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

• 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.

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 TC 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 TC 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).

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