Titanate Ceramics for the Stabilization of Partially Reprocessed Nuclear Fuel Elements

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 Magnéli 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).