Effect of molybdenum on the structure and on the crystallization of SiO2-Na2O-CaO-B2O3 glasses

Crystallization of the poorly durable Na2MoO4 phase able to incorporate radioactive cesium must be avoided in SiO2-Al2O3-B2O3-Na2O-CaO glasses developed for the immobilization of Mo-rich nuclear wastes. Increasing amounts of B2O3 and MoO3 were added to a SiO2-Na2O-CaO glass, and crystallization tendency was studied. Na2MoO4 crystallization tendency decreased with the increase of B2O3 concentration whereas the tendency of CaMoO4 to crystallize increased due to preferential charge compensation of BO4- entities by Na+ ions. Si-29 MAS NMR showed that molybdenum acts as a reticulating agent in glass structure. Trivalent actinides surrogate (Nd3+) were shown to enter into CaMoO4 crystals formed in glasses.