Devitrified and Phase Separated Material Found in Simulated High Level Nuclear Waste Glasses Containing Ca and Zn Additions

Abstract

In the UK, blended high level nuclear waste (HLW) streams from the Magnox and THORP reprocessing plants are currently vitrified using a lithium sodium borosilicate base glass frit. Laboratory and full size non-radioactive simulations (produced on the Vitrification Test Rig at Sellafield [1]) of these compositions have shown that these glasses need to be melted at circa 1050°C to obtain a reasonable viscosity for pouring. Also, at high waste loadings an alkali molybdate phase (termed “yellow phase”) can form in these glasses [e.g. 2, 3]. Vitrification flowsheets are set to avoid yellow phase formation as this phase is highly corrosive to the inconel melter in the molten state and is partially water soluble at ambient temperature and so may challenge product quality.Ca and Zn additions to the base glass frit have been found to reduce viscosity and allow melt homogeneity and pouring at lower temperatures. It was also theorised that Ca additions could increase the solubility of Mo and thus reduce the likelihood of yellow phase formation. The composition of the phase separated material in as-cast and heat treated specimens of Ca and Zn HLW glasses produced at both laboratory and full scale is examined in this work

The Structural Role of Zn in Nuclear Waste Glasses

“…First, the waste loading of U.K. HLW glass product is typically in the range 25–28 wt% (on the basis of metal oxides), with a possible move toward higher incorporation levels in the future. In comparison, the waste loading of the comparable French product is typically 15–18 wt% . Second, although both processes employ a sodium lithium borosilicate glass frit, the composition employed at la Hague contains different amounts of SiO 2 , B 2 O 3 , Li 2 O, and Na 2 O to the U.K. glass and, crucially, also contains additions of ZnO and CaO which are believed to confer certain beneficial effects, as summarized below .…”

Calculating entropies of alkaline earth metal molybdates

“…Immobilizing molybdenum containing nuclear waste as borosilicate glasses can, at high waste loadings, result in the formation of an alkali and alkaline earth metal containing molybdate phase (termed ''yellow phase'') [1]. In the molten state this phase is highly corrosive to the inconel melter.…”

Modeling the Structure of Complex Aluminosilicate Glasses: The Effect of Zinc Addition

“…7,8 Moreover, similar compositions have also recently found interest in the immobilization of nuclear waste. 9 The nonspecific structural role of zinc in silica-based glasses is confirmed by the literature. Cassingham et al, 10 in 2011, revealed the presence of (ZnO 4 ) 6− tetrahedra participating in network formation by combining XANES and EXAFS zinc Kedge measurements on Na 2 O−CaO−ZnO−SiO 2 glasses.…”

“…In the present paper, we aim to unravel the structure of six complex aluminosilicate glasses with different zinc amounts, paying particular attention to the role of this element. These glasses have different applications, in particular, in some ceramic materials like glazes, where glass is the dominant phase (about 90 wt %), affecting in turn the technological properties of the whole material, due to the increased chemical durability and mechanical processing ability as zinc is introduced in the structure. , Moreover, similar compositions have also recently found interest in the immobilization of nuclear waste …”