Sophie Achigar

Daniel Caurant
email: daniel.caurant@chimieparistech.psl.eu

Elise Régnier

Odile Majérus

Dismantling nuclear waste rich in P2O5, MoO3 and ZrO2: How do these oxides incorporate in aluminoborosilicate glasses?

Keywords: Nuclear dismantling, waste vitrification, aluminoborosilicate glass, cesium, MoO3, P2O5, and ZrO2 incorporation Highlights

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Introduction

During the reprocessing of a highly radioactive spent fuel in a nuclear facility, significant fouling of the plant circuits and components occurs. Thus, active deposits remain in the equipment of shut down facilities. When dismantling the reprocessing facilities, these fouling wastes have to be managed [START_REF] Girold | French innovative processes in the field of thermal treatment for decommissioning and legacy waste[END_REF], for which vitrification is considered. The requirements for the containment glass matrix to vitrify these wastes are different from those used for high-level radioactive wastes (HLW) generated by fuel reprocessing. Nevertheless, as for HLW vitrification, it is considered that dismantling waste would be added to a glass frit and melted at about 1100°C in a metallic crucible. However, in the case of dismantling waste, the metallic crucible would play the role of canister after melt cooling [START_REF] Girold | French innovative processes in the field of thermal treatment for decommissioning and legacy waste[END_REF]. As will be pointed out later on in this paper, the challenge here lies in the ability of the glass matrix to accommodate a variable waste stream, as these dismantling wastes generally have much more variable compositions than HLW. For this, an alkali-rich aluminoborosilicate glass matrix was selected. The aim of this work is to study the microstructure (phase separation and crystallization) of several glass series prepared by varying the total and relative amounts of the oxides simulating the dismantling waste (in order to take into account the wide composition range of the waste stream), and to follow the location of the radionuclides in the different phases that may be present.

In this study, we focused on the waste coming from the dismantling operation on the nuclear site of Marcoule in France. These wastes represent deposits that are found in old tanks from the shutdown reprocessing facility named UP1 and were already presented in [START_REF] Girold | French innovative processes in the field of thermal treatment for decommissioning and legacy waste[END_REF]. The main elements of these deposits are Zr, Si, P, Mo, Fe, Na and Al, and the deposit activity is essentially due to the radioactive 137 Cs (Cs2O representing about 1 wt% of the waste). In the real waste, P, Zr, Mo and Cs are thought to be in the following forms: phosphorus as phosphates, zirconium as Zr(HPO4)2, ZrMo2O7(OH)2 and hydroxides, molybdenum as hydroxides and ZrMo2O7(OH)2 and cesium as Cs compounds with both P and Mo, of the type CsxPMoyOz. However, in this study, the elements present in the waste were considered only as oxide forms and added to the batch as commercial raw materials (mainly oxides and carbonates). The glass selected to incorporate the dismantling waste, and that will be mixed as glass frit with this waste in the industrial process, was rich in alkali oxides (33.1 mol% Na2O + Li2O) both to facilitate waste incorporation (formation of a high proportion of non-bridging oxygen atoms) and to limit the melting temperature, and thereby to limit the risks of cesium volatilization. Compared to the HLW coming from the nuclear fuel reprocessing, this type of waste is featured by a wide range of composition [START_REF] Girold | French innovative processes in the field of thermal treatment for decommissioning and legacy waste[END_REF]. As a simplified majoring approach, the variation of each oxide can be considered between 0 and 50 wt% of the waste composition (expressed as oxides, Table 1). In addition, this waste composition involves elements presenting a high field strength. The field strength F of a cation in an oxide material is defined as F = Z/d 2 , with Z the cation charge and d the mean cation-oxygen distance in Å. Thus, cations with a high field strength tend to exert a strong ordering effect on the surrounding oxygen ions and may separate easily from the glassy network. This is the case of Mo 6+ , P 5+ and Zr 4+ cations (F(Mo 6+ ) = 1.89-1.94 Å -2 , F(P 5+ ) = 2.00-2.11 Å -2 , F(Zr 4+ ) = 0.92-0.93 Å -2 [START_REF] Caurant | Glasses, glass-ceramics and ceramics for immobilization of highly radioactive nuclear wastes[END_REF]), which are likely to lead to crystallization or phase separation.

Molybdenum is a well-known element in the nuclear industry and is one of the fission products that are very difficult to incorporate in borosilicate glasses [START_REF] Caurant | Glasses, glass-ceramics and ceramics for immobilization of highly radioactive nuclear wastes[END_REF]. In these glasses, molybdenum forms MoO4 2-entities, which are located in alkali and alkaline earth-rich depolymerized regions outside the silicate or borosilicate network [START_REF] Caurant | Glasses, glass-ceramics and ceramics for immobilization of highly radioactive nuclear wastes[END_REF][START_REF] Caurant | Structural investigations of borosilicate glasses containing MoO3 by MAS NMR and Raman spectroscopies[END_REF][START_REF] Calas | Structural role of molybdenum in nuclear glasses: An EXAFS study[END_REF]. The solubility limit of MoO3 is relatively low in borosilicate matrices, i.e., 1 -3 mol% [START_REF] Nicoleau | Phase separation and crystallization effects on the structure and durability of molybdenum borosilicate glass[END_REF][START_REF] Ohkubo | Structural investigation of aluminoborosilicate glasses containing Na2MoO4 crystallites by solid state NMR[END_REF][START_REF] Kroeker | 133 Cs and 23 Na MAS NMR spectroscopy of molybdate crystallization in model nuclear glasses[END_REF][START_REF] Tan | The incorporation and solubility of sulphate, chloride and molybdate anions in borosilicate and aluminosilicate glasses[END_REF]. Above this limit, phase separation or crystallization of Na2MoO4 or CaMoO4 may occur during the cooling of the material [START_REF] Ohkubo | Structural investigation of aluminoborosilicate glasses containing Na2MoO4 crystallites by solid state NMR[END_REF][START_REF] Kroeker | 133 Cs and 23 Na MAS NMR spectroscopy of molybdate crystallization in model nuclear glasses[END_REF][START_REF] Cousi | Phase separation and crystallisation induced by adding molybdenum and phosphorus to a soda-lime-silica glass[END_REF].

The relative proportions of these phases depend on the amount of B2O3 in the glass [START_REF] Caurant | Effect of molybdenum on the structure and on the crystallization of SiO2-Na2O-CaO-B2O3 glasses[END_REF].

Moreover, even if Mo isotopes are stable [START_REF] Teillac | Nuclear wastes[END_REF], molybdate phases can incorporate radioactive elements, such as 137 Cs [START_REF] Kroeker | 133 Cs and 23 Na MAS NMR spectroscopy of molybdate crystallization in model nuclear glasses[END_REF][START_REF] Greer | Characterisation of heterogeneous molybdate and chromate phase assemblages in model nuclear waste glasses by multinuclear magnetic resonance spectroscopy[END_REF][START_REF] Kroeker | Precipitation of mixed-alkali molybdates during HLW vitrification[END_REF].

