Phase evolution from Ln₂Ti₂O₇ (Ln=Y and Gd) pyrochlores to brannerites in glass with uranium incorporation

Abstract

Ln 2 Ti 2 O 7 (Ln=Y and Gd) pyrochlore glass-ceramics have been fabricated successfully via internal crystallization. Subsequently, the phase evolution from Ln 2 Ti 2 O 7 pyrochlores to Ln 0.5 U 0.5 Ti 2 O 6 brannerites in glass with uranium (U) substitutions on the Ln-site of Ln 2 Ti 2 O 7 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 U 5+ ion has been used to probe the phase evolution, with the corresponding f-f transition band of 2 F 7/2 energy level significantly shifting to longer wavenumbers due to the local coordination environment changing from eightfold coordination in pyrochlore to sixfold coordination in brannerite.

Structural and spectroscopic investigations on the crystallization of uranium brannerite phases in glass

“…The short‐term static leaching tests confirmed that these brannerite GCs are chemically durable and suitable waste forms for immobilizing actinide wastes with processing chemicals. In addition, phase evolution from pyrochlore to brannerite in glass with the incorporation of uranium was also studied . In this work, we aim to further investigate the crystallization of uranium brannerite phases in a borosilicate glass with the incorporation of yttrium (Y)/cerium (Ce)/europium (Eu) focusing on the formation of brannerites, structures, phase compositions, microstructures, vibration modes, and uranium valences.…”

Pyrochlore glass‐ceramics fabricated via both sintering and hot isostatic pressing for minor actinide immobilization

“…In addition, the crystallization of uranium (U) pyrochlore in glass had been conducted to validate the concept, 22 followed by the successful preparation of both U and/or plutonium (Pu) pyrochlore GCs via sintering in air with the partitioning of U/Pu dominantly in the durable pyrochlore phase 22 . Moreover, the phase evolution from pyrochlore to brannerite in glass with the addition of U has been realized confirming the feasibility of having mixed pyrochlore and brannerite in one GC system 26 …”

“…A mixed oxide precursor was adopted to target the crystallization of Eu 2 Ti 2 O 7 pyrochlore in a sodium aluminoborosilicate glass (Na 2 AlBSi 6 O 16 ) with pyrochlore to glass weight ratio of 3:2. The glass composition (with aluminum replacing half of boron in a sodium borosilicate glass) was chosen largely due to its demonstrated excellent chemical durability 22‐26 . The required amounts for a 120.0 g batch (oxide basis) of Na 2 CO 3 , Al 2 O 3 , H 3 BO 3 , SiO 2 , TiO 2 , and Eu 2 O 3 were mixed and calcined at 725°C in air for 2 hours.…”

“…To overcome the disadvantages for either glass or ceramic waste forms, composite materials, for example glass‐ceramics (GCs), have been developed as potential waste forms for actinide waste management 16‐18 . The GC waste forms have the advantage of combining the simple processing and chemical flexibility of glasses (to accommodate processing chemicals) with the excellent chemical durability of ceramics (to host actinides) for the immobilization of actinide‐rich radioactive wastes containing impurities 17‐29 . So far, GCs based on stable titanate mineral phases, such as zirconolite, 17‐21 pyrochlore, 22‐26 and brannerite, 27‐29 have been primarily investigated as potential waste forms for actinide immobilization.…”

“…[16][17][18] The GC waste forms have the advantage of combining the simple processing and chemical flexibility of glasses (to accommodate processing chemicals) with the excellent chemical durability of ceramics (to host actinides) for the immobilization of actinide-rich radioactive wastes containing impurities. [17][18][19][20][21][22][23][24][25][26][27][28][29] So far, GCs based on stable titanate mineral phases, such as zirconolite, [17][18][19][20][21] pyrochlore, [22][23][24][25][26] and brannerite, [27][28][29] have been primarily investigated as potential waste forms for actinide immobilization.…”