Department of Nuclear Engineering and Radiological Sciences

Department of Geological Sciences

The University of Michigan, Ann Arbor, Michigan 48109-2104

Pacific Northwest National Laboratory, P.O. Box 999 (K6-24)

Richland, Washington 99352

Borosilicate glasses are presently in use for the immobilization of high-level nuclear waste. New glass compositions with neutron absorbers, such as Gd, are being developed for the immobilization of actinides, e.g., excess weapons plutonium. We present data on crystalline precipitates that formed in a sodium gadolinium alumino-borosilicate glass (45.39-31.13 wt % Gd2O3, 28.80-34.04 wt % SiO2, 10.75-14.02 wt % Na2O, and 4.30-5.89 wt % Al2O3). Backscattered electron images show that the crystals are elongated, acicular, prismatic, skeletal or dendritic, tens of mm in size, some reaching 200 mm in length. The crystals are chemically homogeneous. An empirical formula based on electron microprobe analysis is NaGd₉(Si_5.25B)O₂₆. Relative to the glass matrix, the precipitated sodium gadolinium silicate phase is much enriched in Gd₂O₃ (81.25 wt %) and depleted in SiO₂ (15.66 wt %) and Na₂O (1.38 wt %). Based on crystal morphology and X-ray diffraction analysis, the crystalline phase is hexagonal, probably a rare earth silicate with the apatite structure A_4-xREE_6+x(SiO₄)_6-y(PO₄)_y(F,OH,O)₂ (where where A = Li, Na, Mg, Ca, Sr, Ba, Pb and Cd, and REE = La, Ce, Pr, Nd, Pm, Sm, Eu and Gd). Silicate apatites are generally durable and are receiving considerable attention as an actinide waste form.