Phosphorus is a non-radioactive element that comes here from the solvent degradation during the Purex process used to extract U and Pu from the spent fuel. The incorporation of phosphorus in nuclear borosilicate glasses has not been widely studied because its content is generally, relatively low in glasses containing HLW (typically < 0.2 mol% of P2O5 exists in the French R7T7 glass). Nevertheless, many studies have been carried out on phosphorus incorporation in soda-lime phosphosilicate bioglasses ([P2O5] < 6 mol%) used for biomedical applications [START_REF] Hench | The story of bioglass[END_REF][START_REF] Fayon | Evidence of nanometric-sized phosphate clusters in bioactive glasses as revealed by solid-state 31 P NMR[END_REF][START_REF] Leonova | Multinuclear solid-state NMR studies of ordered mesoporous bioactive glasses[END_REF][START_REF] Mathew | Direct probing of the phosphate-ion distribution in bioactive silicate glasses by solid-state NMR: Evidence for transitions between random/clustered scenarios[END_REF][START_REF] Stevensson | Assessing the phosphate distribution in bioactive phosphosilicate glasses by 31 P solid-state NMR and molecular dynamics simulations[END_REF]. According to these studies, phosphorus speciation in a silicate network strongly depends on the glass composition. In glasses with high alkali or alkaline earth contents, such as bioglasses, in which alkali and alkaline earth oxides content can be as high as 50 mol%, most of the phosphorus forms isolated orthophosphate PO4 3-units [START_REF] Fayon | Evidence of nanometric-sized phosphate clusters in bioactive glasses as revealed by solid-state 31 P NMR[END_REF][START_REF] Mathew | Direct probing of the phosphate-ion distribution in bioactive silicate glasses by solid-state NMR: Evidence for transitions between random/clustered scenarios[END_REF][START_REF] Stevensson | Assessing the phosphate distribution in bioactive phosphosilicate glasses by 31 P solid-state NMR and molecular dynamics simulations[END_REF][START_REF] Yu | Medium-range structural organization of phosphorusbearing borosilicate glasses revealed by advanced solid-state NMR experiments and MD simulations: Consequences of B/Si substitutions[END_REF]; for example, they represent 96% of P in 45S5 ® bioglass [START_REF] Yu | Medium-range structural organization of phosphorusbearing borosilicate glasses revealed by advanced solid-state NMR experiments and MD simulations: Consequences of B/Si substitutions[END_REF]. In silicate glasses with higher amounts of phosphorus oxide, or lower content of alkali and alkaline earth oxides, a larger proportion of phosphate tetrahedra would be bonded to other phosphate units or to the glassy network [START_REF] Dupree | An MAS NMR study of network -cation coordination in phosphosilicate glasses[END_REF][START_REF] Toplis | A 31 P MAS NMR study of glasses in the system xNa2O-(1x)Al2O3-2SiO2-yP2O5[END_REF][START_REF] Tilocca | Structural effects of phosphorus inclusion in bioactive silicate glasses structural effects of phosphorus inclusion in bioactive silicate glasses[END_REF][START_REF] Muñoz | Influence of phosphorus speciation on the phase separation of Na2O-B2O3-SiO2 glasses[END_REF]. In soda-lime silicate glasses, the nature of phosphate bonds is still debated; some authors argue that these bonds are P-O-P bonds, [START_REF] Dupree | An MAS NMR study of network -cation coordination in phosphosilicate glasses[END_REF][START_REF] Toplis | A 31 P MAS NMR study of glasses in the system xNa2O-(1x)Al2O3-2SiO2-yP2O5[END_REF][START_REF] Schaller | TRAPDOR NMR investigations of phosphorus-bearing aluminosilicate glasses[END_REF] whereas the more recent studies on bioglasses indicate that they could be P-O-Si bonds [START_REF] Nicoleau | Phase separation and crystallization effects on the structure and durability of molybdenum borosilicate glass[END_REF][START_REF] Ohkubo | Structural investigation of aluminoborosilicate glasses containing Na2MoO4 crystallites by solid state NMR[END_REF][START_REF] Kroeker | 133 Cs and 23 Na MAS NMR spectroscopy of molybdate crystallization in model nuclear glasses[END_REF][START_REF] Cousi | Phase separation and crystallisation induced by adding molybdenum and phosphorus to a soda-lime-silica glass[END_REF]. With regard to the connections of phosphate units with other network forming units, such as AlO4, BO3 and BO4, several studies have demonstrated the presence of P-O-Al and P-O-B bonds in aluminosilicate [START_REF] Toplis | A 31 P MAS NMR study of glasses in the system xNa2O-(1x)Al2O3-2SiO2-yP2O5[END_REF][START_REF] Schaller | TRAPDOR NMR investigations of phosphorus-bearing aluminosilicate glasses[END_REF][START_REF] Glatz | Influence of P2O5 on the structure and nucleation of alkali aluminosilicate glasses[END_REF] and borosilicate systems [START_REF] Muñoz | Phosphate speciation in sodium borosilicate glasses studied by nuclear magnetic resonance[END_REF], respectively. According to these different studies, the more numerous the bonds between P and the glassy network are, the higher its solubility would be. Above this limit, the crystallization of alkali or alkaline earth phosphate may occur. In soda silicate glasses, Na3PO4 [START_REF] Muñoz | Influence of phosphorus speciation on the phase separation of Na2O-B2O3-SiO2 glasses[END_REF][START_REF] Glatz | Influence of P2O5 on the structure and nucleation of alkali aluminosilicate glasses[END_REF] and Na4P2O7 [START_REF] Muñoz | Influence of phosphorus speciation on the phase separation of Na2O-B2O3-SiO2 glasses[END_REF] may crystallize, whereas in soda-lime silicate glasses, NaCaPO4 may crystallize [START_REF] Cousi | Phase separation and crystallisation induced by adding molybdenum and phosphorus to a soda-lime-silica glass[END_REF][START_REF] Schuller | Phase separation and crystallization induced by the addition of Mo and P to a calcium-sodium silicate glass[END_REF][START_REF] O'donnell | Effect of P2O5 content in two series of soda lime phosphosilicate glasses on structure and properties-Part II: Physical properties[END_REF].

Zirconium is also a well-known element of nuclear glasses. It is a fission product, although it may also come from either the glass frit added to the waste or the zircaloy clad containing the nuclear fuel [START_REF] Quintas | ZrO2 addition in soda-lime aluminoborosilicate glasses containing rare earths: Impact on the network structure[END_REF]. For instance, in the French R7T7 nuclear glass, the amount of ZrO2 is approximately 1.54 mol% or 2.75 wt% [START_REF] Icenhower | Dissolution rate of borosilicate glass SON68: A method of quantification based upon interferometry and implications for experimental and natural weathering rates of glass[END_REF]. Approximately 20 % of all the Zr occurring in waste solutions as a fission product is radioactive ( 93 Zr), however, it is a weak -emitter with a long half-life [START_REF] Guillaumont | Chemical elements to consider in the back-end nuclear cycle[END_REF]. Moreover, ZrO2 has a very low solubility in water and Zr 4+ ions have a very low mobility in geological fields [START_REF] Quintas | ZrO2 addition in soda-lime aluminoborosilicate glasses containing rare earths: Impact on the network structure[END_REF]. Zr has a higher solubility in silicate glasses, compared to Mo and P, e.g., 15.8 wt% or 8.9 mol% is solubilized in a borosilicate glass of composition 15.8 ZrO2 -8.4 Na2O -2.1 Li2O -17.2 B2O3 -51.1 SiO2 -5.8 others (in wt%) [START_REF] Mckeown | Structural characterization of high-zirconia borosilicate glasses using Raman spectroscopy[END_REF]. This is probably because it is mostly incorporated as a ZrO6 2-octahedron linked to the silicate network (Zr-O-Si bonds) and charge-compensated by alkali or alkaline earth cations [START_REF] Caurant | Structural and crystallization study of a simplified aluminoborosilicate nuclear glass containing rare-earths: effect of ZrO2 concentration[END_REF][START_REF] Connelly | The structural role of Zr within alkali borosilicate glasses for nuclear waste immobilisation[END_REF][START_REF] Galoisy | Evidence for 6-coordinated zirconium in inactive nuclear waste glasses[END_REF][START_REF] Jollivet | An enhanced resolution of the structural environment of zirconium in borosilicate glasses[END_REF][START_REF] Angeli | Influence of zirconium on the structure of pristine and leached soda-lime borosilicate glasses : Towards a quantitative approach by 17 O MQMAS NMR[END_REF][START_REF] Lu | Structural role of ZrO2 and its impact on properties of boroaluminosilicate nuclear waste glasses[END_REF][START_REF] Ferlat | Evidence for symmetric cationic sites in zirconium-bearing oxide glasses[END_REF][START_REF] Pèlegrin | Structural evolution of glass surface during alteration: Application to nuclear waste glasses[END_REF]. Above the solubility limit, phases, such as ZrO2 or ZrSiO4 may crystallize [START_REF] Mckeown | Structural characterization of high-zirconia borosilicate glasses using Raman spectroscopy[END_REF][START_REF] Schuller | Liquid-liquid phase separation process in borosilicate liquids enriched in molybdenum and phosphorus oxides[END_REF][START_REF] Pinet | Glass ceramics containment matrix for insoluble residues coming from spent fuel reprocessing[END_REF].

