Dismantling nuclear facilities leads to radioactive waste which may have highly variable compositions. In this study, we focused on waste coming from the dismantling operation of the spent fuel reprocessing facility UP1 stopped since 1997 (Marcoule, France), that mainly contains Zr, Si, P, Mo, Fe, Na and Al, and whose activity is essentially due to radioactive 137Cs (Cs2O representing about 1 wt% of the waste, but increased to 10 wt% in this study to facilitate Cs detection) [1]. For this waste, vitrification is studied using an in-can vitrification furnace (heated metallic container) designed to be installed directly on the dismantling site [2].

In this work, we firstly studied the ability of an alkali-rich glass matrix belonging to the SiO2-B2O3-Al2O3-Fe2O3-Na2O-Li2O-CaO complex system (Na2O + Li2O  33 mol% (28.5 wt%)) to solubilize P2O5, MoO3, ZrO2 and Cs2O that are present in the waste, by melting a mixture of inactive raw materials at 1100°C. Due to their high field strength, P5+, Mo6+ and Zr4+ ions may exhibit a high tendency to separate from the silicate network in glass structure [1,3,4]. To determine the capacity of this matrix to accept a wide range of waste composition, several glass series were prepared by increasing the total amount of oxides representing the waste (10-30 wt% waste loading) and by varying the relative proportions of P2O5, MoO3 and ZrO2 within the waste from 0 to 50 wt% in order to take into account the potential variability of the waste composition (the case of P2O5 is shown in Fig. 1). Their incorporation in the melt was studied by analyzing the microstructure of quenched glasses by XRD and SEM-EDS, whereas the phase separation and crystallization tendencies during melt cooling in the metallic container were studied by analyzing the microstructure of samples cooled to room temperature at 1°C.min 1 (Fig. 1). It appeared that the investigated glass can accept a wide range of waste compositions without exhibiting heterogeneities. For all the compositions studied, the melt remained homogeneous with 10 wt% waste loading. Nevertheless, during slow cooling, P2O5 and MoO3 may lead to phase separation and crystallization of Na2MoO4, CsLiMoO4, NaCaPO4, NaLi2PO4, and Li3PO4 (Fig. 1). The increasing order of oxides solubility in the glass was found to be the following: MoO3 < P2O5 < ZrO2. A 10 wt% waste loading appears acceptable as almost all the glass compositions resulting from this loading value were homogeneous after slow cooling (only the one with 50 wt% MoO3 leads to molybdate phase separation followed by crystallization of Na2MoO4 and CsLiMoO4) [1]. During these studies, it appeared that ZrO2 never leads to phase separation or crystallization, possibly because of the existence of strong connections between Zr and Si through Zr-O-Si bonds [5].
To complete this work on the complex glass, XRD, Raman and multinuclear (31P, 29Si, 23Na, 27Al, 11B) MAS NMR studies have been performed on a simplified quenched or slowly cooled glass belonging to the SiO2-B2O3-Al2O3-Na2O-CaO system derived from the complex composition by removing among other things iron oxide and by adding increasing P2O5 content (0 – 10 mol%), to investigate the crystallization of phosphate phases and the evolution of the structural environment of phosphorus. It appeared that above 2 mol% P2O5 phase separation followed by crystallization of -Na3PO4 occurs, then followed by NaCaPO4 and even Na4P2O7 for the highest P2O5 contents. Moreover, according to 31P MAS NMR and Raman spectroscopies, it appeared that phosphorus is mainly present in glass structure as PO43- (orthophosphate) entities and probably also as P2O74- (pyrophosphate) mobile entities non-connected to the silicate network and located in depolymerized regions of the glass structure (Fig. 3). The possible existence of P-O-Si, P-O-Al and P-O-B connections will be discussed. Due to the mobilization of Na+ and Ca2+ as charge compensators of phosphate entities in the glassy (Fig. 3) and crystalline phases, an increase of the polymerization of the glassy network as shown by 29Si MAS NMR associated with an increase of the glass transformation temperature from 454°C (0% P2O5) to 553°C (8% P2O5) were also put in evidence.

[1] S. Achigar, D. Caurant, E. Régnier, O. Majérus, Dismantling nuclear waste rich in P2O5, MoO3 and ZrO2: How do these oxides incorporate in aluminoborosilicate glasses? J. Nucl. Mater. 544 (2021) 152731. https://doi.org/10.1016/j.jnucmat.2020.152731

[2] C. Girold, S. Francois, L. Petit, S. Catherin, T. Prevost, E. Fourcy, G.
Lecomte, A. Viand, French innovative processes in the field of thermal treatment for decommissioning and legacy waste, in Waste Manag., Phenix, USA (2018). https://hal-cea.archives-ouvertes.fr/cea-02339255/document

[3] D. Caurant, O. Majérus, E. Fadel, A. Quintas, C. Gervais, T. Charpentier, D. Neuville, Structural investigations of borosilicate glasses containing MoO3 by MAS NMR and Raman spectroscopies, J. Nuclear Mater. 396 (2010) 94. https://doi:10.1016/j.jnucmat.2009.10.059

[4] S. Schuller, O. Pinet, A. Grandjean, T. Blisson, Phase separation and crystallization of borosilicate glass enriched in MoO3, P2O5, ZrO2, CaO, J. Non-Cryst. Solids 354 (2008) 296. https://doi.org/10.1016/j.jnoncrysol.2007.07.041

[5] A. Quintas, D. Caurant, O. Majérus, P. Loiseau, T. Charpentier, J-L. Dussossoy, ZrO2 addition in soda-lime aluminoborosilicate glasses containing rare earths : Impact on the network structure, J. Alloys Compd. 714 (2017) 47. https://doi.org/10.1016/j.jallcom.2017.04.182