Thermodynamics and kinetics of crystallization of near-liquidus crystalline phases in defense nuclear waste glass melts

Vitrification has been identified as the primary mode of nuclear waste immobilization. Two important factors that affect the economic success of the vitrification program are waste loading and melter life. High-level nuclear waste (HLW) streams often contain significant amounts of Fe, Ni, Mn and Cr. Some of these waste streams contain significant amounts of Zr, Th & U. This often leads to the crystallization of iron-chrome spinels or oxides/silicates of Zr, Th and U as the near-liquidus phases. The waste loading that can be achieved is often limited by crystallization of these near-liquidus phases. The settling of these phases can reduce the melter life. Hence understanding the crystallization of these crystalline phases is important in predicting the behavior of the HLW glasses in the melter environment.; Representative HLW glasses were selected and heat treated at different temperatures and times. The compositions of the glassy phase, composition, morphology and volume fraction of the crystalline phases have been analyzed to understand thermodynamics and kinetics of crystallization of these near liquidus phases.; The spinel compounds observed in HLW glasses exhibit extensive solid solution ranges depending on the temperature and glass composition, with Cr/Rh-rich spinels at one end, favored at higher temperatures and Fe-Cr spinels at the other end. For the first time Rh-containing spinel (up to 48 wt% Rh2 O3) has been identified as a liquidus phase. This Rh-rich spinel crystallization increases the liquidus temperature by about 150--200°C, even though Rh2O3 is present as low as 0.05wt%. The economic impact of this is huge.; Using the transformation-time data measured at different temperatures for Zr, Th or U compounds, kinetic parameters were modeled using Avrami equation. Isokinetic crystallization is observed for baddeleyite and thorianite while non-isokinetic behavior is observed for zircon and parakeldyshite. Presence of a significant second phase or dramatic change in the morphology strongly influences the crystallization kinetics. The kinetic parameters depend on the liquidus phase, its volume fraction and temperature. The kinetic parameters obtained for crystallization of Zr, Th and U oxides or silicates can be used for designing HLW glass formulation as well as testing parameters.