In this work, we studied the incorporation of the oxides representing the different elements present in a waste originating from dismantling operations (Table 1) in an alkali-rich aluminoborosilicate glass (Table 2). The aim of the work was to determine how the selected glass could incorporate these oxides, taking into account the high variability of the waste composition. Two parameters were considered to study the incorporation of the oxides in the glass: (i) the waste loading rate (from 10 to 30 wt%) in the glassy matrix and (ii) the waste composition, by increasing selectively the relative proportions of the different oxides present in the waste, focusing particularly on P2O5, MoO3 and ZrO2. For this first study, the incorporation of oxides coming from the waste was studied by mixing oxide + carbonate + phosphate powders at the final glass composition ratio, and by melting this mixture at 1100 °C with oxide powders coming from the frit. The incorporation of the waste into the glass frit, taking into account the real chemical nature of the elements present in the waste and using a true glass frit has not been studied here. This will be done in a separate study. We selected this method of glass preparation because our objective was not here to study the reactivity of the waste (or of the waste simulant) with the glass frit during melting but to study the solubility of the oxides representing the waste both in the melt and in the glass obtained after controlled cooling of the melt at a rate close to natural cooling in industrial canisters (1 °C.min -1 ). The solubility in the melt was studied by examining at room temperature the microstructure of the sample obtained after melt quenching assuming that no important evolution occurs during rapid cooling. All the quenched and slowly cooled samples thus produced were characterized using scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (MEB-EDS) and X-ray diffraction (XRD) to detect and identify the presence of heterogeneities (crystalline or amorphous) and using differential thermal analysis (DTA) to determine the glass transition temperature.

Experimental procedure

Preparation of samples

Table 1 shows the reference waste composition (expressed as oxides) considered for this study, which corresponds to the mean composition from 19 measurements on the UP1 active deposits.

The composition of the glass frit that would be added to the waste is given in Table 2. From these two input data, glass compositions were calculated.

All the glass compositions prepared in this work are given in Table 3. To study the influence of the incorporation rate of the waste into the alkali-rich aluminoborosilicate glass, three glass series (S10, S20 and S30), with increasing waste loading (respectively 10, 20, and 30 wt%), were calculated and synthesized based on the glass frit (Table 2) and reference waste compositions (Table 1). The waste loading was defined in this paper as the ratio between the sum of the oxides mass of the waste and the mass of the glass even if the true waste contains not only oxides as indicated above. This waste loading definition corresponds to the weight contribution of the waste in the final glass, i.e. after decomposition and dissolution in the melt of all the compounds that were present in the real waste. Then, to study the influence of the waste composition variability among these three series, the reference composition of the waste was modified: the relative oxide contents in the waste were modified from 0 to 50 wt% successively for each oxide MyOz. Either this oxide was removed from the composition (this waste composition was designated as "M0") or its content was raised to 50 wt% of the waste (this waste composition was designated as "M50"), while the relative weight ratios of all the other waste components were kept constant. Fig. 1 illustrates this approach in the case of MoO3.

The different samples thus prepared were referred to as Sx.My (with x = 10, 20, or 30; and y = 0 or 50). In detail, for Series S10, in addition to the reference composition R of the waste leading to the sample S10.R, seven oxide contents were modified and 14 compositions were prepared, which included variations of SiO2 (samples S10.Si0 and S10.Si50), Na2O (samples S10.Na0 and S10.Na50), Al2O3 (samples S10.Al0 and S10.Al50), Fe2O3 (samples S10.Fe0 and S10.Fe50), ZrO2 (samples S10.Zr0 and S10.Zr50), P2O5 (samples S10.P0 and S10.P50), and finally, MoO3 (samples S10.Mo0 and S10.Mo50, Fig. 1). Thus, all the waste oxides were modified except B2O3 and Cs2O. In fact, the content of B2O3 was very low, while that of Cs2O was deliberately increased by a factor of approximately 10 (Table 1) compared to its real proportion in the waste, to have a significant amount of cesium (> 0.5 wt% Cs2O, Table 3) in glasses. For Series S20, three compositions were synthesized (S20.R, S20.P50, and S20.Zr50), to study the influence of increasing phosphorus and zirconium contents, and complete the results of Series S10. For Series S30, only two compositions were prepared (S30.R and S30.Zr50), to study the influence of increasing the zirconium content, and complete the results of Series S10 and S20. The sample S30.Zr50 containing 15 wt% (8.54 mol%) of ZrO2 was not characterized in depth in this study because at 1100 °C, the mixture did not melt completely owing to the high refractory nature of ZrO2 (melting point 2715 °C).

For the three series, samples (50 g batches) were prepared using appropriate quantities of reagent grade SiO2 (Alfa Aesar, purity 0.995), H3BO3 (Alfa Aesar, purity 0.99), Na2CO3 (Alfa Aesar, purity 0.995), CaCO3 (VWR, purity 0.99), Li2CO3 (Laboratoire du Bois de Boulogne), Al2O3 (Baikowski, purity 0.995), Fe2O3 (Prolabo, purity 0.99), ZrO2 (Rhône Poulenc), NH4H2PO4 (Alfa Aesar, purity 0.98), Cs2CO3 (Alfa Aesar, purity 0.999), and MoO3 (Sigma Aldrich, purity 0.995) powders. They were melted in air in an electrical furnace at 1100 °C for 2 h in Pt crucibles, and cast on a brass plate. The samples obtained were ground and melted again at 1100 °C for 2 h to ensure their homogeneity. The melt was quenched again by casting on a brass plate at a cooling rate of approximately 10 3 °C.min -1 or rapidly quenched between two brass plates at a cooling rate of approximately 10 4 °C.min -1 to obtain 1-mm thick samples.

The latter operation of increasing the quenching rate was conducted to obtain homogeneous samples, when heterogeneous samples were obtained after only casting. The quenched samples were designated with the subscript "q" in this paper. Then, a fraction of the quenched samples, approximately 2 g, was melted again at 1100 °C in Pt-Au crucibles, and slowly cooled at 1 °C.min -1 from 1100 °C to room temperature to simulate the typical natural cooling rate in the bulk of industrial nuclear glass canisters. These slowly cooled glass samples were designated by a subscript "sc" in this paper. All the quenched and slowly cooled samples were transparent with a brown color, owing to the presence of Fe2O3 and macroscopically homogeneous, except for three slowly cooled samples (S10.Mo50sc, S20.P50sc, and S30.Rsc). The sample S10.Mo50sc, with the highest MoO3 content (5 wt%) was brown and opaque with macroscopic white phases on its surface and samples S20.P50sc and S30.Rsc with respectively the highest P2O5 content and the highest waste proportion, were opaque with a uniform brown color.

Samples characterization

DTA measurements were performed on a DTA/TG STA 449 Netzsch apparatus, using approximately 150 mg of the powdered quenched samples with a controlled grain size of 80 -125 μm, and a heating rate of 10 °C.min -1 to determine the glass transition temperature Tg (measured as the onset of the first endothermic effect). For the samples that were not completely vitreous after quenching, Tg corresponds to the glass transition temperature of the residual glass surrounding the crystals.

