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.

Structural analysis of some sodium and alumina rich high-level nuclear waste glasses

“…), spinel crystallization is known to have little or no effect on durability of nuclear waste glasses [5] because the crystal structure does not require glass network-forming components. Also, CaO content did not play significant role in suppressing [9] on the basis of correlation between ND and OB concept. We could not obtain clear evidence with respect to the impact of Al 2 O 3 on nepheline crystallization in glasses as a function of high-boron content because the glasses with high Al 2 O 3 were poor in boron.…”

“…However, the impact of such a structural change in glasses on their crystallization behavior is yet to be investigated. Furthermore, although the concept of correlation between ND and OB, recently proposed by McCloy et al [9] to explain the tendency of nepheline formation in HLW glasses, is not violated by any of the glasses that precipitated nepheline (17,19,33,35,39). However, it fails to explain the absence of nepheline crystallization in the other glasses.…”

“…Unfortunately, the ND criterion cannot take into account the influence of these oxides while designing the waste glass compositions. In order to overcome the limitations faced by the ND model, McCloy et al [9] suggested a different metric for influence of oxides on the propensity for nepheline formation in glasses through consideration of 'optical basicity' (OB) concept. According to the OB concept, more basic cations (e.g., CaO, K 2 O) are more likely to cause aluminosilicate crystallization as they readily donate valence electrons and thus get removed from the covalent glass network as opposed to more acidic cations (e.g., B 2 O 3 ).…”

Kinetic model for quartz and spinel dissolution during melting of high-level-waste glass batch

“…Solid particles, also called stones, whether incompletely dissolved raw materials or precipitated crystals, are unacceptable in commercial glass products. In HLW glasses, solid particles become an issue only when they present a processing problem or when they compromise glass durability [7,8,9,10].…”

“…Various kinds of solid particles are problematic, such as incompletely dissolved quartz grains [11,12], crystals of spinels [13], or crystals of nepheline [9]. In this study, we confine our attention to particles that enter molten glass from the cold cap, a layer of reacting glass batch floating on the surface of melt.…”