Crystallisation of a simulated borosilicate high-level waste glass produced on a full-scale vitrification line

A simulated (inactive) borosilicate high-level waste (HLW) glass was produced on a full-scale vitrification line with composition simulating vitrified oxide fuel (UO₂) reprocessing waste. As-cast samples were compositionally homogeneous (Type I microstructure) and/or compositionally inhomogeneous displaying compositional ‘banding’ and frequently containing ‘reprecipitated calcine’ (Type II microstructure). Crystal phases identified in as-cast samples were: tetragonal RuO₂, cubic Pd-Te alloy, cubic (Cr,Fe,Ni,Ru)₃O₄, trigonal Na₃Li(MoO₄)₂·6H₂O, ostensibly cubic Zr_{1 − x − y}Ce_xGd_yO_2 − 0.5y and a lanthanoid (Nd,Gd,La,Ce) silicate. Zr_{1 − x − y}Ce_xGd_yO_2 − 0.5y and lanthanoid (Nd,Gd,La,Ce) silicate were found exclusively in the Type II microstructure as component crystal phases of ‘reprecipitated calcine’. Heat treated samples (simulating the retarded cooling experienced by actual (active) borosilicate HLW glasses after pouring) displayed extensive crystallisation and cracking (Type A microstructure) and/or ‘banded’ crystallisation (Type B microstructure) depending on their parent (as-cast) microstructure (Type I and/or Type II respectively). Crystal phases identified in heat treated samples were: tetragonal SiO₂ (α-cristobalite), tetragonal (Na,Sr,Nd,La)MoO₄, cubic Ce_{1 − x − y}Zr_xGd_yO_2 − 0.5y, a Ni-rich phase, a lanthanoid (Nd,Gd,La,Ce) silicate and orthorhombic LiNaZrSi₆O₁₅ (zektzerite). α-cristobalite was found exclusively in the Type A microstructure, while lanthanoid (Nd,Gd,La,Ce) silicate and zektzerite were only found in the Type B microstructure. Potential host phases for HLW radionuclides are: Pd-Te alloy (¹⁰⁷Pd and ⁷⁹Se), (Cr,Fe,Ni,Ru)₃O₄ (⁶³Ni), Zr_{1 − x − y}Ce_xGd_yO_2 − 0.5y (⁹³Zr, Pu and U), both lanthanoid (Nd,Gd,La,Ce) silicates (Am and Cm), (Na,Sr,Nd,La)MoO₄ (⁹⁰Sr, Am and Cm), Ce_{1 − x − y}Zr_xGd_yO_2 − 0.5y (⁹³Zr, Pu and U), the Ni-rich phase (⁶³Ni) and zektzerite (⁹³Zr, ¹²⁶Sn and U). Cracking in samples was attributed to thermal expansion mismatch between the borosilicate HLW glass matrix and RuO₂, cristobalite (both α and β), (Na,Sr,Nd,La)MoO₄ and zektzerite on cooling. There was also a contribution from the cristobalite α-β phase transition.