The microstructure of the quenched and slowly cooled samples was studied using a FEG-SEM Zeiss supra 55VP microscope with a tungsten wire and operating at 15 kV with a working distance of 8.8 mm. EDS analysis was carried out on the slowly cooled samples to determine the nature of the heterogeneities detected on the SEM images. The composition of S10.Mo50sc molybdate phases (see Section 3.3) corresponds to the average of five EDS measurements and the error is the standard deviation of these measurements. To estimate the surface proportion of the molybdate phases on the SEM images of the S10.Mo50sc sample, a set of five hundred images were collected and processed with ImageJ software. The composition of several quenched samples were checked by electron probe microanalysis (EPMA), with a CAMECA SX100 (CAMPARIS, Paris, France) apparatus, operating at 15 kV and 4 nA. The Li2O content was not directly analyzed, but deduced based on the difference from the total oxide content.

According to EPMA performed on several samples (results not shown), it appeared that the analyzed compositions were close to the nominal compositions which indicates that no important volatilization of oxides such as B2O3 and alkali oxides occurred during melting. Thus, hereafter, the actual compositions of the samples of the different series were considered similar to the nominal compositions. EPMA was also used to determine the composition of the residual glass of the slowly cooled sample S10.Mo50sc (see Section 3.3). The result corresponds to an average of 10 measurements and the error is the standard deviation of these measurements.

XRD was conducted at room temperature on an X'Pert PRO MPD PANalytical instrument using Cu-Kα radiation (λKα1 = 0.15406 nm and λKα2 = 0.15444 nm) in Bragg-Brentano (θ-2θ) geometry (U = 40 kV and I = 40 mA), and on a Bruker instrument using Co-Kα radiation (λKα1 = 0.17890 nm and λK α2 = 0.17929 nm) in Bragg-Brentano (θ-2θ) geometry (U = 35 kV and I = 28 mA). For this analysis, the samples were crushed into powders. On the PANalytical diffractometer, the XRD patterns were recorded over 2θ ranging from 10 to 90°, with a step of 0.017°, and an acquisition time per step of 489 s (i.e., a total acquisition time of 5 h). On the Bruker apparatus, the XRD patterns were recorded over 2θ ranging from 10 to 80°, with a step of 0.020°, and an acquisition time per step of 960 s (i.e., a total acquisition time of 5 h 05 min).

For the homogeneity of the results, all the patterns recorded with the Bruker diffractometer were converted to the Cu wavelength. The identification of the crystalline phases present in several samples was performed by comparing the patterns with the International Centre for Diffraction Data (ICDD) database PDF4+2019 using HighScore and DIFFRAC.EVA software packages. To extract the lattice parameters of the crystalline phases, refinements were conducted using the "Le Bail" method [START_REF] Bail | Ab-initio structure determination of LiSbWO6 by X-ray powder diffraction[END_REF], thanks to FullProf software.

Results

Glass transition temperature Tg

Tg was measured for each sample and the results, presented in Table 4, reveal that: (i) increasing the loading rate from 10 to 30 wt% (by comparing S10.Rq, S20.Rq and S30.Rq samples) leads to a significant increase of 44 °C in Tg; (ii) increasing the MoO3 content from 0 to 2.2 mol% (compare S10.Mo0q, S10.Rq and S10.Mo50q samples, Fig. 1) does not have significant impact on Tg that remains close to 420°C; (iii) increasing the P2O5 content from 0 to 4.7 mol% (by comparing S10.P0q, S10.Rq and S10.P50q samples for the Series S10, and S20.Rq and S20.P50q samples for the Series S20) leads to an increase of 13 °C and 39 °C in Tg respectively for the Series S10 and S20; and (iv) increasing the ZrO2 content from 0 to 8.55 mol% (by comparing S10.Zr0q, S10.Rq, S10.Zr50q samples for the Series S10, S20.Rq and S20.Zr50q samples for the Series S20) only leads to a slight increase of 7 °C in Tg. The origin of these different effects will be further detailed and discussed in Section 4.1.

Microstructural analysis

Effect of increasing the waste loading on the microstructure

Here, we present the studies on the incorporation of different loadings, namely 10, 20, and 30 wt% of the oxides representing the reference waste (Table 1) in glasses S10.R, S20.R and S30.R, respectively (Table 3). The incorporation of 10 and 20 wt% of waste in the glass (samples S10.R and S20.R) led to homogeneous glasses, both for the quenched and slowly cooled samples according to both SEM and XRD (not shown here). However, the quenched S30.Rq and the slowly cooled S30.Rsc samples were heterogeneous. For the quenched sample S30.Rq, a high density of nm-sized heterogeneities (size < 100 nm) was clearly visible on the SEM images (Fig. 3a). For this sample, the XRD pattern (Fig. 2) shows crystallization of two phosphate phases, NaCaPO4 and NaLi2PO4. The broad diffraction peaks demonstrate a poor crystallinity or small-size crystallites. The SEM images of the slowly cooled S30.Rsc sample (Fig. 3b) prove the presence of μm-sized heterogeneities with different contrasts, thus reflecting the variation in the composition. The globular shape of a high number of particles suggests that they originated from a phase separation phenomenon in the liquid. For this sample, the diffraction pattern (Fig. 2) reveals the crystallization of both molybdate and phosphate phases; it shows the crystallization of Na2MoO4 (cubic low temperature form), CsLiMoO4, NaCaPO4, NaLi2PO4, and Li3PO4. The EDS analyses confirmed the crystallization of Na2MoO4, CsLiMoO4, NaCaPO4, and alkali phosphates (Mo-rich phases appear in white on the SEM image in Fig. 3b). However, as Li cannot be detected by EDS, the exact nature of the alkali phosphate phase could not be determined by this method.

Effect of varying the waste composition on the microstructure

Glasses with 10 wt% waste loading (Series S10) All the samples were homogeneous after both quenching and slow cooling except for S10.Mo50. The SEM analysis of S10.Mo50q highlighted that a high density of nm-sized heterogeneities was present essentially in the bulk of the sample (Fig. 4a). The brightness of the heterogeneities on the SEM image shows that they contain heavy elements, compared to the surrounding matrix. However, owing to the small size of the phases (<< 1 µm), the EDS analyses could not determine their composition. In addition, XRD did not allow the detection of the crystallized phases either (Fig. 5). However, the crystallization of molybdate phases in the corresponding slowly cooled S10.Mo50sc sample shows that these phases could be molybdate phases (Fig. 4b). The demixtion of a molybdenum-rich phase may have occurred at the beginning of the quenching. As no heterogeneity was detected by SEM close to the sample surface (insert in Fig. 4a), which had a higher quenching rate than the bulk, it confirms that this phase separation only occurred during quenching, whereas the melt was homogeneous at 1100 °C. For the slowly cooled sample S10.Mo50sc, the XRD pattern (Fig. 5) proves the crystallization of Na2MoO4 (cubic low temperature form), Na2MoO4.2H2O, and CsLiMoO4.

