Crystallization of Titanosilicate Glasses for Nuclear Waste Immobilization

Abstract

The effects of different cooling and reheating rates on the phase constitutions of Na 2 O–Al 2 O 3 –TiO 2 –SiO 2 glass‐ceramics containing up to 20 wt% of simulated nuclear fuel recycle waste have been studied using X‐ray diffraction, differential thermal analysis, and scanning and transmission electron microscopy. The metastable formation of a perovskite‐structured phase (nominally CaTiO 3 , but containing ionic substituents) was observed in samples with up to 15 wt% of simulated waste after cooling from the melt at rates between 0.25 and 50°C/min. When the partially devitrified glass was reheated to 1050°C, incomplete conversion of this phase to sphene (nominally CaTiSiO 5 ) occurred by reaction with the silica‐rich glass matrix. The conversion was completed by heating further to 1150°C. Waste loadings ⋝10 wt% produced crystallization of powellite (nominally CaMoO 4 ) in addition to sphene and perovskite, whereas metastable perrierite (a rare‐earth titanosilicate) was also crystallized at waste loadings ⋝15 wt%. New data on elemental partitioning between the crystalline and vitreous phases confirmed earlier results obtained in different atmospheres and with simplified waste compositions and were largely in accord with crystal‐chemical predictions.