Scoping Studies of Wasteform Candidates for Li₂SO₄ Bearing Nuclear Waste Immobilisation

The secondary waste stream generated during Mo-99 production is challenging to immobilise due to the high concentrations of lithium (Li⁺) and sulphate ions (SO₄^2-), its acidic nature and its activity content. Glass and glass-ceramic wasteforms were explored in the current work as candidates to maximise SO₄^2-incorporation, and to provide sufficiently high waste loadings and chemical durability. The base glass systems considered were barium borosilicate glasses with specific compositions selected for targeted phase formation. The highest sulphate incorporation of 2.78 wt.% SO₃ (from waste loading of 11 wt.% as Li₂SO₄) in the glass wasteform was achieved without crystallisation following melting at 1200 °C. An immiscible sulphate layer rich in BaSO₄ and Na₂SO₄ formed on top of the glass at lower temperature (800–1100 °C) and ~65% of the SO₃ was lost during high temperature consolidation. Therefore, tailored glass-ceramic options that can be prepared at lower temperatures with suitably high waste loadings were considered. Single/multi phase ceramics were observed within the glass matrix at 1000 °C with 14-18 wt.% waste loading. Glass and glass-ceramics wasteforms were studied <i>via</i> XRD, SEM-EDS, XRF, Raman and thermal analysis. The chemical durability was assessed using the ASTM C1285 Product Consistency Test (PCT) standard protocol and evaluated relative to accepted high-level nuclear waste glasses.