Donggao Zhao, L.M. Wang, R.C. Ewing

Department of Nuclear Engineering and Radiological Sciences and Department of Geological Sciences

The University of Michigan, Ann Arbor, Michigan

Liyu Li, L.L. Davis, D.M. Strachan

Pacific Northwest National Laboratory, Richland, Washington 99352

The aim of this study is to understand the solubility of actinides in glasses. Hafnium and gadolinium are used as surrogates for Pu(IV) and Pu(III), respectively, but they are also used as neutron absorbers to prevent criticality. An understanding of the solubilities of rare earth neutron absorbers (such as Gd and Hf) in synthesizedA series of sodium alumino-borosilicate glasses provides a scientific basis for developing waste forms for the safe disposal of fission materialwere used as the basis with which to collect data on the dependence of solubility on glass composition. Because of mass losses during synthesis of Gd- and Hf-alumino-borosilicate glasses and compositional heterogeneity of the glasses,Initially, solubility was determined by examining the glasses under an optical microscope to determine if crystals had formed and by using the target glass composition. To be more precise in the determination of the solubility, a more sensitive direct determination of the glass compositionsof the glass compositions is required to verify or confirm the solubilities of Gd and Hf based on the initial/target compositionswas needed. Compositions The compositions of the a representative number of glasses were obtained from with an electron microprobe analysis (EMPA). The glasses analyzed are homogeneous in composition except for a sample containingOne of these glass samples was known to have micrometer-size Gd-rich silicate crystals with an apatite structure. The separated precipitated crystals contained more gadolinium (about 80 wt percentmass% Gd₂O₃) from than the bulk glass matrix (31 to 45 wt percent Gd₂O₃) and made the glass heterogeneous in composition. The glass composition near the precipitated crystals was depleted in Gd and richer in the other glass components. The measured compositions of the remaining glass samples are were mostly close to the target compositions. However, for Hf-containing glasses, the differences between the measured and target compositions are were up to 2 wt %mass% for SiO₂ and for HfO₂. The electron microprobe data also show that Na was lost during the preparation of the high Na₂O-containing (30 mole %) glasseses. The direct measurements of the glasses and crystals by with an electron microprobe EMPA confirmed that glass compositions have, in some instances, changed significantly during experiments preparation and that compositional heterogeneity exists when micrometer-size precipitated crystals formedGd-rich crystals contain boron as well as Si.

MRS 1999 Fall Meeting

Symposium QQ: Scientific Basis for Nuclear Waste Management XXIII