Structural characterization of SiO 2 -Na 2 O-CaO-B 2 O 3 -MoO 3 glasses D. Caurant, O. Majérus, E. Fadel, M. Lenoir CNRS, ENSCP, Laboratoire de Chimie de la Matière Condensée de Paris (UMR-CNRS 7574), 75231 Paris, France C. Gervais CNRS, Université Pierre et Marie Curie, Laboratoire de Chimie de la Matière Condensée de Paris (UMR-CNRS 7574), 75252 Paris, France T. Charpentier CEA Saclay, Laboratoire de Structure et Dynamique par Résonance Magnétique, DSM/DRECAM/SCM-CEA/CNRS URA 331, 91191 Gif sur Yvette, France D. Neuville Laboratoire de Physique des Minéraux et Magmas CNRS-IPGP, 4 Place Jussieu, 75252, France Nuclear spent fuel reprocessing generates high level radioactive waste with high Mo concentration that are currently immobilized in borosilicate glass matrices containing both alkali and alkaline-earth elements [1]. Because of its high field strength, Mo 6+ ion has a limited solubility in silicate and borosilicate glasses and crystallization of alkali or alkaline- earth molybdates can be observed during melt cooling or heat treatment of glasses [2-4]. Glass composition changes can significantly modify the nature and the relative proportions of molybdate crystals that may form during natural cooling of the melt. For instance, in a previous work we showed that CaMoO 4 crystallization tendency increased at the expenses of Na 2 MoO 4 when B 2 O 3 concentration increased in a SiO 2 -Na 2 O-CaO-MoO 3 glass composition [4]. In this study, we present structural results on two series (M x , B y ) of quenched glass samples belonging to this system using 29 Si, 11 B, 23 Na MAS NMR and Raman spectroscopies. The effect of MoO 3 on the glassy network structure is studied and its structural role is discussed (M x series). The evolution of the distribution of Na + ions within the borosilicate network is followed when B 2 O 3 concentration increased (B y series) and is discussed according to the evolution of the crystallization tendency of the melt. For all glasses, ESR was used to investigate the nature and the concentration of paramagnetic species. GLASS PREPARATION AND CHARACTERIZATION METHODS Two series of glasses were prepared for this study all derived from the following composition (mol.%): 58.2SiO 2 - 13.77Na 2 O - 9.81CaO - 18.08B 2 O 3 either by increasing MoO 3 concentration from 0 to 5.0 (M x series with x = 0, 0.87, 1.54, 2.50, 3.62 and 5 mol.% MoO 3 ) or by changing B 2 O 3 concentration from 0 to 24 mol.% (B y series with y = 0, 6, 12, 18 and 24 mol.% B 2 O 3 ) keeping constant MoO 3 concentration (2.50 mol.%). For all samples, 0.15 mol.% Nd 2 O 3 was introduced in composition both to facilitate 29 Si nuclei relaxation during MAS NMR experiments and to perform optical studies not presented in this paper [4]. Glasses were all prepared at 1300°C under air in Pt crucibles using reagent grade SiO 2 , CaCO 3 , Na 2 CO 3 , H 3 BO 3 , MoO 3 and Nd 2 O 3 powders. Depending on glass composition, samples were quenched either as cylinders or disks [4]. Several reference glass samples (borate and silicate glasses) were also prepared for comparison with M x and B y glasses (NMR and Raman spectra). The amorphous character of samples was checked using both X-ray diffraction