Dismantling nuclear waste rich in P2O5, MoO3 and ZrO2: How do these oxides incorporate in aluminoborosilicate glasses?

Highlights: • Incorporation in a glass of a dismantling waste rich in Zr, P and Mo • Impact of waste composition variation and waste loading on glass microstructure • P and Mo are the only elements that may lead to phase separation or crystallization • For Mo-rich compositions Cs may enter into molybdate crystals (CsLiMoO₄) Dismantling nuclear facilities leads to radioactive waste less active but which may have highly variable compositions compared to the high-level radioactive wastes recovered after the reprocessing of spent nuclear fuel. In this work, we studied the ability of an alkali-rich glass matrix belonging to the SiO₂-B₂O₃-Al₂O₃-Fe₂O₃-Na₂O-Li₂O-CaO system to solubilize P₂O₅, MoO₃, ZrO₂ and Cs₂O by melting at 1100°C. Phosphorus, molybdenum, zirconium and cesium are present as a mixture of complex compounds in the real radioactive dismantling waste containing ¹³⁷Cs considered here. To determine the capacity of the matrix to accept a wide range of variations of waste composition and the solubility limits of P₂O₅, MoO₃, and ZrO₂, several glass series were prepared by melting mixtures of raw materials (oxides, carbonates, phosphates) and by increasing the total amount of oxides representing the waste and varying their relative proportions. Their incorporation in the melt was studied by analyzing the microstructure of quenched glasses by XRD and SEM-EDS. In addition, the phase separation and crystallization tendencies during cooling were studied by analyzing the microstructure of glasses cooled at 1°C.min^-1 from 1100°C (representative of cooling in industrials steel canisters). It is shown that the glass can accept a wide range of waste compositions without exhibiting heterogeneities. For all compositions the melt remained homogeneous (study of quenched samples). However, during slow cooling, P₂O₅ and MoO₃ may lead to phase separation and crystallization of Na₂MoO₄, CsLiMoO₄, NaCaPO₄, NaLi₂PO₄, and Li₃PO₄. Cs can be partially incorporated into the molybdenum-rich phase CsLiMoO₄ when MoO₃ content is higher than 1.3wt%. ZrO₂ never lead to phase separation or crystallization, possibly because of the existence of strong connections between Zr and Si through Zr-O-Si bonds whereas P and Mo would be present as PO₄³⁻ and MoO₄²⁻ mobile entities. The increasing order of oxides solubility in the glass is the following: MoO₃2O₅2.