Titanate Ceramics for the Immobilization of Sodium-Bearing High-Level Nuclear Waste

The phase chemistries and microstructures of titanate-based ceramics containing simulated high-level nuclear waste with varying sodium contents were compared. Incorporation of relatively low sodium levels (∼0.9 wt% Na₂O) resulted in more complex phase assemblages. The principal hosts for sodium were hibonite and freudenbergite, and, when iron and sodium were present in combination, loveringite was also stabilized. During fabrication, oxygen potential was controlled by Ti-TiO₂ or TiH₂-Ti₃O₅ solid-state buffers. These metal and hydride oxygen getters behaved similarly, neither disturbing the phase assemblages nor significantly altering the partitioning of waste elements between radiophases. It is believed that the hydrothermal stability of the sodium-bearing ceramics (containing up to 2.7 wt% Na₂O) will be comparable to sodium-free material, since less durable sodium-rich phases are encapsulated in a resistant matrix. Extensive formation of glassy phases may cause embrittlement at higher sodium loadings.