Nepheline Crystallization in Nuclear Waste Glasses: Progress Toward Acceptance of High‐Alumina Formulations

Abstract

Historical data have been critically compiled and analyzed for investigating the quantity of nepheline (NaAlSiO4) precipitated as a function of composition in simulated nuclear waste glasses. To understand compositional effects two primary methods were used: (1) investigating the Al2O3–SiO2–Na2O ternary while filtering for different B2O3 levels and (2) creating a quadrant system consisting of compositions reduced to two representations: (i) the nepheline discriminator (ND) which depends only on the SiO2 content by weight normalized to the total weight of the Al2O3–SiO2–Na2O submixture and (ii) the optical basicity (OB) which contains contributions from all constituents in the glass. Nepheline precipitation is expected to be suppressed at high SiO2 levels (ND > 0.62) or at low basicities (OB < 0.55–0.57). Changes in sodium aluminosilicate glass OB values due to additions of CaO and B2O3 correlate with the observed effects on nepheline formation. It is proposed that additional composition space is available for formulating high waste loading, high‐Al2O3 nuclear waste glasses when consideration is given to location on the Al2O3–SiO2–Na2O submixture as well as OB.