  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, Mo6+ 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 CaMoO4 crystallization
tendency increased at the expenses of Na2MoO4 when B2O3 concentration increased
in a SiO2-Na2O-CaO-MoO3 glass composition [4]. In this study, we present
structural results on two series (Mx, By) of quenched glass samples belonging
to this system using 29Si, 11B, 23Na MAS NMR and Raman spectroscopies. The
effect of MoO3 on the glassy network structure is studied and its structural
role is discussed (Mx series). The evolution of the distribution of Na+ ions
within the borosilicate network is followed when B2O3 concentration increased
(By 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.
