In nuclear borosilicate glasses, when molybdenum is in too high concentration and when it combines with other elements such as alkali and alkaline-earth elements it may form crystalline molybdates, including sodium molybdate, Na2MoO4, during melt cooling. In a nuclear vitrification context, the origin of this phenomenon must be understood to control and to avoid the appearance of this water-soluble crystalline phase. The solubility limit of MoO3 was found to be 2.5 mol% in a simplified SiO2-B2O3-Na2O-CaO nuclear glass at about 1300 degrees C. Higher MoO3 concentrations induced liquid phase separation followed by crystallization of Na2MoO4 and CaMoO4. This study assessed the impact of increasing the CaO and B2O3 content on the tendency of the melts to crystallize and the impact on the glass network structure. Structural analysis (Mo-95 MAS NMR and B-11 MAS NMR) of several glass series and standard SiO2-Na2O-MoO3 or SiO2-CaO-MoO3 glass showed that the nature of the crystallized phases that may appear during cooling of the melt can be controlled by correlation of the proportion of Na+ cations remaining free in the glass network with the soda/lime environment of tetrahedral MoO42- entities.