PNNL

Abstract

La (III) is considered as a surrogate for Pu (III) in the development of radioactive waste glasses. The addition of La (III) in some sodium alumino-borosilicate glasses causes phase separation, but in others, it causes crystallization of sodium lanthanum silicate. In this study, we characterize some clear glasses and some phase-separated La-containing glasses using optical microscopy, X-ray diffraction, and transmission electron microscopy. The baseline glass (without La2O3) in which phase separation first occurs with the addition of La2O3 is more heterogeneous than the baseline glass in which crystallization first occurs with the addition of La2O3. They all consist of two portions: a borate portion and a silicate portion. As La2O3 is added, it is incorporated in the borate portion first, and causes a phase rich in La and B separated from the melt. Increasing excess Na, i.e., Na-Al, or excess Al, i.e., Al-Na, can suppress the phase separation. When more La2O3 is added, it is incorporated in the silicate portion, and causes the crystallization of sodium lanthanum silicate in the melt.