Effect of FLiNaK Salt Additive on the Properties of Borophosphate Glasses

Abstract

This paper investigates borophosphate glasses containing FLiNaK salt additive, a eutectic mixture of alkali metal fluorides (46.5 mol.% LiF, 11.5 mol.% NaF, and 42 mol.% KF) that is a byproduct of developing molten salt nuclear reactors and a form of radioactive waste. The synthesized glasses exhibit a high degree of FLiNaK incorporation, reaching 20 – 25 wt.%, while maintaining a homogeneous, amorphous structure and uniform elemental distribution. However, at FLiNaK concentrations exceeding 25 wt.%, the glasses crystallize, forming an elpasolite (K₂NaAlF₆) phase. X-ray diffraction analysis reveals specific trends in the fluoride content of the glass. These trends suggest a correlation between the transformation of the glass local structure and the formation of a crystalline phase. This relationship will be examined more closely in future studies. Studies of the chemical and mechanical stability of synthesized glasses containing up to 20 wt.% FLiNaK demonstrate that these materials meet regulatory requirements for glass matrices used to immobilize radioactive waste. Additionally, the dependence of these properties on FLiNaK content shows an extremum at a concentration of 7 wt.%. However, no direct correlations with structural changes have been identified.