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A comprehensive study on the development of sodium zirconium phosphate (NZP)‐based glass‐ceramic composites as potential wasteforms for the immobilization of nuclear wastes is reported. Two complementary waste treatment routes, the ex situ and in situ crystallisation of NZP with a sodium aluminoborosilicate glass, were investigated with various processing conditions including sintering temperature, cooling rate and NZP to glass ratios. While the ex situ route with mixing of pre‐made NZP and glass is a robust and reliable means of producing the glass‐ceramic composites, the in situ crystallisation of NZP from an amorphous NZP precursor is a more realistic processing route. The formation of ZrO2 as a minor phase was observed especially for high NZP to glass ratios due to the solubility difference between Zr and P oxides in glass. The addition of extra phosphate can overcome this and yield glass‐ceramic composites with appropriate NZP stoichiometry. Overall, the NZP glass‐ceramic system is versatile offering multiple processing options for nuclear waste management. [ABSTRACT FROM AUTHOR]

Ln2Ti2O7 (Ln=Y and Gd) pyrochlore glass-ceramics have been fabricated successfully via internal crystallization. Subsequently, the phase evolution from Ln2Ti2O7 pyrochlores to Ln0.5U0.5Ti2O6 brannerites in glass with uranium (U) substitutions on the Ln-site of Ln2Ti2O7 has been investigated using X-ray diffraction, scanning electron microscope-electron dispersive spectroscopy, transmission electron microscopy, Raman and diffuse reflectance spectroscopy. Combined characterization by XRD, SEM- EDS and TEM SAED confirms the structures and phase evolution while Raman spectroscopy reveals characteristic vibration modes for both pyrochlore and brannerite. In addition, DRS of the U5+ ion has been used to probe the phase evolution, with the corresponding f- f transition band of 2 F7/2 energy level significantly shifting to longer wavenumbers due to the local coordination environment changing from eightfold coordination in pyrochlore to sixfold coordination in brannerite. [ABSTRACT FROM AUTHOR]

Brannerite-based glass-ceramics have been developed as potential waste forms for the immobilization of actinide-rich radioactive wastes. For the first time, the formation of brannerite phases in glass has been demonstrated using uranium (U) and plutonium (Pu) with additions of gadolinium and hafnium as neutron absorbers. Both XRD and SEM- EDS confirm that brannerite is the dominating phase with compositions close to Y0.5U0.5Ti2O6, Gd0.2Pu0.3U0.5Ti2O6, and Gd0.1Hf0.1Pu0.2U0.6Ti2O6 internally crystallized in the glass. TEM SAED and Raman spectroscopy reveal the typical structure and vibration modes for brannerite. In addition, the presence of U5+ species as designed in the formulations has been confirmed by diffuse reflectance spectroscopy. More importantly, the U and Pu were partitioned exclusively in the ceramic phases with no detectable actinide in the glass. [ABSTRACT FROM AUTHOR]

Glass-ceramics (GCs) based on titanate pyrochlores have attracted recent attention as candidate waste forms for actinide immobilization. As zirconate pyrochlore has a superior radiation resistance, it is anticipated that Zr substitution of Ti in titanate pyrochlore GCs would increase their potential for waste form applications. The concept was primarily addressed via the preparation of Gd 2 Ti 2-x Zr x O 7 (x = 0.1–0.3) GCs to determine the effects of sintering temperature, Zr substitution, and pyrochlore content (50–70 wt%) on the properties of the resultant GCs. XRD and SEM analyses were used to reveal the phase assemblages and microstructures while TEM and Raman spectroscopy were used to analyze the local structures. XRD results confirmed the formation of the targeted pyrochlore as the major phase, with Gd 9.33 (SiO 4) 6 O 2 oxyapatite present as a minor phase. SEM analyses revealed that up to 0.2 formula units of Ti could be substituted by Zr under the processing conditions. The pyrochlore crystallite sizes were largely controlled by the sintering temperature and cooling rate and showed little sensitivity to the glass contents. This work has demonstrated successful substitutions of Ti with Zr in Gd 2 Ti 2 O 7 GCs as potential waste forms for actinide wastes owing to their superior radiation resistance. The feasibility of Zr substitution of Ti in Gd 2 Ti 2 O 7 glass-ceramics for improving radiation resistance of titanate pyrochlore glass-ceramics as potential composite waste forms for actinide immobilization has been investigated. It has been demonstrated that up to 0.2 formula units of Zr can replace Ti in Gd 2 Ti 2 O 7 to form titanate-zirconate pyrochlore glass-ceramics with major crystalline pyrochlores and a minor Gd-silicate oxyapatite phase. [Display omitted] • Zr substitution of Ti in Gd 2 Ti 2 O 7 glass-ceramics for better radiation tolerance. • Up to 0.2 formula units of Zr can replace Ti under the experimental conditions. • Phase assemblage, structure and microstructure have been investigated. • The glass-ceramics contain major pyrochlore and minor oxyapatite. [ABSTRACT FROM AUTHOR]