reaction Chamber from Harrick). The spectra were collected from 25  C to 450  C at every 25  C and the sample was heated at a rate of 20  C/min. A total of 512 scans were recorded per spectrum over the range of 4000e650 cm1 at a resolution of 4 cm1. The sample was placed inside the dome equipped with two ZnSe windows under N2 flow. Reference spectra were recorded on KBr under the same conditions as for samples spectra. All spectra were converted into Kubelka-Munk units. 2.7. Specific surface area The specific surface area of the amorphous phases obtained by pre-heating UO4$4H2O was measured by using the B.E.T. method with an Ankersmit Quanta Sorb Jr. For the different phases, a degassing was made during 30 min at 130  C. Then, the sample was placed in a flow constituted by 30% N2 and 70% He. N2 was physisorbed at  196  C and desorbed at room temperature. The quantity of physisorbed gas permits to determine the specific surface area. 3. Results and discussion 3.1. Thermal decomposition in air

2.3. High temperature X-ray diffraction (HTXRD) High Temperature X-ray Diffraction (HTXRD) experiments were performed in dynamic air (5 L h1) or N2/H2 (95/5) flow (5 L h1) using the same diffractometer equipped with an Anton Paar HTK1200N furnace. Several diagrams were recorded between ambient temperature and 800  C every 25  C, in the range 10e80

Whatever the precursor (nitrate or sulfate), the HTXRD diagrams and the TGA did not reveal any difference. The HTXRD obtained with UO4$4H2O ex uranyl sulfate is reported Fig. 1. The dehydration starts at very low temperature, in fact the X-ray pattern at 50  C shows the presence of both UO4$4H2O and UO4$2H2O and only the dihydrate is present on the patterns

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Fig. 1. HTXRD study for UO4$4H2O ex uranyl sulfate.

between 75 and 175  C. At 200  C, an amorphous phase is formed and no crystallized phase is observed between 200 and 500  C. This amorphous phase crystallizes at 525  C to a mixture of a-UO3 and UO2.9 [26]. The last step at 650  C is the formation of a0 -U3O8 which is the high temperature form of a-U3O8. After return back to room temperature, a-U3O8 is obtained. Indeed, the two structures are closely related. As described by Siegel [34], when temperature increases to 400  C in vacuum, the orthorhombic pseudo-hexagonal lattice of a-U3O8 (space group C2mm) evolves to a more stable form with hexagonal lattice [34] and space group P62m [35]. This transition takes place at lower temperature (208.5  C) in air [36]. The HTXRD results were completed by the thermal decomposition study of UO4$4H2O performed by TG/TD analysis (Fig. 2). The first weight loss can be assigned to the dehydration of the tetrahydrated uranyle peroxide, UO4$4H2O, into the dehydrated form, UO4$2H2O (exp. 9.9 wt%/th. 9.6 wt%). Around 180  C, a second weight loss associated to an endothermic signal can be related to the amorphous phase formation. A continuous weight loss is then observed in the TGA curve. It is followed by a slight decrease which occurs between 430  C and 470  C, attributed to the crystallization of the mix of oxides UO3 and UO2.9. Between 530 and 680  C, the TGA curve reveals two weight losses. The overall experimental weight loss (exp. 1.4 wt%) corresponds to the formation of a0 -U3O8.

It was supposed that the former weight loss (exp. 0.6 wt%) takes place due to the reduction of UO2.9 and the latter (exp. 0.8 wt%) to the reduction of UO3. The corresponding weight percentage of UO2.9 and UO3 would be approximately 60% and 40%. A theoretical weight loss of 14.2 wt% close to the experimental value (exp. 14.4 wt%) was calculated on the basis of a transition of UO4$2H2O into the mixture of (60% UO2.9 þ 40% UO3), considering a loss of two water molecules and 1.06 oxygen atom. 3.2. Evolution of the morphology during thermal decomposition As it is well established that the properties of the final product depend on the starting material, a special attention was paid to the morphological evolution of the uranyl peroxide and phases formed with the heating temperature. For this study, UO4$4H2O ex uranyl sulfate was heated at 70, 300, 525 and 700  C so as to obtain UO4$2H2O, the amorphous phase, the mixture of oxides (aUO3 þ UO2,9) and U3O8, respectively. The compounds were maintained at the desired temperature during 15 min and cooled to room temperature. After heating at 525  C, only a-UO3 is revealed by powder XRD at room temperature. It seems that UO2.9 is not stable at room temperature and re-oxidized into UO3. In the same way, when a0 -U3O8 is cooled to room temperature, only a-U3O8 is observed.