The presence of Na2MoO4.2H2O may be explained by Na2MoO4 hydration in the laboratory atmosphere before the recording of the XRD pattern. The weak peaks at 2θ = 17.5 and 21.7° may be due to the crystallization of the hexagonal high temperature form of Na2MoO4 (metastable at room temperature). Numerous heterogeneities with a highly variable size (from a few hundreds of nm to 100 µm) were visible on the SEM images (Fig. 4b). The globular shape of these heterogeneities indicates the occurrence of phase separation and ripening phenomena in the liquid, during the slow cooling. The EDS analyses of the biggest separated globular particles in the bulk revealed that they are enriched in Mo, Na, and Cs, and probably, Li as well (not detectable by EDS). SEM images also showed that the brightest zones (μm-size) inside these separated particles (Fig. 3b, inset (i)) were enriched in Cs and thus, probably corresponded to CsLiMoO4, whereas the rest of the particles might correspond to Na2MoO4. The presence of CsLiMoO4 in the separated phase was confirmed by a complementary analysis. On the surface of the S10.Mo50sc sample, globular particles larger than the ones seen in the bulk were observed. Thus, it was possible to select and probe using EDS only the brightest phases. Their

Glasses with 20 wt% waste loading (S20 series)

The aim of employing the S20 series was to increase the waste loading in the glass to 20 wt% and to focus on the incorporation of P2O5 and ZrO2, raised by 50 wt% in the waste. These two oxides were chosen because they are only brought by the waste as Cs2O and MoO3 (i.e., these four oxides are not present in the glass frit composition, Table 2). Cs2O and MoO3 were not studied in the S20 series because the Cs2O content was already increased (by a factor 10) in the reference waste compared to the true waste, and MoO3 already led to phase separation and crystallization with a 10 wt% loading rate (S10.Mo50 sample, Figs. 4 and5).

The SEM analyses (not presented) showed that S20.R and S20.Zr50 samples were homogeneous after both quenching and slow cooling. This demonstrates that zirconium remained solubilized in the glass structure even for this high waste loading. This was not the case with phosphorus. Indeed, the SEM analysis of S20.P50q sample (Fig. 6a) highlighted the presence of a high density of nm-size heterogeneities (size < 100 nm). The spherical shape of these phases suggests that they resulted from a liquid-liquid phase separation mechanism. The XRD analyses (Fig. 7) revealed the crystallization of NaCaPO4 and NaLi2PO4. The same phases were detected by XRD for the slowly cooled S20.P50sc sample (Fig. 7). It is important to remark that in spite of the high amount of P2O5 in this sample (10 wt%), the incorporation of cesium in the crystalline phosphate phases was not observed contrary to the case of the molybdate phases for the MoO3-rich S10.Mo50 sample. For the S20.P50sc sample, two different heterogeneities containing crystals are clearly visible on the SEM images (insert in Fig. 6b); one appears darker than the matrix and the other, clearer. According to the EDS measurements, the clearer one was enriched in Na, Ca, and P; and the darker one was enriched in Na and P; therefore, they could be attributed to NaCaPO4 and NaLi2PO4 crystals, respectively (Fig. 7). The diffraction peaks of the slowly cooled samples were slightly sharper than those of the quenched samples (Fig. 7), probably owing to a larger size of the crystallites in the former because of the longer growth time during the slow cooling. The microstructure of S20.P50sc sample was complex (Fig. 6b), and composed of both spherical phases and elongated crystals; the size of the heterogeneities in this sample was approximately 1 μm. The coexistence of these various crystalline microstructures may be owing to the different growth temperatures during the sample cooling, which have led to different viscosities of the supercooled liquid; for example, a dendritic growth would be favored in the most viscous liquids, corresponding to a lower temperature.

Cs partition coefficient between the glass and crystals

As crystallization of a Cs-rich water soluble phase (CsLiMoO4) [START_REF] Klevtsova | Synthesis and crystal structure of CsLiMoO4[END_REF] was highlighted during the slow cooling of S10.Mo50 sample (Fig. 5), it was important to focus on the distribution of Cs, which is the main radioactive element (as 137 Cs isotope) present in the waste, between the globular separated phase and the residual glass (Fig. 4). In the general case, the molar percentage of an oxide MxOy in a specific phase i among a set of n phases is given by: mol% M 𝑥 O 𝑦 (phase 𝑖 𝑇𝑜𝑡 ⁄ ) = 100.

𝑚𝑓 𝑖 (M 𝑥 O 𝑦 ). 𝑉𝑓(𝑖) . 𝑑(𝑖) 𝑀(𝑖)

∑ 𝑚𝑓 𝑗 (M 𝑥 O 𝑦 ) 𝑛 1

.

𝑉𝑓(𝑗) . 𝑑(𝑗) 𝑀(𝑗)

where Tot is the set of all the phases; mfi(MxOy) is the molar fraction of MxOy in phase i; Vf(i)

is the volumetric fraction of phase i; M(i) is the molar weight of phase i; and d(i) is the density of phase i. For calculating the partition coefficient of Cs2O in our case, all the separated particles were considered as one phase (Phase 1), and the glass matrix surrounding these particles as Phase 2. Using the SEM images of the bulk of the S10.Mo50sc sample (Fig. 4b) and ImageJ processing software, the surface fraction of Phase 1 was estimated to be 3 ±1%. As the particle distribution in the sample was isotropic and homogeneous, the surface fraction determined by SEM corresponded to the volume fraction [START_REF] Russ | Practical Stereology[END_REF]. EDS and EPMA analysis showed that Cs2O represented 1.07 ±0.10 mol% of the separated particles (i.e., Phase 1), and 5.4×10 -2 ±1.3×10 -2 mol% of the glass matrix (i.e., Phase 2), respectively. For comparison, the Cs2O nominal concentration in S10.Mo50 sample was 0.1 mol% (Table 3). Thus, the Cs2O concentration was approximately 43% lower in the residual glass than in the global sample. The density of the residual glass matrix was estimated at 2.5 [START_REF][END_REF], and its molar weight was calculated to be 64 g.mol -1 from the composition determined by EPMA. For the sake of simplicity and because the cubic form of Na2MoO4 was the most abundant molybdate phase in S10.Mo50sc sample according to the intensity of the XRD peaks (Fig. 5), the densities and molar weights of all the crystalline phases present in the globular particles were considered similar to those of the cubic Na2MoO4 phase (d = 3.604 and M = 103 g.mol -1 ). According to all these analyses and hypotheses, the calculations revealed that between 17 and 45 mol% of Cs2O migrated into the globular separated phases and thus between 55 and 83 mol% of Cs2O remains in the percolating residual glassy phase. This result will have to be further considered for the chemical durability study that will be done later.

Discussion

Glass transition temperature

Because of the complexity of the glass compositions studied in this work (11-oxide glasses), and particularly, the composition variations that have been considered among the different glass series, it is difficult to easily correlate the variations in Tg with those in the concentration of a particular oxide (Table 4). Nevertheless, we attempted to correlate the strongest Tg variations observed with the waste loading and inside the different glass series with the strongest variations in the oxide concentrations from one glass to another.

A comparison of the Tg of samples S10.Rq (417 °C), S20.Rq (437 °C), and S30.Rq (461 °C) (Table 4) shows that increasing the waste loading led to a significant increase in Tg (+ 44 °C).

According to the molar compositions given in Table 3, the increase in Tg with the increase in the amount of waste is probably mainly due to both a decrease in the Na2O + Li2O concentration (-3.2 mol%) and an increase in the ZrO2 and P2O5 contents (+2.8 and +2.2 mol%, respectively).

Indeed, alkali oxides are well-known for their fluidizing effect in silicate glasses, and ZrO2 and P2O5 are known for their reticulating effect on the silicate network [START_REF] Quintas | ZrO2 addition in soda-lime aluminoborosilicate glasses containing rare earths: Impact on the network structure[END_REF][START_REF] Li | Local structure of Si and P in SiO2-P2O5 and Na2O-SiO2-P2O5 glasses: a XANES study[END_REF].

The Tg values of all the quenched samples of the S10 series are between 391 and 427 °C (Table 4). S10.Na50 has a lower Tg (391 °C) than all the other samples (411 -427 °C), owing to the higher Na2O content in this glass (26.2 mol%, compared to less than 22.7 mol% for all other glasses, Table 3).

