Gadolinium Borosilicate Glass-Bonded Gd-Silicate Apatite: A Glass-Ceramic Nuclear Waste Form for Actinides
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Gadolinium Borosilicate Glass-Bonded Gd-Silicate Apatite: A Glass-Ceramic Nuclear Waste Form for Actinides Donggao Zhao1,3, Liyu Li2, L.L. Davis2, W.J. Weber2 and R.C. Ewing1 1 Department of Nuclear Engineering & Radiological Sciences, University of Michigan, Ann Arbor, Michigan 48109-2104 2 Pacific Northwest National Laboratory, Richland, Washington 99352 3 Current address: Electron Microscopy Center and Department of Geological Sciences, University of South Carolina, Columbia, SC 29208. ABSTRACT A Gd-rich crystalline phase precipitated in a sodium gadolinium alumino-borosilicate glass during synthesis. The glass has a chemical composition of 45.4-31.1 wt% Gd2O3, 28.8-34.0 wt% SiO2, 10.8-14.0 wt% Na2O, 4.3-5.9 wt% Al2O3, and 10.8-14.9 wt% B2O3. Backscattered electron images revealed that the crystals are hexagonal, elongated, acicular, prismatic, skeletal or dendritic, tens of µm in size, some reaching 200 µm 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. INTRODUCTION Considerable effort has been devoted to determining distributions and solubility limits of radionuclides and neutron absorbers in borosilicate glasses in the four-component Na2O-B2O3Al2O3-SiO2 system [1-3]. In recent years, glass compositions in this system have been developed to test the effects of changing compositions on the solubilities of the neutron absorbers Gd and Hf [1-3]. When Gd exceeds its solubility limit in the glass during synthesis of Gd-borosilicate glasses, a Gd-rich crystalline silicate phase precipitates. Likewise, Gd-rich crystalline silicate phases precipitate in Gd-borosilicate glasses during extended times at elevated temperatures [4], as would occur during slow cooling of canisters or during repository storage. The purpose for synthesizing a Gd-borosilicate glass is to incorporate as much Gd as possible into the glass. Therefore, the precipitation of a Gd-rich crystalline phase from the glass matrix was initially considered unwanted. However, as a Gd-rich phase, this crystalline phase may also immobilize actinides, as has been demonstrated previously [4]. An essential issue of the long-term immobilization and disposal of actinides is whether a waste form is sufficiently durable. Durability may be based on a variety of
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