Thermal stability of SiO₂–B₂O₃–Al₂O₃–Na₂O–CaO glasses with high Nd₂O₃ and MoO₃ concentrations

The incorporation of high MoO₃ amounts in borosilicate glasses developed for the immobilization of radioactive waste may lead to the crystallization of Mo-rich phases that may induce a decrease of the long term performances of glasses. It is thus essential to understand their crystallization mechanisms and the possible effect of other abundant fission products present in the wastes (such as rare earths) in order to control or to avoid their formation during glass preparation. This paper presents a study, performed by X-ray diffraction, scanning electron microscopy, Raman and optical absorption spectroscopies of the stability as a function of the thermal treatment temperature T_C of a simplified Mo-rich nuclear waste glass. The impact of the addition of a high amount of Nd₂O₃ on the thermal stability of this glass is studied. For comparison, the thermal stability of a Nd₂O₃-rich glass without Mo is also presented. The crystallization range of all phases formed in these glasses (CaMoO₄, Na₂MoO₄, Ca₂Nd₈(SiO₄)₆O₂ (apatite)) and the evolution of their structure and microstructure as a function of T_C are presented. The introduction of Nd₂O₃ in the MoO₃-rich glass inhibits the crystallization of molybdates (increase of Mo solubility), as long as apatite does not form which suggests that [MoO₄]^2- entities and Nd³⁺ cations are close to each other in the glass structure. Besides, when a high density of apatite crystals form, for instance from glass surface, small Mo-rich partially crystallized globular heterogeneities are observed between these crystals that exacerbate the nucleation of new apatite crystals (nucleating effect). - Highlights: • Effect of MoO₃ and Nd₂O₃ separately or together on glass crystallization is studied. • The crystallization range of molybdates and apatite (Ca₂Nd₈(SiO₄)₆O₂) was determined. • The evolution of crystals structure and microstructure with temperature is presented. • Nd₂O₃ inhibits molybdates crystallization of as long as apatite does not form. • Mo-rich partially crystallized heterogeneities have a nucleating effect on apatite.