Gadolinium Borosilicate Glass-Bonded Gd-Silicate Apatite: A Glass-Ceramic Nuclear Waste Form for Actinides

A Gd-rich crystalline phase precipitated in a sodium gadolinium alumino-borosilicate glass during synthesis. The glass has a chemical composition of 45.39-31.13 wt% Gd2O3, 28.80-34.04 wt% SiO2, 10.75-14.02 wt% Na2O, 4.30-5.89 wt% Al2O3, and 10.75-14.91 wt% B2O3. Backscattered electron images revealed that the crystals are hexagonal, elongated, acicular, prismatic, skeletal or dendritic, tens of mm in size, some reaching 200 mm in length. Electron microprobe analysis confirmed that the crystals are chemically homogeneous and have a formula of NaGd9(SiO4)6O2 with minor B substitution for Si. The X-ray diffraction pattern of this phase is similar to that of lithium gadolinium silicate apatite. Thus, this hexagonal phase is a rare earth silicate with the apatite structure. We suggest that this Gd-silicate apatite in a Gd-borosilicate glass is a potential glass-ceramic nuclear waste form for actinide disposition. Am, Cm and other actinides can easily occupy the Gd-sites. The potential advantages of this glass-ceramic waste form include: (1) both the glass and apatite can be used to immobilize actinides, (2) silicate apatite is thermodynamically more stable than the glass, (3) borosilicate glass-bonded Gd-silicate apatite is easily fabricated, and (4) the Gd is an effective neutron absorber