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

Abstract

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 Zr1−x−yCexGdyO2−0.5y and a lanthanoid (Nd,Gd,La,Ce) silicate. Zr1−x−yCexGdyO2−0.5y 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 Ce1−x−yZrxGdyO2−0.5y, 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), Zr1−x−yCexGdyO2−0.5y (93Zr, Pu and U), both lanthanoid (Nd,Gd,La,Ce) silicates (Am and Cm), (Na,Sr,Nd,La)MoO4 (90Sr, Am and Cm), Ce1−x−yZrxGdyO2−0.5y (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.