Fig. 2. TG/TD analysis performed on UO4$4H2O ex uranyl sulfate.

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When UO4$4H2O is precipitated from an uranyl sulfate solution, needles are formed (Fig. S1a). The needles are 500 nm in length and 100e300 nm of width. The initial needle-like shape is kept in air during the thermal decomposition of uranyl peroxide until the formation of UO3 (Fig. S1(b1) to S1(b3)). This interesting pseudomorphism occurring during the transformation of the actinide precursor into the oxide was already reported for UO4$4H2O [26,37], ammonium diuranate [38e40] and actinides oxalates, precursors of single or mixed actinides oxides [41,42]. After heating at 625  C, needles tend to disappear to the benefit of oval-shaped aU3O8 particles with 50 nm wide and 100 nm long (Fig. S1(b4)) which indicates that sintering starts to occur in the range of temperature [525e625  C]. 3.3. Influence of precursor on morphology and thermal decomposition Initial composition may affect the thermal decomposition of the uranium precursor and the specific surface area of the resulting compounds [43]. Indeed, in the chemical systems studied herein,

the uranyl precursor influences neither the precipitation reaction nor the compound formula but affects morphology in that the use of uranyl nitrate in place of uranyl sulfate leads to uranyl peroxide with a spherical shape instead of needles (Fig. 3a). The spheres have a diameter in the range 50e150 nm approximately. Indeed, when UO4$4H2O ex uranyl nitrate is not washed and heated at 70  C to obtain dihydrated uranyle peroxide, the spherical morphology is kept (Fig. 3b) but if it is washed with water in order to remove residual nitrates and heated at the same temperature, needles are observed (Fig. 3c). These needles are 300 nm long and 50 nm wide. The influence of nitrate was already discussed by Cordfunke and Van der Giessen who found spherical shaped particles by heating UO4$4H2O containing 5% of nitrate [26]. In case of the uranyl sulfate precursor, washing has no influence on the particle shape. Only a little decrease of the needles size is observed. So, impregnating nitrates (with a content of approximately 1 nitrate for 2.67 uranium atom) prevent the needle-like shape of uranyl peroxide. The influence of precursor and washing on the transition temperatures was analyzed by DTA (Fig. 4). Four samples were compared, two obtained from uranyl nitrate and two from uranyl

Fig. 3. Morphology of UO4$4H2O ex uranyl nitrate (washed or not) (a) and UO4$2H2O obtained from UO4$4H2O ex uranyl nitrate not washed (b) and washed (c).

Fig. 4. DTA for UO4$4H2O obtained from different precursors and washing or not.

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sulfate. In each series, one sample was washed and the other one not. For the four samples, the dehydration of UO4$4H2O into UO4$2H2O takes place at 70  C approximately. Comparison of curves (c) and (d) shows that washing has no, or scarcely, influence on the thermal decomposition of UO4.4H2O ex uranyle sulfate. The only difference is a slight increase of the temperature at which the UO3 and UO2.9 mixture forms when the starting compound is not washed. In case of using the nitrate precursor (curves (a) and (b)), washing has a strong influence on thermal decomposition. When nitrates are removed from UO4$4H2O ex uranyl nitrate, the formation of the amorphous phase takes place at a lower temperature and recrystallization of the amorphous into the UO3 and UO2.9 mixture is higher than without washing. It can be supposed that the stability of the amorphous phase decreases in presence of nitrates, when the compound is not washed. U3O8 appears either at 620  C or 655  C by decomposing either UO4$4H2O ex uranyl nitrate or UO4$4H2O ex uranyl sulfate but washing doesn't influence the temperature of formation of U3O8 for a given precursor. 3.4. Amorphous phase study A large experimental effort has been already addressed to determine the nature of the amorphous phase resulting from the

Fig. 5. In situ Raman analysis of the thermal decomposition of uranyl peroxide, UO4$4H2O.

