Processing Impurities as Phase Assemblage Modifiers in Titanate Nuclear Waste Ceramics

The tolerance of titanate nuclear waste ceramics to fluctuations in the concentrations of processing contaminants was monitored using X-ray diffraction, electron microscopy, secondary ion mass spectrometry, and analysis of dissolution liquors. Several waste forms were fabricated, all of which contained idealized Purex waste simulant and, in addition, varying quantities of the common waste stream impurities P₂O₅, MgO, Fe₂O₃, Na₂O, and SiO₂. Incorporation of the oxides individually stabilized new phases including monazite (P₂O₅), pseudobrookite (MgO), loveringite (Fe₂O₃), freudenbergite (Na₂O), and pollucite (SiO₂)—only the latter phase deleteriously affected waste form performance by promoting cesium dissolution. However, when the processing contaminants were added simultaneously, a number of synergetic effects, particularly the stabilization of a soluble glassy phase, resulted in elemental losses which were an order of magnitude greater for some matrix and radwaste species. It was found that up to 25 wt% of the idealized Purex waste could be incorporated in the waste without diminution of its properties.