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Titanate ceramic waste forms were prepared using several combinations of calcination atmosphere (N{sub 2}, N{sub 2} {minus} 3.5% H{sub 2},h{sub 2}) and metallic redox buffers (Ni,Fe,Ti,Al) to examine the dependence of microstructure and durability upon oxygen activity. It was found that the microstructures and phase assemblages were mostly insensitive to the fabrication method, although in detail systematic changes were recognized. The correlation of aqueous durability with oxygen fugacity was not straightforward because of density variations in the hot-pressed ceramics. These fluctuations in density dominated the dissolution characteristics of the waste forms and sometimes obscured the more subtle changes associated with redox potential. It is concluded that although the best durability is achieved at lower fugacities (i.e., Ti metal buffer and H{sub 2} calcination atmosphere), a satisfactory product can be produced using any of the preparative routes examined, provided the material is completely densified.

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 P{sub 2}O{sub 5}, MgO, Fe{sub 2}O{sub 3}, Na{sub 2}O, and SiO{sub 2}. Incorporation of the oxides individually stabilized new phases including monazite (P{sub 2}O{sub 5}), pseudobrookite (MgO), loveringite (Fe{sub 2}O{sub 3}), freudenbergite (Na{sub 2}O), and pollucite (SiO{sub 2})--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.

A titanate ceramic designed to immobilize high-level waste generated by Amine reprocessing of heavy-water reactor fuel and fabricated by uniaxial hot-pressing was characterized. X-ray diffraction and selected-area electron diffraction were used to identify a six-phase assemblage consisting of betafite, a hollandite structure type, perovskite, uraninite, hibonite, and Magneli phases. Secondary electron imaging of polished surfaces revealed many microvoids consistent with a measured density of 5.25 g · cm−2 (∼90% of theoretical density). The waste form was chemically heterogeneous at the hundreds of micrometers scale, as backscattered electron imaging and energy-dispersive X-ray spectroscopy indicated that regions rich in either uranium or titanium oxides were common. Grain sizes ranged from 0.1 to 0.5 μm. All crystals were faceted with the exception of anhedral uraninite grains. Dissolution experiments conducted at 90°C in distilled water gave uranium and titanium loss rates which were solubility limited (10−1 to 10−4 g · m−2· d−1), while cesium dissolution, which was not contrained by solubility limits, was more rapid (23.3 g · m−2· d−1). Hydrothermal testing at 150°C in distilled water resulted in precipitation of uranium(VI) titanate (UTiO5) and brookite (TiO2).