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dehydration of studtite. Depending on the study, the amorphous phase is assigned as an oxide, UO3 [26] or UOx [28], a hydrated oxides, UOx.nH2O with 3 < x < 3.5 [44] or UO3$nH2O [27,31], or a peroxide, U2O7 [25,30,32]. Most of the authors underline the discrepancies of the results that can be explained by the different experimental conditions used for dehydrating UO4.4H2O which vary by the choice of atmosphere, temperature, rate or time of heating, water vapor pressure and peroxide precursor (studtite or metastudtite). The present section takes advantages of characterizing the amorphous phase using various in situ analyses. A new formula for the amorphous phase obtained by calcination of UO4.4H2O, in air, without controlling the water vapor pressure, is proposed. Raman spectroscopy analysis was performed on the range of temperature [25e500  C] (Fig. 5). From 25 to 50  C, two peaks located at 818 and 863 cm1 are observed. They are characteristic of the symmetric stretching frequency of the uranyl cation (ns O^U^O) and the peroxide anion (ns OeO), respectively. Upon increasing the temperature up to 70  C, the Raman strongest band assigned to ns (O^U^O) is shifted by about 10 cm1 to higher wavenumbers while the lowest intensity peroxo stretch (ns OeO) remains about at the same position. These Raman features are in agreement with the studtite transformation into metastudite at 70  C as previously reported in literature [45]. At 120  C, a broad band which maximal intensity is centered at 748 cm1 starts to appear. With increasing the temperature, the bands characteristic of uranyl cation (ns O^U^O) and peroxide anion (ns OeO) tend to disappear while the new broad band at ca. 748 cm1 shifts to lower wavenumbers. At 200  C, there is no more metastudtite. At 300  C, the maximum intensity of the new band, attributed to the amorphous phase, is located at 707 cm1 and at 400  C and higher temperatures, the latter is further downshifted to 675 cm1. This analysis, in agreement with TGA, proves that the amorphous phase is not stable and doesn't keep the same structure upon increasing the temperature. The specific surface area measurements agree with this observation (Fig. S2). With the temperature increase from 250  C to 400  C, the specific surface area increases from 15 m2/g to 18 m2/g. After heating at 500  C, it decreases drastically to 11 m2/g. The evolution of the specific surface area is known to result from two competitive processes which tend either to generate surface by departure of gaseous products or to lose surface by sintering [46]. The specific surface area increase from 250 to 400  C probably occurs due to the loss of water molecules while the decrease observed beyond 400  C can be explained by the crystallization of the high temperature amorphous phase into the UO3 and UO2.9 mixture. The presence of water molecules and hydroxide groups in the amorphous phase was probed by in situ infrared spectroscopy and ex situ 1H NMR analysis of a sample previously heated to 300  C. The IR spectra obtained by heating the sample up to 200  C provide evidence of the similarities between studtite and metastudtite with a strong and narrow band located about at 1600 cm1 (dH-O-H) and two broad bands located about at 3200 and 3500 cm1 (nO-H) [43,47] (Fig. 6). In a study on hydrated uranium oxides, UO3$nH2O, Nipruk et al. [48] assigns the bands around 3500-3600 cm1 to the interlayer H2O (ns OeH and nas OeH), the band at about 3350 cm1 to the water molecules coordinated to uranium (nUO-H), and the two bands at 1620 and 1586 cm1 to non-bonding H2O interacting each other and with the hydroxyl groups UOH belonging to the layers (dH-O-H), respectively. In a previous study on studtite, Bastians et al. [45] suggests that the band at 3147 cm1 arises from the water molecules of crystallization contained in (UO2)(O2)(H2O)2$2H2O because its intensity is decreased in metastudtite compared to studtite. Our study reveals some differences on heating the sample