A comparison of the Tg of S10.Mo0q (0 wt% MoO3), S10.Rq (0.30 mol% MoO3) and S10.Mo50q

(2.22 mol% MoO3) samples with increasing MoO3 content shows that Tg tends to remain constant (417 -420 °C, Table 4). Previous works on different silicate glasses indicated that Tg may decrease with addition of MoO3 [START_REF] Caurant | Glasses, glass-ceramics and ceramics for immobilization of highly radioactive nuclear wastes[END_REF][START_REF] Tan | MoO3 incorporation in magnesium aluminosilicate glasses[END_REF], in spite of the reticulating effect of molybdenum on the silicate network, an evidence of which was provided by 29 Si NMR for soda-lime borosilicate glasses [START_REF] Caurant | Structural investigations of borosilicate glasses containing MoO3 by MAS NMR and Raman spectroscopies[END_REF][START_REF] Caurant | Effect of molybdenum on the structure and on the crystallization of SiO2-Na2O-CaO-B2O3 glasses[END_REF]. This particular effect of MoO3 content on Tg in the present study could be explained by considering an increasing extension in glass structure of alkali-, alkaline earth-, and molybdenum-rich depolymerized domains containing isolated MoO4 2-entities as it was proposed for the addition of MoO3 in soda-lime borosilicate glasses [START_REF] Caurant | Glasses, glass-ceramics and ceramics for immobilization of highly radioactive nuclear wastes[END_REF]. Another structural explanation was given by Tan et al. [START_REF] Tan | MoO3 incorporation in magnesium aluminosilicate glasses[END_REF] for the effect on Tg of MoO3 addition in magnesium aluminosilicate glasses that did not contain alkali oxides. As in our system, the initial content of alkali and alkaline earth oxide is high (about 38 mol% in the S10.Mo0 sample) before MoO3 addition, the depolymerized domains are already extended in glass structure which suggests that MoO3 addition (2.2 mol%, Table 3) does not significantly change the size of these domains; therefore the amount of MoO3 added to the composition would not significantly modify the network structure and Tg would to remain constant.

A comparison of the glass transition temperatures of S10.P0q (414 °C), S10.P50q (427 °C), and S20.P50q (476 °C) (Table 4) shows that increasing the phosphorus oxide content from 0 to 4.7 mol% (Table 3) was responsible for a significant increase in Tg. This evolution is in accordance with the available literature on borosilicate glasses with high a Na2O/B2O3 ratio (> 1) [START_REF] Muñoz | Phosphate speciation in sodium borosilicate glasses studied by nuclear magnetic resonance[END_REF]. It may be explained by the extraction of alkali or alkaline earth ions from the glassy network to compensate the isolated PO4 3-and P2O7 2-units when P2O5 is added to the glass, which thus becomes more polymerized. However, S10.P0q and S10.P50q samples were amorphous, and S20.P50q sample was strongly crystallized (Fig. 7). Thus, for this last sample, Tg corresponds the glass transition temperature of the residual glass. The strong crystallization of phosphate phases could explain the higher Tg increase (+62 °C) between S10.P0q and S20.P50q samples than between S10.P0q and S10.P50q samples (+13 °C). In fact, the crystallization of NaLi2PO4 and NaCaPO4 in the S20.P50q sample (Fig. 7) definitively isolates a significant proportion of the Li + , Na + , and Ca 2+ ions from the glassy silicate network that becomes more polymerized.

Incorporation of molybdenum, phosphorus, and zirconium in glass 4.2.1. Molybdenum incorporation in glass

In this study, molybdate phase separation (followed or not by crystallization) was observed at 2.0 wt% (1.0 mol%) MoO3 in the slowly cooled S30.Rsc sample (Fig. 3), and at 5.0 wt% (2.2 mol%) MoO3 in the quenched S10.Mo50q sample (Fig. 4). In the quenched S10.Mo50q sample, heterogeneities were only observable in the bulk of the sample, and the surface (which had the highest cooling rate) was homogeneous at the SEM scale. This shows that phase separation was not present in the melt at 1100 °C and only occurred during melt cooling.

The solubility limit of MoO3 in this study is in accordance with those given in literature for borosilicate glasses [START_REF] Nicoleau | Phase separation and crystallization effects on the structure and durability of molybdenum borosilicate glass[END_REF][START_REF] Ohkubo | Structural investigation of aluminoborosilicate glasses containing Na2MoO4 crystallites by solid state NMR[END_REF][START_REF] Magnin | Effect of compositional changes on the structure and crystallization tendency of a borosilicate glass containing MoO[END_REF]. For instance, in the soda-lime borosilicate glasses studied by Magnin et al. [START_REF] Magnin | Effect of compositional changes on the structure and crystallization tendency of a borosilicate glass containing MoO[END_REF] (66.4 SiO2 -10.75 B2O3 -14.05 Na2O -8.65 CaO, mol%), the MoO3 solubility limit was about 2.5 mol% for the quenched samples and 1 mol% for the slowly cooled samples. However, higher MoO3 solubility (higher than 5.3 mol%) was reported in magnesium aluminoborosilicate glasses without alkali oxides [START_REF] Tan | MoO3 incorporation in magnesium aluminosilicate glasses[END_REF].

Two molybdate crystalline phases were detected in S30.Rsc and S10.Mo50sc samples, namely Na2MoO4 and CsLiMoO4 (Figs. 2 and5). The crystallization occurred during melt cooling after liquid-liquid phase separation (Fig. 4). The crystallization temperature of pure CsLiMoO4 (860 °C [START_REF] Aleksandrov | Ferroelectric phase transitions in CsLiMoO4 single crystals[END_REF]) is known to be much higher than that of Na2MoO4 (688 °C [START_REF] Petrosyan | Phase diagrams of Na2MoO4-MMoO4 (M-Ca, Sr, Ba)[END_REF]). Thus, CsLiMoO4 may have crystallized first in the separated droplets, and Na2MoO4, later during the cooling, with surrounding CsLiMoO4 crystals having been already formed.

Regarding the Na2MoO4 crystals, two allotropic forms were present. The main one (which crystallized in S30.Rsc and S10.Mo50sc samples (Figs. 2 and5)) is the cubic form, which is stable at the ambient temperature. The second one, close to the high-temperature hexagonal form, only crystallized in the slowly cooled sample S10.Mo50sc (Fig. 5). The unexpected crystallization of such a high-temperature phase in the slowly cooled sample could be explained by the formation of nano-sized high temperature molybdate phases that are stabilized by the surrounding matrix.

In comparison with Na2MoO4, the crystallization of CsLiMoO4 has been rarely reported in literature. For instance, Morgan et al. [START_REF] Morgan | Interactions of simulated high level waste (HLW) calcine with alkali borosilicate glass[END_REF] highlighted the formation of CsLiMoO4 during the incorporation of waste in a sodium lithium borosilicate glass (63.4 SiO2 -20.0 B2O3 -11.1 Na2O -5.5 Li2O, mol%). In the present work, CsLiMoO4 was the only Cs-bearing crystalline phase observed, which suggests that this phase crystallized preferentially to other Cs-bearing phases, such as CsNaMoO4 (observed in other works [START_REF] Kroeker | 133 Cs and 23 Na MAS NMR spectroscopy of molybdate crystallization in model nuclear glasses[END_REF][START_REF] Greer | Characterisation of heterogeneous molybdate and chromate phase assemblages in model nuclear waste glasses by multinuclear magnetic resonance spectroscopy[END_REF], and not here). Moreover, the lattice parameters of the cubic form Na2MoO4 that crystallized in S30.Rsc and S10.Mo50sc samples (9.117 ±0.001 and 9.122 ±0.001 Å, respectively, according to the Le Bail refinement) are higher than that of the pure Na2MoO4 phase (9.109 Å [START_REF] Fortes | Crystal structures of spinel-type Na2MoO4 and Na2WO4 revisited using neutron powder diffraction[END_REF]). This suggests that the big Cs + ions could have partially replaced a fraction of the Na + ions in this phase, and lead to the lattice expansion.

