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As potential waste forms for immobilizing actinide‐rich radioactive wastes, Eu2Ti2O7 (Eu as a surrogate for minor actinides) pyrochlore glass‐ceramics were fabricated via hot isostatic pressing (HIPing) at 1200°C. The structure and microstructure at the reaction interface between the glass‐ceramic waste form and the stainless steel (SS) canister under HIPing conditions were carefully investigated with scanning electron microscopy (SEM), transmission electron microscopy (TEM), and synchrotron single crystal X‐ray diffraction (SC‐XRD). The interactions at the reaction interface led to the formations of a ~10‐µm‐thick Cr2O3 layer as the oxidation front of the SS and a layer of a mixed oxide phase (Eu1.25SiCr0.8Ti1.2O7.5) on the glass‐ceramic side of the reaction interface. The crystal structure of such a unique mixed oxide phase was revealed indubitably with a combination of synchrotron SC‐XRD and TEM assisted with a focused ion beam (FIB) SEM system. The improved structural understanding of the reaction interface will help to support the utilization of HIPing as a versatile hot consolidation process for the treatment of radioactive wastes. [ABSTRACT FROM AUTHOR]

[Display omitted] Pyrochlore glass ceramics have been fabricated via in-situ crystallization under reducing conditions by both sintering and hot isostatic pressing (HIPing) as candidate wasteforms for the acidic waste biproduct of Mo-99 radiopharmaceutical production. The tailored wasteform demonstrates flexibility in the wasteform design to receive the required waste variability, it also suitably passes high-level waste performance requirement, and successfully scales to 1 kg scale with 45 wt.% waste loading. U-rich pyrochlore as the major phase was confirmed by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy, with residual glass and minor secondary phases. The presence of both U4+ and U5+ valences in the wasteforms was revealed by diffuse reflectance spectroscopy. Addition of glass content had little influence on the pyrochlore composition but facilitated minor perovskite formation. The up-scaled dense, HIPed sample showed elemental releases of < 2 g/L for all elements in durability experiments. [ABSTRACT FROM AUTHOR]

Lanthanum zirconate has been prepared via a new chemical synthesis method by combining sol-gel processing and complex precipitation. The synthesis was carried out in aqueous solution under ambient conditions. The synthesized powder possessed the pyrochlore superstructure upon calcination above 1200°C. A suite of characterization techniques, including FTIR, Raman, X-ray and electron diffraction, TEM, SEM, and nitrogen sorption were employed to investigate the microstructural evolution and bulk properties. Dense ceramics (>90% relative density) were obtained after sintering at 1400°C, without need for additional processing (i.e., hot or cold isostatic pressing) or any milling steps. A mechanism has been proposed that elucidates molecular assembly of this chemical synthesis method. [ABSTRACT FROM AUTHOR]

Pyrochlore glass‐ceramics (GCs) have been investigated with samples fabricated via both sintering and hot isostatic pressing (HIPing) of a mixed oxide precursor. It has been demonstrated that sintering at 1200°C in air is necessary to obtain well‐crystallized pyrochlore crystals in a sodium aluminoborosilicate glass through a one‐step controlled cooling. The crystallization, structure, and microstructure of Eu2Ti2O7 pyrochlore as the major phases in residual glass were confirmed with X‐ray diffraction (XRD), scanning electron microscopy‐energy dispersive spectroscopy, transmission electron microscopy, and Raman spectroscopy. The structures of major Eu2Ti2O7 pyrochlore and minor [Eu4.67O(SiO4)3] apatite in both sintered and HIPed samples were refined using synchrotron XRD data. While the processing atmosphere did not appear to affect the cell parameter of the main pyrochlore phase, very small volume expansion (~0.3%) was observed for the minor apatite phase in the HIPed sample. In addition, static leaching of the HIPed sample confirmed that pyrochlore GCs are chemically durable. Overall, pyrochlore GCs prepared via both sintering and HIPing with the Eu partitioning factor of ~23 between ceramics and the residual glass are suitable waste forms for minor actinides with processing chemicals. [ABSTRACT FROM AUTHOR]

Graphical abstract Both Y 2 Ti 2 O 7 and Gd 2 Ti 2 O 7 pyrochlore glass-ceramics have been prepared with a typical oxide route and consolidated via hot isostatic pressing, and characterized as potential waste forms for the immobilization of actinide-rich radioactive wastes. Abstract Both Y 2 Ti 2 O 7 and Gd 2 Ti 2 O 7 pyrochlore glass-ceramics (GCs) have been prepared with an oxide route and consolidated via hot isostatic pressing (HIPing) as waste forms for actinide immobilization. The crystallization of Y 2 Ti 2 O 7 /Gd 2 Ti 2 O 7 pyrochlore as the major actinide host phase in the residue glass has been confirmed with XRD, SEM-EDS and Raman spectroscopy. The static leaching tests confirmed that both GCs are chemically durable with normalized Y/Gd and Ti leach rates ∼10−4–10-5 g m-2 d-1 after 28 days. In addition, the interactions between the 316L stainless steel (SS) HIP-can and the GCs have been investigated. Interactions occur at the interface with a layer of mixed Cr/Ti oxide and CrTi 2 O 5 crystals observed for the Y 2 Ti 2 O 7 GC and a layer (∼10 μm) of Cr 2 O 3 observed for the Gd 2 Ti 2 O 7 GC as the reaction fronts from the SS cans. Furthermore, a layer (∼10 μm) of Gd-(Si/Ti)-oxide phase formed on the GC side of the interface. [ABSTRACT FROM AUTHOR]