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Fig. 6. In situ infrared spectroscopy of the thermal decomposition of uranyl peroxide, UO4$4H2O. In red: studtite, UO4$4H2O (25 and 50  C), light blue: metastudtite, UO4$2H2O (75e250  C), navy blue: amorphous phase (275e450  C) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.).

up to 450  C. A loss of intensity of the H2O stretching vibration located at about 3200 cm1 is first observed due to the dehydration of studtite into metastudtite. Meanwhile, the H2O bending vibration band, dH-O-H, first splitted into two bands (1700 and 1620 cm1) becomes unique which indicates that only one type of interaction involving the water molecules remains [48]. On heating the sample, both of the intensities of the H2O stretching vibration bands become weaker until the one located around 3200 cm1 has almost disappeared. Beyond 300  C, it remains a broad band centered on 3480 cm1. The loss of water molecules at high temperature is accompanied by the appearing of three bands located at 1540, 1415 and 1360 cm1. These are characteristic of CO2 species

adsorbed onto the surface [49]. With two large bands located at 5 and 10 ppm corresponding respectively to water molecules and OH ions (Fig. 7), the 1H NMR analysis confirms the results obtained by in situ infrared spectroscopy. On the basis of the NMR and Infra-Red Spectroscopy results, the high temperature amorphous phase formed in the following of the dehydratation of studtite in air, was formulated UO3-x(OH)2x$zH2O. Beside the experimental results, this new formula refers to crystallized uranium trioxide hydrates that contain OH ions and water molecules as UO2.67(OH)0.66 (x ¼ 0.33, UO3$0.33H2O) [50], UO2(OH)2 (x ¼ 1, UO3$1H2O) [51e54], UO2.25(OH)1.5$1.5H2O

Fig. 7. 1H NMR spectrum of the amorphous phase obtained at 300  C.

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Fig. 8. HTXRD study of UO4$4H2O ex uranyl sulfate in H2/N2.

(x ¼ 0.75, schoepite UO3$2.25H2O) [55], UO2.25(OH)1.5$1.25H2O (x ¼ 0.75, meta-schoepite UO3$2H2O) [56]. It explains the progressive specific surface area increase by the loss of water molecules and hydroxide groups until 400  C while the crystallization of the UO3 and UO2.9 mixture beyond 400  C explains the specific surface area decrease. 3.5. Thermal decomposition in H2/N2 flow From an industrial point of view, the starting material (oxide or peroxide) has to be reduced into UO2 before hydrofluoration into UF4, so the thermal decomposition under reducing atmosphere is of importance. The HTXRD diagrams and the TG/TD analyses of UO4$4H2O ex uranyl sulfate in H2/N2 flow are reported on Figs. 8 and 9, respectively. According to these experiments, the first steps of decomposition in H2/N2 are similar to those observed in air: (i) UO4$4H2O starts to transform into UO4$2H2O at relatively low temperature and the

transformation is complete at 75  C (exp. 8.7 wt%; calc. 9.6 wt%) (ii) UO4$2H2O remains stable until about 150  C, temperature at which it decomposes into an amorphous compound. At this step, some differences arise between the behavior in H2/N2 and in air: (iii) the existence domain of the amorphous phase is 100  C less than that observed from the experiments conducted in air since a-UO3 starts to crystallize at about 420  C instead of 525  C (exp. 13.3 wt%; calc. 13.9 wt%) (iv) a-UO3 is not stable in H2/N2 and UO2.9 was not found during these experiments and the reduced form, U3O8, appears as a-U3O8 at 475  C, instead of a0 -U3O8, at 650  C, in air (v) a-U3O8 is reduced in UO2 around 500  C (exp. 4.4 wt%; calc. 4.3 wt%) as previously found by calcination of U(C2O4)2$2H2O [41]. The thermal analysis (Fig. 9) reveals slightly higher transition temperatures than those observed by HTXRD and experimental weight losses in good agreement with theoretical calculations. The loss of water molecules during the transformations UO4$4H2O / UO4$2H2O / Amorphous / a-UO3 are endothermic phenomena while the reduction mechanisms a-UO3 / a-U3O8 and a-U3O8 /

Fig. 9. TD/TG analysis performed on UO4$4H2O ex uranyl sulfate in H2/N2.