The molybdate phases that were detected in S30.Rsc and S10.Mo50sc samples were only alkalibearing phases, and CaMoO4 crystallization was never observed, whereas several studies reported the preferential crystallization of CaMoO4 in soda-lime aluminoborosilicate glasses [START_REF] Caurant | Effect of molybdenum on the structure and on the crystallization of SiO2-Na2O-CaO-B2O3 glasses[END_REF][START_REF] Brehault | Compositional dependence of solubility/retention of molybdenum oxides in aluminoborosilicate based model nuclear waste glass[END_REF][START_REF] Taurines | Synthesis of powellite-rich glasses for high level waste immobilization[END_REF]. This observation may be explained by the higher alkali oxide content (29 -32 mol%, Table 3) and the lower B2O3 and CaO contents (7.1 -7.9 and 5.2 -6.1 mol% respectively, Table 3) for the compositions studied in the present work, which were disadvantageous for CaMoO4 crystallization [START_REF] Caurant | Structural investigations of borosilicate glasses containing MoO3 by MAS NMR and Raman spectroscopies[END_REF][START_REF] Caurant | Effect of molybdenum on the structure and on the crystallization of SiO2-Na2O-CaO-B2O3 glasses[END_REF].

Phosphorus incorporation in glass

Among the different P-rich compositions prepared in this study, the one with the lowest P2O5 content, leading to phosphate crystallization was S30.R with 6.5 wt% (3.2 mol%) P2O5 (Figs.

2 and 3); on the other hand, no phosphate crystallization was detected in the S10.P50 sample containing 5 wt% (2.2 mol%) P2O5, even after slow cooling. Muñoz et al. [START_REF] Muñoz | Influence of phosphorus speciation on the phase separation of Na2O-B2O3-SiO2 glasses[END_REF] highlighted that phosphorus speciation in silicate glasses strongly depended on the glassy matrix. The higher the Na2O content and the lower the B2O3 content, the lower the solubility limit would be because these conditions favor the formation of phosphate groups isolated from the silicate network (i.e, not connected to the borosilicate network). In quenched sodium borosilicate with a high Na2O/B2O3 ratio (> 1), these authors demonstrated crystallization of Na3PO4 and Na4P2O7 from 3 mol% P2O5 [START_REF] Muñoz | Influence of phosphorus speciation on the phase separation of Na2O-B2O3-SiO2 glasses[END_REF], which is close to the P2O5 content in S30.R sample.

However, in our samples, the crystallization of pure sodium phosphate phases (i.e., Na3PO4, Na4P2O7) was not observed. Because of the large Na2O excess (> 20 mol%), compared to other alkali and alkaline earth oxides (< 15 mol% Li2O + Cs2O + CaO) in S20.P50 and S30.R samples, the crystallization of pure sodium phosphate phases, such as Na3PO4, could be expected as in the study by Glatz et al. [START_REF] Glatz | Influence of P2O5 on the structure and nucleation of alkali aluminosilicate glasses[END_REF] on a P2O5-bearing sodium aluminosilicate glass (51 SiO2 -18,4

Al2O3 -27,6 Na2O -3 P2O5, mol%), or in the study by Muñoz et al. [START_REF] Muñoz | Influence of phosphorus speciation on the phase separation of Na2O-B2O3-SiO2 glasses[END_REF] on a P2O5-bearing sodium borosilicate glass (40 Na2O -20 B2O3 -40 SiO2 + 3 P2O5, mol%). In the present work, the crystallization of NaCaPO4, NaLi2PO4, or Li3PO4 phases, in which phosphorus is only present as non-connected PO4 3-units, and the absence of Na3PO4 suggests a common tendency of calcium, lithium, and phosphorus to separate from the glassy network, together with sodium which could be explained as follows: firstly, it is known that in alkali and alkaline earth-rich silicate glasses, phosphorus is mainly incorporated into the glass structure as isolated orthophosphate PO4 3-units surrounded by alkali and alkaline earth cations for local chargecompensation in depolymerized regions [START_REF] Fayon | Evidence of nanometric-sized phosphate clusters in bioactive glasses as revealed by solid-state 31 P NMR[END_REF][START_REF] Mathew | Direct probing of the phosphate-ion distribution in bioactive silicate glasses by solid-state NMR: Evidence for transitions between random/clustered scenarios[END_REF][START_REF] Stevensson | Assessing the phosphate distribution in bioactive phosphosilicate glasses by 31 P solid-state NMR and molecular dynamics simulations[END_REF][START_REF] Yu | Medium-range structural organization of phosphorusbearing borosilicate glasses revealed by advanced solid-state NMR experiments and MD simulations: Consequences of B/Si substitutions[END_REF]; secondly, owing to the higher field strength of Li + and Ca 2+ , compared to Na + (FLi+ = 0, 23-0, 26 Å -2 , FCa2+ = 0, 33-0, 36 Å -2 , FNa+ = 0, 18-0, 19 Å -2 [START_REF] Caurant | Glasses, glass-ceramics and ceramics for immobilization of highly radioactive nuclear wastes[END_REF]), Li + and Ca 2+ have a higher tendency than Na + to separate from the silicate network, leading either to demixtion or crystallization; lastly, it is also well known that in borosilicate glasses, Na + ions react preferentially with boron atoms to compensate BO4 -units, compared to Li + and Ca 2+ ions [START_REF] Quintas | Effect of compositional variations on charge compensation of AlO4 and BO4 entities and on crystallization tendency of a rare-earth-rich aluminoborosilicate glass[END_REF][START_REF] Caurant | Structural role and distribution of alkali and alkaline-earth cations in rare earth-rich aluminoborosilicate glasses[END_REF]. Thus, all these considerations suggest that lithium and calcium have a higher tendency than sodium to join phosphorus in the depolymerized regions of the glass structure, and then form crystalline phases. Eventually, owing to the high Na2O content in our samples, a significant fraction of Na + ions is probably mixed with Li + and Ca 2+ ions in the neighborhood of orthophosphate units, which explains the presence of sodium in NaCaPO4 and NaLi2PO4 crystals.

With regard to the crystallization mechanism of phosphate phases, the nano-sized separated phases obtained in the quenched S30.Rq and S10.P50q samples suggest that the melt was homogeneous at 1100 °C, but separated and crystallized during cooling. Moreover, in the slowly cooled S30.Rsc and S20.P50sc samples, spherical phosphate droplets and elongated phosphate phases coexist (Figs. 3 and6). The presence of spherical phosphate phases suggests that they were formed by liquid-liquid phase separation, followed by crystallization. The elongated ones may be formed in a more viscous liquid, i.e., at lower temperatures compared to that of the droplets.

Zirconium incorporation in glass

Similar to Mo 6+ and P 5+ ions, Zr 4+ ion is a high-field strength cation present in high proportions in the waste that could separate from the silicate network in the liquid melt, and in the quenched or slowly cooled glass. However, for the glass composition studied in this work, it appeared that ZrO2 did not lead to phase separation or crystallization at least up to 10 wt% (5.4 mol%) in the S20.Zr50 sample, even after slow cooling. This indicates that zirconium was solubilized well in the glassy network probably because it is present as ZrO6 2-entities connected to the silicate network with alkali and alkaline earth ions ensuring local charge compensation, as in the French R7T7 glass and other peralkaline borosilicate glasses [START_REF] Quintas | ZrO2 addition in soda-lime aluminoborosilicate glasses containing rare earths: Impact on the network structure[END_REF][START_REF] Galoisy | Evidence for 6-coordinated zirconium in inactive nuclear waste glasses[END_REF][START_REF] Lu | Structural role of ZrO2 and its impact on properties of boroaluminosilicate nuclear waste glasses[END_REF]. For higher ZrO2 content (e.g., S30.Zr50 sample with 15 wt% (8.55 mol%)), as the sample was not totally melted at 1100 °C, it would have been necessary to increase the melting temperature to reach the liquidus temperature to hope to obtain a glass, but such a temperature would be higher than that of the vitrification process envisaged in this work.

Application to an industrial process: ability of glass to incorporate the dismantling waste

The results obtained for almost all the samples with 10 wt% of waste loading (Series S10) led to homogeneous melts and glasses (at SEM and XRD scale). This shows the ability of the glass composition selected in this work to accept a wide range of waste composition variations.

However, among all the compositions tested in Series S10, one exception was that containing a high proportion of molybdenum, namely S10.Mo50 sample containing 50 wt% MoO3 in the waste (Fig. 1). In this case, the results show that the melt at 1100 °C is homogeneous; however, separated phases and crystallization appeared during melt cooling (Figs. 4 and5). Because the crystallized phases may contained cesium (as in CsLiMoO4), which is the main radioactive element ( 137 Cs) present in the dismantling waste considered in this work, special attention must be paid to this element. The incorporation of cesium in the crystallized phase CsLiMoO4 at 10 wt% waste loading with 50 wt% of MoO3 in the waste was thus the limiting point. To consider a further increase in the waste loading, studies should be conducted on the chemical durability of the waste forms and the Cs-partition in the samples containing 1 wt% of Cs2O (which is closer to the real waste).

Conclusions

The aim of this work was to study the incorporation of ZrO2, P2O5 and MoO3 by melting at 1100 °C glass compositions calculated considering a dismantling waste rich in Zr, P and Mo and an aluminoborosilicate glass frit highly rich in alkali oxides. Twenty glass compositions have thus been prepared based on oxide (mainly) powders mixing. The aim of these twenty compositions was to take into account the wide composition range of the waste and to study the impact of increasing the waste loading from 10 to 30 wt% in the final glass. The incorporation of Zr, P and Mo oxides was studied by analyzing the resulting slowly cooled and quenched glasses by SEM-EDS and XRD.

Molybdenum was the first element of the waste that may lead to phase separation and crystallization when its content in the simulating wastes is increased. In fact, molybdate crystallization appeared at 5 wt% (2.2 mol%) MoO3 for quenched samples, and at 2.0 wt% (1.0 mol%) MoO3 for slowly cooled samples. Two molybdate phases (Na2MoO4 and CsLiMoO4) crystallized. P2O5 may also lead to phase separation and crystallization (NaCaPO4, NaLi2PO4, and Li3PO4). This phenomena first appeared at 6.5 wt% (3.18 mol%) P2O5 for both the quenched and slowly cooled samples. The separation tendency of molybdenum and phosphorus was owing to the high field strength of Mo 6+ and P 5+ ions, and to the mobility of MoO4 1), (d) of the waste when the MoO3 content was increased to 50 wt%. These three waste compositions were used to calculate the glass compositions reffered respectively to as S10.Mo0, S10.R and S10.Mo50 in Table 3. Crystalline phases were identified by comparison with the ICDD files given in parenthesis: ○:

NaLi2PO4 (04-012-1893), ∆ : NaCaPO4 (04-016-6900). 1), (d) of the waste when the MoO3 content was increased to 50 wt%. These three waste compositions were used to calculate the glass compositions reffered respectively to as S10.Mo0, S10.R and S10.Mo50 in Table 3. (cubic form, 00-012-0773), ▲ CsLiMoO4 (00-038-1238). The amorphous halo below the XRD lines is due to the contribution of the glassy phase surrounding the crystals. S30.R q S30.R sc Crystalline phases were identified by comparison with the ICDD files given in parenthesis: ○:

Waste Glass frit

NaLi2PO4 (04-012-1893), ∆ : NaCaPO4 (04-016-6900).

S20.P50 q S20.P50 sc

  Cs2O content (42 wt%) and MoO3 content (50 wt%) were close to those of CsLiMoO4 ([Cs2O] = 47 wt%, [MoO3] = 48 wt%).

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .

 2 Figure 2. XRD pattern of S30.R samples: S30.Rq (quenched from the melting temperature 1100 °C) and S30.Rsc (slowly cooled at 1 °C.min -1 from the melting temperature 1100 °C). Crystalline

Figure 3 .

 3 Figure 3. SEM images (backscattered electrons) of S30.R samples: (a) S30.Rq (quenched from the melting temperature 1100 °C), 20 000, and (b) S30.Rsc (slowly cooled at 1 °C.min -1 from

Figure 4 .

 4 Figure 4. SEM images (backscattered electrons) of S10.Mo50 sample: (a) S10.Mo50q

Figure 5 .

 5 Figure 5. XRD patterns of S10.Mo50 samples: S10.Mo50q (quenched from a melting temperature 1100 °C) and S10.Mo50sc (slowly cooled at 1 °C.min -1 from a melting temperature

Figure 6 .

 6 Figure 6. SEM images (backscattered electrons) of S20.P50 sample: (a) S20.P50q (quenched from the melting temperature 1100 °C), 20 000 and (b) S20.P50sc (slowly cooled at 1 °C.min - 1 from the melting temperature 1100 °C), 100. Insert : zoom of the image, 5 000.

Figure 7 .

 7 Figure 7. XRD pattern of S20.P50 sample: S20.P50q (quenched from the melting temperature 1100 °C) and S20.P50sc (slowly cooled at 1 °C.min -1 from the melting temperature 1100 °C).

Figure 1 Figure 1 .

 11 Figure 1

Figure 2 Figure 2 .

 22 Figure 2

Figure 3 .

 3 Figure 3. SEM images (backscattered electrons) of S30.R samples: (a) S30.Rq (quenched from the melting temperature 1100 °C), x 20 000, and (b) S30.Rsc (slowly cooled at 1 °C.min -1 from

Figure 4 Figure 4 .Figure 5 Figure 5 .Figure 6 Figure 6 . 44 Figure 7 Figure 7 .

 4455664477 Figure 4

Table 1 :

 1 Average nominal composition of the dismantling waste considered in this study (in wt% and mol%). To facilitate the study of cesium incorporation in the different phases that could form in the glasses of the different series, the Cs2O content was increased by a factor of approximately 10 compared to its actual concentration in the decommissioning and dismantling waste.

	2-and

Table 2 :

 2 Nominal composition of the glass frit that would be added to the dismantling waste (Table1) during melting to prepare the nuclear glass (in wt% and mol%).

		SiO2	B2O3	Na2O	CaO	Li2O	Al2O3	Fe2O3
	wt%	48.0	9.5	23.5	6.0	5.0	2.0	6.0
	mol%	48.5	8.3	23.1	6.5	10.1	1.2	2.3

Table 3 .

 3 Nominal compositions in wt% and mol% (mol% are indicated in parentheses) of the

	three series of samples prepared for this study (series S10, S20, and S30 with 10, 20, and 30
	wt% of waste, respectively).

Acknowledgments

This study is a part of the DEM&MELT project, which is a partnership between CEA, industrial partners (Orano, ECM Technologies), and ANDRA. This project is supported by the French government program titled "Programme d'Investissements d'Avenir (PIA)", whose management was entrusted to ANDRA.

The authors want to express their thanks to Charlène Vallat, Lionel Campayo, and Damien Perret of CEA Marcoule for their help in SEM and DTA analyses. The authors also gratefully acknowledge Michel Fialin and Nicolas Rividi (CAMPARIS service, Paris, France) for their help during the EPMA measurements and Maxime Fournier for helpful comments.

Series

Series S10 (