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Fig. 10. Rate of hydrofluorination of UO4$4H2O ant its calcination products versus the heating temperature.

UO2 are exothermic phenomena. In H2/N2, as in air, all the transformations are pseudomorphic which leads to needle like shaped UO2 crystallites giving high performance in hydrofluoration (Fig. S1c and Fig. 10) [24]. 4. Conclusions A scheme of thermal decomposition for the uranyl peroxide tetrahydrate, UO4$4H2O, is presented taking advantage of in situ analyses like thermal analyses and high temperature powder X-ray diffraction. In air, like in H2/N2, the following decomposition sequence can be proposed UO4$4H2O / UO4$2H2O / Amorphous / a-UO3 / a-U3O8. In air, at high temperature, a-UO3 is accompanied with UO2.9 while a0 -U3O8 is found instead of a-U3O8. In H2/ N2, there is an additional step due to the reduction of a-U3O8 which transforms into UO2, as previously observed during the calcination of U(C2O4)2.2H2O. The composition of the amorphous phase formed in air was further investigated by in situ analyses, Raman and InfraRed spectroscopies, combined with BET measurements and 1H NMR of pre-heated samples. The experimental results proved that the composition of the so-called ‘amorphous UO3’ evolves with the temperature increase and that it most probably contains hydroxide ions and water molecules. On the basis of these results and on the chemical formula of crystallized uranium trioxide hydrates containing OH ions and water molecules, the amorphous phase obtained in air, without controlling the water vapor pressure, was formulated UO3-x(OH)2x$zH2O. This result points out that it remains quite difficult to conclude with certainty on the chemical formula of the amorphous phase since it most probably depends on the experimental conditions. The initial morphology of UO4$4H2O can be modified by using either uranyl nitrate or uranyl sulfate, as starting material for the synthesis of uranyl peroxide. Spherical particles are obtained by

using uranyl nitrate whereas needle-like particles are obtained by using uranyl sulfate. The needle-like shape of the crystallites are kept during the decomposition until the formation of the final oxides, U3O8 in air, and UO2 in a H2/N2. This morphology is particularly suited for hydrofluorination of UO2 into UF4. Acknowledgments AREVA and Direction de la Recherche et de l’Innovation (DRI) are acknowledged for funding Rudy Thomas PhD Thesis and supporting this work. The authors thank B. Revel for technical support  de Lille 1’ for access to and the ‘Plateforme RMN de l’Universite re de l’Enspectrometers. Chevreul Institute (FR 2638), Ministe rieur et de la Recherche, Re gion Nord e Pas de seignement Supe Calais and FEDER are acknowledged for supporting and funding partially this work. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jnucmat.2016.11.009. References [1] K. Walenta, Amer. Mineral. 59 (1974) 166e171. [2] M. Deliens, P. Piret, Amer. Mineral. 68 (1983) 456e458. [3] K.-A.H. Kubatko, K.B. Helean, A. Navrotsky, P.C. Burns, Science 302 (2003) 1191e1193. [4] G. Sattonay, C. Ardois, C. Corbel, J.F. Lucchini, M.-F. Barthe, F. Garrido, D. Gosset, J. Nucl. Mater. 288 (2001) 11e19. € lelin, J. Nucl. [5] M. Amme, B. Renker, B. Schmid, M.P. Feth, H. Bertagnolli, W. Do Mater. 306 (2002) 202e212. [6] B. McNamara, E. Buck, B. Hanson, Mater. Res. Soc. Symp. Proc. 757 (2003) 401e406. [7] F. Clarens, J. de Pablo, I. Diez-Perez, I. Casas, J. Gimenez, M. Rovira, Environ. Sc. Technol. 38 (2004) 6656e6661.

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